Neutral Beam Injection Research Articles

Neutralized, intense-ion beams for fusion

A charge- and current-neutralized, intense-ion beam (IIB) will propagate undeflected in a magnetized plasma by the E×B , or diamagnetic, drift for a distance large compared to the beam-ion gyro-radius. This propagation occurs because of collective plasma effects when the beam-energy density is sufficient to sustain the polarization-electric field, or diamagnetic-screening currents. Our principal interest is the E×B drift, with β=Ebeam/Efield<1 . IIBs are characterized by parameters in the range: 0.1–2 MeV ion energy, 1–100 MA m−2 ion-current density, and 17–350 kW average power produced on repetitively-pulsed systems. Multi-MW average-beam power is possible with appropriate modifications to the power supply and ion-source/accelerator. IIBs can be focused geometrically, and/or magnetically, to spot sizes of the order of a few cm’s and their angular divergence is typically, ∼15 mRadians, suitable for long-range propagation in a vacuum beam-line. IIBs lose energy and momentum in the same manner as particles injected by a neutral-beam injector (NBIs), hence could be useful for fusion applications, including: heating, current drive, fueling, profile modifications, etc and such applications have yet to be thoroughly studied. Summarized here are the physical principles accounting for IIB propagation; ion-source designs used to produce the IIB; and pulsed-power methods for energizing the IIB accelerator. This technology base informs scalable metrics for the ion-source/accelerator (excluding the HV power supply and interconnects) used in a conceptual injector that provides the same ion energy and injected power (1 MeV, 17 MW) as NBIs used on ITER. A comparison indicates that the IIB injector would be much smaller in size, lower cost, and have much greater efficiency, while also providing for real-time modulation of the beam energy and intensity and the use of small-diameter injection ports that could minimize fuel contamination and magnetic-field leakage between the IIB injector and tokamak.

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.

Investigation of Cs27LiYCl6:Ce scintillator energy response for D-D fusion neutron spectrometer

During plasma experiments on EAST (Experimental Advanced Superconducting Tokamak), the discharges heated with neutral beam injection (NBI) for plasma results in the production of 2.45 MeV deuterium-deuterium (D-D) fusion neutrons. In order to measure the energy spectrum of D-D fusion neutrons, a new Cs27LiYCl6:Ce (CLYC-7) crystal with 7Li-enrichment has been developed and fabricated into a CLYC-7 scintillation detector. Before utilizing this detector for studying neutron energy spectra, it is crucial to explore the response function of the detector within a relevant neutron energy range. Therefore, the detector has been experimented using the 4.5 MeV electrostatic accelerator at Peking University to investigate its response to fast neutrons, energy resolution, and pulse discrimination capability in the neutron energy range 2.07 MeV–5.51 MeV. The neutron reactions occurring within the CLYC-7 scintillator were simulated using MCNPX, focusing on the reactions 35Cl(n,p) and 35Cl(n,α), as well as the energy response curve and the contributions of different particles (protons, alphas) to the neutron spectrum. The experimental results were successfully validated on EAST, where the 2.45 MeV fusion neutron energy spectrum was measured. That demonstrates the ability of the CLYC-7 scintillation detector to diagnose the fusion neutron energy spectrum for deuterium plasma discharge.

Core electron temperature turbulence and transport during sawtooth oscillations in the DIII-D tokamak

Sawteeth are one of the concerning instabilities in ITER and future burning plasma experiments. Sawtooth dynamics and its interaction with broadband plasma turbulence has been a challenge for predictive simulations of core transport in future fusion devices. This study provides new observations of core turbulence behavior during sawtooth oscillations in DIII-D hydrogen L-mode neutral beam injection heated plasmas in an inner wall limited configuration. A strong correlation of electron temperature and density turbulence levels with the sawtooth oscillation phase has been observed at locations inside the T e inversion radius and/or safety factor q = 1 magnetic surface. The T e turbulence amplitude in the core during the sawtooth ramp exhibits a critical T e gradient behavior inside but not near the T e inversion radius/q = 1 magnetic surface. The most unstable mode calculated from the trapped gyro-landau fluid turbulence simulations reveal a change from low-k ion-type to low-k electron-type modes from pre- to post- sawtooth crash time periods.

