Energy distributions of protons in DCX

Abstract

A particle spectrometer has been used to measure the energy distributions of neutral hydrogen atoms escaping from the 300 keV proton storage ring in DCX as the result of electron capture collisions between trapped protons and background gas molecules. A portion of the atoms were converted to protons by passage through an argon-filled gas cell, and the proton beam was then electrostatically analyzed. Energy distributions of the circulating protons were obtained by transformations applied to the measured distributions. For both gas and carbon arc dissociation the energy distributions were strong functions of the injected H₂⁺ current and the location of the region of sampling relative to the median plane. A number of curves are shown illustrating these dependences. With arc dissociation, the circulating protons lost energy at a rate of about 20 keV/ms with 0.1 mA injected current, and at a rate twice this value when the current was increased to 2.3 mA. Most of the 20 keV/ms loss rate is believed to be due to coulomb collisions of the circulating protons with electrons in the dissociating arc. This loss rate is within a factor of two of that calculated on the basis of loss to electrons of a Maxwellian distribution, well within the accuracy of the arc parameters used in the calculation. Several mechanisms that might account for the additional energy loss rate at higher currents are suggested, but details of the origin of this loss are as yet unclear. The additional energy loss had the consequence of decreasing the mean storage time of the circulating protons. Measurements with gas dissociation also showed an increase in the rate of energy loss with injected current. With either arc or gas dissociation, the response of the energy distributions to changes in injected H₂⁺ current indicated the presence of a non-collisional dispersing mechanism which increases in importance with increases in injected current. The nature of this mechanism is not clear.