Start-up and shutdown thermomechanical transient analyses of the IFMIF European lithium target system

In the framework of the current IFMIF Engineering Validation and Engineering Design Activities (IFMIF/EVEDA) phase, ENEA is responsible for the design of the European concept of the IFMIF lithium target system which foresees the possibility to periodically replace only the most irradiated and thus critical component (i.e., the backplate) while continuing to operate the rest of the target for a longer period (bayonet backplate concept). With the objective of evaluating the performances of the system in terms of temperature, stress and displacement fields evolution during start-up and shutdown phases, an uncoupled thermomechanical transient analysis has been performed in close collaboration with the University of Palermo by means of a qualified finite element (FE) thermomechanical code. The calculations employed a realistic 3D time-dependent FE model which takes into account all the mechanical and thermal loads including the nuclear heating due to neutron and prompt gamma fields during start-up and decay power of activated products during shutdown. The nuclear data have been calculated by ENEA as part of a parallel extensive neutronic analysis carried out through the MCNP transport code and the EASY-2010 activation code package and then passed as input to the thermomechanical FE model. In this paper, the results of the above transient analyses are reported, highlighting the relevant indications obtained with respect to the fulfillment of the design requirements and possible hints for improving the system design. In particular, it is found that ITER design rules taking into account secondary loads are not always fulfilled during the transient, whereas the predicted displacements allow to exclude any contact of the target system with the surrounding structures.     

Dynamic development model for long gap discharge streamer-leader system based on fractal theory

Establishing a long air gap discharge model considering the streamer-leader transition and randomness of the discharge path is of great significance to improve the accuracy of discharge characteristic prediction and optimize external insulation design. Based on fractal theory and thermal ionization theory of streamer-leader transition, this work establishes a dynamic development model for the long air gap discharge streamer-leader system, which includes streamer inception, streamer development, leader inception, development of streamer-leader system and final jump. The positive discharge process of a 3 m rod plate is simulated to obtain the fractal distribution of the discharge path and the law of leader development for comparison with the discharge test results. The results show that the simulation model is similar to test results in the development characteristics of leader path distribution, each stage time and leader velocity. Finally, a simulation calculation of a 50% breakdown voltage of the rod-plate gap and ball-plate gap is carried out, with results fairly consistent with test data, proving the effectiveness and practicality of the model.     

SUNIST Microwave Power System

Experiments on the start-up and formation of spherical tokamak plasmas by electroncy clotron heating alone without ohmic heating and electrode discharge assisted electron cyclotron wave current start-up will be carried out on the SUNIST (Sino United Spherical Tokamak) device.The 2.45 GHz/100 kW/30 ms microwave power system and 1000 V/50 A power supply for electrode discharge are ready for experiments with non-inductive current drive.     

Effects of Internal and External Connection of Electrodes in Diagonal Type Nonequilibrium Plasma MHD Generator

In this paper, by a two-dimensional analysis in the duct cross section perpendicular to the plasma flow, the authors investigate comparatively the current distribution, the electrical efficiency etc. in the diagonal type nonequilibrium plasma generator of the window frame construction and the one of the insulating sidewall. Their numerical computations are made for an example of the cesium-seeded helium in nonequilibrium ionization, where it is assumed that the ionization instability does not occur.

It is shown that the current in the generator duct of the diagonal conducting wall tends to flow uniformly in the cross section of the duct, and on the other hand, the current in the duct of the insulating sidewall does in its central part; thus the performance characteristics of the former becomes better than those of the latter. This tendency increases with the load factor and wall temperature of the generator. Also the generator characteristics are improved with increasing the inclination factor, and the electrode voltage drop decreases with the load factor.     

Study on the discharge mechanism and EM radiation characteristics of Trichel pulse discharge in air

The Trichel pulse stage is an unstable stage of negative corona discharge that can also involve electromagnetic (EM) radiation signals. In this paper, the discharge mechanism and radiation characteristics of the Trichel pulse are studied in the needle-plate electrode configuration. The Trichel pulse current and its EM radiation signals are measured at different applied voltages. The results show that Trichel pulse discharge changes from the random pulse stage to the continuous pulse stage as the applied voltage increases. During these different stages, the normalized shape of the Trichel pulses remains unchanged, while the frequency of the EM radiation generated by the discharge remains unchanged. The discharge mechanism and EM radiation characteristics of the Trichel pulse are theoretically analyzed in the different stages. Both the positive ion sheath and the negative ion cloud play key roles in the formation of the Trichel pulse. The EM radiation signal is generated by the rapidly changing Trichel pulse current, and the Trichel pulse current waveform determines the characteristics of the EM radiation signal.     

