Roderich Keller

Spectroscopic investigation of H− and D− ion source plasmas

Several H I (Balmer), Cs I, Cs II, and Mo I lines emitted by the small‐angle source and 4X source plasmas are studied. After correcting for Stark broadening, the Hα line width gives the H‐atom temeprature kTHO. After correcting for Doppler broadening, the Hβ and Hδ line widths give the electron density ne. For pulsed operation of both sources, kTHo is 1.5 to 2 eV and ne is 1 to 2×1014/cm3,with kTHo and ne scaling approximately with the square root of the discharge current. For the 4X source operated on D2, kTDo and ne are near the values of kTHo and ne obtained for H2 operation. Assuming that the H−/D− ion temperature equals the H/D‐atom temperature, we deduce a lower limit to the H−/D− beam emittance.

Use of a Minimum-Ellipse Criterion in the Study of Ion-Beam Extraction Systems

Ion-beam extraction systems may be optimized by ray-tracing codes. As a general criterion for comparing the geometry-dependent phase-space distributions, we first calculate the minimum-area ellipse that encloses all particles of any given two-dimensional phase-space distribution. Then, the relation between ellipse area and contained beam fraction is established by systematically finding and eliminating those particles that contribute most heavily to the emittance. Prescriptions for finding the minimum ellipse and beam fractions will be presented. The minimum and rms ellipses are compared for two code-calculated distributions that represent ion-beam extraction geometries.

High-intensity ion sources for accelerators with emphasis on H- beam formation and transport (invited)

This paper lays out the fundamental working principles of a variety of high-current ion sources for accelerators in a tutorial manner, and gives examples of specific source types such as dc discharge-driven and rf-driven multicusp sources, Penning-type, and electron cyclotron resonance-based sources while discussing those principles, pointing out general performance limits as well as the performance parameters of specific sources. Laser-based, two-chamber, and surface-ionization sources are briefly mentioned. Main aspects of this review are particle feed, ionization mechanism, beam formation, and beam transport. Issues seen with beam formation and low-energy transport of negative hydrogen-ion beams are treated in detail.

H− Ion Source Development for the LANSCE Accelerator Systems

Employment of H‐ ion sources for the LANSCE accelerator systems goes back about 20 years, to the construction of the Proton Storage Ring (PSR). The standard ion source consists of a filament driven multi‐cusp discharge vessel with a biased converter electrode for negative‐ion production and an 80‐kV extraction system feeding into a 670‐kV electrostatic pre‐accelerator. The source typically delivers 18 mA pulsed beam current into the pre‐accelerator column and reaches up to 35 days between services at 60 Hz pulse repetition rate. Recent development efforts with this source have been dedicated to improved filament material, improved cesium oven geometry and operating the source at elevated temperatures. A second line of development focuses on filament‐less devices driven by a helicon discharge. Performance data obtained with the standard source as well as key results for the helicon experiments are given in this paper; the helicon work is discussed in a separate paper in much greater detail.

Tungsten filament material and cesium dynamic equilibrium effects on a surface converter ion source.

We present results on two different aspects that affect surface converter H(-) ion source performance: tungsten filament material and converter/wall temperature control. On the tungsten material aspect, evidence that filament grain size affects the source performance as well as filament failure modes is shown. Materials with impurity contents that hinder grain growth during conditioning or operation are to be avoided in order to increase the filament lifetime. Regarding the temperature control of the converter and plasma chamber walls, we present results of increased current output of up to 2.5 mA (15%). This is explained by generating increased cesium vapor pressure leading to enhanced sputtering of H(-) ions.

High-brightness, high-current ion sources

Several existing high‐current dc ion sources are presented that yield positive ions and are based on gas discharges. Their common features are then outlined: extraction systems, required plasma qualities, and the production of ions from solid materials. Finally, from well‐established laws, scaling rules for high‐brightness ion beams are derived taking one specific case to determine the numerical constants. These rules predict that the current values of transportable beams scale proportionally to the 3/2 power of the extraction voltage, whereas the brightness strongly decreases with rising voltage.

