Guiding-centre orbits in non-circular axisymmetric tokamak plasmas are studied in the constants of motion (COM) space of (v, ζ, ψm. Here, v is the particle speed, ζ is the pitch angle with respect to the parallel equilibrium current, J‖, at the point in the orbit where ψ = ψm, and ψm is the maximum value of the poloidal flux function (increasing from the magnetic axis) along the guiding-centre orbit. Two D-shaped equilibria in a flux-conserving tokamak having β̄ values of 1.3% and 7.7% are used as examples. In this space, each confined orbit corresponds to one and only one point, and different types of orbit (e.g. circulating, trapped, stagnation and pinch orbits) are represented by separate regions or surfaces in the space. It is also shown that the existence of an absolute minimum B in the higher-β̄ (7.7%) equilibrium results in an orbit topology dramatically different from that of the lower-β̄ case. The differences indicate the confinement of additional high-energy (v → c, within the guiding-centre approximation), trapped, co- and counter-circulating particles, with an orbit ψm falling within the absolute B-well.

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The National Spherical Torus Experiment (NSTX) is a low aspect-ratio fusion research facility whose research goal is to make a determination of the attractiveness of the spherical torus concept in the areas of high-β stability, confinement, current drive, and divertor physics. Remarkable progress was made in extending the operational regime of the device in FY 2002. In brief, β t of 34% and β N of 6.5 were achieved. H-mode became the main operational regime, and energy confinement exceeded conventional aspect-ratio tokamak scalings. Heating was demonstrated with the radiofrequency antenna, and signatures of current drive were observed. Current initiation with coaxial helicity injection produced discharges of 400 kA, and first measurements of divertor heat flux profiles in H-mode were made.

-Rapidly developing diagnostic, operational, and analysis capability is enabling the first detailed local physics studies to begin in high beta plasmas of the National Spherical Torus Experiment (NSTX).These studies are motivated in part by energy confinement times in neutral-beam-heated discharges that are favorable with respect to predictions from the ITER-89P scaling expression. Analysis of heat fluxes based on profile measurements with NBI suggest that the ion thermal transport may be exceptionally low, and that electron thermal transport is the dominant loss channel. This analysis motivates studies of possible sources of ion heating not presently accounted for by classical collisional processes. Gyrokinetic microstability studies indicate that long wavelength turbulence with k θ ρ i ~ 0.1 -1 may be suppressed in these plasmas, while modes with k θ ρ I ~ 50 may be robust. High harmonic fast wave (HHFW) heating efficiently heats electrons on NSTX, and studies have begun using it to to assess transport in the electron channel. Regarding edge transport, H-mode transitions occur with either NBI or HHFW heating. The power required for L-to H-mode transitions far exceeds that expected from empirical ELM-free H mode scaling laws derived from moderate aspect ratio devices. Finally, initial fluctuation measurements made with two techniques are permitting the first characterizations of edge turbulence.