Neutral Beam Injection Experiments 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.

Study of Orbit Motion of Hydrogen and Deuterium Beam Ions Toward Neutral Beam Injection Experiment in Thailand Tokamak-1

Thailand Tokamak-1 (TT-1) is a small tokamak under the operation of the Thailand Institute of Nuclear Technology. In our future plans, TT-1 has the feasibility of being equipped with external heating systems to achieve high-performance plasma operation and conduct physics associated with fast ions generated by the external heating system. One of the potential external heating systems under consideration is a positive-ion-source-based neutral beam injection (NBI) heating system. The primary source of fast ions for the NBI can be either hydrogen or deuterium-doped hydrogen beams. This study examines how hydrogen and deuterium ions, namely protons and deuterons, respectively, move in orbits, using the Lorentz orbit (LORBIT) code to track their full gyromotion. The energy of the ions has been varied between 200 eV and 40 keV. Three trajectory motions have been characterized, i.e., counter-passing ion, co-passing ion, and trapped ion. The average Larmor radius, dependent on the ion energies, has been investigated. The Larmor radius significantly increases with increasing ion energy in the case of trapped ions, while in the case of counter- and co-passing ions, the Larmor radius exhibits a slight increase with increasing ion energy. Furthermore, the pitch angle range of the lost ion in the TT-1 has been investigated. The results demonstrate the feasibility of utilizing a deuterium-doped hydrogen beam in TT-1 to study fast ion physics. To avoid a large number of lost ions, it is possible to use a deuterium-doped hydrogen beam with an energy not exceeding approximately 10 keV in the TT-1.

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.

Hot spots of the main limiter induced by fast ions in experimental advanced superconducting tokamak (EAST)

The main limiter in EAST was observed to endure a high heat load and was cracked near the midplane at the right side during the plasma operation. To explore the heat load carried by fast ion loss toward the main limiter, the neutral beam injection (NBI) and radio frequency power proportion experiment was conducted in EAST where the plasma stored energy and line integrated density were kept almost constant. The hot spot at the right side of the main limiter was observed to be enhanced by co-current perpendicular (co-perp) NBI. An NBI ion loss simulation was performed in the presence of the toroidal field ripple and the Coulomb collision by using the orbit code GYCAVA and the NBI code TGCO. The result indicates that the NBI ion loss by ripple and collision mainly causes a bright area below the midplane of the right side of the main limiter as observed in the EAST experiment. The peak heat load of lost fast ions generated by co-perp NBI is ∼0.5 MW/m2 as obtained by 1 MW of NBI deposited power and comparable with the heat load carried by fast electrons induced by lower hybrid current drive. In addition, increasing the gap between the separatrix and the first-wall limiters in the simulation is found to reduce this heat load.

Enhancing ion extraction with an inverse sheath in negative hydrogen ion sources for NBI heating

Negative hydrogen ion (H−) sources employed in neutral beam injection (NBI) systems are subject to extraction efficiency issues due to the considerable volumetric losses of negative hydrogen ions. Here, we propose to improve the H− extraction by activating an alternative sheath mode, the electronegative inverse sheath, in front of the H− production surface, which features zero sheath acceleration for H− with a negative sheath potential opposite to the classic sheath. With the inverse sheath activated, the produced H− exhibits smaller gyration, a shorter transport path, less destructive collisions, and therefore higher extraction probability than the commonly believed space-charge-limited (SCL) sheath. Formation of the proposed electronegative inverse sheath and the SCL sheath near the H–-emitting surface is investigated by the continuum kinetic simulation. Dedicated theoretical analyses are also performed to characterize the electronegative inverse sheath properties, which qualitatively agree with the simulation results. We further propose that the transition between the two sheath modes can be realized by tuning the cold ion generation near the emissive boundary. The electronegative inverse sheath is always coupled with a plasma consisting of only hydrogen ions with approximately zero electron concentration, which is reminiscent of the ion–ion plasma reported in previous NBI experiments.

