m=2 mode activity during lower hybrid current drive in ASDEX

Zohm, H.; Soeldner, F.; Bruhns, H.; Buechse, R.; Leuterer, F.; ASDEX-Team,

doi:10.1088/0741-3335/33/12/007

Cited by 11 publications

(6 citation statements)

References 11 publications

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“…The internal inductance and core rotation velocity both dropped at first, but as the MHD activity developed at 0.93 s, as seen on the magnetics trace, the rotation velocity halted and there was an increase in l i . Similar behaviour has been reported from ASDEX [34]. Mode locking is well known to cause rotation braking [35,36] but why this affects the current density profile, as seen in the increase in the internal inductance, is not clear.…”

Section: Observed Rotation Time Histories and Profile Evolutionsupporting

confidence: 75%

Observations of counter-current toroidal rotation in Alcator C-Mod LHCD plasmas

et al. 2009

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Following application of lower hybrid current drive (LHCD) power, the core toroidal rotation in Alcator C-Mod L-and H-mode plasmas is found to increment in the countercurrent direction, in conjunction with a decrease in the plasma internal inductance, l i . Along with the drops in l i and the core rotation velocity, there is peaking of the electron and impurity density profiles, as well as the ion and electron temperature profiles. The mechanism generating the counter-current rotation is unknown, but it is consistent with an inward shift of energetic electron orbits, giving rise to a negative core radial electric field. The peaking in the density, toroidal rotation (in the counter-current direction) and temperature profiles occurs over a time scale similar to the current relaxation time but slow compared to the energy and momentum confinement times. Most of these discharges exhibit sawtooth oscillations throughout, with the inversion radius shifting inward during the LHCD and profile evolution. The magnitudes of the changes in the internal inductance and the central rotation velocity are strongly correlated and found to increase with increasing LHCD power and decreasing electron density. The maximum effect is found with a waveguide phasing of 60 • (a launched parallel index of refraction n ∼1.5), with a significantly smaller magnitude at 120 • (n ∼3.1), and with no effect for negative or heating (180 • ) phasing. These results resemble the current drive efficiency which scales as P LH /n e n 2 . Regardless of the plasma parameters and launched n of the waves, there is a strong correlation between the rotation velocity and l i changes, possibly providing a clue for the underlying mechanism.

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Section: Observed Rotation Time Histories and Profile Evolutionsupporting

confidence: 75%

Observations of counter-current toroidal rotation in Alcator C-Mod LHCD plasmas

et al. 2009

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“…Furthermore, stabilization of the MHD activity and a prevention of a disruption will be obtained by reversal of the slowing-down process and prevention of mode locking. Previous experiments showed that it was possible to unlock a mode using NBI [11,12], but at the cost of changing the plasma temperature considerably and, hence, also the resistivity. In TEXTOR we studied pure rotation effects, using additional momentum from NBI, keeping profile changes as small as possible.…”

Section: Introductionmentioning

confidence: 99%

MHD-mode stabilization by plasma rotation in TEXTOR

Vries

Waidmann

Donné

et al. 1996

Plasma Phys. Control. Fusion

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An experimental investigation into rotating MHD modes has been performed in the TEXTOR tokamak. The effects on the stability of the MHD tearing modes of coupling between m/n = 2/1 and 1/1 modes and of the slowing down of the mode rotation by wall friction have been studied. Tangential neutral beam injection (NBI) has been used to change the toroidal rotation to observe the influence of plasma rotation on the development of MHD modes.Two phases in the rotation frequency behaviour of the modes can be distinguished: a slow reversible decrease and a fast exponential decay, which starts with a short transient increase in frequency. The last phase finally results in mode locking, which mostly leads to a disruption.By injecting low power NBI in the last phase of the slowing-down process, mode locking could be prevented. The mode amplitude and rotation frequency have been stabilized, without major changes to the pressure profile. By decreasing the central plasma velocity, using NBI, a coupling of the two modes has been forced. The coupling of the modes had a destabilizing effect on the MHD activity. * This paper is an extended version of a contribution to the 22nd EPS conference on Plasma Physics and Controlled

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“…Experiments on Tore Supra [4] and ASDEX [5] have identified certain "pre-forming" techniques to eliminate the occurrence of MHD instabilities during LHCD. Some of the techniques identified are: step changes in P LH during the discharge; ramping down I p after turning on the LHRF; and intentional distortion of the n || spectrum by reducing the number of waveguide columns energized.…”

Section: Experimental Approachmentioning

confidence: 99%

LHCD during current ramp experiments on Alcator C-Mod

et al. 2017

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Abstract. The lower hybrid current drive (LHCD) system on Alcator C-Mod is capable of sustaining fully non-inductive discharges for multiple current relaxation times ( cr ~ 200 ms) at line averaged densities in the range of 5x10 19 m -3 . Some of these non-inductive discharges develop unstable MHD modes that can greatly reduce current drive performance, particularly in discharges with plasma current of 0.5 MA or less [1,2]. Avoiding these unstable MHD modes motivated an experiment to test if the stable current profile shape of a higher current non-inductive discharge could be achieved in a lower current discharge. Starting from a discharge at 0.8 MA, the plasma current was ramped down to 0.5 MA over 200 ms. The surface voltage of the plasma swings negative during the ramp, with the loop voltage reversal impacting the edge fast electron measurements immediately. Little change can be seen during the I p ramp in the core fast electron measurements, indicating that the loop voltage reversal does not penetrate fully to the magnetic axis on the timescale of the current ramp. The resulting discharge did not exhibit deleterious MHD instabilities, however the existence of this one discharge does not necessarily represent a robust solution to the problem.

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m=2 mode activity during lower hybrid current drive in ASDEX

Cited by 11 publications

References 11 publications

Observations of counter-current toroidal rotation in Alcator C-Mod LHCD plasmas

Observations of counter-current toroidal rotation in Alcator C-Mod LHCD plasmas

MHD-mode stabilization by plasma rotation in TEXTOR

LHCD during current ramp experiments on Alcator C-Mod

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