Experimental determination of the diffusion rate in a mirror machine by means of trapped particle echoes

Echoes associated with the bounce motion of ions are observed in a mirror fusion device. The echoes are initiated by means of two sucessive perturbations of the confining magnetic field. The characteristic time for echo cancellation under diffusion in velocity space is measured. This time is somewhat less than the predicted classical time for Coulomb collisions. The discrepancy may be accounted for either by rf turbulent fields at ω_ci or by impurities both of which provide an enhancement of the diffusion coefficient.