Study on fueling characteristics of supersonic gas puffing applied to large high-temperature plasmas

  Fueling characteristics of supersonic gas puffing (SSGP) applied to large high-temperature plasmas have been investigated in the Large Helical Device (LHD). A fueling efficiency of ~20 % can be achieved in low-density plasmas and it decreases as the target plasma density increases. This is due to the fueling mechanism of SSGP, where the fuel particles are supplied to the plasma edge region and then transported to the core region by diffusion. The fueling efficiency improves depending on the recycling condition and/or the edge density condition of the target plasma. Various phenomena are induced by SSGP, such as the reheat of the plasma stored energy, the strong edge cooling and the nonlocal transport. The electron temperature fluctuations related to the MHD instability are induced after the nonlocal phenomenon. SSGP is also capable of inducing the fast density modulation externally by using fast pulse train. The convergence of gas flow plays an important role in these experiments. An effectiveness of a Laval nozzle in generating the convergent gas flow has been tested by visualizing the gas flow.   Establishment of fueling methods in future thermonuclear fusion reactors is one of the critical issues. In a fusion reactor, the role of fueling device is to supply fuel particles and, consequently, to control the plasma density profiles. Two major fueling methods have been used in the plasma experiments. One is the gas puffing that is a conventional method and has been used since the early period of the fusion plasma study. The conventional gas puffing has a drawback of low fueling efficiency. The other is the ice-pellet injection that can effectively increase the density in the plasma core region. However, the pellet injection device is complicated compared with the gas puffing device. SSGP has been developed as a new fueling method that can combine both advantages of the pellet injection and the conventional gas puffing, i.e., simpleness of the device, high fueling efficiency, and rapid response. In SSGP, high-pressure gas is ejected through a fast solenoid valve equipped with the Laval nozzle. SSGP supplies pulsed convergent gas flow to the plasma.   Before applying SSGP to LHD, the effectiveness of the Laval nozzle has been tested by visualizing the gas flow. Three methods have been applied for visualization, i.e., the shadow graph imaging, the emission imaging using electron beam, and the laser scattering after forming the cluster beam. The cluster beam is formed by selecting the gas species, or by cooling the gas using a refrigerator.   As the first step, the cluster beam ejected through the fast solenoid valve without using the Laval nozzle has been investigated by selecting the gas species capable of forming the cluster at a room temperature in a test chamber. Time-resolved 2-D images of Rayleigh scattering from clusters have been measured by a fast charge coupled device camera. The expansion half angle of the gas flow without the Laval nozzle was 22.5º. The scattering signal was proportional to the averaged cluster size and the number density of clusters. The scattering signals from argon and nitrogen clusters showed approximately cubic dependence on the backing pressure as expected from a model. Meanwhile, stronger pressure dependence than this was found in the case of methane, where the scattering signal increased with the fifth power of the backing pressure at 3.2 MPa – 7 MPa, and it was further enhanced at > 7 MPa. This suggests that a new structure model would be necessary to determine the cluster size of methane, which shows stronger backing pressure dependence than argon and nitrogen.    Next, formation of the hydrogen cluster beam using the Laval nozzle has been investigated at a low-temperature regime ranging from 120 K to 300 K. The Rayleigh scattering signal from hydrogen clusters was detected when the temperature was lower than 178 K, as expected from a calculation result of the cluster formation condition. The scattering signal intensity was inversely proportional to the fifth power of the gas temperature and the cube of the backing pressure as expected from an available cluster model. The divergence of cluster beam has been decreased from 22.5 º to ~5 º after installation of the Laval nozzle.   Based on the test results of the Laval nozzle, fueling characteristics have been investigated in LHD. Since there is no disruption in LHD, the edge density can be significantly increased by supplying particles with a large flow rate. The plasma minor radius of ~0.6 m is much longer than the penetration depth of neutrals supplied by SSGP, of which the typical order is mm in LHD. The fueling efficiency of SSGP depends on the target plasma density and decreases as the density increases. This is due to the fueling mechanism of SSGP, where the fuel particles are supplied to the plasma edge region and then transported to the core region by diffusion. SSGP locally supplies a large number of particles to ...

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