Negative Hydrogen Ion Production Research Articles

Investigation in improving the Cs-free negative hydrogen ion production with short-pulse low power in the afterglow of pulse-power-modulated plasma sources

Abstract Pulse plasma ion sources have been shown to be capable of achieving high negative hydrogen ion densities ( n H − ) or currents. The fast cooling of the electrons and reduction in the electron density (n e) provide favorable plasma conditions to improve the H− production, with a high density ratio of negative hydrogen ions to electrons during the pulse-off period. The purpose of this research is to further improve the performance of volume-produced H− ion sources based on the pulse power modulation. In this study, a time-modulated negative hydrogen plasma sustained by 10 kHz pulse power at the typical gas pressure of 0.3 Pa was numerically investigated using a global model. The model is compared with measurements obtained in a pulsed radio frequency ion source and shows a reasonable agreement. A steplike low-high power strategy in the active glow period and a short-pulse low power in part of the afterglow period are employed to mitigate overshoot of the electron temperature (T e) in the initial stage of the pulse period and to optimize H− production in the afterglow, respectively. The model predicts that a small-magnitude, short-duration low power operation in the afterglow increases n H − and decreases n e from the onset of the low power until the beginning of the next high power pulse, due to a modest temporary increase in the T e (less than 2 eV). It facilitates dissociative electron attachments and thus H− formation. The reduction in the n e indicates that positive ions lost to the walls cannot be compensated by weak ionization. H− production with a low number of co-extracted electrons can be optimized by adjusting the magnitude and duration of the low power in the afterglow.

Work function of the caesiated converter surface at the BATMAN Upgrade H− ion source at different operational scenarios

Since negative hydrogen ion sources for neutral beam injection (NBI) systems rely on the surface production of negative hydrogen ions, Cs is injected to lower the work function of the extraction electrode surface. The adsorbed Cs layers are affected by residual gases from the given non-UHV conditions as well as by reactive hydrogen species during plasma phases, which leads to a complex surface chemistry and the occurrence of temporal changes of the work function. To control the work function and get insight into its temporal dynamics, an absolute work function diagnostic has been developed for ion sources with which measurements can be performed in vacuum phases between pulses. The diagnostic is applied at the BATMAN Upgrade test facility, which is equipped with the prototype RF ion source for the ITER NBI. It is shown that the Cs conditioning of the ion source leads to a dramatic decrease in the work function to ultra-low values < 1.5 eV. First measurements after the application of 1000 s pulses indicate that the ultra-low work function layer is not stable upon long-term plasma exposure and it is revealed that high dynamics of the Cs surface properties are given right after the pulse.

Ultra-low work function of caesiated surfaces and impact of selected gas species

Modern high-power negative hydrogen ion sources rely predominantly on the surface production of negative hydrogen ions. Hence, low work function converter surfaces are mandatory, for which the alkali metal Cs is commonly evaporated into the ion source to lower the work function of refractory metals by surface adsorption. To study the work function behaviour upon caesiation under the typically given non-ultra-high vacuum conditions, investigations are performed at a dedicated laboratory experiment. In a vacuum environment dominated by water vapour, the work function evolution is found to be dependent on the flux ratio of Cs to H2O onto the surface. For sufficiently high flux ratios, ultra-low work functions in the range of 1.25±0.10 eV are generated with excellent reproducibility. In the absence of Cs evaporation, the work function gradually increases under the influence of the residual gases, and re-caesiation processes lead to lower quantum efficiencies and higher work functions of typically 1.9–2.1 eV. While the addition of hydrogen and deuterium gas at several Pa as well as the leakage of inert gases (argon and nitrogen) into the vacuum system have a negligible influence on the caesiation process, small amounts of oxygen with partial pressures of ∼10-2–10-1 Pa lead to an instant reduction of the Cs density in the gas phase by several orders of magnitude and to an increase in the work function of the order of 1 eV. After the oxygen exposure is terminated, however, the Cs density and work function fully recover within several minutes.

3D fluid model analysis on the generation of negative hydrogen ions for negative ion source of NBI

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.

