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Negative hydrogen ion yields at plasma grid surface in a negative hydrogen ion source
Negative hydrogen (H{sup −}) ion yield from the plasma grid due to incident hydrogen ions and neutrals has been evaluated with the surface collision cascade model, ACAT (Atomic Collision in Amorphous Target) coupled to a negative surface ionization models. Dependence of negative ion fractions upon the velocity component normal to the surface largely affect the calculation results of the final energy and angular distributions of the H{sup −} ions. The influence is particularly large for H{sup −} ions desorbed from the surface due to less than several eV hydrogen particle implact. The present calculation predicts that H{sup −} ion yield can be maximized by setting the incident angle of hydrogen ions and neutrals to be 65 degree. The Cs thickness on the plasma grid should also affect the yields and mean energies of surface produced H{sup −} ions by back scattering and ion induced desorption processes.
Electrostatic ion (hydrogen) cyclotron and ion acoustic wave instabilities in regions of upward field-aligned current and upward ion beams
An investigation is made into the stability of electrostatic hydrogen ion cyclotron and ion acoustic waves in a model plasma where an ion beam, population 2, and oppositely directed drifting electrons pass through a stationary ion background, population 1. The excited wave properties are then compared with the characteristics of the unstable modes observed on the S3-3 satellite. Three temperature regimes are studied: (1) Te greater than Ti2 much greater than Ti1, (2) Ti2 greater than Te not less than Ti1, and (3) Te approximately equal to Ti1 greater than Ti2. It is found that the ion beam acts as a free energy source only in regime 1. This regime is also highly unstable to the electrons as a free energy source. Unstable modes in regimes 2 and 3 seem to best satisfy the electrostatic hydrogen cyclotron wave (EHC) properties at 1 earth radius. For these cases the electrons are the free energy source, the beam supplies damping.
Effect of nickel grid parameters on production of negative hydrogen ions
Negative hydrogen ions are produced by plasma-assisted catalytic ionization using a nickel grid. When positive ions passing through the grid are decelerated by an electric field, the extraction current density of passing positive ions is sharply reduced by neutralization and negative ionization of the ions. This phenomenon is found to depend on the specific surface area of the grid and the current density.
Development of activity–descriptor relationships for supported metal ion hydrogenation catalysts on silica
With the increasing attention on single-site heterogeneous catalysts on account of their high specific activity, more effective catalyst design strategies are necessary. We demonstrate a first-principles-calculation-guided active-site design strategy for alkene hydrogenation using silica-supported single-atom single-site heterogeneous catalysts (SSHCs, i.e., M/SiO2). An activity-descriptor relationship for propylene hydrogenation was established using representative catalysts (i.e., Ga3+, Zn2+, Mn2+ and Ti4+/SiO2), correlating the computational activity descriptor (i.e., the reaction free energy of the metal hydride formation) with the experimental reaction rates. Microkinetic modeling confirmed the predicted reaction mechanisms and further explained the rationale of the activity-descriptor relationship. This relationship was then used to predict the trend of activities of a variety of isolated ions on silica, and an In3+/SiO2 catalyst was identified as the most promising candidate based on its predicted activity and synthetic feasibility. The monodispersed In3+/SiO2 catalyst was then synthesized, for the first time, and showed higher propylene hydrogenation activity than the other four single-atom SSHCs, as predicted. Detailed spectroscopic characterization (XAS, XPS, DRIFTS and DRUV-Vis) provided insights into coordination environments and oxidation states of the catalysts, as well as evidence for the hydride intermediate. The successful discovery of the In3+/SiO2 catalyst for propylene hydrogenation using the first-principles-calculations-guided design strategy opens up new opportunities for efficient development of single-atom SSHCs.
Optimization of plasma parameters with magnetic filter field and pressure to maximize H{sup −} ion density in a negative hydrogen ion source
Transverse magnetic filter field as well as operating pressure is considered to be an important control knob to enhance negative hydrogen ion production via plasma parameter optimization in volume-produced negative hydrogen ion sources. Stronger filter field to reduce electron temperature sufficiently in the extraction region is favorable, but generally known to be limited by electron density drop near the extraction region. In this study, unexpected electron density increase instead of density drop is observed in front of the extraction region when the applied transverse filter field increases monotonically toward the extraction aperture. Measurements of plasma parameters with a movable Langmuir probe indicate that the increased electron density may be caused by low energy electron accumulation in the filter region decreasing perpendicular diffusion coefficients across the increasing filter field. Negative hydrogen ion populations are estimated from the measured profiles of electron temperatures and densities and confirmed to be consistent with laser photo-detachment measurements of the H{sup −} populations for various filter field strengths and pressures. Enhanced H{sup −} population near the extraction region due to the increased low energy electrons in the filter region may be utilized to increase negative hydrogen beam currents by moving the extraction position accordingly. This new finding can be used to design efficient H{sup −} sources with an optimal filtering system by maximizing high energy electron filtering while keeping low energy electrons available in the extraction region.
