Penetration of fast hydrogen atoms into a fusion reactor plasma

The depth of penetration of fast hydrogen atoms into a fusion reactor plasma, before they become ionized and held by the magnetic field, is of importance in connection with the heating and refuelling of such a plasma. Here the experimental cross-sections for the important collision processes are reviewed and analytic expressions are formed which describe the data reasonably well.

The ion and electron energy distributions to be expected in both toroidal and mirror reactors are used, with the analytic expressions for the cross-sections, to calculate rate coefficients. The plasma thickness T for an 1/e reduction in intensity of the atom beam is then obtained as a function of beam energy and plasma temperature over the range 0.1 to 2000 keV (D⁺, D^o). As an example a 1 MeV deuterium atom beam entering a toroidal reactor plasma has T=1.7×10¹⁶ and 2.5×10¹⁶cm⁻² for plasma temperatures of 1 and 100 keV, respectively.