Multicusp Negative Ion Source Research Articles

NuCARIBU : An upgrade for the CARIBU facility at the Argonne Tandem Linac Accelerator System

The Californium Rare Isotope Breeder Upgrade (CARIBU) facility is changing the mechanism for creating neutron-rich fission products. Spontaneous fission from a 252Cf source has provided beams to support the low energy and accelerated-beams ATLAS programs. 252Cf has a 2.65-year half-life, requiring the source to be replaced every three years to maintain high beam intensities. Fabricating an appropriately thin 252Cf source to efficiently release the fission products has been challenging. The solution to these problems is nuCARIBU, a new system that provides neutron-induced fission on actinide foils. The Best Cyclotron B6P System (6-MeV proton beam at 0.5mA) is chosen, utilizing a multi-cusp negative ion source extracting into a cyclotron, which uses carbon foils to strip the H- ions to protons. These protons are delivered to a 7Li target to produce neutrons. The fast neutrons are moderated to thermal energies to induce fission in an actinide foil, providing neutron-rich fission products. These subsystems are described in detail along with comparisons between the current CARIBU facility and the presented nuCARIBU facility.This work was supported by the U.S. Department of Energy, Office of Nuclear Physics, under Contract No. DE-AC02-06CH11357

High current DC negative ion source for cyclotron.

A filament driven multi-cusp negative ion source has been developed for proton cyclotrons in medical applications. In Cs-free operation, continuous H(-) beam of 10 mA and D(-) beam of 3.3 mA were obtained stably at an arc-discharge power of 3 kW and 2.4 kW, respectively. In Cs-seeded operation, H(-) beam current reached 22 mA at a lower arc power of 2.6 kW with less co-extracted electron current. The optimum gas flow rate, which gives the highest H(-) current, was 15 sccm in the Cs-free operation, while it decreased to 4 sccm in the Cs-seeded operation. The relationship between H(-) production and the design/operating parameters has been also investigated by a numerical study with KEIO-MARC code, which gives a reasonable explanation to the experimental results of the H(-) current dependence on the arc power.

Effect of high energy electrons on H⁻ production and destruction in a high current DC negative ion source for cyclotron.

Recently, a filament driven multi-cusp negative ion source has been developed for proton cyclotrons in medical applications. In this study, numerical modeling of the filament arc-discharge source plasma has been done with kinetic modeling of electrons in the ion source plasmas by the multi-cusp arc-discharge code and zero dimensional rate equations for hydrogen molecules and negative ions. In this paper, main focus is placed on the effects of the arc-discharge power on the electron energy distribution function and the resultant H(-) production. The modelling results reasonably explains the dependence of the H(-) extraction current on the arc-discharge power in the experiments.

A control system of a negative hydrogen ion sources by using an EPICS-based proportional-integral-derivative

We have developed an EPICS-based control system for a multicusp negative ion source based on a VME IOC software, a sequencer controlling a closed proportional integral (PI) loop, and an interlock. The PI loop was tuned by a step input voltage, and tuning gains of the proportion and integral have been determined. The PI logics, developed by using standard proportional-integral-derivative records, can control the arc current within +/-1% of a given set value.

Plasma Electrode Bias Effect on the $\hbox{H}^{-}$ Negative-Ion Density in an Electron Cyclotron Resonance Volume Source

The role of the plasma electrode (PE) bias in the extraction process in a large-volume hybrid multicusp negative-ion source, which is driven by 2.45-GHz microwaves, is studied. Spatially resolved negative-ion and electron density measurements close to the extractor aperture were performed under various pressures (1-4 mtorr) by means of the electrostatic probe and photodetachment technique. As the low positive voltage applied to the PE is slightly increased (from 4 to 7 V), the electron temperature passes through a minimum (0.2-0.6 eV), while simultaneously, both the negative-ion density and the H extracted current reach a maximum (~1-2 times 109 cm-3 and ~0.5 mA/cm2, respectively). Optimum pressure values for the extracted negative-ion current and the negative-ion density are found between ~1.5 and 3 mtorr. It is deduced that the negative-ion density measured in the center of the source cannot be directly correlated with the ion extracted current. The electron density and the associated extracted electron current linearly decrease as a function of the PE bias. The physical mechanisms explaining the experimental results are discussed.

