Non-adiabatic and stochastic mechanisms for cyclotron resonance trapping and heating in mirror geometries

Numerical orbit calculations-indicate very efficient trapping of injected particles by an r.f. field with a frequency corresponding to the cyclotron frequency within a magnetic mirror. For an initially isotropic velocity distribution the orbit theory indicates that the trapping process is nonadiabatic and consequently that the particles will remain trapped for very long periods of time and be stochastically heated to high energies. The orbit theory also predicts a transition to adiabatic behavior for large initial to heat highly anisotropic plasmas in this manner. A statistical theory has been developed to calculate both the degree of heating and the trapping times. The predicted distribution function is Gaussian in energy rather than Maxwellian. Experiments on r.f. cyclotron resonance capture and heating have been performed. Previously reported work has been extended and considerably higher temperatures ( approximately 100 keV at peak compression of 45 kG) have been found. A clear transition between the Gaussian and Maxwellian energy distribution is found, as predicted by the theory. The absolute magnitude of the stochastic heating ( approximately 8 keV at the end of the heating pulse at cyclotron resonance of 3.7 keV) is in reasonable agreement with the theoretical prediction. The number of trapped particles is found to be greater than 10 12 .