[en] Experiments carried out on the RECE-Berta field reversing electron-ring device have identified a new type of anomalous particle loss caused by externally applied quadrupole-Ioffe magnetic field perturbations. Magnetic probe measurements of the ring magnetic fields and field reversal factor delta (= delta B/sub ring on axis/B/sub 0/) reveal three characteristic anomalous features in the ring decay when quadrupole fields are applied: (1) critical field reversal levels of delta/sub A/ < delta/sub B/ separating; (2) regimes of steady state anomalous decay between and above these levels, and (3) sudden dumps of fast ring electrons immediately before delta/sub A/ and delta/sub B/. Normal classical ring decay caused by collisions of fast ring electrons with neutral background gas is observed when delta < delta/sub A/. As part of an extensive series of experiments, the scaling of the three anomalous features with gas pressure and quadrupole field strength is studied. Additional measurements of x-ray emissions from a radial tungsten limiter target show that the dump of delta/sub A/ is coincident with a highly localized dump of fast electrons onto the limiter. Rings subjected to hexapole field perturbations show qualitatively similar decay anomalies. To obtain an explanation of these losses, the resonant coupling of particle orbital degrees of freedom by the externally applied symmetry breaking Ioffe-multipole field perturbations is investigated theoretically. The conditions for orbital instability of particles moving in the magnetic field of the electron ring and external mirror-multipole coils are calculated analytically, generalizing the results of Cohen et al. to all multipole orders. Unstable orbits occur when particle orbital frequencies w/sub z/ w/sub r/, and w/sub theta/, obey a resonance condition Nw/sub z/ + Lw/sub r/ + Mw/sub theata/ = 0 for theoretically calculated harmonic numbers (N,L,M)

[en] An instability condition is derived for electrostatic waves obeying the dispersion relation ω = ω/sub pe/k/sub parallel//k using a two dimensional electron distribution function, f(v/sub parallel/,v/sub perpendicular/), appropriate for lower-hybrid current drive. It is found that when the raised plateau in f in the velocity range v₁ < v/sub parallel/ < v₂ has a sufficiently large anisotropy, T/sub p parallel/ > T/sub p perpendicular/, this mode can become unstable. If the plateau velocities satisfy the condition v₂/v₁ > 1 + ω/sub ce//ω/sub pe/ then waves exist with phase velocities in the plateau region and these waves are most easily destabilized by the anomalous Doppler resonance, ω - k/sub parallel/v/sub parallel/ = -ω/sub ce/

[en] In microsecond-megavolt-pulse experiments with a magnetically insulated diode, high-energy ion beam densities of up to 300 A/cm² were obtained which are superdense (i.e. above the Child-Langmuir limit) by an order of magnitude. Ion pulse durations of up to 1 μsec resulted in large time-integrated ion yields: up to 120 μAsec/cm² (75 μAsec/cm² of protons) per pulse which may be sufficient for the generation of field-reversing ion rings. Using conducting anodes, megavolt insulation times in the microsecond range were observed for gap spacings of 1.2 to 1.7 cm

[en] Lower-hybrid current drive experiments (LHCD) have been carried out on the Versator II tokamak in which RF injection for pulse lengths longer than the plasma L/R time has generated large increases in the net toroidal current. Incremental increases, ΔI/I, of more than 35% have been obtained. These current rises are strongly dependent on the relative phasing between waveguides, Δphi =-90⁰. In typical cases, Thomson scattering measurements during RF drive show a decrease in the central electron temperature from 240 eV +- 45 eV to 120 eV +- 20 eV due to the spontaneous reduction in ohmic heating power during RF drive

[en] The scientific goal of the UCSD-DIII-D Collaboration during this period was to understand the coupling of the core plasma to the plasma-facing components through the plasma boundary (edge and scrape-off layer). To achieve this goal, UCSD scientists studied the transport of particles, momentum, energy, and radiation from the plasma core to the plasma-facing components under normal (e.g., L-mode, H-mode, and ELMs), and off-normal (e.g., disruptions) operating conditions

[en] A magnetic field has been applied to the 800 MHz four-waveguide lower-hybrid heating grill on the Versator II tokamak to suppress plasma formation and RF breakdown in the evacuated waveguides during tokamak operation. Without an auxiliary magnetic field, the incident power density is limited typically to 0.13 kW/cm². When the imposed magnetic field in the antenna is above the electron-cyclotron resonance value, the grill operates without RF breakdown or plasma formation at an incident power level of 0.42 kW/cm², limited only by the available RF power. A power density of 0.94 kW/cm² has been achieved when all the RF power was injected into one waveguide, again with no evidence of plasma formation

[en] An instability condition is derived for electrostatic waves obeying the dispersion relation ω = ω/sub p//sub e/k/sub parallel//k using a two dimensional electron distribution function, f(v/sub parallel/,v/sub perpendicular/), appropriate for lower-hybrid current drive. It is found that when the raised plateau in f in the velocity range v₁< v/sub parallel/< v₂ has a sufficiently large anisotropy, T/sub p//sub parallel/>T/sub p//sub perpendicular/, this mode can become unstable. If the plateau velocities satisfy the condition v₂/v₁>1+ω/sub c//sub e//ω/sub p//sub e/ then waves exist with phase velocities in the plateau region and these waves are most easily destabilized by the anomalous doppler resonance, ω-k/sub parallel/v/sub parallel/ = -ω/sub c//sub e/

[en] The effects of losses of the fast electron current carriers are included in a calculation of the efficiency of RF current drive. The analytical expressions obtained for the current drive efficiency J/P_A and bulk heating rate P_D/P_A are explicitly dependent on the fast electron confinement time, τ_F. In most present tokamak experiments, fast particle losses are predicted to reduce the current drive efficiency from 50% to more than an order of magnitude. However, in future fusion plasmas fast electron confinement appears to be sufficient to allow nearly ideal efficiencies to be achieved. (author). Letter-to-the-editor. 4 refs, 2 figs

[en] Increases in average density have been observed during lower-hybrid current drive (LHCD) on the Versator II tokamak and in other LHCD experiments where inductive and lower-hybrid current drive are combined. In the present paper results of an extensive study of this density increase are presented showing that the increase is the result of an improvement in particle confinement time, tau/sub p/, compared to ohmic discharges with comparable parameters. The improved confinement appears when the Parail-Pogutse relaxation mode is stabilized during LHCD

[en] Measurements of the momentum relaxation rate of suprathermal electrons in the VERSATOR II tokamak have been made using a novel cyclotron resonance technique. Attenuation of probe beams of X-mode radiation is interpreted as cyclotron absorption, yielding a measure of the distribution of current carriers. Differential measurements of two counter-propagating beams eliminate non-resonant effects. A distribution of electrons is produced by lower hybrid current drive and is observed to relax in the absence of wave power. The rate of momentum relaxation is anomalous but consistent with the observed current drive efficiency. (author). Letter-to-the-editor. 25 refs, 5 figs