Negative Ion Source Research Articles

Analysis and optimization of cesium dynamics for the HUST negative ion source

The high-density beam in the NBI negative ion source relies on surface production on the converter with low work function, which is facilitated by cesium injection. Maintaining the appropriate cesium conditions during the source operation sequence is essential for achieving optimal cesium coverage on the converter. To better understand cesium cycle in the source and to enhance beam production performance during operation sequences, analysis of cesium dynamics and optimization of cesium operation have been carried out. The analysis model is established based on the Monte-Carlo method with cesium sources, cesium transport and surface process during the operation sequence considered. Cesium distribution and converter surface coverage can be obtained from the model using input parameters from the HUST negative ion source. For the operation in the HUST negative ion source, the influence of temperature settings and time sequence is discussed, with suggested parameters for cesium injection proposed to achieve appropriate cesium coverage on converter surface through operation sequences.

Comprehensive research facility for negative ion source neutral beam injection at CRAFT: design and first operations

Abstract Neutral beam injection (NBI) has been proven as a reliable heating and current drive method for fusion plasma. For the high-energy NBI system (particle energy > 150 keV) of large-scale fusion devices, the negative ion source neutral beam injection (NNBI) system is inevitable, which can obtain an acceptable neutralization efficiency (> 55%). But the NNBI system is very complex and challengeable. To explore and master the key NNBI technology for future fusion reactor in China, an NNBI test facility is under development in the framework of the Comprehensive Research Facility for Fusion Technology (CRAFT). The initial goal of CRAFT NNBI facility is to achieve a 2 MW hydrogen neutral beam at the energy of 200–400 keV for lasting 100 s. In the first operations of the CRAFT NNBI facility, a negative ion source with dual RF drivers was developed and tested. By using the 50 keV accelerator, the long-pulse and high-current extractions of negative hydrogen ions have been achieved and the typical values were 55.4 keV, 7.3 A (~ 123 A/m2), 105 s and 55.0 keV, 14.7 A (~ 248 A/m2), 30 s, respectively. By using the 200 keV accelerator, the megawatt-class negative hydrogen beam has also been achieved (135.9 keV, 8.9 A, 8 s). The whole process of the gas neutralization of negative ion beam, electric removal of residual ions, and beam transport have been demonstrated experimentally.

A novel plasma source concept for negative ion generation in neutral beam injectors for fusion applications

Abstract A low-temperature plasma can very well be confined by a simple magnetic dipole, such as in the Van Allen belts of Earth’s magnetoshpere. This configuration can be reproduced in laboratory as a small experimental device, designed in such a way that the magnetic field lines remain within a vacuum-tight container and are virtually not intercepted by the container wall. In this paper we propose to use a dipole field for the realization of an efficient negative Ion source. To this purpose, we analyze the plasma confinement capabilities of such plasma source, in order to assess the equilibrium pressure and estimate the particle trajectories and drifts.

A new attempt to further reduce the work function of the cesiated surface: The strongest electronegative element fluorine co-adsorption with cesium on Mo (001) surface

The efficiency of hydrogen negative ion production in negative ion sources depends on the work function of the molybdenum surface. Deposition of cesium (Cs) atoms on the Mo surface reduces the work function from over 4.0 eV to about 1.50–1.80 eV. However, the Fermi level remains below the affinity level (−0.75 eV) of hydrogen negative ions. To explore further reduction methods, the work function of Cs co-adsorbed with fluorine on the Mo (001) surface was studied using DFT calculations. The results show that at 4/16 θ Cs coverage, adding fluorine can reduce the work function, achieving the lowest value of 1.40 eV at 8/16 θ F coverage. Analysis reveals a significant vacuum-inward dipole moment with fluorine-Cs co-adsorption, favorably reducing the work function. However, fluorine’s strong electronegativity also creates a surface-directed dipole moment, increasing the work function. Introducing fluorine is beneficial only at 4/16 θ Cs coverage, indicating that precise control of Cs coverage is necessary to achieve lower work functions.

