Dependence of the sheath potential at the first wall on the wall temperature

The sheath potential near the first wall of a fusion reactor can be very sensitive to the temperature of the first wall due to the thermionic emission of electrons. The presence of this effect would lead to lower incident ion energies and could be utilized to reduce ion sputtering at the limiter blade of fusion reactors. The expression for the sheath potential at the first wall in the presence of thermionic emission has been derived. It was found that the value of the sheath potential could be reduced by ≅2kΒ/sub t/e/sup SE/ /e Volts due to thermionic emission (where kΒ /sub is the Boltzmann constant, t/e/sup SE/ is the electron temperature at the sheath edge, and e is the elementary charge). Thermionic emission is very sensitive to the temperature of the first wall. The transition temperature for a significant reduction in the sheath potential due to thermionic emission was found to be ≅ 1500 K for tungsten and dependent on the edge density (n/sub e//sup SE/) and temperature. This transition temperature could be in the range of 700-1100 K for materials with a low work function (≅1.5 -2.5 eV)