Neutral Beam Injection System Research Articles

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.

Plasma discharge experiments of a Faraday shieldless driver for high-power and long-pulse radio frequency ion source for NBI application.

Due to the high stability and less maintenance, the radio frequency (RF) driven ion source is preferred for the neutral beam injection (NBI) system. In a popular design of the RF ion source for NBI application, a Faraday shield (FS) is installed inside the RF plasma driver to protect the discharge tube. However, the FS also brings some drawbacks, such as lowering the RF power transfer and increasing the processing difficulty. A prototype of the RF plasma driver without FS and with mature manufacturing technology has been developed by using a water-cooled discharge tube. After basic testing, this prototype was further tested under the RF plasma discharge experiments in views of high power, long pulse, and long term. The reliability and its plasma characteristics were the focus of these experiments, also for the hidden issues. The results show that the prototype could generate stable and high-density plasma without any damage or sputtering mark. An RF plasma discharge of 50 kW and 20s has been achieved. The expected frequency tuning was equally effective on the prototype. Moreover, compared to the RF plasma driver with FS, the prototype could produce higher electron density in the extraction region under the same RF power. Of course, some of the shortcomings of the prototype have also been exposed in the experiments and will be improved in subsequent experiments.

Design concept of intelligent integrated control system for neutral beam injection

Abstract Due to the specificity of neutral beam injection (NBI) system, the control of its actual physical system is achieved by the integrated control system (ICS). The current NBI ICS adopts a distributed design structure to balance the system load, which is also the mainstream system design and architecture model of ICS around the world. However, from a practical point of view, the distributed system architecture is not perfect. In the long run, upgrading the system intelligence and automation capabilities is an inevitable choice for the future, so it is necessary to explore ways to transform and upgrade the ICS. At present, Internet of Things (IoT), as an important part of the new generation information technology, can realize the control of everything through data exchange. This is because on the one hand, IoT has wide compatibility and powerful scenario-based capabilities, it not only has the advantages and characteristics of distributed design, but also can pull the NBI subsystems into the same level scenario and lay the foundation for the further construction of digital NBI; on the other hand, the intervention of artificial intelligence makes IoT have some new typical characteristics such as intelligent sensing, ubiquitous connectivity, precise control, digital modeling, real-time analysis and iterative optimization, which is enough to pull the current NBI ICS into a new intelligent control era. Finally, it is worth mentioning that due to its inherent design structure and functional characteristics, ICS tends to be broadly generic, so it is not exclusively used for NBI operation in nuclear fusion, and it can provide some insight into other application areas.

Particle transport in reduced turbulence neutral beam heated discharges at Wendelstein 7-X

A spontaneous reduction in anomalous particle transport in the plasma core is seen experimentally in reproducible, purely neutral beam heated plasma phases at Wendelstein 7-X (W7-X). Heating and fueling the plasma exclusively with the neutral beam injection system for several seconds leads to continuously peaking plasma density profiles with strong gradients inside mid minor radius. A significant acceleration of the density peaking occurs after a certain onset time and is examined with a detailed particle transport analysis in several discharges. By invoking the particle continuity equation, the total experimental radial electron flux is deduced from the time evolution of the electron density profile and the radially resolved particle sources. Subtracting the modeled neoclassical particle flux contribution gives the anomalous particle flux. Exploiting the evolving plasma conditions, anomalous diffusion and convection coefficients are computed from the flux variation with density and density gradients. In several discharges a significant and consistent change of the anomalous transport coefficients is seen when crossing a specific normalized density gradient length.

Beam Optics Analysis for the 120-keV Multiaperture Accelerator of Neutral Beam Injector

Neutral beam injection is an important auxiliary heating method for magnetic confinement nuclear fusion. To further optimize the steady-state operation of the high-temperature plasma in the Experimental Advanced Superconducting Tokamak (EAST), a higher beam energy is required for the high-power, long-pulse neutral beam injection system. The most critical technology to achieve this goal is the upgrade of the ion source accelerator. This paper analyzes the beam optics of the multiaperture accelerator for the extraction of a deuterium ion beam of 120 keV/50 A with a divergence angle of less than 1 deg root-mean-square. The effects of the number of grids, shapes of plasma grids (PGs), grid gaps, voltage ratio, and plasma parameters on beam optics were assessed by numerical simulation. The results allowed optimization of the PG shape; they also indicated that the tetrode configuration can reduce the beam divergence angle effectively while the triode configuration can extract a higher current. These conclusions provide guiding significance for the selection and parameter design of the EAST neutral beam injector ion positive source with a 120-keV accelerator.