Multi-pellet injection into the NBI-heated phase of TJ-II plasmas

A pellet-induced enhanced confinement (PiEC) phase, with general characteristics similar to those reported for the stellarator W7-X, is observed after single pellet injection (>1019 H atoms) into the neutral beam injection heated phase of plasmas in the mid-sized heliac-type stellarator TJ-II. In addition to a step-like increase in density, plasma diamagnetic energy content rises significantly with respect to that of reference discharges, energy confinement time is similarly enhanced when compared to International Stellarator Scaling law predictions (Yamada et al 2005 Nucl. Fusion 45 1684) renormalized for TJ-II, and the triple product, n e · T i · τ E, exhibits a clear bifurcation towards an improved confinement branch when compared to the branch product predicted by the same law. In this work, multiple pellets are injected in series into NBI-heated plasmas in the TJ-II and post-injection plasma performance is reported and discussed. For instance, a charge-exchange recombination spectroscopy diagnostic reveals significantly increased core ion temperatures after pellet injection compared to temperatures achieved in comparable reference plasmas, this pointing to increased ion energy content and improved ion energy confinement during a PiEC phase. It is also found that enhanced performance is independent of whether co- or counter-NBI heating beam is employed. Finally, record stored diamagnetic energy content and plasma beta values are achieved when the largest available pellets are employed. The results indicate that pellet injections extend the operational regime well beyond limits previously achieved in TJ-II without pellets.

Achieving stationary high performance plasmas at Wendelstein 7-X

This work reports on recent results on the search for high performance plasma scenarios at the magnetically confined stellarator fusion device Wendelstein 7-X. In four new designed scenarios, the development from transient toward stationary plasmas of improved performance has been realized. In particular, a high performance duration of up to 5 s, an energy confinement time of 0.3 s, a diamagnetic energy of 1.1 MJ, a central ion temperature of 2.2 keV, and a fusion triple product of 3.4×1019m−3· keV · s have been achieved, and previously observed limitations of the machine have been overcome, regarding both the performance and its duration. The two main experimental techniques for stationary high performance are neutral beam injection core fueling on the one hand and the use of a magnetic field configuration with internal islands on the other hand. Two of the developed scenarios are expected to be extendable straightforward toward a duration of several tens of seconds, making use of the long pulse operation capabilities of W7-X.

Static performance prediction of long-pulse negative ion based neutral beam injection experiment

The mission of negative ion-based neutral beam injection (NNBI) is to conduct experiments with pulses lasting thousands of seconds. It is crucial to develop a simplified physical calculation model for the long-pulse negative ion source in the current NNBI device. This model will be used to evaluate the advantages and disadvantages of the selected parameters prior to the experiment, and to assist in adjusting and establishing the experimental parameters for the long-pulse ion source experiment. This paper presents the development of a static performance prediction model using a back propagation neural network. The model assesses the yield of negative hydrogen ions and the quantity of electrons in the ion source under specific parameter conditions, utilizing various experimental parameters as input. The experimental data used for this model are derived from historical data generated during the operation of the 2022 NNBI experiment. The test results indicate that under the current optimal hyperparameter condition, the prediction accuracy of H− ion current (I_H−) is 80.84%, and the prediction accuracy of extraction grid electronic current (I_EG) is 77.57%. This can effectively prevent invalid shots, accurately assess the advantages and disadvantages of the input parameters, and enhance the performance of the long-pulse NNBI device.

Bayesian inference of electron density and ion temperature profiles from neutral beam and halo Balmer-α emission at Wendelstein 7-X

By employing Bayesian inference techniques, the full electron density profile from the plasma core to the edge of Wendelstein 7-X (W7-X) is inferred solely from neutral hydrogen beam and halo Balmer-α (Hα) emission data. The halo is a cloud of neutrals forming in the vicinity of the injected neutral beam due to multiple charge exchange reactions. W7-X is equipped with several neutral hydrogen beam heating sources and an Hα spectroscopy system that views these sources from different angles and penetration depths in the plasma. As the beam and halo emission form complex spectra for each spatial point that are non-linearly dependent on the plasma density profile and other parameters, a complete model from the neutral beam injection and halo formation through to the spectroscopic measurements is required. The model is used here to infer electron density profiles for a range of common W7-X plasma scenarios. The inferred profiles show good agreement with profiles determined by the Thomson scattering and interferometry diagnostics across a broad range of absolute densities without any changes to the input or fitting parameters. The time evolution of the density profile in a discharge with continuous core density peaking is successfully reconstructed, demonstrating sufficient spatial resolution to infer strongly shaped profiles. Furthermore, it is shown as a proof of concept that the model is also able to infer the main ion temperature profile using the same data set.