Tomography of fast ion distribution function under neutral beam injection and ion cyclotron resonance heating on EAST

In magnetic confinement fusion devices, velocity-space tomography of fast-ion velocity distribution function is crucial for investigating fast-ion distribution and transport. In the neutral beam injection (NBI) and ion cyclotron resonance heating (ICRF) synergistic heating experiments in Experimental Advanced Superconducting Tokamak (EAST), high-energy particles with energy exceeding the particle energy in NBI are observed. Simulations of synergistic effect on fast-ion velocity distribution function given by TRANSP also show the existence of particles with energy higher than the particle energy in NBI. To investigate the behaviors of fast ion distribution and calculate the velocity distribution functions under different heating conditions, the first-order Tikhonov regularization tomographic inversion method with higher inversion accuracy is introduced by comparing various regularization techniques. The limitations of the dual-view fast-ion D&lt;sub&gt;α&lt;/sub&gt; (FIDA) diagnostic measurements in velocity space are addressed by incorporating prior information such as null measurement and the known peaks and effectively mitigate the occurrence of artifacts. This method is first employed in the case of NBI heating. The NBI peak is successfully reconstructed at the expected location in velocity space, which shows significant improvement in the inversion results. In order to further validate the synergistic effect of NBI-ICRF heating and study the mechanism of fast ion distribution under synergistic heating, the combination of FIDA and neutron emission spectrometer (NES) is applied to the first-order Tikhonov regularization tomographic inversion method for enhancing the coverage of velocity space, through which the issue of artifacts in the inversion results is significantly improved, and thus the precision of the obtained fast-ion velocity distribution functions is enhanced. Based on the benefit described above, the method of combining NES diagnosis and FIDA diagnosis is used to obtain fast-ion velocity distribution functions in the NBI and ICRF synergistic heating discharge. The synergistic heating effect is manifested in the fast-ion velocity distribution. The availability of this inversion method in reconstructing fast-ion velocity distribution functions during high-performance operation of NBI-ICRF synergistic heating in the EAST experiment is confirmed. In the next-step EAST research, high performance discharge will demand more efficiency NBI and ICRF synergistic heating, the present work builds the stage for investigating the underlying mechanism of synergistic heating and the intricate behaviors associated with fast ion distribution and transport.

Toroidal plasma acceleration due to NBI fast ion losses in LTX-β

The recent Lithium Tokamak Experiment-Beta (LTX-β) upgrade includes the addition of neutral beam injection (NBI) in the same direction as the plasma current (co-I P ) and a new toroidal Mirnov array for MHD characterization. In initial NBI experiments, a spontaneously rotating n = 1 MHD mode is seen to accelerate during NBI in the counter-beam direction, accompanied by a rise in electron density consistent with the beam-injected inventory but without a clear increase in plasma pressure. Together with analytic and numerical modeling of beam optics and fast ion confinement, these observations indicate the prompt loss of all or nearly all beam ions. However, the same modeling also suggests that planned upgrades to the Ohmic heating system should provide the fast ion confinement necessary for beam heating and core fueling. A simple analytic model relates the momentum confinement time to the observed evolution of mode rotation due to the combination of NBI momentum coupling, fast ion loss , and anomalous viscous torques, yielding values consistent with past measurements of electron energy confinement time τ E,e .

Development of a wide-range neutron flux monitoring system in EAST

A wide measurement range for a neutron flux monitoring (NFM) system is developed in the Experimental Advanced Superconducting Tokamak (EAST) for fusion neutron yield measurement, comprising an 3He proportional counter, a 235U fission chamber (FC), and a wide dynamic range signal-processing circuit. Two high-sensitivity 3He detectors operate in counting mode, which is suitable for low neutron yield (Yn) conditions with ohmic and radio-frequency heating of plasma. Two other FCs are adapted with newly developed digitizers, operating in counting and campbelling modes to satisfy the conditions of wide dynamic range measurement required by neutral beam injection (NBI) experiment. The anti-magnetic-field interference of the FC is examined using two permanent magnets, whose efficiency are calibrated with 2.46 MeV neutrons produced by a Van de Graaff accelerator. Furthermore, the NFM system is implemented in-situ calibration with a 252Cf neutron source and a remote handling. As a routine diagnostic, the NFM system is successful obtaining the fusion neutron yield for ion cyclotron resonance heating (ICRH) and NBI heating performance evaluation in EAST plasma experiment.

Development of neutron activation system on EAST.

