LBL-30814

MECHANICAL DESIGN AND FABRICATION OF A BARIUM SURFACE CONVERSION H- ION SOURCE

R. P. Wells and T. A. Stevens Lawrence Berkeley Laboratory

Berkeley, California 94720

C. F. A.. van Os Lucas Labs

470 B Lakeside Dr Sunnyvale, CA 94086

USA

Abstract The details of a prototype surface conversion ion source

designed for continuous operation are described. The source consists of an annular, convectively water-cooled, magnetic multi- cusp driver chamber separated by a magnetic field from an inner chamber. A barium coated spherically contoured disc shaped converter in the inner chamber produces a stream of H- ions focused at the exit aperture. The converter is electrically biased up to 300 volts negative with respect to the source potential. Jacketed, water- cooled permanent magnets near the source exit deflect electrons out of the beam path. Techniques for fabrication and maintenance of a clean barium metal surface are also discussed.

Tntroduction

High energy neutral beams of up to 1300 kV are required for the next generation of magnetic fusion experiments'. Low neutralization efficiency at energy levels in excess of 160 kV precludes the use of positive-ion based neutral beams. However, negative-ion beams have relatively high neutralization efficiencies up to very high beam energies2. The Magnetic Fusion Energy Group (MFE) at Lawrence Berkeley Laboratory (LBL) is in the process of developing a Barium Surface Conversion Ion Source (BaSCS), shown in figurel, to meet this need. Two basic types of negative- ion sources are presently under development at LBL and other laboratories world-wide. The first, and most widely studied, is volume production while the second approach is surface conversion. The surface conversion source has an important advantage in that it can run at gas filling pressures of 1/5 to 1/10 that of volume sources thus producing less background gas in the accelerator where ion- neutral collisions result in beam stripping.

Figure 1. Barium Surface Conversion Source (BaSCS) mounted on test facility with top end cap removed.

The process by which negative-ions form in a surface conversion source is as follows: Positive ions are accelerated toward a metal electrode (converter) which is biased negative with respect to the plasma by a few hundred volts. These ions impinge on the

91CH3035-3$03.00 0 1992 IEEE 82

converter where they are implanted in the surface. Subsequent impinging ions sputter these adsorbed particles off the surface resulting in a flux leaving the converter. A fraction of the exiting particles experience a resonant charge exchange with the metal surface and become negatively charged. These negative ions then accelerate away from the converter surface toward the extraction aperture 3.

Surface conversion charge exchange has been successfully demonstrated with a variety of metald. The general trend is that the lower the work function of the metal, the greater the negative-ion yield. A cesium covered molybdenum converter has been used to produce more than 1 ampere of H- from a source operating at l ~ l O - ~ Torr5.6. More recent work by van Os and others3.6.7 has demonstrated a significant level of negative-ion production from barium coated electrodes.

Descriotion

The BaSCS is comprised of an outer annular "driver" chamber surrounding an inner converter region. Sixteen rows of samarium-cobalt magnets are uniformly spaced around the outer wall and in,? radial pattern on the end caps. These magnets form a magnetic bucket" which confines the primary electrons by effectively reducing the electron loss area (anode) to the width of the magnet cusp at the wall. Inner anode rings attached to either end cap provide additional anode area and serve to isolate the primary plasma and evaporated tungsten in the driver region from the converter region. These inner anodes do not contain permanent magnets. The geometry of the prototype BaSCS is illustrated schematically in figure 2.

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Figure 2. Schematic cross section of BaSCS in side view.

A solenoid electromagnet at the back, converter end, of the source provides a means of adjusting the strength of the "filter" field in the opening between the driver and converter chambers. The filter field causes primary electrons to be reflected while ions, which have a much larger gyroradius, pass through. Low energy electrons also pass into the converter region by a collisional random walk process.

The magnetic field is conducted to the filter region by pole pieces consisting of 1008 steel cylinders. The pole pieces can be electrically biased negative with respect to the plasma generator anode thus helping to pull positive ions into the converter region. The converter, as mentioned above, can be biased up to 300 volts negative with respect to the plasma potential thus accelerating positive ions towards the surface and driving negative-ions towards the extraction aperture.

