[en] It is shown that the current distribution in a typical Tokamak plasma can be measured by a light scattering technique. The direction of the total magnetic field is measured accurately enough that the magnitude of the small poloidal component can be found. The field direction is measured by observing the scattered frequency spectrum of CO₂ laser light. The usual Gaussian spectrum becomes modulated at the electron cyclotron frequency when the difference between the incident and scattered wave vectors is nearly perpendicular to the magnetic field. The harmonics can be superimposed with a Fabry-Perot interferometer and their collective width resolved as the scattering direction is changed. The signal-to-noise ratio is high only when the detector is shielded against background radiation. 14 refs., 11 figs

[en] One technique to extend microwave scattering as a probe of long wavelength density fluctuations in magnetically confined plasmas is to consider the launching and scattering of extraordinary (X-mode) waves nearly perpendicular to the field. When the incident frequency is less than the electron cyclotron frequency, this mode can penetrate beyond the ordinary mode cutoff at the plasma frequency and avoid significant distortions from density gradients typical of tokamak plasmas. In the more familiar case, where the incident and scattered waves are ordinary, the scattering is isotropic perpendicular to the field. However, because the X-mode polarization depends on the frequency ratios and the ray angle to the magnetic field, the coupling between the incident and scattered waves is complicated. This geometrical form factor must be unfolded from the observed scattering in order to interpret the scattering due to density fluctuations alone. The geometrical factor is calculated here for the special case of scattering perpendicular to the magnetic field. For frequencies above the ordinary mode cutoff the scattering is relatively isotropic, while below cutoff there are minima in the forward and backward directions which go to zero at approximately half the ordinary mode cutoff density

[en] Wave reflections from density fluctuations with magnitudes and frequencies typical of drift waves have been investigated. Both drift waves and the reflected phase and amplitude depend on the density gradient scale length, and this common feature implies that both the angular deviation of a normally propagating wave and the phase change on reflection are of order unity. Thus the surface will always appear ''rough'' and amplitude variations will always be large. For smaller amplitude waves numerical solutions of the one dimensional full wave equation for the propagation near cutoff frequencies has been used to show how to interpret reflectometry measurements. For density perturbations with wavelengths near the density scale length, the external fluctuating phase can be simply interpreted in terms of a fluctuating density near the cutoff layer. However, the amplitude of the phase response falls substantially as the fluctuation wavelength, Λ, approaches the free space wavelength of the reflected wave, λ_o, and the location of the maximum response moves out in front of the cutoff layer following the wave matching condition k_Λ = 2k ∼ 2η(x)k₀. Similarly, correlation measurements of density fluctuations from probe waves of different wavelengths are shown to be limited to about four times the average reflected wavelength. 12 refs., 9 figs

[en] The processes leading to plasma heating when a large amplitude magnetic pulse interacts with an already magnetized low density plasma are studied. The pulse is produced in a theta-pinch geometry and propagates radially inward in the nature of a cylindrically imploding shock. Only rarely are conditions found, however, that lead to a quasi-steady state behind the pulse, so it is not strictly speaking a shock wave

[en] Experiments conducted on the PLT tokamak have shown that both plasma-limiter interaction and the gross energy confinement time are functions of the gas influx during the discharge. By suitably controlling the gas influx, it is possible to contract the current channel, decrease impurity radiation from the core of the discharge, and increase the gross energy confinement time, whether the aperture limiters were of tungsten, stainless steel or carbon

[en] Recent experimental results for ICRF heating in PLT are presented. For the two-ion regime in D-H or D-³He plasmas minority H and ³He ions are found to absorb the rf power and transfer it to the deuterons and electrons in accordance with Fokker-Planck theory. The deuteron heating rate is approx. 3 eV x 10¹³ cm^-3/kW for H and approx. 6 eV x 10¹³ cm^-3/kW for ³He minorities. Neutron fluxes of approx. 3 x 10¹¹ sec^-1 corresponding to a T/sub d/ approx. 2 keV (ΔT/sub d/ approx. 1.2 keV) have been produced with P/sub rf/ approx. = 620 kW at anti n/sub e/ approx. = 2.9 x 10¹³ cm^-3. Neutron energy spectra and mass sensitive charge exchange spectra indicate Maxwellian deuteron distributions. In addition, D-³He fusion reaction rates greater than or equal to 10¹² sec^-1 have been produced by the energetic ³He ions. For the second harmonic regime, initial heating results for an H plasma at P/sub rf/ approx. = 140 kW are consistent with the Fokker-Planck theory and the bulk heating rate is comparable to that of D heating in the D-H minority regime

[en] The evolution of the density profile in PLT during intense gas puffing is documented and analyzed. Measurements of the spectrum of low energy edge neutrals and of the change in central neutral density indicate that charge-exchange processes alone cannot account for the central density rise. The transient density profile changes can be reproduced numerically by a diffusivity of approx. 10⁴ cm²/s, and a spatially averaged inward flow of 10³ cm/s. These transport coefficients are 10 → 10² times larger than neoclassical. The ion energy confinement is reduced, the small scale density fluctuations are increased, and runaway electrons losses are increased during the density rise

[en] In order to investigate the effects of a light-element limiter on plasma parameters, aluminum rail limiters interchangeable with Mo rails were installed top, bottom, and outside directions in the ST tokamak. The inside limiter remained a fixed Mo rail. Compared with discharges produced immediately before and after with the usual Mo limiters, the ''aluminum'' discharges showed an increase of T/sub e/ (by factors of 1.4-2 near the center) and of energy confinement (by factors of 2 to 3 in el. energy/power input, depending on time of observation). H₂ and He discharges showed practically identical effects. In plasma composition, the Mo concentration dropped significantly, but Fe only slightly if at all; the Al concentration was about 3-5 percent (i.e., large compared to the heavier metals), whereas oxygen, about 4 to 8 percent to start with, dropped to insignificance, probably as a result of Al evaporation. The z/sub eff/ from resistivity increased 20-30 percent although the resistance dropped because of the higher T/sub e/. The improved T/sub e/ and energy confinement are thought to be the result of cumulative effects of more favorable radial current and power input distributions rather than direct energy losses by radiation

[en] One technique to extend microwave scattering as a probe of long-wavelength density fluctuations in magnetically confined plasmas is to consider the launching and scattering of extraordinary (X-mode) waves nearly perpendicular to the field. The geometrical factor is calculated for the special case of scattering perpendicular to the magnetic field. For frequencies above the ordinary-mode cut-off the scattering is relatively isotropic, while below cut-off there are minima in the forward and backward directions which go to zero at approximately half the ordinary-mode cut-off density. (author)

[en] Two different discharges in the ST Tokamak are described: A discharge dominated by high-Z (iron) impurity (Case A), and one dominated by low-Z (oxygen) impurity (Case B). The oxygen-dominated discharge shows higher electron temperature and longer containment time than the iron-dominated discharge. Suppression of Fe and Mo impurities has been achieved by pulsing in air for a few hundred shots