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 the edge region within a short time on the order of ms. The fueling efficiency of ~20 % can be achieved by SSGP, which is more than twice higher than that of ordinary gas puffing.  Two kinds of improvement in the fueling efficiency of SSGP have been observed. The fueling efficiency improved suddenly when the target plasma was close to the density limit. In the case of hydrogen SSGP, abrupt increase in the density increase rate was observed with the plasma shrinkage. As a result of this, the fueling efficiency was improved for two times even though the difference in the number of supplied particles was less than 20 %. The fueling efficiency also improved when the edge-density was kept high and a strongly hollow density profile was maintained. The fueling efficiency of helium SSGP is indeed higher than that of hydrogen SSGP because of the high recycling property.  SSGP can be also used for physics experiments. For example, the nonlocal transport phenomenon and electron temperature fluctuations were triggered by SSGP. After a short-pulse SSGP, the core electron temperature increased while the edge electron temperature decreased. SSGP triggered a longer core temperature increase than that triggered by a small impurity pellet injection. The temperature profile, which was relatively flat inside the half minor radius before SSGP, becomes parabolic after non-local transport was triggered. Fluctuations were excited in the electron temperature signals around the half minor radius. The frequency of these fluctuations increased from ~400 Hz to ~1 kHz within ~0.1 s and the amplitude decreased correspondingly. The temperature fluctuations inside and outside of the half minor radius had opposite phases. Magnetic fluctuations resonating near the half minor radius were observed simultaneously with the electron temperature fluctuations.  Fast density modulation experiments using SSGP has been also carried out in the LHD. The fast density modulation of 50 ~ 200 Hz was excited by SSGP. High-frequency injection (500 Hz) was confirmed in preliminary experiments on the test stand. The achieved frequency of 200 Hz is still smaller than that demonstrated in the test stand of 500 Hz. The optimum flow rate for fast density modulation in LHD was 40 – 200 Pa·m3/s. In the case of 50 Hz injection, higher harmonics of up to 200 Hz was also observed simultaneously.SSGP has an additional effect of edge cooling that will be beneficial for divertor heat load reduction. The edge temperature during or just after SSGP becomes more than 50 % lower than that in the case of conventional gas puffing, compared at the similar edge density. This strong edge cooling effect will be beneficial for divertor heat load reduction, which is inevitable in future fusion reactors, since the low edge temperature enhances the radiation loss in the ergodic layer and reduces the conduction loss to the divertor plates.  In conclusion, the fueling efficiency of SSGP is more than twice higher than that of conventional gas puffing. The fueling efficiency improves when the target plasma is close to the density limit during SSGP and the edge-density is kept high and a strongly hollow density profile is maintained. In plasma physics experiments, SSGP caused various interesting phenomena by supplying the short convergent gas flow. SSGP is also available as the fueling method which triggers a variety of plasma responses by supplying a large numbers of particles in a short time. The scientific knowledge obtained in this study will be beneficial for future fusion reactors.

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