Negative Hydrogen Ion Source Research Articles

Influence of H2 and D2 plasmas on the work function of caesiated materials

Caesium-covered surfaces are used in negative hydrogen ion sources as a low work function converter for H–/D– surface production. The work function χ of the converter surface is one of the key parameters determining the performance of the ion source. Under idealized conditions, pure bulk Cs has 2.14 eV. However, residual gases at ion source background pressures of 10−7–10−6 mbar and the plasma surface interaction with the hydrogen discharge in front of the caesiated surface dynamically affect the actual surface work function. Necessary fundamental investigations on the resulting χ are performed at a dedicated laboratory experiment. Under the vacuum conditions of ion sources, the incorporation of impurities into the Cs layer leads to very stable Cs compounds. The result is a minimal work function of χvac ≈ 2.75 eV for Cs evaporation rates of up to 10 mg/h independent of substrate material and surface temperature (up to 260 °C). Moreover, a distinct degradation behavior can be observed in the absence of a Cs flux onto the surface leading to a deterioration of the work function by about 0.1 eV/h. However, in a hydrogen discharge with plasma parameters close to those of ion sources, fluxes of reactive hydrogen species and VUV photons impact on the surface which reduces the work function of the caesiated substrate down to about 2.6 eV even without Cs supply. Establishing a Cs flux onto the surface with ΓCs ≈ 1017 m−2 s−1 further enhances the work function obtaining values around 2.1 eV, which can be maintained stable for several hours of plasma exposure. Hence, Cs layers with work functions close to that of pure bulk Cs can be achieved for both H2 and D2 plasmas. Isotopic differences can be neglected within the measurement accuracy of about 0.1 eV due to comparable plasma parameters. Furthermore, after shutting down the Cs evaporation, continuing plasma exposure helps against degradation of the Cs layer resulting in a constant low work function for at least 1 h.

Towards powerful negative ion beams at the test facility ELISE for the ITER and DEMO NBI systems

The test facility ELISE represents an important step in the European R&D roadmap towards the neutral beam injection (NBI) systems on ITER. ELISE provides early experience with operation of large radio frequency (RF) driven negative hydrogen ion sources. Starting with first plasma pulses in March 2013, ELISE has demonstrated stable 1 h plasma discharges with repetitive 10 s beam extraction pulses every 3 min in hydrogen and deuterium at the pressure of 0.3 Pa required by ITER. Stable ion currents of 9.3 A and 5.8 A have been extracted using only one quarter of the available RF power and reducing the extraction voltage in order to control the co-extracted electrons. The best hydrogen pulse for the required 1000 s for hydrogen gave an extracted current of 21.4 A and resulted in an accelerated current of 17.9 A, using only 53 kW per driver. Linear scaling towards full RF power (90 kW/driver) predicts that the target value of the negative ion current (H−: 33 A extracted, 23 A accelerated; D−: 28 A extracted and 20 A accelerated) can be achieved or even exceeded. Issues in long pulse operation are the caesium dynamics and the stability of the co-extracted electron current, for which the caesium management and the magnetic field configuration are promising tools for optimisation. Operation at high RF power for long pulses has highest priority for the next experimental campaign. In parallel or in a later stage, ELISE could serve as a test bed for studies on a DEMO NBI system. Examples are concepts concerning RF efficiency, operation with largely reduced caesium consumption or with caesium alternatives, and neutralization of the accelerated ion beam by a laser neutralizer in order to improve efficiency and reliability of NBI systems. Lab scale experiments on these topics are carried out presently in parallel with ELISE operation.

