Negative Ion Source Research Articles

Continuous pulse advances in the negative ion source NIO1

Consorzio RFX and INFN-LNL have designed, built and operated the compact radiofrequency negative ion source NIO1 (Negative Ion Optimization phase 1) with the aim of studying the production and acceleration of H- ions. In particular, NIO1 was designed to keep plasma generation and beam extraction continuously active for several hours. Since 2020 the production of negative ions at the plasma grid (the first grid of the acceleration system) has been enhanced by a Cs layer, deposited though active Cs evaporation in the source volume. For the negative ion sources applied to fusion neutral beam injectors, it is essential to keep the beam current and the fraction of co-extracted electrons stable for at least 1 h, against the consequences of Cs sputtering and redistribution operated by the plasma. The paper presents the latest results of the NIO1 source, in terms of caesiation process and beam performances during continuous (6 ÷ 7 h) plasma pulses. Due to the small dimensions of the NIO1 source (20 cm×∅10 cm), the Cs density in the volume is high (1015 ÷ 1016 m-3) and dominated by plasma-wall interaction. The maximum beam current density and minimum fraction of co-extracted electrons were respectively about 30 A/m2 and 2. Similarly to what done in other negative ion sources, the plasma grid temperature in NIO1 was raised for the first time, up to 80 °C, although this led to a minimal improvement of the beam current and to an increase of the co-extracted electron current.

Plasma emission monitored via optical emission spectroscopy during the Cs conditioning at SPIDER

The SPIDER test facility is the full-size ITER neutral beam injector (NBI) ion source, required to provide 355 A/m2 extracted negative ion current density in hydrogen (285 A/m2 in deuterium) with an electron-to-ion ratio lower than 0.5 (one in deuterium). The negative ion source is attached to a three-grids extraction and acceleration system. The operational conditions for the cases presented in this work involve short pulses (up to about 30 s length) repeated every five to six minutes. The duty cycle can be adjusted. In order to fulfil the requirement on the extracted negative ion current with reduced amount of co-extracted electrons, the evaporation of caesium into the ion source through Cs ovens (three in SPIDER) and the optimisation of Cs conditioning techniques are mandatory. At SPIDER, the plasma is monitored via optical emission spectroscopy techniques measuring the plasma emission in a line-of-sight (LOS) integrated manner in several positions inside the ion source. In particular, close to the extraction region, two sets (centred at 5 mm and at 35 mm distance from the plasma grid) of four horizontal LOSs are used to retrieve the vertical profile of the plasma emission. During the Cs conditioning campaign performed at SPIDER, although the extraction capabilities were reduced due to technical problems, the RF power coupled to the plasma reached 400 kW with all four RF generators working simultaneously. The negative ions produced by surface emission affects the plasma radiation, and the ion source performances in terms of extracted negative ions and co-extracted electrons. The aim of this work is to study the evolution of the plasma emission over the initial phase of the Cs conditioning campaign. The effect of Cs conditioning on the extracted negative ions and electrons is also presented and discussed.

Enhancing ion extraction with an inverse sheath in negative hydrogen ion sources for NBI heating

Negative hydrogen ion (H−) sources employed in neutral beam injection (NBI) systems are subject to extraction efficiency issues due to the considerable volumetric losses of negative hydrogen ions. Here, we propose to improve the H− extraction by activating an alternative sheath mode, the electronegative inverse sheath, in front of the H− production surface, which features zero sheath acceleration for H− with a negative sheath potential opposite to the classic sheath. With the inverse sheath activated, the produced H− exhibits smaller gyration, a shorter transport path, less destructive collisions, and therefore higher extraction probability than the commonly believed space-charge-limited (SCL) sheath. Formation of the proposed electronegative inverse sheath and the SCL sheath near the H–-emitting surface is investigated by the continuum kinetic simulation. Dedicated theoretical analyses are also performed to characterize the electronegative inverse sheath properties, which qualitatively agree with the simulation results. We further propose that the transition between the two sheath modes can be realized by tuning the cold ion generation near the emissive boundary. The electronegative inverse sheath is always coupled with a plasma consisting of only hydrogen ions with approximately zero electron concentration, which is reminiscent of the ion–ion plasma reported in previous NBI experiments.

