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A. L. Rosenberg, for the NSTX Team With contributions from:

Supported by. Columbia U Comp-X General Atomics INEL Johns Hopkins U LANL LLNL Lodestar MIT Nova Photonics NYU ORNL PPPL PSI SNL UC Davis UC Irvine UCLA UCSD U Maryland U New Mexico U Rochester U Washington U Wisconsin Culham Sci Ctr Hiroshima U HIST Kyushu Tokai U

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A. L. Rosenberg, for the NSTX Team With contributions from:

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  1. Supported by Columbia U Comp-X General Atomics INEL Johns Hopkins U LANL LLNL Lodestar MIT Nova Photonics NYU ORNL PPPL PSI SNL UC Davis UC Irvine UCLA UCSD U Maryland U New Mexico U Rochester U Washington U Wisconsin Culham Sci Ctr Hiroshima U HIST Kyushu Tokai U Niigata U Tsukuba U U Tokyo JAERI Ioffe Inst TRINITI KBSI KAIST ENEA, Frascati CEA, Cadarache IPP, Jülich IPP, Garching U Quebec Fast Ion Absorption of the High Harmonic Fast Wave in the National Spherical Torus Experiment A. L. Rosenberg, for the NSTX Team With contributions from: J. E. Menard, J. R. Wilson, S. S. Medley, R. Andre,M. Bitter,D. S. Darrow,J. Egedal,R. W. Harvey,E. F. Jaeger,B. P. LeBlanc, T. K. Mau,C. K. Phillips, M. H. Redi, A. L. Roquemore, P. M. Ryan, S. A. Sabbagh, and D. W. Swain 45th Annual Meeting of Division of Plasma Physics American Physical Society October 27 – 31, 2003 Albuquerque, New Mexico

  2. Introduction and Motivation • Ion absorption critically important to assessing viability of HHFW to heat and drive current in STs • ECH cut off, LHCD & ICRF have low e- CD efficiency • HHFW predicted to be completely absorbed in one pass • Experimental evidence of HHFW interaction with NBI • Neutral Particle Analyzer (NPA) scannable at midplane • neutron rates, Fast Lost Ion probes • Modeling supports observations • HPRT, CQL3D, CURRAY, AORSA

  3. NSTX top-down layout

  4. HHFW antenna array delivers controllable phase 12 element antenna for 6 MW @ 30 MHz Antenna phasing controls heating or current drive co-CD B counter-CD Heating -20 0 20 kz (m-1) Antenna extends almost 90° toroidally w/WD = 9-13

  5. HHFW generates a fast ion tail with NBI • Neutral beam at 80 keV • D+ tail extends to 130 keV • Tail saturates in time during HHFW • Typical density, temperature, Ip • Tail decays on collisional time scale

  6. HHFW enhances neutron rate No RF vs. TRANSP Measured RF vs. no RF RF vs. TRANSP 105908 105906 RF Measurement Measurement no RF TRANSP TRANSP Measured neutron rate for RF shot significantly exceeds TRANSP prediction without RF After RF turnoff, rate decays close to measured and predicted no RF value

  7. Enhancements in tail and neutron rate simultaneously observed • NPA horizontally scanned from 108249-51, shots otherwise set up identically

  8. Ray tracing predicts fast ion absorption competitive with electrons • HPRT computes hot plasma absorption over cold ion/hot electron ray path • 25-50 rays used • Fast ions modeled as effective Maxwellians • Tfast & nfast from TRANSP • Fast ions dominate central absorption, electrons further off-axis • Ti,th = 2 Te, no thermal ion absorption Shot 105908 Time 195 ms ne0 = 3.2  1013 cm-3 nf0 = 2.5  1012 cm-3 Te0 = .6 keV Tf0 = 17.3 keV Pe = 55% Pfi = 45% Dfast e- r/a p

  9. Agreement between HPRT, CURRAY, AORSA, and CQL3D for RF + NBI • HPRT, CURRAY, AORSA find 30-35% fast absorption • CQL3D finds similar ion deposition profile using model f(v), 3 harmonics

