Investigation of discharge initiation by ICRF antenna on URAGAN 3M

Introduction The experiments on ICRF discharge initiation were performed on the l = 3 URAGAN 3-M (U-3M) torsatron with a natural helical divertor located in Kharkiv, Ukraine in frame of the EU (Belgium) Ukrainian collaboration on ICRF Plasma Production. The experimental motivations were: (i) to study the dependencies of the breakdown time on the neutral gas pressure and the antenna power and (ii) investigate the hypothesis for the gas breakdown trigger as the moment of LHR generation in the antenna-field vicinity [1] by Langmuir probe measurements. The standard Frame antenna [2] was operated at f = 8.6MHz, variable RF power (P = 15−130kW, RF voltage at generator, VRF = 3−9kV), confining magnetic field BT = 0.01−0.72T to produce RF plasma in hydrogen at a continuous gas flow with pressure range pH2 ≈ 1 ·10−3−2 ·10−2Pa. Whereas sustaining a fully ionized hydrogen RF plasma takes place in the Alfvén wave range of frequencies (ω < ωci) for the given operation frequency at higher magnetic fields (BT ∼ 0.72T). The hydrogen gas RF breakdown moment is characterized by dominant concentration of the hydrogen molecular ions H 2 [1]. This results in the breakdown performance in the U-3M case in the frequency range ω > ωci, which allows existence of the LHR for all tested BT values.