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.

Hybrid model predictive control techniques for safety factor profile and stored energy regulation while incorporating NBI constraints

A novel hybrid Model Predictive Control (MPC) algorithm has been designed for simultaneous safety factor (q) profile and stored energy (w) control while incorporating the pulse-width-modulation constraints associated with the neutral beam injection (NBI) system. Regulation of the q-profile has been extensively shown to be a key factor for improved confinement as well as non-inductive sustainment of the plasma current. Simultaneous control of w is necessary to prevent the triggering of pressure-driven magnetohydrodynamic instabilities as the controller shapes the q profile. Conventional MPC schemes proposed for q-profile control have considered the NBI powers as continuous-time signals, ignoring the discrete-time nature of these actuators and leading in some cases to performance loss. The hybrid MPC scheme in this work has the capability of incorporating the discrete-time actuator dynamics as additional constraints. In nonlinear simulations, the proposed hybrid MPC scheme demonstrates improved q-profile+w control performance for NSTX-U operating scenarios.

Preliminary design and optimization of heat transfer element for calorimeter in neutral beam injection system

Abstract The calorimeter is the primary high heat flux component of the neutral beam injection (NBI) system. To meet the requirements of China’s future high-power and long-pulse NBI system for efficient heat removal from the calorimeter, this study proposes a preliminary design for a calorimeter heat transfer element (HTE) mainly formed by vacuum brazing the front plate and back plate, featuring internal hypervapotron channels to enhance heat transfer. Parametric design and numerical simulations are applied to find the optimal channel parameters, including channel height (h), fillet radius (r), and slot width (s). Response surface analysis is used to process the results of the design schemes. The thermohydraulic performance of the optimized design under different channel flow velocities and heat flux densities is further analyzed. The results indicate that the optimal parameters are as follows: h=5 mm, r=1 mm, and s=1.5 mm. The h and s mainly influence HTE’s maximum temperature and pressure drop per unit length, respectively. The optimized design can meet the design criteria with a normal channel flow velocity of 10 m/s. To prevent the HTE pressure drop from surpassing 2 MPa, the channel flow velocity should not exceed 12 m/s. The design and optimization results of this study can serve as guidance for the engineering design of the calorimeter in China’s future NBI system.

Engineering design and manufacturing of a radio frequency plasma driver without Faraday shield for NBI application

A radio frequency (RF) driven ion source is favorable for high-power and long-pulse neutral beam injection (NBI) system. To reduce the damage caused by the isotropic heat and particle fluxes from the plasma, the traditional RF plasma driver of the NBI ion source introduces a Faraday shield (FS) for protection. However, the simulations and experiments show that the FS seriously affects the RF coupling and around 50 % of the input power deposits on the FS. The water-cooled discharge tubes with a double-layer ceramic structure have been developed to avoid using the FS structure. It's expected to improve the RF power transfer efficiency and to promote the development of high-power and long-pulse RF ion source. To verify the feasibility of the double-layer ceramic structure, the paper has carried out the fluid-thermal-structural coupling analysis and the fracture mechanical analysis on two proposed types: the alumina ceramic brazing structure and the silicon nitride ceramic bonding structure. The sample tests and the prototype manufacturing have been completed. Preliminary tests of the plasma discharge on the prototype of the alumina ceramic brazing structure have been launched. A plasma discharge of 30 kW and 20 s have been achieved.

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.

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.

Discharge characterization of two-region arc plasma (TRAP) ion source

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.

Characterization of hydrogen plasma bulk in 2.45 GHz ECR ion source

Electron cyclotron resonance ion source (ECRIS) as the front-end for the neutral beam injection (NBI) system has production potential of hydrogen ion beam in tokamaks. In this article, two methods are hired to evaluate the plasma ion fractions: H+, H2+ and H3+ with respect to the ECRIS operating parameters. The methods are matured by utilizing 28 important volume and surface collisions processes. We encrypted a global 0D numerical model (GM) based on solving the nonlinear equations of plasma species as the first method. Next, a 3D plasma simulation (PS) is applied through the finite element method (FEM) by COMSOL Multiphysics. Then, the results are compared to the experimental data.

