Search

By operating a magnetized coaxial gun in a pulsed mode it is possible to produce large voltage pulses of duration $\ensuremath{\sim}500\text{ }\ensuremath{\mu}\mathrm{s}$ while reaching a few kV, giving a discrete input of helicity into a spheromak. In the sustained spheromak physics experiment (SSPX), it is observed that pulsing serves to nearly double the stored magnetic energy and double the temperature. We discuss these results by comparison with 3D MHD simulations of the same phenomenon.

Magnetic fluctuations have been reduced to approximately 1% during discharges on the Sustained Spheromak Physics Experiment by shaping the spatial distribution of the bias magnetic flux in the device. In the resulting quiescent regime, the safety factor profile is nearly flat in the plasma and the dominant ideal and resistive MHD modes are greatly reduced. During this period, the temperature profile is peaked at the magnetic axis and maps onto magnetic flux contours. Energy confinement time is improved over previous reports in a driven spheromak.

A laser diagnostic yielding detailed space-tme resolution of the ion velocity distribution function reveals that ions ejected by electrostatic instabilities can form a frequency-coherent beam circulating diamagnetically at large radii around the unstable region. This results in anomalous transport with fluxes and over distance scales much larger than conventional processes.

In thermal-barrier experiments in the tandem mirror experiment upgrade axial confinement times of 50 to 100 ms have been achieved. During enhanced confinement we measured the thermal-barrier potential profile using a neutral-particle-beam probe. The experimental data agree qualitatively and quantitatively with the theory of thermal-barrier formation in a tandem mirror.

Tokamak discharges using the expanded boundary divertor in the DIII-D device exhibit H-mode confinement. With neutral-beam power up to 6 MW, energy confinement remains comparable to the Ohmic value at a plasma current of 1 MA. Confinement is also independent of plasma density and toroidal field. Confinement increases with plasma current, but the exact functional dependence is, as yet, uncertain. These results show that the H mode can be achieved in a reactor-compatible open divertor configuration.

The first observation of $H$-mode confinement with electron heating as the sole auxiliary heating method has been made in divertor discharges in the DIII-D tokamak. These discharges exhibit the usual characteristics of the $H$ mode, including improved confinement of particles and energy, when electron cyclotron heating is added at a power level above 0.7 MW. The $H$-mode transition is accompanied by the development of an electron temperature pedestal of 0.25 keV and a dramatic steepening of the density gradient near the separatrix.

Neutral-beam current-drive experiments in the DIII-D tokamak with a single null poloidal divertor are described. A plasma current of 0.34 MA has been sustained by neutral beams alone, and the energy confinement is of $H$-mode quality. Poloidal $\ensuremath{\beta}$ values reach 3.5 without disruption or coherent magnetic activity suggesting that these plasmas may be entering the second stability regime.