Off-axis fishbone-like instability and excitation of resistive wall modes in JT-60U and DIII-D

Abstract

An energetic-particle (EP)-driven "off-axis-fishbone-like mode (OFM)" often triggers a resistive wall mode (RWM) in JT-60U and DIII-D devices, preventing long-duration high-b N discharges. In these experiments, the EPs are energetic ions (70-85 keV) injected by neutral beams to produce high-pressure plasmas. EP-driven bursting events reduce the EP density and the plasma rotation simultaneously. These changes are significant in high-b N low-rotation plasmas, where the RWM stability is predicted to be strongly influenced by the EP precession drift resonance and by the plasma rotation near the q ¼ 2 surface (kinetic effects). Analysis of these effects on stability with a self-consistent perturbation to the mode structure using the MARS-K code showed that the impact of EP losses and rotation drop is sufficient to destabilize the RWM in low-rotation plasmas, when the plasma rotation normalized by Alfvén frequency is only a few tenths of a percent near the q ¼ 2 surface. The OFM characteristics are very similar in JT-60U and DIII-D, including nonlinear mode evolution. The modes grow initially like a classical fishbone, and then the mode structure becomes strongly distorted. The dynamic response of the OFM to an applied n ¼ 1 external field indicates that the mode retains its external kink character. These comparative studies suggest that an energetic particle-driven "off-axis-fishbone-like mode" is a new EP-driven branch of the external kink mode in wall-stabilized plasmas, analogous to the relationship of the classical fishbone branch to the internal kink mode.

Disruptions, Disruptivity, and Safer Operating Windows in the High-β Spherical Torus NSTX

“…On the other hand, if these perturbations are not eliminated, then operation with q min >1.5 may be more appropriate. We also note in passing that EPMs have previously been implicated in the triggering of NTMs in ASDEX-Upgrade [165] and RWMs in DIII-D and JT-60 [166]. Similarly, ELMs have been indicated in the triggering of the dangerous 2/1 NTM in DIII-D [167] and JET [168], RWMs in DIII-D [80], and core n=1 kink modes in JET [169].…”

Physics of Alfvén waves and energetic particles in burning plasmas

“…However, even stronger effects are expected when resonant interactions are accounted for with thermal ions at the bounce or transit frequencies (Bondeson and Chu, 1996;Liu et al, 2004), or with either trapped thermal ions or electrons at the precession frequency (Hu and Betti, 2004). Experimental evidence also suggests the existence of EP driven external kink modes Okabayashi et al, 2011), which are the EPM counterpart of the resistive wall mode (RWM) (Pfirsch and Tasso, 1971). Recent reviews of the physics of internal kink (sawtooth) stabilization (Chapman et al, 2007;Graves et al, 2010Graves et al, , 2012 and analyses of high-β regimes for the DEMOnstration Power Plant (DEMO) (Chapman et al, 2011) confirm the necessity of thorough kinetic models for the description of the plasma operation control in burning plasmas.…”

Measurements of fast-ion transport by mode-particle resonances on DIII-D

“…Bursts, similar to traditional q = 1 fishbones, are observed on DIII-D in high-beta plasmas with q > 1 and heated by NBI [32,33]. Dubbed 'off-axis fisbones,' they have a fishbone-like magnetic signal and eigenfunction situated on the q = 2 surface.…”