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-βN 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-βN 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=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=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=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.
Highlights
The steady-state advanced tokamak (AT) regime is considered to be one of the most attractive candidates for fusion energy production
In JT-60U, frequent excitation of the energetic particles (EPs)-driven mode in the early phase of the discharge was a roadblock to reproducible achievement of long-pulse steady-state high-bN discharges.14,16,17. This EP-driven mode is named the “energeticparticle-driven wall mode (EWM)” in JT-60U emphasizing their observation in the wall-stabilized high-bN plasmas, while in DIII-D the term “off-axis-fishbone-driven” resistive wall mode (RWM) is used, since the mode has frequency chirping, similar to the classical fishbone even with qð0Þ well above unity
The dTe-electron cyclotron emission (ECE) signals [Figs. 3(a) and 3(e)] show that the mode excitation took place with the peak located around r=a 1⁄4 0.5–0.6 in JT-60U (r=a is defined as volume-averaged radius) and q 1⁄4 0.5–0.6 in DIII-D (q is defined with normalized toroidal flux)
Summary
The steady-state advanced tokamak (AT) regime is considered to be one of the most attractive candidates for fusion energy production. In JT-60U, frequent excitation of the EP-driven mode in the early phase of the discharge was a roadblock to reproducible achievement of long-pulse steady-state high-bN discharges.14,16,17 This EP-driven mode is named the “energeticparticle-driven wall mode (EWM)” in JT-60U emphasizing their observation in the wall-stabilized high-bN plasmas, while in DIII-D the term “off-axis-fishbone-driven” RWM is used, since the mode has frequency chirping, similar to the classical fishbone even with qð0Þ well above unity.. We use the terminology “off-axis-fishbone-like mode (OFM)” or “off-axis-fishbone” describing these bursting modes It has been observed at each EP-driven MHD event in DIII-D, that the plasma rotation was reduced simultaneously with the rapid decrease of neutron emission rate, based on the fast sampling of charge exchange recombination (CER) light measurement.
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