H. Fujita

K.N. Sato

S. Yoshimura

T. Misawa

Y. Ohtsu

National Institute for Fusion Science (NIFS-Repository)

§ 22. Basic Process of Sol id HydrogenAblation by PlasmaFujita, H., Ohtsu, Y., Misawa, T. (Saga Dniv.),Sato, K.N. (Kyushu Dniv.),Yoshimura, S.From the viewpoint of perfonnance of nuclear fusionplasmas, pellet injection experiments have been activelycarried out in many toroidal devices in the sense of the controlof density profile, obtaining high density or improvedconfinement, and diagnostic purposes. However, there exists,so far, only an empirical scaling and the essential part of solidhydrogen ablation by plasmas, such as the interaction betweenpellet and plasma, have not been clarified until now. Forinstance, obsetvation of so-called ''Tail Mode" phenomena,which may be the result of charge exchange equilibrium stateand the plasma rotation by the potential, can be affected by thedensity profile of the edge plasma [1]. Thus, the study ofinteraction between plasma and solid is one of the mostinteresting issues to be investigated in the sense offundamentalplasma science.In this research, an accumulation of data on theinteraction between plasma and solid hydrogen is planned bymeasuring the fundamental process of pellet injection into aninductively coupled plasma (ICP) and/or an electron cyclotronresonance (ECR) plasma. Both ICP and ECR plasmas arepossible to get high-density of 1012 cm-3 and unifonn densityprofile at low pressure of a few mTorr. These plasmas areconsidered to be utilized as target plasmas to simulate edgeplasmas.In this year, the ICP discharge is realized at a lowpressure (a few mTorr) and a radio frequency (RF: 13.56MHz)power using a helical antenna. We report the experimentalresults on the fundamental characteristics of ICP as simulatedtarget plasma for solid hydrogen pellet injection.Figure 1 shows the ICP experimental apparatus. Theeffect of magnetic configurations on plasma characteristics hasbeen examined in the ICP. Figure 2 shows axial profile ofelectron density at Ar gas pressure of 15 mTorr and RF powerof 150W at various magnetic field configurations shown in Fig.1. Here, Z=O denotes the distance from the end plate as shownin Fig.l. It is seen that electron densities at all type magneticfield configurations increase gradually with away from the endplate. Electron density at Type 3 is the lowest in three type308magnetic field configurations because a magnetic confinementof electrons is weak owing to the low magnetic flux density.On the other hand, electron density at Type 1 is about twotimes higher than that at Type 2 whereas the magnetic fluxdensities of both Type 1 and Type 2 are almost the samevalues.In the future work, we will include the effect ofmagnetic field on the pellet injection through the comparisonof these results. Thus, the detailed data will be accumulated bypractical experiments. Furthennore, the pellet injections intothese two plasma devices are expected to be effective becausethese plasmas are unifonn in density profile and those densitiesare possible to be controlled with wide range. As the next step,the pellet injection into "HYPER-I" device (lle=1011_1013 cm-3,diameter L=30cm) will be prepared.References[1] K.H.Finken and K.N.Sato: American Inst. OfPhys. No.345,p172 (1995).Magnetic coil181 181 181 181 181 181,r Type 1m \. 4&m .1 ...oFig.1 Experimental apparatus.COAt 15mTorrRF 150W10 15 20 25Z(cm)Fig.2 Axial profile ofelectron density at various magneticfield configurations.

Basic Process of Solid Hydrogen Ablation by Plasma

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