Development of a compact torus injection system for the Keda Torus eXperiment

A compact torus injection system, KTX-CTI, has been developed for the planned injection experiments on the Keda Torus eXperiment (KTX) reversed field pinch (RFP) device to investigate the physics and engineering issues associated with interaction between a compact torus (CT) and RFP. The key interests include fueling directly into the reactor center, confinement improvement, and the injection of momentum and helicity into the RFP discharges. The CT velocity and mass have been measured using a multichannel optical fiber interferometer, and for the first time the time evolution of the CT density profile during CT propagation is obtained. The effects of discharge parameters on the number of injected particles, CT velocity and CT density have been characterized: the maximum hydrogen CT plasma mass, ${m}_{{\rm{CT}}},$ is 50 μg, corresponding to 30% of the mass in a typical KTX plasma; the CT velocity exceeds 120 km s⁻¹. It is observed for the first time that multiple CTs can be produced and emitted during a very short period (<100 μs) in one discharge, which is significant for the future study of repetitive CT injection, even with an ultra-high frequency.     

Design of a Wedge-Shaped Toroidal Field Winding for KTX Device

The KTX device is a reversed field pinch(RFP)device currently under construction.Its maximum plasma current is designed as 1 MA with a discharge time longer than 100 ms.Its major radius is 1.4 m and its minor radius is 0.55 m.One of the most important problems in the magnet system design is how to reduce the TF magnetic field ripple and error field.A new wedgeshaped TF coil is put forward for the KTX device and its electromagnetic properties are compared with those of rectangular-shaped TF coils.The error field Bn/Btof wedge-shaped TF coils with6.4 degrees is about 6%as compared with 8%in the case of a rectangular-shaped TF coil.Besides,the wedge-shaped TF coils have a lower magnetic field ripple at the edge of the plasma region,which is smaller than 7.5%at R=1.83 m and 2%at R=1.07 m.This means that the tokamak operation mode may be feasible for this device when the plasma area becomes smaller,because the maximum ripple in the plasma area of the tokamak model is always required to be smaller than 0.4%.Detailed analysis of the results shows that the structure of the wedged-shape TF coil is reliable.It can serve as a reference for TF coil design of small aspect ratio RFPs or similar torus devices.     

Measurement and Processing of Fast Pulsed Discharge Current in Plasma Focus Machines

The fast pulsed electric discharge current drives all physical processes in the plasma focus device; in turn all physical processes in the focus affect the current waveform. Thus the discharge current waveform is the most important indicator of plasma focus performance. This underlies the importance of properly measuring, processing and interpreting the discharge current waveform. This paper reports the measurement of fast pulsed discharge current by the Rogowski coil, in two different modes: the current transformer, ??I?? mode, and current derivative, ??Idot?? mode. The processing and interpretation of recorded current waveform to obtain useful information about the physical processes in the plasma focus device are discussed. The current transformer with a large number of turns and a sub-1 Ohm terminator has good high frequency response, necessary for the sharp current dip region when dI/dt exceeds 2?×?10¹¹?A/s. However the signal is ??noisy?? in the current dip region. Several methods to extract the current dip from the noise are discussed and examples of how low pass filters affect the signals are shown. The dI/dt coil, the Rogowski coil in ??Idot?? mode, with a few turns terminated by 50-Ohm is also described. Integrating the 1?GSa/s digital waveform does remove the high frequency noise components, yet the extracted waveform shows sharp angular features indicative of the retention of short-time features. This makes the dI/dt coil superior to the current transformer. A 7-turn coil is tested against the Lee Model code and found to be suitable to measure the plasma focus discharge current.     

