Numerical investigation of the impact of conductivity modelling in fluid simulations of RF drivers for giant 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−.

Giant negative ion sources, producing high-current of several tens amps with high energy of several hundreds keV to 1 MeV, are required for a neutral beam injector (NBI) in a fusion device. The giant negative ion sources are cesium-seeded plasma sources, in which the negative ions are produced on the cesium-covered surface. Their characteristic features are discussed with the views of large-volume plasma production, large-area beam acceleration, and high-voltage dc holding. The international thermonuclear experimental reactor NBI employs a 1 MeV-40 A of deuterium negative ion source, and intensive development programs for the rf-driven source plasma production and the multistage electrostatic acceleration are in progress, including the long pulse operation for 3600 s. Present status of the development, as well as the achievements of the giant negative ion sources in the working injectors, is also summarized.

At CERN, a high performance negative ion (NI) source is required for the 160 MeV H− linear accelerator named Linac4. The source should deliver 80 mA H− ion beams within an emittance of 0.25 mm·mrad. For this purpose two ion sources were developed: IS01 is based on the NI volume production and IS02 provides additional NI by surface production via H interaction on a cesiated Molybdenum plasma electrode. The development of negative ion sources for Linac4 is accompanied by modelling activities. ONIX code has been modified and adapted to investigate the transport of NI and electrons in the extraction region of the CERN negative ion sources. The simulated results from modeling of IS01 and IS02 extraction regions, which were obtained in 2012 during source commissioning, are presented and benchmarked with experimental measurements obtained after 2013. The formation of the plasma meniscus and the screening of the extraction field by the source plasma are discussed. The NI production is compared between two types of sources, the first one based on volume production only and the second one encompassing NI cesiated surface production. For the IS02 source, different states of conditioning were simulated by changing the NI emission flux from the plasma electrode and Cs+ density in the bulk plasma region. The numerical results show that in low work function regime, with high NI surface emission rate of 3000 A m−2 and Cs-density of nCs+ = 3.8 × 1016 m−3, the total extracted NI current could reach ~80 mA. At the less favorable Cs-coverage, when the surface NI emission rate becomes significantly lower, namely 300 A m−2 with nCs+ = 3.3 × 1015 m−3, the total extracted NI current only reaches ~20 mA. A good agreement between simulation and experimental results is observed in terms of extracted NI current for both extraction systems, including the case of reversed extraction potential that corresponds to positive (H+) ion extraction.

Large-scaled hydrogen negative ion source development is reviewed for a negative ion based neutral beam injector (NBI) in the large helical device (LHD) fusion machine. The target performance of the ion source is characterized by a high current of 30–40 A with a relatively low energy of 120–180 keV. A series of negative ion source development is conducted with a one-dimensionally reduced size of ion sources which still have a large beam area of 25 cm×26 cm or 50 cm with multi apertures. We employed a cesium-seeded volume production source with an external magnetic filter for the source development. Improvement of the arc plasma confinement is effective to produce a high-current negative ion of 16 A with a current density of 31 mA/cm2 at a low operational gas pressure below 0.4 Pa. Suppression of the accelerated electrons is achieved both by strengthening the magnetic field at the extraction grid and by shaping the inside of the extraction grid aperture to shield the secondary electrons against the acceleration electric field. Multi beamlets delivered from a large area are finely focused with the aperture displacement technique applied to the grounded grid. Based on these results, the LHD-NBI negative ion source was designed and fabricated with a beam area of 25 cm×125 cm. The LHD-NBI source produced 25 A of negative ions with an energy of 104 keV at a low gas pressure of 0.3 Pa. A long-pulse negative ion beam of 81 keV–1.3 MW was produced for 10 s. Four sources were installed to the LHD-NBI system, and around 4 MW of neutral beams were injected into the LHD plasmas with an energy of 100–110 keV in the first period for the NBI experiments. The LHD-NBI ion source is still being developed to improve its performance, and the key issues for the improvement are discussed.

Fires in recycling facilities are known for producing noxious fumes, having high heat release rates (HRRs), being difficult to extinguish and causing significant devastation to business operations. This research aims to provide experimental data and numerical models from which performance‐based designs can be developed for recycling facilities. Through validating simulation models against experimental data, ignition parameters are verified and can be used as inputs for future larger‐scale modelling of flame spread and behaviour. In previous research, cone calorimeter tests were conducted in which the thermal properties (HRR curves, heat of combustion [HOC], time to ignition [TTI]) of recycled plastic pellets were determined. In this research ignition and flame spread are studied, which includes flame spread experiments in a horizontal trough. This data is used to calibrate the density, thermal conductivity, and specific heat capacity by comparing the TTI observed from cone calorimeter testing to that observed in simulated models. The HRR curves and HOC, in addition to the calibrated density, thermal conductivity and specific heat capacity, are used for fire dynamic simulator (FDS) models presented in this paper to simulate and observe the horizontal flame spread rate of shallow samples of plastic pellets made of recycled material. This paper then compares experimental and numerical horizontal flame spread rates and burning behaviour observed. The experimental and simulated horizontal flame spread rates are in the same order of magnitude, 0.2 and 0.35 cm/min respectively, highlighting that the ignition, initial flame spread and model parameters have been suitably captured.