Thermal Analvsis

The BaSCS is to designed to operate continuously at arc power levels of up to 60 kW. The magnetic bucket arrangement concentrates the heat flux from the arc on very localized magnetic cusps on the surfaces of the chamber resulting in high peak power densities. To produce a narrow cusp magnets must be positioned close to the anode surface necessitating a relatively thin wall section at the magnet. Figure 3 is a picture of the bucket wall prior to assembly. A plot of the calculated cusp width as a function of distance from the magnet surface is shown below. A 1 mm cusp width was adopted for the BaSCS and thus, from figure 4, a magnet to the inner wall surface distance of 4 mm.

dissipation problem is less serious. The primary anode surfaces (wall, end caps and inner anode) are constructed from OFHC copper due to its excellent heat conductivity and good vacuum compatibility. To dissipate the fairly significant heat load, all copper parts of the source are actively cooled using forced-convection water cooling.

A key criteria of the design is limiting the temperature of the samarium-cobalt magnets to less than 100OC. The cross-section of a representative portion of the bucket wall was modeled thermally with ANSYS8 using the 2D finite element mesh shown in figure 5. The selection of a thick, 2 cm, wall and OFHC copper allowed the use of a single cooling passage per magnet row. The magnet cavity is dimensioned to give a 0.3 mm gap between the magnet and the front surface of the magnet cavity thus moving the location of the controlling temperature from the hot front of the magnet cavity to the cooler side wall.

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t q = 1 4 0 0 WJ"

Figure 5. ANSYS model for the thermal analysis of the bucket wall.

Finite element modeling was performed using a range of convective heat transfer coefficients, or equivalently a range of water flow rates, and cooling passage sizes. The diameter of the cooling passage and the flow rate represent a compromise between heat transfer efficiency and compact design. The 9.5 mm diameter passage requires a 0.25 Vs flow rate to produce a heat transfer coefficient of 1.5 W/cm2OC. At this flow rate and given the available pressure differential of 0.41 MPa (60 psi), 8 cooling channels can be connected in series. Thus the entire bucket wall can be cooled bv two parallel circuits. For additional safety, a flow rate per circuit df 0.38 Us. is used. As evident from figure 5, the maximum surface temperature is 156OC while the temperature of the side wall of the magnet cavity ranges from 97°C to 85OC.

Figure 3. BaSCS driver chamber wall prior to assembly.

Distance from Magnet Surface, cm

Figure 4. Magnetic cusp width as a function of distance from the magnet surface.

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Given the 60 kW heat load and assuming a uniform distribution over the total cusp length gives a peak power density of 720 Wkm2 at the wall and end cap surfaces. To allow for possible non-uniformities in the flux distribution a 1400 W/cmz heat flux was used as a design value. Since the inner anode lacks magnets a more diffuse heat flux distribution is expected and, therefore, the heat

Figure 6. Bottom end cap prior to brazing, inner surface up.

Cooling of the magnet cusps on the bucket's end caps is

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accomplished by two small, 4.7 mm diameter, water channels straddling each magnet cavity. The end cap cooling channels communicate with matching channels within the inner anode rings. Photographs of the mating surfaces of the bottom end cap and the inner anode ring are shown in figures 6 and 7, respectively. Water enters from the outer edge of the end cap, flows along one side of the magnet cavity, into the inner anode and back out through a matching passage. As would be expected, the dual channel design resulted in lower magnet cavity temperatures than single channel design for comparable heat transfer coefficients. The increased path length and decreased passage diameter necessitated dividing the coolant to each end cap into a series/parallel arrangement having 4 parallel circuits.

Figure 7.

Heat dissipation on the inner anode is not as severe a problem as other portions of the driver chamber. While the average heat flux on the inner anode is expected to be comparable to other regions of the anode it lacks magnets and therefore the flux is not highly concentrated. Additionally, the cross-section for heat conduction to the cooling passages is thick, 1 cm, and uniform giving rise to lower temperature differentials.