Compact toroid challenge experiment with the increasing in the energy input into plasma and the level of trapped magnetic field

The present work reports on compact toroid hydrogen plasma creation by means of a specially designed discharge system and results of magnetic fields introduction. Experiments in the compact toroid challenge (CTC) device at P.N. Lebedev Physical Institute (FIAN) have been conducted since 2005. The CTC device differs from the conventional theta-pinch formation in the use of an axial current for enhanced efficiency. We have used a novel technique to maximize the flux linked to the plasma. The purpose of this method is to increase the energy input into the plasma and the level of trapped magnetic flux using an additional toroidal magnetic field. A study of compact torus formation with axial and toroidal currents was done and a new method is proposed and implemented.     

Power handling of the bulk tungsten divertor row at JET: First measurements and comparison to the GTM thermal model

The design of the tile assemblies of the bulk tungsten divertor row in JET was improved in the course of several experiments as far as the power and energy performances are concerned: many prototypes were exposed to high heat fluxes in several electron and ion beam facilities during the development phase. These experiments were carried out in parallel with extensive modelling of the complete tungsten tile assembly in the so-called Global Thermal Model (GTM). The goal was to understand the heat flow from the plasma-facing surface through the supporting structure down to the base plate of the JET MkII divertor sufficiently to be able to later interpret operational data from the torus. Temperatures measured in the torus are in good agreement (?10/+15%) with the model. Some characteristic times show stronger deviations, with no incidence on the highest temperature at all times.     

Theoretical model and experimental investigation of optically triggered hollow cathode discharge formation

In order to investigate the process of optically triggered discharge formation,a model of ion space-charge formation based on classical plane electrodes and revised for a characteristic hollow-cathode discharge(HCD)configuration is proposed in this paper.The primary modified factor in our model is the penetrating electric-field parameter,which influences the ionization of trigger electrons and is calculated via particle simulation.Optical-trigger experiments are carried out using different voltages and under different seed-electron conditions,provided by two different photocathodes,Cu and Mg.The ion-accumulation rates calculated by our model are compared to the discharge-formation time,which is deduced from optical-trigger experiments.The results demonstrate that the process of positive space-charge formation is dominant in the HCD formation process or trigger delay,which is highly dependent on the seeding-electron density and applied voltage,and can therefore be quantitatively described by our model.Additionally,electron-beam generation is investigated by optically triggered HCD experiments on Mg-and Cu-photocathode-based devices.The results show that a more efficient trigger device is capable of generating an electron beam with higher amplitude and density.     

Response of organic photodiode fabricated directly on plastic scintillator to X-rays

Organic photodiodes (OPDs) have attracted considerable interest in the field of photonic devices because they are useful for large and flexible photodetectors. To assess their applicability to radiation measurement, the authors have been studying their response to X-ray irradiation. For a previous study, OPDs with a bulk heterojunction structure were fabricated. Then, their response to X-rays was evaluated. However, the X-ray-induced current was extremely low and the necessity to increase the efficiency was apparent. In this study, to increase X-ray-induced current, the authors fabricated OPDs directly on plastic scintillator plates. The device structure was a plastic scintillator substrate (1 or 5 mm)/ITO (150 nm) or IZO (100 nm)/PEDOT: PSS (30 nm)/P3HT:PCBM (200 nm)/Al (70 nm). The fabricated devices were irradiated with X-rays. Then their responses were evaluated. Results demonstrated that the X-ray-induced current can be increased by fabricating OPDs directly on plastic scintillator plates. The measured energy responses of the devices were compared with the energy deposition calculated using EGS5 code. Both results show coincident tendencies. However, because the collected charges were smaller than expected from simulations in the low-energy region, further study should be conducted to specify the factors.     

