Study of energetic electrons during lower hybrid current drive experiments at 2.45 GHz from measurements of synchrotron radiation spectra in the PLT tokamak

Abstract

The spectrum of the synchrotron radiation in the extraordinary mode is measured using a rapid scanning Michelson interferometer during lower hybrid current drive experiments at 2.45 GHz in the PLT tokamak. The total population of the superthermal electrons scales with plasma density ne as for ne 1.5 × 1013 cm−3. When the input radiofrequency power PRF is varied from about 50 kW to 400 kW, the total population of the super-thermals increases linearly, from about 8 × 1015 electrons to 2.8 × 1016 electrons; T⊥min remains constant at about 80 keV, when PRF ≤ 155 kW, and thereafter it decreases almost linearly to about 50 keV, when PRF ≃ 400 kW. The population of the superthermal electrons is found to be maximum for a relative phasing between the waveguides of about 45-60°; the minimum perpendicular temperature is insensitive to the variation of this phasing. At intermediate densities (ne ∼ (1-1.5) × 1013 cm−3), when the RF wave is launched from the outside coupler it yields a population of superthermal electrons about 40% higher than when the RF wave is injected from the top of the torus; the minimum perpendicular temperature remains invariant in the two situations.