Effect of cathode shape on the lifetime of arc discharge negative Penning ion source

Internal Penning ion source for the production of negative hydrogen ions is commonly used in various compact cyclotrons. The ion source generally operates at high power DC discharge to provide a high negative hydrogen ion beam current. In this case, the cathode will undergo intense ion sputtering and evaporation due to fierce ion collision. Then, the sputtered material from the cathode will deposit on the inner wall of the ion source discharge chamber. These deposits may lead to slight changes in the structure of the discharge chamber and affect the ion beam extraction stability. Additionally, these deposits would accidentally break and spall, inducing sudden changes in the electric field distribution or even short circuits of the discharge. As a result, the working life of the ion source is dramatically declined. Thus, the relationship between the cathode size and the plasma state was theoretically explored in this study to prolong the ion source’s lifetime. The theory suggests that the impact power of ions on the cathode can be significantly reduced when the cooling capacity of the cathode is reduced. Meanwhile, the cathode temperature is slightly increased, and the discharge voltage declines. Experiments of 4.8 mm and 6 mm in diameter of the cathode were conducted by changing the diameter of the cathode to influence the radiation cooling efficiency. Specifically, the beam current difference was within ±15% before and after changing the cathode size in the same discharge current when the small cathode was used; the beam current of the small cathode was more than 10% (even 40%) better than that of the big one at the same operating power. Before the ion source short-circuited at the same discharge current, the small and big cathode groups operated 5536 μAh and 3000 μAh, respectively. Another test demonstrated that after working for 50 h at 1.8 A discharge current, the mass loss of small cathode was 0.0456 g, and the mass loss of big cathode was 0.0798 g.

Analysis of the chemical network in a volume-production high-current negative hydrogen ion source

Complex simulations (e.g. multi-dimensional fluid models) usually prevent the use of very large chemistry models. In this work, the important physicochemical processes in a volume-production high-current negative hydrogen ion source (HCNHIS) are identified for a pressure range of 1–100 Torr using a global model. The particle species include H2(υ = 0–14), H(n = 1–3), , , H+, H−, and electrons. The simulation results indicate that for the production of negative hydrogen ions through dissociative attachments, the high vibrational levels of hydrogen molecules are important at low pressures, and the ground state and low vibrational levels of hydrogen molecules play an increasingly important role with increasing pressure. The reactions involving the ground state, low vibrational levels, and neighboring levels of each level tend to dominate the production of vibrational levels and transitions between them in the volume-production HCNHIS. With increasing pressure, the electron energy dissipation shifts from dissociation of H2(υ = 0) and electron excitation of H2(υ = 0) through an indirect mechanism followed by radiative decay (the EV process) to electron excitation of H2(υ = 0) through a resonant mechanism (the eV process). A valuable subset of reactions provided by the simplified model is proposed in the investigated pressure range, which could be incorporated in more complex simulations. The results obtained with the simplified model under different simulation conditions are within an acceptable error margin of the results for the full model, which indicates that the robustness of the simplified model of chemical reactions is guaranteed to some extent.

Efficiency investigation of a negative hydrogen ion beam production with the use of the gasdynamic ECR plasma source

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.

Study of gasdynamic electron cyclotron resonance plasma vacuum ultraviolet emission to optimize negative hydrogen ion production efficiency.

Negative hydrogen ion sources are used as injectors into accelerators and drive the neutral beam heating in ITER. Certain processes in low-temperature hydrogen plasmas are accompanied by the emission of vacuum ultraviolet (VUV) emission. Studying the VUV radiation, therefore, provides volumetric rates of plasma-chemical processes and plasma parameters. In the past, we have used gasdynamic ECR discharge for volumetric negative ion production and investigated the dependencies between the extracted H- current density and various ion source parameters. It was shown that it is possible to reach up to 80 mA/cm2 of negative ion current density with a two electrode extraction. We report experimental studies on negative hydrogen ion production in a high-density gasdynamic ECR discharge plasma consisting of two simple mirror traps together with the results of VUV emission measurements. The VUV-power was measured in three ranges-Lyα, Lyman band, and molecular continuum-varying the source control parameters near their optima for H- production. It was shown that the molecular continuum emission VUV power is the highest in the first chamber while Lyα emission prevails in the second one. Modifications for the experimental scheme for further optimization of negative hydrogen ion production are suggested.

