The goal is to estimate the feasibility of using electrostatically plugged cusps and mirrors to confine thermonuclear plasmas. Assuming the density of ’’trapped electrons’’ to be small, the present work describes a set of equations for determining equilibrium particle densities, temperatures, and potential barrier heights. The equations are solved for specific cases involving neutral beam injection, and the ratio Q of fusion power output to injection power input is determined. Favorable Q values are found for applied voltages exceeding 250 kV.