Experimental study on plasma characteristics in the extraction region of a high-power RF negative ion source based on electrostatic probe

Abstract The plasma characteristics of the extraction region in high-power RF negative ion source have a significant impact on the production and extraction of negative hydrogen ions. This study utilized electrostatic probe to investigate the effect of RF power, source pressure, magnetic filter field and bias voltage on the plasma parameters of the extraction region (without cesium), and also studied the variation of plasma parameters with position and time. The results indicate that as RF power increases, the plasma density in the extraction region significantly rises, but it also leads to an increase in the electron temperature ( T e ) of the extraction region (which increases the loss of negative hydrogen ions); increasing the source pressure can effectively increase the electron density ( N e ) in the extraction region and reduce T e as expected; increasing the magnetic filter field can effectively reduce the T e in the extraction region, but after the plasma grid current exceeds 1900 A, the change in T e is not significant; although increasement of the bias voltage can effectively suppress the N e in the extraction region, it causes T e to rise; moreover, the distribution of plasma parameters in the extraction region is relatively uniform; in addition, the variation of plasma parameters with time is not obvious. These findings not only deepen the understanding of the physical mechanisms of plasma behavior in the extraction region of negative ion sources, but also provide important theoretical and experimental bases for the optimization of high-power RF negative ion sources.

Development and Insulation Performance Evaluation of Experimental Sample for a Direct Current High-Voltage Transmission Line

For the negative-ion-based neutral beam injection system, the direct current (DC) high-voltage transmission line (HVTL) is the link between the radio frequency (RF) negative-ion source system and the power supply system, which not only realizes the function of the power transmission between the power supply system and the RF negative-ion source system, but also provides transmission channels for high-pressure cooling water, working gas, and the measurement and control signals needed for the operation of the RF negative-ion sources. In this study, the experimental sample for the DC HVTL is developed based on the insulation simulation design, and an insulation performance evaluation test bed of the experimental sample is designed and built. The insulation performances of the experimental sample at different SF6 gas pressures are investigated, and the leakage current laws of the experimental sample at different applied voltages and different SF6 gas pressures are obtained. The test results show that the maximum leakage current is 472 μA at a loading voltage of 500 kV, which proves that the experimental sample for the DC HVTL satisfies the requirements of the insulation design.

Optimization of a large-area grid electrode for negative ion source in fusion neutral beam injector

At present, the CRAFT (Comprehensive Research Facility for Fusion Technology) dual-driver negative ion source for neutral beam injection applications has made progress. In order to solve the problem of heat removal function in large-area extraction grid, the multi-physics simulation, manufacturing and testing are carried out. Two simulation conditions before Cs seeding (Je∼350 A/m2, JH-∼50 A/m2) and after sufficient Cs conditioning (Je∼150 A/m2, JH-∼250 A/m2) were considered to evaluate the power load on EG, which peak power density were 11 and 6 MW/m2 respectively and were concentrated on locate near the two aperture rows. This paper mainly studies three designs of minor cooling channels (double bending channels, double straight channels, and single straight channel). First, the grid temperature, thermal stress and thermal deformation are compared with an inlet water rate of 1.21 kg/s at 35 °C, and the channel forming methods and welding types mechanisms are compared. The simulation results show that the thermos-mechanical property of double bending channels is the best, and that of double straight channels is slightly worse. In terms of processing and manufacturing, the yield of double bending channels is lower than double straight channels, and it is easy to leak during the experiment.

Determination of warfarin in volumetric absorptive microsampling by liquid chromatography-tandem mass spectrometry

ObjectiveThis study aims to develop and validate bioanalytical method for quantifying warfarin in VAMS samples using liquid chromatography tandem mass spectrometry (LC-MS/MS), directly implementing the method to patients receiving warfarin therapy. MethodsThe UPLC-MS/MS method was developed and optimized, with quercetin as the internal standard. Sample preparation was carried out using protein precipitation with methanol-acetonitrile (1:3 v/v). ResultsChromatographic separation was achieved using Acquity® UPLC BEH C18 column with 0.1 % formic acid-acetonitrile-methanol (30:69:1 v/v) as mobile phase, in isocratic elution. Multiple Reaction Monitoring (MRM) detection was done using m/z values of 307.10 → 161.06 for warfarin and 301.03 → 150.98 for quercetin as internal standard, using Electrospray Ionization (ESI) negative ion source. The clinical application of the bioanalytical method was carried out on 25 patients receiving warfarin therapy at Universitas Indonesia Hospital and warfarin levels were well within the calibration range from 6.05 to 431.39 ng/mL. ConclusionA novel method has been developed to analyze warfarin in VAMS samples. This method has been fully validated according to guideline from FDA 2022 and is linear in the range of 5–500 ng/mL and the value of r ≥ 0.9977, and successfully applied for the analysis of warfarin in VAMS samples of clinical patients.