Re-evaluation of shielding ability and induced activity of KSTAR with increased neutron yields

To determine the viability of the 2-fold neutron yield increase of the Korea Superconducting Tokamak Advanced Research (KSTAR) owing to the installation of a second neutral beam injection system to increase the plasma density and a divertor material change from graphite to tungsten to reduce the heat load, we performed a radiological evaluation and shielding analysis of an existing building structure and the KSTAR with an increased neutron yield per shot. According to the shielding analysis results, the weekly and annual doses outside the building did not exceed the criteria, and the fluxes and doses in the plasma experimental room were unaffected by the divertor material change. The maximum dose rate in the working area in the tokamak was 2.16E+03 μSv/h at the surface-plasma-facing components. However, the dose rate was reduced to the background level one month after a 1-shot operation of 300 s. Ar-41 was the major nuclide for the air activation calculations; however, the effluent level exceeded the derived air concentration after the 1-shot operation ended. The residual activity concentration of Ar-41 was reduced below the derived air concentration after 243 min. The annual operating condition of the KSTAR is suggested based on the evaluation results.

Influence of surface work function and neutral gas temperature on mechanism of H− production in negative hydrogen ion sources

Negative hydrogen ion sources (NHISs) based on surface production with cesium (Cs) seeded can fulfill the demanded parameters for neutral beam injection systems for ITER. In this study, the Global Model for Negative Hydrogen Ion Source based on volume-produced H− ions is developed to include surface-produced H− ions and is validated against experimental data obtained in a planar inductively coupled plasma discharge used for study of Cs effect on H− production. The H− density predicted by the model decreases three times with surface work function from 2.1 to 4.5 eV, achieving good agreement with the experimental results, as surface conversion yield of particles to H− ions shows exponential decline with surface work function. The model predicts the rise in neutral gas temperature remarkably enhances surface production but reduces volume production of H− ions, because of increase in surface conversion yield of H atoms to H− ions and in electron temperature, respectively. The dependences of H− production on surface work function and neutral gas temperature are analyzed by evaluating creation rates of the H− ions from different reaction pathways. The developed model can be applied for prediction of H− production in NHISs and ultimate parameter optimization of negative ion beams for fusion reactors.

Progress of Lyman-alpha-based beam emission spectroscopy (LyBES) diagnostic on the HL-2A tokamak

An edge Lyman-alpha-based beam emission spectroscopy (LyBES) diagnostic, using a heating NBI (neutral beam injection) system, is currently under development on the HL-2A tokamak. The 20-channel edge LyBES, which is intended to measure the density fluctuation in plasma edge (from R = 1960 mm to R = 2026 mm) with an improved spatial resolution of 3.3 mm, is a complement to the existing conventional beam emission spectroscopy (BES) diagnostic. In this article, we introduce the progress of LyBES diagnostic, including the collection optics, the monochromator, and the detector system. The reflectance of the collection mirrors is measured to be ~82% at 122 nm, and the aberration geometrical radius of the collection optics is tested to be ~150 μm in the aimed area. The linear dispersion of the LyBES monochromator is designed to be ~0.09 nm mm−1. The bandwidth of the detector system with the 5×107 V A−1 preamplifier gain is measured to be ~280 kHz, and the peak-to-peak noise of the detector system is tested to be ~16 mV. The finalized design, components development and testing of the LyBES diagnostic have been completed at present, and an overall performance of the LyBES diagnostic is to be confirmed in the next HL-2A campaign.

Particle injection methods in 3D-PIC MCC simulations applied to plasma grid biasing

In negative ion sources for the ITER Neutral Beam Injection system, the co-extraction of electrons is one of the main limiting factors. The current of co-extracted electrons can be decreased by applying a positive bias voltage to the Plasma Grid (PG) with respect to its source walls. Simulations using three-dimensional Particle-in-Cell Monte Carlo Collision (3D-PIC MCC) model are a powerful tool for studying the extraction region of such ion sources. However, the inclusion of both PG and source walls in the simulation domain is difficult due to numerical constraints. This study uses the 3D-PIC MCC code ONIX to explore the effects of particle injection models on plasma characteristics, using a flux injection model to regulate particle influx for a flat transition in potential from the bulk plasma to the simulation domain. Biasing of the PG above floating potential is possible using the flux injection scheme and results in a notable reduction in co-extracted electrons, corroborating with established experimental observations.

