Surface Conversion Source Research Articles

Helicon plasma generator-assisted surface conversion ion source for the production of H− ion beams at the Los Alamos Neutron Science Center

The converter-type negative ion source currently employed at the Los Alamos Neutron Science Center (LANSCE) is based on cesium enhanced surface production of H(-) ion beams in a filament-driven discharge. In this kind of an ion source 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 deposits on the H(-) converter surface and degrades its performance. Therefore, we have started an ion source development project focused on replacing these thermionic cathodes (filaments) of the converter source by a helicon plasma generator capable of producing high-density hydrogen plasmas with low electron energy. In our studies which have so far shown that the plasma density of the surface conversion source can be increased significantly by exciting a helicon wave in the plasma, and we expect to improve the performance of the surface converter H(-) ion source in terms of beam brightness and time between services. The design of this new source and preliminary results are presented, along with a discussion of physical processes relevant for H(-) ion beam production with this novel design. Ultimately, we perceive this approach as an interim step towards our long-term goal, combining a helicon plasma generator with an SNS-type main discharge chamber, which will allow us to individually optimize the plasma properties of the plasma cathode (helicon) and H(-) production (main discharge) in order to further improve the brightness of extracted H(-) ion beams.

Measurements on H− sources for spallation neutron source application

Lawrence Berkeley National Laboratory is engaged in the development of H− ion sources for the upgrade of the Los Alamos Neutron Science Center (LANSCE) facility and the spallation neutron source (SNS) to be built in the U.S. For the upgrade of the LANSCE facility, the H− ion generator has to deliver an output current of 40 mA. The repetition rate must be 120 Hz at a pulse length of 1 ms (12% duty factor). Furthermore, the normalized emittance must be less than 0.1π mm mrad. During the last years, the Ion Beam Technology Group of the LBNL improved the so-called surface conversion source for the generation of higher H− currents. In the first part of this article, we discuss the operation conditions of the source at the required 40 mA output current. The ion source for the 1 MW spallation neutron source is required to provide 35 mA of H− beam current at 6% duty factor (1 ms pulses at 60 Hz) with a normalized rms emittance of less than 0.2π mm mrad. The H− beam will be accelerated to 65 keV and matched into a 2.5 MeV RFQ. The ion source is expected to ultimately produce 70 mA of H− at 6% duty factor when the SNS is upgraded to 2 MW of power. For this application, a radio-frequency driven, magnetically filtered multicusp source is being developed at LBNL. Experimental results (including emittance measurements) on the performance of the prototype ion source operated at the demanded beam parameters will be presented in this article.

Development of the next generation H− ion source for the LANSCE facility

The upgrade of the Los Alamos Neutron Science Center (LANSCE) will require an ion source producing high intensity H− beams. The new LANSCE source in particular will need to generate 40 mA of H− beam current at a duty factor of 12% (1 ms pulse at 120 Hz). To achieve this, the present Los Alamos results were first reproduced employing a prototype surface conversion source similar to the existing LANSCE source. Using these results as a benchmark for further testing, it was discovered that by moving the filament cathode into the chamber’s cusp-field, the H− ion yield was enhanced. Considering this result, two extension chambers were added with movable magnetic filters at each end of the source to further improve the H− beam current. In addition, construction has been started on a new prototype axial source which will enable more plasma ions to funnel across the filter field into the central region where the converter is located. Results of the magnetic filter operation as well as the new axial source design will be presented.

Angular and energy distributions of surface produced H− and D− ions in a barium surface conversion source

The production of negative hydrogen and deuterium ions is studied in the foundation for fundamenteel Onderzoek der Materie surface conversion experiment. In this experiment a Barium surface is used to convert positive ions into negative ions. We have developed a detector able to analyze the characteristics of the surface produced negative ion beam. Measurements with this detector show that in barium surface conversion sources negative ions can be formed by recoiling and direct reflection of positive ions from the barium surface. The broad angular distribution of the surface produced negative ions is determined to extend over 16°.

Energetic “self-extracted” H - ions in a Ba surface conversion source

The production of negative hydrogen and deuterium ions is studied in the FOM surface conversion experiment (FOMSCE). In this experiment a barium surface is used to convert positive ions to negative ions. We have developed a detector able to analyze the characteristics of the surface-produced negative ion beam. Measurements with this detector show that in barium surface conversion sources negative ions can be formed by recoiling and direct reflection of positive ions from the barium surface.

