A noninductive current drive concept, based on internal pressure-driven currents in a low-aspect-ratio toroidal geometry, has been demonstrated on the Current Drive Experiment Upgrade (CDX-U) [Forest et al., Phys. Rev. Lett. 68, 3559 (1992)] and further tested on DIII-D [in Plasma Physics and Controlled Nuclear Fusion Research, 1986, Proceedings of the 11th International Conference, Kyoto (International Atomic Energy Agency, Vienna, 1987), Vol. 1, p. 159]. For both experiments, electron cyclotron power provided the necessary heating to breakdown and maintain a plasma with high-βp and low collisionality (εβp∼1, ν*≤1). A poloidal vacuum field similar to a simple magnetic mirror is superimposed on a much stronger toroidal field to provide the initial confinement for a hot, trapped electron species. With application of electron cyclotron heating (ECH), toroidal currents spontaneously flow within the plasma and increase with applied ECH power. The direction of the generated current is independent of the toroidal field direction and depends only on the direction of the poloidal field, scaling inversely with magnitude of the later. On both CDX-U and DIII-D, these currents were large enough that stationary closed flux surfaces were observed to form with no additional Ohmic heating. The existence of such equilibria provides further evidence for the existence of some type of bootstrap current. Equilibrium reconstructions show the resulting plasma exhibits properties similar to more conventional tokamaks, including a peaked current density profile which implies some form of current on axis or nonclassical current transport.

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Following boronization, tokamak discharges in DIII-D have been obtained with confinement times up to a factor of 3.5 above the ITER89-P L-mode scaling and 1.8 times greater than the DIII-D/JET Hmode scaling relation. Very high confinement phases are characterized by relatively high central density with n e (0) ~ 1 xlO 20 m~3, and central ion temperatures up to 13.6 keV at moderate plasma currents (1.6 MA) and heating powers (12.5-15.3 MW). These discharges exhibit a low fraction of radiated power, F< 25%, Z e nr(0) close to unity, and lower impurity influxes than comparable DIII-D discharges before boronization.PACS numbers: 52.55.Fa, 52.25.Fi, 52.25.Vy In order to achieve ignition in proposed future fusion devices such as the Burning Plasma Experiment (BPX) and the International Thermonuclear Experimental Reactor (ITER), global energy confinement significantly better than the low-mode (L-mode) scaling relation is required in discharges with a low influx of impurities and low dilution of hydrogenic species [1]. Following boronization we have recently obtained discharges in the DIII-D tokamak with a very high confinement quiescent phase. These discharges have been repeated over many experimental days. We refer to this very high confinement phase as the VH mode. A number of tokamaks have obtained a high confinement mode (// mode) [2] with energy confinement times approximately a factor of 2 greater than for the L mode. In F/Z-mode discharges global energy confinement times are as much as a factor of 3.5 above ITER89-P [1] L-mode scaling and 1.8 times greater than the DIII-D/JET //-mode thermal confinement scaling relation [3]. This dramatic improvement in confinement quality is of great importance since the triple product noT,TE (related to the ratio of fusion power to heating power) in a tokamak fusion system increases as the square of the confinement enhancement factor over the L-mode scaling. Moreover, the VH phase of these discharges has shown less radiated power loss than is usually observed in comparable quiescent, i.e., ELM-free, //-mode discharges. [Edge-localized modes (ELMs) are transient phenomena which can occur in the outer plasma region and produce enhanced particle and energy transport, //-mode behavior is often described by the presence or absence (quiescent phase) of ELMs.] Temperature and density profiles show steep edge gradients extending further into the plasma than in the normal H mode, indicating a thicker edge transport barrier region.Boronization is a plasma-assisted chemical vapor deposition (CVD) process which deposits a thin, amorphous boron or boron-carbon film on all plasma facing components [4,5]. The boronization process was first implemented and later optimized in the TEXTOR tokamak at Forschungszentrum Julich GmbH [4]. Boronization in DIII-D (in collaboration with Julich) was accomplished using a glow discharge [6] in a helium-diborane gas mixture, 90% He and 10% B 2 D 6 , at a pressure of 5x10 ~3 mbar. A film of 100 nm average thickness was deposited. Depth profiles of a sample...

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Results are presented from an experiment in which a large amount of deuterium gas was continuously puffed at the scrape-off layer and exhausted from the DIII-D pump divertor ('puff and pump' technique). The experiments were conducted with a single null divertor configuration during edge localized mode (ELM)ing H mode. Steady state density conditions were attained during the strong deuterium puff. The deuterium puff reduced the central plasma density of argon, puffed continuously as a trace impurity, by as much as a factor of 20, and the central argon radiated power density by as much as a factor of 10