Structural Analvsis

From a structural stand point the ion source is a pressure vessel. Atmospheric pressure on the outside of the bucket will tend to buckle the cylindrical wall and generate. a concave displacement of the end caps. Since the bucket wall has a free length of 16 cm and a nominal wall thickness of about 2 cm, buckling is not a concern. However, bowing of the end caps could be a problem especially since these plates are fully annealed during the fabrication process.

Due to the complex geometry of the end cap and inner anode assembly a 3D finite element structural analysis was performed using ANSYS to determine the stress and deflection of this structure. Since the material in question is annealed OFHC copper handbook values for elastic modulus must be used with care. OFHC copper has a limit of proportionality of approximately 1.38 MPa (2000 psi). For stress in excess of this value the effective elastic modulus is significantly reduced. Using the lower bound for the 0.2% offset yield stress of 48 MPa (7000 psi) and simplifying by assuming linear stress-deflection behavior gives an effective modulus of 2 x 104 MPa (2.9 x 106 psi).

The finite element results led to increasing the wall thickness of the inner anodes so as to stiffen the end caps. The increase from 7 mm to 10 mm in wall thickness decreased the peak values of von Mises equivalent stress from 50 MPa (7250 psi) to 40 MPa (5800 psi). The calculated maximum axial deflection was 0.7 mm at the inner diameter of the end cap. For added safety, the end caps were bolted to the adjacent flux return plates.

Source Fabrication

Inner anode rings prior to brazing.

Source Chamber

To prevent contamination and/or oxidation of the barium

surface converter, all materials and fabrication techniques were ultra- high vacuum (UHV) compatible. The bucket wall was machined from a single forged (ring-rolled) billet of OFHC copper. The rough forging was reduced in wall section by machinin about 2 cm off both the ID and OD. A helium leak check (10-18Atm-cm3/s sensitivity) was performed after this initial machining as well as after final machining..

The end caps were machined from OFHC copper plate. A pair of 4.7 mm diameter cooling passages were drilled parallel to each magnet cavity. These holes stop just short of the inner radius. Cross-drilled holes communicate with milled slots on the inside surface of the plate. The slots in the end plate match holes drilled through the inner anode cylinders. Each pair of cooling passages is connected by a slot milled in the far end of the cylinder. A cap ring seals the exposed slots. The assembly was joined in a single step vacuum braze operation using Palcusil 15 braze alloy. Vacuum integrity was confirmed by a post braze helium leak check.

The seal between the bucket wall and the end caps was made by viton O-rings. To minimize out gassing the O-rings were vacuum baked at 100°C for 24 hours prior to installation. O-rings are also used to seal the filament chuck feed-throughs to the outside of the bucket wall. All O-rings were installed without vacuum grease.

MametFilament Chuck Assembly

The filament chucks use a squirt-tube arrangement to cool the copper conductor. The squirt tube is formed by drilling a blind hole in a 6.35 mm copper rod the end of which is threaded to accept a molybdenum chuck. The rod extends through and is vacuum brazed to an LBL stock alumina insulated vacuum feed-through. Water is supplied by a thin wall 4 mm OD stainless steel tube that runs inside the copper rod to within 1.3 cm of the end of the bore

The base of the filament vacuum feed-throughs were welded via steel transition pieces to the 1018 steel flux return plates. These plates were grooved to accept the samarium-cobalt permanent magnet arrays. Each flux return plate holds a row of 6, 1.27 x 1.9 x 2.5 cm, magnets.

Electron Suppressor and Filter

To limit the flow of electrons two rows of samarium-cobalt magnets straddle the exit path from the source Energetic electrons approaching the exit will be reflected by this field while negative- ions will pass through. These filter magnets are enclosed within a stainless steel tube. The magnets are cooled by water passing between the square sided magnet and the round bore of the tube. The corners of the magnets fit into L-shaped notches electric discharge machined (EDM) along the length of the tube's bore.

To assure a reliable vacuum tight joint, end caps were welded to the filter magnet tubes with the magnets in place. Welding took place in an argon filled glove box with argon flowing over the magnets to prevent them from overheating. After welding the residual strength of the magnets was measured and compared to a back-up set. No degradation was observed. The "filter rods", with their cooling tubes in place were then welded into mating holes in the bottom flux return plate. Four radially drilled holes in this plate provide a route for water flow to and from the filter rods.