Recovery of saturated signal waveform acquired from high-energy particles with artificial neural networks

Artificial neural networks(ANNs) are a core component of artificial intelligence and are frequently used in machine learning. In this report, we investigate the use of ANNs to recover the saturated signals acquired in highenergy particle and nuclear physics experiments. The inherent properties of the detector and hardware imply that particles with relatively high energies probably often generate saturated signals. Usually, these saturated signals are discarded during data processing, and therefore, some useful information is lost. Thus, it is worth restoring the saturated signals to their normal form. The mapping from a saturated signal waveform to a normal signal waveform constitutes a regression problem. Given that the scintillator and collection usually do not form a linear system, typical regression methods such as multi-parameter fitting are not immediately applicable. One important advantage of ANNs is their capability to process nonlinear regression problems.To recover the saturated signal, three typical ANNs were tested including backpropagation(BP), simple recurrent(Elman), and generalized radial basis function(GRBF)neural networks(NNs). They represent a basic network structure, a network structure with feedback, and a network structure with a kernel function, respectively. The saturated waveforms were produced mainly by the environmental gamma in a liquid scintillation detector for the China Dark Matter Detection Experiment(CDEX). The training and test data sets consisted of 6000 and 3000 recordings of background radiation, respectively, in which saturation was simulated by truncating each waveform at 40% of the maximum signal. The results show that the GBRF-NN performed best as measured using a Chi-squared test to compare the original and reconstructed signals in the region in which saturation was simulated. A comparison of the original and reconstructed signals in this region shows that the GBRF neural network produced the best performance. This ANN demonstrates a powerful efficacy in terms of solving the saturation recovery problem. The proposed method outlines new ideas and possibilities for the recovery of saturated signals in high-energy particle and nuclear physics experiments. This study also illustrates an innovative application of machine learning in the analysis of experimental data in particle physics.     

Theoretical model and experimental investigation optically triggered hollowcathode discharge formation

In order to investigate the process of optically triggered discharge formation, a model of ion space-charge formation based on classical plane electrodes and revised for a characteristic hollow-cathode discharge(HCD) configuration is proposed in this paper. The primary modified factor in our model is the penetrating electric-field parameter, which influences the ionization of trigger electrons and is calculated via particle simulation. Optical-trigger experiments are carried out using different voltages and under different seed-electron conditions, provided by two different photocathodes, Cu and Mg. The ion-accumulation rates calculated by our model are compared to the discharge-formation time, which is deduced from optical-trigger experiments.The results demonstrate that the process of positive space-charge formation is dominant in the HCD formation process or trigger delay, which is highly dependent on the seeding-electron density and applied voltage, and can therefore be quantitatively described by our model.Additionally, electron-beam generation is investigated by optically triggered HCD experiments on Mg-and Cu-photocathode-based devices. The results show that a more efficient trigger device is capable of generating an electron beam with higher amplitude and density.     

A new consecutive energy calibration method for X/γ detectors based on energy continuously tunable laser Compton scattering light source

In this paper, we present an energy calibration method based on steep Compton edges of the laser Compton scattered(LCS) photon energy spectra. It performs consecutive energy calibration in the neighborhood of certain energy, hence improves calibration precision in the energy region. It can also achieve direct calibration at high energy region(several MeV) where detectors can only be calibrated by extrapolation in conventional methods.These make it suitable for detectors that need wide-range energy calibration with high precision. The effects of systematic uncertainties on accuracy of this calibration method are studied by simulation, using the design parameters of a LCS device—SINAP Ⅲ. The results show that the SINAP Ⅲ device is able to perform energy calibration work over the energy region of 25–740 keV. The precision of calibration is better than 1.6% from 25 to 300 keV and is better than 0.5% from 300 to 740 keV.     

Tangential and Vertical Compact Torus Injection Experiments on the STOR-M Tokamak

This paper describes the setup and results of compact torus (CT) injection experiments on the STOR-M tokamak. Tangential CT injection into STOR-M induced H-mode-like phenomena including doubling the electron density, reduction in the Hα radiation level, suppression of the floating potential fluctuations, suppression of the m = 2 Mirnov oscillations, and increase in the global energy confinement time. Experimental setup, bench-test results, and some preliminary injection data for vertical CT injection experiments on STOR-M will be shown. In addition, numerical simulations of the CT trajectories in tokamak discharges for both tangential and vertical injection geometries will be discussed.     

