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The Korea Atomic Energy Research Institute (KAERI) is developing a novel Two-Region Arc Plasma Ion Source (TRAP) as a negative hydrogen (deuterium) ion source for a Neutral Beam Injection (NBI) system in a fusion tokamak. The TRAP ion source is based on a two-region configuration, comprising a high energy electron region that creates highly vibrationally excited molecules and a low electron temperature region that generates negative ions by attaching electrons to molecules. This configuration can be achieved by optimizing the filament position and magnetic cusp field. In order to optimize the TRAP configuration, the plasma parameters are investigated under various operating conditions, such as filament position, gas pressure, and arc power. Electron density and temperature are determined using Langmuir probe measurements. In this paper, the detailed experimental results are described and discussed.

[Introduction] High energy, strong current, and long pulse neutral beam negative ion source technology are one of the core technologies for achieving combustion conditions in magnetic confinement fusion reactor plasma. To meet the beam diagnostic requirements of the long pulse high-power negative ion source experimental platform under the parameters of 200 kV/20 A, a calorimeter target has been developed for intercepting negative ion beams or neutral beams, diagnosing the power density distribution and beam divergence angle of the two beams, as well as the neutralization efficiency of negative ions and other performance parameters. [Method] Based on the existing structure of the experimental platform vacuum chamber and the size of the extraction electrode, the Matlab program was used to obtain the power density distribution and beam spot size of the negative ion beam generated by the negative ion source at the front end of the calorimeter target at a beam divergence angle of 1° under this parameter. Subsequently, the physical structure of the calorimeter target with a V-shaped target plate was designed. On this basis, Workbench software was used to simulate the thermal load of the oxygen-free copper V-shaped target plate structure under full power operation and obtain the temperature distribution of the calorimeter target during long pulse operation with a water flow rate of 80 m3/h. The highest temperature was 610 °C. [Results] Based on the simulation calculation results, and in combination with the engineering structure and diagnostic requirements of the experimental platform, the engineering design of the calorimeter target was completed. [Conclusion] The calorimeter target adopts magnetic fluid vacuum sealing to achieve the opening and closing of the V-shaped target plate, and a thermocouple array is arranged on the back of the target plate to monitor the temperature of the target plate in real time. The calorimeter target engineering has a compact structure, and its installation size can be compatible with ion beam diagnostic vacuum chambers and neutral beam diagnostic vacuum chambers, meeting diagnostic requirements and enabling safe operation with long pulses.

To realize an initial objective of the negative ion-based neutral beam injection (N-NBI) at the Comprehensive Research Facility for Fusion Technology (CRAFT) test facility, which targets an H0 beam power of 2 MW at an energy of 200–400 keV and a pulse duration of 100 s, it is crucial to study the cesium dynamics of the negative ion source. Here a numerical simulation program CSFC3D is developed and applied to simulate the distribution and time dynamics of cesium during short pulses. The calculations show that most of the cesium on the plasma grid (PG) area originates from the release of cesium that is accumulated within the ion source in the plasma phase. Increasing the wall temperature reduces the loss of cesium on the wall of the ion source. Furthermore, the thickness of the cesium monolayer is directly influenced by the PG temperature. Both simulated and experimental results demonstrate that maintaining the PG temperature between 180 °C and 200 °C is essential for enhancing the performance of the ion source and optimizing the cesium behavior.

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.

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As an effective methods of plasma heating, neutral beam injection (NBI) systems based on negative hydrogen ion sources will be utilized in future magnetic-confinement nuclear fusion experiments. Because of the collisions between the fast negative ions and the neutral background gas, the positive ions are inevitable created in the acceleration region in the negative NBI system. These positive ions are accelerated back into the ion source and become high energy backstreaming ions. In order to explore the characters of backstreaming ions, the track and power deposition of backstreaming H+ beam is estimated using the experimental and simulation methods at NNBI test facility. Results show that the flux of backstreaming positive ions is 1.93 % of that of negative ion extraction from ion source, and the magnet filed in the beam source has an effect on the backstreaming positive ions propagation.

Based on the DFT method, the effects of copper and tungsten impurities present in the negative ion source of neutral beams on the cesiated surface were studied, including their effects on the adsorption energy of cesium on the surface and the surface work function. The results indicate that copper impurities significantly increase the average adsorption energy of cesium, whereas tungsten has limited enhancement on the average adsorption energy of cesium and may even reduce it. The work function calculations show that, at cesium coverages below 4/16 θ, copper impurities cause a significant increase in the work function. However, at cesium coverages above 6/16 θ, high-coverage copper impurities lead to a further decrease in the work function, causing the cesium coverage corresponding to the lowest work function to shift towards higher cesium coverage. Under any tungsten impurity and cesium coverage, tungsten impurities can significantly increase the surface work function, with the maximum increase reaching 0.50 eV. The dipole moment density analysis shows that in most cases, impurities significantly reduce the dipole moment density of the cesiated surface, except when the coverage of cesium and copper impurities is above 6/16 θ. The charge transfer results show that the copper impurity layer has more positive charge compared to the tungsten impurity layer, which significantly affects the dipole moment density of the surface system. In addition, adsorbed atoms cause electrons in the molybdenum atomic layer to migrate to the surface, resulting in the molybdenum substrate having a pronounced negative dipole moment.

