Interaction Between UHF Radiation and a Turbulent Plasma

Previous work revealed an anomalously low low-frequency conductivity of plasma in the Alpha apparatus. Our purpose in this work was to determine the UHF conductivity. For the wavelengths 8 and 4 mm, we observed the passage of signals through the plasma, the thermal radiation of the plasma and, because of the plasma, the changed Q-factor of the discharge chamber which is regarded as a large volume resonator. Simultaneously the average plasma density was determined from the attenuation of the beam of hydrogen atoms passing through it. During the active discharge period, the plasma density was between 1013 and 10H cm-3, and the electron temperature 20 to 40 eV. A forcefree configuration of fields and currents existed in the discharge, and the plasma filled comparatively evenly thé discharge chamber of about 6 m3. In the decaying plasma, following the discharge current, interferometric measurements of the electron concentration agreed with density measurements that used the attenuation of an atomic beam. In the active period of the discharge, interferometric measurements were impossible because of the strong turbulence of the plasma, so the electron concentration was determined from the attenuation of the atomic beam. From the Q-value of the discharge chamber during the discharge, one can determine the absorption coefficient of the UHF signal in the plasma. If the average density is known from the attenuation of the atomic beam, it is possible to find the effective frequency of electron collisions. This may also be found from the dc conductivity of the plasma, which is determined by electrical measurements. On thecother hand, from the observed densities and electron temperatures one can calculate the frequency of electron collisions for a totally ionized hydrogen plasma. Both methods of determining experimentally the effective collision frequency gave much higher values than found by calculation. We may thus assert that there existed in our plasma an electron slowing-down mechanism which was more effective than Coulomb collisions with singly-charged ions. This caused the anomalously low conductivity and anomalously high absorption coefficient in the UHF range. As well as measuring the Q-factor, we measured the attenuation of 8 and 4-mm signals passing through the plasma. The signal was strongly attenuated at plasma densities which were far from critical. For example, the power of a 4-mm signal was reduced by 10 decibels on passing through a layer of plasma 1 m thick, whose density was 1.5 to 2. 0 x 1013 cm-3 whereas the critical density for this range is 7 x 1013 cm-3. Thus, determination of the electron concentration by 'cut-off' of the signal gives much higher results. Attenuation of the signal at concentrations far from critical may be caused both by absorption in the plasma and by scattering from non-homogeneous regions of the plasma. Calculation shows that the observed attenuation cannot be explained by absorption, even with the anomalously high measured absorption coefficient. Most likely, attenuation is a result of the scattering of UHF radiation from non-homogeneous regions in a turbulent discharge. A further, more careful study of this scattering may give a method for investigating the turbulent state of the plasma. (author)