[en] In MIX-1, a washer gun produced plasma is injected through a long (1.3m) uniform guide field region into a 60 cm mirror cell. After injection, the system consists of a mirror-trapped plasma and an adjacent plasma column in the guide field region. The mirror cell plasma (n/sub e/ ≅ 10/sup 10/-10/sup 12/cm/sup -3/, T/sub i/ ≅ 100 eV, T/sub e/ ≅ 10 eV) is unstable to the drift cyclotron loss cone (DCLC) mode, while the neighboring guide region plasma (n/sub e/ ≅ 10/sup 11/-10/sup 12/cm/sup -3/, T/sub i/ ≅ T/sub e/ ≅ 10 eV) is microstable. Coupling of wave energy from the unstable mirror region into the guide plasma has been investigated for various magnetic field configurations. In the case B(Guide) > B(Mirror midplane), DCLC activity at the mirror midplane ion cyclotron frequency generates slow modes which propagate into the guide region with little attenuation. For B(Guide) < B(Mirror midplane), a resonance layer appears between the mirror cell and guide region at which strong ion cyclotron absorption is observed. In this case, weak conversion into a mode which has properties similar to an electrostatic ion cyclotron wave also occurs. This latter configuration is especially relevant to tandem mirror systems, in which a low field central cell is adjacent to a high field end cell which may be microunstable

[en] A model which treats the formation of wave sidebands on a propagating electron plasma wave in the case where the plasma density is modulated by a low-frequency perturbation is developed. A new mechanism for production of an asymmetric distribution of sidebands appears, and circumstances under which the original wave may reconstitute itself are described. An experimental study of electrostatic plasma waves in a weakly ionized plasma, where the density is modulated either by external means or by the presence of an unstable drift wave, is in agreement with the model

[en] A new ICRF antenna set has been installed which consists of two dual half turn antennas in each end cell as well as four dual half turn antennas in the central cell with multiphasing capability. Also, higher end cell magnetic fields now make it possible to consider heating the electrons in the end cells at the fundamental frequency rather than the second harmonic. Critical to the understanding of this new configuration is the determination of the behavior of the electron temperature in the end cells as well as throughout the machine. High energy density in the end cells (T/sub i/ ≅ 1 pounds≤V, is contained in a 5 x 10/sup 12/cm/sup -3/) makes it impossible to measure the electron temperature by standard electrostatic probes. Thompson scattering, which analyzes ruby light scattered off of the end cell electrons, is used to determine the local electron temperature. The east end cell Thomson scattering system uses a 5 joule, 30 nanosecond Q-switched ruby laser. The laser beam scattering volume is located on axis of the end cell and 5cm toward the central cell from the end cell midplane. The scattered light is collected at a 90 degree angle from the incident laser beam and is analyzed in a 5 channel triple grating polychrometer

[en] Full text: Discharges with high edge electron temperatures and flat radial electron temperature profiles, extending to the last closed flux surface, and into the low field side scrape-off layer, have now been achieved in the Lithium Tokamak eXperiment (LTX), with lithium-coated walls. Flat temperature profiles are a long-predicted consequence of low recycling boundary conditions. Temperature profiles are measured in repeated discharges with Thomson scattering; data from several discharges is averaged at each time point to improve accuracy at low density. Modelling indicates that the ion temperature profiles are also flat, which should eliminate temperature gradient-driven instabilities. The confined plasma therefore appears to be (separately) isothermal in the electron and ion populations. The edge density is very low, with a density profile which decreases approximately linearly with the poloidal flux. So far experiments are transient. Gas puffing is used to increase the plasma density. After gas injection stops, the discharge density is allowed to drop, and the edge is pumped by the low recycling lithium wall. The core impurity content, even in low density plasmas without fuelling, and edge electron temperatures of 200 eV, remains low. Z_eff is approximately 1.5, with most of the increase from oxygen, followed by carbon. The smallest fraction of the Z_eff increase, especially in the core, is from lithium. An upgrade to LTX, which includes a 35 A, 20 kV neutral beam injector to provide core fuelling and auxiliary heating, is underway. Two beam systems have been loaned to LTX by Tri Alpha Energy. With core fuelling provided by the neutral beam, an equilibrium similar to the ^Isomak [2] - a tokamak discharge in thermodynamic equilibrium - may be accessible in LTX, for the first time. A widened operational window, in both toroidal field and plasma current, is also planned, as well as eventual operation in diverted geometry. Results from the most recent experimental campaign will be described, as well as the upgraded configuration of LTX. (author)

