Simulation study of helium ash removal by ICRF heating in LHD and rippled tokamak plasmas

The accumulation of helium ash produced by DT fusion burning dilutes the fuel. Therefore, avoiding helium ash accumulation is essential in maintaining stable plasma burning in magnetic confinement fusion reactors. Hamamatsu et al. [PPCF1998] proposed removing Helium ash by ICRF heating with toroidal ripples as an active method to remove helium ash from burning plasmas efficiently. 

The He$^+$ ions, generated by the charge exchange of He$^{2+}$ with neutrals, are accelerated by RF waves and escape from the plasma region by orbit loss. This method would work well in the stellarator configurations because of the 3D magnetic configuration.

We study helium ion removal by ICRF heating, assuming the He beam injection in the deuterium plasma of LHD. 

We apply the GNET code[Murakami, NF2000], which can solve the drift kinetic equation in 5D and evaluate the behavior of He$^+$ ions. The maximum removal rate is obtained at a resonance field of around 2.56 T (the central magnetic field: 2.75T). At the optimum heating position, the removal rate increases by about 2.5 power of the heating power. We also perform the He ash removal simulation assuming ripple-tokamaks of LHD size varying the ripple amplitude, and the results are compared with the LHD configuration ones.