Beam extraction under Cs-free conditions in HUST negative ion source

A radio-frequency driven negative hydrogen ion source is under development at Huazhong University of Science and Technology, which aims to investigate the physics of production and extraction of negative ions. The source operational parameters are investigated to investigate the source plasma characteristics and to optimize the extraction properties before cesium injection, which means, in this study, that negative ions were mostly produced by reactions in the plasma volume. The magnetic filter field generated by a plasma grid (PG) current flowing through the PG can cool down the electron temperature below 1 eV, but simultaneously increased the plasma density in the extraction region. The plasma asymmetry in the direction perpendicular to the PG filter field, is discussed and compared with that in other ion sources. As the PG current increased, the asymmetry factors tended to increase first and then decrease. Biasing the PG positively with respect to the source body can reduce the co-extracted electrons effectively, resulting in reducing the electron to H− ion current ratio. An extracted negative beam with a current density of about 19 A m−2 and an energy of 12 keV was achieved under 20 kW RF power and 0.3 Pa filling pressure, with a ratio of electron current to ion current of about 54.

H- beam formation simulation in negative ion source for CERN's Linac4 accelerator

The caesiated surface negative ion (H-) source is the first element of CERN's LINAC4 a linear injector designed to accelerate negative hydrogen ions to 160 MeV. The IS03 ion source is operated at 35 mA beam intensity and reliably feeds CERN's accelerator chain, H- ions are generated via plasma volume and caesiated molybdenum (Cs-Mo) plasma electrode surface mechanisms. Studying the beam extraction region of this H- ion source is essential for optimizing the H- production. The 3D Particle-in-cell (PIC) Monte Carlo (MC) code ONIX (Orsay Negative Ion eXtraction [1]), written to study H- beam formation processes in neutral-beam injectors for fusion, has been adapted to single aperture accelerator H- sources. The code was modified to match the conditions of the beam formation and extraction regions of the Linac4 H- source [2]. A set of parameters was chosen to characterize the plasma and to match the specific volume and surface production modes. Simulated results of the extraction regions are presented and benchmarked with experimental results obtained at the Linac4 test stand [3].

First Results of Magnetic Filter Field at RF Negative Ion Source

First Results of Magnetic Filter Field at RF Negative Ion Source

First H- beam extracted from the non-caesiated external RF-coil ion source at ISIS

A high power, high duty-cycle, non-caesiated negative hydrogen ion source is in development at the ISIS pulsed spallation neutron and muon facility. Stable operation of an inductively-coupled plasma pulsing at 50 Hz, 1 ms, 70 kW has been perfected with no apparent lifetime issues. Novel features compared to similar H- sources include a very low power microwave ignition gun, an adjustable permanent magnet filter field and multiple large 3D-printed components. This paper details the initial commissioning efforts required to generate a beam. An H- beam current of 10 mA was extracted on the first try in pure volume-production mode. Parameter sweeps and optimisation to reach the required 35 mA were put on hold to address ancillary equipment problems. Mitigation of RF pickup required major mechanical changes such as additional screening and magnetic shielding. Extraction voltage holding capabilities were investigated to cope with co-extracted electron current loading in excess of 0.7 A. Preliminary studies of the biased plasma electrode current showed that co-extracted electrons could be suppressed by increasing the magnetic filter field and gas flow.

Towards self-consistent modelling of negative ion beam acceleration

Neutral beams are a valuable option to provide heat and current drive to a magnetically confined plasma. In addition they can be used as a diagnostics for the positive ion temperature and, in case of nuclear fusion reactions, for the amount of helium ash. In the case of ITER, stringent requirements are set on the acceptable beam divergence and aiming in order to propagate such beams up to the tokamak itself. Besides the well-known dependence of the beamlet optics on the beam energy and perveance, other important features affecting the optics are the ion spatial and velocity distribution at the plasma boundary facing the extractor, the so-called plasma meniscus. In a caesiated negative ion source, this distribution is influenced both by the birth energy of the negative ions and by the ion transport in the plasma itself. In this work we use a test particle code to investigate the negative ion transport up to the plasma meniscus and we use a standard ray-tracing code to trace the test particles that reached the meniscus inside the electrostatic accelerator. The coupling of the two codes is iterated up to numerical convergence. By this procedure, the angular distribution of the accelerated particles is found to be much more similar to the experimental ones than the one obtained by simple ray-tracing simulations.