Negative hydrogen ion sources for particle accelerators: Sustainability issues and recent improvements in long-term operations
High brightness, negative hydrogen ion sources are used extensively in scientific facilities operating worldwide. Negative hydrogen beams have become the preferred means of filling circular accelerators and storage rings as well as enabling efficient extraction from cyclotrons. Several well-known facilities now have considerable experience with operating a variety of sources such as RF-, filament-, magnetron- and Penning-type H- ion sources. These facilities include the US Spallation Neutron Source (SNS), Japan Proton Accelerator Research Complex (J-PARC), Rutherford Appleton Laboratory (RAL-ISIS), Los Alamos Neutron Science Center (LANSCE), Fermi National Accelerator Laboratory (FNAL), Brookhaven National Laboratory (BNL), numerous installations of D-Pace (licenced by TRIUMF) ion sources used mainly on cyclotrons and, most recently, the CERN-LINAC-1 injector. This report first summarizes the current performance of these ion sources in routine, daily operations with attention toward source service-periods and availability metrics. Sustainability issues encountered at each facility are also reported and categorized to identify areas of common concern and key issues. Recent ion source improvements to address these issues are also discussed as well as plans for meeting future facility upgrade requirements.
Study on space charge compensation in negative hydrogen ion beam
Negative hydrogen ion beam can be compensated by the trapping of ions into the beam potential. When the beam propagates through a neutral gas, these ions arise due to gas ionization by the beam ions. However, the high neutral gas pressure may cause serious negative hydrogen ion beam loss, while low neutral gas pressure may lead to ion-ion instability and decompensation. To better understand the space charge compensation processes within a negative hydrogen beam, experimental study and numerical simulation were carried out at Peking University (PKU). The simulation code for negative hydrogen ion beam is improved from a 2D particle-in-cell-Monte Carlo collision code which has been successfully applied to H{sup +} beam compensated with Ar gas. Impacts among ions, electrons, and neutral gases in negative hydrogen beam compensation processes are carefully treated. The results of the beam simulations were compared with current and emittance measurements of an H{sup −} beam from a 2.45 GHz microwave driven H{sup −} ion source in PKU. Compensation gas was injected directly into the beam transport region to modify the space charge compensation degree. The experimental results were in good agreement with the simulation results.
Effects of discharge chamber length on the negative ion generation in volume-produced negative hydrogen ion source
In a volume-produced negative hydrogen ion source, control of electron temperature is essential due to its close correlation with the generation of highly vibrationally excited hydrogen molecules in the heating region as well as the generation of negative hydrogen ions by dissociative attachment in the extraction region. In this study, geometric effects of the cylindrical discharge chamber on negative ion generation via electron temperature changes are investigated in two discharge chambers with different lengths of 7.5 cm and 11 cm. Measurements with a radio-frequency-compensated Langmuir probe show that the electron temperature in the heating region is significantly increased by reducing the length of the discharge chamber due to the reduced effective plasma size. A particle balance model which is modified to consider the effects of discharge chamber configuration on the plasma parameters explains the variation of the electron temperature with the chamber geometry and gas pressure quite well. Accordingly, H{sup −} ion density measurement with laser photo-detachment in the short chamber shows a few times increase compared to the longer one at the same heating power depending on gas pressure. However, the increase drops significantly as operating gas pressure decreases, indicating increased electron temperatures in the extraction region degrade dissociative attachment significantly especially in the low pressure regime. It is concluded that the increase of electron temperature by adjusting the discharge chamber geometry is efficient to increase H{sup −} ion production as long as low electron temperatures are maintained in the extraction region in volume-produced negative hydrogen ion sources.
Hydrogen atom temperature measured with wavelength-modulated laser absorption spectroscopy in large scale filament arc negative hydrogen ion source
The velocity distribution function of hydrogen atoms is one of the useful parameters to understand particle dynamics from negative hydrogen production to extraction in a negative hydrogen ion source. Hydrogen atom temperature is one of the indicators of the velocity distribution function. To find a feasibility of hydrogen atom temperature measurement in large scale filament arc negative hydrogen ion source for fusion, a model calculation of wavelength-modulated laser absorption spectroscopy of the hydrogen Balmer alpha line was performed. By utilizing a wide range tunable diode laser, we successfully obtained the hydrogen atom temperature of ∼3000 K in the vicinity of the plasma grid electrode. The hydrogen atom temperature increases as well as the arc power, and becomes constant after decreasing with the filling of hydrogen gas pressure.
Production of negative hydrogen ions on metal grids
Negative hydrogen ions are produced on a nickel grid with positive-ion irradiation. In order to investigate the production mechanism, a copper grid without the chemisorption of hydrogen atoms and positive helium ions without negative ionization are used for comparison. Positive hydrogen ions reflected on the metal surface obtain two electrons from the surface and become negatively ionized. It is found that the production yield of negative ions by desorption ionization of chemisorbed hydrogen atoms seems to be small, and the production is a minor mechanism.
