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January 5-6, 2011 Cornell Gun Review DAVE DOWELL COMMENTS ................................................................................................................................ 2

ELECTROSTATIC DESIGN: .................................................................................................................................................. 2 CERAMIC: ..................................................................................................................................................................... 2 VACUUM: ..................................................................................................................................................................... 3 SOLENOID: .................................................................................................................................................................... 3 CATHODE: .................................................................................................................................................................... 4 TESTING: ...................................................................................................................................................................... 4

CARLOS HERNANDEZ-GARCIA COMMENTS ......................................................................................................... 5

A) HIGH CURRENT OPERATION.......................................................................................................................................... 5 Some questions: .................................................................................................................................................... 7

B) MARK II GUN DESIGN .................................................................................................................................................. 7 COLLABORATIONS: ......................................................................................................................................................... 7

MATT POELKER COMMENTS .............................................................................................................................. 8

BRUCE DUNHAM PART1: RESULTS FROM MARK I GUN .......................................................................................................... 8 KARL SMOLENSKI PART1: POSSIBLE FEATURES OF A MARK II GUN ............................................................................................ 9 IVAN BAZAROV: EMITTANCE DISCUSSION .......................................................................................................................... 10 JARED: GUN ELECTRODES .............................................................................................................................................. 10 KARL SMOLENSKI PART 2: MAGNETS FOR EMITTANCE OPTIMIZATION ..................................................................................... 10 BRUCE DUNHAM PART2: HIGH VOLTAGE ISSUES ................................................................................................................ 11 JARED PART2: ION BOMBARDMENT ................................................................................................................................. 11 YULIN LI: VACUUM ISSUES ............................................................................................................................................. 11 XIANGHONG: PHOTOCATHODES ...................................................................................................................................... 12 LUCA CULTRERA: CSK2SB PHOTOCATHODES ...................................................................................................................... 12 STRAY NOTES: .............................................................................................................................................................. 12 CONCLUSIONS: ............................................................................................................................................................ 13

CHARLIE SINCLAIR COMMENTS ......................................................................................................................... 14

1. 400C bakeout: ............................................................................................................................................ 14 2. Non-recoverable QE damage: .................................................................................................................... 14 3. Ions damage from downstream of the gun: ............................................................................................... 15 4. Boiloff nitrogen gas: ................................................................................................................................... 15 5. N2 activation of GaAs: ............................................................................................................................... 15 6. Passive cooling with LN2: ........................................................................................................................... 16 7. Anode material: .......................................................................................................................................... 16 8. Titanium for electrode material: ................................................................................................................ 16 9. NEG activation power too high: ................................................................................................................. 16 10. GaAs roughness effect on emittance: .................................................................................................... 17 11. Various faces of GaAs to try: .................................................................................................................. 17 12. Large cryopump assessment: ................................................................................................................. 18 13. Conductive glass for electrodes: ............................................................................................................ 18

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14. Spherical cathode shape for focusing: ................................................................................................... 18 15. XRF studies of photocathodes: ............................................................................................................... 19 16. Divots: .................................................................................................................................................... 19 17. Anodizing vs masking: ............................................................................................................................ 20 18. Conditioning resistor: ............................................................................................................................. 20 19. SF6 high pressure vessel: ....................................................................................................................... 20 20. Antimonide cathodes: ............................................................................................................................ 20 21. Electric power to the cathode: ............................................................................................................... 21 22. Shield rings in segmented ceramic: ........................................................................................................ 21 23. General remark: ..................................................................................................................................... 21 24. SRIM: ...................................................................................................................................................... 21 25. Follow-up: .............................................................................................................................................. 21

WILL WALDRON COMMENTS ............................................................................................................................ 23

HIGH VOLTAGE DESIGN: ................................................................................................................................................ 23 TESTING: .................................................................................................................................................................... 25 COLLABORATIONS: ....................................................................................................................................................... 25

Dave Dowell comments

Electrostatic Design: -Excellent analysis to determine optimal angle for the cathode focusing -The adjustable gap should be pursued given its potential benefits. - The effort required to add an intermediate electrode doesn’t seem justified by the marginal gain in cathode field. It causes strong defocusing leading to beam loss and likely emittance growth. These problems are potentially solved by adding more electrodes to move the defocus further from the cathode, making the gun design looking like the old vacuum tubes. Perhaps a multi-electrode design should be investigated for future guns, but a single intermediate electrode for the MKII doesn’t seem warranted. -However, there is a strong need for additional focusing near the cathode. Unfortunately, electrostatic focusing by curving the cathode reduces the cathode surface field too much. This means adding external focusing with either a magnetic solenoid or additional electrostatic lenses. -General recommendation concerning focusing optics: Leave the cathode flat, continue to use electrostatic focusing with ~26 degree angle, include focusing anode solenoid as proposed in solenoid section below to reduce magnetic field at cathode. Investigate magnetic shielding to reduce cathode field to acceptable value.

Ceramic: -Ceramic punch-through: Based upon the Japanese results, this problem will be greatly mitigated by the new ring shields between the HV stalk and the ceramic column. However there remains the

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potential for thermal loading on and even penetrating the shield rings due to field emission from the HV stalk. Thought should be given to any instrumentation which can indentify excessive thermal load before the ceramic is damaged. -Continuously monitor the HVPS ripple and try to determine if this is causing the gun breakdowns. -Implement a fault system that quickly shutters/latches off on the next laser pulse if there is any RF fault in the cryomodules.

Vacuum: -The initial simulations of ion back bombardment are very interesting and instructive and should be pursued. In particular the phenomenon should be further modeled to understand if the computed ion flux is consistent with the observed QE loss. The agreement between simulation and observation of the pattern at the cathode center suggests this is the correct effect, but it would be good to verify the numbers. -Biasing the anode or another electrode after the anode to reduce the ion flux sounds like a good idea. - The gun vacuum appears to be good enough since most of the gun pressure during operation comes from the beamline downstream of the gun. Although difficult, effort should concentrate on improving the vacuum downstream of the gun. Fortunately the good lifetime performance of CsK2Sb should relieve the overall vacuum requirement.

Solenoid: -The gun solenoid will require a bucking coil. The embedded bucking coil adds too much complication and shouldn’t be implemented. Consider making/forming the bucking field with permanent magnets instead of a coil. Also try using magnetic shielding like soft iron and mu-metal to cancel out the field. -What is the beam size at the anode solenoid vs. bunch charge and average current (and beam size on cathode)? Show the beam envelope from the cathode through to the buncher cavity for these various conditions and indicate the locations of the anode hole, the anode solenoid. -Is it truly necessary to have the solenoid field extend toward the cathode? This only increases the cathode field to buck out. Having the iron/magnetic gap face the beam axis instead of the cathode will reduce the magnetic field on the cathode and perhaps allow passive magnetic shielding instead of an embedded coil.