The negative beam source with single driver for CRAFT NNBI: design and conditioning results

The Comprehensive Research Facility for Fusion Technology (CRAFT) is a large scientific facility that is preferentially deployed for the construction of major national science and technology infrastructures. A negative beam source based neutral beam injector (NNBI) with beam energy of 200–400 keV, beam power of 2 MW and beam duration of 100 s is one of the devices. A giant radio frequency (RF) based negative beam source was designed for the CRAFT NNBI system. In order to understand the physics and pre-study the engineering problems for RF negative beams source, several beam sources with different scales will be designed and tested. According to the R&D schedule, a negative beam source with a single driver was designed, developed and tested first. The single driver beam source is a quarter of the size of the full size negative beam source. It contains an RF driver, an expansion chamber and a negative ion accelerator with three electrodes, which are a plasma grid (PG), extraction grid (EG) and ground grid (GG). To enhance the negative ion production, Cesium is injected into the plasma chamber and a magnetic filter filed is produced by current flow through the PG to decrease the electron temperature. The negative beam source was tested on the test facility after assembly, including RF plasma generation, negative ion production, extraction and acceleration. The characteristic of plasma discharge, beam extraction and acceleration was studied without and with Cs injection. The long pulse of 105 seconds negative ion beam was achieved successfully. The extracted negative ion current is 4 A and the ratio of electron and negative ion is around 0.8. It lays a good foundation for the R&D of negative ion source with a multi-driver for the CRAFT NNBI system. The details of the design and experimental results of beam source was shown in this paper.

Surface production of negative deuterium ions from plasma-exposed boron doped diamond and graphite: work function measurements using photoemission yield spectroscopy

Negative-ion sources are of considerable interest for applications such as materials processing and neutral beam injection for magnetic confinement fusion. The efficient production of negative ions in these sources often relies on surface production. Work function measurements are critical to enable a detailed understanding of the mechanisms that underpin this. In this study we used a combination of photoemission yield spectroscopy and the Fowler method to determine the work functions of boron doped diamond (BDD) and highly oriented pyrolytic graphite (HOPG) directly after exposure to a low-pressure inductively coupled deuterium plasma (150 W, 2 Pa). A magnetised retarding field energy analyser is used to measure the negative ion current from the samples. During plasma exposure, samples are biased at −130 V or −60 V and their temperature is varied between 50 ∘C and 750 ∘C. The results show that the increasing work function of the plasma exposed HOPG occurs over the same sample temperature range as the decreasing negative-ion current. In contrast, the work function of BDD does not show a clear relationship with negative-ion current, suggesting that different mechanisms influence the negative-ion production of metal-like HOPG and dielectric-like BDD. The necessity for an additional fitting parameter for the Fowler fits to BDD suggests that its electronic properties are changing under plasma exposure, unlike HOPG. For both materials, the maximum photocurrent measured from the samples displays a strong similarity with negative-ion current, suggesting they are driven by a common mechanism. The in-situ measurement of the work function using non-invasive techniques is of interest for the development of negative ion sources.

Characteristics of the pressure profile in the accelerator on the RF negative ion source at ASIPP

Neutral beam injection (NBI) systems based on negative hydrogen ion sources—rather than the positive ion sources that have typically been used to date—will be used in the future magnetically confined nuclear fusion experiments to heat the plasma. The collisions between the fast negative ions and neutral background gas result in a significant number of high-energy positive ions being produced in the acceleration area, and for the high-power long-pulse operation of NBI systems, this acceleration of positive ions back to the ion source creates heat load and material sputtering on the source backplate. This difficulty cannot be ignored, with the neutral gas density in the acceleration region having a significant impact on the flux density of the backstreaming positive ions. In the work reported here, the pressure gradient in the acceleration region was estimated using an ionization gauge and a straightforward 1D computation, and it was found that once gas traveled through the acceleration region, the pressure dropped by nearly one order of magnitude, with the largest pressure drop occurring at the plasma grid. The computation also revealed that the pressure drop in the grid gaps was substantially smaller than that in the grid apertures.

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.

