Stochastic electron acceleration by an electron cyclotron wave propagating in an inhomogeneous magnetic field

Stochastic acceleration of electrons trapped in an inhomogeneous magnetic field is observed. A right-hand circular polarized microwave is used to excite an electron cyclotron wave, and a plasma with a temperature ≤ 6 eV and a density ≤ 7 x 1011 cm-3 is produced. The electrons trapped in the weak mirror interact with the electron cyclotron wave in their bouncing motion, and are accelerated to a few tens of keV range, emitting soft X-rays by bremsstrahlung. A simple discrete map based on the bounce period is derived to analyze the wave-particle system, and is found to be reduced to the Fermi map. The experimental results are understood with a knowledge of numerical experiments on this map, and the upper limit of acceleration is determined by the upper boundary of the stochastic region in the phase space. The theoretical scaling of the maximum electron energy with respect to wave power shows a good agreement with the experimental results. (author)