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-Study higher-order fields of the anode solenoid using a rotating coil. A short (~2 cm long) rotating coil will allow measuring the z-dependence of the multi-pole fields. Investigate having these measurements done at SLAC. -Consider adding normal and skew quadrupole correctors for the anode solenoid.

Cathode: -Great progress has been made in fabricating and beam testing of CsSbK2 cathodes. The ~5% QE’s are quite good for the first cathodes. -To achieve higher QE’s the cathode needs to be doped with a gas such as oxygen or water. Since both are difficult to control, Jym Clendenin has suggested using NF3 as a dopant. -Cathode cooling scheme seems good enough for now.

Testing: -The ongoing cathode studies of the lifetime and thermal emittance of CsK2Sb and other cathode materials is an excellent cathode research program. The recent results with CsK2Sb are very encouraging! Great progress in such a short time! -Test the presence of anomalous quadrupole fields by reversing the polarity of each solenoid and determine if the emittance changes. This should be done at high bunch charge when the beam is large in the solenoid since the effect scales like the square of the beam size at the solenoid. If the test indicates there is a quadrupole field then consider adding normal and skew quad correctors. -While difficult to implement, it would useful to be able to view the shield rings from the top of the ceramic stack with cameras through view ports. This will allow a detailed measurement of the thermal load pattern on the rings. See figure:

Reoriented Iron Core

Reoriented Magnetic Gap

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Carlos Hernandez-Garcia comments You are setting new standards and we look forward to your continuing success. I am very pleased to learn that you are engaging with material science experts for cathode technology and with high voltage experts for their advice on operational issues. These are disciplines that need to come together in DC guns for achieving unprecedented emittance and high current. Remarkable progress. Congratulations! I am dividing my notes into two sections: a) High current operation, and b) Mark II gun design.

a) High Current Operation First effort should be focused on eliminating, or minimizing the cathode arcing, which JLab believes it is actually triggered by the booster, and not the gun. When there is an arc in the SRF coupler, the machine protection systems shuts off the RF, causing the beam to decelerate and expand, impacting the SRF cavity and desorbing significant amount of gas (Hydrogen?) that reaches the gun, therefore increasing the pressure in the cathode-anode gap and finally triggering a cathode discharge. GaAs vaporizes instantly since the gun is still at voltage. To address this issue we propose:

IR cameras

shield rings

HV stalk

cathode

ceramic insulatorstack

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• Condition the SRF couplers prior to high current run. In the JLab case, the couplers were

conditioned up to 30kW CW, but probably needed to be condition to 50kW for 10 mA CW beam.

• Monitor the cavities tuners while ramping up the current. In the JLab FEL, the tuners are mechanical and take seconds to respond.

• If the waveguide is under vacuum, monitor the vacuum conditions and interlock the ion pump read back to the machine protection system to shut the beam off when a pressure rise reaches certain value.

Second, but probably has significant contribution to cathode arcing, is electron beam halo induced by drive laser scatter light.

• Even though in the present configuration there are AR coated windows (with 0.2% reflectivity at best), drive laser beam reflections can be significant. With a very small tilt in the window with respect to the laser beam path, the reflected beam off the cathode strikes the window at a slightly different location. This reflection is reflected back to the cathode a few mm off the main laser spot location; the arrival of laser pulses reflected by the window will have a different timing than the main pulse as well. It is therefore strongly encouraged to polarize the laser beam and to use windows at the Brewster angle.

• It seems that laser beam shaping optics and optical transport system are significantly decreasing the power delivered to the cathode. For the purpose of demonstrating high current and for studying QE lifetime, it may seem reasonable to work on the laser shaping off line, focusing on optimizing the laser transport system to deliver high power, and to stabilize the beam spot positioning in the cathode, which can be done by an active feedback system. This will have a significant effect during high current runs, i.e. if the spot moves on the cathode, the electron beam path will wander off the orbit set for minimal pressure rise.

• At some point of course, the demanding requirements for emittance must be addressed, and this is when laser beam shaping optics will play a vital role. As a suggestion, beam shaping can take place before amplification, but it will be a feedback process since amplification will also alter the pulse shape.

• The combination of laser shaping optics, stability and high power impose stringent requirements on the overall drive laser system. We strongly recommend devoting considerable resources (engineers, post-docs, laser optics experts, etc) to address this important issue that is often overlooked. The DC gun community would benefit tremendously from this effort.

• Even if the gun vacuum is better than currently achieved (which is very good), vacuum in the beamline between the gun and the booster needs to be improved, especially pumping speed. It is difficult to do bakes with the amount of diagnostics and beam optics, but this is something that needs to be considered.

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Some questions: • Has dark current (field emission, no beam) being observed, from the CsK2Sb cathode? If

so, it would be interesting to compare with that from GaAs at the same gun voltage. • How is the injector beam optics setup? Is it setup for emittance compensation? If so,

might not be best solution for high current operations since beam transverse size can be very large especially at the first solenoid. In the JLab FEL, first solenoid is set to minimize spot size at buncher, then second solenoid to minimize emittance at the injector exit.

b) Mark II Gun design • Stay away from the vacuum embedded solenoid. Although simulations show ~20%

emittance improvement, I fear that powering up the solenoid and dealing with vacuum issues will consume significant efforts that can be put in other gun systems

• I would opt for the floating anode rather than the movable anode, which also seems mechanically challenging. It would be advisable to start with a fixed anode-cathode gap and anode-cathode geometry, then perform all emittance and beam characterization to infer from simulation what would be the best geometry for a particular bunch charge, and operate there.

• The dual gap electrode seems adding complexity to the system with little gain towards emittance or even operational voltage. It would also present challenges with field asymmetries. If this topic is for a thesis project, it can be developed on a separate, dedicated chamber with ~100kV or less to demonstrate principle. SLAC (more precisely Charlie Sinclair) developed a similar dual gap gun, but it had cylindrical geometry.

• JLab makes available all drawings used in the design of the resistor assembly for the FEL gun, in which the conditioning (100 M-Ohm resistor) can be “shorted out” with the running (~400 Ohm) resistor by means of internal DC motors driven by insertion rods into the SF6 pressure vessel. This allows changing resistors without opening the SF6 pressure vessel.

Collaborations: The JLab FEL will be happy to provide any further insights into solving the cathode arcing problem during high current runs. I would be most enthusiastic to collaborate with Cornell and CEBAF’s polarized source group in testing a CsKSb cathode in the CEBAF inverted gun test chamber. One could imagine driving the cathode from Ithaca to Newport News in a vacuum suitcase.

