1. Critical RMP Field and Magnetic Island Amplitude Prior to Disruption on the COMPASS Tokamak
T. Markovič1,2, P. Cahyna1, R. Panek1, M. Peterka1,2, P.C. de Vries3, J. Havlicek1, A. Havranek1,4, M. Hron1, Y. Liu5, V. Weinzettl1, and the COMPASS team1
100 m vysoko, metru za sekundu.
1Institute of Plasma Physics of the CAS, Prague, Czech Republic
2Faculty of Mathematics and Physics, Charles University in Prague, Prague, Czech Republic
3ITER organization, St. Paul Lez Durance, France
4Faculty of Electrical Engineering, Czech Technical University in Prague, Prague, Czech Republic
5CCFE, Culham Science Centre, Abingdon, United Kingdom
21st MHD Stability Control Workshop / San Diego / 7th November 2016

2. Outline COMPASS tokamak. RMP coils and power sources.
Saddle loop diagnostics.
Locked mode generation by RMP.
n = 1,2 magnetic component, RMP-induced disruptions.
Spin-up after RMP, sawteeth activity drop.
Avoidance of RMP-induced disruptions.
q95 vs. IRMP conditions.
q95 vs. resonant perturbation magnitude.
Critical magnitude MHD disruption precursor studies
Summary
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3. ITER-like geometry with a single-null-divertor (H, He, D)
COMPASS tokamak
ITER-like geometry with a single-null-divertor (H, He, D)
Two NBIs enabling either co- or balanced injection
Ohmic and NBI-assisted H-modes [1]
New comprehensive set of diagnostics focused on the edge, SOL and divertor plasma
Major radius [m]
Minor radius [m]
Plasma current [MA] < 0.4
Magnetic field [T] < 2.1
Triangularity ~ 0.4
Elongation < 1.8
Pulse length [s] < 1.0
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[1] R Pánek, et al., Plasma Phys. Control. Fusion 58, 01401, (2016).
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4. RMP system of tokamak COMPASS
Experiments on COMPASS  2 RMP configurations so far, n = 2:
On-midplane – Bottom and top row + midplane row.
Off-midplane, even configuration.
Off-midplane, odd configuration in near future. + _ +
RMP coils – close to plasma separatrix.
Also coils on HFS (near future). + +
Power supplies:
4 independent power sources
Imax = 4 kA per RMP coil.
Current waveforms
AC RMP experiments in near future.
100 m vysoko, metru za sekundu. _ +
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5. + + - - + - + - Diagnostic saddle loops
100 saddle loops currently operational:
SL_11
SL_03
SL_01
Located outside vacuum vessel – skin effect cut-off of (>1’s kHz) frequencies.
Measurement of poloidal distribution of n = 1, 2 component of total field.
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n = 1 mode + - + -
Vacuum RMP approximation
Plasma RMP response
n = 2 mode
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6. RMP-induced locked modes
L-mode diverted plasma, on-midplane RMP, <ne> = 3-5*1019 m-3, varying q95.
Linear plasma response:
Typical response.
Brespn2 ~ IRMP in anti-phase to original perturbation.
Btotn1 ≈ Bvacn1
Due to n=2 Bresp
Mode locking by RMP:
Initially linear character.
Upon locking – significant change in Brespn2 and Brespn1
Variable onset delay.
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Locking onset
Locking onset
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7. RMP-induced locked modes
RMP field off  harmonic n = 1 oscillations of increasing frequency and decreasing magnitude.
 Spin-up of previously locked magnetic island.
Small in-vessel coil – spectrogram shows island frequency and magnitude returns to its pre-RMP values.
Central radiation  Te drop upon locking (sawteeth and background)
RMP pulse
Mode-locking
Mode-locking
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End of RMP
RMP pulse
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8. RMP-induced disruptions
If RMP magnitude for given q95 too high  disruption of the discharge.
Mode locking
Disruption
Multiple strong n = 1 non-harmonic peaks typically observed during the mode-locked phase – also visible on Dα signal.
Typical signature – Iplasma peak (Vloop drop) prior disruption due to sudden current profile redistribution.
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21st MHD Stability Control Workshop / San Diego / 7th November 2016
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9. RMP-induced disruptions
Device operation-wise – mapping combinations of operational parameters that did not lead to disruption.
On-midplane configuration – very limited IRMP for low q95.
Off-midplane configuration – no long-lasting mode locking observed  either entirely linear or disruption.
Disruptive
Linear
Absence of large midplane coil  higher RMP field by top/bottom row coils possible.
 But why is it so?
Disruptive
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Linear
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10. RMP magnitude on resonant surfaces
MARS-F [2] vacuum RMP simulations:
Spectra of n = 2 RMP – similar for both configurations.
Midplane coil row  larger RMP magnitude [3].
B1 magnitude on m/n = 3/2 instead of IRMP to plot linear/disruptive response:
m/n = 3/2
Shift of resonant surfaces due to q95 decrease
RMP field on resonant surface – more relevant to predict plasma behavior.
m/n = 3/2
Disruptive
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Linear
[2] Y. Liu, et al., Phys. Plasmas 7, 3681, (2000).
[3] T. Markovic, et al., Nucl. Fus. 56, , (2016).
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11. MHD disruption precursor studies
Statistics of critical magnetic island amplitude prior to thermal quench.
n = 1 measurements on both LFS and HFS:
Higher magnitude on LFS – closer proximity of sensors to plasma.
Purpose – multi-machine (JET, AUG, COMPASS) scaling of the critical precursor amplitude for ITER baseline scenario [4].
Full poloidal coverage of vacuum vessel allows more detailed studies (in progress).
Current peak
Signals of saddle loops at 4 toroidal quadrants
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Precursor phase
[4] P.C. de Vries, et al., Nucl. Fusion 56, , (2016).
Start of thermal quench
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12. Summary Tokamak COMPASS: Flexible operation, compact tokamak.
Variable m-number spectra of RMP fields (n = 1,2).
Over 100 saddle loops with full chamber coverage.
Crossing critical RMP magnitude for given q95 – tearing mode locking and/or discharge disruption.
Mode-locking thresholds:
Seemingly different for each of the 2 RMP configurations.
Identical from the viewpoint of RMP magnitude on resonant surfaces.
Studies of critical size of disruption precursor:
Part of multi-machine (+AUG, JET) scaling coordinated by P.C. de Vries.
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