A beam of microwave radiation is a powerful and penetrating means of exploring the density and temperature oflaboratory plasmas while causing minimal perturbation of the plasma. To a wave of frequency greater than the electron plasma frequency the plasma behaves like a dielectric, causing a change in the "optical" path which, when measured by interference techniques, yields the average electron density. The attenuation of the probing wave can give the collision frequency and hence the plasma temperature.
Propagation EquationsRepresenting a plane wave by the notation eiwt-yr we can determine the propagation constant y from the well-known Appleton-Hartree dispersion relation. If we restrict consideration to the case of propagation transverse to the confining magnetic induction field with the E-vector of the wave parallel to this field, the dispersion relation is given bywhere v is the collision frequency. For the plasma used in these experiments v is much less than the wave frequency wand the plasma frequency wp = (nee2/~ome)t.In the three frequency regions of interest, w>wp' w = wp' and w< wp' the attenuation constant a and the phase constant (3 are found from equation (1):(3 = (w~v/2cw2) (w~/w2_1)-t.(I)) Equation (3) gives the electron density in terms of the measured phase shift and (2) gives the collision frequency from the measured attenuation and known density. Provided the electron-ion collision frequency is somewhat. larger than the electron-neutral gas collision frequency the electron temperature (Te' in degrees Kelvin) can be calculated from the expression (Delcroix 1960) v = (2·6n In A)jTe3/2, collisions/s,
An extensive experimental program has been carried out on the MHD activity occurring in the LT-4 plasma during ohmic heating discharges, under.a wide range of reproducible conditions, for 3·6 > q(a) > 2·5. Perturbations are studied by using large arrays of individually monitored magnetic coils, distributed both poloidally and toroidally, and two toroidally separated soft X-ray poloidal arrays. The results show that the behaviour can be broadly classified into four distinct regimes within each of which the observable MHD phenomena are remarkably reproducible, and whose boundaries can be very well described by the single parameter q(a). Although most of the observations conform with those on other tokamaks, some appear inconsistent with the usual interpretation, for example, the assumption of rigid toroidal rotation. Further, while internal m/ n = 1/1 and m = 0 sawteeth activity at the q = 1 surface are clearly defined, the magnetic signals usually ascribed to a 2/1 mode when q( a) .;; 3 are clearly shown to have a predominant 3/1 structure, while activity at the q = 2 surface appears relatively weak.
A microwave beam (8-mm wavelength) was propagated parallel to the confining magnetic field in a plasma. To identify the transmission bands, the electron density and the magnetic field were slowly varied (in time) to separate the transmission bands of the left-hand circularly polarized wave from the right hand wave, above and below the electron cyclotron resonance. The experimental data show good agreement with the dispersion relations derived more than thirty years ago for ionospheric propagation.
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