Gas discharge ion source. I. Duoplasmatron.
The effects of the plasma expansion cup on the operation of a duoplasmatron ion source have been investigated by measuring the total ion current and the distributions of the ion energy, mass, and current density. A copper expansion cup did not affect the magnetic field near the anode of the ion source and consequently the ion current density distribution was sharply peaked near the center of the cup. Ion energy distributions were approximately symmetrical about anode potential. The dominant ionic species were D(+) (3) and D(+) at low and high arc currents, respectively. Changes in the electrical potential of the copper cup with respect to the anode produced negligible changes in the above data. A mild steel plasma expansion cup caused the magnetic field to diverge and intercept the cup walls, resulting in ion current density distributions that were flatter and more amenable to focusing than the ones with the copper cup. With the steel cup at anode potential, the ion mass distribution was similar to that from the copper cup; however, the ion energy distribution was asymmetrical about the anode potential with a peak about 10-20 V above anode potential. The total ion current from this mode of operation was about one-third the value from the copper cup. If the steel cup assumed floating potential, about 50 V below anode potential, the total current increased to the level observed from the copper cup and the ion energy distribution was similar to that observed with the copper cup but the current density distribution was much flatter than that of the copper cup. The ion mass distribution was 60%-70% atomic ions over the entire arc current range investigated. Based on these data, a modified plasma expansion cup was designed with tapered steel walls lined with a boron nitride insert. The overall performance of the duoplasmatron ion source with this cup was superior to any of the previous three modes of operation. The improved design uses a molybdenum anode insert which is far superior to copper in regard to erosion over extended operating times. A total beam current of 200 mA was produced at an arc current of 24 A.
Intense Neutron Source Target Test Facility: A 200 kV, 200 mA dc, Deuterium Ion Accelerator
A 200 kV, 200 mA dc, deuterium ion accelerator has been constructed to evaluate high-temperature hydride materials such as scandium and erbium for use as targets in D-T neutron generators. The metal hydride film is deposited on a water-cooled, copper substrate designed to handle power densities of 40 MW/M2 while maintaining a surface temperature of ¿ 450°C. The goal of this program is to develop a target for a D-T neutron generator capable of producing 101 neutrons/s for use in cancer therapy. The accelerator employs a small duopigatron ion source which is capable of delivering a 200 mA deuterium ion beam that is composed of 60% atomic ions. The ion source is held at positive high voltage and a beam is formed with a single stage accel gap. An extractor electrode is biased negative about 2 kV to suppress secondary electrons and to define the point along the beam axis where the beam is space-charge neutralized by the trapped secondary electrons. The distance from the plasma expansion cup to the target is 0.6 m. Ion beam diameters at the target have been determined from measured infrared radiance profiles of the target due to beam heating. Data are presented on ion beam diameters for several operating conditions and accelerator geometries. D-D neutron output rates of 6 x 1010 neutrons/s have been measured with a 200 kV, 180 mA beam on erbium and a 180 kV, 160 mA beam on scandium.
Multimomentum 650 mA ion source
A high-current well-focused multimomentum source of molecular ions has been developed as part of a high-energy energy-spread injection system. The multimomentum injection system is designed to produce a trapped plasma with continuous distributions of momenta and energies in order to reduce velocity-space instabilities. The ion source beam was delivered at 16-20 keV. It was composed of nine or more different species of molecular and atomic ions of hydrogen, deuterium and other gases which were extracted and focused identically and simultaneously. Maximum beam divergence was 2° from the beam axis. Average beam intensity was 130 mA/cm2 in a 2 cm dia. beam 40 cm from the source. Electrostatic focusing was accomplished by an array of 6 mm dia. accel-decel extraction and focusing apertures. The beam was 85% composed of molecular ions, of which either the diatomic or triatomic component could be maximized by an adjustment of the gas fed to a reflex arc. Operating with a mixture of hydrogen and deuterium gas, the relative intensities of the several mass-to-charge ratio beam components indicated a random distribution of protons and deuterons.