Investigations on Cs-free alternatives for negative ion formation in a low pressure hydrogen discharge at ion source relevant parameters

Caesium (Cs) is applied in high power negative hydrogen ion sources to reduce a converter surface’s work function and thus enabling an efficient negative ion surface formation. Inherent drawbacks with the usage of this reactive alkali metal motivate the search for Cs-free alternative materials for neutral beam injection systems in fusion research. In view of a future DEMOnstration power plant, a suitable material should provide a high negative ion formation efficiency and comply with the RAMI issues of the system: reliability, availability, maintainability, inspectability. Promising candidates, like low work function materials (molybdenum doped with lanthanum (MoLa) and LaB6), as well as different non-doped and boron-doped diamond samples were investigated in this context at identical and ion source relevant parameters at the laboratory experiment HOMER. Negative ion densities were measured above the samples by means of laser photodetachment and compared with two reference cases: pure negative ion volume formation with negative ion densities of about and the effect of H− surface production using an in situ caesiated stainless steel sample which yields 2.5 times higher densities. Compared to pure volume production, none of the diamond samples did exhibit a measurable increase in H− densities, while showing clear indications of plasma-induced erosion. In contrast, both MoLa and LaB6 produced systematically higher densities (MoLa: ×1.60; LaB6: ×1.43). The difference to caesiation can be attributed to the higher work functions of MoLa and LaB6 which are expected to be about 3 eV for both compared to 2.1 eV of a caesiated surface.

Diagnostics tools and methods for negative ion source plasmas, a review

Plasma parameter measurements for negative hydrogen (H−) ion sources have been playing an important role in clarifying fundamental physics related to negative ion production and destruction processes. Measured data of beam properties, such as H− ion current density with the co-extracted electron current and the emittance, were correlated to local concentration of charged particles and temperature often characterized by Langmuir probes and optical emission spectrometry. Langmuir probes coupled to pulse lasers quantified local H− ion densities from early days of H− ion source development, while the cavity ring down photodetachment method removed Langmuir probes from contemporary large-size high power density ion sources. Technological progress has made source plasma diagnostics possible during beam extraction, which has thrown light on the transport of H− ions during the application of the extraction electric field. The advancement of plasma diagnostics for high intensity H− ion sources are summarized in this report together with recent results from the research and development negative ion source being operated for collaborative research programs at National Institute for Fusion Science.

Response of H− ions to extraction field in a negative hydrogen ion source

In order to investigated the dynamics of H− ions and understand the extraction process inside filament-arc-driven plasmas in a Cs-seeded negative ion source, diagnostic experiments using a directional Langmuir probe combined with photodetachment measurement have been conducted. Two-dimensional flow pattern of H− ion flow has been obtained as well as the profile of H− ion density. The result shows that H− ions come from the direction of plasma grid surface, flow to the plasma in the extraction region. During beam extraction, H− ion flow turns to the beam direction. The stagnation point of H− ion flow is at ∼20mm apart from the plasma grid. Around the stagnation point, the maximum decrement of H− density caused by beam extraction appears. The results indicate that H− ions are extracted from the plasma volume. The extraction process occurs in a region near the plasma grid aperture with boundary of ∼20mm apart from the plasma gird.

Characteristics of a high-power RF source of negative hydrogen ions for neutral beam injection into controlled fusion devices

An injector of hydrogen atoms with an energy of 0.5–1 MeV and equivalent current of up to 1.5 A for purposes of controlled fusion research is currently under design at the Budker Institute of Nuclear Physics, Siberian Branch, Russian Academy of Sciences. Within this project, a multiple-aperture RF surface-plasma source of negative hydrogen ions is designed. The source design and results of experiments on the generation of a negative ion beam with a current of >1 A in the long-pulse mode are presented.

A system of distributed cesium feeding for increasing the efficiency of powerful sources of negative hydrogen ions

Injection of high-power flows of high-energy neutral atoms is often required in modern facilities designed for fusion research. At energies of several hundreds keV, obtaining neutral atom flows is ineffective and converting them from negative ion beams is expedient. The efficiency of negative-ion production increases by many times if a cesium film is applied to the surface on which negative ions are produced. An optimized system of distributed cesium feeding is described. Using this system, it is possible to transport cesium vapor through large distances and its uniform application to large-area emitting surfaces in high-power sources of negative hydrogen ions.