Low conductivity (1M-ohm) water (LCW) is used exclusively as the cooling medium for the source. Welding and/or brazing may leave some types of stainless steels vulnerable to intergrannular corrosion (sensitized) by LCW. Unstabilized austenitic stainless steels with carbon contents of more than 0.3% become sensitized when exposed to temperatures between 425°C to 830°C9 To minimize the possibility of producing sensitized joints extra low carbon, 304L, stainless tubing was used exclusively. A low carbon magnetic stainless steel, 17-4 PH, was selected as the bottom flux return plate.

A pair of molybdenum electrodes in the shape of split cylinders immediately inside the filter magnets collect electrons that are not reflected. These electrodes are electrically isolated by bushings. The electrical connection is made by a copper wire attached to each electrode. The wire is fed through an alumina

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bushed hole in the adjoining flux retum plate to a commercial UHV vacuum feed-through.

Electromagnet

Bench top tests of a mock-up of the magnet coil and pole geometry established that the desired magnetic field rigidity of 100 to 200 gausscm would be produced by a coil of no more than 1500 amp turns. Given that the source is to run CW a water cooled conductor was selected. Hollow magnet conductor (bus bar) is not readily available in small quantities so our selection was limited to sizes on hand. For this application we selected the smallest available conductor, 4.2 mm square with a bore of 2.4 mm. With the conductor cross section fixed the design became one of choosing the number of turns so as to produce a compact coil while staying within the capacity of an existing power supply (10 volts at 175 amps). A 14 turn, 107 A coil was selected. At full current the heat produced is a modest 96 W.

Converter

At present the converter load lock assembly, shown below, has been designed and is in the process of being fabricated. This assembly will be used to transport and house the converter from application of the barium surface through operation and removal. Since the load lock will be used to hold the converter during periods of non-operation, such as evenings and weekends, it must be designed for low outgassing and have adequate pumping. The goal is to produce a

The load lock is an all stainless steel hard-sealed enclosure with two titanium sublimation filaments, one on either side of a central baffle. The baffle limits the gas load on the barium surface from outgassing by the considerable surface area of the bellows. The upstream filament resides within the load lock chamber and uses the water cooled chamber walls as the gettering surface. The downstream filament is housed within a water cooled appendage to prevent flakes of titanium from fouling the valve seat

The design of the barium converter is still in process. However, a "dummy" converter was built and installed to allow testing of the driver in advance of full source operation. The dummy converter is an non-coated water cooled OFHC copper disk.

Torr pressure with the converter isolated.

I S O L A T I ON VALVE^ LCONVERTER

Figure 8. Load lcckkonverter assembly. Discussion

The BaSCS has operated at an arc power levels in excess of 20 kW for tens of minutes at a time. The maximum plasma density produced in the driver region was about 150 mA/cm2 with a 90 mA/cm* density at the converter. Several problems with the source design have become evident, some even before operation began. A

three axis hall probe measurement of the magnetic field revealed that a significant radial field exists in a large portion of the "field free" region of the driver. Additionally, after several minutes of operation, the electromagnet's pole pieces become very hot and expand into the adjacent inner anode.. Water cooling has been added to the back (converter side) pole piece and a molybdenum shield has been installed over the other. The replacement of the solenoid coil with a permanent magnet "cage" to separate the driver and converter regions is under consideration. This change should eliminate the radial field in the "field free" region near the filaments while also eliminating the uncooled pole pieces.

In general, the BaSCS has demonstrated poor arc efficiency, primarily due to the large anode area of the inner anodes. An electrically floating shield was added over the outside surface of the anodes and some improvement was noted. The addition of magnets to form a multi-cusp arrangement on the inner anode is under consideration.

Acknowledge men@

This work was supported by the Director, Office of Energy Research, Office of Fusion Energy, Development and Technology Division, of the U.S. Department of Energy under Contract No. DE-AC03-76SF00098.

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MIXED-ENERGY HYDROGEN BEAMS FOR CTR

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