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. Highlights: • A simple 1D calculation is used to predict the pressure gradient in a high-power negative ion source accelerator. • The calculation has the primary pressure drop occurring in the aperture, whereas experiments show that it occurs mainly in the grid gaps. • The pressure decreases linearly when the pumping speed is neglected, whereas experiments indicate that it actually decreases exponentially. [ABSTRACT FROM AUTHOR]

Comprehensive Research Facility for Fusion Technology (CRAFT) is an integration of different demonstrating or testing facilities, which aim to develop the critical technology or composition system towards the fusion reactor. Due to the importance and challenge of the negative ion based neutral beam injection (NNBI), a NNBI test facility is included in the framework of CRAFT. The initial object of CRAFT NNBI test facility is to obtain a H0 beam power of 2 MW at the energy of 200–400 keV for the pulse duration of 100 s. Inside the negative ion accelerator of NNBI system, the interactions of the negative ions with the background gas and electrodes can generate abundant stray electrons. The stray electrons can be further accelerated and dumped on the electrodes or eject from the accelerator. The stray electrons, including the ejecting electrons, cause the unwanted particle and heat flux onto the electrodes and the inner components of beamline (especially the temperature sensitive cryopump). The suppression of the stray electrons from the CRAFT accelerator is carried out via a series of design and simulation works. The paper focuses the influence of different magnetic field configurations on the stray electrons and the character of the ejecting electrons.

BackgroundNegative ion sources driven by radio frequency (RF) waves have become the preferred solution for future neutral beam injection systems.PurposeThis study aims to monitor the plasma discharge state of each exciter of a high power RF negative ion source by developing a plasma luminescence monitoring system based on a photodiode is designed and constructed.MethodsThe intensity of plasma emission was closely related to the number of specific collisions, collision particle density, and collision particle energy. Therefore, the intensity of plasma emission was applied to monitoring plasma parameters qualitatively, and a photodiode-based multichannel plasma luminescence monitoring system was designed and implemented to monitor the plasma discharge state of each exciter of a high power RF negative ion source. Based on a reasonable collision radiation model, plasma parameters were quantitatively obtained by analyzing the intensity of plasma characteristic spectral lines, and the influence of the filtering magnetic field generated by plasma current on plasma emission signals as experimental tested.ResultsExperimental results this monitoring system demonstrate that the real-time intensity information of plasma emission from different positions is successfully collected, and subsequently presented and saved in the form of voltage signals for real-time monitoring and post-data processing by the host computer. The intensity of plasma emission has a good linearity with RF discharge power.ConclusionsThe plasma light monitoring system of this study can accurately and real-time measure the excitation, maintenance, and extinction processes of plasma in the RF exciter.

The production of H− ions in negative ion sources relevant for particle accelerator facilities and neutral beam injection systems is based predominantly on the surface conversion of H atoms at a low work function surface covered with caesium (the plasma grid (PG)). Therefore, the H atom density n H and energy distribution function (EDF) close to the PG determine the amount of surface produced H− ions. As a direct method for the density and EDF determination, two-photon absorption laser induced fluorescence (TALIF) on H atoms was implemented at the ion source of the teststand BATMAN Upgrade (BUG) being the first time that this was accomplished at an H− ion source. Several challenges had to be overcome concerning the application of the diagnostic at the complex facility and the evaluation of the fluorescence signals against a bright H α background. The observed line profiles suggest a Maxwellian EDF with an H atom temperature of (2000 ± 500) K. The presence of highly energetic H atoms (measured by optical emission spectroscopy, (OES)) could not be resolved by the TALIF system due to the insufficient signal-to-noise ratio. Atomic densities were measured for H2 and D2 plasmas for varying ion source parameters at BUG resulting in values between 3 × 10 18 m−3 and 1.1 × 10 19 m−3 for hydrogen. For the operation with deuterium, 30 % higher atomic densities are observed for similar ion source parameters which agree well with the previous results obtained with OES. [ABSTRACT FROM AUTHOR]

One of the requirements of Heating and current drive Neutral Beam injectors for ITER is a beam homogeneity greater than 90%, to achieve an optimal beam transmission while keeping the heat load consistently low on the acceleration electrodes. The large size and complexity of ITER negative ion source play a key role in determining the homogeneity of the negative ion current of each of the 1280 beamlets and their divergence, and it is studied in the full-scale prototype source SPIDER. In this work the plasma properties are studied by spectroscopic and electrostatic measurements in the drivers, where the plasma is generated, and in the expansion region, where the plasma drifts and negative ions are produced, and they are correlated with the properties of the beam. The non-homogeneous plasma density profile is related to the non-homogeneous availability of negative ions along the beam vertical profile, with and without cesium evaporation. Visible tomography, a technique capable of characterizing isolated beamlet properties, is used to study the beam’s dependence on plasma uniformity along the entire beam profile. Using these tools, it has been demonstrated how an increase in plasma density is linked to an improvement in beam homogeneity. The latter has been directly correlated with plasma homogeneity. The magnetic filter field and biases of the plasma grid and bias plate are responsible for the variation in plasma density and its homogeneity. Non-uniformities in the plasma’s top/bottom and left/right distributions have been studied and partially addressed experimentally. The first issue was resolved by adjusting the radio-frequency power supplied to the plasma in different vertical regions, while the second issue was addressed by reversing the direction of the magnetic filter field and increasing the plasma density.

