Sept. 27, 2009 Oak Ridge, TN, USA

1. SNS External Antenna Source Problems and Solutions R.F. Welton, J. Carmichael, N. J. Desai, R. Fuga, R.H. Goulding, B. Han, Y. Kang, S.W. Lee, S.N. Murray, T. Pennisi , K. G. Potter, M. Santana, and M.P. Stockli • Detailed source description • Brief history of testing the AlN source • Problems encountered during 1st operation of AlN source on SNS: • Water leaks • Antenna failures • Magnet heating • Plasma ignition • Beam decay • Proposed solutions Sept. 27, 2009 Oak Ridge, TN, USA

2. SNS External Antenna Source

3. Presentation_name

4. A brief history of testing of the AlN external antenna source • AlN Source first tested on test stand January 2008 • Early testing on stand (Jan-July 2008) focused on the elemental Cs system – 100mA achieved from Ni collar! • Later testing on stand (July 2008 to present) focused on using baseline chromate Cs system • Early tests on SNS front end spanned only a few days and showed that ~40mA could be transported through the RFQ (July 2008) • 4 production versions of the source were prepared and testing on the stand began in Nov 2008 • Front end operation spanned Feb – April 2009. We noted a 97% availability and • 1 water leaks in cooling jacket (later found bolts not tight) • 2 antenna failures • 2 p-gun failures • Unexplained ~10% / week beam decay

5. For Ref: Operational history of the AlN source

6. Improved Plasma Chamber Cooling Jacket • Early versions were made from unannealed polycarbonate which developed small fractures from residual stress and deformation under water pressure leading to small leaks which would terminate source operation • The current version employs a both dimensionally stable PEEK and stainless steel materials, see photo below • Jacket is performing well during initial testing on the test stand • No failures of revised water jackets yet noted Original (Blue) Revised (Red) Deformation Stress

7. PEEK Cooling Jacket and SS Manifold

8. Improved RF Antenna and Ferrite Backing • Several antenna failures were noted during operation - burning of the polyolefin insulation • It was discovered that during antenna fabrication inner and outer turns were mixed resulting in RF electric fields which exceeded the breakdown strength of air. • Inner and outer turns were then separated (as in the original design) using a 3/32 inch Teflon separator and no failures were noted thereafter over several experimental runs • To further reduce the peak and overall electric fields of the antenna a 3.5-turn antenna and Teflon holder have been designed • Alternatively, potting the antenna also remains a promising approach • Ferrites backing the antenna will enhance the inductive coupling efficiency with the plasma and are in new design

9. Improved Magnet Cooling and Confinement • Multicusp confinement magnets in the external antenna source occasionally overheat. New magnet configurations provide better cooling as well as higher magnetic confinement fields. Combines magnet holders and water manifold.

10. Improved Magnet Cooling Octapole Hexapole • RF power dissipation on the surfaces of the magnet holder was calculated from CST MICROWAVE STUDIO and used as loads for Cosmos Floworks…. Circumferential (outer) 111 W Toward ANT 138 W • Thermal simulations show that for a net RF power of 50 kW and cooling of 1.5 GPM the maximum temperature where the magnets contact the holder as <50 ºC and ΔT of water as 1.5 ºC. • Actual temperature is less since the plasma and ferrites will significantly reduce heating. Total Surface Power 460 W Circumferential (inner) 121 W

11. Improved Magnet Confinement • The new configuration offers 1.7 (octapole) and 2.3 (hexapole) -fold increase in magnetic plasma confinement fields as well as improved cooling. Current Hexapole Proposed Octapole Proposed Hexapole

12. Improving Plasma Ignition… • Reliable plasma ignition remains as a major problem with this source • Currently CW plasma guns (shown above) affixed to the rear of the source • Alternatively CW 13 MHz can be used for plasma ignition.

13. Improving Plasma Ignition • Plasma guns based on Cu cathodes have shown long-lifetimes but are not ideal since they sputter-inject noticeable layers of Cu into the plasma chamber. Figure shows gun emission current for a 4-month run. • Mo cathodes offer less sputtering but some seem to exhibite an unexplained poisoning effect after 1-2 weeks of operation rendering the source inoperable. • The application of 13 MHz RF directly to the primary antenna requires high RF CW power-levels of 0.5-1 kW (increasing with time) for reliable ignition and suffers from cross talk issues between the 2 and 13 MHz systems. Presentation_name

14. Improving Plasma Ignition • Several approaches are being pursued: • Identifying and testing, on a multiport chamber (shown to the right), long-lived, low-sputtering cathode materials for the existing plasma guns, which includes W, Ni and SiC based on feedback from the thyratron community. • Obtaining a commercial electron gun (see Heatwave slides) • Operating the pulsed 2 MHz system also in a low-level, cw mode. • An 13MHz RF plasma gun has been designed to replace the existing DC plasma gun on the source.

15. Improving Plasma Ignition • An RF plasma gun will take advantage of the existing 13MHz RF system used with the baseline source. This should allow significant decoupling of the 2 and 13 MHz RF systems, reduced RF power requirement due to operation closer to the Paschen minimum (higher pressure) and power deposition into a separate cooling system. RF Plasma Gun Plasma chamber of source

16. Improving Plasma Ignition – Commercial Options – AS&E, Heatwave Proposal Electrons emitted from the directly-heated dispenser cathode (red), are focused by the low-albedo, water-cooled, back-plane repeller (blue), so that they flow to the right through the non-intercepting aperture (green) into the main ionization chamber. Ions flowing through the aperture to the left also are focused so that they miss the cathode and impinge on the back-plane repeller. Secondary electrons produced by ions striking the back-plane repeller add to the constant current supplied by the directly-heated cathode. Since the low-temperature, long-life cathode produces a constant supply of electrons, any variation in this secondary electron emission current is inconsequential. Figure 1. 2-inch Diameter x 3-inch Long Optics Model Cathode current is controlled by the potentials, jCATH and jBPR Electron and ion focusing is controlled by the potentials, jCATH and jBPR These potentials can be established using a single power supply and voltage divider CONFIDENTIAL AND PROPRIETARY

17. Improving Plasma Ignition – Commercial Options – AS&E, Heatwave Proposal CONFIDENTIAL AND PROPRIETARY • Long life, reproducible, reliable directly-heated dispenser cathodes1 • 900-1200°C operation • 3 A/cm2 or more CW. > 40,000 hours lifetime • Stanford Linear Accelerator (SLAC) has 8 A/cm2 for over 45,000 hours • Reproducibility is well proven • Can be let up to air and reactivated repeatedly • The actual design will be scaled to fit the existing system and optimized by determining, in axis-symmetric space, the actual plasma emission surface meniscus formed at the non-intercepting aperture by balancing: • The ion current density available from the plasma • Bohm relationship • The space-charge limited ion extraction current density • Child’s law

18. RF Plasma Gun New external antenna plasma chamber assembly Integrated Solid Model of Improvements

19. Summary • We have focused on addressing each issue which arose during FE operation • An integrated model of the complete core assembly has been developed • Currently changes are being reviewed and detailed by the SNS Mech Group • Looking forward to collaborating with this community!

20. Remaining issue: Beam Decay with time (~10% / week) • Cause unknown • Could result from changing Cs conditions • Impurities • From plasma gun 5 W (Cu  Mo) ? • Leaks? • Out gassing of something hot e.g an o-ring? • AlN tube? • Gasses from hot LEBT? • Cs migration to/from AlN tube? • Could result from changing plasma conditions • Changing coupling? • Amplifier / matching network / antenna • Need to look carefully at the data