Negative Ion Accelerator Research Articles

The impact of neutral beam parameters on current drive and neutron yield in DEMO-FNS tokamak

DEMO-FNS (DEMO Fusion Neutron Source) will be a hybrid reactor designed to combine fusion and fission technologies. In a hybrid reactor the power is mainly produced by a fission blanket which is exposed to the neutron flux generated by confined fusion plasma used as a Fusion Neutron Source (FNS). The steady-state operation of the FNS is easier to achieve as compared to pure fusion reactors, allowing the FNS to be more compact. The DEMO-FNS will produce 40 MW of fusion power. The neutral beam injectors (NBI) are designed to provide a steady state plasma heating, rotation, fueling, current drive and neutron generation in DEMO-FNS. Four injector units will deliver 30 MW power in deuterium with energy Eb = 500 keV. The NBI concept and main components are inspired from ITER HNBI design implementing acceleration of negative ions with their neutralization on gas.The high density of injected power in DEMO-FNS leads to a very specific operation scenario with ahigher ratio of high-energy ions in the plasma with respect to conventional tokamaks, where the mainpart of neutron flux will result from fusion between hot-tail ions and relatively coldbackground. The current drive and neutron yield produced by the tangentially injected beam in asteady-state operation are strongly affected by magnetic configuration and plasma kineticprofiles. The fast ions deposition and performance in plasma are limited by the losses associatedwith the neutral beam shape and aiming. The main channels of direct beam losses includeshine-through losses, as well as fast ion orbital losses. The detailed evaluation of NB parametersand geometry effects is a necessary step of scenario optimization.BTR code (Beam Transmission or, in earlier versions, Beam Transmission with Reionization) is intended for injector beamlines study, and BTR detailed beam model can be also applied to perform a detailed analysis of beam losses and performance in plasma. The analysis of beam in plasma shows the ways towards higher values of beam driven current (NBCD) and neutral yield; the beam-in-plasma behaviour can be tuned for a given tokamak configuration. The NB tracing model workflow is implemented in the BTOR suite (Beam in TORoids). The results for DEMO-FNS are compared with the NBI performance in ITER tokamak, as both machines are beam-driven and implement a similar NBI concept.

Comparative Analysis of High-Frequency Plasma Drivers with Various Protective Screens for Atomic Injectors with Multi-Second Pulse Duration

Atomic beam injection is one of the main methods of plasma heating in thermonuclear facilities. An injector of high-energy hydrogen atoms for plasma heating based on the acceleration and neutralization of negative hydrogen ions is being developed at the Budker Institute of Nuclear Physics, Siberian Branch of the Russian Academy of Sciences. The injector uses a surface plasma source, in which a plasma flow is created using a radio-frequency driver: an induction radio-frequency (RF) discharge, ignited inside a cylindrical ceramic chamber when an RF voltage is applied to an external antenna. As a part of this work, a new version of the RF driver is being developed. A protective screen is used to prevent overheating and erosion of the ceramic wall of the driver. The operation of RF driver with different protective screens is studied. These screens reduce the efficiency of RF power transmission into the discharge, but make it possible the operation of the ion source in multi-second or steady-state pulses.

Fast and cold negative ion and neutral atom beams from a water spray

High-power lasers are routinely used to generate energetic positively charged ions, and this paper reports on the observation of negative ions in these experiments. A large number of negative ions and neutral atoms at MeV energies was obtained from the interaction of a high intensity laser pulse with a water spray along with positive ions [Abicht et al., Appl. Phys. Lett. 103, 253501 (2013)]. Beams of negative ions and neutral atoms have the same properties as beams of positive ions. However, the mechanism of negative ion formation and acceleration is still under discussion. In order to gain more information about physics of generation negative ions and neutrals, we present a new experiment where all species, positive, negative ions, and neutrals, are spatially separated, and the electron capture and loss of each in water spray is evaluated. The formation of negative ions and neutral atoms of hydrogen and carbon with energies up to 140 keV and 1.2 MeV, respectively, is confirmed. It is suggested that the electrification of spray droplets plays a decisive role in these charge-exchange processes.