We studied the work function of cesium deposition and co-deposition with the electronegative element on the plasma grid (PG) using the first-principles calculations. The impurity particles may exist in the background plasma and vacuum chamber wall, and the work function of the PG will be affected. The results indicate that the minimum work functions of pure cesium deposition on Mo (110), W (110), and Mo (112) are reached at a partial monolayer. They are 1.66 eV (σ = 0.56 θ), 1.69 eV (σ = 0.75 θ), and 1.75 eV (σ = 0.88 θ), respectively. An appropriate co-deposition model consisting of cesium with electronegative elements can further decrease the work function. The coverage of cesium and electronegative elements are both 0.34 θ in all the co-deposition models. The F-Cs co-deposition model where the Cs atom and F atom are aligned along the surface normal obtains the lowest work function. They are 1.31 eV for F-Cs on Mo (110), and 1.23 eV for F-Cs on W (110), respectively. The change in work function is linearly related to the change in dipole moment density with a slope of −167.03 VÅ. For pure cesium deposition, two factors control the change in dipole-moment density, one is the electron transfer between adsorbates and the substrate, and another one is the restructuring of surface atoms. There are two additional factors for the co-deposition model. One is the intrinsic dipole moment of the double layer, the other is the angle between the intrinsic dipole moment and the surface. The latter two factors play important roles in increasing the total dipole moment.

It has been proposed that the negative ion yield of a caesium sputter ion source could be enhanced by promoting neutral caesium atoms to electronically excited 7p states supporting resonant ion pair production. We have tested this hypothesis by illuminating the cathode of a caesium sputter ion source with an adjustable wavelength laser and measuring its effect on the extracted beam currents of Oâˆ', Hâˆ' and Brâˆ' anions. The laser exposure causes the beam currents to increase but the effect is independent of the wavelength in the range of 440â€"460 nm, which leads us to conclude that there is no evidence for resonant ion pair production. The photon-induced beam current enhancement scales with the applied laser power and, depending on the ion source conditions, can more than double the extracted beam current. We present a qualitative explanation for the observed effect. The model, based on photoelectron emission and subsequent increase of the caesium sputtering rate, thus liberating negative ions from the cathode, is supported by the data demonstrating that the caesium sputter ion source can produce Brâˆ' beams without thermal surface ionization as a source of Cs+ ions, i.e. in external laser-driven mode without heating the surface ionizer. [ABSTRACT FROM AUTHOR]

The RF-driven negative ion source has a steady state characteristic and will be a promising ion source for neutral beam injection (NBI) in the future. Cesium (Cs) injection is an efficient method to enhance the yield of negative ions during plasma discharge. In order to support the engineering and physical research in this field, the Cs oven prototype has been developed for negative ion source based neutral beam injector at ASIPP. This article presents the design details of a Cs oven system, including the constant temperature control system, the mechanical structure of Cs oven, and the surface ionization detector (SID). SID is a measurement method for the cesium flux in the nozzle. The experiment results of constant temperature control system show that the control accuracy and function meet the requirements of device operation. Meanwhile, the simulation analysis of Cs vapor concentration has been carried out in this paper. According to the simulation results, the graph of total Cs flux is given in the article, which presents the reference for the subsequent device testing. [ABSTRACT FROM AUTHOR]

• Simple heat flux reconstruction algorithm from IR data • Minimization of errors through simulation-based inference • Beam optic studies of single or multi-beamlet beams • Calorimetric full beam transmission studies • Comparison with other available calorimetric diagnostics 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. [ABSTRACT FROM AUTHOR]

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.

The SPIDER experiment, hosted in the Neutral Beam Test Facility of Padua, Italy, is the full-scale prototype of the negative ion source for ITER neutral beam injectors. Inside the ion source, eight radio frequency (RF) drivers are responsible for producing plasma via electromagnetic induction. The Faraday Shield lateral wall (FSLW) is a cup-shaped component of SPIDER RF drivers made out of electrodeposited copper, protecting the surrounding components from direct contact with plasma. Electromagnetic coupling with plasma is not perfect inside the driver. Indeed, RF power in the order of tens of kW is deposited on the driver components during nominal operation, thus requiring them to be actively cooled. During SPIDER 2018-2021 experimental campaign, an unexpected issue with the cooling systems resulted in the structural failure of the FSLW of four drivers. Numerical analyses initially suggested the hypothesis of a localized damage, implying the possibility of repairing the damaged components rather than replacing them with new ones. Measurements of hardness and grain size collected on the surface of the FSLWs have confirmed this hypothesis and served as a basis for the development of the refurbishment strategy. Eventually, the damaged FSLWs were successfully repaired and reinstalled in SPIDER. [ABSTRACT FROM AUTHOR]