A CW radiofrequency ion source for production of negative hydrogen ion beams for cyclotrons
A CW 13.56 MHz radiofrequency-driven ion source RADIS for production of H{sup −} and D{sup −} beams is under development for replacing the filament-driven ion source of the MCC30/15 cyclotron. The RF ion source has a 16-pole multicusp plasma chamber, an electromagnet-based magnetic filter and an external planar spiral RF antenna behind an AlN window. The extraction is a 5-electrode system with an adjustable puller electrode voltage for optimizing the beam formation, a water-cooled electron dump electrode and an accelerating einzel lens. At 2650 W of RF power, the source produces 1 mA of H{sup −} (2.6 mA/cm{sup 2}), which is the intensity needed at injection for production of 200 µA H{sup +} with the filament-driven ion source. A simple pepperpot device has been developed for characterizing the beam emittance. Plans for improving the power efficiency with the use of a new permanent magnet front plate is discussed.
Comparison of coincident OGO 3 and OGO 4 hydrogen ion composition measurements
Simultaneous hydrogen ion composition measurements by upper ionospheric polar orbiting OGO 4 and eccentric orbiting magnetospheric OGO 3 at midlatitude
Numerical Modeling of Ion-Optical Systems for Extraction of a Beam of Negative Hydrogen Ions and Its Acceleration and Matching with a Radio-Frequency Quadrupole Accelerator
A source of negative hydrogen ions is constructed at the Institute for High Energy Physics in order to raise the intensity of the U-70 accelerator complex using multiturn charge-exchange injection. The 3D ion-optical IBSimu simulation package was used to model and optimize the system for extraction of an ion beam from plasma, its acceleration to 100 keV, and matching with a radio-frequency quadrupole (RFQ) accelerator. The system of extraction of an ion beam with a current of 50 mA and electron-flow suppression was modeled. The magnetic transport channels with two solenoids and six quadrupole lenses were compared. The end goals of analysis of the matching channel were to obtain the needed Twiss parameters and to minimize the emittance growth at the RFQ input.
Some deductions from a measurement of the hydrogen ion distribution in the high atmosphere
Hydrogen ion distribution in upper atmosphere
Electron-irradiation-induced Cr segregation in Fe-Cr model alloy pre-implanted with hydrogen ions
Highlights: • Streaks appeared in the diffraction patterns of hydrogen-implanted Fe-Cr alloy after electron irradiation in an HVEM. • A kind of needle-like Cr precipitates give rise to the streaks. • The streaks are explained reasonably by shape effect of precipitates. - Abstract: Reduced activation ferritic/martensitic (RAFM) steels are widely accepted as the candidate structural materials for fusion reactors. To gain knowledge about Cr behaviour of these steels in such environments, Fe-Cr binary alloys have to be investigated. In this study, Fe-10 at.% Cr ferritic model alloy pre-implanted with hydrogen ions was electron-irradiated in a high voltage electron microscope (HVEM) at the temperature ranging from 300 °C to 600 °C. Streaks were observed in the diffraction patterns in the model alloy after electron irradiation at 450–550 °C and explained reasonably by shape effect. The analysis of transmission electron microscopy (TEM) reveals that the observed streaks are induced by a kind of needle-like precipitates with the directions along <100>. The dependence of the length and density of the precipitates on the temperature were studied. In addition, contrast experiments were conducted with pure Fe and Fe-10 at.% Cr alloy with and without hydrogen ion implantation, respectively. After electron irradiation, no streak was observed in the hydrogen-implanted pure Fe and Fe-10 at.% Cr alloy without the hydrogen implantation. Therefore, it is assumed that the needle-like precipitates could be the complex of Cr and H, which segregate and precipitate along <100> direction.
Laser photodetachment diagnostics of a 1/3-size negative hydrogen ion source for NBI
To investigate the flows of charged particles in front of the plasma grid (PG) in a negative hydrogen ion source, the information of the local densities of electrons and negative hydrogen ions (H-) are necessary. For this purpose, the laser photodetachment is applied for pure hydrogen plasmas and Cs-seeded plasma in a 1/3-size negative hydrogen ion source in NIFS-NBI test stand. The H- density obtained by photodetachment is calibrated by the results from cavity ring-down (CRD). The pressure dependence and PG bias dependence of the local H- density are presented and discussed. The results show that H- density increases significantly by seeding Cs into the plasma. In Cs-seeded plasma, relativity exists between the H- ion density and plasma potential.
Use of low energy hydrogen ion implants in high efficiency crystalline silicon solar cells
This program is a study of the use of low energy hydrogen ion implantation for high efficiency crystalline silicon solar cells. The first quarterly report focuses on two tasks of this program: (1) an examination of the effects of low energy hydrogen implants on surface recombination speed; and (2) an examination of the effects of hydrogen on silicon regrowth and diffusion in silicon. The first part of the project focussed on the measurement of surface properties of hydrogen implanted silicon. Low energy hydrogen ions when bombarded on the silicon surface will create structural damage at the surface, deactivate dopants and introduce recombination centers. At the same time the electrically active centers such as dangling bonds will be passivated by these hydrogen ions. Thus hydrogen is expected to alter properties such as the surface recombination velocity, dopant profiles on the emitter, etc. In this report the surface recombination velocity of a hydrogen emplanted emitter was measured.