K. N. Leung

Extraction of volume‐produced H− ions from a multicusp source

H− ions formed by volume processes are extracted from a multicusp ion source. It is shown that a permanent magnet filter together with a small positive bias voltage on the plasma grid can produce a very significant reduction in electron drain as well as a sizable increase in H− ions available for extraction. A further reduction in electron current is achieved by installing a pair of ceramic magnets at the extraction aperture. The combined arrangement improves the ratio of extracted H− ion current to electron current to about unity.

Self‐extraction negative ion source

A large multicusp negative ion source has been constructed to generate H− or D− ions for high‐energy neutral beam systems. With the presence of cesium, the source has produced a steady‐state H− ion beam current greater than 1 A with an impurity level less than 1%. The vertical and horizontal profiles of the self‐extracted beam have been measured with a gridded Faraday cup. The gas efficiency of the source and the electron component in the accelerated beam have been measured for two different pumping conditions.

Characteristics of the Berkeley multicusp ion source

The performance of a cubical permanent magnet generated line-cusp ion source has been investigated for use with neutral beam injectors. This source has been operated with discharge currents greater than 500 A and ion current densities higher than 400 mA/cm2 at the extraction grid. The uniformity of the density profile across the extraction area is found to be dependent on the gas pressure. By using a fast Langmuir probe sweeping circuit, the electron temperature and the plasma density and potential have been analyzed for different discharge powers and gas pressures. The heat load on the plasma grid when it is electrically floating or connected to the negative cathode has been compared calorimetrically. The use of lanthanum hexaboride and impregnated oxide cathodes have been investigated for the purpose of long pulse operation. The phenomenon of mode flipping is found to occur quite frequently during a discharge with these magnetic-field-free cathodes. Species composition as a function of discharge power and chamber length is measured by a mass spectrometer.

Optimization of H−production from a small multicusp ion source

A small multicusp source has been developed to generate volume‐produced H− ion beams in pulsed operation. To obtain high H− current densities (J−>250 mA/cm2), this source requires relatively high gas pressure and high discharge power. Experiments have been conducted to improve the arc and gas efficiencies, the beam pulse shape, and the H− to electron ratio in the extracted beam by optimizing the filter magnetic field, the thickness and axial position of the extraction aperture in the plasma electrode, and by mixing xenon or other elements with hydrogen in the discharge. The biggest improvement is achieved by adding cesium to the source, resulting in a fivefold increase in the extractable H− current and a substantial drop in the e/H− ratio. In order to improve the lifetime of the cathode, both filament and coaxial type LaB6 cathodes have been developed and have been operated successfully in this H− source.

Effect of a magnetic filter on hydrogen ion species in a multicusp ion source

Hydrogen ion species and discharge characteristics have been compared for two different magnet geometries in a multicusp ion source. One magnet configuration indicated that the H2+ ion percentage in the extracted beam could be reduced by eliminating the high energy ionizing electrons near the ion extraction region. To accomplish this and maintain the desired features of both magnet geometries, a magnetic filter was installed near the source exit. With this combined arrangement, we found that the H+ ion percentage and discharge condition were much improved and a more uniform density profile across the extraction plane was achieved. The atomic species fraction can then be further enhanced by the addition of low‐energy (∼16 eV) primary electrons into the source plasma chamber.

Production of H− ions with addition of cesium or xenon to a hydrogen discharge in a small multicusp ion source

The effect on H− ion production by adding cesium or xenon to a hydrogen discharge, in a small magnetically filtered multicusp ion source, has been investigated. Addition of cesium vapor to the hydrogen discharge resulted in a factor of 16 increase in H− output relative to the uncesiated discharge for the same operating parameters. Likewise, the addition of xenon gas to a hydrogen discharge resulted in a factor of 2.7 increase in H− output over pure hydrogen operation at optimum H2 source pressure, while maintaining the same arc parameters. Operation of the source with the plasma electrode at optimum bias voltage was essential to obtain this result.

rf driven multicusp H− ion source

LBL-29315 Preprint Lawrence Berkeley Laboratory UNIVERSITY O F C A L I F O R N I A J Accelerator & Fusion Research Division Submitted to Review of Scientific Instruments RF Driven Multicusp H Ion Source K.N. Leung, G J . DeVries, W.F. DiVergilio, R.W. Hamm, C A . Hauck, W.B. Kunkel, D.S. McDonald, and M.D. William June 1990 Prepared for the U.S. Department of Energy under Contract Number DE-AC03-76SF00098.

Multicusp negative ion source

The characteristics of a steady state, surface conversion, multiline‐cusp negative ion source have been investigated for the production of high‐energy neutral beams. With the presence of cesium, this source has been operated at a neutral pressure of 1×10−3 Torr to generate a H− ion current greater than 400 mA. The percentage of impurities in the self‐extracted negative ion beam has been analyzed by a mass spectrometer. A technique for reducing the background electron density at the source exit is also presented.

Directly heated lanthanum hexaboride filaments

Some physical properties of lanthanum hexaboride filaments, when operated as cathodes in a gas discharge, are presented. These directly heated hairpin‐shaped filaments have been tested in different types of ion sources and are shown to be capable of long‐pulse or cw discharge operations. The design of a shaped lanthanum hexaboride filament for the purpose of further extension of lifetime is also described.

Development of a sealed-accelerator-tube neutron generator

Sealed-accelerator-tube neutron generators are being developed in Lawrence Berkeley National Laboratory (LBNL) for applications ranging from neutron radiography to boron neutron capture therapy and neutron activation analysis. The new generation of high-output neutron generators is based on the D-T fusion reaction, producing 14.1-MeV neutrons. The main components of the neutron tube--the ion source, the accelerator and the target--are all housed in a sealed metal container without external pumping. Thick-target neutron yield computations are performed in this paper to estimate the neutron yield of titanium and scandium targets. With an average deuteron beam current of 1 A and an energy of 120 keV, a time-averaged neutron production of approximately 10(14) n/s can be estimated for a tritiated target, for both pulsed and cw operations. In mixed deuteron/triton beam operation, a beam current of 2 A at 150 keV is required for the same neutron output. Recent experimental results on ion sources and accelerator columns are presented and discussed.