Potential development and electron energy confinement in an expanding magnetic field divertor geometry

The formation of electrostatic potential in an expanding magnetic field divertor is numerically simulated using a kinetic model. As theoretically expected, the electrostatic potential is formed in the expanding magnetic field, which, in combination with the Debye potential near target walls, repels electrons back and balances electron and ion currents. Going beyond the existing theoretical description of the pre-sheath potential formation limited to the asymptotically low electron flow (u(e) << v(Te)), we demonstrate the limit of applicability of asymptotic theory and study pre-sheath potential in practically important range of electron flow [0 < I-e < 2I(sat), where I-sat = en root(T-e + T-i/m(i) is the ion saturation current]. Results of the asymptotic theory are fully reproduced at the low side of this range (I-e << I-sat), whereas at high electron current range I-e similar to I-sat, the pre-sheath potential substantially decreases. The formation of the pre-sheath potential minimizes the interaction of plasma electrons with the material walls and reduces the Debye sheath potential. Reducing the Debye potential forms favorable conditions for eliminating arcing and cold electron emission from the walls. In these favorable conditions, electron thermal losses at the wall could be reduced to minimal theoretical limit of similar to 5 - 8T(e) per lost ion.