Negative Ion-based Neutral Beam Research Articles

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.

Preliminary experimental results of tungsten wire calorimeter for CRAFT NNBI

The comprehensive Research Facility for Fusion Technology(CRAFT) has been carried out since 2019 in China and as an important component of CRAFT, a negative ion-based neutral beam injection sysgem(NNBI) (200–400 keV, 2 MW) is under construction. In order to realize online diagnosis of beam uniformity, a tungsten wirecalorimeteris developed. The tungsten wire calorimeter is characterized by double-layer tungsten wire mesh structure. During beam bombardment, the tungsten wire mesh is heated and emits light, a visible light camera is used to photograph the tungsten wire calorimeter. The beam profile is obtained by digitizing the light intensity distribution map. In this paper, through the thermal simulation anslysis of tungsten wire,the installation position of tungsten wire calorimeter are determined and the spatial resolution under different beam divergence are also analyzed.The tungsten wire calorimeter was tested on the beam source test platform, the light intensity distribution maps of the tungsten wire calorimeter under 15 keV–40 keV beam energy were obtained. The work above will accumulate experience for the operation of tungsten wire calorimeter for full scale beam source in the next stage, and provide technical means for the conditioning of negative beam sources.

Simulations for the cesium dynamics of the RF-driven prototype ion source for CRAFT N-NBI

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

Simulation study of interaction between energetic particles and magnetohydrodynamic modes in the JT-60SA inductive scenario with a flat q≈1 profile

Interactions between energetic particles (EPs), an internal kink mode, and other magnetohydrodynamic (MHD) instabilities in the inductive scenario of JT-60SA (scenario 2) are simulated with MEGA, a global EP–MHD hybrid code. For this scenario, it was predicted by TOPICS, an integrated transport code that the internal kink mode can be unstable and the sawtooth relaxation results in a flat safety factor (q) profile with q ≈ 1 for . In this equilibrium, it is found in the simulation results that the stability of the internal kink mode depends strongly on the bulk plasma pressure gradient (). In the n = 1 simulations where the toroidal mode number is restricted to n = 0 and 1, the pressure-driven internal kink mode is dominant. In the presence of co-passing EPs generated by the negative-ion-based neutral beam (NB), these EPs transfer energy to the internal kink mode; however, the EP driving rate () is much lower than the driving rate from the bulk plasma pressure gradient (). The mode’s frequency is less than 1 kHz because the toroidal orbit frequency (ω φ ) and poloidal orbit frequency (ω θ ) of the co-passing EPs are approximately equal within the q = 1 surfaces. This mode affects the EP and bulk plasma pressure redistributions. The feasibility of stabilizing the internal kink mode using trapped EPs is also investigated. It is found that the trapped EPs with energy 85 keV generated by the positive-ion-based NBs cannot stabilize the internal kink mode. Stabilization is observed when the injection energy is greater than 500 keV. In the multi-n simulations, where modes are retained, the most unstable modes are high n interchange modes with poloidal number m = n whose linear growth rates exceed that of the pressure-driven internal kink mode observed in the n = 1 simulations. The overlapping of these modes creates a stochastic magnetic field, leading to stronger EP and bulk plasma pressure redistributions than those observed in the n = 1 simulations. During the nonlinear phase, the transition from the high m = n modes to the low m = n modes is observed where the dominant mode is the mode with an internal kink-like structure. These low m = n modes are generated by the nonlinear coupling of the high m = n modes. The EP kinetic effect has a minor contribution to the dynamics of these nonlinearly generated m = n modes.

Structural design and strength analysis of the calorimeter pipeline joint for the negative neutral beam injection system of China fusion engineering test reactor

The negative ion-based neutral beam injection (NNBI) verification prototype system is one of the core components of China Fusion Engineering Test Reactor (CFETR). To effectively cut off the beam and debug the neutral beam device, the power measurement system plays an indispensable role. In this study, we examine the detailed design of the pipeline joint structure of the calorimeter verification prototype, focusing on the impact of the structural design of the target panel pipeline joint on the structural strength and manufacturing process required by the cooling circular pipeline joint of the calorimeter. Based on this, the manufacturing process and strength of the integrated pipeline structure of the target panel are inspected under a design pressure of 4 MPa, including theoretical analysis of the integrated pipeline pressure-bearing structural design, welding technology design, and finite element simulation under high pressure. Accordingly, the feasibility of the final manufacturing scheme and key technologies are analyzed. The pressure test of the sample under an internal N2 pressure of 4 MPa verifies that its structure can meet the design requirements, thereby providing reference for the effective structural design and manufacturing of the NNBI calorimeter prototype.

