Study of Various Modes of Operation of an IEC Device

Inertial electrostatic confinement fusion devices are compact sources of neutrons, protons, electrons and X-rays. Such sources have many applications. Improving the efficiency of the device also increases the number of applications of this device. Hence a thorough understanding of the operation of this device is needed. In this paper we study the various modes in which an IEC device can be operated. The device seems to perform better when RF power is used. Furthermore, the amount of current that the power source can drive at a given pressure and grid voltage is dependent on the frequency applied to the grid—the higher the frequency the higher the cathode current. The device has been tested up to 320 kHz and the power supply current kept increasing up to this frequency. The higher limit was not reached by the present RF power supply. Another effect observed with the use of RF power is that the ionization source is more homogeneous relative to electron source ionization. This could mean that the heat load on the cathode is more evenly distributed. Such an even heat distribution would allow higher power operation.     

The Use of an Electron Microchannel as a Self-Extracting and Focusing Plasma Cathode Electron Gun

A new and simple type of electron gun is presented.Unlike conventional electron guns,which require a heated filament or extractor,accelerator and focusing electrodes,this gun uses the collimated electron microchannels of an inertial electrostatic confinement(IEC) discharge to achieve the same outcome.A cylindrical cathode is placed coaxially within a cylindrical anode to create the discharge.Collimated beams of electrons and fast neutrals emerge along the axis of the cylindrical cathode.This geometry isolates one of the microchannels that emerge in a negatively biased IEC grid.The internal operating pressure range of the gun is 35-190 m Torr.A small aperture separates the gun from the main vacuum chamber in order to achieve a pressure differential.The chamber was operated at pressures of 4-12 m Torr.The measured current produced by the gun was 0.1-3 m A(0.2-14 m A corrected measurement) for discharge currents of 1-45 m A and discharge voltages of 0.5-12 k V.The collimated electron beam emerges from the aperture into the vacuum chamber.The performance of the gun is unaffected by the pressure differential between the vacuum chamber and the gun.This allows the aperture to be removed and the chamber pressure to be equal to the gun pressure if required.     

Preliminary Results of Experimental Studies from Low Pressure Inertial Electrostatic Confinement Device

In this study, Turkey’s first low pressure inertial electrostatic confinement (IEC) device, constructed at the Saraykoy Nuclear Research and Training Center (SNRTC-IEC), is introduced and the first results are reported. This device was designed for neutronic fusion studies in terms of D–D reaction. The SNRTC-IEC device consists of spherical chamber 300 mm in diameter and a grid-type spherical cathode in which high negative voltage is applied at the center of chamber. The outer surface of the device held at ground potential has 10 ports to connect the vacuum pump, high voltage load, residual gas analyzer, ion sources and other peripherals. Cathode voltage is 85 kV and it is particularly emphasized that the SNRTC-IEC device is studied at low pressure (1?10 × 10^?4 mbar). The maximum total neutron production rate is measured at around 2.4 × 10⁴ neutrons per second for the medium grid cathode.     

Study of Fuel Ratios on the Fusion Reactivity in an Inertial Electrostatic Confinement Device Using a Residual Gas Analyzer

Gridded Inertial Electrostatic confinement (IEC) devices are of interest due to their flexibility in burning advanced fuels, their tuning ability of the applied voltage to the reaction cross-section. Although this device is not suitable for power production in its present form, it does have several near term applications. The number of applications of this device increases with increasing fusion reactivity. These devices are simple to operate but are inherently complicated to understand and an effort to incrementally understand the device to improve its operational efficiency is underway at University of Wisconsin, Madison. Of all the parameters under study we are focusing on the effects of flow rate and flow ratio on the fusion reactivity in the present paper. Experiments were conducted to understand the influence of fuel flow ratio on the fusion reactions. The residual gas analyzer (RGA) was used to study the impurity concentration as the flow ratio was changed. It was observed that the higher flow rate resulted in reduced impurity levels and hence an increase in fusion rate. Several different species of gases were detected, some of these molecules formed inside the RGA analyzer. The flow ratio scan revealed that the optimum mixture of D₂ with ³He to be D₂:³He::1:2 for maximum D–³He fusion rate.     

