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DIII-D experiments at low density (${n}_{e}\ensuremath{\sim}{10}^{19}\text{ }\text{ }{\mathrm{m}}^{\ensuremath{-}3}$) have directly measured whistler waves in the 100--200 MHz range excited by multi-MeV runaway electrons. Whistler activity is correlated with runaway intensity (hard x-ray emission level), occurs in novel discrete frequency bands, and exhibits nonlinear limit-cycle-like behavior. The measured frequencies scale with the magnetic field strength and electron density as expected from the whistler dispersion relation. The modes are stabilized with increasing magnetic field, which is consistent with wave-particle resonance mechanisms. The mode amplitudes show intermittent time variations correlated with changes in the electron cyclotron emission that follow predator-prey cycles. These can be interpreted as wave-induced pitch angle scattering of moderate energy runaways. The tokamak runaway-whistler mechanisms have parallels to whistler phenomena in ionospheric plasmas. The observations also open new directions for the modeling and active control of runaway electrons in tokamaks.

A class of small aspect ratio (A) stellarators is found that does not rely on quasisymmetrization to achieve good confinement. These systems depart from canonical stellarators by allowing a small net plasma current. An optimization procedure with bounce-averaged omnigeneity and other desirable physical properties as target criteria is described and applied to show the existence of a compact plasma device having A{approx_lt}3 , high {beta} (ratio of thermal energy to magnetic field energy), and low plasma current. The added design flexibility afforded by the plasma current leads to an attractive low-A hybrid device which is stable to magnetohydrodynamic ballooning modes for {l_angle}{beta}{r_angle}{lt}6{percent} . {copyright} {ital 1998} {ital The American Physical Society}

Alpha-particle-driven toroidal Alfven eigenmodes (TAEs) have been observed for the first time in deuterium-tritium (D-T) plasmas on the tokamak fusion test reactor (TFTR). These modes are observed 100–200 ms following the end of neutral beam injection in plasmas with reduced central magnetic shear and elevated central safety factor [q(0) > 1]. Mode activity is localized to the central region of the discharge ({lt}0.5) with magnetic fluctuation level B⊥/B∥ ~ 10⁻⁵ and toroidal mode numbers in the range n=2–4, consistent with theoretical calculations of α -TAE stability in TFTR.

Alfvenic modes are observed due to neutral beam injection for the first time in a reversed field pinch plasma. Modeling of the beam deposition and slowing down shows that the velocity and radial localization are high. This allows instability drive from inverse Landau damping of a bump-on-tail in the parallel distribution function or from free energy in the fast ion density gradient. Mode switching from a lower frequency toroidal mode number n = 5 mode that scales with beam injection velocity to a higher frequency n = 4 mode with Alfvenic scaling is observed.

Energetic electrons generated by electron cyclotron resonance heating are observed to drive instabilities in the quasihelically symmetric stellarator device. The coherent, global fluctuations peak in the plasma core and are measured in the frequency range of 20-120 kHz. Mode propagation is in the diamagnetic drift direction of the driving species. When quasihelical symmetry is broken, the mode is no longer observed. Experimental observations indicate that the unstable mode is acoustic rather than Alfvenic.

Compact stellarator configurations have been obtained with good neoclassical confinement that are stable to both pressure- and current-driven modes for high values of beta. These configurations are drift-optimized tokamak-stellarator hybrids with a high-shear tokamak-like rotational transform profile and /B/ that is approximately poloidally symmetric. The bootstrap current is consistent with the required equilibrium current and, while larger than that in existing stellarators, is typically only a small fraction (1/3-1/5) of that in an equivalent tokamak. These configurations have strong magnetic wells and consequently high interchange stability beta limits up to beta=23%. Because of the reduced bootstrap current, these configurations are stable to low-n ideal MHD kink modes with no wall stabilization for values of beta ( approximately 7%-11%) significantly larger than in an equivalent advanced tokamak.

During neutral beam injection coherent magnetohydrodynamic activity occurs at low and medium \ensuremath{\beta}, which is driven by the energetic beam particles. As interpretation is given in terms of marginally stable global Alfv\'en eigenmodes (GAE), which are destabilized by Landau resonances of the far circulating particles. Toroidicity induced gaps for toroidal Alfv\'en eigenmodes do not occur in W7-AS but GAE modes in gaps below the shear Alfv\'en continua can be excited. This is investigated within a gyrofluid model, which has been used sucessfully for tokamak cases previously.

In the ELMO bumpy torus, neoclassical transport coefficients depend critically on the ambipolar electric field. These coefficients, calculated for arbitrary radial electric fields, are applied in a one-dimensional radial-transport calculation which, for the first time, treats the electric field self-consistently. This purely classical model predicts many features of experimentally operation including a steady-state solutions with radially inward-pointing ambipolar fields.