[en] Experiments performed in a single-ended Q-machine examine the effect of a large-amplitude electrostatic lower hybrid wave on the dispersion of the current-driven ion acoustic instability. Nonlinear coupling of the modes provides a mechanism for suppression of the ion acoustic instability through a reduction in ion-acoustic mode phase velocity and an associated increase in ion Landau damping. An analysis which includes the effects of electron collisional or transit-time damping on the interaction is presented. This analysis demonstrates the resonant character of the interaction and allows quantitative evaluation of the lower hybrid power levels necessary for stabilization of the ion acoustic mode. (author)

[en] We describe a technique to routinely measure the scattering matrix of a radio frequency (rf) current drive antenna in a magnetic fusion experiment during high power operation. This technique can be used under any conditions of phase or amplitude of excitation. It involves modulation of the phase of the rf voltage applied to each of the antenna ports by a few degrees, measurement of the complex forward and reflected voltages, detection of the phase modulation, and solution of a 2x2 matrix problem to yield the scattering matrix. Further calculation then yields the antenna close-quote s impedance matrix. Because it involves only a small modulation, this method can be used to routinely monitor matching, decoupling, or plasma loading and to provide input to tuning calculations so that the matching and phasing conditions can be maintained. We show results for the case of 400 kW Alfvgrave en wave current drive experiments on the Phaedrus-T tokamak. We also show how the results can be used to tune the antenna. copyright 1997 American Institute of Physics

[en] The three-dimensional mode structure of the drift cyclotron loss-cone (DCLC) instability is characterized in an axisymmetric mirror machine. It is shown, for this experiment, that (1) the azimuthal mode structure does not assume the geometry associated with an azimuthal normal mode phi-tilde -- f (r,z)exp(imθ) with a single integral value of m, (2) the presence of a cool-ion plasma stream does not, in general, stabilize the mode globally, and (3) the axial mode characteristics have finite structure

[en] Suppression of the drift-cyclotron loss-cone instability (DCLC) in an axisymmetric mirror plasma has been observed for frequencies in the vicinity of the bounce frequency of electrostatically trapped thermal electrons. The location and width of the frequency region of suppression is in good agreement with calculations of bounce-resonance Landau damping

[en] Theoretical studies of fast wave damping on electrons are presented. Different regimes of importance for fast wave plasma heating and current drive in tokamaks are considered. Various cases of Maxwellian and multi-Maxwellian, isotropic and anisotropic electron distribution functions are investigated. In all cases, fast wave damping on electrons is due to three terms: Landau damping, transit-time magnetic pumping (TTMP), and the cross-term effect. Expressions for all these terms are derived for all regimes considered. Although in most cases Landau damping, TTMP, and the cross-term effect are the same order of magnitude, regimes are found that can be referred to as the pure Landau damping regimes, or pure TTMP regimes. A key parameter that defines the character of fast wave damping is found: δ_α=ε₂/γ_αεε₃. In simplified form, and far from resonances, it can be written as δ_α∼-2ω²/Ω_iΩ_eβ_α, where α specifies the appropriate regime

[en] Phaedrus B is an RF-sustained tandem mirror with ICRF heating in the central cell and in each end cell. Presently there are two sets of ICRF antennas in the central cell. Each set consists of two closely spaced (7.5 cm center to center) double half-turn antennas, with leads azimuthally displaced by 90⁰ to avoid antenna - antenna inductive coupling. The two antenna sets are located 50 cm to either side of midplane in the central cell. Currently there are two transmitters available for ICRF experiments in the central cell, each capable of delivering > 200 KW to the plasma. When Phaedrus B is operated without ICRF in the end cells, MHD stability of the central cell plasma depends on ICRF-driven radial ponderomotive force effects. Stability is dependent on both near field and far field (wave propagation) effects. In previous experiments, it was observed that stability was a strong function of (ω/ω/sub ci/), where ω is the ICRF frequency and ω/sub ci/ is the ion cyclotron frequency in the central cell (typically 8 x 10/sup 6/ rad/sec in Phaedrus B). This effect was attributed to the change in near and wave fields, and their radial gradients, as (ω/ω/sub ci/) is varied through unity. Here the authors present experiments in which the relative phase of two antennas in the central cell is varied, thereby selecting particular wave modes, and their associated radial field profiles, for preferential excitation. Such selectivity allows investigation of ponderomotive force effects without the changes in plasma parameters associated with varying the ratio (ω/ω/sub ci/)