Large aperture electrostatic dust detector

Diagnosis and management of dust inventories generated in next-step magnetic fusion devices is necessary for their safe operation. A novel electrostatic dust detector, based on a fine grid of interlocking circuit traces biased to 30 or 50 V has been developed for the detection of dust particles on remote surfaces in air and vacuum environments. Impinging dust particles create a temporary short circuit and the resulting current pulse is recorded by counting electronics. Up to 90% of the particles are ejected from the grid or vaporized suggesting the device may be useful for controlling dust inventories. We report measurements of the sensitivity of a large area (5 × 5 cm) detector to microgram quantities of dust particles.     

Study of the key factors affecting the triple grid lifetime of the LIPS-300 ion thruster

In order to ascertain the key factors affecting the lifetime of the triple grids in the LIPS-300 ion thruster,the thermal deformation,upstream ion density and component lifetime of the grids are simulated with finite element analysis,fluid simulation and charged-particle tracing simulation methods on the basis of a 1500 h short lifetime test.The key factor affecting the lifetime of the triple grids in the LIPS-300 ion thruster is obtained and analyzed through the test results.The results show that ion sputtering erosion of the grids in 5 kW operation mode is greater than in the case of 3 kW.In 5 kW mode,the decelerator grid shows the most serious corrosion,the accelerator grid shows moderate corrosion,and the screen grid shows the least amount of corrosion.With the serious corrosion of the grids in 5 kW operation mode,the intercept current of the acceleration and deceleration grids increases substantially.Meanwhile,the cold gap between the accelerator grid and the screen grid decreases from 1 mm to 0.7 mm,while the cold gap between the accelerator grid and the decelerator grid increases from 1 mm to 1.25 mm after 1500 h of thruster operation.At equilibrium temperature with 5 k W power,the finite element method(FEM)simulation results show that the hot gap between the screen grid and the accelerator grid reduces to 0.2 mm.Accordingly,the hot gap between the accelerator grid and the decelerator grid increases to 1.5 mm.According to the fluid method,the plasma density simulated in most regions of the discharge chamber is 1?×?10~(18)-8?×?10~(18)m~(-3).The upstream plasma density of the screen grid is in the range 6?×?10~(17)-6?×?10~(18)m~(-3)and displays a parabolic characteristic.The charged particle tracing simulation method results show that the ion beam current without the thermal deformation of triple grids has optimal perveance status.The ion sputtering rates of the accelerator grid hole and the decelerator hole are 5.5?×?10~(-14)kg s~(-1)and 4.28?×?10~(-14)kg s~(-1),respectively,while after the thermal deformation of the triple grids,the ion beam current has over-perveance status.The ion sputtering rates of the accelerator grid hole and the decelerator hole are 1.41?×?10~(-13)kg s~(-1)and 4.1?×?10~(-13)kg s~(-1),respectively.The anode current is a key factor for the triple grid lifetime in situations where the structural strength of the grids does not change with temperature variation.The average sputtering rates of the accelerator grid and the decelerator grid,which were measured during the 1500 h lifetime test in5 k W operating conditions,are 2.2?×?10~(-13)kg s~(-1)and 7.3?×?10~(-13)kg s~(-1),respectively.These results are in accordance with the simulation,and the error comes mainly from the calculation distribution of the upstream plasma density of the grids.     

Effects of Grid Structures and Dielectric Materials of the Holder in an Inertial Electrostatic Confinement (IEC) Fusion Device

An inertial electrostatic confinement fusion device modeling has been carried out. Finite element method is used in a 3D media in order to identify the potential and electrical fields inside the device. The effects of different materials on the electrostatic features are found. In addition, different geometric arrangements for cathode sphere (i.e. inner grid) are considered for the determination of spatial potential and electrical field. The effects of dielectric materials such as porcelain and polystyrene as the cathode holder have also been explored. It is found that porcelain giving a minimal potential value at the center of inner grid in order to confine ions. Increase in number of vertical rings on the inner grid diminishes the bottom corner of electrical potential, thereby ions are scattered to the entire region of the inner grid, thereby, an optimal shape for the inner grid can be adjusted for a better ion core at the middle of the grid for the confinement.     