The RF-driven negative ion source has a steady state characteristic and will be a promising ion source for neutral beam injection (NBI) in the future. Cesium (Cs) injection is an efficient method to enhance the yield of negative ions during plasma discharge. In order to support the engineering and physical research in this field, the Cs oven prototype has been developed for negative ion source based neutral beam injector at ASIPP. This article presents the design details of a Cs oven system, including the constant temperature control system, the mechanical structure of Cs oven, and the surface ionization detector (SID). SID is a measurement method for the cesium flux in the nozzle. The experiment results of constant temperature control system show that the control accuracy and function meet the requirements of device operation. Meanwhile, the simulation analysis of Cs vapor concentration has been carried out in this paper. According to the simulation results, the graph of total Cs flux is given in the article, which presents the reference for the subsequent device testing.

BackgroundNegative ion source based on neutral beam injector is an important auxiliary heating and current driving system for the China Fusion Experimental Test Reactor (CFETR). Due to the high beam energy and large beam spot of negative ion based neutral beam injection system (NNBI), electrical deflection of residual ions has become the preferred scheme for residual ion dumping.PurposeThis study aims at the conceptual design of NNBI electrostatic residual ion dump system to deflect residual ions for neutralization of the beam.MethodsAccording to the development objectives of NNBI, three polar plates and two channels were adopted for the conceptual design of electrostatic residual ion dump (ERID). A negative voltage was applied to the intermediate plate to form a deflection electric field between the channels. When the mixed beam passed through the deflection channel, the residual ions were deflected to the panel due to the action of electric field force. Through theoretical calculation and beam deposition simulation, the structure of the ERID and more uniform heat load distribution on its surface were preliminarily determined.Results & ConclusionsThe simulation results show that the ERID can deflect the residual ions under the parameter design, providing a reference for future engineering design.

One‐dimensional fluid model for a planar sheath in front of a negative ion‐emitting electrode surface immersed in a collision‐less, non‐magnetized, electronegative plasma is presented. It was found that the positive ion speed at the plasma–sheath boundary (PSB) increases linearly with negative ion emission from the electrode but attains a saturation value as soon as a virtual cathode is formed near the electrode surface. The effect of negative ion emission on the pre‐sheath region shows that the potential drop increases across the pre‐sheath in accordance with the rise in positive ion speed at the PSB. The sheath width obtained using the present model shows a similar trend as the Child‐Langmuir law, but its magnitude is found to be consistently higher compared with a non‐emitting electrode. A plausible explanation has been given to explain these effects. [ABSTRACT FROM AUTHOR]

For a critical plasma test equipment JT‐60SA, vacuum electric breakdown around apertures is a serious problem for developing a high‐energy and high‐power negative ion source accelerator. To improve the high voltage performance of the accelerator, we investigated the relation of a breakdown electric field in a vacuum with the electric field integral value of electrode's surface for multi‐apertures electrode. The results of breakdown tests for various types of multi‐aperture electrode show that the electric field integration ES had a relation with the vacuum breakdown field and field emission current characteristics. The reduction in the ES with the relaxation of electric fields improved the breakdown performance. Moreover, the low value of ES decreased the field enhancement factor 'β' and the emission area 'A' of field electron emission from the electrode. Therefore, we found that the breakdown electric field can be improved by reducing the prebreakdown current that flows before the occurrence of breakdown by reducing the ES. © 2019 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc. [ABSTRACT FROM AUTHOR]

The ITER Heating Neutral Beam (HNB) injector is required to deliver 16.7 MW power into the plasma from a neutralised beam of H−/D− negative ions, produced by an RF source and accelerated up to 1 MeV. To enhance the H−/D− production, the surface of the acceleration system grid facing the source (the plasma grid) will be coated with Cs because of its low work function. Cs will be routinely evaporated in the source by means of specific ovens. Monitoring the evaporation rate and the distribution of Cs inside the source is fundamental to get the desired performances on the ITER HNB. In order to proper design the source of the ITER HNB and to identify the best operation practices for it, the prototype RF negative ion source SPIDER has been developed and built in the Neutral Beam Test Facility at Consorzio RFX. A Laser Absorption Spectroscopy diagnostic will be installed in SPIDER for a quantitative estimation of Cs density. By using a wavelength tunable laser, the diagnostic will measure the absorption spectrum of the 852 nm line along 4 lines of sight, parallel to the plasma grid surface and close to it. From the absorption spectra the line-integrated density of Cs at ground state will be measured. The design of this diagnostic for SPIDER is presented, with details of the layout and of the key components. A preliminary installation of the diagnostic on the test stand for Cs ovens is also described, together with its first experimental results; the effect of ground state depopulation on collected measurements is discussed and partially corrected. [ABSTRACT FROM AUTHOR]

