A global model of high current negative hydrogen ion source

Negative hydrogen ions are produced and extracted using an aluminum plasma grid, without cesium through irradiation with hydrogen plasma. The negative ion amount increases sharply, up to ∼115 μA·cm−2, at a discharge power of 1 kW with an electric field downstream of the grid. These negative ions are extracted with high purity. In addition, it is suggested that negative ions are produced by positive hydrogen ions and hydrogen atoms. As high-density plasmas enhance the negative ion production, the current density of negative ions is expected to increase with plasma discharge power.

The injection of energetic neutral hydrogen atoms plays an important part for plasma heating in fusion experiments. In order to fulfill the requirements of the ITER neutral beam injection (NBI), a RF‐driven ion source based on the generation of negative ions prior to neutralization has been successfully developed at IPP Garching. Negative hydrogen ions are generated on a cesiated converter surface (plasma grid) by neutral particles and positive ions and are then transported to the extraction apertures, where the ion beam formation process takes place. Numerical models are necessary to include the relevant physical aspects of these processes. The Monte Carlo transport code CSFLOW is used to describe the dynamical behavior of the cesium distribution on the source walls during vacuum operation. The negative ion transport process is simulated by means of the probabilistic ion transport code TRAJAN, focussing on the effects of aperture diameter variations in mono‐ and multiaperture extraction systems. A simulation of ion beam formation is carried out with the KOBRA3 ray tracing code, which allows a full 3d potential solution without any symmetry restriction. This is necessary to simulate beam steering effects by a non‐axisymmetric magnetic field and electrode configuration for the design of the extraction systems for future negative ion experiments.

An overview of high current (>1 mA) negative hydrogen ion (H–) sources that are currently used on particle accelerators. The current understanding of how H– ions are produced is summarised. Issues relating to caesium usage are explored. The different ways of expressing emittance and beam currents are clarified. Source technology naming conventions are defined and generalised descriptions of each source technology are provided. Examples of currently operating sources are outlined, with their current status and future outlook given. A comparative table is provided.

A radio-frequency (RF) inductively coupled negative hydrogen ion source (NHIS) has been adopted in the China Fusion Engineering Test Reactor (CFETR) to generate negative hydrogen ions. By incorporating the level-lumping method into a three-dimensional fluid model, the volume production and transportation of H− in the NHIS, which consists of a cylindrical driver region and a rectangular expansion chamber, are investigated self-consistently at a large input power (40 kW) and different pressures (0.3–2.0 Pa). The results indicate that with the increase of pressure, the H− density at the bottom of the expansion region first increases and then decreases. In addition, the effect of the magnetic filter is examined. It is noteworthy that a significant increase in the H− density is observed when the magnetic filter is introduced. As the permanent magnets move towards the driver region, the H− density decreases monotonically and the asymmetry is enhanced. This study contributes to the understanding of H− distribution under various conditions and facilitates the optimization of volume production of negative hydrogen ions in the NHIS.

The RF-based single-driver negative hydrogen ion source test bed ROBIN is a 100 kW, 1MHz negative hydrogen ion source test bed at IPR, Gandhinagar. The inductive RF coupling produces plasma in the driver, expanding into an expansion chamber coupled to a three-grid extractor accelerator system. A magnetic filter field, transverse to the plasma flow direction from the RF driver, is produced magnetically using magnet boxes fitted in the diagnostic flange, also called filter field flange. The diagnostic flange is located between the exit of the expansion chamber and the entry to the three-grid system consisting of a plasma grid (PG), extraction grid, and grounded grid. The filter field in conjunction with the bias voltage of a few tens of volts applied to the plasma grid with respect to the source body has a strong influence on the control of plasma electrons, plasma drifts, and plasma confinement, which, in turn, influence plasma uniformity, and so beam profile uniformity, beam divergence, and beam transmission. The present design of ITER sources also envisages magnets lined along the expansion chamber for plasma confinement. Their effectiveness on operations is an important aspect of studies, being performed on fusion-relevant negative ion test beds globally. A systematic study on similar lines has been carried out on the ROBIN test bed and is reported in this paper. The magnets are arranged in boxes and fitted on the walls of the ROBIN expansion chamber extending up to the diagnostic flange. Several magnetic configurations, line and cusp arrangements, have been studied. Significant effects on plasma uniformity across the ion extraction plane in front of the PG are observed. As a consequence, the ratio of co-extracted electron current to negative hydrogen ion (H- ion) current in terms of their current densities (je-/jion) is also influenced by the magnetic configurations. The findings presented here may be relevant for achieving improved H- uniformity along the extraction plane, thereby helping in improved beam divergence and reduced transport losses from larger sources.