A neutron activation system (NAS) has been developed on the Experimental Advanced Superconducting Tokamak (EAST) for the investigation of time-integrated neutron yield. It is a robust neutron diagnostic with high precision and a wide dynamic range. Some high purity materials with a proper nuclear reaction energy threshold and cross section are tested as neutron detectors in the NAS, and they are delivered to the tokamak device by a pneumatic transfer system. The length of the pneumatic pipeline is about 50 m, and the transfer time ranges from 10 s to 20 s. The decay gamma rays of the activated samples are measured with a high-purity germanium spectrometer, and its detection efficiencies are 6.9% at 336 keV and 1.7% at 1779 keV, respectively. Indium and silicon samples with a reaction threshold of 0.3 MeV and 4.0 MeV, respectively, were successfully implemented in the diagnosis of DD and DT neutron yield in the EAST. The neutron flux of the NAS was compared with the count of the fission chamber in the EAST neutral beam injection experiment, and the DD neutron yield evaluated by the NAS ranges from 1.9 × 1012 n/shot to 5.9 × 1014 n/shot during recent experimental campaign.

Validating fast-ion wall-load IR analysis-methods against W7-X NBI empty-torus experiment

The first neutral beam injection (NBI) experiments in Wendelstein 7-X (W7-X) stellarator were conducted in the summer of 2018. The NBI system is used to heat the magnetically confined plasma by neutralising an accelerated hydrogen ion beam and directing it into the plasma, where the resulting energetic ions release their energy to heat the plasma. The modelling of the NBI fast ion experiments has commenced, including estimation of the shine-through and the orbit-losses. The stellarator has a wide-angle infra red (IR) imaging system to monitor the machine plasma facing component surface temperatures. This work validates the NBI model “Beamlet Based NBI (BBNBI)” and the newly written synthetic IR camera model. The validation is accomplished by comparing the measured and the synthetic IR camera measurements of an experiment where the NBI was injected into the vacuum vessel without a plasma. A good qualitative and quantitative match was found. This agreement is further supported by spectroscopic and calibration measurements of the NBI and and IR camera systems.

Perturbative studies of toroidal momentum transport in KSTAR H-mode and the effect of ion temperature perturbation

Perturbative experiments have been carried out using tangential neutral beam injection (NBI) and non-resonant magnetic perturbation (NRMP) to analyze the momentum transport properties in KSTAR H-modes. Diffusive and non-diffusive terms of momentum transport are evaluated from the transient analysis. Although the operating conditions and methodologies applied in the two cases are similar, the momentum transport properties obtained show clear differences. The estimated momentum diffusivity and pinch obtained in the NBI modulation experiments is larger than that in the NRMP modulation experiments. We found that this discrepancy could be a result of uncertainties in the assumption for the analysis. By introducing time varying momentum transport coefficients depending on the temperature gradient, the linearized equation shows that if the temperature perturbation exists, the evolution of toroidal rotation perturbation could be faster than the transport rate of mean quantity, since the evolution of toroidal rotation perturbation is related to , a momentum diffusivity from perturbative analysis. This could explain the estimated higher momentum diffusivity using time independent transport coefficients in NBI experiments with higher ion temperature perturbation compared to that in NRMP modulation experiments. The differences in the momentum transport coefficient with NRMP and NBI are much reduced by considering time varying momentum transport coefficients in the time dependent transport simulation.

Fusion Research in Ioffe Institute

In this paper we present the fusion related activities of the Plasma Physics Division at the Ioffe Institute. The first experiments on lower hybrid current drive (LHCD) in a spherical tokamak performed at the Globus-M tokamak (R = 0.36 m, a = 0.24 m, Bt = 0.4 T, Ip = 200 kA) with a novel poloidally oriented grill resulted in an RF driven current of up to 30 kA at (100 kW, 2.5 GHz), exceeding the modelling predictions. At the FT-2 tokamak (R = 0.56 m, a = 0.08 m, Bt = 3 T, Ip = 30 kA) experiments with a traditional toroidally oriented grill revealed no strong dependence of the LHCD density limit on the H/D ratio in spite of LH resonance densities differing by a factor of 3. Microwave Doppler reflectometry (DR) at the Globus-M, and DR and heavy ion beam probe measurements at the tokamak TUMAN-3M (R = 0.53 m, a = 0.24 m, Bt = 1.0 T, Ip = 190 kA) demonstrated geodesic acoustic mode (GAM) suppression at the L to H transition. Observations at FT-2 using Doppler Enhanced Scattering showed that the GAM amplitude is anti-correlated both spatially and temporally to the drift turbulence level and electron thermal diffusivity. For the first time turbulence amplitude modulation at the GAM frequency was found both experimentally and in global gyrokinetic modelling. A model of the L–H transition is proposed based on this effect. The loss mechanisms of energetic ions' (EI) were investigated in the neutral beam injection (NBI) experiments on Globus-M and TUMAN-3M. Empirical scaling of the 2.45 MeV DD neutron rate for the two devices shows a strong dependence on toroidal field and plasma current justifying the Bt and Ip increase by a factor of 2.5 for the proposed upgrade of Globus-M. Bursts of ∼1 MHz Alfvenic type oscillations correlating with sawtooth crashes were observed in ohmic TUMAN-3M discharges. The possibility of low threshold parametric excitation of Bernstein and upper hybrid waves trapped in drift-wave eddies resulting in anomalous absorption in electron cyclotron resonance heating (ECRH) experiments in toroidal plasmas was identified theoretically. A novel method of radial correlation Doppler reflectometry is shown to be capable of measuring the turbulence wave-number spectrum in realistic 2D geometry. On the progress in design and fabrication of three diagnostics for ITER developed in the Ioffe institute is reported: neutral particle analysis, divertor Thomson scattering and gamma spectroscopy.