Development of an r.f. driven negative hydrogen ion source

Particle-in-cell (PIC) codes are used since the early 1960s for calculating self-consistently the motion of charged particles in plasmas, taking into account external electric and magnetic fields as well as the fields created by the particles itself. Due to the used very small time steps (in the order of the inverse plasma frequency) and mesh size, the computational requirements can be very high and they drastically increase with increasing plasma density and size of the calculation domain. Thus, usually small computational domains and/or reduced dimensionality are used. In the last years, the available central processing unit (CPU) power strongly increased. Together with a massive parallelization of the codes, it is now possible to describe in 3D the extraction of charged particles from a plasma, using calculation domains with an edge length of several centimeters, consisting of one extraction aperture, the plasma in direct vicinity of the aperture, and a part of the extraction system. Large negative hydrogen or deuterium ion sources are essential parts of the neutral beam injection (NBI) system in future fusion devices like the international fusion experiment ITER and the demonstration reactor (DEMO). For ITER NBI RF driven sources with a source area of 0.9 × 1.9 m2 and 1280 extraction apertures will be used. The extraction of negative ions is accompanied by the co-extraction of electrons which are deflected onto an electron dump. Typically, the maximum negative extracted ion current is limited by the amount and the temporal instability of the co-extracted electrons, especially for operation in deuterium. Different PIC codes are available for the extraction region of large driven negative ion sources for fusion. Additionally, some effort is ongoing in developing codes that describe in a simplified manner (coarser mesh or reduced dimensionality) the plasma of the whole ion source. The presentation first gives a brief overview of the current status of the ion source development for ITER NBI and of the PIC method. Different PIC codes for the extraction region are introduced as well as the coupling to codes describing the whole source (PIC codes or fluid codes). Presented and discussed are different physical and numerical aspects of applying PIC codes to negative hydrogen ion sources for fusion as well as selected code results. The main focus of future calculations will be the meniscus formation and identifying measures for reducing the co-extracted electrons, in particular for deuterium operation. The recent results of the 3D PIC code ONIX (calculation domain: one extraction aperture and its vicinity) for the ITER prototype source (1/8 size of the ITER NBI source) are presented.

In negative hydrogen ion sources, the kinetic energy of the atoms is directly related to the negative ion yield at the caesiated converter, with a larger contribution from hot atoms. The energy distribution of hydrogen atoms is related to the formation process: either the kinetic energy release, resulting from dissociation of the hydrogen molecules or molecular ions, or the proton neutralization either in the volume or during reflection at walls. The interpretation of recent experimental measurements related to the translational energy distribution of atoms or positive ions could profit from accurate inclusion of the initial energy distribution in numerical models. In this work, we focus on the calculation of the kinetic energy release for the various dissociation channels due to electron impact on H2 and H2 +, in the Franck-Condon and delta approximation. Since in negative ion sources non-equilibrium vibrational distributions of H2 are found, the energy distribution of fragments is calculated for all vibrational levels. The inverse cumulative distribution functions related to the main dissociation processes are given, as well as the cumulative distributions for all dissociation channels by electron impact, for simple implementation in Monte Carlo numerical simulations. Finally, the application of the method to few cases of interest for negative ion sources is discussed.

At CERN, a high performance negative ion (NI) source is required for the 160 MeV H− linear accelerator Linac4. The source is planned to produce 80 mA of H− with an emittance of 0.25 mm mradN-RMS which is technically and scientifically very challenging. The optimization of the NI source requires a deep understanding of the underling physics concerning the production and extraction of the negative ions. The extraction mechanism from the negative ion source is complex involving a magnetic filter in order to cool down electrons’ temperature. The ONIX (Orsay Negative Ion eXtraction) code is used to address this problem. The ONIX is a selfconsistent 3D electrostatic code using Particles-in-Cell Monte Carlo Collisions (PIC-MCC) approach. It was written to handle the complex boundary conditions between plasma, source walls, and beam formation at the extraction hole. Both, the positive extraction potential (25kV) and the magnetic field map are taken from the experimental set-up, in construction at CERN. This contribution focuses on the modeling of two different extractors (IS01, IS02) of the Linac4 ion sources. The most efficient extraction system is analyzed via numerical parametric studies. The influence of aperture’s geometry and the strength of the magnetic filter field on the extracted electron and NI current will be discussed. The NI production of sources based on volume extraction and cesiated surface are also compared.

It is well known that cesium seeding in volume hydrogen negative ion sources leads to a large reduction of the extracted electron current and in some cases to the enhancement of the negative ion current. The cooling of the electrons due to the addition of this heavy impurity was proposed as a possible cause of the mentioned observations. In order to verify this assumption, we seeded the hydrogen plasma with xenon, which has an atomic weight almost equal to that of cesium. The plasma properties were studied in the extraction region of the negative ion source Camembert III using a cylindrical electrostatic probe while the negative ion relative density was studied using laser photodetachment. It is shown that the xenon mixing does not enhance the negative ion density and leads to the increase of the electron density, while the cesium seeding reduces the electron density.