A model for real time, in situ estimation of cesium coverage on metal substrate using infrared imaging under vacuum.

The present work is to develop an infra-red (IR) camera based in situ diagnostic tool for the determination of cesium (Cs) coverage suitable for ion source applications. Cs seeding is done to reduce the surface work function that enhances the surface assisted negative hydrogen ion production. The temporal Cs deposition on a metal surface (for, e.g., tungsten or molybdenum) follows Langmuir adsorption isotherm (LAI) kind of behavior. The surface temperature varies while the Cs deposition is reflected in the IR camera temperature measurements for a constant surface emissivity value. In this paper, a model on the relationship between Cs coverage in correlation with surface emissivity and temperature variation based on the theory of LAI is presented. A surface ionization probe (SIP) in the form of a cathode-anode assembly together with an IR camera viewing arrangement is designed to measure the Cs flux and the surface temperature simultaneously to test our model. In the present experiment, the Cs flux measurement using SIP is validated with a standard quartz crystal microbalance (QCM). The proposed model would be useful to correlate Cs coverage on plasma grid-like surface conditions under negative ion source relevant vacuum conditions.

Computational characterization of plasma transport across magnetic filter in ROBIN using PIC-MCC simulation

The RF based negative ion source experimental facility ROBIN has been setup to understand and investigate the different issues related to production, transport and extraction of negative hydrogen ions in negative ion sources for fusion applications. The source consists of a driver, an expansion chamber, a magnetic filter and extraction system consisting of different grids. The first phase of the ROBIN experiments have been conducted for hydrogen plasma generation and measurement of different plasma parameters without negative ion generation/ extraction. We require accurate characterization of plasma to completely understand the experimental results, the complex physics, and plasma dynamics. As a first step in this direction, we have developed an in-house parallel 2D-3v Particle-In-Cell with Monte Carlo Collision (PIC-MCC) code to understand the transport across magnetic filter under conditions similar to real ROBIN operational parameters (magnetic field, pressure, power, density etc.). Simulation results from our 2D-3v PIC-MCC code reproduces the qualitative as well as quantitative behaviors observed during the first phase of the ROBIN experiments such as decrement in electron temperature and similar plasma density profile across the magnetic filter. Our exhaustive PIC simulations helps us in quantifying the plasma transport across magnetic filter as well as the actual input power that is coupled to the plasma. Electron energy distribution function (EEDF) obtained from our simulations shows a Maxwellian distribution and is in agreement with similar experimental calculations reported in the literature. Several case studies have been performed to understand the role of the magnetic filter profile on plasma transport, which will help in planning future experiments by using the magnetic filter as a switching mechanism to achieve the required density and electron temperature profiles for efficient operation of ROBIN.

The mechanism of negative and positive hydrogen ions production on the Ni surface

The studies about surface production of hydrogen negative ions play an important role in many technologies. In the present paper, a low energy molecular hydrogen ion beam was irradiated to the porous Ni sheet. It was found that negative atomic hydrogen ions were detected at the back of the porous Ni sheet when the energy of incident ion beam was lower than 180 eV. When the energy of the incident ions increased, the positive ions dominated at the back of the porous Ni sheet. The production of negative hydrogen on Ni surface can't be explained by comparing the electron affinity of Ni and H. The charge transferring during the process of H atoms reflected from Ni surface was calculated by using constrained DFT (cDFT). The results show the production of negative hydrogen on Ni surface may well be due to heterogeneous electron density on the Ni surface. However, the influence of incident energy on the production of negative and positive hydrogen ions is unable to be explained by cDFT. The problem may be solved by using TDDFT which is able to capture the transfer of energy to electrons during the collision between atoms.