The negative beam source with single driver for CRAFT NNBI: design and conditioning results

The Comprehensive Research Facility for Fusion Technology (CRAFT) is a large scientific facility that is preferentially deployed for the construction of major national science and technology infrastructures. A negative beam source based neutral beam injector (NNBI) with beam energy of 200–400 keV, beam power of 2 MW and beam duration of 100 s is one of the devices. A giant radio frequency (RF) based negative beam source was designed for the CRAFT NNBI system. In order to understand the physics and pre-study the engineering problems for RF negative beams source, several beam sources with different scales will be designed and tested. According to the R&D schedule, a negative beam source with a single driver was designed, developed and tested first. The single driver beam source is a quarter of the size of the full size negative beam source. It contains an RF driver, an expansion chamber and a negative ion accelerator with three electrodes, which are a plasma grid (PG), extraction grid (EG) and ground grid (GG). To enhance the negative ion production, Cesium is injected into the plasma chamber and a magnetic filter filed is produced by current flow through the PG to decrease the electron temperature. The negative beam source was tested on the test facility after assembly, including RF plasma generation, negative ion production, extraction and acceleration. The characteristic of plasma discharge, beam extraction and acceleration was studied without and with Cs injection. The long pulse of 105 seconds negative ion beam was achieved successfully. The extracted negative ion current is 4 A and the ratio of electron and negative ion is around 0.8. It lays a good foundation for the R&D of negative ion source with a multi-driver for the CRAFT NNBI system. The details of the design and experimental results of beam source was shown in this paper.

An integration design model for a large-scale negative ion accelerator of neutral beam injection system for fusion application

Design processes of a large-scale negative ion accelerator for neutral beam injection (NBI) application involve a series of physics and engineering issues, which include high voltage holding, background gas and stripping losses, beamlets optics and steering, particle and power flux on the grid electrodes, heat removal, thermal deformation, and stress of the grids. A self-consistent design model covering all these critical issues has been developed, where the results of one design aspect can be directly plugged into another one as the input conditions with little approximation or assumption. This design model has been applied to the negative ion accelerator of the NBI test facility of CRAFT (Comprehensive Research Facility for Fusion Technology), which is designed to produce a negative hydrogen ion beam of 25 A with the particle energy of 400 keV and the pulse duration of 3600 s. The accelerated current density is required to be 210 A/m2 from 768 apertures with a diameter of 14 mm. The evaluated results of the CRAFT accelerator design are quantitatively analyzed. Additionally, the modeling is applied to a large-scale and relatively complete structure of the multi-grid electrodes. Hence, some nonuniformities or special distributions appear in different design issues, which were not noticed in the reference works.

Tutorial: Modeling of the extraction and acceleration of negative ions from plasma sources using particle-based methods

In this Tutorial, we consider plasma sources with applications to fusion devices and high energy accelerators. These ion sources typically produce negative ions from hydrogen-isotope gases, which are extracted through one or multiple apertures and accelerated to high kinetic energies. Next, they are either double stripped of two electrons to form positive ions used as precursors in accelerator devices or neutralized to produce a neutral beam injected in tokamak reactors. Contrary to the working conditions of most ion sources where volume production prevails, the mechanism of negative ion production by dissociative electron attachment on vibrationally excited molecules inside the plasma volume of fusion-type hydrogen-fueled high power discharges is mostly balanced by their destruction by detachment before being extracted rendering this means of producing negative ions rather inefficient. Surface production through the transfer of electrons from low work function metallic materials to the impacting atoms is the alternative solution to fulfill the requirements for the applications concerned. Negative ions are produced close to the aperture from which they are extracted. As a result, the analysis and understanding of the extraction mechanisms through experimental diagnostics is rather difficult due to the lack of accessibility and can only give a partial view. In addition, most of the experimental work is focused on the validation of requirements for the applications and not to the investigation of the fundamental processes that take place inside these types of sources. This Tutorial is focused on the description and understanding of the physical mechanisms behind the extraction and acceleration of negative ions from hydrogen plasma sources through modeling methods. We describe the numerical techniques of particle-based methods with a specific emphasis on particle-in-cell Monte Carlo collision algorithms. An analysis of the physical processes involved in driving the negative ions from the plasma source, across the apertures and inside the accelerator as reported in the literature, is presented in detail. This Tutorial concludes with additional and future works to be addressed in the coming years.

Suppression of stray electrons in the negative ion accelerator of CRAFT NNBI test facility

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.

