Gyrobroadening of fast ion energy spectrum due to relativistic mass variation

A gyrobroadening process causing the perpendicular energy of fast ions to be broadened by cyclotron instabilities due to the relativistic mass variation of the fast ions in a magnetized plasma is reported. At low cyclotron harmonics, a two-gyro-stream instability can be excited with the involvement of thermal ions if the condition lfwcf < w < lswcs is satisfied. The fast ions can be anomalously thermalized. Their lost energy goes to bulk ion heating. While the fusion produced protons can satisfy the condition, the alphas cannot. The low harmonics are linearly stable for the alphas. Therefore, for low harmonics one has to inject waves to affect the alpha dynamics. However, the alphas can excite high harmonic cyclotron waves in the lower hybrid regime, in which thermal ions respond as cold plasma. The interaction becomes selective. Only alphas with high pitch angle are involved. The implications of this process for fusion ignition and fast ion confinement are assessed