Flux savings from inductive drive of a Transient CHI started plasma

1. NSTX Supported by Flux savings from inductive drive of a Transient CHI started plasma College W&M Colorado Sch Mines Columbia U Comp-X General Atomics INEL Johns Hopkins U LANL LLNL Lodestar MIT Nova Photonics New York U Old Dominion U ORNL PPPL PSI Princeton U Purdue U SNL Think Tank, Inc. UC Davis UC Irvine UCLA UCSD U Colorado U Maryland U Rochester U Washington U Wisconsin Culham Sci Ctr U St. Andrews York U Chubu U Fukui U Hiroshima U Hyogo U Kyoto U Kyushu U Kyushu Tokai U NIFS Niigata U U Tokyo JAEA Hebrew U Ioffe Inst RRC Kurchatov Inst TRINITI KBSI KAIST POSTECH ASIPP ENEA, Frascati CEA, Cadarache IPP, Jülich IPP, Garching ASCR, Czech Rep U Quebec R. Raman, B.A. Nelson, D. Mueller, M. Bell, T.R. Jarboe University of Washington Princeton Plasma Physics Laboratory NSTX XP Review 8 May 2009

2. CHI Start-up and coupling to induction demonstrated on NSTX • Increased emission from low-z impurities from the lower divertor as the injector current is increased • Increased emission from low-z impurities as an absorber arc is generated • Lithium improved the reproducibility of CHI but did not increase the initial CHI produced current (as seen with Ti-gettering on HIT-II)

3. CHI Start-up produces current by generating an electrode discharge across the lower divertor electrodes • Quality of lower divertor electrodes (especially the presence of low-z impurities) determines the resulting temperature and current decay rates of resulting plasma. • Low-z impurities are an issue for any start-up plasma as input power is low (metal electrodes preferred for CHI) CHI for an ST: T.R. Jarboe, Fusion Technology, 15 (1989) 7 Transient CHI: R. Raman, T.R. Jarboe, B.A. Nelson, et al., PRL 90, (2003) 075005-1

4. Most of the injector current signal in this discharge is due to the presence of an absorber arc

5. Low-z Impurity Radiation Should be Reduced for Inductive Coupling • Low-z impurity radiation increases with more capacitors • Future improvements • Test CHI in NSTX with partial metal outer divertor plates as part of liquid Li divertor upgrades • High Te in spheromaks (500eV) obtained with metal electrodes • Discharge clean divertor with high current DC power supply • Use 350kW ECH during FY11

6. CHI Run Plan for FY09 aims to reduce low-Z impurities through divertor plate conditioning • CHI discharges on HIT-II and in Spheromaks have often seen improvements in plasma performance as the same discharge is repeated many times • This is easier on metal electrodes as the surface layer of low-Z deposits can be removed with several plasma conditioning shots • On carbon surfaces with trapped oxygen, electrode cleaning could be more difficult requiring more dedicated plasma conditioning shots • For FY09, the DC rectifier supply used before FY04 will be used to produce ~9kA injector current discharge for 200-400ms durations • Each of these discharges would be the equivalent of about 30 high current discharges that could be run using the capacitor bank • In addition about 10 high-current, high power upper single null discharges would be run to reduce low-Z surface impurities from the upper divertor plates.

7. Overall Run Plan • Obtain reference discharges • Reproduce CHI-only and CHI coupled to inductive discharges run in FY08 under the present divertor conditions • Reproduce high-current CHI-only discharges produced during FY07 and vary the toroidal field • Run electrode conditioning discharges • Inductive USN discharges to clean upper divertor • CHI discharges using DC rectifier supply to clean the lower divertor plates • Repeat reference discharges • Initial at similar levels of Li as before, then with increased amounts of Li • Use Li-dropper when it becomes available to coat Li on upper divertor • Use absorber PF coils when then become available (to be developed as part of XP-927) • Produce reference discharges with reversed TF • These will be repeated after the LLD installation in FY10 for a direct comparison of the benefits of a partial metal divertor plate