2-D Fluid Model for Discharge Analysis of the RF-Driven Prototype Ion Source for ITER NBI (SPIDER)

The experimental fusion reactor ITER will be heated by injection of a fast neutral beam generated by acceleration and neutralization of negative ions. The negative ion source used for this purpose (SPIDER), constructed at the Consorzio RFX (Italy), consists of eight driver volumes where radio frequency (RF) power is inductively coupled to the plasma electrons and an expansion chamber containing a magnetic filter (MF). This article presents self-consistent 2-D fluid description of the source, including neutral gas flow, plasma chemistry, and plasma transport through the magnetic filter. The numerical method is based on finite volume approximation, and nine-point discretization is used to account on anisotropy due to magnetic field. Semi-implicit numerical solver allows for large time steps producing steady-state solution in a reasonable time (few hours for the typical mesh). Simulation results for the simple configuration show code numerical stability and efficiency. Influence of neutral gas pressure, RF power, bias potential, and magnetic field on the plasma properties is investigated. It has been found that the reduced electron mobility perpendicular to the magnetic field reduces electrostatic plasma screening, allowing for easier extraction of negative charges from the plasma by means of an applied voltage.

Challenges toward improvement of deuterium-injection power in the Large Helical Device negative-ion-based NBIs

Improvement of deuterium injection power in the negative-ion-based NBIs (n-NBIs) for the Large Helical Device (LHD) are reported. Co-extracted electron current at acceleration of deuterium negative ions (D− ions) limits the injection power. The electron current is reduced by decreasing the extraction gap, and the injected D− current evaluated from the injection power increased from 46 to 55 A. Greater electron reduction was achieved by installing a structure named an ‘electron fence’ (EF), with which D− beam power was successfully improved from 2.0 MW to 3.0 MW. The injection power in three configurations − without EF, with EF of 5 mm and 7 mm distance from the plasma grid (PG) surface − have been compared in both cases of hydrogen and deuterium operations, and it was found that the configuration with the EF of 5 mm distance was the best to satisfy the performance for both of hydrogen and deuterium injections. Although the co-extracted electron current is reduced in the negative ion sources applied for JT-60SA and ITER by utilizing the PG filter, it is possible to achieve more effective electron reduction by combining the PG filter and the EF.

Development of high-voltage negative ion based neutral beam injector for fusion devices

A prototype of a powerful high-voltage neutral beam injector, based on acceleration of negative hydrogen ions and their neutralization, is under development at Budker Institute of Nuclear Physics (BINP). The design of the BINP high-voltage injector includes several innovative components, important for injector operation stability and overall efficiency. It includes a multi-aperture long-pulse surface-plasma negative ion source with thermostabilized grid, ithe magnetic system with concaved field lines in the ion-optic system (IOS) and the distributed cesium deposition system. The injector scheme incorporates a wide-aperture low-energy beam transport (LEBT) section, plasma target for negative ionneutralization, and recuperators of non-neutralized ions. Several test stands were constructed at BINP for injector component studies. This paper describes the results of experiments on negative ion beam production, transport through LEBT, ion acceleration to energy up to 240 keV and transport through the high voltage beam transport (HEBT) section to the distance ∼10 m from the source. The parameters of the transported beam, which were measured at several points along the beam line and at the beam dump calorimeter, are presented. The beam transport efficiency as a function of various ion source, LEBT and HEBT parameters is measured and compared with the calculated values. The results dof plasma neutralization target study are presented as well.

Benchmark of beam acceleration codes on a high voltage negative ion accelerator for fusion with a new hypothesis on the beam meniscus

It is a widely recognized phenomenon that, in negative ion sources for NBI, the magnetic fields in the source cause an asymmetry of the current density at the meniscus, which affects the beam optics. In order to achieve a more realistic estimation of the negative ion beam optics, the ion current density profile at the beamlet meniscus is sometimes assumed to be non-symmetric with respect to the beamlet axis. The new hypothesis tested in this paper is an empirical law obtained by recent investigations on a similar Kamaboko ion source, which relates this asymmetry to the magnetic field strength at the meniscus and to the power injected in the ion source. For the first time this hypothesis is used to directly and correctly predict the value of the beamlet deflection.The MeV ion source Test Facility (MTF) is a high voltage negative ion beam accelerator dedicated to fusion experiments in QST, Naka, Ibaraki, Japan. MTF can be operated as a three- or five-stage multi-aperture accelerator with a configuration and beam energy comparable to the future ITER NBI. In 2019, experiments were conducted on MTF aiming at accelerating a H- beam with high energy and good optics for a long duration of the beam (around 100 s). These experiments allowed to measure the beamlet divergence and deflection as well as the heat loads due to particle impinging on the accelerator grids, over a wide range of operating parameters. The predictions of several beam simulation codes (Opera, COMSOL, SLACCAD, EAMCC3D) have been compared to the experimental measurements, which allows to obtain a good agreement in terms of beam optics and a fair agreement in terms of heat loads.