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Matt Poelker comments (I provide my editorial comments and notes from each presentation, and then give some “conclusions”)

Bruce Dunham part1: Results from Mark I gun

• How to monitor HV power supply while operating? Stability, ripple, total current draw. • Current surge from cathode, a very nasty problem. Strong desire to understand nature

of this problem and ensure it doesn’t happen again. Turn OFF laser and ramp voltage down? But there is a realistic concern that it will not be possible to respond quickly enough to avert disaster.

• How to keep laser power constant? Noise eater, but this means less power available at photocathode. Laser power scarce today and will be even scarcer at higher current.

• Demonstrated 25mA average current with RF structure, from GaAs photocathode. Getting very close to Boeing record.

• Demonstrated 8mA w/CsK2Sb, good lifetime. This is a very nice result and a potential “game changer”. CsK2Sb: 2.7% QE at 532nm, 4% QE at 520nm

• Two modes of operation, both with 77pC bunch charge: 50MHz (4mA avg current) and 1.3GHz (100mA avg. current).

• Commented that it was difficult to find operating conditions that accommodate a range of bunch charge (i.e., what you get when you change beam current by changing the power of the laser light using an attenuator). Considering using a pulse picker so that average beam current could be varied, but the bunch charge stays constant. Seems like a good idea.

• 10mm active area, out of ~ 25mm total. 2 to 3mm laser beam diameter • Also used a laser beam that was larger than active area: 3mm active area and 4mm laser

spot. Poor man’s way to generate top-hat profile. • Observed FE from defects on GaAs wafer @ 350kV bias voltage. No FE @ 250kV bias • Gun capacitance ~ 100pF. Kaisser HV supply ~ 100pF capacitance • Gun vacuum with valve closed = 7x10-12Torr, 1x10-11Torr with valve open • With original insulator (no conductivity), they had punch through at 450kV. Alumina

97% (alumina = Al2O3). What is the other 3%? • Same thing happened with bulk conductivity insulator: Conclusion, “conductive”

doesn’t help? Why not? What is the material that provides conductivity? Carbon? • Punch through likely related to high gradient associated with support tube passing

through insulator bore. Gradient highest at this location. For new gun, go to 24” diameter insulator to reduce gradient significantly. For new gun, highest gradient will be at cathode/anode gap. But this means wire seal flanges…

• Don’t use CPI for brazing, they don’t have big enough oven.

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• We discussed the purpose of the conditioning resistor: not there to “fold back” gun voltage and halt FE, as I originally thought. Rather it is there to absorb the stored energy of the gun and HV supply when some catastrophic event occurs.

• 100MOhm seems to be the ballpark correct size for a conditioning resistor

Karl Smolenski part1: Possible features of a Mark II gun

• Goal: operation at 500kV and 100mA, 1.3GHz rep rate, 2 to 3ps pulses, 77 pC/bunch. Also want to bias to 750kV and operate at voltages > 500kV, but not necessarily 100mA.

• Do you think you suffer from drive laser mirror too near the beam? By suffer, I mean do you hit it with beam and degrade vacuum, hastening QE decay?

• At your laser powers, do you think the photocathode is heating up? Did you evaluate the effectiveness of the Mk I cathode puck cooling? Yes, it is adequate.

• Do you think you suffer emittance degradation because of asymmetric side ceramic gun design?

• Can a running resistor be used for electron bunch shaping, and thereby get rid of (lossy?) laser table optical elements? Or is there too much capacitance in system? It would introduce energy spread.

• What is JAEA up to? Have they operated their segmented insulator gun with beam at 500kV?

• On subject of wire seals, I note Russia/Novosibirsk just welds chambers together. Of course this is a pain when the system needs to come apart.

• Does a really big ion pump help or hurt? • Guard rings made of copper. Charge from FE will bleed away through HV power supply.

But will the FE beam simply drill a hole through copper guard rings? And then through the insulator again?

• Why not rotate the NEG modules to completely eliminate line of sight from wall to cathode/anode gap.

• Cornell considering a double walled gun to implement cooling of the interior vacuum chamber, to reduce outgassing rate. P. Redhead “Extreme High Vacuum” lists outgassing rate vs wall temp.

• Considering TiN coating to act as diffusion barrier • I wonder if it would be worthwhile to eliminate cathode/support tube adjustment

features, just weld support tube to top flange, get rid of conductance pump-out holes and have the tube be part of the vacuum chamber. This would (significantly?) reduce the surface area of the gun chamber and therefore improve vacuum. It would also allow you to get your cathode chiller closer to the puck. Electrode alignment could happen at anode or via some bellows attached near the cathode flange or insulator.

• How good does the stainless steel need to be for cathode and support tube, to minimize FE? 316LN VIM/VAR, etc., Vendors only sell this in large quantity, and this amounts to lots of money, 150k$ No one knows specs for good steel, in terms of eliminating FE.

• What about Li glass electrode? It would be smooth. Harvey Gould at Berkley

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• Solenoid embedded within the cathode electrode and another outside the vacuum near anode. This allows use of flat cathode electrode and higher field strength near photocathode. Solenoids provide the focusing to capture beam. But can’t have magnetic field at photocathode surface. How do you know field has been bucked?

• I suggested permanent magnets but these work for only one beam condition • Tilt-able anode might help to optimize emittance during off-axis running • Anode on moveable gap: another knob to shape beam, optimize emittance, and

perhaps to assist HV conditioning (small gaps = large fields at low voltage) • Normal operating conditions call for 50mm gap. This seems small – CEBAF gap = 60mm

and we have ~ 8MV/m at just 200kV bias. • Considering a gun design with an intermediary electrode and two gaps.

Ivan Bazarov: Emittance discussion

• Striving to obtain small emittance at high average current. Best emittance obtained at high gun bias voltage

• Gun geometry optimization: vary the cathode focusing angle, the gap and the photocathode recess

• Space charge limit: I am puzzled, thinking that for small beam size and short pulses, Cornell would be operating way below space charge limit.

• Measured emittance > predicted emittance. Sorting through possible explanations. Dave Dowell mentioned problems with solenoids at LCLS, a small quadrupole field served to degrade emittance appreciably.

Jared: Gun Electrodes

• Modeling gun, first pass - no space charge, just single electron trajectories • Optimize gun electrode shape : goal to minimize focal length (z-location where electron

crosses axis) • Anode shape basically doesn’t matter, at least with single particle trajectories and no

space charge • How will you know the bucking coil is properly bucking? • Exploring three-electrode, two-gap gun

Karl Smolenski part 2: Magnets for emittance optimization

• Use a flat cathode electrode to maximize the field at photocathode. Then use magnet near anode to focus the beam, and a magnet inside cathode electrode (or behind cathode) to buck the field at the photocathode

• Also considering a design with intermediate electrode.