Comparison of Measured Soft X-Ray Yield Versus Pressure for NX1 and NX2 Plasma Focus Devices Against Computed Values Using Lee Model Code

The soft X-ray yield versus pressure curves of NX1 and NX2 plasma focus machines have been measured and published for different pressures and electrode configurations. In this work, the numerical experiments are carried out, using Lee model code. The Lee model code is configured for each of these devices NX1 and NX2 by fitting computed total discharge current waveform against a measured total discharge current waveform. The computed soft X-ray yield versus pressure curves are compared with the laboratory measured soft X-ray yield versus pressure data. The comparison shows agreement between computation and measurement of several important features of the yield versus pressure curves.     

Modelling of the Mark I pool suppression system

A mathematic model of the Mark I type pool suppression system is presented, together with its computer implementation. The time-dependent compressible gas dynamics through the vent system and the bubble formation are described.The pressure, velocity and mass flow rates in the system have been determined. Scaling laws and compressibility effects are studied. The analysis forms the starting point for the determination of the pool swell and the dynamic loading of the torus shell.     

Thermal fluid multiphysics optimization of spherical tokamak centerpost

An experimental Fusion Nuclear Science Facility (FNSF) is required that will create the environment that simultaneously achieves high energy neutrons and high ion fluence necessary in order to bridge the gaps from ITER to the realization of a fusion nuclear power plant. One concept for achieving this is a high duty cycle spherical torus. This study will focus on thermal modeling of the spherical torus centerpost using computational fluid dynamics to effectively model the thermal transfer of the cooling fluid to the centerpost. The design of the fluid channels is optimized in order to minimize the temperature in the centerpost. Results indicate the feasibility of water cooling for a long-pulse spherical torus FNSF.     

Performance evaluation of the backprojection filtered （BPF） algorithm in circular fan-beam and cone-beam CT

In this article we introduce an exact backprojection filtered （BPF） type reconstruction algorithm for cone-beam scans based on Zou and Pan＇s work. The algorithm can reconstruct images using only the projection data passing through the parallel PI-line segments in reduced scans. Computer simulations and practical experiments are carded out to evaluate this algorithm. The BPF algorithm has a higher computational efficiency than the famous FDK algorithm. The BPF algorithm is evaluated using the practical CT projection data on a 450 keV X-ray CT system with a flat-panel detector （FPD）. From the practical experiments, we get the spatial resolution of this CT system. The algorithm could achieve the spatial resolution of 2.4 lp/mm and satisfies the practical applications in industrial CT inspection.     

Research plan for divertor simulation making use of a large tandem mirror device

We are planning to start a study of divertor simulation under the closely resemble to actual fusion plasma environment making use of the advantage of open magnetic field configuration and to contribute the solution for realizing the divertor in ITER as a future research plan of Plasma Research Center of the University of Tsukuba. In the research plan, the concepts of two divertor devices are introduced. One has an axi-symmetric divertor configuration with the separatrix which is similar to toroidal divertor of torus systems and the other is a high heat flux divertor simulator by using an end-mirror exit of the existing tandem mirror device. Development of magnetic field configuration for ensuring the MHD stability is under way and a designed example is investigated under the optimal condition for plasma production. Consideration of plasma heating scheme using Fokker-Planck simulation code was successfully performed at both axi-symmetric divertor and end-mirror regions. Preparative experiments using calorimeter, Mach probe and high-speed camera have been started at the end-mirror region and the heat flux density of the level in 1-10 MW m⁻² was achieved in standard hot-ion mode plasma-confining experiments, which gives a clear prospect of generating the required heat flux density for divertor studies.     

