Negative-ion-based Neutral Beam Injection 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.

Auto-Recording System for Long-Pulse Negative Ion–Based Neutral Beam Injection Experimental Data

For long-pulse negative ion–based neutral beam injection (NNBI) experiments, an automatic recording system was designed. The data processing algorithm of neutral beam injection was implemented on the basis of existing methods, and different experimental data were processed in a mode-based method according to the physical characteristics of the neutral beam injection experiment. Based on MDSplus segment storage technology, the range of recorded data was determined by intercepting the segment sequence number of the time series data. LabVIEW was used to achieve the efficient and accurate transmission of experimental processing data. A set of tests demonstrates that the NNBI auto-recording system meets the requirements of the NNBI experience in data processing accuracy (normalized mean square error [NMSE] < 0.04), data transmission accuracy (>98%), and data affect presentation. This system not only obviously improves the work efficiency, but also provides more reliable data sources for the intelligent analysis and processing of the NNBI system.

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.

Functional optimization for a Beam Driven Plasma Neutralizer in DEMO Neutral Beam Injector

The Beam Driven Plasma Neutralizer (BDPN) has been proposed as a more efficient alternative to the gas neutralizer for negative-ion based Neutral Beam Injection (NNBI). In this paper we model the performance of an entire NNBI beamline with a BDPN. We simultaneously consider all the relevant physics and engineering aspects, the most important being the plasma density and degree of ionization inside the BDPN as a function of its geometry and feed gas flow, the geometrical transmission of the beamline, the dependence of the neutral gas distribution in the beamline on the geometry of the beamline components and gas flows, and the species evolution of the extracted D− beam through this neutral and charged particle distribution. Furthermore, we calculate the heat loads expected on the BDPN parts and on the NBI components located downstream of it and study the effect of the magnetic cusp field across the BDPN entrance on beamline transmission. While our results constitute an optimization only under the applied boundary conditions, we find that the beamline with a BDPN increases the system’s wall plug efficiency by about 13% to 0.34 from the 0.30 estimated for a gas neutralizer.

Pumping performance analysis of negative ion based neutral beam injector cryosorption pump prototype

A cryosortion pump creates a vacuum gradient environment in the process of neutral beam generation and transmission. Its pumping performance is related to the neutral beam quality. Here, the structural design and manufacturing method of the cryosorption pump were demonstrated based on the actual needs of negative ion based neutral beam injector (NNBI).Then,in order to verify the rationality of cryosorption pump's design, the test system of the cryosorption pump test prototype was built based on the flow-meter method to perform experiments to study the factors affecting the pumping performance. The results show that the prototype's stable pumping performance under different temperature, vacuum pressure, gas composition and the important characteristics of effective pumping speed for hydrogen and its isotopic gasses. The maximum error between the actual and the theoretical pumping speeds is maintained within 35%. The physical mechanism that temperature fluctuation and impurity gas helium can obviously affect the pumping performance of the cryosorption pump is revealed.It is concluded that the cryosorption pump prototype has excellent pumping performance, verifying the rationality of the structural design and providing a reference for the performance evaluation of a NNBI cryosorption pumps.

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.

Development of experimental control system for regenerative heating process of high-speed cryo-adsorption pump

The vacuum system of the Negative-ion-based Neutral Beam Injector (NNBI) verification prototype of the China Fusion Engineering Test Reactor (CFETR) employs a built-in ultra-high-speed cryo-adsorption pump utilizing forced-flow cooling as the main pump, which functions through four major processes: cooling, stable operation, regeneration, and temperature rise. The research analyzes the key technical problems of engineering that must be solved,such as natural regeneration takes a longer time with temperature uncontrollable, lead to low experiment efficiency. An efficient and safe controllable technical method for the regeneration heating process was proposed,and regeneration heating experimental control system, including an equipment layer, control layer, data acquisition and monitoring layer, was designed to control the regeneration heating process of the cryo-adsorption pump. The signals collected by temperature sensors are processed and transmitted through signal isolators to the host computer for display and storage.The target temperature is manually set on the host computer. An improved generalized predictive control (GPC) algorithm and a fuzzy proportional integral derivative (PID) control algorithm are used in the control layer to control the heating process.The designed activation regeneration heating control experimental system was embedded in the comprehensive experimental test bench of cryo-adsorption pump for experimental verification, and the test experiment was carried out.The results show that the system can control the regenerative heating process in an efficient, safe and stable way. This lays the foundation for further development of control functions in the regeneration process of the cryo-adsorption pump equipped in the NNBI verification prototype, and provides a design basis for the vacuum system of fusion engineering experimental reactors.