Sputtering of a hydrogenated barium surface in a negative ion surface conversion source

Both the production of negative hydrogen ions on a barium surface and surface sputtering under low energy (∼ 200 eV) intense particle bombardment is studied in the FOM Surface Conversion Experiment. We have observed a decrease in barium sputter-yield when a barium surface is exposed to an intense positive hydrogen or deuterium ion flux, extracted from a plasma. This effect is attributed to the loading of hydrogen into barium, causing enhanced preferential sputtering of hydrogen. The sputtering process is observed by means of optical emission spectroscopy. Whenever the surface is saturated with hydrogen, the barium line intensity is constant in time, and is proportional to a calculated solid-body sputter-coefficient. The scaling with calculated sputter-coefficient is a feature which is also found during argon ion bombardment. The effects of dynamic hydrogen loading of a barium converter surface are observed with three different diagnostic techniques, and shown to be consistent. In a high pressure experiment it is indicated that it might be possible to determine the sputtered barium ion flux with a Langmuir probe. The steady state negative ion output of our surface conversion negative source amounts to 17 mA/cm 2 of H − and 12 mA/cm 2 of D − at an operating pressure of 4 × 10 −3 mbar.

Modeling of H− surface conversion sources; binary (H-Ba) and ternary (H-Cs/W) converter arrangements

The production process for the formation of H− ions in a surface conversion source is sputtering of hydrogen atoms from the converter surface layers by incident positive ions, followed by electron attachment via resonant charge exchange with the converter surface. The sputtering process is in direct relation to the converter surface composition. New experimental data led us to identify two different classes of converters: metallic converters, like solid barium(binary) and adlayer converters, like cesium on tungsten (ternary). For a binary converter the hydrogen in the surface layers is directly sputtered by the incoming ions. Consequently, the negative ion yield scales with the hydrogen concentration in the surface layers. In the cesium/tungsten system (ternary) the hydrogen at the surface is believed to be sandwiched between the cesium adlayer and the tungsten surface. Hence, the negative ion yield scales with the sputter coefficient of hydrogen on adsorbed cesium. This is experimentally confirmed.

Plasma-generator-induced effects on the dynamics of a negative-ion surface conversion source

The production of H− ions at a barium, magnesium, strontium, molybdenum, and lanthanum-hexaboride converter immersed in a hydrogen plasma have been investigated for pulsed and dc discharges. The scaling of the negative-ion output with incident positive-ion current on the converter depends greatly on the actual geometry and type of plasma generator. A less than linear scaling is observed in a multicusp system using filaments or rf to generate the discharge. A more than linear scaling is obtained in a configuration where the filament and the converter are separated by a permanent-magnet filter. For a rf plasma generator, there is a pronounced optimum in the relative position of antenna and converter.

Development of high current and high brightness negative hydrogen ion sources

Negative hydrogen ions have found important applications in particle accelerators and in fusion research. These ions can be generated from two different types of ion sources — the surface conversion source and the volume production source. Recent experiments demonstrate that H − current exceeding 1 A can be obtained from both types of ion sources. Because of the lower H − ion temperature and the fact that they can be operated without cesium, volume H − sources are highly desired. However, further technology must be developed on the control of electrons and the reduction of gas flow before this type of sources becomes practical units of a multiampere neutral beam injection system.

Importance of heavy ion bombardment for H− formation in surface conversion sources

The interaction between the plasma and the converter in a surface conversion negative ion source is studied. The converter consists of a porous tungsten button, through which liquid cesium diffuses towards the surface. This scheme results in a low cesium density in the discharge. Xe+ and Ar+ ions are used to simulate the sputtering effect of Cs+ ion bombardment of the converter. The extracted H− beam energy profile and the conversion efficiency appeared to depend strongly on the heavy ion flux to the converter surface. The total H− current increases with an order of magnitude with increasing heavy ion flux. A maximum current density of 8 mA/cm2 of H− ions has been extracted.

Cesium coverage on molybdenum due to cesium ion bombardment

Cesium coverage due to Cs+ bombardment of a polycrystalline molybdenum surface was investigated in the incident energy range below 500 eV. When the surface is exposed to a Cs+ beam the work function decreases until steady state is reached with a total dose of less than ≊1×1016 ions/cm2. The steady state work function reaches a minimum at an incident energy of ≊100 eV and slowly increases with bombarding energy. Cesium coverage of a target is proportional to the ratio (1−β)/γ, where β is the reflection coefficient and γ is the sputter yield. This ratio is large for molybdenum because of its low mass and high binding energy. Implications of these results for H− surface conversion sources are discussed.