Final design and manufacturing of the neutralizer for the negative ion-based neutral beam injector test facility of CRAFT

The Comprehensive Research Facility for Fusion Technology (CRAFT) is a highly integrated research and development base. It consists of 20 different demonstrating or testing facilities that address most of the critical technologies and systems for the fusion reactor in China. A negative ion-based neutral beam injector (NNBI) test facility is established in CRAFT. A gas neutralizer is applied to the CRAFT NNBI test facility for the neutralization of the negative hydrogen ion beam. The target of the neutralization efficiency is more than 50%. The length of the neutralizer is 3 m. The neutralizer is divided into two adjacent vertical channels and the total opening is 1.7 m × 0.32 m. The neutralizer is mainly composed of channel plate, support structure, beam scraper, cooling water pipes, and gas inlet pipes. Based on the design requirements of the neutralizer, the support structure and cooling water pipes were optimized, to reduce the total weight. The structural strength of all components was simulated and compared to the preliminary design. The manufacturing of the neutralizer is ongoing. Some manufacturing challenges have been overcome, such as deep hole drilling. Several 1.7 m long cooling channels were successfully drilled in a copper panel and the ratio of depth to diameter was 94.

Overview of photo-neutralization techniques for negative ion-based neutral beam injectors in future fusion reactors

Overview of photo-neutralization techniques for negative ion-based neutral beam injectors in future fusion reactors

Dynamic Voltage-Balancing Method of Series Diode Rectifier in Inverter-Type AGPS

In negative ion-based neutral beam injector (N-NBI) system, acceleration grid power supply (AGPS) provides ultrahigh dc voltage for accelerator. Because the power supply often occurs in the ignition fault similar to the load short circuit, it will produce complex and violent electromagnetic transient process. The diode rectifier is the last stage of it and the diode rectifier is directly connected to the load of the AGPS. The working voltage of the rectifier reaches hundreds of kV, and a large number of diodes are used in the device in series. Therefore, it is necessary to fully consider the influence of distributed capacitance on voltage balancing and improve the static and dynamic voltage-balancing reliability of diodes. However, discussions about solutions for the dynamic voltage imbalance in the reverse recovery process caused by parasitic capacitance are not sufficient. This article presents a voltage-balancing method in the form of parameter relationships between non-isoparametric resistance-capacitance ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RC</i> ) snubber circuits. Under the assumption of uniform voltage-sharing, the equivalent parallel circuit of each diode is obtained through constructing isoelectric points, accordingly, the parameter relationships are deduced. In this article, both the distributed capacitance to ground and the distributed capacitance to high voltage are considered in the derivation process; the derived non-equal parameter <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RC</i> voltage-balancing method includes not only the buffer capacitance parameter relationship, but also the damping resistance parameter relationship and upper limit, which innovatively explores the important influence of damping resistance on the voltage-balancing effect. Dual-pulse tests on series-connected diodes prototype are carried out; experimental results show that voltage balance ratio <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\alpha $ </tex-math></inline-formula> of the prototype is only 0.80 with the traditional isoparametric <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RC</i> circuit in parallel, while <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\alpha $ </tex-math></inline-formula> is up to 0.99 with the non-isoparametric <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RC</i> circuit designed through the proposed method. The results will be valuable for improving the reliability and device utilization rate of series-connected diodes rectifiers in inverter-type AGPS.

Thermal analysis and optimization of the calorimeter verification prototype of a negative ion-based neutral beam injection system