It is well known that cesium seeding in volume hydrogen negative ion sources leads to a large reduction of the extracted electron current and in some cases to the enhancement of the negative ion current. The cooling of the electrons due to the addition of this heavy impurity was proposed as a possible cause of the mentioned observations. In order to verify this assumption, we seeded the hydrogen plasma with xenon, which has an atomic weight almost equal to that of cesium. The plasma properties were studied in the extraction region of the negative ion source Camembert III using a cylindrical electrostatic probe while the negative ion relative density was studied using laser photodetachment. It is shown that the xenon mixing does not enhance the negative ion density and leads to the increase of the electron density, while the cesium seeding reduces the electron density.

Summary form only given, as follows. The dissociative attachment of low energy electrons to molecules or atoms of a gas is the preferable mechanism in negative ion sources. One of the methods to obtain low energy electrons is the return of an electron trajectory in the device. The presence of a great number of low energy electrons secures the effective interaction in return region. In this paper the results of an experimental study of negative hydrogen ion source, using electrons of a Penning discharge plasma, are represented. The positive potential of the source anode was (0.5-1) kV, the strength of external magnetic field was (0.1-0.6) kOe, the pressure of gas (hydrogen) was (10/sup -4/-10/sup -3/) Torr. In such discharges the electrons oscillate between reflecting electrodes and have a minimal energy in the return region. The coordinates of the return point were defined by the distribution of the electrical field near the cathode. That is why the source cathode, placed near the extractor, was made in the form of a hollow cylinder with a metal-hydride disk as a bottom. The cylinder had zero potential. The metal-hydride disk had a negative potential of (10-100) V. The point of electron return was placed near the extractor chick by means of variation of metal-hydride disk negative potential. The gas entrance was carried out as a result of hydrogen desorption from the metal-hydride. Rotational and vibrational excitation of desorbed hydrogen molecules increases the cross section of the dissociative attachment process, and therefore the quantity of extracted negative hydrogen ions increases too.

In this article we report on surface scattering experiments at graphite as to model the interactions at divertor surfaces in tokamaks and conversion electrodes in negative ion sources. The formation of negative and positive hydrogen ions on a graphite surface during positive hydrogenous ion scattering at a projectile energy of 400 eV is observed. A remarkably high H− and H+ ion fraction, both varying between 10% and 30% of all scattered particles, is found during scattering of positive hydrogenous ions.

HELicon Experiment for Negative ion source (HELEN-I) with a single driver is developed with a focus on the production of negative hydrogen ions. In the Helicon wave heated plasmas, very high plasma densities (1018–1019 m−3) can be attained with electron temperatures as low as ∼1 eV in the downstream region. These conditions favor the production of negative hydrogen ions. In HELEN-I device at IPR, helicon plasma is produced using Hydrogen gas by applying a RF power (PRF) of 800–1000 W at 13.56 MHz frequency. A Nagoya-III antenna is used to excite m = 1 helicon mode in the plasma. A permanent ring magnet creates the required axial magnetic field. The plasma is confined by a multi-cusp field configuration in the expansion chamber. The transition from inductively coupled mode to Helicon mode is observed near PRF ∼ 700 W with plasma density ∼1018 m−3 and electron temperature ∼5 eV in the driver and ∼1 eV in the expansion volume. Negative hydrogen ion density, averaged over the line of sight, is measured in the expansion chamber by employing an optical emission spectroscopy diagnostic technique using Hα/Hβ ratio and a laser photodetachment based cavity ring down spectroscopic diagnostic technique. The measured value of negative hydrogen ion density is in the order of 1016 m−3 at 6 m Torr pressure and does not vary significantly with power, pressure and downstream axial magnetic field variation in the helicon mode. The negative ion density measurements are compared with theoretically estimated values calculated using a particle balance method considering different reaction rates responsible for negative hydrogen ion creation and destruction. It is to be noted that, at present, cesium is not injected in the plasma discharge to enhance H− ion density.