SPIDER on-line data visualization tool

SPIDER (Source for Production of Ion of Deuterium Extracted from Radio Frequency plasma), has been operating in Padova since May 2018, with the aim to test and develop the ITER-scale radio-frequency negative ion source, to study the beam characteristics and to verify the source operation. The source produces more than 500 gigabyte per operational day. Extracting as much information as possible from a set of selected signals (plasma light, voltages and currents of power supplies, cameras, tomography, beam emission spectroscopy, caesium oven parameters and others) during an experimental day allows a more efficient use of the largest neutral beam operating source. The simultaneous visualization of signals, related to pre-defined times of interest arrays, monitors the state of the beam and source features from quite different points of view. However, the time evolution of signals of interest often does not represent the expected format for data analysis. The experimental pulse can be divided in phases called ‘blips’, which correspond to a given experimental configuration typically lasting 40 s. Blips are separated by time periods, required for changing the experiment settings to different configurations. Some punctual information is required for visualization, while derived information is then stored in a relational database for subsequent analysis.The derivation of punctual values was performed offline, with the disadvantage of not being able to derive results of interest during the experiment itself. The new tool derives blip-related information during the pulse, as soon as required data have been stored in the pulse file. The availability of such tool represents a big improvement in the exploitation of the scientific results of SPIDER that can be used in this way during the experimental session itself. Furthermore new features are being developed regarding the addition of other signals including secondary ones and the access to the database from multiple platforms.

Status of SPIDER beam source after the first 3.5 years of operation

SPIDER is the negative ion source testbed for ITER neutral beam injector. It is currently the largest negative ion source ever built, equipped with a 100 keV accelerator, aiming at producing a negative ion beam with an extracted current density in hydrogen of 355 A/m2, beam-on time of one hour. During the first 3.5 years of operation several improvements on the SPIDER source and its operating conditions were implemented. This paper focuses on the description of materials degradation phenomena in SPIDER and on the lessons learnt that were derived from components inspection during the already planned shutdown in 2022. The aim of these inspections was to improve the design of some components and to repair or refurbish damaged components. In particular, this paper describes the characterisation of damaged molybdenum coating, testing of surface cleaning techniques and experimental evidence of caesium vapour uneven distribution inside the plasma chamber. Sputtering due to beam particles was also investigated experimentally to observe its effect on materials used in SPIDER.

Control method of DC component for NNBI accelerator power supply

In the high voltage power supply of the negative ion source neutral beam injection system (NNBI) accelerator, the three-phase three-level neutral point clamping (NPC) inverter, as an inverter module, has a detectable DC component in its output. When it flows into the step-up transformer, it will lead to increased transformer loss and excitation saturation, thus reducing the service life of the transformer and increasing potential safety hazards. This paper analyzes the causes of the DC component of the NPC inverter, and a reliable DC component suppression method based on double-loop control is proposed. A suitable PI controller can effectively suppress the output DC component during the power supply operation by establishing the transfer function between the output DC component and the regulation rate of the switch’s duty cycle. Finally, the prototype experiments verify the effectiveness of the control method successfully.

Investigating radioactive negative ion production via double electron capture

The relative cross sections for radioactive negative ion production via double electron capture have been measured for collisions between a 40 keV projectile beam of uranium-238 and potassium vapor. This was performed at the collinear resonance ionization spectroscopy (CRIS) experiment at CERN-ISOLDE and is a step towards measuring the electron affinities (EAs) of elements that cannot be efficiently produced in negative ion sources at radioactive ion beam (RIB) facilities. This includes short-lived radioactive isotopes that have low production quantities and heavy and superheavy elements that systematically have smaller EAs than work functions of available ion source materials. Negative ions are particularly sensitive to electron–electron correlation effects, which make such studies ideal for benchmarking atomic structure models that go beyond the independent particle model. While the EAs of most light elements have been measured, experimental investigations on heavier elements, namely the actinides, remain scarce due to their radioactive nature and production difficulty. By developing negative ion production by charge exchange, we aim to make these studies feasible at RIB facilities.

Design of The Power Supply and Distribution Monitoring System for NNBI

The power supply and distribution system provides powerful support for the efficient operation of the negative ion source neutral beam injector (NNBI) experiment. To centrally manage the power supply equipment of each subsystem of NNBI, a power supply, and distribution monitoring system based on WinCC software and SQL Server database is developed. The system has an excellent man-machine interaction interface and realizes the monitoring and management of energy consumption in the power supply and distribution system. It has circuit process monitoring, fault alarm, operation log, historical data query, and other functions. After testing, the system can run stably and reliably control each device. At the same time, the system has good compatibility and expansibility and meets the needs of intelligent management and monitoring of the power supply and distribution system.