Process analysis and control flow design of the 1kW @ 4.5K helium refrigerator for NNBI

A helium cryogenic system with a cooling capacity of 1kW@4.5K was designed in order to support the NNBI(Negative ion based Neutral Beam Injection system) test facility, cryopumping test facility and beam source test facility, which consists of a helium compressor with its storage and recovery system, 1kW@4.5K helium refrigerator, a cryogenic distribution valve box, cryogenic transfer lines and a helium Dewar, etc. Based on the SIMATIC S7-1500H PLC, the cryogenic control system can realize seven major operational modes, including the independent refrigeration mode and independent liquefaction mode, etc. On the basis of process analysis, the control flow of the NNBI helium cryogenic system in each operational modes was designed and will be presented in this paper. It will direct the control system to realize automatic control in multiple operational modes.

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.

100 s negative ion accelerations for the JT-60SA negative-ion-based neutral beam injector

In the negative-ion-based neutral beam injector (N-NBI) of JT-60SA, a hydrogen negative ion beam with energy of 500 keV and a current density of 154 A m−2 for 118 s has been successfully demonstrated. This achievement exceeds the requirements (500 keV, 130 A m−2, 100 s) for the first time. To maintain stable negative ion production for a long pulse, the optimal temperatures of the chamber wall and plasma grid were analytically examined and were experimentally demonstrated. It was confirmed that the temperature during the beam pulse should be <50 °C for the chamber wall and >200 °C for the plasma grid. Damage to the filament cathode due to an abnormal discharge, so-called arcing, has been mitigated by developing a fast cut-off system of the arc current for around 100 μs after the arcing. To maintain sufficient voltage holding capability and to reduce the grid heat load due to the beam in the accelerator, techniques developed for the beam acceleration have been applied to this test. As a result of the integration of these techniques, a stable beam over 100 s has been demonstrated successfully. This is the first achievement over a 100 s stable beam with intensity of >75 MW m−2, which is the required practical level in the N-NBI. These results contribute to the coming NBI system for ITER and DEMO.

Design of the data acquisition system of acceleration grid power supply for CFETR N-NBI prototype

The Acceleration Grid Power Supply (AGPS) of the China Fusion Engineering Test Reactor (CFETR) negative-ion-based neutral beam injector (N-NBI) prototype is a high-voltage power supply rated at -200 kV/25 A. The output voltage measurement and state detection of AGPS are required to safely operate for the N-NBI system. Hence, a high-performance data acquisition system is essential. The performance requirements of the data acquisition system becomes remarkable when the output voltage increases and electromagnetic interference enhance. This paper proposes an effective data acquisition method based on field programmable gate array (FPGA) technology and PCI extensions for instrumentation (PXI) series hardware used in the existing control system. Compared with traditional voltage-to-frequency (VF) conversion methods, the frequency output by the proposed method can maintain a consistently high standard when a lower reference output voltage is required. The principle test and high voltage test on the pulse step modulation (PSM) power supply of the J-TEXT Tokamak indicate that the delay of the data acquisition system is no more than 20 μs, which fulfills the requirements.

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.

Design and control of the accelerator grid power supply-conversion system applied to CFETR N-NBI prototype

The acceleration grid power supply (AGPS) rated 200 kV/25 A is a key component devoted to supply the acceleration grids of the China fusion engineering test reactor negative-ion-based neutral beam injector (N-NBI) prototype system. This paper focused on the design and control of the AGPS conversion system (AGPS-CS), with emphasis on the requirement of the wide range output voltage and rise time. A voltage regulation switch at the front of step-down transformer is applied to optimize the grid current and DC-link voltage. Moreover, a new feedforward control strategy with piecewise PI compensator is proposed to improve the characteristics of AGPS. The simulation results of the proposed AGPS-CS are presented, proving the performance of the power supply to achieve the desired requirements.

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.

Physical design of the neutralizer for CFETR negative ion based neutral beam injection prototype

A research project of the CFETR Negative ion based Neutral Beam Injection (NNBI) prototype has been started in China. The objectives of the CFETR NNBI protype are to produce a negative hydrogen ion beam of >20 A to 200–400 kV for 3600 s and to attain a neutralization efficiency of >50%. Thus, a neutral beam power of >2 MW is foreseen onto the calorimeter (i.e., beam dump). A gas neutralizer is applied inside the beamline of CFETR NNBI prototype, to achieve the neutralization of negative ion beam through the charge-exchange collisions with the gas target. The most critical issues for the gas neutralizer are to balance the contradictive requirements of high neutralization efficiency, low gas inflow, and low heat load. During the physical design of the neutralizer for CFETR NNBI prototype, two proposals have been considered and studied in detail. One has single beam channel and other one has two narrow channels separated by a panel. The narrow channel design is benefic to decrease gas conductance, and thus reduce the required gas inflow to attain the maximal neutralization efficiency. However, the inserted panel will restrict the extraction area of the beam source, where the aperture columns corresponding to the panel should be masked to avoid the direct beam interception. But the leading edge of the inserted panel will still suffer a high thermal load due to the stray particles from the beam source. A 3D model of the whole beamline of CFETR NNBI prototype has been developed to study the gas flow and the beam transport in the beamline. Based on this model, these two neutralizer designs have been evaluated in detail, in terms of the relation between neutralization efficiency and gas inflow, and the power load on the neutralizer components.