Ripple Induced Gyrostochastic Loss Of Trapped Particles In Tokamaks

Rapid loss of fusion alpha particles in tokamaks prevents transfer of energy to the background plasma, necessary for the maintenance of temperatures high enough to allow nuclear burn. It has been known for some time that the existence of magnetic field ripple with magnitude larger than a critical threshold value allows poloidally trapped high energy particles to stochastically diffuse out of the plasma on a time scale too short for significant energy transfer. This diffusion process is well understood, and provides a limit on the magnitude of toroidal field ripple, currently an important part of tokamak design. We find in the present work that the cyclotron motion causes a much larger loss of fusion alpha particles, originating from deep within the plasma. This process may also be important for neutral beam heating particles in present tokamak and stellarator research devices, and for the effect of compressional Alfven waves on trapped particles. The low threshold for gyrostochastic diffusion also has implications for thermal ion transport in 3-D error fields.