Electron cyclotron heated plasmas in the Helically Symmetric Experiment (HSX) feature strongly peaked electron temperature profiles; central temperatures are 2.5 keV with 100 kW injected power. These measurements, coupled with neoclassical predictions of large “electron root” radial electric fields with strong radial shear, are evidence of a neoclassically driven thermal transport barrier. Neoclassical transport quantities are calculated using the PENTA code [D. A. Spong, Phys. Plasmas 12, 056114 (2005)], in which momentum is conserved and parallel flow is included. Unlike a conventional stellarator, which exhibits strong flow damping in all directions on a flux surface, quasisymmetric stellarators are free to rotate in the direction of symmetry, and the effect of momentum conservation in neoclassical calculations may therefore be significant. Momentum conservation is shown to modify the neoclassical ion flux and ambipolar ion root radial electric fields in the quasisymmetric configuration. The effect is much smaller in a HSX configuration where the symmetry is spoiled. In addition to neoclassical transport, a model of trapped electron mode turbulence is used to calculate the turbulent-driven electron thermal diffusivity. Turbulent transport quenching due to the neoclassically predicted radial electric field profile is needed in predictive transport simulations to reproduce the peaking of the measured electron temperature profile [Guttenfelder et al., Phys. Rev. Lett. 101, 215002 (2008)].

By adding a current-carrying central rod to the basic rotamak apparatus, a magnetic configuration has been produced which is that of a spherical tokamak (ST) maintained in steady state by the application of a rotating magnetic field. The noteworthy reproducibility of the rotamak-ST discharges has facilitated the measurement of the time-averaged magnetic field components throughout a poloidal plane. These measurements, together with an assumption of axisymmetry, have enabled the field lines of an ST to be directly reconstructed from experimental data for the first time. [S0031-9007(98)07036-7] PACS numbers: 52.55.Fa, 52.55.HcThe investigation of plasma/field configurations of the compact torus variety is of current interest in the field of fusion research. Two configurations of this genre are the field reversed configuration (FRC) which does not have an externally applied toroidal magnetic field and the spherical tokamak (ST) which possesses such a field.The rotamak [1] is a compact torus configuration having the unique and distinctive feature that the steady toroidal plasma current is driven in a steady-state, noninductive fashion by means of the application of a rotating magnetic field (RMF) [2]. The toroidal current ring is kept in horizontal and vertical equilibrium by an externally applied magnetic field and, if conditions are appropriate, it can reverse this equilibrium field thus generating a compact torus configuration of the FRC type. Some of the latest results describing the operation of the rotamak as an FRC can be found in [3].The ST is the low aspect ratio limit of the tokamak. It has the advantages of simple construction, lower magnetic fields, and improved stability over the conventional tokamak. Interest in this particular compact torus concept is growing apace, supported in part by a favorable report [4] which highlights its potential as an economic fusion power plant. FIG. 1. The Flinders Rotamak-ST. By means of a simple modification, a steady toroidal magnetic field can be added to the basic rotamak apparatus and the configuration then becomes that of an ST maintained in steady state by means of the application of the RMF. Such a modified rotamak apparatus was indeed the first spherical tokamak experiment [5]. FIG. 2. Time history of (a) the rod current, ( b) the vertical field, and (c) the driven toroidal plasma current. The RMF was applied during the period 30 -70 ms. 2072 0031-9007͞98͞81(10)͞2072(4)$15.00

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 Alfvénic.

In the rotamak concept, a rotating magnetic field is used to drive toroidal plasma current in a compact torus device in a non-inductive manner. The latest results from a 0.5 m diameter rotamak apparatus are presented. These show that, for a given filling pressure of hydrogen, it is possible to drive more current, whilst simultaneously preserving the compact torus configuration, by increasing the amount of RF power transferred to the plasma. Attention is drawn to the fact that a fair evaluation of the rotamak concept requires experimentation at higher RF power levels than are presently available

Energetic-particle driven instabilities have been extensively observed in both tokamaks and stellarators. In order for such devices to ultimately succeed as D-T fusion reactors, the super-Alfvénic 3.5 Mev fusion-produced alpha particles must be sufficiently well confined. This requires the evaluation of losses from classical collisional transport processes as well as from energetic particle-driven instabilities. An important group of instabilities in this context are the discrete shear Alfvén modes, which can readily be destabilized by energetic particles (with velocities of the order of v Alfvén ) through wave-particle resonances. While these modes in three-dimensional systems have many similarities to those in tokamaks, the detailed implementation of modeling tools has required development of new methods. Recent efforts in this direction will be described here, with an emphasis on reduced models.