Spectroscopic Measurements on an H- Ion Source Discharge

Spectral emission lines from an H- Penning surface-plasma source (SPS), the 4X source, are examined in the visible and near ultraviolet. Electron distribution temperatures are deduced from integral line-strength measurements. These temperatures are surprisingly low, about 0.5 eV. Electron density values of about 1.5 × 1014 cm-3 and H-atom energies between 2 and 2.6 eV are determined from the measured Balmer-line profiles. Assuming the H- energy is identical to the H-atom energy, an emittance limit of 0.006¿·cm·mrad is deduced for this source with a 5.4-mm aperture.

An alternate approach to the production of radioisotopes for nuclear medicine applications

There is a growing need for the production of radioisotopes for both diagnostic and therapeutic medical applications. Radioisotopes that are produced using the (n,γ) or (γ,n) reactions, however, typically result in samples with low specific activity (radioactivity∕gram) due to the high abundance of target material of the same element. One method to effectively remove the isotopic impurity is electro-magnetic mass separation. An Ion Source Test Facility has been constructed at TRIUMF to develop high-intensity, high-efficiency, reliable ion sources for purification of radioactive isotopes, particularly those used in nuclear medicine. In progress studies are presented.

Helicon Plasma Generator‐Assisted Negative Ion Source Project at Los Alamos Neutron Science Center

Helicon plasma generators are widely used for plasma processing applications due to their long life‐time and capability of creating high‐density plasmas efficiently. The aim of the helicon plasma generator‐assisted negative ion source project at Los Alamos Neutron Science Center (LANSCE) is to use these features for producing intense beams of H− ions. Our development work builds upon pioneering experiments previously conducted at Lawrence Berkeley National Laboratory (LBNL) with a 2.45 GHz electron cyclotron resonance plasma generator. In the new approach a helicon plasma generator is used as a plasma cathode injecting electrons into a multi‐cusp H− ion source. The secondary source can be operated without filaments or any other consumable parts and, consequently, the life‐time of the ion source can be extended significantly. The development of the ion source is aimed to meet the beam production goals of the LANSCE 800 MeV linear accelerator refurbishment project i.e. 20 mA of H− beam with normalized area emittance (95 % of the beam) less than 1.1 π⋅mm⋅mrad and a duty factor of 12 %. The operation principle of the source, the test stand design and the status of the development work will be presented in this article.Helicon plasma generators are widely used for plasma processing applications due to their long life‐time and capability of creating high‐density plasmas efficiently. The aim of the helicon plasma generator‐assisted negative ion source project at Los Alamos Neutron Science Center (LANSCE) is to use these features for producing intense beams of H− ions. Our development work builds upon pioneering experiments previously conducted at Lawrence Berkeley National Laboratory (LBNL) with a 2.45 GHz electron cyclotron resonance plasma generator. In the new approach a helicon plasma generator is used as a plasma cathode injecting electrons into a multi‐cusp H− ion source. The secondary source can be operated without filaments or any other consumable parts and, consequently, the life‐time of the ion source can be extended significantly. The development of the ion source is aimed to meet the beam production goals of the LANSCE 800 MeV linear accelerator refurbishment project i.e. 20 mA of H− beam with normalized area ...

First Results with a Surface Conversion H− Ion Source Based on Helicon Wave Mode‐Driven Plasma Discharge

The currently employed converter‐type negative ion source at Los Alamos Neutron Science Center (LANSCE) is based on cesium enhanced surface production of H− ion beams in a filament‐driven discharge. The extracted H− beam current is limited by the achievable plasma density, which depends primarily on the electron emission current from the filaments. The emission current can be increased by increasing the filament temperature but, unfortunately, this leads not only to shorter filament lifetime but also to an increase in metal evaporation from the filament, which degrades the performance of the H− conversion surface. In order to overcome these limitations we have designed and tested a prototype of a surface conversion H− ion source, based on excitation of helicon plasma wave mode with an external antenna. The source has been operated with and without cesium injection. An H− beam current of over 12 mA has been transported through the low energy beam transport of the LANSCE ion source test stand. The results o...