Corrosion phenomena and deposits in ITER Neutral Beam Test Facility primary cooling circuits

The ITER Neutral Beam Test Facility (NBTF) is hosted in Padua and includes two experiments: MITICA, the 1 MeV full-scale prototype of the ITER HNB injector, and SPIDER, the 100 keV full-size ITER Radio Frequency (RF) negative ion source. SPIDER and MITICA experiments are actively cooled by Ultrapure Water (UPW) to electrically insulate in-vessel components that are biased to high voltage levels. Water conductivity is an important monitored parameter to ensure components' insulation by limiting the leakage current of active cooled equipment. A very low conductivity water is especially important in MITICA, where components need to operate up to 1 MVdc, an insulation level beyond the actual industrial standard. Careful selection of suitable materials for any in-vessel (vacuum insulation) and out-vessel component (air insulation) is of utmost importance, as their interaction with water and different environment may affect their chemical and mechanical properties. Additionally, materials selection can severely influence water chemical characteristics: cooling circuits are made of metals (copper, aluminium, steel) and insulating materials (plastic, rubber) when connecting parts at different electric potentials.During the first years of cooling plant exploitation, it was shown that water degrades more quickly than estimated by design. The Primary Circuits (PCs) that showed the most severe water degradation during operation are SPIDER and MITICA power supply ones, respectively called PC01 and PC08. This paper describes the results of specific experimental tests performed on MITICA PC08 to evaluate possible causes of water degradation and detect sources of contaminants that might compromise future experimental campaigns. The cooling circuit was subdivided in different sections and water circulation tests were performed at constant temperature and flowrate. Water samples were collected and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) analyses were performed to quantify the type and amount of metals released. Moreover, components samples collected along their cooling circuits were characterized by Scanning Electron Microscope (SEM) technique to detect undesired contaminants and immersed in UPW to study their corrosion behaviour using metal release tests.

A calorimetric evaluation method for beam targets with IR imaging: Implementation for the negative ion source BATMAN Upgrade

A comprehensive calorimetric evaluation method is presented for the beam target installed in the testbed BATMAN Upgrade (BUG). BUG hosts the prototype RF-driven negative ion source for the ITER Neutral Beam Heating System. In nominal conditions BUG features an array of negative ion beamlets (5 horizontal × 14 vertical) with 20 mm spacing among them. Each beamlet is accelerated electrostatically up to a maximum of 45 kV and transports currents in the range from 0.01 to 0.05 A. BUG has two beam targets at different distances from the beam acceleration system. The furthest (2.08 m downstream) is a water-cooled diagnostic CW-calorimeter, which can measure the heat flux distribution of the beam with a resolution of 20 × 20 mm² over a surface of 600 (H) × 800 (V) mm² and water calorimetry. The closest (0.86 m downstream) beam target with sub-millimetric resolution consists of a (376 × 142 × 20 mm³) tile made of a composite anisotropic material with carbon fibers oriented in the thickness direction (1D-CFC). It can only be exposed to beams for limited time, therefore it is retractable. Infrared (IR) imaging of beam targets yields time-resolved high-resolution spatial temperature footprints. A simple algorithm is proposed to reconstruct the heat flux at the front of a beam target, from the IR footprints at the rear. The reconstructed heat flux profiles are lower and broader due to heat conduction effects in the beam target. A correction algorithm is proposed through simulation-based inference of 2D axisymmetrical FEM simulations that reduces the power and shape fit errors over an order of magnitude, to a few percent. The calorimetric evaluation method is applied to experimental shots and two advanced heat flux distribution analysis are shown. The most relevant error sources are discussed and quantified to assess systematic errors. Finally, the method is validated in BUG comparing total beam power estimations with the two calorimetric diagnostics from the CW-calorimeter, showing good agreement.

Towards ITER-Relevant CW Extraction at Negative Ion Sources for Fusion

Negative hydrogen or deuterium ion sources for neutral beam injection (NBI) systems used at fusion devices are based on the surface production process at a caesiated low work function converter surface. While producing a stable and globally homogeneous negative ion beam is not an issue, during long pulses typically a pronounced increase in the co-extracted electrons is observed, limiting the pulse length or the achievable performance. This effect is particularly pronounced in deuterium and it is attributed to an increasing work function of the converter surface. In the last years the negative ion source test facilities at IPP Garching, BATMAN Upgrade (using the small prototype source) and ELISE (using a source of the same width but only half the height of the ITER NBI source) have been converted into CW machines, making possible investigating at ITER conditions counter-measures for the increase in the co-extracted electrons. Investigations are performed, mainly at ELISE, on homogenizing and stabilizing the co-extracted electrons by affecting the ion source plasma close to the converter surface by means of biasing (additional) surfaces in the plasma.

