Analysis of the plasma meniscus in a hydrogen electronegative plasma

The emitting surface of an ion beam (plasma meniscus) in the vicinity of the extraction aperture is theoretically investigated. In the present study, the electrons, H+ ions and volume produced H- ions are contained in the electronegative plasma. By solving the one-dimensional Poisson equation, an analytical formula is obtained from the balance between the current reaching a wall and the space charge limited current of negatively charged particles. The analytical formula indicates that the distance, d (eff), between the plasma meniscus and the extraction grid includes the ratio of masses of the negative ions and electrons, which is different from that for a plasma containing electrons and single positive ions, and depends on the ratio of the negative ion density to the electron density, alpha. With increase in the value of alpha, the distance d (eff) becomes shorter, and thus the penetration of the electric field for negative ion extraction into the source plasma becomes smaller. This tendency is confirmed by using a three-dimensional particle-in-cell simulation from the results of the contour map of the electrostatic potential near the plasma grid and the location of the plasma meniscus. The reasons why the distance d (eff) becomes shorter with increase in the value of alpha are considered to be the larger space charge effect of the negative ions compared with that of the electrons and the decrease in the plasma potential.