Influence of the magnetic field topology on the performance of the large area negative hydrogen ion source test facility ELISE

In negative hydrogen ion sources a too high amount or a too rapid increase in co-extracted electrons can prevent achieving the required negative ion current density and restrict the pulse duration. One important measure for reducing and stabilizing the co-extracted electron current is a magnetic filter field. The half-ITER-size NNBI test facility ELISE—in which the filter field is created by a current flowing through the extraction system—is used for performing experiments on the reaction of the source performance on modifying the filter field topology; external magnet bars are attached to the source in different polarities and extensive parameter variations have been done in volume and surface operation. A significant correlation of the extracted ion and electron currents and their temporal stability with the field topology is seen: with the external magnets strengthening the standard filter field a strong reduction of the co-extracted electrons and additionally a reduced increase in these electrons during the pulses is observed. In this configuration it was possible to perform the very first 1 h deuterium pulse in ELISE—an important step toward fulfilling the requirements to the ion source for ITER NBI.

Tomographic reconstruction of the beam emissivity profile in the negative ion source NIO1

A versatile negative hydrogen ion source named NIO1 of a moderate size (130 mA total extracted H− current, 9 apertures, 60 kV total acceleration) has been developed and installed at Consorzio RFX. It will allow great experimental flexibility, very beneficial for studying several important issues related to beam extraction, optics and performance optimization, in view of SPIDER and MITICA, the two full-scale experiments for the ITER neutral beam injector under construction at RFX. The main target of emission tomography applied to an ion beam is the reconstruction of the emissivity profile, from which the ion density distribution can be obtained. The measurement of the beam density profile and of its uniformity throughout the pulse duration with a non-invasive diagnostic, such as tomography, would represent an effective method for monitoring the ion source operation and for malfunction detection. The application of this diagnostic to the NIO1 beam will represent the experimental verification of the possibility to reconstruct a multi-beamlet profile, in the interest of the next tomography systems for SPIDER and MITICA. In this paper, a feasibility study of the tomographic diagnostic for NIO1 is presented. A tomography code based on algebraic reconstruction techniques has been developed for this purpose and the transport of the nine H− beamlets is simulated with a Monte Carlo particle tracking code from the ion source to the tomography plane, where the beam emissivity profile to be reconstructed is calculated. The reference emissivity profile is reconstructed by the tomography code considering different possible layouts of the detection system, in order to find the best compromise between the quality of reconstructions and the complexity of the diagnostic. Results show that a tomography system based on six linear CCD cameras should be capable of reconstructing the NIO1 emissivity profile with an rms error lower than 10%. How instrumental noise in the integrated signals affects the reconstructed beam emissivity profiles is also studied. A simple low-pass filter is found effective if the noise level is less than 10%; otherwise a more sophisticated filtering technique must be considered.

Extraction of negative charges from an ion source: Transition from an electron repelling to an electron attracting plasma close to the extraction surface

Large-scale sources for negative hydrogen ions, capable of delivering an extracted ion current of several ten amperes, are a key component of the neutral beam injection system of the upcoming ITER fusion device. Since the created heat load of the inevitably co-extracted electrons after magnetic separation from the extracted beam limits their tolerable amount, special care must be taken for the reduction of co-extracted electrons—in particular, in deuterium operation, where the larger amount of co-extracted electrons often limits the source performance. By biasing the plasma grid (PG, first grid of the extraction system) positively with respect to the source body, the plasma sheath in front of the PG can be changed from an electron repelling towards an electron attracting sheath. In this way, the flux of charged particles onto the PG can be varied, thus changing the bias current and inverse to it the amount of co-extracted electrons. The PG bias affects also the flux of surface-produced H − towards the plasma volume as well as the plasma symmetry in front of the plasma grid, strongly influenced by an E→×B→ drift. The influence of varying PG sheath potential profile on the plasma drift, the negative hydrogen ion density, and the source performance at the prototype H − source is presented, comparing hydrogen and deuterium operation. The transition in the PG sheath profile takes place in both isotopes, with a minimum of co-extracted electrons formed in case of the electron attracting PG sheath. The co-extracted electron density in deuterium operation is higher than in hydrogen operation, which is accompanied by an increased plasma density in deuterium.

