Order of magnitude improvements in the level and curation of current driven by lower hybrid waves have been chieved in the PLT tokamak. Steady currents up to 175 kA have been maintained for three seconds and 400 kA for 0.3 sec by the rf power alone. The principal current carrier appears to be a high energy (-100 keV) electron component, concentrated in the central 20-40 cm diameter core of the 80 cm PLT discharge. DISimtOTI OF THIS DOCUMENT is mump .\JLA

A simple technique for reducing the re-cycling rate and impurity concentration in a tokamak plasma is described. An active metal coating, in this case titanium, evaporated onto the surface of the vacuum vessel, provides a trap for neutral hydrogen and impurity atoms which would otherwise freely penetrate the plasma. With this treatment, the plasma density decays with time after the ionization of the initial filling gas is completed, in contrast to typical standard discharges which have a rising density throughout the entire period of the discharge, indicating a large gas influx. These discharges are observed to have resistances close to that of a pure hydrogen plasma, Zeff ≃ 1.0. There is a corresponding reduction in the intensity of highly ionized spectral lines of oxygen and iron as evidence of reduced impurity concentrations. The value of the effective ion charge, Zeff, can be varied, by pulsing controlled amounts of impurity gases into the hydrogen plasma.

Radial transport of medium- and high-Z ions during co- and counter-neutral-beam heating in the PLT tokamak is studied, using molybdenum and scandium ions as tracer elements. The time evolution of the radial profiles of several ionization stages of both elements, injected by laser blowoff during the neutral-beam heating, is measured under three significantly different beam-plasma combinations. No noticeable differences in the radial profiles attributable to the beam direction are observed. However, a given injected amount resulted in considerably larger interior concentrations of the tracer element in the counter-beam heating cases, suggesting larger penetration of the plasma periphery. Computer simulation with the MIST code suggests a net inward drift of the order 103 cm·s−1 superposed to a diffusion coefficient of the order 104 cm2·s−1 for both scandium and molybdenum ions. Injection of larger amounts of the tracer element, sufficient to cause measurable central electron temperature changes, resulted in dramatic changes in ion-state distributions, making some appear peaked in the centre while others disappeared. This effect could be produced with both co- and counter-beam heating, but with lesser amounts in the latter case. It is interpreted as rearrangement of the ionization balance, rather than any preferential accumulation of the injected element.

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Experimental results from the Adiabatic Toroidal Compressor (ATQ tokamak are used to obtain empirical scaling laws for the average electron temperature and electron energy confinement time as functions of the average electron density, the effective ion charge, and the plasma current. These scaling laws are extended to include dependence upon minor and major plasma radius and toroidal field strength through a comparison of the various tokamaks described in the literature. Electron thermal conductivity is the dominant loss process for the ATC tokamak. The parametric dependences of the observed electron thermal conductivity are not explained by present theoretical considerations. The electron temperature obtained with Ohmic heating is shown to be a function of current density -which will not be increased in the next generation of large tokamaks. However, the temperature dependence of the electron energy confinement time suggests that significant improvement in confinement time will be obtained with supplementary electron heating.

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In a toroidal fusion reactor, the plasma temperature will be maintained by the Coulomb collisions of the suprathermal a-particle population produced by DT reactions in the plasma. To attain fusion-reactor temperatures, a similar heating method can be used: namely, injection of a suprathermal ion population by means of neutralatom beams. These ions need serve merely as a Temp. Phys. !L2, 359 (1973). We obtain the following deviations from the classical Lorenz number: for Fe, 2.8%; for Co, 4.5%; and for Ni, 0.4%, Part of these deviations could be explained by specimen quality or real metal effects, as well as additional errors in the resistivity determination. . 20, 474 (1966). ^Some of our data (for example, Fig. 3) also suggest the existence of a first transition at Q 0 < Q c . This could correspond to a transition to superfluid turbulence, which gives further support to our association of Q c with normal-fluid turbulence. 12 J. T. Tough and C. E. Oberly [J. Low Temp. Phys. £, 161 (1972)] have discovered a dimensionless parameter P which seems to characterize the transition to normalfluid turbulence in the bulk liquid in the range 1-2 K. The parameter P does not seem to describe our data very well (5

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