• We have designed the magnetized-sheet plasma. • The negative ions produce by volume production without Cs seeding. • The negative hydrogen /deuterium ions are successfully extracted. • This study clarified the basic characteristics of the developed system. The use of negative ion sources without Cs seeding is desired in neutral beam injectors of fusion experiments. We have designed a magnetized-sheet plasma that produces negative ions by volume production without Cs seeding. The developed system was experimentally tested on steady-state hydrogen and deuterium plasmas. The negative hydrogen ion beam was extracted by a two-grid extraction system located at the periphery of the sheet plasma. The negative hydrogen/deuterium ions were successfully extracted from the sheet plasma. The current densities of the ion beams increased with increasing discharge current. At an extraction voltage of 7 kV and a discharge current of 80 A, the approximate current densities of the negative hydrogen and deuterium ion beams were 3.8 and 1.2 mA/cm2, respectively. [ABSTRACT FROM AUTHOR]

• Status of MITICA facility. • SPIDER experimentation started. • First scan of plasma light and spectroscopic signals vs RF power and filter field. • SPIDER short-term planning. To reach fusion conditions and control plasma configuration in ITER, a suitable combination of additional heating and current drive systems is necessary. Among them, two Neutral Beam Injectors (NBI) will provide 33 MW hydrogen/deuterium particles electrostatically accelerated to 1 MeV; efficient gas-cell neutralisation at such beam energy requires negative ions, obtained by caesium-catalysed surface conversion of atoms inside the ion source. As ITER NBI requirements have never been simultaneously attained, a Neutral Beam Test Facility (NBTF) was set up at Consorzio RFX (Italy), including two experiments. MITICA is the full-scale NBI prototype with 1 MeV particle energy. SPIDER, with 100 keV particle energy, aims at testing and optimising the full-scale ion source: extracted beam uniformity, negative ion current density (for one hour) and beam optics (beam divergence <7 mrad; beam aiming direction within 2 mrad). This paper outlines the worldwide effort towards the ITER NBI realisation: the main results of the ELISE facility (IPP-Garching, Germany), equipped with a half-size source, are described along with the status of MITICA; specific issues are investigated by small specific facilities and by joint experiments at QST and NIFS (Japan). The SPIDER experiment, just come into operation, will profit from strong modelling activities, to simulate and interpret experimental scenarios, and from advanced diagnostic instruments, providing thorough plasma and beam characterisation. Finally, the results of the first experiments in SPIDER are presented, aimed at a preliminary source plasma characterisation by plasma light detectors and plasma spectroscopy. [ABSTRACT FROM AUTHOR]

The ion source NIO1 (Negative Ion Optimization 1) is a versatile multiaperture H− source capable of continuous regime operation, with the plasma generated by a 2 MHz/2.5 kW radiofrequency (rf) power supply and extraction of nine beamlets. It aims to partly reproduce the conditions of much larger ion sources, built or in construction for the neutral beam injectors of fusion devices, in a compact and modular ion source, where effects of individual source components can be rapidly verified and compared to simulation code results. Several modifications of the magnetic configuration (both inside the ion source and the embedded magnets inside the accelerator grids) were investigated. Saturation of beneficial effect of filter field at large strength (>0.01 T) leads us to use softer magnetic filters, and the advantages of a crossed deflection field are noted. The rf system takes full advantage of the generator bandwidth (0/ + 20 kHz used). The result database and the integration of major diagnostic systems with the control system are also summarized, with some details on voltage holding and beam uniformity. [ABSTRACT FROM AUTHOR]

• ELISE allows routine operation at high RF power for long plasma pulses up to 3600 s. • Negative hydrogen ion current densities above ITER requirements can be accelerated. • Large amount and non-uniformity of co-extracted electrons is the main challenge. • Suppression of co-extracted electrons can be influenced by magnetic fields. • Series of high performance beams can be sustained by dedicated caesium conditioning. Neutral Beam Injection (NBI) for ITER shall deliver in total 33 MW heating power to the plasma with two injectors at a beam energy of 1 MV. Taking neutralization efficiency and all losses along the beam path into account a negative ion current density of 329 A/m2 (H− for 1000s) and 286 A/m2 (D− for 3600 s) has to be extracted from each ion source (size 1 × 2 m2) with a beam uniformity >90%. For the optimization of the radio frequency driven (RF) negative ion source and its extraction system the ELISE test facility has been set up at IPP as a size scaling experiment with a source area of 0.9 × 1 m2 and an extraction area of 0.1 m2. Since its experimental start in 2013 continuous progress was achieved improving the system technically for long pulse operation at high RF power (total 300 kW for 4 drivers) as well as approaching ITER´s physics requirements for the ion beam. For hydrogen ˜90% of the required ion current density can be extracted for long pulses (1000s); the accelerated current density even exceeded the ITER target. For deuterium operation the main challenge is the amount, temporal in-stability and spatial asymmetry of co-extracted electron current which limit the applicable RF power and the source performance. Shaping the magnetic field topology and controlling the electric fields inside the source together with an improved caesium conditioning procedure have been identified as possible tools to improve the source performance. [ABSTRACT FROM AUTHOR]

