Characterization of Rail Gun Plasmas for Simulating Edge Localized Mode Plasma

Rail gun;ELM plasma;Plasma jet;Quadruple Langmuir probe;PIC simulation;ELM control;622

공과대학 에너지시스템공학부 ;

University:서울대학교 대학원

关键词: Rail gun;    ELM plasma;    Plasma jet;    Quadruple Langmuir probe;    PIC simulation;    ELM control;    622;   

Others  :  http://s-space.snu.ac.kr/bitstream/10371/123452/1/000000010229.pdf

美国|英语

来源: Seoul National University Open Repository

【 摘 要 】

Damages of divertor targets by plasma loads during edge localized modes (ELMs) are critical issue in fusion engineering because they reduce the lifetime of the divertor target. In the case of ITER, the plasma load on divertor target during ELM is up to tens of GW/m2 and its duration is approximately a few hundred microseconds. To evaluate the damages of divertor target during ELMs experimentally, various irradiation systems using a pulsed laser or particle beams have been proposed and tested. Among them, a pulsed plasma gun is thought to be suitable to simulate the ELM situation because it can give the particle load as well as the thermal heat load on the divertor target. Therefore, in this thesis, a small-sized rail gun is adopted to simulate the heat loads during ELMs and explore how to relieve the heat loads.The rail gun is designed and fabricated in configuration of two parallel electrodes: the separation between the electrodes is 1 cm wide and the length of the rail is 30 cm long. The rail gun is installed at the top of cylindrical chamber so that the plasma jet is ejected vertically downward from the muzzle of the rail gun. The electrical current is fed to the electrodes by fast discharge of electrical energy stored in low-inductance capacitor. Initiation of discharge is accomplished by fast injection of gas at the breech of the rail using a piezoelectric valve. Argon and hydrogen are used as working gases.Diagnostics of the plasma jet ejected from the muzzle of the rail gun is carried out with a quadruple Langmuir probe and a fast camera. The fast camera is also used to diagnose the formation and motion of the plasma channel between the electrodes, revealing almost linear increase in velocity with time. Dynamic motion of the plasma channel inside the rail is confirmed by a simple equivalent circuit model for parallel-plate rail gun, showing good agreements between the numerical calculation and the experimental observation. The quadruple Langmuir probe is used to measure the spatio-temporal changes for the properties of the plasma jet during the propagation in open space by moving the probe position vertically. With appropriate data processing, it provides the information on the time-dependent plasma properties, i.e. electron density, electron temperature and ion drift velocity, which are critical in studying the ELM-like plasmas. From a lot of experiments conducted using gases with large difference in mass, i.e. hydrogen and argon, it is suggested that the plasma gun developed in this study can be separately applicable for the different topics on researches of ELM plasma: ELM control experiments for argon and heat load tests for hydrogen. The argon plasma jet with ion velocity of 10 km/s is suitable for investigation of the control of ELM-like plasma because the ion drift velocity is similar to the ELM filament velocity approaching to the divertor target. While, the ion drift velocity for the hydrogen plasma jet is much higher up to 120 km/s, so that it is possible to use for simulating the heat load impacting on the divertor surface. The ion drift velocity, or equivalently ion kinetic energy, is easily increased by increasing the length of the rail as well as the discharge current. Therefore, the plasma gun developed and characterized in the present study is expected to be well utilized for the versatile researches on the future experiments on the ELM control and mitigation.