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Bruce Dunham part2: High Voltage Issues

• Goal: keep gradient < 10MV/m at support tube at 750kV bias voltage • Triple point = metal, vacuum, insulator joint • Need to shunt the energy away from delicate stuff when disaster looms. • Caddock sells HV resistors and varistors. Varistors clamp voltage, nominal 1800V value.

They start to conduct at higher voltage. I guess we’d rather have a current clamp…. • What is the stored energy of the system? ~ 6 Joules. During HV processing, you want to

reduce the stored energy delivered to the gun (i.e., delicate stuff). We do this with an in-line conditioning resistor, nominal value 100MOhm.

• Problem: the resistor is removed for running, so there’s less protection. • For next gun, initially the HV system will be tested without the cathode electrode and

support tube installed. • Floating ammeter to measure HV power supply current • Max FE ~ 100uA but typically much less. Max pressure during processing 1x10-8Torr.

Quickly reaches 250kV, then ramp > 250kV at rate 5 to 10kV/hour. • I wondered if HVPS ripple varies with gun current (Glassman says it does for their 150kV

supply). So maybe ripple increased at high current, and beam became un-manageable?

Jared part2: Ion bombardment

• I suggested H2 ions become trapped interstitial defects that degrade diffusion length and reduce QE. Bill Schaff commented that dopant species can trap H2 ions, some dopants do this better than others. Interesting test: purchase GaAs with different dopant species and compare lifetime under similar vacuum conditions.

• Discussion about different cleave plane surfaces. Note mechanism referred to as ion channeling. Even a small change in crystal orientation can have huge. Another interesting test: purchase GaAs samples with different cleave plane orientations and evaluate lifetime under similar vacuum conditions.

• Jared presents the beginnings of a nice model describing ion bombardment. Step 1, how many ions are created by beam as it travels from cathode to anode and out the gun. Step 2, he tracks these ions back the photocathode. Next, he should consider what these ions do at the photocathode, e.g., using SRIM code.

Yulin Li: Vacuum Issues

• Presents a nice pressure profile from gun chamber along A1 beamline. He wishes pressure in A1 beamline was better. Alas, no room for differential pump or NEGs

• Obviously, improving A1 pressure is the best place to focus attention since gun vacuum is considerably better

• Cool everything to reduce outgassing rate? • 400C prebake of A1 components is worthwhile, but not all components can be baked so

hot.

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• Perhaps reduce the tuning range of buncher, to eliminate surface area and eliminate small space that is tough to pump.

Xianghong: Photocathodes

• Wobble stick with fork added to the prep chamber for swapping pucks • Anodize the whole wafer, cut circle (how is it protected during cutting?), then mount to

puck, indium solder, then remove anodization at end with ammonium hydroxide (and hydrogen peroxide and clean water from Millipore?).

• Wafers can be H cleaned using cracker near prep chamber • QE 10% at 520nm so doing pretty well, i.e., the method works. • Good results activating with XeF2 and N2!! (Appl Phys Lett 92, 241107 (2008)) • Charlie suggests using GaAs with 111A cleave plane and with low dopant. This material

gave high QE and low dopant will provide smaller band bending, so perhaps less surface charge limit?

Luca Cultrera: CsK2Sb photocathodes

• Presents historical table of QE from alkali photocathodes • Cornell results to far: CsK2Sb 4.5%QE at 520nm, 3.2%QE at 532nm • Luca deposits the three chemicals multiple times during the activation • Silicon is the substrate but would prefer something else, with a crystal structure that

matches the CsK2Sb lattice. Didn’t like moly substrate. • Pressure inside grow chamber 4x10-10Torr and dark lifetime infinite. • They have grown about 12 photocathodes so far, all with SAES dispensers. Indeed, the

SAES dispensers run out quickly so he tack welds ~ 4 of them in series. Each dispenser holds about 4mg. Alvatec dispensers hold ~ 100mg

• Cs3Sb QE 0.5% at 532nm • NaCsSb QE 2% at 532nm • Dave Dowell described Boeing CsK2Sb photocathodes

o First they grew CsK2Sb photocathode in prep chamber, using CsK eutectic source o Then “poison” the photocathode with water “dopant” inside the RF gun, watch

QE drop to half. o Then move back to prep chamber and heat to 150C, watch QE go back UP o Cool to 135C and just add Cs and K (no Sb) and repeat this a few times until

photocathode gets “crusty”. o He measured QE 12% at 527nm

Stray notes:

• Joe Grames can explain fast and slow QE lifetime due to Gaussian profile of laser beam. In the beginning, lots of beam from center, then you start to get more beam from edges. He will write up and send out to interested parties.

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• Charlie recommends against using effusion style alkali source for potassium, it would need to be very hot.

Conclusions:

• Must make evaluation of CsK2Sb photocathode top priority, as it will determine what vacuum level you can live with, how much effort to devote to gun/beamline vacuum improvement. It might determine what level of FE you can live with. This photocathode might not get damaged so badly if you encounter another “current surge”, although there probably aren’t many things that can survive kA current….

• How to know the HV power supply is functioning properly, and what steps can you take to avert destruction of photocathode: close laser shutter and ramp down voltage? This might not happen fast enough. Similarly, what machine protection do you implement to protect system from other problems that could degrade vacuum near gun, like RF cavity trip, laser position suddenly moves to edge of photocathode, or a magnet turns OFF, or IOC crashes.

• If emittance degradation comes from “elsewhere”, how do you propose to find it/fix it? • Vacuum:

o obvious goal, improve A1 beamline vacuum so that gun vacuum does not go UP when valve is opened. 400C pre-bake as much of the A1 beamline as you can before putting things together. I don’t have as much hope re: NEG coating….we don’t get much pump speed from our coatings at JLab.

o Consider making the cathode support tube part of the vacuum chamber outer wall. Weld the support tube to the top flange, have it be hollow. I suspect this would eliminate significant amount of surface area (note, the interior of the support tube has no pumping, and all gas originating from tube needs to pass by photocathode to be pumped away)

o With a hollow support tube, I suspect you could more aggressively actively cool the photocathode puck, which might be useful at 100mA.

• The segmented insulator is not a “done deal” in terms of fixing punchthrough. What is your plan C? Two-gap gun?

• Nebulous comment: gradient versus voltage - is one factor significantly more important than the other? Where does most emittance degradation come from? Would you be better off with a lower voltage gun but with small gap/higher gradient versus a higher voltage gun but with larger gap/lower gradient?

• We noted the “discord” within community about GaAs QE decay mechanism(s). Community needs more experiments that shed light on ion bombardment. I was a little surprised to hear Charlie say vacuum can’t be used to explain poor gun lifetime….i.e., QE decays faster than can be accounted for by vacuum and ion bombardment.