Initial plasma start-up using partial solenoid coils in Versatile Experiment Spherical Torus (VEST)

Initial plasma start-up experiments based on ohmic discharge using partial solenoid coils located at both vertical ends of a center stack have been carried out in Versatile Experiment Spherical Torus (VEST) at Seoul National University. Ohmic discharges with the help of microwave pre-ionization have been performed according to the pre-programed start-up scenario which was experimentally verified by a series of vacuum field measurements using an internal magnetic probe array. A plasma current of around 0.4 kA has been achieved by ohmic discharge using partial solenoid coils, under the toroidal magnetic field of 0.1 T. The vacuum field calculation and fast camera image have revealed that the small plasma current even with significant amount of loop voltage up to 9.7 V is attributed to the imbalance of poloidal field for equilibrium. Modification of the start-up scenario and upgrade of power supplies are proposed to be carried out in order to achieve higher plasma current in the future experiments.     

Characteristics of long-gap AC streamer discharges under low pressure conditions

The generation and propagation of a streamer is a significant physical process of air gap discharge. Research on the mechanism of streamers under low-pressure conditions is helpful for understanding the process of long-gap discharge in a high-altitude area. This paper describes laboratory investigations of streamer discharge under alternating current(AC) voltage in a low pressure test platform for a 60 cm rod–plane gap at 30 kPa, and analyzes the characteristics of streamer generation and propagation. The results show that the partial streamer and breakdown streamer all occur in the positive half-cycle of AC voltage near the peak voltage at 30 kPa. The partial streamer could cause the distortion of current and voltage waveform, and it appears as the branching characteristic at the initial stage. With the extension of the streamer, the branching and tortuosity phenomena become gradually obvious, but the branching is suppressed when the streamer crosses the gap. The low-pressure condition has little influence on the tortuosity length and the tortuosity number of the streamer, but affect the diameter of streamer obviously.     

An equivalent model of discharge instability in the discharge chamber of Kaufman ion thruster

The industrial application of the Kaufman ion thruster in its arc stage is limited owing to the instability of the discharge pulse. Presently, a complete prediction model that can predict the discharge pulse in the high-current stage does not exist. In this study, a complete prediction model for the pulse in the ion thruster is established using the zero-dimensional plasma discharge model and equivalent circuit model. The zero-dimensional plasma discharge model is used to obtain the corresponding plasma parameters by calculating the beam current, discharge current, voltage, and gas flow under actual working conditions. The input parameters of the equivalent circuit model are calculated using empirical formulae to acquire the estimated discharge waveforms. The pulse waveforms obtained using the model are found to be consistent with the experimental results. The model is used to evaluate the process of rapid changes in plasma density. Additionally, this model is employed to predict changes in the pulse waveforms when the volume of the discharge chamber and grid plate transmittance are changed.     

Study on the influence of the discharge voltage on the ignition process of Hall thrusters

A high-speed charge-coupled device camera was used to capture images of the plume and acceleration channel of a Hall effect thruster during ignition at different discharge voltages. To better understand the influence of changes in the discharge voltage on the plasma parameters during thruster ignition, a particle-in-cell numerical model was used to calculate the distribution characteristics of the ion density and electric potential at different ignition moments under different discharge voltages. The results show that when the discharge voltage is high, the ion densities in the plume and acceleration channel are significantly higher at the initial phase of thruster ignition; with the gradual strengthening of the ignition process, the propellant avalanche ionization during thruster ignition occurs earlier and the pulse current peak increases. The main reason for these phenomena is that the change in the discharge voltage results in different energy acquisitions of the emitted electrons entering the thruster channel.     