Progress of the ELISE test facility: towards one hour pulses in hydrogen

In order to fulfil the ITER requirements, the negative hydrogen ion source used for NBI has to deliver a high source performance, i.e. a high extracted negative ion current and simultaneously a low co-extracted electron current over a pulse length up to 1 h. Negative ions will be generated by the surface process in a low-temperature low-pressure hydrogen or deuterium plasma. Therefore, a certain amount of caesium has to be deposited on the plasma grid in order to obtain a low surface work function and consequently a high negative ion production yield. This caesium is re-distributed by the influence of the plasma, resulting in temporal instabilities of the extracted negative ion current and the co-extracted electrons over long pulses. This paper describes experiments performed in hydrogen operation at the half-ITER-size NNBI test facility ELISE in order to develop a caesium conditioning technique for more stable long pulses at an ITER relevant filling pressure of 0.3 Pa. A significant improvement of the long pulse stability is achieved. Together with different plasma diagnostics it is demonstrated that this improvement is correlated to the interplay of very small variations of parameters like the electrostatic potential and the particle densities close to the extraction system.

How does a probe inserted into the discharge influence the plasma structure?

Shielding the bias applied to the probe by the sheath formed around it and determination of parameters of unperturbed plasmas are in the basis of the probe diagnostics. The results from a two-dimensional model of a discharge with a probe inserted in it show that the probe influences the spatial distribution of the plasma parameters in the entire discharge. The increase (although slight) in the electron temperature, due to the increased losses of charged particles on the additional wall in the discharge (mainly the probe holder), leads to redistribution of the plasma density and plasma potential, as shown by the results obtained at the floating potential of the probe. The deviations due to the bias applied to the probe tip are stronger in the ion saturation region of the probe characteristics. The pattern of the spatial redistribution of the plasma parameters advances together with the movement of the probe deeper in the discharge. Although probe sheaths and probe characteristics resulting from the model are shown, the study does not aim at discussions on the theories for determination of the plasma density from the ion saturation current. Regardless of the modifications in the plasma behavior in the entire discharge, the deviations of the plasma parameters at the position of the probe tip and, respectively, the uncertainty which should be added as an error when the accuracy of the probe diagnostics is estimated do not exceed 10%. Consequently, the electron density and temperature obtained, respectively, at the position of the plasma potential on the probe characteristics and from its transition region are in reasonable agreement with the results from the model of the discharge without a probe. Being in the scope of research on a source of negative hydrogen ions with the design of a matrix of small radius inductive discharges, the model is specified for a low-pressure hydrogen discharge sustained in a small-radius tube.

A simple spectroscopic method to determine the degree of dissociation in hydrogen plasmas with wide-range spectrometer.

A new and simple method for determining the degree of dissociation in hydrogen plasmas is presented. In this method, wide-range spectrum covering from an atomic H-γ line (434.05 nm) to molecular Fulcher-α band (600-640 nm) is measured simultaneously by a wide-range miniature spectrometer. Since the wide-range spectrum measured by the miniature spectrometer is too broadened to resolve respective lines in the Fulcher-α band, a synthetic spectrum method is applied to improve the accuracy in the Q-branch of Fulcher-α band intensity measurement. In order to reduce the influence from other transitions or anomalous P- and R-branch of Fulcher-α spectrum, the Fulcher-α spectra of which vibrational states are higher than 1 (υ ≥ 1) are synthesized using the rotational temperature obtained by the 0-0 Fulcher-α spectrum. The degree of dissociation is determined from the intensity ratio between H-γ line and the synthesized Fulcher-α band spectrum. A comparative study carried out in a volume-produced negative hydrogen ion source shows that the degree of dissociation determined by this method agrees well with the measured values using a spectrometer with high spectral resolution. The present method is expected to be useful to characterize the plasma sources with molecular species since it provides important parameters for understanding neutral particle behaviors.