• IPP Garching has recently upgraded the RF-driven negative ion source testbed BATMAN. • Beam uniformity is going to be investigated using a new tungsten wire calorimeter. • The new TWC is positioned only 19 cm downstream from the extraction system. • The new TWC allows observation of the beamlets for the first time in this testbed. • The new TWC is absolutely calibrated by ohmic heating of a tungsten wire. Within the framework of the ion source development for the ITER and DEMO Neutral Beam Injection (NBI) systems, IPP Garching has recently upgraded the radiofrequency-driven negative ion source testbed BATMAN. One of the requirements for the ITER NBI system is to produce a beam power density homogeneity above 90% over its large beam size extracted from an ion source of 1 × 2 m2. The beam uniformity is going to be investigated, on a smaller scale, in BATMAN Upgrade with the new ITER-like beam optics. The testbed is equipped with several beam diagnostic tools to measure beam power, uniformity and divergence. A new tungsten wire calorimeter (TWC) has recently been developed to characterize the beam quantitatively and with an improved spatial and temporal resolution. The TWC consists of an array of thin tungsten wires with a diameter of 0.3 mm, placed in the beam path. The wires are heated by the beam up to 3000 °C and emit visible light, which is observed by an optical camera. While the existing TWC provides a qualitative impression of beam power characteristics and is placed at a distance of almost 2 m from the beam extraction system, the new TWC is positioned only 19 cm downstream from the extraction system and allows observation of the single beamlets for the first time in this testbed. In order to determine the correlation between the pixel intensity measured by the camera and the power density impinging on the wires, one wire is equipped with an ohmic heating system which allows heating up the wire with a known power, thus enabling calibration of the diagnostic tool. In this paper the design, including FEM simulations of the wire thermal behavior, manufacturing and installation of the new TWC inside the BATMAN Upgrade testbed are described and the first, very promising, observations of beamlet intensities are presented. [ABSTRACT FROM AUTHOR]

The test facility BATMAN was dedicated since its start in 1996 to the development of radio frequency driven negative hydrogen ion sources for ITER NBI with focus on formation and extraction of negative ions, technological developments and improved concepts. During 2017, the test facility was upgraded in order to replace the former extraction system with a new ITER-like extraction system comparable in size to one ITER beamlet group and having the option to flow a current through the grid creating a magnetic filter field as foreseen for the ITER sources. In addition, the beam diagnostics has been extended: beam emission spectroscopy is located at two positions from the grounded grid (26 cm and 129 cm) with spatial resolution in vertical direction. A newly developed tungsten wire calorimeter is placed just 19 cm downstream the grounded grid to provide quantitative measurements of individual beamlets, whereas the tungsten wire calorimeter at 180 cm distance is still in use for qualitative beam profile diagnostics. Together with a beam dump calorimeter with a crosswise arrangement of thermocouples, beam divergence and uniformity can be studied. This is accompanied by modeling the beamlet transport from the extraction system up to the calorimeter. Results from the first experimental campaign are reported, being very promising for detailed understanding of features measured on large beams. [ABSTRACT FROM AUTHOR]

• SPIDER beam non-uniformity and divergence not compliant yet with ITER requirements. • Additional diagnostics, e.g. fixed and insertable langmuir probes, RFEA. • Purpose: assess overall vertical and local beam non uniformities and beam divergence. • Method: improve spatial resolution, with more profiles and other plasma parameters. Each ITER Neutral Beam Injector (NBI) will provide 16.5 MW hydrogen/deuterium particles, electrostatically accelerating negative ions to 1 MeV. The challenging ITER NBI requirements have never been simultaneously attained and are expected to be demonstrated at the ITER Neutral Beam Test Facility (NBTF), at Consorzio RFX (Italy). NBTF includes the MITICA experiment, full-scale NBI prototype with 1 MeV particle energy, and SPIDER, with 100 keV particle energy, devoted to test and optimise the full-scale ion source. The four years of SPIDER operation demonstrated beam acceleration with high current density, but also highlighted that the non-uniformity and divergence of the beam are too large to comply with the ITER requirements, and that the current density is still lower; moreover, these findings can be traced back to non-homogeneity of the source plasma, where negative ions are produced. SPIDER then entered a shutdown dedicated to the improvement of the various plants and of the diagnostic system. The proposed additional diagnostic systems are integrated with the existing ones and mainly aim at increasing the spatial resolution of the plasma measurements, at investigating profiles in other directions and at measuring other parameters. In parallel, most of the already existing diagnostics will be refurbished or improved. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract The negative ion source test facility ELISE represents an important step in the European R&D roadmap for the neutral beam injection (NBI) systems of ITER. Its aim is to consolidate the design and to gain early experimental experience with a large and modular Radio Frequency (RF) negative ion source and an ITER like extraction system of the same width but half the height of the ITER source (0.9 × 1 m2). H− and D− beams can be extracted and accelerated up to 60 kV for 10 s every ∼150 s out of the continuously operating plasma source. For short plasma pulses (10 s), the extracted negative ion current densities in hydrogen have almost reached the ITER requirement (329 A/m2 H‾, 286 A/m2 D‾). Also the required long pulse source operation up to 1000 s (H-)/3600 s (D-) could be demonstrated on ELISE with repetitive beam blips, but with reduced current densities. The main limitations are the amount and temporal stability of co-extracted electrons, especially in deuterium operation. This co-extracted electron current has to remain below the extracted ion current to avoid thermal overloading of the extraction grid. Magnetic field configurations, electric potentials of source surfaces close to the extraction system and caesium management are under investigation as tools for source performance optimization. Furthermore RF issues such as heating of source components, RF breakdowns and RF matching have been solved for high power source operation. [ABSTRACT FROM AUTHOR]

