Neutral atom and negative hydrogen ion production with a Hall accelerator

A Hall accelerator was studied with the objective in mind of injecting its output into a cesium charge exchange cell and producing negative ions. Operation of the high current Hall accelerator resulted in the observation of an ionizational instability. Elimination of this instability is possible in one of two ways. Either the arc current can be decreased to a very low level or an extension can be placed in front of the anode and the arc current increased to such a value that the mean free path for ionization is approximately equal to the separation between this extension and the anode. This latter method preionizes the gas before it enters the accelerating stages. Also by nulling the axial magnetic field between the two stages the source efficiency is greatly increased. High current operation also results in the predominant number of particles ejected from the source being neutral. At 0.1 ms into the pulse the source output consists of 940 +- 340 equivalent amperes of atomic hydrogen neutrals with an average energy of 1000 +- 300 eV and 24 A of protons with an energy of 1300 +- 260 eV. At 1 ms the output decreases to 320 +- 110 equivalent amperes of atomic hydrogen neutrals with an average energy of 590 +- 180 eV and 16 A of protons with an average energy of 870 +- 170 eV. The arc currents and voltages at 0.1 ms and 1 ms are approximately 1 kA at 1700 V and 670 A at 1600 V respectively. Two possible explanations are set forth to describe the large flux of neutral particles observed. One is charge exchange of protons on atomic hydrogen within the source. The other is wall neutralization of the accelerated ions after they are driven to the walls via instabilities and normal trajectories