带搅混格架5×5棒束流场的激光多普勒测量

采用激光多普勒测速（LDV）系统,对带有2道搅混格架的5×5棒束格架下游4个横截面的流场进行了轴向流速测量。实验测得了各个截面上的平均轴向速度分布和轴向脉动速度均方根（RMS）分布。通过比较不同截面的平均速度与RMS速度的差异,分析了定位格架下游流场的演变规律;比较了2道格架下游的实验数据,分析了上游流场对格架效应的影响。本文实验数据是在充分重复性实验的基础上获得的,可以为计算流体动力学（CFD）结果的验证和评价提供基准参考。      

基于超单元技术的乏燃料贮存格架多自由度非线性抗震分析

基于有限元软件ANSYS超单元分析技术,建立乏燃料贮存格架整池超单元模型,对格架地震响应进行更精确的多自由度模拟。考虑了格架间流-固耦合影响,对整池格架进行非线性时程抗震分析,获得了格架的跳起位移、碰撞加速度、滑移位移等计算结果,并将超单元模型抗震分析结果扩展到格架详细模型,得到格架地震下的变形和应力。分析结果表明:基于超单元分析建模的技术方法显著提高了分析效率,格架响应计算也更为精确,可用于乏燃料贮存格架的抗震设计。      

Analysis of Unsteady Turbulent Triple Jet Flow with Temperature Difference

Suitable analysis methodology is required to obtain detailed information about magnitude and frequency of temperature variation of flow field for the study of thermal stripping phenomena. The large eddy simulation (LES) is applied to analyze unsteady turbulent triple jet water flow which can be a direct cause of thermal stripping. Current analyses are performed with different sub-grid scale models, number of grids, time increments, and inlet temperature intensities to find the effects of these on prediction. Predicted results of the LES are compared with experimental results. The LES successfully produces a time history of turbulence variables, which can be used to evaluate magnitude and frequency of instantaneous temperature. The LES tends to predict higher levels of root mean square temperatures compared to those of an experiment, indicating very active mixing effect among triple jets. The LES is found to be able to provide reliable frequency information about temperature fluctuation. The different sub-grid scale models show no significant difference in prediction ability and other variations of the LES prediction show no significant difference in prediction either. However, cases using the fine grid and the small time increment are slightly better than others. Further study is desired with different levels of inlet temperature intensities and separate sub-grid scale models for temperature field.     

A study of the influence of different grid structures on plasma characteristics in the discharge chamber of an ion thruster

The grid structure has significant effects on the discharge characteristics of an ion thruster.The discharge performances of a 30 cm diameter ion thruster with flat,convex and concave grids are studied.The analysis results show that the discharge chamber with a convex grid has a larger’magnetic-field free area’ than the others,and the parallelism of the magnetic-field isopotential lines and anode is generally the same in the three models.Plasma densities of the three structures at the grid outle...     

Spacer Grid Effects during a Reflood in an Annulus Flow Channel

An experimental study has been performed to investigate the effects of a spacer grid in an annulus flow channel with a uniform power shaped single rod during a bottom-reflood phase. The ranges of the experimental parameters are 2-8 cm/s for the flooding velocity, 20-80°C for the inlet subcooling temperature, and 500-700°C for the initial wall temperature. Two types of spacer grids, i.e., a swirl-vane type grid and a straight egg-crate type spacer grid, have been tested to compare the differences in their thermal hydraulic behavior through the spacer grids. Flow patterns and a rewetting front behavior near a spacer grid are remarkably altered with the types of spacer grids used. In the case of a low flooding rate and a high wall temperature condition, the cooling capacity of the swirl-vane spacer grid is better than that of the straight egg-crate type grid. Rewetting velocities through the swirl-vane spacer grids are faster than those through the other types of grids. The cladding temperature of the heater rod near a spacer grid shows a different pattern with the types of spacer grid used.     

Effects of various spacer grid modeling on the neutronic parameters of the VVER-1000 reactor

The main objective of this paper is to study the effects of various spacer grid models on the neutronic parameters of a VVER-1000 reactor. Specifically, the data of the nuclear power plant at the Bushehr site, which is of a VVER-1000 type, will be studied. Three models, representing the spacer grids along the fuel assemblies are presented. These three models are the homogeneous and the heterogeneous local spacer grid models and the shroud spacer grid model. In the homogeneous and the heterogeneous models, the spacer grids are considered at their actual locations in the axial direction. The only difference between the two models is that in the homogeneous model, the spacer grids are homogenized with the coolant while in the heterogeneous model, the spacer grids are modeled around the fuel cells at their exact axial positions. In the shroud model, the spacer grids are modeled in the shroud region containing the coolant and are not necessarily placed at their appropriate axial positions.     

