K. W. Ehlers

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.

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.

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.

Ion Source for the Production of Multiply Charged Heavy Ions

A pulsed, cold‐cathode ion source has been developed to produce multiply charged ions for use in a linear accelerator. Milliampere currents of such ions as He42+, C122+, N143+, O163+, Ne203+, and smaller currents of S325+ and A406+ have been produced in focused beams.Mass spectra of the various ionization states produced by the arc are presented. Constructional details of the source and the associated electronic equipment are given, and operating characteristics of the source and ion extraction are discussed.

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.

Optimization of H− production in a magnetically filtered multicusp source

Different techniques to increase the H− yield in a multicusp source have been investigated. It is shown that the extracted H− current can be enhanced by optimizing the discharge voltage, the neutral gas pressure, or by employing a magnetoelectrostatic containment scheme. A factor of 6 increase in H− output has been achieved by placing the extractor very close to the magnetic filter. It is also found that the extracted electron current is much reduced at this optimum geometry.

Extraction of H− beams from a magnetically filtered multicusp source

H− ions produced by volume processes have been extracted from a multicusp source operated with a magnetic filter and accelerated to form a high‐quality beam. It was found that the H− beam current increased almost linearly with the discharge current. An H− ion current density of 38 mA/cm2 was obtained with a discharge current of approximately 350 A. The accelerated beam was mass analyzed and the emittance of the H− component was measured. The electron component and the percentage of ion impurity in the extracted beam were also determined.

Small multicusp H− source

High‐quality H− beams have been generated from a multicusp source equipped with a permanent magnet filter. It is shown that a large improvement in H− yield can be achieved by employing a small multicusp source, fabricated with the proper wall material and extraction chamber length. From this small source, H− current densities higher than 250 mA/cm2 have been extracted from a 1‐mm‐diam aperture for a discharge voltage of 150 V and a discharge current of 450 A. When the source is operated with deuterium, the extractable negative ion current density is reduced by approximately 30%.