• I recommend saving the solenoid approach to improving emittance for a Mark3 gun design. Similarly, I would put off the three-electrode, two-gap gun design for later. It will be challenge enough to build new gun with larger-bore segmented insulator.

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• Would it be worthwhile to commission the gun with DC light from simple Verdi laser? No space charge problems. Should be easier to set up the beam. A 10W laser probably lives somewhere at Cornell, plenty of power to make 100mA, which could look good on paper. i.e., it’s not the beam you want, but it’s still impressive.

• What is nature of catastrophic failure of photocathode? Stored energy ~ 6 Joules (100pF and 350kV), with kA instantaneous current. What triggers the release of this current? A rough surface? How to prevent this unregulated, uncontrolled liberation of energy? Obviously this stuff is on your mind, unfortunately, I don’t have much to offer you in the way of assistance….

Charlie Sinclair comments (The points below are not rank ordered.)

1. 400C bakeout: I believe that the results obtained during Park’s sabbatical year on the use of 400 C air (or vacuum) bakeout clearly demonstrate how to reach very low outgassing rates from stainless steel vacuum chambers. I also believe that obtaining an excellent gun vacuum is highly desirable in the long run. Therefore, I believe that every effort should be made to design the new gun to have only thin walls. I have looked enough at the problem to believe it can be accomplished to a high degree. I also think that it would probably pay off to do the same thing for Siddarth’s energy analyzer system. If the large “coffin” that we used for the 400 C firing still exists, it is pretty straightforward to fire a large number of modest sized parts in a single run, so it doesn’t have to be particularly burdensome.

2. Non-recoverable QE damage: The GaAs cathode that showed “non-recoverable” damage in the central region should be atomic hydrogen cleaned, since so far, “non-recoverable” damage means only that the QE cannot be recovered by heat cleaning. There is fairly good evidence that the majority of the ion back bombardment damage comes from downstream of the anode aperture. I haven’t seen an RGA spectrum from this region, but I would bet methane is present to some degree. If this is so (or perhaps is there is water), the damaged region could have carbon contamination from cracking of the methane (or oxygen from water??). Such carbon contamination was observed years ago in the polarized source at Mainz. Carbon is very tightly bound to GaAs, and cannot be removed by heat cleaning. After we demonstrated the effectiveness of hydrogen cleaning at JLab, the Mainz group hydrogen cleaned one of their cathodes

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with “non-recoverable” damage and found that it allowed complete restoration of the QE. You may find the same. It would be an interesting result in and of itself. The question of the exact details of what ion back bombardment does to the GaAs is not definitively answered. Some believe that the ion impact physically damages the crystal structure, in a way that reduces the diffusion length. I believe that if such structural damage occurs, it is unlikely to be restored by a heat cleaning cycle. If instead, the QE can be completely restored after hydrogen cleaning, I think this will be evidence that can place a limit on the amount of structural damage done to the crystal.

3. Ions damage from downstream of the gun: The evidence that your ion back bombardment damage arises from downstream of the anode aperture is fairly strong. I believe the new gun should incorporate either a biased anode, or an electrode immediately following the anode, that can be operated at a voltage high enough to prevent these ions from reaching the cathode. This, possibly combined with vacuum improvements downstream of the gun, may make it possible to operate from the electrostatic center – something that I believe is a legitimate long term goal. It is probably wise to design and build an electrode to mount immediately downstream of the present anode, and have it ready for installation whenever the A1 vacuum is opened. If you change to button BPMs instead of the present striplines, this might be a good time for such an installation.

4. Boiloff nitrogen gas: The point was made that nitrogen from the Dewar boiloff is of poor quality, in terms of both chemical purity and possible particulate contamination. As this nitrogen is routinely used for venting vacuum systems, and has been used to blow dust off gun parts, it makes some sense to determine how bad the problem actually is, and fix it to the maximum extent practical. For the sorts of vacua you need to be able to create, it is essential that high quality venting gas be used. If particulates are a real possibility from your venting source, you should like use a filter like those the SRF people employ.

5. N2 activation of GaAs: The result of a GaAs photocathode being activated with cesium and nitrogen is fascinating, and should be repeated with care to assure that it is correct. One needs to be sure there is no water impurity, for example, as this might be an oxygen source for activation. If indeed it is found that cathodes can be activated by cesium and pure nitrogen, this result should be published. You might also consider contacting Pianetta’s group to see if they have any interest in doing their excellent XPS studies on a nitrogen activated cathode (possibly with a Cornell collaborator?). I believe that this group has X-ray beam line facilities at SSRL to do such studies regularly.

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6. Passive cooling with LN2: It was mentioned that it may be practical to cool the

photocathode by placing a liquid nitrogen cooled plate at the front of the gun, along with cooling the anode itself. This is a quantitative question, and if it should prove feasible, it should likely be implemented in the new gun. It may be practical to test the idea by cooling the anode area in the present gun, since the area that can be cooled already presents a large solid angle to the cathode. If this works, it would significantly reduce the complexity of the cathode mounting in the new gun, and would allow removal of the long copper rod in the present gun.

7. Anode material: The issue of the proper material for the anode was raised. It has been stated that a genuine temperature rise has been observed during processing in the present gun. I firmly believe that Be is a wise choice for the anode material, and that this choice is strongly reinforced by the observation of temperature rise in the anode. Be has excellent thermal diffusivity, is a hard material, and the penetration depth of electrons is the best (highest) of any practical material. It might be worthwhile to understand outgassing from Be, and whether there is any treatment (high temperature vacuum bakeout??) that could reduce this outgassing if it is significant. I attempted to learn about outgassing Be while I was still at Cornell, but was unsuccessful. I’m certain people at Brush must know the answer, if you can find the right one to talk to.

8. Titanium for electrode material: I think the question regarding the suitability of Ti4V6Al for the high voltage electrodes is unsettled. Admittedly the first electrode set had high particulate contamination when it was removed. This is believed (but not proven?) to be from the coating material of the first ceramic, which was mechanically sanded in an attempt to make the surface resistivity more uniform. JLab observed that clean titanium alloy electrodes showed lower field emission than stainless electrodes in their polarized guns. JLab does not use Ti electrodes because they found that after exposure to cesium, the field emission was worse than from similarly cesium-exposed stainless. In the JLab guns where this was seen, the photocathodes were fabricated in situ in the gun – i.e. the entire electrode was exposed to cesium during cathode activation. This issue is not relevant to the load-locked gun situation at Cornell (or now at JLab, for that matter). So – I’d think seriously about trying with titanium again. Whether there is any utility in trying some of the pure titanium that the Japanese have found to perform so well is another question. It might be well worth looking into, possibly in the new high field, high voltage test stand.

