Managed by UT-Battelle for the Department of Energy Sept. 27, 2009 Oak Ridge, TN, USA 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 1.Detailed source description 2.Brief history of testing the AlN source 3.Problems encountered during 1 st operation of AlN source on SNS: 1.Water leaks 2.Antenna failures 3.Magnet heating 4.Plasma ignition 5.Beam decay 4.Proposed solutions

2Managed by UT-Battelle for the Department of Energy SNS External Antenna Source

3Managed by UT-Battelle for the Department of Energy Presentation_name

4Managed by UT-Battelle for the Department of Energy 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 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

5Managed by UT-Battelle for the Department of Energy For Ref: Operational history of the AlN source

6Managed by UT-Battelle for the Department of Energy 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 Deformation Stress Original (Blue) Revised (Red)

7Managed by UT-Battelle for the Department of Energy PEEK Cooling Jacket and SS Manifold

8Managed by UT-Battelle for the Department of Energy 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

9Managed by UT-Battelle for the Department of Energy 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.

10Managed by UT-Battelle for the Department of Energy 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…. 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. Toward ANT 138 W Circumferential (outer) 111 W Total Surface Power 460 W Circumferential (inner) 121 W

11Managed by UT-Battelle for the Department of Energy Improved Magnet Confinement Current Hexapole Proposed Octapole Proposed Hexapole The new configuration offers 1.7 (octapole) and 2.3 (hexapole) -fold increase in magnetic plasma confinement fields as well as improved cooling.

12Managed by UT-Battelle for the Department of Energy 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.

13Managed by UT-Battelle for the Department of Energy Presentation_name 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 kW (increasing with time) for reliable ignition and suffers from cross talk issues between the 2 and 13 MHz systems.

14Managed by UT-Battelle for the Department of Energy 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. Improving Plasma Ignition

15Managed by UT-Battelle for the Department of Energy 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

16Managed by UT-Battelle for the Department of Energy 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. CONFIDENTIAL AND PROPRIETARY Figure 1.2-inch Diameter x 3-inch Long Optics Model Cathode current is controlled by the potentials, CATH and BPR Electron and ion focusing is controlled by the potentials, CATH and BPR These potentials can be established using a single power supply and voltage divider Improving Plasma Ignition – Commercial Options – AS&E, Heatwave Proposal

17Managed by UT-Battelle for the Department of Energy Long life, reproducible, reliable directly-heated dispenser cathodes 1 – °C operation –3 A/cm 2 or more CW. > 40,000 hours lifetime –Stanford Linear Accelerator (SLAC) has 8 A/cm 2 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 Childs law CONFIDENTIAL AND PROPRIETARY Improving Plasma Ignition – Commercial Options – AS&E, Heatwave Proposal

18Managed by UT-Battelle for the Department of Energy Integrated Solid Model of Improvements RF Plasma Gun New external antenna plasma chamber assembly

19Managed by UT-Battelle for the Department of Energy 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!

20Managed by UT-Battelle for the Department of Energy 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