A two-dimensional air streamer discharge modified model based on artificial stability term under non-uniform electric field at low temperature and sub-atmospheric pressure

In this paper, an improved air discharge fluid model under non-uniform electric field is constructed based on the plasma module COMSOL Multiphysics with artificial stability term, and the boundary conditions developed in the previous paper are applied to the calculation of photoionization rate. Based on the modified model, the characteristics of low temperature subatmospheric air discharge under 13 kV direct current voltage are discussed, including needle-plate and needle-needle electrode structures. Firstly, in order to verify the reliability of the model, a numerical example and an experimental verification were carried out for the modified model respectively. Both verification results show that the model can ensure the accuracy and repeatability of the calculation. Secondly, according to the calculation results of the modified model, under the same voltage and spacing, the reduced electric field under low temperature subatmosphere pressure is larger than that under normal temperature and atmospheric pressure. The high electric field leads to the air discharge at low temperature and sub atmospheric pressure entering the streamer initiation stage earlier, and has a faster propagation speed in the streamer development stage, which shortens the overall discharge time. Finally, the discharge characteristics of the two electrode structures are compared, and it is found that the biggest difference between them is that there is a pre-ionization region near the cathode in the needle-needle electrode structure. When the pre-ionization level reaches 10~(13) cm~(-3), the propagation speed of the positive streamer remains unchanged throughout the discharge process, and is no longer affected by the negative streamer. The peak value of electric field decreases with the increase of pre-ionization level, and tends to be constant during streamer propagation. Based on the previous paper, this paper constructs the air discharge model under non-uniform electric field, complements with the previous paper, and forms a relatively complete set of air discharge simulation system under low temperature and sub atmospheric pressure, which provides a certain reference for future research.     

Influence of pump starting times on transient flows in pipes

The purpose of this study is the numerical modelling of the transient flow in a cylindrical pipe caused by the start-up of a centrifugal pump. The pump starting produced by the speed of the electric motor, which rises from zero up to the rated speed, influences the flow in the hydraulic installation and forced it to follow the starting dynamic law of the motor. The governing equations for such flows are two coupled hyperbolic partial differential equations which are the equations of continuity and motion. The head and the discharge are considered as two principal dependent variables. Under the effect of the friction force, due to the fluid viscosity, the transient flow is progressively dumped until reaching the final steady-state flow. Two boundary conditions are considered: a pump with a known motor torque at the upstream end and a constant level reservoir at the downstream end of the pipe. The motor torque is considered for different starting regimes: very rapid, rapid, medium and slow. In order to predict the dynamic behaviour of the flow during the pump start-up transient conditions, the mathematical model is solved by the method of characteristics. The computed head and discharge, caused by the starting of the pump, are presented at some cross sections of the pipe and the results show that the evolution of the hydraulic variables is well influenced by the pump starting time.     

Volume Diffuse Dielectric Barrier Discharge Plasma Produced by Nanosecond High Voltage Pulse in Airflow

Volume diffuse dielectric barrier discharge (DBD) plasma is produced in subsonic airflow by nanosecond high-voltage pulse power supply with a plate-to-plate discharge cell at 6 mm air gap length.The discharge images,optical emission spectra (OES),the applied voltage and current waveforms of the discharge at the changed airflow rates are obtained.When airflow rate is increased,the transition of the discharge mode and the variations of discharge intensity,breakdown characteristics and the temperature of the discharge plasma are investigated.The results show that the discharge becomes more diffuse,discharge intensity is decreased accompanied by the increased breakdown voltage and time lag,and the temperature of the discharge plasma reduces when airflow of small vclocity is introduced into the discharge gap.These phenomena are because that the airflow changes the spatial distribution of the heat and the space charge in the discharge gap.     

Experimental study on plasma actuation characteristics of nanosecond pulsed dielectric barrier discharge

Combining high-speed schlieren technology and infrared imaging technology, related research has been carried out on the influence of parameters such as actuation voltage, repetition frequency, and electrode size of an actuator on the discharge characteristics, induced flow field characteristics, and thermal characteristics of nanosecond pulsed dielectric barrier discharge. The results show that increasing the value of the actuation voltage can significantly increase the actuation intensity, and the plasma discharge area is significantly extended. Increasing the repetition frequency can increase the number of discharges per unit time. Both will cause more energy input and induce more changes in the flow field. The effect of temperature rise is more significant. The width of the covered electrode will affect the potential distribution during the discharge process, which in turn will affect the extension process of the plasma discharge filament. Under the same actuation intensity, the wider the covered electrode, the larger range the induced flow field and temperature rise is. Preliminary experimental analyses of high-frequency actuation characteristics, temperature field characteristics, flow field characteristics and actuation parameter settings provide support for the parameter selection and partial mechanism analysis of plasma anti-icing.     

