An instability with a lower frequency than the toroidicity-induced Alfvén eigenmode was initially identified as a beta-induced Alfvén eigenmode ͑BAE͒. Instabilities with the characteristic spectral features of this ''BAE'' are observed in a wide variety of tokamak plasmas, including plasmas with negative magnetic shear. These modes are destabilized by circulating beam ions and they transport circulating beam ions from the plasma core. The frequency scalings of these ''BAEs'' are compared to theoretical predictions for Alfvén modes, kinetic ballooning modes, ion thermal velocity modes, and energetic particle modes. None of these simple theories match the data.

DISCLAIMER I T h i s report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

No abstract

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...

Recent measurements of the two-dimensional (2-D) spatial profiles of divertor plasma density, temperature, and emissivity in the DIII-D tokamak [J. Luxon et al., in Proceedings of the 11th International Conference on Plasma Physics and Controlled Nuclear Fusion (International Atomic Energy Agency, Vienna, 1987), p. 159] under highly radiating conditions are presented. Data are obtained using a divertor Thomson scattering system and other diagnostics optimized for measuring the high electron densities and low temperatures in these detached divertor plasmas (ne⩽1021 m−3, 0.5 eV⩽Te). D2 gas injection in the divertor increases the plasma radiation and lowers Te to less than 2 eV in most of the divertor volume. Modeling shows that this temperature is low enough to allow ion–neutral collisions, charge exchange, and volume recombination to play significant roles in reducing the plasma pressure along the magnetic separatrix by a factor of 3–5, consistent with the measurements. Absolutely calibrated vacuum ultraviolet spectroscopy and 2-D images of impurity emission show that carbon radiation near the X-point, and deuterium radiation near the target plates contribute to the reduction in Te. Uniformity of radiated power (Prad) (within a factor of 2) along the outer divertor leg, with peak heat flux on the divertor target reduced fourfold, was obtained. A comparison with 2-D fluid simulations shows good agreement when physical sputtering and an ad hoc chemical sputtering source (0.5%) from the private flux region surface are used.

The non-inductive current drive from directional fast Alfven and electron cyclotron waves was measured in the DIII-D tokamak in order to demonstrate these forms of radiofrequency (RF) current drive and to compare the measured efficiencies with theoretical expectations. The fast wave frequency was 8 times the deuterium cyclotron frequency at the plasma centre, while the electron cyclotron wave was at twice the electron cyclotron frequency. Complete non-inductive current drive was achieved using a combination of fast wave current drive (FWCD) and electron cyclotron current drive (ECCD) in discharges for which the total plasma current was inductively ramped down from 400 to 170 kA. For steady current discharges, an analysis of the loop voltage revealed up to 195 kA of non-inductive current (out of 310 kA) during combined electron cyclotron and fast wave injection, with a maximum of 110 kA of FWCD and 80 kA of ECCD achieved (not simultaneously). The peakedness of the current profile increased with RF current drive, indicating that the driven current was centrally localized. The FWCD efficiency increased linearly with the central electron temperature as expected; however, the FWCD was severely degraded in low current discharges owing to incomplete fast wave absorption. The measured FWCD agreed with the predictions of a ray tracing code only when a parasitic loss of 4% per pass was included in the modelling along with multiple pass absorption. Enhancement of the second harmonic ECCD efficiency by the toroidal electric field was observed experimentally. The measured ECCD was in good agreement with Fokker-Planck code predictions

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.

The results of electron cyclotron current drive experiments on the T-10 tokamak are presented.The total RF power was up to 2.5 MW, the electron temperature was up to 7 keV and the maximum driven current was 110 kA. The current drive efficiency qcD was approximately 0.1 A/W. The value of qcD and its dependence on the plasma parameters agree satisfactorily with the linear theory, corrected for the finite confinement time of resonant electrons. In discharges with large beta poloidal, ,9, = 3, complete replacement of the inductive current by non-inductive electron cyclotron current drive and bootstrap current was obtained.