Nonlinear absorption of high power free-electron-laser-generated microwaves at electron cyclotron resonance heating frequencies in the MTX tokamak

We report the first measurements of single-pass propagation of intense (>1-GW, 20-ns) microwaves through a high-density [${\mathit{n}}_{\mathit{e}}$=(0.5–2.4)×${10}^{20}$ ${\mathrm{m}}^{\mathrm{-}3}$] tokamak plasma at 140 GHz, the fundamental electron cyclotron resonance frequency. Compared with low-power pulses, these free-electron-laser generated intense pulses were not as strongly absorbed by the plasma, in agreement with predictions of nonlinear theory. Enhanced absorption of intense pulses was achieved by increasing the gradient in the parallel index of refraction (${\mathit{N}}_{\mathrm{\parallel }}$) of the microwave beam.