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Recent observations on the HL-2A tokamak give new experimental evidence of resonant tearing modes (r-TMs) caused by the redistribution of energetic ions (EIs) due to sawtooth collapses in neutral beam injection plasmas. The $m/n = 2/1$ ( m and n are the poloidal and toroidal mode numbers) r-TMs with frequencies chirping down rapidly are found after strong bursts of fishbone modes and closely followed strong sawtooth collapses. The frequencies of fishbone modes and r-TMs chirp down from 10 to 7 kHz and 5 to 2 kHz rapidly, higher and lower than the 4–6 kHz existing weak resistive TMs. The fishbone modes propagate in ion diamagnetic drift directions in poloidal. In contrast, r-TMs propagate in the electron diamagnetic drift direction, though r-TMs are excited by EIs. It is suggested that counter-passing EIs, which originated from redistribution of EIs due to fishbone modes and combined with sawtooth collapses, excite r-TMs. Simulation results from the M3D-K code reveal that the counter-passing EIs play an important role in the excitation of r-TMs. The results can help broaden understanding of the interaction mechanisms between energetic particles and TMs, and open up a new perspective on the excitation mechanism of r-TMs.

The objectives of NSTX-U research are to reinforce the advantages of STs while addressing the challenges. To extend confinement physics of low- A , high beta plasmas to lower collisionality levels, understanding of the transport mechanisms that set confinement performance and pedestal profiles is being advanced through gyrokinetic simulations, reduced model development, and comparison to NSTX experiment, as well as improved simulation of RF heating. To develop stable non-inductive scenarios needed for steady-state operation, various performance-limiting modes of instability were studied, including MHD, tearing modes, and energetic particle instabilities. Predictive tools were developed, covering disruptions, runaway electrons, equilibrium reconstruction, and control tools. To develop power and particle handling techniques to optimize plasma exhaust in high performance scenarios, innovative lithium-based solutions are being developed to handle the very high heat flux levels that the increased heating power and compact geometry of NSTX-U will produce, and will be seen in future STs. Predictive capabilities accounting for plasma phenomena, like edge harmonic oscillations, ELMs, and blobs, are being tested and improved. In these ways, NSTX-U researchers are advancing the physics understanding of ST plasmas to maximize the benefit that will be gained from further NSTX-U experiments and to increase confidence in projections to future devices.

The resonant interaction between $m/n = 2/1$ tearing mode (TM) and energetic ions redistributed by $m/n = 1/1$ fishbone/sawtooth collapse has been observed recently in HL-2A neutral beam injection-heated high-density plasmas, where m is the poloidal mode number and n is the toroidal mode number. This event essentially results from a synergistic effect with a multiple-mode nature. Understanding such experimental phenomena, especially when using high-power auxiliary heating, is crucial for achieving high-performance plasmas with high confinement and high β _N in the prospective thermonuclear fusion reactors. In this work, based on the typical experimental phenomenon of synergy between fishbone/sawtooth and TM on HL-2A, a self-consistent nonlinear hybrid-kinetic simulation is conducted utilizing the experimental parameters and profiles before the occurrence of this synergy as the code M3D-K input. In particular, we focus chiefly on the phase-space transport, loss and redistribution of energetic ions induced by two cases, one with only $m/n = 2/1$ TM and one with synergy between $m/n = 1/1$ fishbone/sawtooth and $m/n = 2/1$ TM. It is found that the synergy of the fishbone/sawtooth and $m/n = 2/1$ TM on energetic-ion transport, loss and redistribution is important and can dramatically enhance the loss rate of energetic ions compared with the case with only $m/n = 2/1$ TM. It is due to the fact that such synergy can open up an efficient energetic-ion loss channel which connects the plasma center and edge. The phase space transport and redistribution of energetic ions induced by the two different cases are also respectively analyzed and compared in detail. Because of the overestimation of amplitudes of fishbone/sawtooth and TM in the simulations, the obtained energetic-ion transport level is only limited in the context of the numerical framework. These findings indicate that the synergy triggering the sawtooth collapse may lead to an enhancement of the energetic-ion transport and can help us understand the underlying mechanisms of synergy-induced energetic-ion transport.

The resonant interaction between energetic-particles and tearing mode is an unresolved physics issue at present. It is found for the first time in tokamaks that an unstable tearing mode with slowly rotating m/n  =  2/1 helicity, where m/n represent poloidal/toroidal mode-numbers, interacts with energetic-ions and results in amplitude-bursting/frequency-chirping fishbone-like activities. Nonlinear hybrid kinetic-MHD simulations with M3D-K code prove that the co-passing energetic-ions are responsible for the drive of tearing modes, and the wave-particle resonance condition is satisfied at , where , and are the toroidal, poloidal angular frequencies of energetic-ions and the mode frequency respectively. These findings can help the understanding of tearing mode induced energetic-particle loss and particle acceleration during the tearing mode reconnection in laboratory and space plasmas. [ABSTRACT FROM AUTHOR]

We report an experimental result on the stabilization of the energetic–ion driven internal kink mode (ion fishbone) by electron cyclotron resonance heating (ECRH), observed for the first time in a toroidal plasma. The mode asserts itself a resistive branch close to the marginal stability point. The resulting fishbone mode depends not only on the injected power but also on the radial deposition location of ECRH, and the instability can be completely suppressed when the injected ECRH power exceeds certain threshold. Analysis by the fishbone dispersion relation, including the resistive effect, suggests that the magnetic Reynolds number plays a key role in the mode stabilization—it weakens the mode growth-rate and enhances the critical energetic–ion beta without changing the energetic–ion population. This ion fishbone stabilization mechanism can be important for future devices such as ITER, which has significant ECRH capability. [ABSTRACT FROM AUTHOR]