Preface

Proceedings of the 20th International Conference on Ion Sources, Victoria, BC, Canada, 2023ICIS’23 was the 20th event in a biennial series of conferences that are dedicated to Ion Sources and their applications, and the first in-person conference of this series after the COVID 19 pandemic. The conference was hosted by TRIUMF (www.triumf.ca), Canada’s Particle Accelerator Centre in Victoria, BC, Canada, September 17 to September 22, 2023. The ICIS series covers the physics and technology of ion sources for scientific research and applications. With the conference being the main event within the ion sources community it offers a forum for exchanging of new ideas and establishing of collaborations.The conference was held at the Victoria Conference Centre in Victoria BC, Canada, with support from the adjacent Fairmont Empress hotel. The venue is located on the traditional, ancestral, and unceded territory of the Lkwungen People, also known as the Songhees and Esquimalt First Nations communities.The biennial conference series covers the physics and technology of ion sources for scientific research and applications. New developments and advancements have been presented and discussed. As in previous conferences of this series, all aspects of ion sources have been covered in the categories:• Fundamental processes in ion sources and plasmas• Production of high intensity ion beams• Production of highly charged ion beams• Negative ion sources• Ion sources for fusion• Radioactive ion sources, charge breeders and polarized beams• Beam formation, extraction, transport, and diagnostics• Applications of ion sources• Key technologies for ion sourcesThe conference was organized in a hybrid mode and a zoom link was made available to participants, who could not attend in person. A total of 178 participants have registered for the conference, of whom 20 were students and 23 attended from remote. Fifty contributions have been presented in plenary sessions of which 20 were invited by the international advisory committee. The remaining 30 have been selected by the Scientific advisory committee. 123 contributions have been presented at 2 poster sessions. The high level and interesting contributions triggered lively discussions during all sessions. Thank you to all who contributed to this.Additional events included a tour of the TRIUMF Laboratory on September 17 just before the conference, and a reception on the same evening. In the afternoon of Wednesday, September 20 delegates were able to enjoy the beautiful surrounding Vancouver Island by either participating in a whale watching tour or a nature hike along the shoreline. In the evening of this day the conference banquet was held in the Fairmont Empress hotel.Fifteen students participated in the student poster competition. Members of both the international advisory committee and the scientific program committee acted as judges. Thank you to all the students who have contributed and the judges. Three winners have been awarded.List of International advisory committee, Scientific program committee, Local organizing committee are available in this pdf.

Plasma homogeneity over the extraction beamlet groups at the half size ITER negative ion source at ELISE test facility

The negative ion source for neutral beam injection at ITER has to provide high-intensity and low-divergence negative hydrogen and deuterium ion beams. Extracting the negative ions from the plasma is inevitably accompanied by co-extraction of electrons, which can limit the source performance, especially in deuterium. Reducing the co-extracted electrons is done by applying a magnetic filter field. On the one hand, the filter field reduces the electron temperature and the amount of co-extracted electrons, on the other hand, it introduces × B→ drifts. The drifts create a vertical plasma inhomogeneity in front of the extraction area and, consequently, an asymmetry of the co-extracted electrons. This motivated detailed studies on the vertical plasma inhomogeneity over the beamlet groups in the ELISE ion source using a movable Langmuir probe in deuterium plasma. It is demonstrated that the vertical distribution of the plasma potential changes together with the sheath, forming at the plasma grid – from electron repelling to attracting sheath. A repelling sheath reduces the flux of electrons from the plasma to the grid surface and consequently, more electrons are co-extracted. The attracting sheath collects the majority of electrons on the grid and reduces the co-extracted electrons.

HVE ion sources for medium and high-energy accelerator systems

Since decades, High Voltage Engineering (HVE) manufactures particle accelerator systems for research and industry. HVE’s product line includes Singletron and Tandetron accelerator systems with terminal voltages of up to 6 MV. They are dedicated to a wide range of applications including ion implantation and irradiation, Ion Beam Analysis (IBA), Accelerator Mass Spectrometry (AMS), and neutron reference fields. In this paper, we give an overview of the different positive and negative ion sources that are applied in these systems. We focus especially on the recent development and the performance of a compact 2.45 GHz permanent magnet ECR bipolar ion source (HVE Model SO-160), used for negative light-ion injection into tandem accelerators. It generates high-current, low-emittance light-ion beams at 30 keV energy. The novelty in the design of this ion source is that it combines direct negative extraction for H − with positive extraction for He + that is followed by charge exchange to He − in a Na-based electron donor canal. The source produces in excess of 250 eµA of H − and more than 20 eµA of He −. It is expected the design allows for substantially higher currents, especially for H −.