Generation of hydrogen ionic plasma superimposed with positive ion beam

In this study, a hydrogen ionic plasma with relatively low residual fractional electron concentration (ne/n+ ∼ 10−2) is generated using an aluminum plasma grid for the production of negative hydrogen ions and a control grid for negative ion extraction and electron removal. The ionic plasma is composed of negative and positive ions, containing molecular ions. Negative ions are in part produced using positive ions with several electron volts. A positive ion beam with 50 eV or more contributes to increase the density of the ionic plasma. The positive ion beam energy and the control grid bias voltage are tuned in such a way that a high-density ionic plasma is maintained.

Characterization of hydrogen plasma in a permanent ring magnet based helicon plasma source for negative ion source research

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.

Production of Negative Hydrogen Ions Within the MMS Fast Plasma Investigation Due to Solar Wind Bombardment

AbstractThe particle data delivered by the Fast Plasma Investigation instrument aboard National Aeronautics and Space Administration's Magnetospheric Multiscale (MMS) mission allow for exceptionally high‐resolution examination of the electron and ion phase space in the near‐Earth plasma environment. It is necessary to identify populations which originate from instrumental effects. Using Fast Plasma Investigation's Dual Electron Spectrometers, we isolate a high‐energy (approximately kiloelectron volt) beam, present while the spacecraft are in the solar wind, which exhibits an azimuthal drift with period associated with the spacecraft spin. We show that this population is consistent with negative hydrogen ions H− generated by a double charge exchange interaction between the incident solar wind H+ ions and the metallic surfaces within the instrument. This interaction is likely to occur at the deflector plates close to the instrument aperture. The H− density is shown to be approximately 0.2–0.4% of the solar wind ion density, and the energy of the negative ion population is shown to be 70% of the incident solar wind energy. These negative ions may introduce errors in electron velocity moments on the order of 0.2–0.4% of the solar wind velocity and significantly higher errors in the electron temperature.

Experimental benchmark of the NINJA code for application to the Linac4 H− ion source plasma

For a dedicated performance optimization of negative hydrogen ion sources applied at particle accelerators, a detailed assessment of the plasma processes is required. Due to the compact design of these sources, diagnostic access is typically limited to optical emission spectroscopy yielding only line-of-sight integrated results. In order to allow for a spatially resolved investigation, the electromagnetic particle-in-cell Monte Carlo collision code NINJA has been developed for the Linac4 ion source at CERN. This code considers the RF field generated by the ICP coil as well as the external static magnetic fields and calculates self-consistently the resulting discharge properties. NINJA is benchmarked at the diagnostically well accessible lab experiment CHARLIE (Concept studies for Helicon Assisted RF Low pressure Ion sourcEs) at varying RF power and gas pressure. A good general agreement is observed between experiment and simulation although the simulated electron density trends for varying pressure and power as well as the absolute electron temperature values deviate slightly from the measured ones. This can be explained by the assumption of strong inductive coupling in NINJA, whereas the CHARLIE discharges show the characteristics of loosely coupled plasmas. For the Linac4 plasma, this assumption is valid. Accordingly, both the absolute values of the accessible plasma parameters and their trends for varying RF power agree well in measurement and simulation. At varying RF power, the H− current extracted from the Linac4 source peaks at 40 kW. For volume operation, this is perfectly reflected by assessing the processes in front of the extraction aperture based on the simulation results where the highest H− density is obtained for the same power level. In surface operation, the production of negative hydrogen ions at the converter surface can only be considered by specialized beam formation codes, which require plasma parameters as input. It has been demonstrated that this input can be provided reliably by the NINJA code.