Study of ion-ion plasma formation in negative ion sources by a three-dimensional in real space and three-dimensional in velocity space particle in cell model

Recently, in large-scale hydrogen negative ion sources, the experimental results have shown that ion-ion plasma is formed in the vicinity of the extraction hole under the surface negative ion production case. The purpose of this paper is to clarify the mechanism of the ion-ion plasma formation by our three dimensional particle-in-cell simulation. In the present model, the electron loss along the magnetic filter field is taken into account by the “τ///τ⊥ model.” The simulation results show that the ion-ion plasma formation is due to the electron loss along the magnetic filter field. Moreover, the potential profile for the ion-ion plasma case has been looked into carefully in order to discuss the ion-ion plasma formation. Our present results show that the potential drop of the virtual cathode in front of the plasma grid is large when the ion-ion plasma is formed. This tendency has been explained by a relationship between the virtual cathode depth and the net particle flux density at the virtual cathode.

Depth of Influence on the Plasma by Beam Extraction in a Negative Hydrogen Ion Source for NBI

Depth of Influence on the Plasma by Beam Extraction in a Negative Hydrogen Ion Source for NBI

Recent Studies of Hydrogen Negative Ion Source and Beam Production for NBI in Large Helical Device

Recent Studies of Hydrogen Negative Ion Source and Beam Production for NBI in Large Helical Device

Oscillatory instability development in extraction system of a negative ion source.

Conditions of oscillatory instability development in the extraction system of a negative hydrogen ion source based on a volume-produced plasma are studied. Such an ion source is characterized by the presence of the parent gas in the extraction system due to the leakage from the gas-discharge chamber. The secondary electrons in the area of the ion-optical system become the reason of oscillation appearance and possible beam current modulation. Analytically the range of the stable beam propagation is found. The instability increment is shown to be rather small. Maximum increment of the oscillations corresponds to the beam velocity equal to the thermal velocity of plasma electrons. The group velocity of the oscillations is close to the beam velocity so the oscillations are convective. Simulation of the low energy beam propagation is performed in COMSOL Multiphysics, the beam current modulation being observed.

Status of the RF-driven H− ion source for J-PARC linac

For the upgrade of the Japan Proton Accelerator Research Complex linac beam current, a cesiated RF-driven negative hydrogen ion source was installed during the 2014 summer shutdown period, with subsequent operations commencing on September 29, 2014. The ion source has been successfully operating with a beam current and duty factor of 33 mA and 1.25% (500 μs and 25 Hz), respectively. The result of recent beam operation has demonstrated that the ion source is capable of continuous operation for approximately 1100 h. The spark rate at the beam extractor was observed to be at a frequency of less than once a day, which is an acceptable level for user operation. Although an antenna failure occurred during operation on October 26, 2014, no subsequent serious issues have occurred since then.

Optimization of plasma parameters with magnetic filter field and pressure to maximize H⁻ 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(-) populations for various filter field strengths and pressures. Enhanced H(-) 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(-) sources with an optimal filtering system by maximizing high energy electron filtering while keeping low energy electrons available in the extraction region.

Spectroscopic study of neutral species in a planar-coil inductive discharge in hydrogen

This contribution reports on an experimental study of characteristics of the neutrals in a single element of a matrix source of negative hydrogen ions. The investigations are carried out in hydrogen discharges maintained at a frequency of 27 MHz, applied power varied in the limits P = (50–150) W and pressure in the range p = (90–160) mTorr. An optical emission spectroscopy diagnostics, based on analysis of the spectral line profile and the intensity distribution of the Fulcher-α molecular band, is used. The profile of the atomic line reveals the existence of thermal as well as of non-thermal (fast) hydrogen atoms in the discharge. For processing the experimental data a line shape model, accounting for details of the plasma kinetics and the fine structure of the line, is used. The temperature of the atoms in the excited n = 3 state, the mean energy of the fast atoms, the ratio between the densities of these two groups of atoms and the relative population of the fine structure components of the n = 3 hydrogen state are determined. The atomic temperature is estimated to be equal to the temperature of the excited (n = 3) atoms. The molecular temperature is determined from the analysis of the intensity distribution of the molecular Fulcher-α band and is found to be about two times lower than the atomic temperature.