Destabilization of internal kink by LHCD suprathermal electron pressure L. Delgado-Aparicio 1

1. Destabilization of internal kink by LHCD suprathermalelectron pressure L. Delgado-Aparicio1 S. Shiraiwa2,L. Sugiyama3, R. Granetz2, R. Parker2,J. Irby2, S. G. Baek2, I. Faust2,G. Wallace2, R. Mumgaard2, S. Scott1, D. A. Gates1,N. Gorelenkov1,N. Bertelli1,C. Gao2,M. Greenwald2, Hubbard2, J. Hughes2,E. Marmar2,P. Phillips4, M. L. Reinke2, J. E. Rice2, W. Rowan4, R. Wilson1, S. Wolfe2and S. Wukitch2 1PPPL2MIT-PSFC 3MIT-LNS 4U-Texas-Austin 55th Annual Meeting of the APS Division of Plasma Physics   Monday–Friday, November 11–15, 2013; Denver, Colorado

2. Outline • Background and motivation • Measurements of fishbone-like activity in C-Mod • a) Main spectroscopic diagnostics • b) Core buildup before mode onset • c) Fluctuation measurements • d) Effects of non-Maxwellian fast electrons • Calculating the fast-electron pressure. • Summary and future work.

3. Background and motivation • Instabilities can distort the orbits of fast particles causing an off-axis redistribution of plasma thermal energy to the detriment of the centralb. • Understanding the formation and stability of 3D helical modes in the core of an axisymmetrictoroidal configuration, remains one of the challenges of fusion research. • Important for a burning plasma such as ITER since these 3D structures could occupy a significant volume of the plasma (rq=1~a/2). • The study of modes generated by energetic electronsremains a much less explored field than energetic ions (e.g. fishbones). • These studies are relevant to the investigation of trapped alpha particle interactions with low-f MHD modes in burning plasmas. • ECRH/ECCD or LHCD provide large sources of energetic electrons (10-500 keV) which can trigger these fishbone-like modes

4. SXR tomographic System (XTOMO) Diagnostic suite installed in C-Mod enables 3D studies of internal kink Two-color interferometer (TCI) Hard x-ray camera

5. MHD survey during LHCD: observations of precursors, sawteeth and (1,1) internal kinks DVloop~-0.3-0.5 V DVloop~-0.3-0.5 V

6. Ip~0.5 MA, Bt~5.4 T, n||,LHCD~1.6, Te0~2.3-2.9 keV, ne0=(1.3-1.7)×1020m-3. • Periodic (1,1) kink-like mode grows during the sawtooth ramp. • Train of successive m=1bursts can appear during a sawtooth-free phase. • Time-scales associated to crash and damping of the mode are different (e.g. tSC≲50 ms, tFB~1 ms). • Delayed off-axis SXR suggest a redistribution of particles/heat. LHCD is driving fishbone-like internal kink(iK) modes Two different modes ≠ (1,1) impurity long-lived mode.

7. Thomson Scattering ne & Te profiles For the two-conditions of interest the plasmas have different densities & temperatures during fishbone-like MHD • But the same central pressure! • ne0Te0~3.7×1020 keV/m3 • ne0Te0~3.7×1020 keV/m3 • ne1Te1~3.2×1020 keV/m3 • ne1Te1~3.2×1020 keV/m3 • ∇ne,q=1=-(5-6)×1020 m-4 • ∇Te,q=1=-(16-17.6) keV/m q=1

8. The coexistence between the periodic kink-like modes and sawteeth is different than that of long-lived (1,1) modes recently found in C-Mod Impurity snakes survive sawtooth crashes with minimal impact L. Delgado-Aparicio, et al., RSI’12, PRL’13, NF’13

9. A hybrid sawtooth-internal-kink-mode is possible The (1,1) fishbone-like activity can coexist with sawteeth, suggesting that the two modes have independent driving mechanisms.

10. Sawtooth precursor grows from oscillations remaining from incompletely damped kink mode • iK=internal kink • SC=sawtooth crash • hM=hybrid mode crash. • All three (1,1) modes have distinct amplitudes but nearly identical frequencies (small differences may arise from the sawtoothrecovery). • TheiK mode grows, briefly saturates, then damps and disappears. • Growth and damping occur over 1-2 ms intervals similar to the growth of the precursor, but much slower than the 20-40ms of the sawtooth crash.

11. Sawtooth-free scenario still shows kink-like mode • A sawtooth stabilized scenario shows only a train of consecutive internal kink-like bursts. • The amplitude of the kink-like mode is nearly “constant” with no obvious signs of frequency chirping which offers a first clue of its non-resonant nature. • Various diagnostics show little evidence of any abrupt change that might correspond to a rapid “crash” phase. • Instead, observation suggests a (1/1) kink that grows steadily to a maximum amplitude, then gradually diminishes to zero.