反应堆核加热冷启动压力控制及超压问题仿真分析

基于反应堆核加热冷启动过程操纵和控制要求，开展了反应堆核加热冷启动过程压力自动控制方法研究，完成了系统压力自动控制方法设计与控制仿真验证；同时对冷启动水密实状态的超压问题进行了仿真分析，提出了防止超压事故的联锁控制方法。结果表明，当核功率不超过一定功率水平时，压力自动控制方法可实现反应堆核加热冷启动过程系统压力的有效控制。      

The current pulses characteristics of the negative corona discharge in SF6/CF4 mixtures

This paper presents the results of numerical investigation of the current pulses characteristics in SF₆/CF₄ mixtures for the negative point-plane corona discharge. The pressure and the temperature of gas mixtures are 0.4 MPa and 300 K, respectively. The CF₄ content varies from20% to 80%. The 2D axisymmetric geometry with point-plane electrodes is investigated, and the three drift-diffusion equations are solved to predict the characteristics of the negative corona...     

Arc characteristics during the instability stage on transverse magnetic field contacts

The arcing process greatly affects the breaking ability after current zero. The instability stage is the transition stage from the ignition to the movement stage, which affects the arc movement characteristics. In this paper, the arc characteristics during the instability stage on spiral-type contacts were investigated using a high-speed video camera. A multi-column parallel instability mode and a single-column instability mode were found during the instability stage. The arc appearance and constriction degree changed rapidly. The arc voltage usually increased accompanied by fluctuations. In addition, it was found that the current significantly influenced the arc mode and duration in the instability stage. With increased peak current, the probability of a single-column instability mode increased, and the fluctuation range and average time decreased.     

Operation with 1 MA plasma current in EAST

Discharge with a plasma current of 1 MA at a line-averaged density of 1.8×1019m-3 was realized in EAST, a fully superconducting tokamak. The key issues to achieve the discharge with 1 MA plasma current include both early shaping and LHCD assistance during start-up phase to extend the voltage margin of PF coils for easier plasma control, an optimization of the control methodology for PF coils to avoid over-current fault and a very good wall condition. A better wall condition was achieved mainly by extensive Lithium coating. Both stationary H-mode and diverted plasma discharge of 100 s were also obtained.     

On the thermomechanical behavior of the European target assembly design of IFMIF-EVEDA lithium test loop under start-up transient scenarios

Uncoupled thermomechanical transient analyses have been carried out to investigate the behavior of IFMIF-EVEDA lithium test loop bayonet backplate target assembly under two selected start-up transient operational scenarios. The first transient scenario considered foresees that the target assembly, starting from the initial uniform temperature of 50 °C, is heated up uniquely by convective heat transfer with lithium, flowing from inlet to outlet nozzle at its reference nominal temperature and pressure, until its nominal steady state thermal field distribution is reached. The second transient scenario foresees, more realistically, that the target assembly, starting from the uniform temperature of 50 °C, is initially warmed-up by electric heaters mounted onto its main accessible surfaces and, subsequently, by convective heat transfer with lithium reference flow, until nominal steady state conditions are reached. Heaters have been supposed to operate in an on/off stepwise mode, resulting to be alternatively switched on and off in order to allow the target assembly thermal field to grow up minimizing thermal gradients. To this purpose, a parametric analysis has been performed to realistically assess, for each electric heater, its heat flux and duty-cycle. Numerical results obtained are presented and critically discussed.     

泳池式反应堆自启停控制技术研究

反应堆实现自动启停，可以有效减轻运行人员工作强度，减少误操作，提高反应堆启动运行的安全可靠性。本文基于对典型泳池式反应堆的工艺特点以及启动操作的分析，对泳池式反应堆自启停系统的控制范围、层次结构、断点、典型控制逻辑进行研究，并搭建泳池式反应堆自启停的仿真测试系统。该自启停系统能够实现泳池式反应堆的自动启停，启停过程无人工操作，降低人员误操作可能性。      

反应堆核加热冷启动压力控制及超压问题仿真分析

Based on the control requirements of the reactor cold start-up process with nuclear heating, the automatic pressure control of reactor cold start-up with nuclear heating is studied in this paper, and the method for the system pressure automatic control in the process of cold start-up with nuclear heating are studied, and the automatic control method is designed and the control simulation verification is completed. At the same time, the problem of overpressure in cold start-up with water compaction state is simulated and analyzed. The interlock control methods to prevent overpressure accidents are proposed. The results show that, when the reactor power does not exceed a certain power level, the automatic pressure control method can realize the effective control of the reactor pressure during the cold start-up with nuclear heating.