Physics and instabilities of low-temperature E × B plasmas for spacecraft propulsion and other applications

Low-temperature E×B plasmas are used in various applications, such as Hall thrusters for satellite propulsion, ion sources and magnetron discharges for plasma processing, and negative ion sources for neutral beam injection in fusion. The plasmas in these devices are partially magnetized, meaning that the electrons are strongly magnetized while the ions are not. They are subject to various micro- and macro-instabilities that differ significantly from instabilities in fusion plasmas. These instabilities are often triggered by the large difference in electron and ion drift velocities in the E×B direction. The possibility of maintaining a large electric field in the quasineutral plasma of Hall thrusters despite anomalous electron transport, or the presence of strong double layers associated with the azimuthal rotation of plasma structures (“rotating spokes”) in magnetron discharges and Hall thrusters are examples of the very challenging and exciting physics of E×B devices. The turbulence and instabilities present in E×B plasma devices constitute a major obstacle to the quantitative description of these devices and to the development of predictive codes and are the subject of intense research efforts. In this tutorial, we discuss the key aspects of the physics of low-temperature partially magnetized E×B plasmas, as well as recent advances made through simulations, theory, and experiments in our understanding of the various types of instabilities (such as gradient-drift/Simon-Hoh and lower hybrid instabilities, rotating ionization waves, electron cyclotron drift instability, modified two-stream instability, etc.) that occur in these plasmas.

Design of a large nonevaporable getter pump for the full size ITER beam source prototype

An improvement for the vacuum system of the multidriver radio frequency (RF) prototype negative ion source SPIDER has been developed, to allow operating at high RF power, while minimizing the breakdown probability on the outside of the plasma source. A first-of-its-kind large nonevaporable getter (NEG) pump, based on a modular design of 384 cartridges totaling about 350 kg of ZAO® alloy (composed by Zr-Ti-V-Al) with an installed pumping speed at a room temperature of 330 m3/s for hydrogen, will complement the existing pumping system, based on eight cryogenic pumps and four turbomolecular pumps totaling about 90 m3/s in hydrogen. The vessel pressure during absorption is expected to be between 20 and 40 mPa, while during the getter regeneration, the peak operating pressure will be around 2 Pa. The NEG pump will use an additional vessel module, with integrated thermal shields to protect the in-vessel components during activation and regeneration of the pump, to be carried out at temperatures in the range of 550–600 °C. Integrated thermal analyses were carried out to verify the low heat load on pre-existing in-vessel components with a low limit of acceptable temperature, and to provide boundary conditions for the mechanical verifications of the pump structure. The scenario of cyclic hydrogen load was calculated considering the SPIDER operation modes, the expected gas throughput, and the cumulated load to the pump, to determine the regeneration temperature and auxiliary pumping necessary to make the regeneration duration compatible with the high availability of the system. The upgrade of the auxiliary pumping system is also described, as well as the mitigation of air or water exposure risk during regeneration of the NEG pump.

Study of positive ion transport to the plasma electrode in giant RF negative ion sources

Negative ion sources are fundamental components of neutral beam injectors (NBI), one of the main heating systems for fusion reactors. SPIDER is the full-scale prototype negative ion source for ITER NBIs. It is hosted in Padua as part of the Neutral Beam Test Facility (NBTF). It aims to extract up to 330Am−2 of negative hydrogen ions from an inductively coupled plasma, generated inside 8 cylindrical drivers. The negative ion production is enhanced by caesium evaporation inside the source.In caesium-seeded negative ion sources, negative ions are produced close to the extraction apertures, and they are mainly generated by surface conversion of neutral atoms and positive ions impinging on the ion source walls, particularly on the plasma grid. The conversion yields depend on the energy distribution of these precursors, and so does the energy of those particles which are reflected as negative ions. The positive ion flow in the extraction region may also impact on the extraction probability of negative ions, via momentum transfer. Besides, in giant multi-driver RF sources such as SPIDER, a gradient of plasma potential is present in the expansion region Sartori et al. (2021), affecting the positive ion transport towards the caesiated plasma electrode and their energy.To approach this complex problem, a 3D test-particle Monte Carlo code for tracing plasma motion in SPIDER was developed. Positive ions species are generated in different positions within the plasma source volume and are tracked under the influence of electric and magnetic fields. Then, Monte Carlo collisions are used to simulate the interaction with predetermined backgrounds of plasma and neutrals, with profiles derived from experimental data. The particles are traced until they hit the ion source walls. Finally, the energy distribution of the different particle species impinging on the plasma grid (PG) are determined, and used to assess the generation and the energy distribution of the produced H−.