... except I just realized that the final expression for gain minus loss is wrong: redoing the calculation I get -4 * E_0 * l_D / ( exp( d / l_D ) + 1 ) So ch

... except I just realized that the final expression for gain minus loss is wrong: redoing the calculation I get -4 * E_0 * l_D / ( exp( d / l_D ) + 1 ) So charges traversing the whole thing lose energy. You still get a net momentum due to the mass difference between electrons and protons, but in the opposite direction. :/