An energy-principle-based dispersion relation is derived for the resistive wall mode, which incorporates both the drift kinetic resonance between the mode and energetic particles and the resistive layer physics. The equivalence between the energy-principle approach and the resistive layer matching approach is first demonstrated for the resistive plasma resistive wall mode. As a key new result, it is found that the resistive wall mode, coupled to the favorable average curvature stabilization inside the resistive layer (as well as the toroidal plasma flow), can be substantially more stable than that predicted by drift kinetic theory with fast ion stabilization, but with the ideal fluid assumption. Since the layer stabilization becomes stronger with decreasing plasma resistivity, this regime is favorable for reactor scale, high-temperature fusion devices.

The toroidal Alfvén eigenmode (TAE), excited by trapped energetic particles (EPs), is numerically investigated in a tokamak plasma, using the non-perturbative magnetohydrodynamic-kinetic hybrid formulation based MARS-K code (Liu et al 2008 Phys. Plasmas 15 112503). Compared with the fixed boundary condition at the plasma edge, a free boundary enhances the critical value of the EPs kinetic contribution for driving the TAE. Free boundary also induces finite perturbations at the plasma edge as expected. An anisotropic distribution of EPs, in the particle pitch angle space, strongly enhances the instability and results in a more global mode structure, compared with the isotropic case. The plasma resistivity is also found to play a role in the EPs-destabilized TAE. In particular, the mode stability domain is mapped out, in the 2D parameter space of the plasma resistivity and a quantity defining the width of the particle distribution in pitch angle (for anisotropic distribution). A resonance layer in the poloidal mode structure, with the layer width increasing with the plasma resistivity, appears at the large width of the particle distribution in pitch angle space. A mode conversion, from the modified ideal kink by the EPs kinetic effect to the TAE, is also observed while increasing the birth energy of EPs. Computational results suggest that the TAE mode structure can be modified by certain key plasma parameters, such as the EPs kinetic contribution, the equilibrium pressure, the plasma resistivity, the distribution of EPs, as well as the birth energy of EPs. Such modification of the eigenmode structure can only be obtained following the non-perturbative hybrid approach (Wang et al 2013 Phys. Rev. Lett. 111 145003, Wang et al 2015 Phys. Plasmas 22 022509), as adopted in this study. More importantly, numerical results show that near the marginal stability point, the dominant poloidal harmonics of the TAE overlap with each other, and are localized at the tip positions of the Alfvén continua. This kind of TAE structure in high beta plasma with unstable ideal kink is substantially different from that of the conventional TAE.

During the 2016 experimental campaign of NSTX-Upgrade (NSTX-U), long L-mode and reproducible sawtoothing plasmas have been achieved that were previously not accessible on NSTX. This provides a good opportunity to investigate the conditions of sawtooth appearance and to study their effects on fast ion confinement and redistribution in spherical tokamaks. The Fast-Ion D-alpha (FIDA) and Solid State Neutral Particle Analyzer (SSNPA) diagnostics on NSTX-U each has two subsystems with one subsystem more sensitive to passing particles and the other one more sensitive to trapped particles. It has been observed on both diagnostics that the passing particles are strongly expelled from the plasma core to the plasma edge during sawtooth crashes while trapped fast ions are weakly affected. The tangential-FIDA (t-FIDA) system which is most sensitive to passing particles saw a signal drop in the region inside the inversion radius (˜125cm), while an increase at the outer region. The neutron rate can drop as much as 13% during sawtooth crashes. This phenomenon is similar to previous observations in DIII-D and ASDEX Upgrade conventional tokamaks. Detailed data analysis and modelling are being performed to quantity the effects of sawtooth crashes on fast-ion redistribution and to compare with the Kadomtsev sawtooth model. *Work supported by US DOE.

The mission of the spherical tokamak NSTX-U is to explore the physics that drives core and pedestal transport and stability at high- and low collisionality, as part of the development of the spherical tokamak (ST) concept towards a compact, low-cost ST-based pilot plant. NSTX-U will ultimately operate at up to 2 MA and 1 T with up to 12 MW of neutral beam injection power for 5 s. NSTX-U will operate in a regime where electromagnetic instabilities are expected to dominate transport, and beam-heated NSTX-U plasmas will explore a portion of energetic particle parameter space that is relevant for both -heated conventional and low aspect ratio burning plasmas. NSTX-U will also develop the physics understanding and control tools to ramp-up and sustain high performance plasmas in a fully-noninductive fashion. NSTX-U began research operations in 2016, but a failure of a divertor magnetic field coil after ten weeks of operation resulted in the suspension of operations and initiation of recovery activities. During this period, there has been considerable work in the area of analysis, theory and modeling of data from both NSTX and NSTX-U, with a goal of understanding the underlying physics to develop predictive models that can be used for high-confidence projections for both ST and higher aspect ratio regimes. These studies have addressed issues in thermal plasma transport, macrostability, energetic particlet-driven instabilities at ion-cyclotron frequencies and below, and edge and divertor physics.