Fast Ion-Driven Instabilities and Losses in NSTX

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 MHD-induced Fast Ion Losses on NSTX E. D. Fredrickson, D. Darrow, N. N. Gorelenkov, S. Medley, J. Menard, L. Roquemore 45th Annual Meeting of Division of Plasma Physics American Physical Society October 27 – 31, 2003 Albuquerque, New Mexico

2. Will fast ion driven instabilities be important in the NSST, CTF? • Fast ion instabilities important for fusion reactors; • Fast ion losses raise ignition threshold, first wall damage • Potentially beneficial (alpha-channeling)? • ST's, with intrinsic low field, are particularly susceptible to fast ion driven instabilities. • Vfast ion / Valfvén>1; fast ion energy available to drive modes • r* = rfi/aplasma; sets mode structure scale, fast ion transport • Also affected by q and density profiles, thermal ion/electron b. • Need to understand instability drive, loss mechanisms. • Result important for conventional aspect ratio reactors (ITER).

3. A broad spectra of kinetic instabilities are seen on NSTX “mostly harmless” Usually do not result in large fast ion losses. However, in some cases… Gorelenkov - GI2.001, Taylor - QP1.048, Fu - QP1.122

4. Neutron rate drops, Da bursts imply fast ion loss in some shots • Neutrons are beam-target; drops mean fast ion loss • dS dnfi • Da bursts probably due to fast ions hitting wall or divertor plates • Can be confused with ELMs • In L-mode, sometimes correlated with Da drops. • Loss also seen in iFLIP, redistribution in NPA.

5. TAE bursts responsible for most of fast ion loss here • Neutron drops ≈ 10-15 %. • Period is ≈ 10 ms. • In steady-state, predicted reduction in fast ion beta of 40 %. • TAE have strong bursting character with multiple modes present. • Fishbones have small neutron drops

6. Ip = 0.8 MA, Pb = 3.4 MW, btor = 16%) Large fast ion losses also with " fishbones” • Up to 20% drops in the neutron rate, with f.b. periods of ≈ 10 ms; steady-state reduction in fast ion population of ≈50%. • Why do some fishbones cause neutron drops, others no drop? • Even no-drop f.b.'s affect CAE. • Do CAE trigger f.b.’s? • Correlation of (large) fishbones with H-mode transitions?

7. Strong frequency chirping correlated with fast ion loss • In this case, multiple modes also seem to co-exist. • Lack of neutron drop may mean loss of lower energy ions; but not according to NPA. • It would be very nice to have good internal data. Ip = 0.8 MA, Pb = 6 MW, btor = 18%) S. Medley - NPA

8. TAE, f.b. and CAE/GAE interact • TAE bursts precede fishbones. • TAE-induced fast-ion losses don't affect CAE. • Fishbone-induced bursts cause drops in CAE amplitude, small effect on neutron rate. Ip = 0.65 MA, Pb = 3.6 MW, btor = 10%)

9. "Not your typical fishbones…" • Retain strong frequency chirping and periodic bursting character. • Toroidal mode numbers up to n=5 have been seen. • Often with q(0) > 1 and m >1 • Onset frequency well above precession drift frequency. • Probably bounce resonance* • May be relevant for ITER… (Ip =1.0 MA, Pb = 1.7 MW, btor = 11%) *Fredrickson, Chen, White, Nucl. Fusion

10. Summary • "Collective" fast ion losses seen on NSTX • "fishbone" induced losses of up to 20% and new fishbone physics • TAE induced losses of 10-15% per burst • Coupled TAE/fishbone induced losses • Losses are significant, either because they may: • Raising ignition threshold • Increase first wall power loading • Need physics basis to extrapolate to NSST or ITER

11. Bounce-resonance, as well as precession, can drive f.b.’s • For effective drive, need large average bounce angle… • …and bounce frequency in reasonable range. • For most conventional, beam heated tokamaks, this is not the case. • However, ITER may be a different story.

12. Principal diagnostics of mode structure are Mirnov arrays, soft x-ray cameras • Mirnov coils have limited poloidal coverage; plasma-coil separation varies significantly. • Soft x-ray data dependent on plasma conditions; H-modes are bad.

13. Mode amplitude typically largest near core, data limited. • Data acquisition rate ≈ 200 kHz. • Horizontally viewing camera. • High frequency f.b.s also seen; much weaker. • No obvious phase inversion; no island? • Signal low near edge. Soft x-ray data from JHU cameras.

14. Fishbones appear to be strongly ballooning? • Interpretation a little tricky due to varying plasma-coil separation. • Outboard poloidal wavelength reasonably long. • Inboard wavelength irrelevant?

15. TAE/fishbones synergistic? • Correlation of f.b. and TAE bursts suggests coupling. • Attribution of cause of fast ion losses difficult to make. • TAE bursts cause initial, fast drop, fishbones later, slower drop. • Neutron drops related to loss of fastest ions; slower ions may also be lost.

16. “Small” losses can add up • Fast ion population is modeled as constant source and exponential decay, S(t) = S0(1-et/t). • 20% drops every 18 ms with ts ≈ 36 ms means average drop of 30%. • Lots swept under rug, but coincidently the prediction is pretty good… (btor = 16%)

17. Many modes are present simultaneously during losses • Largest amplitude (on Mirnov) at lower n, but higher m’s should have faster fall-off from plasma edge.

18. Fast ion loss parameters • Vfast ion / VAlfvén; • How much, if any, of energy in fast ion distribution available to drive waves. • Fusion a’s are all at full energy, only fraction of beam power at full energy. • r* = rfi/aplasma; • Measure of mode structure, effectiveness at causing fast ion losses. • But high b q-profile may ameliorate,

19. MHD-induced Fast Ion Losses on NSTX E. D. Fredrickson, D. Darrow N. N. Gorelenkov, S. Medley, J. Menard, L. Roquemore Princeton Plasma Physics Laboratory, Princeton, New Jersey 08543 45th Annual Meeting of APS-DPP Albuquerque, New Mexico Oct. 27-31, 2003