Measurements and Simulations of Neutron Emission Versus Deuterium Filling Pressure in Plasma Focus Device PF-24

A series of experiments were carried out using the middle energy (dense) plasma focus device PF-24 with deuterium as a working gas under pressure in a range between 2 and 5 mbar for 17 kV of charging voltage. The relationship between these operating deuterium pressures in reference to the total neutron yield (Y_n) was estimated. The 5-phase Lee code was used to simulate the measured discharge current and neutron yield (Y_n) using a phenomenological beam-target neutron generating mechanism, which was incorporated in the model. Comparison of the Y_n versus pressure using fitted model parameters was made at each point of pressure. The good agreement between measured and computed Y_n values was achieved for discharges with lower neutron emission. The measured Y_n (below 2.6?×?10⁹ n/discharge—the median value) has been reproducible by the model for 73% of simulated discharges, while above the median value its prediction were incorrect. The kinetic plasma parameters which were measured and computed using the Lee code for different pressures are: the time to a current sheath collapse (t_c), the average axial current sheath velocity (v_z) and the so called velocity factor (RF). Good agreement was found in the whole range of deuterium pressures between the computed and measured results for these kinematic quantities. Presented findings in this work suggest that the character of neutron emission is more complex than it would seem from classical interpretation of neutron production based on a beam-target model.     

高能X射线CT技术在辐照后核燃料组件检测中的发展及应用

定期检测辐照后核燃料组件对保障反应堆安全运行和开展高燃耗下核燃料组件的性能研究具有重要意义。为了能在不拆卸、不破坏燃料组件的情况下更好地观察燃料组件及其内部燃料棒的缺陷及结构变化等信息,高能X射线计算机断层扫描(X射线CT)技术作为一种有效手段可用于辐照后核燃料组件的检测。日本多年来一直致力于该技术的研究工作,成为世界上唯一一个研制出用于辐照后燃料组件检测的高能、高分辨率X射线CT检测装置且应用于快中子反应堆现场检测的国家。为此,本文梳理日本近几十年来相关研究成果,介绍日本原子能研究开发机构(JAEA)研发的燃料组件高能X射线CT装置结构、工作原理、研究现状及部分应用实例,以期对我国核燃料组件无损检测技术的发展提供参考、借鉴。     

Why Compact Tori for Fusion?

A compact torus (CT) has a toroidal magnetic and plasma geometry, but is contained within a simply-connected vacuum vessel such as a cylinder. Spheromaks and field-reversed configurations fall into this category. Compact tori are translatable and have a high engineering beta. The primary benefit of CTs for fusion is the absence of toroidal field and Ohmic Heating coils and the many problems brought on by them. Studying fusion-relevant plasma in simply-connected geometries affords the world fusion program both physics and technology opportunities not found in other configurations. This paper outlines the technology and physics opportunities of compact tori, and presents a cost model based on geometry for comparison with less compact configurations.     

Characterization of hydrophobic bonded silicon wafers

Direct bonding of silicon-to-silicon has been recognized as an interesting method for creating novel device geometries and structures and it has so far been used for the preparation of power devices and sensors. The influence of the bonded interface on electrical performance is then of great interest. In this contribution the interface region of hydrophobic bonded n-type silicon wafers have been studied and a comparison is made between samples before and after an exposure to low doses of 9.5 MeV protons to see the effect of the interface on point defect kinetics. The samples were studied using current–voltage (IV), capacitance–voltage (CV), deep level transient spectroscopy (DLTS), secondary ion mass spectrometry (SIMS) and scanning electron microscopy (SEM). During reverse bias there is a dramatic increase in leakage current when the depletion region reaches the bonded interface region. Due to the high leakage currents DLTS measurements could not be performed directly at the interface. However, in contrast to previous studies, no deep levels are discovered in the interface region of non-irradiated samples and, furthermore, no influence of the bonded interface on the concentration and depth distribution of irradiation induced defects could be detected. This suggests that the irradiation induced defects are unaffected by the bonded interface. At the interface a boron peak is detected by SIMS.