9. NEG activation power too high: The total power delivered to activate the NEG arrays in the present gun is quite high (ca. 7 kW, I think). During activation, there are various

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sounds that come from within the gun chamber reflecting, I believe, motions that occur from the thermal expansion (or possibly things that are constrained breaking?). It always made me nervous to activate the NEGs. Now that you are redesigning the NEG mounting a bit, by removing the ground connection, you might also think of a mounting scheme that would allow the NEGs to expand thermally easily – I’m sure it could be done with a little thinking. You could also think of buying and mounting simple strip, rather than the complete modules – it might save space and simplify mounting.

10. GaAs roughness effect on emittance: Siddarth’s results on the thermal emittance being driven by surface roughness resulting from the high temperature cleaning cycle are VERY IMPORTANT. If he is correct, and in fact the mean transverse energy (MTE) from a very smooth GaAs cathode is only a few meV, it would be a result of highest importance for the high brightness electron beam business. Presently there are a number of attempts being made at various laboratories to generate brighter electron beams. If the MTE of GaAs could be shown to be very small, that would, I believe, provoke a lot of activity focused on moving GaAs photoemission guns to the next stage of development. Thus, it is of highest importance, in my mind, to put this result on a very firm foundation. One question in my mind is whether or not the surface roughening is random, or is on 110 facets. The early literature on GaAs photocathodes clearly reflects that they knew about surface roughening, but it is stated in several quarters that this roughening was due to faceting on the 110 planes. It is important to understand whether the roughness is random or 110 facets. If it is 110 facets, then presumably making a photocathode on the 110 surface should have far less roughening. It may also be worthwhile to try to determine if there is a useful temperature at or below which the roughening does not occur (useful meaning that it would still “clean” the surface prior to cathode activation). If atomic hydrogen cleaning is used, I think it is necessary to follow it with a heat cycle to remove the hydrogen trapped in the GaAs. I did studies of hydrogen removal following atomic hydrogen cleaning (using deuterium) at JLab, and perhaps that data could be resurrected. If I remember correctly, 400 to 450 C is adequate to remove the hydrogen. Alternatively, there may be people in the semiconductor community that already know the answer to this issue, since atomic hydrogen cleaning has been used to clean substrates prior to epi growth in the past. Would another III-V compound have different thermal roughening characteristics? I know that InP has to be cleaned at a much lower temperature than GaAs (but maybe it roughens at lower temperatures as well?).

11. Various faces of GaAs to try: I believe that there is some merit in studying the preparation of NEA cathodes on at least three of the four crystallographic faces of GaAs.

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You presently use 2o off 100 – a common substrate for growth. 110 is believed to give a lower QE than 100, but may behave significantly differently with regard to surface roughening. 111A is believed to have the smallest band bending, and thus could use a significantly lower dopant level. Years ago, I made cathodes on these surfaces. I am particularly interested in 111A as a vehicle to reduce the dopant density significantly. As far as I know, 111B gives a lower QE, and doesn’t seem interesting to study, but who knows? I wouldn’t make this a main line activity, but it would be a good thing to study with Siddarth’s energy analyzer.

12. Large cryopump assessment: I am personally skeptical about the utility of a large cryo pump, and the issue of a suitable valve for such a pump is certainly problematic. It does have one possible advantage in that it might better reduce the methane levels. Personally, I think it would be interesting to explore a large-ish magnetic levitation turbo pump. A 500 l/s pump would have a more modest throat compared to the cryo pump, so an all metal valve might be more affordable. I think it is interesting to explore the elimination of ALL methane, particularly if the presence of carbon is shown to be the source of “unrecoverable” ion back bombardment damage. It is said in many places that the methane arises from the ion pump itself, but I am not aware of an actual demonstration of this. I think it would be interesting to do a small, dedicated experiment to determine if eliminating the ion pump could eliminate the methane. I can describe how I would do this measurement if you would like. This could have a real impact on the vacuum design of any future gun.

13. Conductive glass for electrodes: I believe there is some merit in figuring out if electrodes of conductive glass might be helpful in reducing field emission at the field strengths desired in a gun. Many years ago, electrostatic particle separators using conductive glass plates were employed to separate pions and kaons for bubble chamber experiments. These devices had huge electrode areas – much larger than the total in a HV gun. If the technology could be reproduced, it might provide a path to reducing or eliminating the field emission problems in very high voltage guns.

14. Spherical cathode shape for focusing: The use of a spherical cathode might help a great deal with focusing in the gun. Such a cathode shape would be easy for the alkali antimonide case, I’d think, and I would bet could be done with GaAs, though it would not be so easy. I don’t know about the issues of operating a spherical cathode with illumination of an off axis small spot, however. It may also be interesting to explore the use of transmission cathodes. These would certainly pose difficulties, but would also have a number of advantages.

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15. XRF studies of photocathodes: I believe that the use of X-ray resonance fluorescence

should be considered as a way to understand things like cesium depletion and chemical contamination resulting from ion back bombardment. The setup is pretty easy, and can be done at CHESS. I mentioned this to Don Bilderback, and he said that CHESS now has a new, state-of-the-art system. Darren Dale is the CHESS contact person for this. If it were desired to look for elements with a Z below about 12 or so, it would be necessary to do so in a vacuum, and this might be worthwhile (looking for F, O, C….). JLab might be able to provide cathodes that have clear evidence for ion back bombardment damage. This could be a good student project.

16. Divots: I don’t know that anyone convincingly understood the business about both visible mechanical damage to the cathode (divots) and complete cathode QE destruction arising from an event in which the SRF trips off. However, it is true that the vacuum pressure within a cryomodule prior to its cooldown is not particularly low – 10-7 mbar or so is typical. Following cooldown, this gas condenses out, but it is only weakly sorbed (physisorbed) on the cold surfaces. A sudden change in the RF field could desorb significant quantities of gas. To give some idea of this, there are 3.5 x 1019 molecules per Torr-liter of gas. If I say that the sorbed gas in the first cavity is 10-7 Torr times ~ 10 liters, then you might imagine desorbing on the order of 1013 molecules, of which roughly half come out the front end. These would be moving with thermal velocities, so would take a msec or two to reach the cathode. The solid angle of the anode aperture as seen by the cavity exit isn’t too large, but with the high beam current, you could easily imagine ionizing a fair fraction of the backstreaming gas, leading to a blast of ions entering the gun and being accelerated to the photocathode. It is easy to imagine that this could be enough to destroy the QE, and to possibly trip the HV DC power supply on the gun. Ions striking the GaAs will surely create electron-hole pairs, which could lead to a surge in emission. Presumably in an RF trip, a similar blast of gas would come from the exit end of the cryomodule. It might be easier to look for this blast of gas at the exit end, rather than in the A1 section which is so very crowded. What might initiate this RF trip? It would be interesting to know how large a nearly instantaneous current change could be tolerated by the RF controls – I would think that the RF control people could give a quantitative answer to this question. Could a relatively sudden change in reflected power, due to the current change, be the cause? Finally, it is hard to imagine what causes the “divots” seen at JLab (but missing at Cornell?). Xianghong told me he checked the energy necessary to remove the amount of material Jared saw, and it was on the mj level. At 250 kV, the energy stored in the gun capacitance is a bit over 3 j, so

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that isn’t apparently an issue. What it would take for the much larger JLab divots I don’t know.

