| To confine a plasma by means of the radiation pressure exerted by an intense, non-uniform electromagnetic field, it is necessary to use a cavity resonator of high quality factor to support the field. A plasma of significant density, however, can detune the resonant frequency of the cavity mode by an amount which is many times greater than the bandwidth of the cavity. Experiments which employ fixed frequency RF sources are thus severely limited in the range of plasma densities which can be studied.;To solve the detuning program, a system was developed which comprised a spherical cavity of .16 m radius and a closely coupled, specially built magnetron RF source of about 10 kW power. By using the cavity as the frequency determining element of the magnetron, it was possible to maintain the confining fields at frequency shifts of as much as 6 percent of the vacuum resonant frequency. A plasma was produced in this system by using the RF fields to break down a neutral gas. By exciting the cavity in the spherical cavity TM031 mode (f = 1.46 GHz), which has a confining quasipotential well, the breakdown resulted in a plasma of a density near the cutoff value of about 2.5 x 10('10)/cc. Under some experimental conditions the plasma evolved into a quiescent state which closely resembled the configuration predicted by RF confinement theory.;To model the experimental results, a computer code was developed to solve the ideal MHD equilibrium equations in spherical geometry by the finite difference method. The results of the model showed good agreement with the experimental results. Further, the calculations gave results which suggest that RF confined equilibria may exist for plasma densities far in excess of the cutoff density. |