The neutral beam needs to be cut off through the calorimeter before the neutral beam is injected into the Tokamak, so that the neutral beam injector can be debugged and tested independently of the Tokamak. The calorimeter can open or close the channel of the beam entering the Tokamak. The long pulse and strong heat flow of high-energy particles directly bombard the calorimeter, which needs to bear a power density of at least 6 MW/m2 which lasts 3600 s and circulates every half an hour, necessitating high requirements for the cooling and heat transfer capacity of the calorimeter. Therefore, optimization analysis must be performed for a single slender swirl tube to improve the heat transfer capacity of the CFETR NNBI calorimeter verification prototype. In this paper, the comprehensive heat transfer capacity is taken as the optimization index, and the key parameters are optimized and analyzed, including the shape of the swirl tube and the parameters of the tapes, such as the length, type, pitch, perforation scheme, number and placement direction. The simulation results show that by changing the heat exchange tube shape, the heat exchange capacity of oval tubes without tape is increased by 33.5% compared with that of circular tubes without tape. When the number of tapes or sides increases, the heat transfer efficiency increases only slightly. As the tape pitch decreases, the field synergy angle decreases, and the heat transfer effect improves. Comparing punched tapes with unperforated tapes, the heat transfer capacity of the inserted tapes with pitches of 28 mm and 15 mm is similar. The original swirl tube of the calorimeter verification prototype of the NNBI-designed cooling circular tube is inserted with full-length bilateral tape at a pitch of 56 mm. The resulting average wall temperature is 424 K. Finally, an oval tube is selected, with half-length bilateral perforated tape, the pressure inlet is set to 2 MPa, and the pressure outlet is set to 0.8 MPa, with a tape pitch of 20 mm. The resulting average wall temperature of the swirl tube is 404 K. Compared with the original model, the average wall temperature decreases by 11.4%, and the peak wall temperature of 601 K is lower than the softening temperature of 823 K of the CuCrZr wall material, which effectively improved the heat transfer performance. To prove the heat exchange effect of the improved structure, a velocity of 13 m/s is selected as the inlet boundary condition. Compared with the original model, the average wall temperature of the optimized model is reduced by 30.8%, which is consistent with the conclusion of a given inlet and outlet pressure. These simulation conclusions provide a reference for the future design of calorimeters that bear high current and long pulse bombardment.

Neutron characterization analysis of the negative ion-based neutral beam verification facility for CFETR

Neutral beam injection heating with negative ion source is one of the important auxiliary heating system to achieve the high parameter steady- state operation goal for CFETR. A Negative ion-based Neutral Beam Verification Facility (NNBVF), a conceptual design for testing the engineering feasibility of neutral beam injection systems, is proposed for the Chinese Fusion Engineering Test Reactor (CFETR). The neutral-beam injectors used for CFETR will operate with deuterium. Inevitably, a large number of neutrons are generated between beam deuterons and deuterons implanted in the calorimeter. Neutron source is one of the important input data for radiation analysis. The neutron characterization on the calorimeter surface were explored by using a multiaperture Gaussian divergence model and the local mixing model. The total neutron intensity reach up to 1.2 × 1014 n/s for 400 keV deuteron beam. Initial results revealed that the aspect ratio of the neutron beam profile decreases with increasing beam divergence. The shape of the neutron profile on the calorimeter surface remains the same for the beam density distributions. This implies the neutron intensity emitted from the target is suitable for the beam density distribution diagnostic applications.

Thermo-mechanical design of the neutralizer for CRAFT negative ion-based neutral beam injection system

The Comprehensive Research fAcility for Fusion Technology (CRAFT) has been launched in China and a negative ion-based neutral beam injection system (NNBI) is an important component of CRAFT. The primary target of the CRAFT NNBI is to realize a neutral hydrogen beam of >2 MW at the beam energy of 400 keV for the pulse duration of 100 s. In the CRAFT NNBI, the neutralization of the negative ion beam will be accomplished in a gas neutralizer mainly consisting of two vertical channels. Each channel has a rectangular cross-section of 0.145 m × 1.7 m and a length of 3 m, which is formed by several copper panels. A key issue of the neutralizer is to remove the heat load from the incident ion beam. This paper presents the thermo-mechanical design process of the neutralizer of the CRAFT NNBI. Because the heat load densities on the lateral walls of the panels are relatively low, their surface temperature can be well controlled under the safe values of copper by deep drilling of the cooling channels. The largest problem is the leading edge of the panels, where the heat load density is extremely high. Thus, the leading edge element (LEE) is carefully designed to avoid damage to these panels. The LEE is made of CuCrZr alloy and the twisted tape is inserted into the cooling channel to promote the turbulence and consequently the heat transfer capability. Finally, the thermo-mechanical properties of the heat load components of the neutralizer can be controlled under the required values in terms of the surface temperature, water temperature, stress, and deformation.

