The influence of electron emission on heat load to the plasma facing materials under space charge limited condition with an oblique magnetic field

摘要

The effect of electron emission on the potential distribution in the sheath and heat load on divertor plates has been studied taking into account the occurrence of space charge limited conditions (SCLC) and its extreme case of a virtual cathode under the existence of an oblique magnetic field. Interactions among heat flux from plasma, surface temperature, and thermo-electron emission are simulated considering self-consistency among electron transport, deposited heat and material thermal response for Be, C, Mo and W under the condition of plasma temperature T_e = 40 eV, plasma density n_o = 10~(19) m~(-3) and magnetic field strength B = 2 T with an angle between magnetic field and surface #alpha# = 10 deg. At lower surface temperatures, secondary electron emission dominates the total electron emission, whereas thermo-electron emission is superior at high surface temperatures. Both are promptly returned to the surface by an oblique magnetic field but the effect is more significant on the secondary electrons. In order to attain the SCLC condition, a substantial amount of thermo-electron emission (and hence higher surface temperature) is required. Heat flux from plasma and the surface temperature increase with time due to the increase of total electron emission. However, once SCLC is attained, the heat flux tends to saturate at a certain temperature. For C and W, with the aid of an oblique magnetic field, the SCLC condition is very likely maintained and consequently W surface would be free from the melting. Be and Mo, on the other hand, the amount of emitted electrons does not seem enough to maintain the SCLC and their surface temperature should be elevated.