Design and Application of Experimental Configuration Query Software for EAST NBI

To verify the rationality of experimental configurations before each experiment discharge of Experimental Advanced Superconducting Tokamak (EAST) Neutral Beam Injection (NBI) and to query the experimental configurations under historical shot numbers, an experimental configuration query terminal is needed during NBI experiments. In this paper, the experimental configuration query software is presented which is realized with multithreading technology, TCP protocol, Mysql database and so on. Multithreading technology achieves that different service requests can be dealt by different threads for improving the processing efficiency of the program. TCP protocol ensures reliable network communication between the server and the client. The configurations set by experimental operators are saved to Mysql database of the server with different data tables. The server works under Linux operating system while the client works on Windows operating system. With the help of this application, the current and historical experimental configurations are displayed and queried, thus on the one hand, the current experimental configurations especially the settings of timing sequence and amplitude for NBI each power supply system are verified before each NBI discharge; on the other hand, the typical history configurations can be queried for experimental reproducibility. This application is very practical significance for EAST NBI experiments.

Development of Equipment Management and Monitoring Software for EAST NBI

For the sake of achieving to manage and monitor the experimental equipment scientifically, as well as relevant experiment information for Neutral Beam Injection (NBI) system, NBI equipment management and monitor software is developed for managing and monitoring the experimental equipment between the remote server control layer and the field management and control layer. Experiment results show that managing equipment scientifically can ensure equipment security and promote the progress of the experiment smoothly. NBI Equipment management and monitoring software is programmed by C language which runs on Windows system and can work steadily and reliably now during the NBI experiments.

Study of advanced neutron emission spectrometry diagnostics for tokamak fusion experiments

The research progress of fusion neutron emission spectrometers in the EAST and HL-2A takamaks is reviewed. Considering the requirements for plasma physics research and neutron diagnostic development for the ITER device, two digital liquid scitillation spectrometers and a stilbene crystal spectrometer were set up for the discharges with the low neutron yield (10⁹-10¹¹ s^-¹) on EAST and HL-2A. We present some neutron spectrometry diagnostic results, such as the time traces of fusion neutron count rates during the EAST discharges, the evaluation of the heating effect in HL-2A neutral beam injection experiments and the neutron diagnostics of sawtooth instability in EAST plasmas. The ion temperature diagnostics of the core plasma were made by using the stilbene neutron spectrometer and the liquid scintillation neutron spectrometer in EAST, and the ion temperature measurement with the neutron emission spectrometer for the plasma with low temperature, which was less than 1 keV, was sfor the first time. A double ring neutron time-of-flight array spectrometer was designed and constructed on EAST for the fast ion physics and tokamak auxiliary heating research.

Merging Experimental Study of High-Beta ST Formation for Non-Inductive Plasma Start-Up Assisted by NBI in TS-4

Spherical tokamak (ST) merging start-up for high-beta ST formation was developed in the TS-4 merging/reconnection experiment under a high external toroidal magnetic field. We optimized the formation of a stable high-beta ST (βt > 30%) by adjusting the initial parameters of two low-beta STs. We precisely analyzed the produced high-beta STs by MEUDAS using the measured and fitted profiles, and found that they exhibited low-plasma internal inductance because of a hollow current profile, a paramagnetic toroidal field, and weakly reversed shear. A pressure-driven instability analysis indicates that the merging STs become unstable when their q-values are less than 1. In the first high-power neutral beam injection experiment (#1 and #2, PNBI ∼ 0.4MW) in TS-4, the decay time and magnetic flux of the produced STs were improved, mainly because of pre-ionization and merging/reconnection effects. c

Technical Design of Arc-Discharge and Deceleration Power Supply for MW Level NBI System on HL-2A Tokamak