17. Anodizing vs masking: Presently you anodize to prevent photoemission from the larger radius area of the cathode. It is also possible to accomplish this by masking the area illuminated by cesium during activation, as it has been convincingly demonstrated that cesium does not migrate on the surface. If there is any problematic field emission from the nominally unactivated surface, the questions arises as to which is better – cesiated anodized GaAs or uncesiated, unanodized GaAs. My suspicion is that the uncesiated area may have lower dark current. If dark current from this unactivated region should prove a problem, it would be worthwhile to determine which treatment is best from a dark current perspective.

18. Conditioning resistor: It looks like you are getting on top of the conditioning resistor problem. If problems persist, I believe it would be a reasonably straightforward matter to construct a resistor using a column of water with a modest amount of a dissolved salt – maybe CuSO4.

19. SF6 high pressure vessel: Since you are building new SF6 chambers, it might be worthwhile to determine exactly what pressure of SF6 is actually required. The 5 atm. absolute number presently used is known to be very conservative. Paul Bannister at Kaiser told me that he would simply start reducing the pressure in small increments until he observed the growth of “hash” on the voltage feedback signal, and then run at a pressure somewhat above this level, where the hash was absent. The field along the power supply circuit board stack is actually only about 2.5 MV/m at 750 kV, and it certainly doesn’t require 5 atm. to keep that in check. You might be able to safely reduce the specifications for the SF6 pressure vessels, thus possibly cutting the costs. At TRIUMF, they looked into the insulating gas issues in some detail, and came up with some very interesting literature on the topic – much more recent and detailed that the old Air Force report that Kaiser Systems used at the time they designed the supply. It turns out that a relatively small amount of SF6 in dry nitrogen is almost as good as pure SF6. I can provide you the reference on this point. It might be a way to reduce costs. Finally, you might look into the way the JLab FEL stores their SF6, using a large storage bag rather than compressing all the way back into the bottles at a much higher pressure. This could greatly reduce the recovery time, and likely reduce the loss per recovery cycle as well.

20. Antimonide cathodes: Given the hint of a very good lifetime for the antimonide cathode, it likely makes sense to continue work with only this cathode in at least the

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near term. That might allow much more rapid progress with many of the outstanding issues (very large emittance, power supply instabilities, RF trip events, reaching higher average current, commissioning diagnostics, etc., etc.) while the GaAs cathode work, vacuum studies, etc. could continue off line. A high quality set of injector results from this cathode would be a major contribution to the field, even if the emittance were not as small as one ultimately hopes for.

21. Electric power to the cathode: There is a strong desire for some modest electrical power to be available at the cathode terminal. I believe that this could be generated with modest effort and little or no impact on the power supply performance by simply putting a turn or two around the ferrite core of the DC power supply at the cathode end. This could perhaps be best studied on the second supply intended for use in the test lab. Among other things, this power could serve a solenoid or a cathode cooling system.

22. Shield rings in segmented ceramic: Little was said about the surface finish of the copper shield rings inside the new segmented ceramic. The pictures shown of these looked like a relatively crude surface – perhaps spun metal? If so, is there any undesirable material worked into the rings? Does the surface finish matter? Copper is pretty soft material, so is relatively easy to make a good surface by polishing. If this were to prove easy to do, it might be worth the effort. It would be sad to have the rings prove to be a source of problems.

23. General remark: Overall, I feel this was an excellent review of an absolutely state-of-the-art facility. The presentations were uniformly good, and there was ample time to ask questions. When the speaker didn’t know the answer to a question, they said so. It was very refreshing to participate in such an excellent review.

24. SRIM: I am committed to prepare the material I have on Ivan Temnykh’s results with SRIM, and the few studies we did with XRF. I have a full plate the week of 1/10, but will get to it shortly.

25. Follow-up: • Dave Dowell commented on the Japanese success with their segmented

insulator. I urge caution here - the Japanese paper stated that they had only 5 pA of average FE current at full voltage. That isn't going to put a hole in anything. The real question is what they did to get that 5 pA average FE current - and can they do it a second time!

• Several reviewers commented on a very fast cutoff of the laser light from the cathode in the event of an RF trip. I guess I missed wherever it was indicated that you don't have this - I must have been asleep! Anyway, it was most

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definitely my intention to have a very rapid laser cut off. This was to be done in two stages - first a fast Pockels cell cuts the light off, and second, a mechanical optical block is inserted in the optical beam line. This is done because the Pockels cell is not the way to cut the light off for an extended time. You need to be sure the mechanical block can take the optical power. My recollection is that we (i.e. Dimitre) had purchased the components to do this. Anyway, if you don't have a fast optical shutoff, it needs to be on your to do list with some reasonable priority.

• Dowell suggested putting pole pieces on the proposed solenoid immediately after the anode aperture, to limit the field that could reach the cathode, possibly eliminating the need for a sucking solenoid. That is a good suggestion. Dowell in his report shows a simple three sided pole piece, but you could go much further, and make a smaller solenoid gap. I think there is a very good chance that you can make a near post-anode solenoid that will not require a bucking solenoid (and its a quantitative question that is readily answered).

• There was mention of how to do a slight oxidation of the K2CsSb cathode, to improve its QE. Both O2 and H2O are very difficult to control, and it was suggested to try NF3, which may work. Another possibility which I think is quite certain to work is N2O.

• Carlos mentioned the possibility of shaping the optical pulse prior to amplification. I asked about this back during the time of Shian's thesis. At the time, it was said to not work, but perhaps it is worth revisiting - I don't know if the technology has changed enough so this could work (probably can't do longitudinal shaping, as you need the short pulse for SHG), but at the time, it didn't look like you could reasonably amplify a transversely shaped pulse.

• Matt mentioned the use of TiN as a diffusion barrier. A fellow at BNL has done a lot of publication on this point. It may be a diffusion barrier for some gases, but it is not for H2, I believe. Mat also said that NEG coating isn't a good idea because yo don't get much pumping speed. I don't know if he measured it, but others have measured it, and it is what you would expect. Perhaps even more important, a 10-20 micron NEG coating is the first thing a gas molecule see as it tried to diffuse out of a metal wall. That's a mighty thick layer of NEG to get through. It may not be a diffusion barrier, but it is a hell of a barrier. I think it may be useful to look into in places (and it is pretty easy to do).

