Destabilization of internal kink by LHCD suprathermal electron 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, A. Hubbard2, J. Hughes2, E. Marmar2, P. Phillips4, M. L. Reinke2, B. J. E. Rice2, W. Rowan4, R. Wilson1, S. Wolfe2 and S. Wukitch2

1PPPL 2MIT-PSFC 3MIT-LNS 4U-Texas-Austin

55th Annual Meeting of the APS Division of Plasma Physics   Monday–Friday, November 11–15, 2013; Denver, Colorado

Outline

① Background and motivation

② Measurements of fishbone-like activity in C-Moda) Main spectroscopic diagnosticsb) Core buildup before mode onsetc) Fluctuation measurementsd) Effects of non-Maxwellian fast electrons

③ Calculating the fast-electron pressure.

④ Summary and future work.

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① Background and motivation

① Instabilities can distort the orbits of fast particles causing an off-axis redistribution of plasma thermal energy to the detriment of the central b.

② Understanding the formation and stability of 3D helical modes in the core of an axisymmetric toroidal configuration, remains one of the challenges of fusion research.

③ Important for a burning plasma such as ITER since these 3D structures could occupy a significant volume of the plasma (rq=1~a/2).

④ The study of modes generated by energetic electrons remains a much less explored field than energetic ions (e.g. fishbones).

⑤ These studies are relevant to the investigation of trapped alpha particle interactions with low-f MHD modes in burning plasmas.

⑥ ECRH/ECCD or LHCD provide large sources of energetic electrons (10-500 keV) which can trigger these fishbone-like modes

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Diagnostic suite installed in C-Mod

enables 3D studies of internal kink

SXR tomographicSystem (XTOMO)

Two-color interferometer (TCI)

Har

d x-

ray

cam

era

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② 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

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LHCD is driving fishbone-like internal kink (iK) modes

① 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.

② Periodic (1,1) kink-like mode grows during the sawtooth ramp.

③ Train of successive m=1 bursts can appear during a sawtooth-free phase.

④ Time-scales associated to crash and damping of the mode are different (e.g. tSC 50 ≲ ms, tFB~1 ms).

⑤ Delayed off-axis SXR suggest a redistribution of particles/heat.

6⑥ Two different modes ≠ (1,1) impurity

long-lived mode.

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

Thomson Scatteringne & Te profiles

q=1

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L. Delgado-Aparicio, et al., RSI’12, PRL’13, NF’13

Impurity snakes survive sawtooth crashes with minimal impact

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

A hybrid sawtooth-internal-kink-mode is possible

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The (1,1) fishbone-like activity can coexist with sawteeth,

suggesting that the two modes

have independent

driving mechanisms.

Sawtooth precursor grows from oscillations remaining from incompletely damped kink mode

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① iK=internal kinkSC=sawtooth crashhM=hybrid mode crash.

② All three (1,1) modes have distinct amplitudes but nearly identical frequencies (small differences may arise from the sawtooth recovery).

③ The iK mode grows, briefly saturates, then damps and disappears.

④ Growth and damping occur over 1-2 ms intervals similar to the growth of the precursor, but much slower than the 20-40 ms of the sawtooth crash.

Sawtooth-free scenario still shows kink-like mode

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① A sawtooth stabilized scenario shows only a train of consecutive internal kink-like bursts.

② 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.

③ Various diagnostics show little evidence of any abrupt change that might correspond to a rapid “crash” phase.

④ Instead, observation suggests a (1/1) kink that grows steadily to a maximum amplitude, then gradually diminishes to zero.

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

buildup

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SXR and AXUV reconstructions confirm corebuild-up and redistribution

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① Redistribution of particles/heat was of the order of -20% in

the core and +10% at q~1(redistribution/flatt

ening)

② Confirm that mode is only core localized.

③ DPrad,0~300 kW/m3

⇒dne ✓

④ Prad is “constant” elsewhere.

⑤ Core Prad sould be due to changes in Ar

and Mo emission.

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

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Core displacement resembles internal kink

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

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Two-color interferometer (TCI) singles-out coredensity perturbation

① 10-channel interferometer (Dr~1.1 cm) measures fundamental frequency at ~17 kHz.

② Mode at ~34 kHz is just a geometrical effect.17

Only central sightlines observe dne perturbation

HFS LFS

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Combining functional form with TCI data ⇒dne/ne0 perturbation is as large as 4%

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q=1

REMINDER: LHCD pulls a tail of the electron energy distribution function with energetic

electrons reaching 120-140 keV

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GPC- and FRC-ECE systems are sensitive to fast non-Maxwellian electrons from LHCD

① Te(R,t) diagnostics agree well within 10-15% during Ohmic phase.

② Non-thermal effects on ECE radiometers during LH difficult the estimates of Te(R,t).

③ Why the changes at the core?21

Initial growth of the mode begins at the maximum of Te,0 while the temperature falls to a

minimum at its disappearance

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④ The relation holds independent of the appearance of sawteeth.

① DC-offset includes the radiation effects

of the non-Maxwellian EEDF.

② Data from faster 2nd radiometer confirms

slow modulation.

③ For the first time, observations

demonstrate a direct dynamic

relation between fast electrons and the destabilization and saturation of

the kink.

CORE fishbone-like MHD is also detected in EDGE channels of high resolution

FRC-ECE radiometer

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Downshift of gyrofrequency due to relativistic effects

is possible

Relativistic effects downshift the frequency

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ECEsightline

(1,1) MHDactivity

Measured

③ Fast-electron pressure might have contribute in driving the internal kink

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Similar signals are measured for electron energies down to 80 keV. Below this energy the plasma is optically thick.

ECEsightline

(1,1) MHDactivity

Measured

GENRAY/CQL3D code calculates the anisotropic electron energy distribution function

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The moments of the EEDF give the electron pressure:

2-3% extra pressure from ~100 keV electrons will contribute to driving the fishbone-like internal kink

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The moments of the EEDF give the

electron pressure:

I

① The differences between the pressure integrands show that

the contribution to pe,hot is mainly from

energies between 30 and 300 keV, with a

strong peak at around 100 keV.

② Coincidentally, the slowing-down time for

100 keV energetic electrons is on the order of 4-5 ms.

LHCD-driven fishbone-like MHD in C-MOD does not result from wave-particle interaction

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① 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.

② 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.

③ Perpendicular resonances that rely on w e,i∗ appear unlikely.

④ A very fast de-trapping rate means that few trapped e- exist for perpendicular resonance, unlike in many ECRH plasmas. Parallel resonances are also unlikely.

⑤ 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.

⑥ 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.

⑦ In addition, the lack of significant frequency chirping in the C-Mod mode also suggests that perpendicular resonance is not important.

Summary

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① A new type of instabiity with a (1, 1) internal kink-like form has been observed on Alcator C-Mod with LHCD.

② Core buildup precedes fishbone-like mode formation. Measured fishbone-like perturbation is of the form J1(lr)cosq.

③ 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.

④ Instead, the mode is destabilized by the suprathermal electron pressure due to the LHCD accelerated electrons.

⑤ The electron energies (80-120 keV) involved in the mode-onset have been measured for the first time using the downshift of electron gyrofrequency due to relativistic effects.

⑥ 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.

REQUESTS

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