12. Fishbone-like mode shows in both HFS and LFS and after core buildup during LHCD buildup

13. SXR and AXUV reconstructions confirm corebuild-up and redistribution • Redistribution of particles/heat was of the order of -20% in the core and +10% at q~1(redistribution/flattening) • Confirm that mode is only core localized. • DPrad,0~300 kW/m3 • ⇒dne✓ • Prad is “constant” elsewhere. • Core Pradsould be due to changes in Ar and Mo emission.

14. Reconstructions of fishbone-like mode resembles (1/1) kink traveling in the e-diamagnetic direction

15. Core displacement resembles internal kink

16. Fit to fishbone-like perturbation reveals “J1(lr)cosq” form of m=1 eigenfunction

17. Two-color interferometer (TCI) singles-out coredensity perturbation • 10-channel interferometer (Dr~1.1 cm) measures fundamental frequency at ~17 kHz. • Mode at ~34 kHz is just a geometrical effect.

18. Only central sightlines observe dne perturbation HFS LFS

19. Combining functional form with TCI data ⇒dne/ne0 perturbation is as large as 4% q=1

20. REMINDER: LHCD pulls a tail of the electron energy distribution function with energetic electrons reaching 120-140 keV

21. GPC- and FRC-ECE systems are sensitive to fast non-Maxwellian electrons from LHCD • Te(R,t) diagnostics agree well within 10-15% during Ohmic phase. • Non-thermal effects on ECE radiometers during LH difficult the estimates of Te(R,t). • Why the changes at the core?

22. Initial growth of the mode begins at the maximum of Te,0 while the temperature falls to a minimum at its disappearance • DC-offset includes the radiation effects of the non-Maxwellian EEDF. • Data from faster 2ndradiometer confirms slow modulation. • For the first time, observations demonstrate a direct dynamic relation between fast electrons and the destabilization and saturation of the kink. The relation holds independent of the appearance of sawteeth.

23. CORE fishbone-like MHD is also detected in EDGE channels of high resolutionFRC-ECE radiometer

24. Downshift of gyrofrequency due to relativistic effectsis possible (1,1) MHD activity ECE sightline Measured Relativistic effects downshift the frequency

25. Fast-electron pressure might have contribute in drivingthe internal kink (1,1) MHD activity ECE sightline Measured Similar signals are measured for electron energies down to 80 keV. Below this energy the plasma is optically thick.

26. GENRAY/CQL3D code calculates the anisotropic electron energy distribution function The moments of the EEDF give the electron pressure:

27. 2-3% extra pressure from ~100 keV electrons will contribute to driving the fishbone-like internal kink The moments of the EEDF give the electron pressure: The differences between the pressure integrands show that the contribution to pe,hotis mainly from energies between 30 and 300 keV, with a strong peak at around 100 keV. Coincidentally, the slowing-down time for 100 keV energetic electrons is on the order of 4-5 ms. I

28. LHCD-driven fishbone-like MHD in C-MOD does not result from wave-particle interaction The C-Mod fishbone- like mode appears instead to be a non-resonant internal kink driven by the fast electron pressure contribution to the central b. The local diamagnetic drift frequency of the background plasma is smaller than the plasma toroidal rotation mainly due to the weak density and temperature gradients. Perpendicular resonances that rely onw∗e,i appear unlikely. A very fast de-trapping rate means that few trappede- exist for perpendicular resonance, unlike in many ECRH plasmas.Parallel resonances are also unlikely. LH-driven parallel velocities are distributed over a wide range (3-10)vth, so that there are relatively few particles at any given resonant velocity. In contrast, ion fishbones driven by parallel wave-particle resonance in NBI heated plasmas, have a high power source of fast parallel ions concentrated at the beam energy that provides a strong candidate for resonance. In addition, the lack of significant frequency chirping in the C-Mod mode also suggests that perpendicular resonance is not important.

29. Summary Anew type of instabiity with a (1, 1) internal kink-like form has been observed on Alcator C-Mod with LHCD. Core buildup precedes fishbone-like mode formation. Measured fishbone-like perturbation is of the form J1(lr)cosq. A number of measurements at high spatial and temporal resolution directly connect the mode to changes in the fast electron dynamics and strongly suggest that it is not driven by wave-particle resonances. Instead, the mode is destabilized by the suprathermalelectron pressure due to the LHCD accelerated electrons. The electron energies (80-120 keV)involved in the mode-onset have beenmeasured for the first time using the downshift of electron gyrofrequency due to relativistic effects. The independence of the fast electron drive from the thermal pressure that drives the conventional internal kink explains the varied co-existence with the sawtooth crash and precursor oscillations.

30. REQUESTS