Simulations of Tungsten Re-deposition Using a Particle-In-Cell Code with Non-uniform Super Particle Sizes

Tungsten (W), a promising candidate as divertor plasma facing material in magnetic fusion devices, is anticipated to promptly redeposit when sputtered or evaporated from surface due to its small ionization energy and long gyro radius. Using an artificial factor for the reaction cross sections, effects of ionization lengths to the re-deposition rate was studied by a newly developed particle-in-cell code. Treating numbers of particles in a super particle, electric charge, and mass as particle variables in the code, a special scheme for ionization and recombination was developed and used for the calculation. Simulations on W test particles with imaginary properties (neglecting the electric force) revealed the effects of ejection angles. Simulations with secondary electrons from surface showed that the sheath potential is weakened and the re-deposition rate becomes small. It was found that the multi-ionization as well as the ionization mean-free-path influences the re-deposition rate in both simulations. (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)