• Matt stated that he had 8 MV/m in their 200 kV gun, with a 60 mm gap. He may have a peak field of 8 MV/m on his Pierce electrode, but he most certainly doesn't have 8 MV/m on the cathode itself. Even a flat-flat electrode design would only give 3.33 MV/m at the cathode!

• Matt made the following comment: "I was a little surprised to hear Charlie say vacuum can’t be used to explain poor gun lifetime….i.e., QE decays faster than can be accounted for by vacuum and ion bombardment." I don't know what he was talking about, and I doubt I said what was reported. What I did talk about was that when you do things right, the vacuum is not the source of cathode

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decay - i.e. the dark lifetime is extremely long, as has been clearly demonstrated in the JLab guns (and probably the Cornell gun, though it has not been measured nearly as well at CU). So - its not vacuum per se that is the culprit - it is ion back bombardment, and ONLY ion back bombardment that is the problem. One can argue about what ion back bombardment does, but it is clearly the issue.

• In Jared's talk about focusing in the gun, he mentioned the use of curved cathodes. His pictures showed a curved cathode that was mounted into a Pierce electrode, with a distinct discontinuity where the cathode and the Pierce electrode met. This is not the way these things are done. The tangent of the cathode sphere should become the angle of the Pierce electrode - i.e. no change in shape at this junction. The focusing is MUCH stronger in this geometry. This geometry is normally used in really high power guns, where you need LOTS of current - like, say, a big klystron. The anode is an aperture in a concentric sphere of smaller radius. The field does NOT have to be reduced at the cathode - the field is simply that which you would get from a given voltage across two concentric spheres separated by the A_K gap. In your case, you would not (probably) want a particularly small radius to the sphere to get the needed beam convergence. As you go to larger and larger sphere radii, you approach the flat cathode case. My guess is that this is certainly worth playing with a bit - I'd guess that you could move toward a not particularly small radius of curvature and still get a high field and a lot of beam convergence. Another aspect that may be beneficial is that, particularly if the ions come from after the anode, they strike a fairly small part of the cathode (due to the large convergence), leaving much of the cathode undamaged. As I say, someone should play with this for a couple hours - it would be easy to do with the alkali antimonides, and I think I know how to go about it for GaAs.

Will Waldron comments

High Voltage Design: The general design looks solid and Cornell’s effort to work with Kaiser on the power supply and Friatec on the ceramic column should be commended. A lot has been learned from the first generation gun. Consider a modification to the lowest field shaping ring at the ground end of the column to reduce the peak field on this non-critical surface. From simulations, it looks like the

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peak field is >6MV/m. This can be made significantly lower by reducing the ring ID and/or increasing the radius of curvature for the surface facing the HV stalk. The stated requirement for the field on the graded insulator column triple points is <2MV/m. My opinion is that this is too high by an order of magnitude. A general rule of thumb is <1MV/m along the insulator vacuum surface and <0.5MV/m at triple points. There are other factors which affect the reliability of a vacuum insulator (e.g., field angle), but low fields at triple points are particularly important. The presented simulations did not highlight the triple point fields, but it does appear that the triple point fields of the design are reasonably low. From a conventional graded HV insulating column design standpoint, the field shaping rings are going in the wrong direction. This means that field emitted electrons under the field shaping rings can bombard the insulator surface and potentially create an electron avalanche. The fields under the rings appear reasonably low, so this may not be an issue. I understand that these rings were designed to protect the insulator surface from line-of-sight from potentially emitted electrons from the HV stalk where fields are the highest in the system (>6MV/m). This may be the dominating factor for HV breakdown of the column, so I do NOT recommend changing the design. I suggest that the flanges on the column be made in such a way that the column can be flipped upside down in case the field angle on the vacuum surface of the ceramic dominates the breakdown mechanism. This may also be achieved with some sort of adapter flange. The goal of a varistor (MOV) string is to protect an insulating ring from a high voltage transient and to protect the remaining insulating rings after one or more have already shorted out. The careful balance is to minimize the effect of the leakage current during normal operation and be close enough to the knee that when a fault occurs, the MOV string can effectively respond to reduce the voltage and shunt fault currents. The value of the grading resistors must be considered when determining the MOV string design. The presented values for grading resistors appear to be on the high side when considering an effective MOV string design. I would not consider the implementation of the MOV string a requirement, but if designed correctly, it can minimize column damage from faults. The Nichrom resistor assembly looks like an excellent solution to the underrated series resistor of the past. The discrete Caddock resistor array also looks like a reasonable backup or something to start with. Make sure there are holes in the acrylic tube around the Caddock resistor array so that SF6 can fully penetrate the assembly and any potential ionization that occurs can be cleared by the recirculating SF6.

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I suggest field shaping rings outside of the grading resistors and MOV strings to reduce local fields on these components with small features in the SF6 tank. These rings will hide these enhancements from the walls of the tank. If the high current voltage breakdown events are only occurring in the existing system when running high currents and always result in pitted cathode damage, I suspect this is a result of halo particles or a misteered beam which desorbs gas from the anode or beampipe walls. Fast vacuum diagnostics could potentially identify where the pressure burst starts. A revisiting of the beam envelope and focusing schemes may also shed light on this issue.

Testing: A suggested sequence for testing to avoid ambiguity if problems arise during fully integrated testing:

1. Start with the column and PS without the HV stalk. This will quantify the performance of the HV system without other potential problems induced by the HV stalk or the beam.

2. Proceed to testing the column and PS with the HV stalk but without an

emitting cathode. This will include the effect of the HV stalk which may be emitting electrons, but does not include beam induced problems in the vacuum.

3. Finally, the fully integrated test with an emitting cathode.

I support the idea of looking for light/x-rays from the OD of the column and second David Dowell’s suggestion of looking at the heating of the column grading rings with IR cameras. I agree that this may be difficult to implement if the cameras have to look inside the vacuum. This could be done through windows in the column end plate. Another idea is that the column grading rings may conduct enough heat to the SF6 side of the column to see a relative temperature difference between rings from the SF6 side. I cannot recall if testing the Kaiser power supply into faults was discussed, but I suggest testing at the factory into arcs to quantify the behavior, test fault detection, and quantify turn-off times after fault detection.

Collaborations:

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Other than the existing discussions between Cornell and LBNL on high voltage issues and photo-guns, there are staff within the Heavy Ion Fusion Science program at LBNL who regularly optimize multi-gap ion and electron sources. If there is further interest in this area, this topic could be discussed.