First result of CRDS system with cesium operation for radiofrequency negative ion source test facility at ASIPP

From September 2017, an advance study project of Negative ion-based Neutral Beam Injection (NNBI) prototype for the China Fusion Engineering Test Reactor (CFETR) was supported by the ministry of science and technology of China. The entire test facility will be integrated at Institute of Plasma Physics, Chinese Academy of Sciences (ASIPP). In order to meet requirement of the development of negative ion based neutral beam injection system for CFETR, a negative ion source test facility with radio frequency source is being built. At the same time, cavity ring-down spectroscopy (CRDS) is employed for measurement of negative ion density. CRDS is an ultrasensitive absorption diagnostic technique for density measurement. Based on photo-detachment process of hydrogen negative ions, CRDS can measure integrated line-of-sight density of hydrogen negative ions in high power ion source. In this paper, the principle of CRDS diagnostic system has been introduced. Based on previous tests, the system setup and improvement are mentioned. In this paper, typical ring-down signals and time evolution of density of hydrogen negative ions are reported. With the first time cesium seeding, density of hydrogen negative ions increased obviously. It increased from 6.3E15 m−3 to 1.8E16 m−3, about 2.8 times under the 40 kW RF power discharge. Several days condition up, the density of hydrogen negative ions is up to 3.4 E16 m−3 under the 50 kW RF power discharge.

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.

Characterization of ZAO® sintered getter material for use in fusion applications

The use of non-evaporable getter (NEG) pumps is common in many UHV applications, and the properties of the new sintered getters based on the ZAO®alloy make them appealing for applications dealing with large fluxes of hydrogen and its isotopes, as typically occurs in fusion research. In particular, the use of NEG pumps is interesting for the vacuum system of negative ion-based neutral beam injectors, which require huge deuterium gas throughputs for long durations. The key advantages of a NEG pumping solution are the high specific pumping speed and capacity, the ability to withstand a large number of absorption/desorption cycles, the ease of integration. In addition, NEG do not release hydrogen unless power is supplied to heat them, addressing an important safety issue related to uncontrolled release in case of power outage or subsystem failures.This paper reports the experimental characterization of ZAO®sintered getters, in pressure regimes and sorption amounts relevant for the use in the next generation NBI. Single discs and complete pumps were characterized in different conditions of H2/D2 pressure between 10−4 and 10-1Pa and operating temperatures up to 150°C. In particular, the feasibility of getter regeneration within time windows compatible with current NBI operation scenarios was demonstrated.

Reconnecting instabilities in JT-60SA during current ramp-up with off-axis N-NB injection

We report results of global resistive magnetohydrodynamic (MHD) simulations, which predict that wide-spread reconnecting instabilities may cause an internal disruption when 5 MW off-axis negative-ion-based neutral beam (N-NB) injection is used during the current ramp-up phase of a JT-60SA plasma. The simulations include fast ions as passive test particles and were initialized with plasma profiles obtained from an integrated transport simulation. The beam-driven current produces a nonmonotonic safety factor profile with three q = 2 rational surfaces. The resulting double or multiple kink-tearing instabilities redistribute both the bulk and fast particles in the inner half of the plasma. While the radial profiles appear to be flattened, the actual field topology and particle distributions are found to saturate in helical states. Moreover, because magnetic drifts are still large during the current ramp-up phase, there is a large difference between the topology of the magnetic field and 500 keV deuteron orbits. Fast ion phase space islands may thus appear and overlap at different times and in different regions than their magnetic counterparts.

Development of a negative ion-based neutral beam injector in Novosibirsk

A 1000 keV, 5 MW, 1000 s neutral beam injector based on negative ions is being developed in the Budker Institute of Nuclear Physics, Novosibirsk in collaboration with Tri Alpha Energy, Inc. The innovative design of the injector features the spatially separated ion source and an electrostatic accelerator. Plasma or photon neutralizer and energy recuperation of the remaining ion species is employed in the injector to provide an overall energy efficiency of the system as high as 80%. A test stand for the beam acceleration is now under construction. A prototype of the negative ion beam source has been fabricated and installed at the test stand. The prototype ion source is designed to produce 120 keV, 1.5 A beam.

