the cebaf 200kv inverted gun p. adderley, m. bastaninejad, j. clark, j. grames, j. hansknecht, j....
TRANSCRIPT
The CEBAF 200kV Inverted Gun
P. Adderley, M. BastaniNejad, J. Clark, J. Grames, J. Hansknecht, J. McCarter, M. Poelker, M. Stutzman, R. Suleiman, K. Surles-Law
PAC 11, New York City, March 28 – April 1
Benefit of Higher Gun Bias Voltage• Reduce space-charge-induced emittance growth,
maintain small transverse beam profile and short bunchlength
• In other words, make a “stiff” beam right from the gun• Particularly important for high bunch charge beam
• CEBAF guns have always operated at 100kV (b = 0.55)• Expect better transmission for Qweak at 140kV (and ILC
Baseline design) (b = 0.62)• Later, we envision an improved CEBAF photoinjector
with a 200kV gun and SRF capture section (b = 0.69) • Indentify what it takes to reach 350kV bias voltage or
higher (b= 0.8+). For ILC, CLIC, EIC, etc.,
Biggest obstacle: Field emission and HV breakdown…
which lead to Photocathode Death
0 50 100 150 2000
50
100
150
200
250
300Bunchlength Vs Gun Voltage
200KeV115KeV100KeV85KeV70KeV
Ave. Gun Current (µA)
Ele
ctro
n B
un
chle
ng
th (
ps)
0 50 100 150 2000
50
100
150
200
250
300Electron Bunchlength vs Gun Voltage
115kV
100kV
85kV
70kV
Ave. Gun Current (uA)
Ele
ctro
n B
un
chle
ng
th (
ps)
Measurements at CEBAF/JLab PARMELA Simulation Results
Benchmarking PARMELA Simulation Results Against Beam-Based Measurements at CEBAF/Jefferson Lab – work of Ashwini Jayaprakash, JLab
Message: Beam quality, including transmission, improves at higher gun voltage
0 50 100 150 2000
102030405060708090
100
Transmission vs Gun Voltage115kV100kV85kV70kV
Ave. Gun Current (uA)
Tran
smiss
ion
(%)
0 50 100 150 2000
102030405060708090
100
Transmission Vs Gun Voltage
200KeV115KeV100KeV85KeV70KeV
Ave. Gun Current (µA)
Tran
smiss
ion
(%)
Similar Trends
“Inverted” Gun
e-
Present Ceramic• Exposed to field emission• Large area• Expensive (~$50k)• Lots of metal at HV
Medical x-ray technology
New design
New Ceramic• Compact• ~$5k• Less metal at HV• No SF6 of N2
We had low level
field emission
Move away from “conventional” insulator used on most GaAs photoguns today – expensive, months to build, prone to damage from field emission.
High gradient locations not related to beam optics, lots of metal to polish
Old Gun Design
• Inverted Gun#2 at Test Cave• Conditioned to 225kV, with large
grain Nb electrode • InvGun2 performs well at 140kV• Problematic ops at 200kV. Small
field emission (~nA) and occasional vacuum bursts that ruin QE
• Preparing for “plan B”
• Inverted Gun#1 installed at CEBAF, operational since July 23, 2009
• Extractor gauge 2x10-12 Torr (raw value)
• 100kV Lifetime ~ 70C at 150uA avg. current
• Conditioned to 150kV, operated briefly at 130kV. Ready for Qweak
ILC-funded Project
Why a Niobium Cathode Electrode?
Work of Riad Suleiman
How much field emission is acceptable?
unfortunately, it seems if you can measure it, it’s too much
Field Emission measured at (floating) anode versus Gun Bias Voltage
Macor spacer
Our design has one region of “unintended” high gradient – could be
problematic…..exploring new designs via electrostatic modeling
Work of Ken Surles-Law
Plan B didn’t work
After re-BCP, there’s no FE at 225kV
5 January, 2011
Injector Test Cave
SUITCASE
4mA at High Polarization*Parameter Value
Laser Rep Rate 1500 MHz
Laser Pulselength 50 ps
Wavelength 780 nm
Laser Spot Size 350 mm
Current 4 mA
Duration 1 hr
Charge 20 C
Lifetime 80 C#How long at 1mA? 10.5 days
High Initial QE
* Note: did not actually measure polarization
# prediction with 10W laser
Improve Lifetime with Higher Bias Voltage?
Hypothesis: Double the gun voltage, halve the # of “bad” ions, improve
lifetime by 2
Ionization cross section for H2
100kV
250kV
Most ions created at low energy, < 10kV
Low energy ion column for 100kV gun
Low energy ion column for 200kV gun
Ion
ener
gy
Prolong Photocathode Charge Lifetime
Beam Current: 2.0 mAVacuum: 8.0 × 10-12 Torr
At 200 kV, only 60% of ions are created compared to 100 kV, longer lifetime
Conclusion
Emittance and Brightness Normalized Emittance from GaAs:
20
,, 4 cm
Tk
E
q
e
effB
syxn
q Bunch Charge (= 0.4 pC, 200 μA and 499 MHz)
Es Electric Field at GaAs surface
Teff Effective Temperature of GaAs (= 300 – 400 K, 780 nm)
kBTeff Thermal Energy (= 34 meV)
Normalized Brightness:
sn EB 2
1
Brightness is proportional to
Gun HV
Gun HV (kV) Es (MV/m) εn (μm)
100 2.0 0.011
140 2.8 0.009
200 4.0 0.008
Anode won’t always capture all FE…. Better to look for x-rays….
2”
50$
Building an inexpensive radiation monitoring system: lots of GM tubes, powered by one HV supply and data stream to computer via RS232
Charge Lifetime vs Bias Voltage2mA avg. current (unpolarized, DC beam), 350um laser spot and 5mm active area
Redesigning electrode to get rid of high gradient region at electrode/insulator joint, gradient ~ 15MV/m (more later)
Poor lifetime at 200kV likely due to low-level field emission (next slides)
And vacuum bursts often killed QE completely…
Work of Riad Suleiman
• 13 mm• 7 mm• 5 mm
Now we mask…
DC beam from bulk GaAs, green light and 350um spot.
Similar (good) results as with older guns, at 100kV
Finite lifetime due to ion back-bombardment
InvGun2 : Lifetime at 2mA and 100kV bias:Versus Laser Position and Active Area