Complementary measurements of ion energy distributions in a magnetically confined high-temperature plasma show that magnetic reconnection results in both anisotropic ion heating and the generation of suprathermal ions. The anisotropy, observed in the C(+6) impurity ions, is such that the temperature perpendicular to the magnetic field is larger than the temperature parallel to the magnetic field. The suprathermal tail appears in the majority ion distribution and is well described by a power law to energies 10 times the thermal energy. These observations may offer insight into the energization process.

A new mechanism for intrinsic plasma flow has been experimentally identified in a toroidal plasma. For reversed field pinch plasmas with a few percent β (ratio of plasma pressure to magnetic pressure), measurements show that parallel pressure fluctuations correlated with magnetic fluctuations create a kinetic stress that can affect momentum balance and the evolution of intrinsic plasma flow. This implies kinetic effects are important for flow generation and sustainment.

An overview of recent results from the MST programme on physics important for the advancement of the reversed field pinch (RFP) as well as for improved understanding of toroidal magnetic confinement more generally is reported. Evidence for the classical confinement of ions in the RFP is provided by analysis of impurity ions and energetic ions created by 1 MW neutral beam injection (NBI). The first appearance of energetic-particle-driven modes by NBI in a RFP plasma is described. MST plasmas robustly access the quasi-single-helicity state that has commonalities to the stellarator and 'snake' formation in tokamaks. In MST the dominant mode grows to 8% of the axisymmetric field strength, while the remaining modes are reduced. Predictive capability for tearing mode behaviour has been improved through nonlinear, 3D, resistive magnetohydrodynamic computation using the measured resistivity profile and Lundquist number, which reproduces the sawtooth cycle dynamics. Experimental evidence and computational analysis indicates two-fluid effects, e.g., Hall physics and gyro-viscosity, are needed to understand the coupling of parallel momentum transport and current profile relaxation. Large Reynolds and Maxwell stresses, plus separately measured kinetic stress, indicate an intricate momentum balance and a possible origin for MST's intrinsic plasma rotation. Gyrokinetic analysis indicates that micro-tearing modes can be unstable at high beta, with a critical gradient for the electron temperature that is larger than for tokamak plasmas by roughly the aspect ratio.

Fig. 1. An isometric view of the HSX stellarator device showing the key features of the device beginning with the plasma boundary, the vacuum chamber, and the main and auxiliary magnet coils along with the support structure.Abstract-The Helically Symmetric Experiment (HSX) is a quasi-helically symmetric (QHS) stellarator currently under construction at the Torsatron/Stellarator Laboratory of the University of Wisconsin-Madison. This device is unique in its magnetic design in that the magnetic field spectrum possesses only a single dominant (helical) component. This design avoids the large direct orbit losses associated with conventional stellarators. The restoration of an ignorable coordinate in the confining magnetic field makes this device analogous to an axisymmetric q = 1=3 tokamak with respect to neoclassical confinement. A XISYMMETRIC toroidal fusion research devices (e.g., tokamaks) possess magnetic field symmetries which ensure good single-particle confinement and are conceptually simple in magnet coil design. However, the pulsed nature of inductive current drive, the large source of free energy in the plasma current which can drive instabilities (disruptions), the requirements for auxiliary and divertor magnet coil sets for plasma shaping, and for stability and position control, force increased engineering costs and complexities for such devices. Stellarators, however, have long been considered attractive fusion reactor candidates from the point of view of no required ohmic current, permitting true steady-state operation. The magnet coil sets for such devices, although complicated, are no more so than present-day tokamak devices. In conventional axisymmetric toroidal systems such as tokamaks, two magnetic field components produce the confining fields. They are 1) a conventional toroidal magnetic field and 2) a "poloidal" magnetic field produced by an ohmic current induced in the toroidal direction. The axis of symmetry therefore, is the toroidal direction. Classical stellarators, however, use helical coils to produce both the poloidal and toroidal magnetic fields which have magnetic field variations in the toroidal direction and therefore a breaking of the symmetry as compared to a tokamak. This gives rise to significant deviation of the particle drift surfaces off the magnetic flux surfaces 0093-3813/99$10.00

The behavior of energetic ions is fundamentally important in the study of fusion plasmas. While well-studied in tokamak, spherical torus, and stellarator plasmas, relatively little is known in reversed field pinch plasmas about the dynamics of fast ions and the effects they cause as a large population. These studies are now underway in the Madison Symmetric Torus with an intense 25 keV, 1 MW hydrogen neutral beam injector (NBI). Measurements of the time-resolved fast ion distribution via a high energy neutral particle analyzer, as well as beam-target neutron flux (when NBI fuel is doped with 3-5% D 2 ) both demonstrate that at low concentration the fast ion population is consistent with classical slowing of the fast ions, negligible cross-field transport, and charge exchange as the dominant ion loss mechanism. A significant population of fast ions develops; simulations predict a super-Alfv enic ion density of up to 25% of the electron density with both a significant velocity space gradient and a sharp radial density gradient. There are several effects on the background plasma including enhanced toroidal rotation, electron heating, and an altered current density profile. The abundant fast particles affect the plasma stability. Fast ions at the island of the core-most resonant tearing mode have a stabilizing effect, and up to 60% reduction in the magnetic fluctuation amplitude is observed during NBI. The sharp reduction in amplitude, however, has little effect on the underlying magnetic island structure. Simultaneously, beam driven instabilities are observed as repetitive $50 ls bursts which coincide with fast particle redistribution; data indicate a saturated core fast ion density well below purely classical predictions. V C 2013 AIP Publishing LLC [http://dx. INTRODUCTIONThe envisioned burning plasma experiment, regardless of magnetic concept, relies on sufficient confinement of the charged fusion product for plasma self heating. As such, the confinement of fast ions and their impact on the bulk plasma are crucial issues.A tremendous body of work demonstrates that fast ions in a tokamak plasma (born from fusion reactions, ICRF, or NBI) are generally well confined and thermalize via classical Coulomb collisions. However, a sufficiently intense fast ion population can excite collective instabilities that can lead to resonant fast ion transport. 1 A new body of work on the effects of a large fast ion population in the reversed field pinch (RFP) configuration has recently been opened. Despite the RFP's weak toroidal field and multiple resonant tearing modes which could diminish fast ion confinement, 2,3 NBI-born fast ions in low concentration are observed to slow classically and have a confinement time much larger than thermal particles. 4 The dearth of transport within the modestly stochastic magnetic field is understood to result from the decoupling of the fast ion orbits from the magnetic perturbations. The ions are routinely confined for up to a classical slowing time. 5 In this work, we investigate the effect of...