Study on stray electrons ejecting from a long-pulse negative ion source for fusion

The negative ion based neutral beam injection is a desirable plasma heating and current drive method for the large-scale magnetic fusion devices. Due to the strict requirements and difficult development of the negative ion source for fusion, a long-pulse negative ion source has been developed under the framework of the Comprehensive Research Facility for Fusion Technology in China. This negative ion source consists of a single radio frequency (RF) driver plasma source and a three-electrode accelerator. The typical extraction and acceleration voltage are 4–8 kV and 40–50 kV, respectively. During one shot of the long-pulse (∼100 s) beam extraction, the gas pressure in the vacuum vessel increased sharply and the temperature of the cryopump rise from 8 K to 20 K. Moreover, the vessel wall appeared a high temperature after several long-pulse shots. A self-consistent simulation of beam-gas interaction revealed that the heat loads on the vessel wall should be caused by the stray electrons ejecting from the accelerator. Those stray electrons are mainly generated via the stripping or ionization collisions and strongly deflected by the downstream side of the deflection magnetic field for the co-extracted electron. The location of hot spots measured by infrared thermography is consistent with the simulation results. To solve this problem, a series of electron dumps are designed to avoid the direct impinging of the ejecting electrons on the cryopump and the vessel wall. And the results suggest that the hot spots are almost eliminated.

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.

Correlation of source parameters and beam properties in the early operation of the full size ITER negative ion beam source

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.

Properties test of 1D carbon-carbon composite tiles for the diagnostic calorimeter of CRAFT NNBI test platform

An important aspect of the Comprehensive Research Facility for Fusion Technology (CRAFT) project is to conduct research on neutral beam injection based on accelerated negative ions, which is one of the main auxiliary heating methods. To evaluate beam characteristics, a diagnostic calorimeter based on a unidirectional carbon fiber-carbon matrix (CFC) composite has been developed in the negative ion source test facility. The CFC calorimeter can diagnose key features of a particle beam, including its divergence and uniformity. The CFC calorimeter has been developed by a worldwide research team of neutral beam injection (NBI) due to its high spatial resolution, which is determined by the thermal properties of CFC materials. This paper aims to verify the thermal properties of CFC materials from TianCe Technology Co., Ltd. by comparing data acquired by infrared (IR) camera with simulations. Experimental and simulation results show that the CFC material provided by the company satisfies the requirements of experiment.

Predicting ion cyclotron emission from neutral beam heated plasmas in Wendelstein7-X stellarator

Measurements of ion cyclotron emission (ICE) are planned for magnetically confined fusion plasmas heated by neutral beam injection (NBI) in the Wendelstein 7-X stellarator (W7-X). Freshly injected NBI ions in the edge region, whose velocity-space distribution function approximates a delta-function, are potentially unstable against the magnetoacoustic cyclotron instability (MCI), which could drive a detectable ICE signal. Prediction of ICE from NBI protons in W7-X hydrogen plasmas is challenging, owing to the low ratio of the ions’ perpendicular velocity to the local Alfvén speed, v⊥(NBI)/VA≃0.14 . We address this from first principles, using the particle-in-cell kinetic code EPOCH. This self-consistently solves the Lorentz force equation and Maxwell’s equations for tens of millions of computational ions (both thermal majority and energetic NBI minority) and electrons, fully resolving gyromotion and hence capturing the cyclotron resonant phenomenology which gives rise to ICE. Our simulations predict an ICE signal which is predominantly electrostatic while incorporating a significant electromagnetic component. Its frequency power spectrum reflects novel MCI physics, reported here for the first time. The NBI ions relaxing under the MCI first drive broadband field energy at frequencies a little below the lower hybrid frequency ωLH, across the wavenumber range kωc/VA=40 –60, where ω c and VA denote ion cyclotron frequency and Alfvén velocity. Nonlinear coupling between these waves then excites spectrally structured ICE with narrow peaks, at much lower frequencies, typically the proton cyclotron frequency and its lower harmonics. The relative strength of these peaks depends on the specifics of the NBI ion velocity-space distribution and of the local plasma conditions, implying diagnostic potential for the predicted ICE signal from W7-X.