Investigation of the Radio-Frequency Discharge in a High Current Negative Hydrogen Ion Source With a Global Enhanced Vibrational Kinetic Model

A numerical investigation of the radio-frequency hydrogen discharge in the high current negative hydrogen ion source (HCNHIS) is presented using a global enhanced vibrational kinetic model (GEVKM). The HCNHIS consists of a high-pressure (2–65 torr) radio-frequency discharge chamber where the main production of high-lying vibrational states of the hydrogen molecules occurs. The hydrogen plasma flow in the discharge chamber is reduced by a series of bypass tubes and enters through a nozzle into a low-pressure (1–15 mtorr) negative hydrogen ion production chamber where H− are generated mainly by the dissociative attachment of low-energy electrons to rovibrationally excited hydrogen molecules. The GEVKM is applied to the HCNHIS discharge and involves volume-averaged equations for 21 hydrogen species (atoms, ions, and molecules in excited states) and electrons. The GEVKM is supplemented with outlet boundary conditions for the nozzle and bypass tubes of the HCNHIS and accounts for compressibility, viscous, and rarefaction effects. GEVKM simulations of the RF discharge are performed with inlet flow rates of 5–1000 sccm and absorbed powers of 200–1000 W using the HCNHIS-2 design which is configured with an extractor grid attached to a short negative ion production region. These simulations investigate the effects of the absorbed power and the inlet flow rate on the chemical composition, electron and heavy particles temperature, wall temperature, the maximum extractable H− current in the discharge chamber, as well as optimum operational parameters of HCNHIS-2. GEVKM simulations of the HCNHIS-2 discharge are used to obtain estimates of the H− current and compared with Faraday cup measurements taken at the extraction grid.

Negative hydrogen ions extracted from metal grids and an ionic plasma owing to diffusion

Metal grids made of nickel, copper, titanium, and iron are used for production of negative hydrogen ions through the irradiation of positive ions. It is easy to determine the existing negative ions in the diffused plasma through the Langmuir probe measurement; however, the negative current, indicating the diminution of the extraction current from the grid, is found to not clearly depend on the work function and electronegativity.

First experiments on applying the gasdynamic ECR ion source for negative hydrogen ion production

First experiments on applying the gasdynamic ECR ion source for negative hydrogen ion production

Enhancement of negative hydrogen ion production at low pressure by controlling the electron kinetics property with transverse magnetic field

In a volume production H− ion source, independent control of electron energy distribution between the driver region and the extraction region is crucial for the efficient production of H− ions due to its unique volume production mechanism. However, at the low pressure regime compatible to ITER operation, it is difficult to control electron energy distribution separately because the nonlocal property dominates the electron kinetics. In this work, we suggest a new method to control the locality of electron kinetics. In this method, an additional pair of permanent magnets is introduced in the vicinity of the skin layer, differently from the conventional method in which the magnetic filter field was strengthened in the extraction region. This magnetic field shortens the energy relaxation length and changes the electron kinetics from nonlocal to local even for low pressure discharges. In this paper, we show that the locality of electron kinetics can be effectively controlled by the additional magnetic field near the skin layer by measuring the electron temperature profile along the center of the discharge chamber as well as by comparing electron energy probability function shapes for different strengths of magnetic field. Using this new method, we demonstrate that control of locality of electron kinetics can greatly enhance the production of H− ions in the extraction region by measuring H− ion beam current extracted from the plasma source.

Radiofrequency and 2.45 GHz electron cyclotron resonance H− volume production ion sources

The volume production of negative hydrogen ions () in plasma ion sources is based on dissociative electron attachment (DEA) to rovibrationally excited hydrogen molecules (H2), which is a two-step process requiring both, hot electrons for ionization, and vibrational excitation of the H2 and cold electrons for the formation through DEA. Traditionally ion sources relying on the volume production have been tandem-type arc discharge sources equipped with biased filament cathodes sustaining the plasma by thermionic electron emission and with a magnetic filter separating the main discharge from the formation volume. The main motivation to develop ion sources based on radiofrequency (RF) or electron cyclotron resonance (ECR) plasma discharges is to eliminate the apparent limitation of the cathode lifetime. In this paper we summarize the principles of volume production dictating the ion source design and highlight the differences between the arc discharge and RF/ECR ion sources from both, physics and technology point-of-view. Furthermore, we introduce the state-of-the-art RF and ECR volume production ion sources and review the challenges and future prospects of these yet developing technologies.