Beam divergence of RF negative hydrogen ion sources for fusion

Neutral beam injectors (NBI) for fusion facilities have strict requirements on the beam divergence (7 mrad for the ITER NBI at 1 MeV). Measurements of the single beamlet divergence of RF negative ion sources (at lower beam energy < 100 keV) show significantly higher values (9–15 mrad), also larger than filament arc sources at similar beam energies. This opened up questions whether the higher divergence is caused by different measurement or evaluation techniques, whether it is a direct cause of the RF source, e.g. due to a higher temperature of negative ions or an oscillating extraction meniscus, and whether it is a problem at all after full acceleration. In a joint effort between the labs modeling and diagnostic capabilities at the NNBI test facilities have been strongly extended and evaluation methods benchmarked. Particularly challenging is the strong increase in beamlet divergence at a lower filling pressure, seen both in filament arc and RF sources.Beside the source and beam investigations carried out in SPIDER (with selected, isolated apertures rather than the total of 1280 apertures) at Consorzio RFX, the IPP test facilities ELISE (640 apertures) and BATMAN Upgrade (70 apertures) contribute to the physics understanding of the beam optics in RF sources. The determination of the beam divergence is not straight-forward because effects originating from measuring the divergence of multiple beamlets (Beam Emission Spectroscopy) and/or constraints from the individual diagnostic (lateral heat conductance in CFC tiles) lead to difficulties. Still, the divergence requirement is not met at the limited total beam energy available at ELISE and BATMAN Upgrade (< 60kV). However, variation of the beam energy show a decrease of the divergence for higher energies and beam simulation for the ITER NBI accelerator predict that the divergence requirement will be met after full acceleration of the negative ion beam.

Simple 3D PIC Analysis for Beam Phase Space Oscillation in RF Driven Negative Hydrogen Ion Source

Temporal oscillation of the negative hydrogen ion (H−) beam in the Radio Frequency (RF) ion source is investigated by a simple 3D3V Particle-In-Cell (PIC) model. The model solves electron, proton, and H− transport processes in the extraction region. The calculation domain is from the vicinity of the beam aperture to the ground electrode. To understand the relation between the plasma density oscillation and the extracted H− beam characteristics, the input electron and proton fluxes from the driver region are varied parametrically with the fundamental J-PARC RF frequency (2 MHz). The numerical results give an idea of the main physical processes between the oscillations of the plasma parameters and the extracted H− ion trajectories depending on the plasma meniscus shape in the different RF phases.

Determining absolute VUV fluxes for assessing the relevance of photon-surface interaction in ion sources

A portable device was developed to quantify VUV fluxes flexibly at ion source setups. It consists of a VUV sensitive photodiode and optical filters for wavelength selection and is calibrated against a VUV spectrometer down to 46 nm for a variety of discharge gases, including Ar, N2, O2 and H2. It was applied to the negative hydrogen ion source at BATMAN Upgrade to quantify the VUV radiation emitted by the driver as well as in front of the extraction surface (plasma grid, PG). The combined VUV fluxes impinging on the PG with photon energies larger than 6.6 eV has a comparable magnitude as the ion flux. It could be shown that the recently confirmed influence of the ion source plasma on the surface work function of the PG can at least partly be ascribed to the VUV radiation from the driver and that photo-emitted electrons from the PG should not play a role in the sheath physics.

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.

TRIUMF’s H-/D- Ion Source Development to Date

The TRIUMF Stable Ion Source group has been developing negative and positive ion sources for decades, including a few arc-discharge H-/D- ion sources and a microwave-driven H-/D- ion source for medical cyclotrons [1] and other applications. The smallest ion source with a 125 cc plasma chamber can produce up to 5 mA continuously. The largest ion source with a 1200 cc plasma chamber is able to produce 60 mA with increased arc power and enhanced magnetic confinement. The filament-less microwave ion source is capable of producing up to 5 mA H- current for years without any manual intervention. A historical overview of H-/D- source development at TRIUMF is presented. A summary of employed optical and diagnostics components is also presented

Evaluation of H-/D- Density Using Langmuir Probe Measurement in a Cs Seeded Negative Ion Source

An electron reduction model is reintroduced for Langmuir probe plasma density profile measurement. The model is compared with conventional laser photo-detachment measurements and can predict negative ion density in the 2-3 x 1017m−3 range using correlation factors for hydrogen and deuterium cases. The calibration and correction procedure is demonstrated for application to ion sources.