Research On Beam Profile Diagnosis For High Power Negative Ion Source

Neutral Beam Injection(NBI)is an important means of fusion plasma auxiliary heating,and the ion source is its core component.The RF negative ion source is recognized as a reference ion source for ITER(International Thermonuclear Experimental Reactor)due to its simple structure,reliable operation,and long-term maintenance-free operation..In order to explore the key technologies of the plasma production driven by ratio frequency(RF)and negative ion generation,Huazhong University of Science and Technology(HUST)began to construct a RF driven negative hydrogen ion source experimental setup in 2011.This paper focuses on the beam profile diagnosis technology of HUST negative ion source,which can measure beam power,beam profile distribution homogeneity and divergence,This is of great significance for the optimization of the ion source extraction system structure design and operating parameters.The research is mainly divided into three parts:(1)The extraction system of HUST ion source is made up of three-electrode with multi cells and has a large extraction area,based on which the beam profile distribution at different distances downstream of the ground grid(GG)is studied by a simplified beamlet geometric optical model.It is found that the further the profile positon away from the GG,the severer overlap of the beamlet occurs.The profile distribution gradually changes from the multiGaussian peak to the flat-topped peak,and finally becomes the single Gaussian peak,which provides the position selection and design scheme for beam profile diagnosis.Based on the beam profile distribution along the beamline,two beam intercepting profile diagnostic tools were designed.(2)The calorimetry diagnostic system is mainly used to measure the beam power and profile distribution far away from the GG.The beam is intercepted by the quasi-adiabatic copper target,and the temperature rise of the copper target is measured by a thermocouple array,by which the power density can be calculated.And there is a water-cooled heat system which can not only cool down the cooper target but also measure the total beam power.Based on the fluid-solid coupling thermal analysis in ANSYS CFX,the beam power density calculation model of the quasi-adiabatic calorimeter was modified.The feasibility of simplifying the water-cooled structure was verified.A 17-channel thermocouple temperature signal synchronous acquisition device was designed and built.(3)Multi-tungsten wire array beam profile diagnostic system is mainly used to measure the beamlet profile,aiming to obtain its position,divergence and intensity.The working principle is based on the heating up of tungsten wire.When the wires are heated up to a temperature above 900 K,they emit radiation in the visible spectrum,which can be can be caught up by a CCD camera.Based on COMSOL Multiphysics thermal analysis,the influence caused by tungsten wire's radius and beam power to time and space resolution was researched.It was found that the speed of heat conductivity is much slower than that of wire's emission of light,which proves a good spatial resolution for beamlets in short time scales(~1 s)of tungsten wire.The structure and layout of the multi-tungsten wire diagnostic device were designed,and a CCD image processing program was written based on MATLAB.