The effect of wall loading limitations and choice of beta on the feasibility of advanced fuel fusion reactors

This paper investigates the effect of wall loading limitations and choice of plasma stability index beta on the feasibility of advanced fuel fusion reactors. Two new conceptual tools are introduced to facilitate this analysis: the “effective reactivity,” which includes all of the reaction-relevant parameters that determine the fusion power density, and the “critical radius,” which is the maximum allowable minor radius of a fusion reactor, beyond which the power generated in the plasma will exceed allowable loadings of radiant energy or neutrons on the first wall. It is shown that if high beta (greater than 0.2) fusion reactors are feasible, the high reactivity of the DT reaction cannot be fully exploited because of wall loading limitations. In addition, some high beta reactors with advanced fuels are also found to be wall loading limited, and to have excess reactivity, which can be traded off for lower magnetic fields, longer particle containment times, etc. Under certain circumstances, the reduced materials problems associated with some advanced fuels may outweigh the reactivity advantage of the DT reaction, and make one of them the reaction of choice for high beta fusion reactors.     

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.     

Particle-in-cell simulation of magnetic-assisted electrostatic confinement device

In order to enhance the fusion reaction rate in inertial electrostatic confinement devices, it is necessary to increase the ion density with low cathode current and low background pressure. In order to accomplish the requirement, the authors suggest magnetic-assisted electrostatic confinement (MEC) scheme. The MEC relies on controlling the ion motion by applying an axial magnetic field to a system with cylindrical electrodes. In order to clarify the fundamental performances of the MEC device, particle-in-cell simulation was carried out. By reducing the background pressure, the ion confinement was improved resulting in the increase of the ion density. However, the ion density saturated due to space charge limitation. The estimated fusion reaction rate was about 5 × 10⁶ 1/s/m when the cathode voltage was −100 kV, the magnetic field was 200 mT, and the cathode current was 100 mA/m. The reaction rate, however, is expected to become higher since the ion density limitation is moderated by the electron which is not considered in the present analysis.     

Time-optimal control of nuclear reactor power with adaptiveproportional-integral-feedforward gains

A time-optimal control method which consists of coarse and fine control stages is described. During the coarse control stage, the maximum control effort (time-optimal) is used to direct the system toward the switching boundary, which is set near the desired power level. At this boundary, the controller is switched to the fine control stage in which an adaptive proportional-integral-feedforward controller is used to compensate for any unmodeled reactivity feedback effects. This fine control is also introduced to obtain a constructive method for determining the (adaptive) feedback gains against the sampling effect. The feedforward control term is included to suppress the over- or undershoot. The estimation and feedback of the temperature-induced reactivity are also discussed     

反应堆棒束通道搅混翼数值研究

定位格架上的搅混翼是核反应堆燃料组件中的关键部件,其性能对棒束通道热工水力特性有重要的影响。以带单层定位格架的5×5棒束为研究对象,对搅混翼排布方式及末端形状对格架下游的流场和温度场的影响进行数值模拟研究。计算结果表明,改变搅混翼的排布方式,压降几乎不受影响,但格架下游流场和传热情况却因排布方式的改变而发生显著变化;将搅混翼末端形状改为弧形,压降较典型撕裂型搅混翼没有明显差异,但换热情况得到明显改善。      

棒束通道内定位格架搅混特性PIV可视化研究

定位格架作为燃料组件中重要的组成部件之一,不仅在结构上固定燃料棒,而且在燃料组件内热工水力性能同样显著,特别是对工质的搅混性能直接关系到反应堆的经济性和安全性,因此有必要对燃料组件内定位格架搅混特性进行研究。本文通过粒子图像测速（PIV）技术开展了棒束通道内定位格架上下游流场的可视化研究,对比了有无格架棒束通道内流场的分布特征,定量分析了定位格架对棒束通道流场搅混的贡献。对不同流速下定位格架下游横纵速度的沿程变化特性进行研究,发现了不同流速作用下定位格架对横向、轴向速度的促进和抑制规律。另外,通过速度均方根对下游的湍流特性进行了评估。实验结果可为数值计算提供全场的数据验证,并可为定位格架设计和优化提供基础。     

Investigation of Arc Regulation of Hot Cathode Ion Source with Langmuir Probe

The neutral beam injector (NBI) is one of the main plasma heating methods for nuclear fusion devices. For the hot cathode high current ion source, the arc current and beam current tends to increase during the beam pulse. In order to gets long pulse beam extraction, the arc regulation technology is employed. The Langmuir probes are installed on the experimental advanced superconducting tokamak-NBI ion source, to feedback control the arc discharge and beam extraction. The experimental results show that, the long pulse of 100 s ion beam is extracted with beam energy of 30 keV with arc regulation. More the results are presented in this paper.