Chundong Hu

Radio frequency negative ion source is the best choice for neutral beam injection (NBI) in future nuclear fusion. Cs injection is an important means to increase the yield of negative hydrogen ions. In addition, the stability of the ion source is influenced by the distribution of Cs inside the source in the vacuum phase and plasma phase. Liquid Cs is heated into Cs vapor and then diffused into the source through a nozzle, the shape of which has a direct effect on the angular distribution of the Cs flux. This article studies the ballistic transport process of simulating three types of nozzles: cylindrical, bent, and edge-cut by developing a Monte Carlo program. The results show that the angular distribution of Cs flux from cylindrical nozzles approximates the cosine square distribution in the case of the same nozzle diameter. In comparison, the peak coverage density increases with a certain shift of the distribution state relative to the cosine square for bent and edge-cut nozzles. By analyzing the flux distribution of different nozzles, this study provides a reference for subsequent simulations of Cs and the design of Cs injection systems.

Excellent heat transfer performance has increasingly become a key issue that needs to be solved urgently in the development process of large-scale fusion equipment. The study of heat transfer performance improvement to scientifically and reasonably determine the design parameters of the high heat flow (HHF) components of fusion reactors based on the efficient in-depth analysis of the heat transfer mechanism and its sensitive factors is of great significance. In this paper, a liquid-vapor two-phase flow model with subcooled boiling for a large length-diameter ratio swirl tube structure in the HHF calorimeter component is proposed to analyze the effects of key design parameters (such as inlet temperature of cooling water flow, swirl tube structure parameters, etc.) on its heat transfer performance. Then, considering the high computational cost of the liquid-vapor two-phase flow model, and in order to improve the efficiency of the sensitivity analysis of these design parameters, the polynomial response surface surrogate model of heat transfer performance function was constructed based on Latin hypercube sampling. On this basis, by combining the proposed surrogate model, the sensitivity index of each design parameter could be obtained efficiently using the Sobol global sensitivity analysis method. This method could greatly improve the calculation efficiency of the design parameter sensitivity analysis of HHF components in the fusion reactor, which provides vital guidance for the subsequent rapid design optimization of related components.

The RF-driven negative ion source has a steady state characteristic and will be a promising ion source for neutral beam injection (NBI) in the future. Cesium (Cs) injection is an efficient method to enhance the yield of negative ions during plasma discharge. In order to support the engineering and physical research in this field, the Cs oven prototype has been developed for negative ion source based neutral beam injector at ASIPP. This article presents the design details of a Cs oven system, including the constant temperature control system, the mechanical structure of Cs oven, and the surface ionization detector (SID). SID is a measurement method for the cesium flux in the nozzle. The experiment results of constant temperature control system show that the control accuracy and function meet the requirements of device operation. Meanwhile, the simulation analysis of Cs vapor concentration has been carried out in this paper. According to the simulation results, the graph of total Cs flux is given in the article, which presents the reference for the subsequent device testing.

The deflection magnet (DM) is the most important component of the Neutral Beam Injection (NBI) system of Experimental Advanced Superconducting Tokamak (EAST), which can magnetically deflect the un-neutralized charged particles after the neutralized process of the beam is extracted from the ion source, and then form a neutral beam injected into the tokamak. Under the operating conditions of the NBI system, by using the thermocouple monitoring system in the experiment, it can be found that the currently operating DM beam collimator has a quite high temperature rise. It is necessary to redesign the DM beam collimator to improve its heat transfer performance. The parallel arrangement of multiple rows of tubes is proposed as the basic method for the redesign of the beam collimator of DM, the thermal-fluid-structure analysis model of this redesign model is established and its temperature field, pressure field and stress field are analyzed. Taking the surface temperature of the beam collimator, the overall dimension after the total tube combination and the pressure drop of the whole structure of collimator as the optimization objectives, and setting the fluid velocity, the tube’s inner diameter and the number of tube rows as the design variables, the optimized design scheme of the beam collimator structure is obtained. From the results of simulation, the new structure can better meet the operation requirements of DM, and its maximum temperature rise is well controlled, which is expected to meet the long pulse operation requirements of the NBI system. The proposed simulation and design optimization method can provide a certain reference for the design and optimization of other high-heat-flux structures in complex large-scale neutral beam systems in the future.

Cryosorption pumps create a vacuum by adsorbing gas at low temperature through porous solid adsorbents. The transmission probability of gas molecules and heat loads of cryosorption pumps are important factors affecting its performance. Herein, Molflow software based on the Monte Carlo principle is used to analyze the effects of the structural design of cryosorption pumps on transmission probability. The influence of structural design on radiation heat transfer is analyzed by ANSYS Steady-State Thermal software. This provides a reference for the design of a cryosorption pump to validate the prototype of a neutral beam injector for the China Engineering Fusion Experimental Reactor (CFETR).

During the process of beam extraction in positive ion source under high voltage region, a large number of electrons are produced in the gaps of grids. After back-streaming acceleration, these electrons go back to arc chamber or impinge grids and then heat back plate or grids, which are harmful for the safety of ion source. Under the situation of poor beam extraction optics, a large part of the primary beam ions bombard the surface of suppressor grid (SG). And this process produces a large number of electrons. Due to the huge extracted voltage, the secondary electron emission coefficient of the SG surface is also high. As a result, the grids' current grows. According to the measurement of the current of SG and the calculation of the perveance of the corresponding shoot, the effect of ion beam divergence angle on back-streaming electrons can be analyzed. When the beam divergence angle increases, the number of back-streaming electrons increases rapidly, and grids' current changes significantly, especially the current of gradient grid and SG. The results can guide the parameters operating on the ion source for Experimental Advanced Superconducting Tokamak-neutral beam injection (EAST-NBI) and find the reasonable operation interval of perveance and to ensure the safety and stable running of the ion source, which has great significance on the development of long pulse, high power ion source.

An important feature of the China Fusion Engineering Test Reactor (CFETR) project is the additional heating obtained from the injection of neutral beams based on accelerated negative ions. For the neutral beams based on negative ions, the most important measurements are beam uniformity, beamlet divergence, and stripping losses. According to the CFETR requirement, the maximum allowed beam divergence angle and beam nonuniformity are 6 mrads and ±10%, respectively. As one-dimensional (1-D) carbon tiles have large ratio between perpendicular conductivity and parallel conductivity and high stability, they can be used for beam uniformity and beamlet divergence measurement. This paper investigates the influence on the response of 1-D carbon tile having the thermal characteristics and features of some dedicated diagnostics. Simulations show that it will be possible to verify experimentally whether the beam meets the requirement about the maximum allowed value. This work lays a foundation for design and application of high-precision beam diagnostic targets.