Electrostatic Confinement of an Alkali-Metal Plasma

Abstract

PHYSICAL REVIEW LETTERS 20 SEPTEMBER 1971 ary, though there is a small displacement of the minimum. Furthermore, as is evident from Fig. 3, it is possible to obtain an analytic representation for B y (x) valid very close to the point where B y = 0 in the plasma. If the B y (x) profile is symmetric, then by iteration the entire profile may be deduced from a numerical fit with the observed polarization results.For experiments in which narrower spatial density profiles are obtained, we expect increasing applicability of the method (of course, the upper hybrid resonance must remain accessible). Finally, we suggest an experiment that can indicate whether a local or an integrated effect is being measured for the wave polarization. In this experiment the relative phase of the second-harmonic ordinary and extraordinary waves is measured. A small phase difference implies |^|«1, and hence Eq. (8) is applicable (the difference must of course be less than 2n).We mention briefly another method, using an incident ordinary wave, that may be appropriate to experiments having sufficiently narrow density profiles.The frequency of the wave is chosen such that the ordinary wave must cross a point in the plasma, x 0 , where v = v co (cutoff frequency for the extraordinary wave), i.e., N x (x o )=0. The E ± component generated for x <x 0 is absorbed at the upper hybrid resonance (or converted into longitudinal waves). If wave tunneling is small, then very close to the cutoff, E x = 0 and E y = 6(x 0 )En. As before, if conditions for x >x 0 approach those of free space we again have a local determination of the magnetic field direction from a polar-Until quite recently, research aimed toward improving plasma confinement has concentrated largely on magnetic field geometries and schemes. Some attention has been given to electrostatic ization measurement of the transmitted wave, E y (b)/E g (b) = 0(x o ). The utility of this method is more restricted than the harmonic technique as free-space propagation is more readily obtained in the latter case for x>x 0 .The imposition of a hollow longitudinal beam of 2-kV electrons about a Q-machine potassium plasma column serves to deepen the radial potential well across that column. Transverse plasma flux is consequently decreased, while the peak ion density and endplate collector current rise an order of magnitude. A calculation is made of the transverse flux of ions in the plasma column with energy sufficient to surmount the potential well. 786