Angle-resolved energy distribution functions of positive and negative hydrogen ions produced from a rough-finished Si surface under 1 keV proton irradiation have been measured. The corresponding distribution from a crystalline surface and a carbon surface are also measured for comparison. Intensities of positive and negative ions from the rough-finished Si are substantially smaller than those from crystalline Si. The angular distributions of these species are broader for rough surface than the crystalline surface. No significant temperature dependence for positive and negative ion intensities is observed for all samples in the temperature range from 300 to 400 K.

Negative hydrogen (H−) 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− ions. The influence is particularly large for H− ions desorbed from the surface due to less than several eV hydrogen particle implact. The present calculation predicts that H− 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− ions by back scattering and ion induced desorption processes.

Negative hydrogen ion sources are of great demand in modern physics as injectors into accelerators and drivers for neutral beam injectors for fusion devices. It has been shown earlier that the use of the gasdynamic ECR discharge provides the opportunity to extract up to 80 mA/cm2 of negative ion current density. We studied experimentally the volumetric negative hydrogen ion production and vacuum ultraviolet emission in a gasdynamic ECR discharge. The high-density plasma was sustained by the pulsed 37 GHz / 100 kW gyrotron radiation in a magnetic configuration consisting of two consecutive simple mirror traps. The future prospects of the volumetric H− source based on the gasdynamic ECR discharge related to the transition from pulsed to continuous operating mode with the use of an improved magnetic confinement system are discussed. Numerical simulation of the negative hydrogen ion beam extraction at the continuous operating mode facility was performed. Optimal configuration of the extraction electrodes and the electron damping magnets was found.

Negative ion beam extraction in volume mode on the RF negative ion source at ASIPP

The development of a large area (Asource,ITER = 0.9 × 2 m2) hydrogen negative ion (NI) source constitutes a crucial step in construction of the neutral beam injectors of the international fusion reactor ITER. To understand the plasma behaviour in the boundary layer close to the extraction system the 3D PIC MCC code ONIX is exploited. Direct cross checked analysis of the simulation and experimental results from the ITER-relevant BATMAN source testbed with a smaller area (Asource,BATMAN ≈ 0.32 × 0.59 m2) has been conducted for a low perveance beam, but for a full set of plasma parameters available. ONIX has been partially benchmarked by comparison to the results obtained using the commercial particle tracing code for positive ion extraction KOBRA3D. Very good agreement has been found in terms of meniscus position and its shape for simulations of different plasma densities. The influence of the initial plasma composition on the final meniscus structure was then investigated for NIs. As expected from the Child–Langmuir law, the results show that not only does the extraction potential play a crucial role on the meniscus formation, but also the initial plasma density and its electronegativity. For the given parameters, the calculated meniscus locates a few mm downstream of the plasma grid aperture provoking a direct NI extraction. Most of the surface produced NIs do not reach the plasma bulk, but move directly towards the extraction grid guided by the extraction field. Even for artificially increased electronegativity of the bulk plasma the extracted NI current from this region is low. This observation indicates a high relevance of the direct NI extraction. These calculations show that the extracted NI current from the bulk region is low even if a complete ion–ion plasma is assumed, meaning that direct extraction from surface produced ions should be present in order to obtain sufficiently high extracted NI current density. The calculated extracted currents, both ions and electrons, agree rather well with the experiment.

The correlation between the work function of a plasma grid surface in a Cs-introduced volume-production-type negative hydrogen ion ( H-) source and the extracted H- current has been investigated experimentally. The quantum efficiency of a Cs/Mo surface has been measured as a function of the photon energy in a high-vacuum test chamber, and a relationship between the photoelectric work function and the quantum efficiencies at the wavelengths of an Ar+ laser has been obtained. Using this relationship, the work function of the plasma grid surface has been determined by measuring quantum efficiency using an Ar+ laser. When the increment of the H- current is plotted against the measured work function, an exponential dependence on the decrease of the work function is observed. The experimental result cannot be explained by a theoretical H- formation model when one assumes that H- are produced from H atoms leaving from the surface with the energy corresponding to the temperature of H atoms in the plasma. When H- are produced with the kinetic energy substantially larger than thermal energy, the theoretical result shows good agreement with the experimental result.