The large high current ICP negative (H−, D−) ion sources will produce the neutral beams needed for fusion plasma heating in ITER. To reach very high intensity negative ion beams a cesium coated surface of the plasma grid is usually used. The cesium coating, however, poses some problem in the maintenance and operation of the ion source and for that alternative ways to enhance the ion current extraction should be investigated. An alternative way to enhance the beam current could be the use of a Planar Ion Funnel (PIF) extraction system recently proposed for applications where a very high extraction efficiency are required [1]. In this contribution the idea of applying the PIF extraction in an ICP negative ion source to further increase the ion current extracted will be discussed. Some trajectory simulations to better show the goodness of this proposal will also be presented.

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.

The neutralizers in ion thruster systems do not generate thrust force. Hence, the power consumption of a neutralizer limits the thrust efficiency of the ion thruster system. Therefore, an ion thruster system is proposed that uses a negative ion source that generates thrust force as well as neutralizes the positive ion beam. In this study, in order to verify the feasibility of this ion thruster system by ground experiments, a negative fullerene ion source was developed and three experiments were conducted. First, in order to demonstrate that the satellite can accelerate ions without becoming charged, the positive and negative ion beams were accelerated in the floating condition using a feedback circuit. Second, in order to verify whether the formation of the virtual anode could be suppressed, the space potential and the beam profile in the downstream region of the ion beam were measured. Third, in order to demonstrate that the thruster system generates thrust force, the thrust with a beam target was measured. From the aforementioned results, it was concluded that the experimental verification of the feasibility of the ion thruster with combined positive and negative ions without a neutralizer was successful.

The knowledge of Cs dynamics in negative hydrogen ion sources is a primary issue to achieve the ITER requirements for the Neutral Beam Injection (NBI) systems, i.e. one hour operation with an accelerated ion current of 40 A of D− and a ratio between negative ions and co-extracted electrons below one.Production of negative ions is mostly achieved by conversion of hydrogen/deuterium atoms on a converter surface, which is caesiated in order to reduce the work function and increase the conversion efficiency. The understanding of the Cs transport and redistribution mechanism inside the source is necessary for the achievement of high performances. Cs dynamics was therefore investigated by means of numerical simulations performed with the Monte Carlo transport code CsFlow3D.Simulations of the prototype source (1/8 of the ITER NBI source size) have shown that the plasma distribution inside the source has the major effect on Cs dynamics during the pulse: asymmetry of the plasma parameters leads to asymmetry in Cs distribution in front of the plasma grid. The simulated time traces and the general simulation results are in agreement with the experimental measurements.Simulations performed for the ELISE testbed (half of the ITER NBI source size) have shown an effect of the vacuum phase time on the amount and stability of Cs during the pulse. The sputtering of Cs due to back-streaming ions was reproduced by the simulations and it is in agreement with the experimental observation: this can become a critical issue during long pulses, especially in case of continuous extraction as foreseen for ITER. These results and the acquired knowledge of Cs dynamics will be useful to have a better management of Cs and thus to reduce its consumption, in the direction of the demonstration fusion power plant DEMO.

The RF-based single-driver negative hydrogen ion source test bed ROBIN is a 100 kW, 1MHz negative hydrogen ion source test bed at IPR, Gandhinagar. The inductive RF coupling produces plasma in the driver, expanding into an expansion chamber coupled to a three-grid extractor accelerator system. A magnetic filter field, transverse to the plasma flow direction from the RF driver, is produced magnetically using magnet boxes fitted in the diagnostic flange, also called filter field flange. The diagnostic flange is located between the exit of the expansion chamber and the entry to the three-grid system consisting of a plasma grid (PG), extraction grid, and grounded grid. The filter field in conjunction with the bias voltage of a few tens of volts applied to the plasma grid with respect to the source body has a strong influence on the control of plasma electrons, plasma drifts, and plasma confinement, which, in turn, influence plasma uniformity, and so beam profile uniformity, beam divergence, and beam transmission. The present design of ITER sources also envisages magnets lined along the expansion chamber for plasma confinement. Their effectiveness on operations is an important aspect of studies, being performed on fusion-relevant negative ion test beds globally. A systematic study on similar lines has been carried out on the ROBIN test bed and is reported in this paper. The magnets are arranged in boxes and fitted on the walls of the ROBIN expansion chamber extending up to the diagnostic flange. Several magnetic configurations, line and cusp arrangements, have been studied. Significant effects on plasma uniformity across the ion extraction plane in front of the PG are observed. As a consequence, the ratio of co-extracted electron current to negative hydrogen ion (H- ion) current in terms of their current densities (je-/jion) is also influenced by the magnetic configurations. The findings presented here may be relevant for achieving improved H- uniformity along the extraction plane, thereby helping in improved beam divergence and reduced transport losses from larger sources.