Suppression of fast electron leakage from large openings in a plasma neutralizer for N-NB systems

To produce highly ionized plasmas at low operating pressure in a plasma neutralizer of negative ion based neutral beam (N-NB) systems, it is a critical issue to suppress leakage of fast electrons through large openings as the beam entrance/exit. The authors propose to form weak transverse magnetic fields without a significant beam deflection, called the shield field, across the large openings of the neutralizer. A numerical study showed that the shield field of only few tens of Gauss is sufficient to suppress the fast electron leakage from the openings. By measuring of an electron energy distribution function (EEDF), it was confirmed that such a weak magnetic field is enough to repel the fast electrons back into the neutralizer plasma. As the result, the plasma density increased with the shield field strength and saturated at 30 G. The plasma density reached 50% higher value than that without the shield field. Thus it was found that reflected fast electrons by the shield field of only 30 G work effectively for the plasma generation. It was also estimated that such a weak magnetic field sufficiently suppresses the deflection of a 1 MeV beam. This weak magnetic field would be applicable to the plasma neutralizer for the fusion demonstration (DEMO) plant.

Energetic ion transport by abrupt large-amplitude event induced by negative-ion-based neutral beam injection in the JT-60U

To investigate energetic ion transport induced by bursting modes in the frequency range of Alfvén eigenmodes, which is called abrupt large-amplitude events (ALEs) driven by negative-ion-based neutral beam (N-NB) injection, neutron emission profile measurement and charge exchange (CX) neutral particle (flux) measurement using a natural diamond detector have been performed simultaneously in JT-60U. It is found from the CX neutral particle (flux) measurement that energetic neutral particles in a limited energy range (100–370 keV) are enhanced due to ALEs, and the neutron radial profile is flattened. The change in the energetic ion density profile inferred from these measurements indicates that ALEs expel energetic ions from the core region of the plasma and induce both redistribution and loss of energetic ions. It has been shown that the energy range of transported energetic ions is consistent with a resonance condition between energetic ions and ALEs, and the energetic ion transport results from the resonant interaction between energetic ions and ALEs. Further, a fraction of the energetic ion loss has been quantitatively estimated to be ∼4%.

Overview of JT-60U results leading to high integrated performance in reactor-relevant regimes

Recent JT-60U results leading to high integrated performance are reported with emphasis on the projection to the reactor-relevant regime. Negative-ion-based neutral beam (NB) and electron cyclotron (EC) power increased up to 6.2 MW and 3 MW, respectively. A high βp H-mode plasma with full non-inductive current drive has been obtained at 1.8 MA, and the fusion triple product reached 3.1 × 1020 m−3 keV s. A high beta with βN = 2.7 was maintained for 7.4 s. Neoclassical tearing mode suppression with EC was accomplished using a real-time feedback control system, and improvement in βN was obtained. The stable existence of a current hole was observed. A high DT-equivalent fusion gain of 0.8 was maintained for 0.55 s in a plasma with a current hole. Current profile control in high bootstrap current reversed shear plasmas was demonstrated using N-NB and LH. A new operation scenario has been established in which a plasma with a high bootstrap current fraction and internal transport barriers (ITBs) is produced without the use of an OH coil. An ECCD study was undertaken in a reactor-relevant high Te regime. A new type of Alfvén eigenmode mode has been proposed and has been found to explain the observed frequency chirp quite well. High confinement reversed shear plasmas with Te > Ti were obtained. Ar exhaust with EC heating was obtained in a high βp mode plasma. Impurity accumulation related to strong ITBs in a reversed shear plasma and degradation of ITB by ECH in a weak positive shear plasma have been found. Dedicated measurement of edge localized mode dynamics and scrape-off-layer plasma flow have advanced understanding of the physics. N-NB heating in an Ar-seed plasma extended the density region to 95% of the Greenwald density, with HHy2 = 0.9. Enhancement of pedestal pressure was obtained with an increase in βp in a high triangularity configuration.

Accelerator R&D for JT-60U and ITER NB systems

High energy accelerator and high current ion source development have been carried out at JAERI for negative-ion based neutral beam (N-NB) systems of JT-60U and ITER. One of R&D issues on the ITER prototype 5-staged electrostatic accelerator was voltage holding capability of the accelerator insulator column. By installing a stress ring, which reduces electric field concentration at the triple junction (interface of metal flange, insulator, and vacuum), 300 kV is held at each stage instead of rated voltage of 200 kV. At present the accelerator insulator column sustains 900 kV stably with the rings in all five stages. In the JT-60U N-NB system, a beamlet deflection by distorted electric field was found at the bottom of the extractor. Correcting the distorted field, reduction in beam divergence was confirmed for the overall beam. As the result, the heat load on the NB port limiter of JT-60U, located about 20 m away from the accelerator, was reduced to less than a half of the previous value before the correction. Consequently, we have succeeded in 10 s continuous injection of H 0 beam with the NB power of 2.6 MW at 355 keV.