Conceptual design of a beam source for negative neutral beam injector of CRAFT facility

The Comprehensive Research Facility for Fusion Technology (CRAFT) is a large scientific device that is preferentially deployed in China's "Thirteenth Five-Year Plan" for the construction of major national science and technology infrastructures. A beam source of negative neutral beam injector (NNBI) is required for the CRAFT device with the beam energy of 400 keV, neutral beam power of 2 MW, and beam duration of 100 s. The beam source is the key component of NNBI system. A radio frequency (RF) based negative beam source was designed for the CRAFT NNBI system. It contains a large plasma generator and a negative ion accelerator. The plasma generator contains a large expansion chamber and four RF drivers which arrange in a vertical column (1 × 4). The accelerator has a similar design to ITER. Consider the 400 keV beam energy, the accelerator was designed with four layers of electrodes, which are plasma grid, extraction grid, acceleration grid, and ground grid. The detailed structures of negative beam source were designed initially with the RF plasma generator and the negative ion accelerator.

Experimental and numerical investigation on the asymmetry of the current density extracted through a plasma meniscus in negative ion accelerator

In multi-beamlet negative ion accelerators for neutral beam injectors, the transverse magnetic field necessary for suppressing the co-extracted electrons induces a deflection of the negative ion beamlets that must be corrected. For the design, particle-tracing simulation codes are used to compute ion trajectories and optical properties of the beamlets in the acceleration stage. In these codes, uniform boundary conditions are normally assumed for the ion current density distribution at the surface (called meniscus), where negative ions are extracted from the plasma and form beamlets, which are accelerated across the apertures of the accelerator grids. Recently, experimental campaigns dedicated to the accurate measurement of the beamlet deflection in the acceleration phase revealed higher deflection than foreseen by simulations. In this work, we demonstrate that an agreement with the experimental data can be obtained by incorporating in the numerical simulations a non-radially symmetric distribution of the ion current density extracted across the meniscus surface. In the first part, the asymmetry of the ion current density at the meniscus is studied in an empirical way, by analysing and fitting the experimental results obtained with different operating parameters. In the second part, we show that the ion current asymmetry estimated by this procedure is well consistent with the flow pattern of H− ions calculated in the meniscus zone using a detailed particle in cell (PIC) model of the ion source in the presence of transverse magnetic field.

Achievement of high power and long pulse negative ion beam acceleration for JT-60SA NBI.

Long pulse acceleration of hydrogen negative ion beams with the power density over 70 MW/m2 and the pulse length over 100 s has been demonstrated for the first time by using a multi-aperture 3-stage accelerator. Such long pulse acceleration was achieved by integrating the design of beam optics and voltage holding capability to meet the requirements of JT-60SA. By using the newly designed accelerator for JT-60SA, voltage holding at 500 kV with beam acceleration was stably sustained even after 5 g of cesium was seeded, and heat load on each acceleration grid was reduced below the allowable level for long pulse, less than 5% of total acceleration power. As a result, 500 keV, 154 A/m2 for 118 s beam acceleration was achieved, which satisfies the requirement of the negative ion source for JT-60SA. This pulse length of such high-power density beams is longest in the world. In addition, the result contributes to the long pulse acceleration of multi-stage electrostatic accelerators, such as 1 MeV negative ion accelerator for ITER.

Development of continuous wave high voltage negative ion beam injector for tandem accelerator.

A vacuum-insulated tandem accelerator, delivering the continuous wave 8 mA, 2 MeV proton beam, is operated regularly at the Budker Institute of Nuclear Physics, where a 10 mA, 25 keV negative ion injector is used. Recently, a new injector with an upgraded negative ion source and beam preacceleration has been developed to increase the tandem accelerated current. The transport line of the new injector is composed of a bending magnet with 90° ion beam turn, an acceleration tube for negative ion acceleration to the energy up to 150 keV, and a 0.6 m long transport section. The H- ion beam production, its acceleration, and transport were studied at a test stand, which is equipped with electrical and optical diagnostics. The data on 14 mA, 133 keV continuous wave negative ion beam production and transport are presented. The undesirable coacceleration of secondary electrons, produced in the acceleration tube, was recorded as well. The coaccelerated electrons' current contributed up to 2% of the total accelerated beam at the operational vacuum in the low energy beam transport. The coaccelerated electrons were removed from the beam with a magnetic filter. The numerical modeling of the beam transport was carried out. A reasonable agreement between the modeled and experimental data was obtained.

