Observation of fishbone-like internal kink modes during LHCD operation in Alcator C-Mod

Transcription

1 Observation of fishbone-like internal kink modes during LHCD operation in Alcator C-Mod L. Delgado-Aparicio 1 S. Shiraiwa 2, S. G. Baek 2, L. Sugiyama 3, R. Parker 2, R. Mumgaard 2, R. Granetz 2, I. Faust 2, S. Scott 1, D. A. Gates 1, N. Gorelenkov 1, N. Bertelli 1, M. Bitter 1, C. Gao 2, M. Greenwald 2, K. Hill 1, A. Hubbard 2, J. Hughes 2, J. Irby 2, E. Marmar 2, O. Meneghini 4, N. Pablant 1, P. Phillips 5, J. E. Rice 2, W. Rowan 5, J. Walk 2, G. Wallace 2, R. Wilson 1, S. Wolfe 2 and S. Wukitch 2 1 PPPL 2 MIT-PSFC 3 MIT-LNS 4 G. Atomics 5 U-Texas- Austin 40 th EPS conference on Plasma Physics, Espoo, Finland, 1 st -5 th July 2013

2 Outline 1 Background and motivation 1 Ion-fishbones (quick review) 1 Measurements of fishbone-like activity in C-Mod a) Core buildup before mode onset c) Fluctuation measurements d) Effects of non-maxwellian fast electrons 4 Working hypothesis on fishbones like observations. 5 Summary 2

3 1 Background and motivation 1 Conventional fishbones correspond to the so-called energetic-ions particle driven instabilities. 2 NBI or ICRH provide large sources of energetic ions compared to those found in Maxwellian distribution functions. 1 Ion-fishbones can induce re-distribution (flattening) of fast ions and thus cause neutron losses. 2 The study of modes generated by energetic electrons remains a much less explored field than energetic ions. 1 These studies are relevant to the investigation of trapped alpha particle interactions with low-f MHD modes in burning plasmas. 3

4 2 Three models to interpret ion-fishbones a) Models based on trapped particles (v ) Precessional fishbones: The frequency is fixed by the precession frequency of deeply trapped ions such that the mode is a continuum resonant mode. Diamagnetic fishbones: The frequency is close to the ion diamagnetic frequency such that the mode is described as a discrete gap mode. b) Models based on circulating particles (v ) Pressure fishbones: The destabilization is due to fast ions injected parallel to the magnetic axis (β hot ); trapped 4 particles are virtually absent.

5 3 MHD survey during LHCD: observations of precursors, sawteeth and electron-fishbones V loop ~-0.3 V V loop ~-0.3 V 5

6 LHCD is apparently driving fishbone-like MHD 1 I p ~0.5 MA, B t ~5.4 T, n,lhcd ~- 1.6, T e0 ~ kev, n e0 =( ) m -3. V loop ~-0.5 V V loop ~-0.5 V 1 A fishbone-like mode grows during the sawtooth ramp. 2 A successive train of m=1 fishbone-like bursts can appear during a sawtooth-free phase. 3 Time-scales associated to sawtooth crash and damping of the fishbone are different (e.g. τ SC 50 µs, τ FB ~1 ms). 1 Delayed off-axis SXR suggest there is a redistribution of particles/heat. 2 Different kind of heat pulses affect the background plasma. 6

7 SXR brightness shows that mode onset occurs after core build-up and affects both LFS and HFS The periodic fishbone-like bursts occur every ~4 ms (characteristic τ fast,e?). 7

8 Tomographic reconstructions confirm core-build-up before mode onset Redistribution/flattening of particles/heat was of the order of - 20% in the core and +10% at q~1 (Also observed in AXUV P rad ) Before After 100 kw/m3 8

9 Two-color interferometer (TCI) probes δn e ~1-2% core perturbation 1 10-channel interferometer ( r~1.1 cm) measures fundamental frequency at ~17 khz. 2 Mode at ~34 khz is just a geometrical effect. 9

10 SXR brightness suggest fishbone formation resembles (1/1) kink mode (1/1) kink traveling in the e-diamagnetic direction 10

11 Core displacement resembles internal kink 11

12 Fit to fishbone-like perturbation reveals J 1 (λr)cosθ form of m=1 eigenfunction 12

13 A hybrid sawtooth-fishbone mode is also possible 13

14 Properties of sawtooth precursors, fishbones and hybrid modes can be slightly different 1 Fishbone-like phenomena and sawteeth coexist ( snake); Hypothesis: δw TOT =δw F + δw K 1 Fishbone frequency ~14 khz. Precursor frequency ~15 khz. 3 f~1 khz? 4 Amplitude crash of hybrid mode is greater than individual fishbones and sawteeth. 14

15 Mode onset occurs when core ECE temperatures peak peaking of fast electron density 1 The GPC-ECE system has a limited time resolution. 2 Activity in the GPC temperature: Is from thermal plasma or nonthermals electrons. 1 Measured also by FRC-ECE. 2 The growth of the mode coincides with a maximum of the GPC temperature (maximum of the fast electron pressure?). 15

16 CORE fishbone-like MHD is detected in EDGE channels of high res. FRC-ECE radiometer Downshift of gyrofrequency due to relativistic effects is possible 16

17 4 Working hypothesis for e-fishbones in C-Mod Ion fishbones Trapped ions rnbi v resonant conditions Circulating ions tnbi v pressure: β hot,i Similarity between ion and electron effects Electron fishbones Trapped e - ECRH v resonant conditions Circulating e - LHCD v pressure: β hot,e 17

18 5 Summary 1 First observations of fishbone-like phenomena in Alcator C-Mod (no mag.) Two different kinds of (1,1) modes (trigger, ampl., τ, crash, and heat pulse). Core buildup precedes fishbone-like mode formation. Measured fishbonelike perturbation is of the form J 1 (λr)cosθ. Energies of circulating electrons possibly responsible for driving the fishbone-like mode are of the order of kev. 5 First estimates of β θ,hot ~5% > β θ,hot,crit. 18