Impact of T i/T e ratio on ion transport based on EAST H-mode plasmas

At the EAST tokamak, the ion temperature (T i) is observed to be clamped around 1.25 keV in electron cyclotron resonance (ECR)-heated plasmas, even at core electron temperatures up to 10 keV (depending on the ECR heating power and the plasma density). This clamping results from the lack of direct ion heating and high levels of turbulence-driven transport. Turbulent transport analysis shows that trapped electron mode and electron temperature gradient-driven modes are the most unstable modes in the core of ECR-heated H-mode plasmas. Nevertheless, recently it was found that the T i/T e ratio can increase further with the fraction of the neutral beam injection (NBI) power, which leads to a higher core ion temperature (T i0). In NBI heating-dominant H-mode plasmas, the ion temperature gradient-driven modes become the most unstable modes. Furthermore, a strong and broad internal transport barrier (ITB) can form at the plasma core in high-power NBI-heated H-mode plasmas when the T i/T e ratio approaches ~1, which results in steep core T e and T i profiles, as well as a peaked n e profile. Power balance analysis shows a weaker T e profile stiffness after the formation of ITBs in the core plasma region, where T i clamping is broken, and the core T i can increase further above 2 keV, which is 80% higher than the value of T i clamping in ECR-heated plasmas. This finding proposes a possible solution to the problem of T i clamping on EAST and demonstrates an advanced operational regime with the formation of a strong and broad ITB for future fusion plasmas dominated by electron heating.

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.

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.

3D modeling of a double-driver ion source considering ion magnetization: an investigation of plasma symmetry modulation methods

A three-dimensional fluid model of a double-driver negative hydrogen ion source for China Fusion Engineering Test Reactor (CFETR) neutral beam injection is developed. In this model, the magnetic filter field is generated by 16 permanent magnets, which are surrounded by a soft iron. In order to accurately describe the transportation of charged species in the presence of strong magnetic field, both the electron magnetization and ion magnetization are taken into account, and the accuracy of the model has been proved by comparison with experimental data. By employing this model, the spatial distributions of the plasma parameters have been investigated, and three methods are proposed to optimize the symmetry at the bottom of the expansion region of a double-driver source. The results indicate that by adjusting the power of Driver I while keeping the power of Driver II constant, the symmetry of the electron density and negative hydrogen ion density could be improved. Furthermore, the inclusion of partition improves the symmetry of the electron temperature and density but has no impact on the regulation of the negative hydrogen ion density distribution. Finally, the application of magnetic shield can not only improve the symmetry of the electron density and negative hydrogen ion density, but also increase their densities at the bottom of the expansion region.

Real-time control of NBI fast ions, current-drive and heating properties

Conventionally, neutral beam injection (NBI) in tokamaks is controlled via engineering parameters such as injection voltage and power. Recently, the high-fidelity real-time NBI code RABBIT has been coupled to the discharge control system of ASDEX Upgrade. It allows to calculate the NBI fast-ion distribution and hence the properties of NBI in real-time, making it possible to control them directly. We successfully demonstrate control of driven current, ion heating and stored fast-ion energy by modifying the injected beam power. A combined ECRH and NBI controller is also successfully tested, which is able to adjust the heating mix between ECRH and NBI to match a certain desired ion heating fraction at given total power. Further experiments have been carried out towards control of the ion heat flux (i.e. ion heating plus collisional heat transfer between ions and electrons). They show good initial success, but also leave room for future improvements as the controller runs into instabilities at too high requests.

Design of a Plasma neutraliser for a Fusion reactor or as an upgrade to the ITER heating neutral beam injectors

The global efficiency of neutral beam injectors to be used on a fusion reactor needs to be of the order of, or greater than, 60 %, which is considerably higher than the efficiency of presently operating NBIs. The required efficiency can only be achieved by increasing the neutralisation efficiency from the ≈56 % obtained with the gas neutralisers currently used, which can be done using a plasma or photon neutraliser. This paper examines possible designs of a plasma neutraliser taking into account the many constraints imposed by operation in the environment around a fusion reactor, and a schematic design is proposed that meets those constraints. The design chosen uses a beam driven plasma neutraliser, which requires no additional power systems, diagnostics or interlocks.Using a plasma neutraliser is shown to improve the global efficiency to ≈55 %. Although that is below the desired value, reasons why that might be acceptable are discussed.This paper also discusses the possible use and benefits of such a neutraliser as an upgrade of the ITER heating neutral beam system.