Penetration of high-frequency waves into a weakly inhomogeneous magnetized plasma at oblique incidence and their transformation to Bernstein modes

Abstract

Propagation of high-frequency electromagnetic waves in a weakly inhomogeneous magnetized plasma is investigated. We suppose the density gradient to be perpendicular to an external magnetic field, and the waves to be incident obliquely upon the plasma from vacuum. We find that the transmission coefficient of the ordinary wave through the plasma resonance is approximately equal to one in a fairly wide range of angles of incidence γ near the value γ0 = arcsin (Ωc/)(Ωo + Ω) ½. The transmitted ordinary wave is fully transformed into an extraordinary wave at higher densities. Then it propagates back to the region of a smaller density, and is fully transformed into the Bernstein modes in the vicinity of the hybrid resonance. Complications connected with the evanescent layer, which arise when the high-frequency energy is transmitted into the plasma in the form of the extraordinary wave, can thus be removed by using the ordinary wave with the angle of incidence chosen appropriately.

Emission of electron Bernstein waves in plasmas

“…This occurs when the O mode is launched at an oblique angle relative to the total magnetic field. 3 From ray tracing analysis we have also shown that EBWs are locally and strongly absorbed at the Doppler shifted electron cyclotron resonance or its harmonics. The strong and localized absorption implies that thermal emission of EBWs can occur for frequencies corresponding to the local Doppler-shifted electron cyclotron frequency.…”

ECRH/EBWH system for NSTX-U

“…The heating pulse duration used for the plasma startup experiments will be 10-50 ms. For EBW heating and current drive studies during the plasma current flattop the gyrotron pulse length will be increased to 1-5 s, and a second 1 MW gyrotron will be installed. Coupling to the EBWs in the plasma will be accomplished via O-mode to slow X-mode to EBW (O-X-B) double mode conversion [10][11][12]. An actively-cooled, concave steerable mirror launcher with a focal length of about 45 cm, capable of withstanding a 5 s duration, 2 MW heating pulse, will focus the 28 GHz power on the EBW mode conversion layer.…”

On electron Bernstein waves in spherical tori

“…Then the O mode power can couple to the X mode which in turn can undergo resonance absorption. The necessary condition for optimum coupling of the O mode power to the X mode is [3] n ,c = ck…”