  10. Ray-tracing and full wave codes see similar significant 2D effect in propagation • N = 24, t = 5%, fast ion shot HPRT 50 rays (99% absorbed) AORSA

  11. Quantitative comparison tools for neutron rates, NPA • TRANSP • Monte Carlo, FLR, finite banana width, time-dependent, asymmetric equilibria • No package to calculate effect of HHFW on fast ion dist. fct. • CQL3D – Harvey (CompX) • Only code w/Fokker-Planck treatment of HHFW-NBI • No neutrals, FLR, banana width (other than loss), time-dependent profile, or asymmetric equilibria effects • Only 3 consecutive harmonic numbers currently possible • TRANSP-based NPA Simulator – Andre (PPPL) • Standalone now reads in CQL3D fast ion dist. function • Function averaged in pitch-angle space to counter unphysical particle localization in ST without FLR effects; unnormalized

  12. Significant RF-induced neutron rate enhancement predicted and observed • Equilibria at t = 235 ms used • Time-weighted solution also using t = 180 ms equilibria • Error in absolute neutron rate at least +/- 10 % • Time dependence, total RF in bulk plasma significant effects

  13. NPA simulator in good agreement with measurement • Agreement above 80 keV within error bars • Discrepancy below ~30 keV likely due to large uncertainty in 1D wall neutral model • Half-to-full RF power spans error bars; edge absorption observed full power half power

  14. Tail reduced with lower B-field, higher t • Larger t expected to promote greater off-axis electron absorption • Reduces fraction of power available to core fast ion population

  15. Can ion loss account for tail reduction with lower B0? • Two codes used to check ion loss at B0=4.5 kG vs. 3.5 kG for 80-120 keV ions • CONBEAM – Egedal (MIT-PSFC) • Loss fraction at 120 keV B0 = 4.5 kG: 21% • EIGOL – Darrow (PPPL) • Loss fraction at 120 keV B0 = 4.5 kG: 17% • Tail at highest B0 stronger by factor ~8 • Small change in loss fraction insignificant compared to major tail reduction • Changes in tail population due to RF, not fast ion loss B0 = 3.5 kG: 25% B0 = 3.5 kG: 23%

  16. Observation of less fast ion absorption at higher t consistent with ray-tracing theory t = 5%, B0=4.5 kG t = 9%, B0=3.5 kG HPRT HPRT Shot 108250 Shot 108252 Pe = 62% Pfi = 37% Pe = 73% Pfi = 24% e- e- Dfast Dfast r/a r/a • Lower on-axis absorption for lower B, higher t predicted

  17. Smaller neutron rate enhancement at higher t predicted and observed Ratio btw high/low B0 Absolute neutron rate • Measured neutron rate ratio is very similar to simulated ratio

  18. Smaller RF-induced fast ion tail at higher t predicted and observed • Good agreement between experiment and theory for t scan

  19. k|| has little observed effect on fast ions • Theory predicts greater ion absorption with lower k|| • Surprisinglylittle variation in tail, small neutron enhancement with higher k||

  20. Greater fast ion absorption predicted at lower k||, in contrast to measurement HPRT • CURRAY, AORSA, and CQL3D also show this trend

  21. Larger neutron rate enhancement at lower k|| predicted, smaller rate observed Neutron rate (1013 n/s)

  22. Much larger fast ion tail at higher k|| predicted, but smaller tail observed • No k|| evolution measurement available • Edge-coupling effects, theory breakdown at low k||? • To be further investigated

  23. Summary • Clear RF-induced fast ion tail observed with NBI • Neutron rate and modeling support this observation • Tail formation suppressed with higher t, consistent with theory • Observed effect of k|| scan opposite to theory • Edge coupling or theory breakdown at low k||an issue? • Good agreement between HPRT, AORSA, CURRAY, CQL3D for HHFW + NBI

  24. Effective Maxwellian matches TRANSP f(E) well

  25. NPA scan indicates induced tail well off-axis • Depletion in particle flux with NPA Rtan further off-axis • Tail extends to same energy range • Future: scan over wider range of r/a

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