Simulation and measurement of rarefied gas flow and neutral density profiles through a large multiaperture multigrid negative ion accelerator

In large multi-aperture, multi-grid negative-ion accelerator for fusion application, the background gas density in the electrostatic accelerator causes a loss of negative-ion current and the formation of dangerous stray particles. In addition, to sustain in dynamic equilibrium a sufficient gas pressure in the plasma ion-source, a very large pumping speed is typically necessary. This paper presents and compare simulations and measurements of gas flows and background pressure profiles, through the electrostatic accelerator of the SPIDER beam source, the full-size prototype source of the ITER neutral beam injectors. The gas pressure profiles through the ion accelerator were measured by multiple movable capacitive pressure gauges, mapping the uniformity of the profiles along nine beamlets at three different filling pressures. The gas flow simulations in molecular flow regime were performed with a large-scale view-factor model, and with a single-aperture periodic test particle Monte Carlo model. The models reproduced the measured pressure profiles but additionally, provided also the profiles of local gas density. The gas conductance and the profiles calculated with the full-scale gas flow model correctly reproduce the measured profiles, and the transversal non-uniformities over the 1.6 m × 0.6 m cross section. The calculated gas density profile is also verified by comparing the two different numerical approaches. The gas flow model validated at room temperature can be used to simulate the pressure profile during operation, in presence of gas heating and dissociation by the source plasma.

Physics and engineering design of 400 keV H- accelerator for negative ion based neutral beam injection system in China.

A research project of the China Fusion Engineering Test Reactor (CFETR) Negative ion-based Neutral Beam Injection (NNBI) prototype has been started in China. The objectives of the CFETR NNBI prototype are to produce a negative hydrogen ion beam of >20 A up to 400 keV for 3600 s and to attain a neutralization efficiency of >50%. In order to identify and optimize the design of the negative ion accelerator, a self-consistent model has been developed to consider all key physics and engineering issues (electric and magnetic fields, background gas flow, beam optics, beam-gas interaction, secondary particle trajectories, power deposition on grids, heat removal design, and mounting pattern). This paper presents the primary results by applying the self-consistent model to the current design of the 400 keV H- accelerator of the CFETR NNBI prototype.

Negative ion beam source as a complex system: identification of main processes and key interdependence

Complex network theory offers new tools to tackle controllability of systems such as processes affecting generation, extraction and acceleration of negative ions in ion beam sources. A model has therefore been developed and tested for the negative ion beam source NIO1. Preliminary results show good agreement between model predictions and experimental behaviour of the source. A subset of driver processes which in principle allow the system to be controlled has been identified. One of the driver processes related to the meniscus formation has been found to play a key role in the global controllability of the system, so that a detailed discussion of the physics behind its formation and uniformity has been provided.

Improving a negative ion accelerator for next generation of neutral beam injectors: Results of QST-Consorzio RFX collaborative experiments

In the framework of the collaboration between Consorzio RFX (Padova, Italy) and QST (Naka, Japan) two experimental campaigns have been organized on the Negative Ion Test Stand (NITS) in Naka, employing an ITER-like multi-beamlet configuration. These campaigns were aimed at compensating for the undesired magnetic deflection of the ion beam. The experiments have clarified the influence of operating parameters on the optical properties of the ion beam. The physical phenomena acting on the optics have been investigated, in particular the effect of an asymmetry of the ion current density at meniscus. Comparisons with simulations have been made and the modeling has been improved consequently. During the second experimental campaign, a perfect compensation of the ion deflection has been achieved.

Extraction of positive and negative ion beams from large area plasma sources

In this study we investigate extraction and acceleration of positive and negative ions from a pulsed inductively coupled plasma (ICP) in oxygen and hydrogen. Experiments were performed in a directed ribbon beam system where ions can be produced at an incident angle non-normal to the target. Positive and negative ions were transported to the wafer. The RF plasma source, extraction electrodes and wafer were biased synchronously. The energy of extracted ions closely follows the amplitude of the applied bias voltage and it ranges from few hundreds of volts to 10 kV. The peak beam current density can reach 100 A m−2. The ion beam angles, ion current, and ion composition are reported. The injection physics requires correlating the positive and negative ion and electron densities near the extraction opening with the extracted currents. The plasma was modeled using CRTRS, a plasma fluid code that self-consistently solves for ICP power deposition, electrostatic potential, and plasma dynamics. A new perspective on the possible production of angled ion beams on surfaces is discussed.