Neutral beam injection (NBI) is one of the most effective ways to heat and drive plasma in a tokamak. The mega watt level neutral beam injector on the HL-2A tokamak consists of four high-power ion sources. Each source is supplied by discharge, beam extraction and auxiliary power supplies. Some circuit topologies and control sequences designed for the system are presented in this paper. Some important technologies such as the notching circuit, insulated gate bipolar transistor (IGBT) series-connected switch, high-frequency switching power supply and control system based on a digital signal processor (DSP) have been used. The system can be effectively used for high current ion beam extraction, protection, ion optics and so on. The power system has been safely used in HL-2A high-parameter NBI experiments for three years. The power of NBI can be kept at higher than 0.75 MW for 1 second and the ion beam power extracted from the ion source is higher than 2 MW. The ion temperature of the plasma center is close to 2.0 keV. These results show that the design of this power system is reasonable and reliable, and it can fully meet the system requirements for NBI of the HL-2A tokamak.

First neutral beam injection experiments on KSTAR tokamak

The first neutral beam (NB) injection system of the Korea Superconducting Tokamak Advanced Research (KSTAR) tokamak was partially completed in 2010 with only 1∕3 of its full design capability, and NB heating experiments were carried out during the 2010 KSTAR operation campaign. The ion source is composed of a JAEA bucket plasma generator and a KAERI large multi-aperture accelerator assembly, which is designed to deliver a 1.5 MW, NB power of deuterium at 95 keV. Before the beam injection experiments, discharge, and beam extraction characteristics of the ion source were investigated. The ion source has good beam optics in a broad range of beam perveance. The optimum perveance is 1.1-1.3 μP, and the minimum beam divergence angle measured by the Doppler shift spectroscopy is 0.8°. The ion species ratio is D(+):D(2)(+):D(3)(+) = 75:20:5 at beam current density of 85 mA/cm(2). The arc efficiency is more than 1.0 A∕kW. In the 2010 KSTAR campaign, a deuterium NB power of 0.7-1.5 MW was successfully injected into the KSTAR plasma with a beam energy of 70-90 keV. L-H transitions were observed within a wide range of beam powers relative to a threshold value. The edge pedestal formation in the T(i) and T(e) profiles was verified through CES and electron cyclotron emission diagnostics. In every deuterium NB injection, a burst of D-D neutrons was recorded, and increases in the ion temperature and plasma stored energy were found.

Evaluation of the Heating Effect in Neutral Beam Injection Experiments of the GAMMA 10 Tandem Mirror

Neutral beam injection heating has been studied for high-density mode plasma in the GAMMA 10 tandem mirror. During NBI heating, the increase in diamagnetism was found to be larger for the high-density mode plasma than the hot-ion mode plasma. A numerical model using zero-dimensional particles and energy balance equations was developed to investigate NBI heating in the central cell area. The evolution of total energy density was found to be consistent with that of diamagnetism in the low-density hot-ion mode. The initial ion temperature is a key parameter for enhancing beam heating. In high-density mode, discrepancies were found between the measured diamagnetism and the calculated total energy density. It is probably because that time behavior of the electron temperature is not suitable.

Plasma Potential Evolution Study by HIBP Diagnostic During NBI Experiments in the TJ-II Stellarator

The heavy ion beam probe diagnostic is used in the TJ-II stellarator to study directly the plasma electric potential with good spatial (up to 1 cm) and temporal (up to 2 μs) resolution. Singly charged heavy ions, Cs+, with energies of up to 125 keV are used to probe the plasma column from the edge to the core. Both electron cyclotron resonance heating (ECRH) and neutral beam injection (NBI)-heated plasmas (PECRH = 200 to 400 kW, PNBI = 200 to 400 kW, ENBI = 28 keV) have been studied.Low-density ECRH [[over bar]n = (0.5 to 1.1) × 1019 m-3] plasmas in TJ-II are characterized by positive plasma potential on the order of 1000 to 400 V. A negative electric potential appears at the edge when the line-averaged density exceeds 0.5 × 1019 m-3. Further density rises are accompanied by a decrease in the core plasma potential, which becomes fully negative for plasma densities [over bar]n ≥ 1.5 × 1019 m-3. The NBI plasmas are characterized by a negative electric potential across the whole plasma cross section from the core to the edge. In this case, the absolute value of the central potential is on the order of -500 V. These results show a clear link between plasma potential and density in the TJ-II stellarator.