A bstract The ionization source is an essential component of most explosive detectors based on negative ion mobility spectrometry. Conventional ion sources suffer from such inherent limitations as special safety regulations on radioactive sources or generating interfering ions (for non-radioactive sources) such as corona discharge operating in the air. In this study, a new negative ion source is introduced for ion mobility spectrometry that is based on thermal ionization and operates in the air, applicable to explosives detection. Our system consists of a heating filament powered by an isolated power supply connected to negative high voltage. The ionization is assisted by doping chlorinated compounds in the gas phase using chlorinated hydrocarbons in contact with the heating element to yield Cl − reactant ions. Several chlorinated hydrocarbons are evaluated as the reagent chemicals for providing Cl- reactant ions, of which CCl 4 is identified as the best ionizing reagent. The ion source is evaluated by recording the ion mobility spectra of common explosives, including TNT, RDX, and PETN in the air. A detection limit of 150 pg is obtained for TNT. Compared to other ionization sources, the new source is found to be low-cost, simple, and long-lived, making it suited to portable explosives detection devices. [ABSTRACT FROM AUTHOR]

The extraction grid power supply (EGPS) is important for neutral beam injection of tokamak, which is used to extract the negative ions from the plasma source. A set of power supply with pulse step modulation topology at 16 kV/20 A has been designed for the negative ions extraction at the Institute of Plasma Physics, Chinese Academy of Sciences, which meets the requirements of fast dynamics, low ripple, and good output operation. The development of EGPS is presented in this paper. Two different voltage ratings and series-connected switched power supply (SPS) modules are fed from multisecondary transformer. Insulated-gate bipolar transistors are applied to the switch of SPS module. Rise time and fall time of EGPS out voltage can be set from few microseconds to milliseconds. When the overcurrent occur, the output voltage falls to zero less than $5~\mu \text{s}$. The dynamic performance tests with dummy load of EGPS are presented. The testing results indicate that the power supply meets the demands of negative ions extraction. The design provides a foundation for the next phase of extraction experimental operation for radio frequency ion source. [ABSTRACT FROM AUTHOR]

The full-size radio frequency (RF) negative ion source used in comprehensive research facility for fusion technology (CRAFT) has four drivers, which are arranged in the vertical direction. In order to study the physical and engineering issues, a quarter-size negative ion source was designed, developed and tested in ASIPP. This source has only one driver, making it impossible to research radio frequency interference and large-area plasma uniformity when multiple drivers work together. Therefore, a half-size negative ion source with dual drivers is developed in this study. The half-size negative ion source with dual drivers has been successfully developed and used as a module. Two half-size negative ion sources were assembled after the improvement to complete the overall design of the full-size negative ion source, which is of great significance to the entire beam source project. This paper mainly introduced the structural design, structural finite element analysis and key component manufacturing process test of the half-size negative ion source. [ABSTRACT FROM AUTHOR]

A long pulse acceleration of high-power-density hydrogen negative ion beams of 184 MW/m 2 (0.97 MeV, 190 A/m 2 ) has been successfully achieved up to a facility limit of 60 s by using a 5 stage multi-aperture multi-grid accelerator, which is the first demonstration of the ITER relevant beam with a pulse length over the time constant of cooling capability of the water-cooled acceleration grids. In order to extend the pulse length of such high power density beams, the extraction and acceleration geometries were tuned to match the beam optics to high power density by increasing the transmission at the extractor and the acceleration electric field at the first acceleration gap. After the modifications, the available current density at the same acceleration voltage was increased by 36%, and grid heat loads at each acceleration grid were reduced to less than 3% of the electric input power, which satisfied an allowable level for long pulse acceleration. Furthermore, since the conditioning procedure has been established for long pulse acceleration, a rapid conditioning of total 16 operational days up to 1MeV, 60 s class has been achieved, which was about half of previous results. Because JT-60SA and ITER NBIs are designed by the same concept, these results are applicable to the modifications and operations toward these NBIs directly. [ABSTRACT FROM AUTHOR]