Estimation of the cusp loss width in negative-ion sources

To estimate loss width in the cusp magnetic field in multicusp negative-ion sources, diffusion of plasma across magnetic field is investigated analytically. The transport process of plasma depends on various plasma conditions. Diffusion coefficients are classified by degrees of plasma ionization. In weakly ionized plasma, where a case of electron-neutral particle collision is dominant as compared with electron-ion collision, there is a possibility that ambipolar diffusion or electron short circuit occurs in plasma. This strongly depends on the conditions of the plasma, the system length, and the wall material. On the other hand, in fully ionized plasma, only ambipolar diffusion occurs automatically due to momentum conservation of electron and ion. In not fully but strongly ionized plasma, plasma diffusion depends on electron-neutral particle collision, ion-neutral particle collision, and electron-ion collision. Being based on the classification of the diffusion coefficients, we derive expressions of loss widths for various plasma conditions.

Negative ion production in multicusp sources

The study of negative ion production in the multicusp negative ion sources (MS) was done by the directed deposition of well-defined amount of cesium into MS, by the cesium recovery from the polluted layers in the MS and by the plasma grid masking. The data obtained evidences the surface-plasma mechanism of negative ion production in the MS. It is shown that a dynamic cesium-tungsten coverage on a plasma grid surface provides the enhanced H− production in the MS. The Cs+W coverage is produced on the hot plasma grid surface due to coadsorption of cesium and of tungsten, evaporated from filaments. The permanent flux of cesium to plasma grid coverage is produced by the thick Cs+W reservoir with a high (75%) percentage of cesium on the cold anode surface. A relatively high cesium seed with a rate of about 20 mg/1 h/30 shots operation is necessary to support the increased H− production in the regular MS. Electrode processing by an additional discharge recovers the cesium from the aged coverage and enhances the H− production without an additional cesium seed. Recovery processing permits the use of deposited cesium more efficiently and to minimize cesium addition during the MS long-term operation. An essential decrease of cesium escape to accelerator and an improvement of injector high voltage operation is produced by introducing the shutter mask at plasma grid surface. The shutter mask experiment displayed, that >60% of H− beam current was born on the hot shutter mask surface. The source operation with a high negative shutter mask biasing and an increased hydrogen pressure resulted in a 35% higher H− yield, than of regular MS.

Cesiated hollow cathodes in the multicusp ion source

A cesiated hydrogen hollow cathode (CHC) was tested for plasma injection in the multicusp negative ion source (MS). The CHC arc with hydrogen feed and cesium seeding through the CHC volume was explored. One cathode unit (40 mm length, 19 mm in diameter, emission opening area 1–3 mm2) with no special cooling provided the MS discharge operation with direct current up to 30 A, and up to 60 A in the long-pulse mode. High efficiency of negative ion production in the MS discharge, driven by a CHC plasma injection was recorded.

Investigation of a large volume negative hydrogen ion source

The electron and negative ion densities and temperatures are reported for a large volume hybrid multicusp negative ion source. Based on the scaling laws an analysis is made of the plasma formation and loss processes. It is shown that the positive ions are predominantly lost to the walls, although the observed scaling law is n+∝I0.57d. However, the total plasma loss scales linearly with the discharge current, in agreement with the theoretical model. The negative ion formation and loss is also discussed. It is shown that at low pressure (1 mTorr) the negative ion wall loss becomes a significant part of the total loss. The dependence of n−/ne versus the electron temperature is reported. When the negative ion wall loss is negligible, all the data on n−/ne versus the electron temperatures fit a single curve.

A Cusped Field H- Ion Source for LAMPF

The addition of the Proton Storage Ring (PSR) to LAMPF requires the development of a more intense H/sup -/ ion source. The goals of this development program are to produce a beam of sufficient quality to match the acceptance of the accelerator and of sufficient intensity and duty factor to satisfy PSR operation as well as existing LAMPF requirements. The ion source concept best suited to these demands is the multi-cusp negative ion source being developed at LBL. The H/sup -/ ion source development was carried out on the high-voltage test stand in the LAMPF injector complex. The test stand, upgraded for the project, provides both unanalyzed and mass-analyzed beams to appropriate emittance scanning stations. Beam currents are measured using both beam current toroids and biased Faraday cups in both sections of the transport line. The emittance data are processed by the SEL 840 control computer at LAMPF. Ion source testing has been carried out using the extraction optics originally developed for the LAMPF duoplasmatron. Optics questions are being addressed by the plasma simulation code SNOW developed at Sandia. The accelerating column design will seek to minimize the divergence of the extracted beam and the predictions from SNOW will bemore » tested on the stand with actual H/sup -/ beams.« less

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.

Electron suppression in a multicusp negative ion source

Three different techniques for reducing the background electron density at the exit region of a multicusp negative ion source are described.