Theory of high-frequency heating of an inhomogeneous high-temperature plasma

Abstract

We investigate Čerenkov absorption by a plasma of the energy of an electromagnetic field created by azimuthal electric currents flowing in a long coil around a plasma cylinder. The density of external currents has the form of a traveling wave j=j0 cos (kz—ωt) δ (r—R) (R is the radius of the coil). The frequency of the field is considerably smaller than the ion cyclotron frequency. Gas-kinetic plasma pressure n0(Te + Ti) is assumed small compared with the magnetic pressure H02/8π. Plasma density n0, electron temperature Te and ion temperature Ti are taken to be functions of the radius.In the present study expressions have been found for the energy absorbed by plasma per unit time. Absorption is maximal when the phase velocity of the wave ω/k is of the order of the thermal velocity of the ions νi = (Ti/mi)½ (mi is the ion mass). If the radius of the plasma is of the order of the radius of the coil and of the order of the wavelength 1/k, then for ω/k ∝ νi the mean energy gained by one particle per unit time, in order of magnitude, equals dW/df = H∼2ωTi/H02 where H∼ = 4πj0/c is the amplitude of the alternating magnetic field.For a strongly non-isothermal plasma (Te ≫ Ti) heating increases drastically if the phase velocity of the wave is near the velocity of sound in a collisionless plasma νs = (Te/mi)½Expressions have also been obtained for energy absorbed by an inhomogeneous plasma for coils of finite lengths.