The test facility ELISE (Extraction from a Large Ion Source Experiment) at IPP Garching, Germany, aims to demonstrate ITER-relevant negative ion beam parameters which are required for the NBI system of ITER. ELISE is equipped with a Radio Frequency driven source and an ITER‐like extraction system with half the ITER size. An H − or D − beam can be extracted for 10 s every 3 min from the continuously operating plasma source. The duration of the beam pulses is currently limited by the power supplies available at IPP. Although up to now record-setting 1 h plasmas have been produced in H − as well as in D − , long plasma pulse operation with multiple beam blips showed a key issue: the co-extracted electron current during the extraction phase is strongly dynamic and temporally instable, particularly in D − . These instabilities are likely caused by back-streaming positive ions and Cs dynamics in the source. In order to investigate the source physics in long beam pulses, an upgrade of ELISE using a steady state high voltage power supply is envisaged. This upgrade requires a new steady state diagnostic calorimeter for which a few concepts are being investigated, which make use of several thermocouples, IR thermography and water calorimetry to measure beam intensity, divergence, profile and homogeneity. In addition the suitability of a tungsten wire calorimeter to characterize the steady state beam is being examined. Shielding of delicate components in the beam line, e.g. a large DN 1250 mm gate valve, by means of suitable protection scrapers, is being considered. A selection of these technical solutions is presented and discussed in the paper. [ABSTRACT FROM AUTHOR]

Indian negative-ion source development program has gained momentum with planned integration of Indian Test Facility (INTF) for International Thermonuclear Experimental Reactor (ITER)—Diagnostic Neutral Beam (DNB) characterization at the Institute for Plasma Research (IPR). Eight RF driver-based negative-ion source, being developed for DNB will be tested and operated in INTF. The Two driver-based Indigenously built Negative ion source (TWIN) source provides a bridge between the operational single driver-based negative-ion source test facility, ROBIN in IPR, and an ITER-type multidriver-based ion source. The source is designed to be operated in continuous mode with 180 kW, 1 MHz, 5-s on/600-s off duty cycle and also in 5-Hz modulation mode with 3-s on/20-s off duty cycle for three such cycles. The complete design of TWIN source and its test facility, from conceptual to detailed engineering, has been carried out in IPR. The manufacturing design has been optimized to match the capability of Indian manufacturers, without compromising on the specifications. Some examples of optimization are: 1) an improvised design of the Faraday shields where electrodeposition has been replaced by vacuum brazing; 2) a simplified design of the side walls of the plasma source, where jointing process is simplified, without the application of Electron Beam Welding; and 3) introduction of a fiber reinforced polymer-based integrated electrical and vacuum isolation scheme that replaces the application of a large ceramic. Finite-element analysis (FEA) based on heat load and structural load calculation ensure the functionality and structural integrity of each component of the source. Due to nonnuclear environment in TWIN source experimental area, vacuum brazing is an acceptable manufacturing process. The contract for manufacturing of the ion source has been awarded to an Indian manufacturing company for the first indigenous production of a large-size fusion grade ion source. TWIN source is designed in such a way that it can be operated both in air mode (RF driver coil antennas are exposed to air) as well as vacuum mode. (Whole source is immersed inside vacuum.) The TWIN source shall be manufactured as per ASME guidelines for pressure vessel. Experiments on the TWIN source are foreseen in the last quarter of 2015, as all the auxiliary systems such as 180-kW, RF generator system, vacuum vessel with pumping station, cooling water system, data acquisition and control system, and other power supply systems are already installed in the lab premises. This paper discusses the FEA-based engineering design, simplified manufacturing design, manufacturing experience with highlighting quality control, and the system integration activities undertaken for the TWIN source test facility. [ABSTRACT FROM PUBLISHER]

A helicon plasma system is being designed and developed at CPP-IPR. The design parameters of the system are deduced from the dispersion relation of bounded helicon waves and the required magnetic fields are simulated by using Poisson Superfish code. The Helic code is used to simulate the power deposition profile for various conditions and to investigate the optimum values of chamber parameters for effective coupling of radio frequency (RF) power to plasma. The helicon source system is aimed at carrying out ion–ion plasma studies in electronegative gases such as Hydrogen, Oxygen and Chlorine. The system mainly consists of a source chamber in which helicon plasma will be produced by injecting RF power at a frequency of 13.56 MHz through a right helical antenna in presence of a DC magnetic field followed by an expansion chamber in which it is expected to produce negative ions along with the positive ions. Installation of the various parts of the system is in progress. The details of the design and development of the system is presented in this article. [ABSTRACT FROM AUTHOR]

In order to study the generation and extraction of negative ions for neutral beam injection application, a prototype RF-driven negative ion source and the corresponding test bed are under construction at Institute of Plasma Physics, Chinese Academy of Sciences (ASIPP). The target of the negative ion source is extracting a negation ion beam of 350 A/m 2 for 3600 s plasma duration and 100 s beam duration. According to the required parameters of test bed, the design of power supply system is put forward for earlier study. In this paper, the performance requirements and design schemes of RF power supply for plasma generation, impedance matching network, bias voltage power supply, and extraction voltage power supply for negative beam extraction are introduced in details. The schemes provide a reference for the construction of power supply system and lay a foundation for the next phase of experimental operation. [ABSTRACT FROM AUTHOR]