Non-Maxwellian ion distribution function in edge pedestal

Gradient scale length of plasma density and temperature in the edge pedestal can be comparable to the banana orbit width of thermal ions. In the present-day large size tokamaks and future fusion reactors including ITER, the ion orbits in the H-mode pedestal can execute a significant radial excursion before experiencing Coulomb collision. Under such circumstances, a local thermal equilibrium may not be possible for ions. When Coulomb collisions are negligible, the ion distribution function becomes a canonical distribution function. In the large collisionality limit, the ion distribution function approaches a local Maxwellian. We use the XGC (X-point included Guiding Center[1]) code to study the property of ion distribution function in a quiescent pedestal plasma with steep density and temperature gradients. Deviation of the ion distribution function from a local Maxwellian or canonical Maxwellian distribution functions are studied as function of collision frequencies. It is shown that the non-Maxwellian property of the ion distribution function is greatly influenced by the orbit squeezing and expansion from radial electric field shear. [1] C.S. Chang, S. Ku, et al, Phys. Plasmas 11, 2649 (2004)