Influence of cusp magnetic field configuration on plasma parameters of radio frequency driven negative ion source for CRAFT NNBI

Radio Frequency (RF) driven negative ion source is the preferred ion source for negative ion based Neutral Beam Injection system (NNBI). The plasma generator, which consists of RF driver(s) and an expansion chamber, is one of the key parts for NNBI. The distribution of cusp magnetic field which is generated by permanent magnets installed at the lateral wall of the expansion chamber is a common issue, due to the great influence on electron density, electron temperature and spatial uniformity of plasma density in the extraction area. Two kinds of configuration of cusp magnetic field (checkerboard type and symmetric type) and three types of arrangement of permanent magnets (with three, four and five rows of magnets) have been carried out with the approach of finite element and the experimental performance. On the RF negative ion source testbed, a moveable Langmuir probe has been adopted to investigate the characteristic of electron density and temperature near the extraction area. Since the symmetric type with three rows of permanent magnets generates the distribution of cusp magnetic field that contribute to extract the plasma from RF driver and improve the probability of particle collision easily, the higher electron density and the lower electron temperature was obtained via simulation. The experimental conclusion is that the symmetric type with three rows of permanent magnets leads to a higher electron density and lower electron temperature, showing a great agreement with simulation results. The results are helpful to optimize the structure and performance of high power ion source, and provide support for the development of large area RF negative ion sources with multi-driver.

Calculation of negative ion beam loss for two region arc plasma negative ion source

Korea Atomic Energy Research Institute (KAERI) is currently developing a Two Region Arc Plasma (TRAP) negative ion source for neutral beam injection (NBI) systems of a fusion tokamaks which is characterized by two distinct regions with different plasma temperatures without the use of external filter field magnets. The source has recently been designed, manufactured, and installed for testing. Plasma discharge characterization experiments of the TRAP ion source are presently in progress, with planned beam extraction experiments. The beam extractor of the TRAP ion source is capable of accelerating hydrogen ions up to 30 keV and comprises four grids with slit-type aperture structure. Understanding the negative ion beam loss in the extractor is crucial for expecting potential issue within the beam extractor and comprehending the plasma discharge characteristics through negative ion beam. Using a 3-D Monte Carlo program (molflow+), we calculated the vacuum pressure distribution within the TRAP ion source experimental device, and based on these results, we calculated the loss of negative ion beams within the beam extractor and the neutralization efficiency from the beam exit to the target, an effect arising from residual gases. The computational results reveal that approximately 20 ∼ 50 % of the negative ion beam is lost within the beam extractor, and a reduction of more than 60 % in negative current occurs from the beam exit grid to the target within the test chamber due to neutralization of negative ions.

Ion flux measurements using a Mach-Langmuir probe in the ITER prototype neutral beam injection ion source

Neutral beam injection systems as foreseen for ITER use radio-frequency (RF) ion sources at low pressure, where negative hydrogen ions are mainly produced via surface conversion of neutral atoms and positive ions at a plasma facing grid (PG). Up to now there is only limited knowledge about how fluxes and directed velocities of the positive ions are affected by external parameters such as power, pressure and the horizontal magnetic filter field which causes plasma drifts and vertical asymmetries in the vicinity of the PG. For this reason a combined Mach-Langmuir-probe diagnostic is used at multiple positions in the expansion and close to the extraction system in the prototype RF ion source (1/8 of the full ITER ion source size) to measure the positive ions directed velocity and flux as well as the plasma parameters simultaneously. With increasing RF power the flux towards the PG is found to increase linearly, its magnitude being controlled by the plasma density. Towards ITER-relevant pressures the ion flux decreases, in contrast to the directed velocity, which increases non-linearly, reaching around 5 km s−1 at a pressure of 0.3 Pa. The magnetic filter field is discovered to strongly bent down the ion flow in front of the PG. As a result, the ions at the lower half of the PG flow almost exclusively parallel to it, wherefore the flux which impinges onto the lower PG half is reduced by around one order of magnitude.