P.B. Parks

R. Ishizaki

National Institute for Fusion Science (NIFS-Repository)

§18. Fluid Simulation on Pellet Ablation with Atomic Process Ishizaki, R., Parks, P.B. (General Atomics) Refueling is one of essential methods in order to control plasma density and sustain steady state plasmas. A gas puffing has been successful for building and sustaining a plasma density in an experimental system of past gen-eration. However, in a large scale experimental system, e.g., LHD, the plasma sources induced by the gas puffing are strongly localized near the plasma surface. Then, a pellet injection is placed as a fundamental tool and has been mainly used to obtain a high density plasma and control a density profile [1]. When a pellet consisting of solid hydrogen is heated by an energy flux, it changes to vapor and subsequently to plasma through atomic process. In the present work, the ablation cloud consists of molecules, atoms, ions and electrons. Effects of atomic physics on the ablation pro-cess arc considered. It is assumed that the species have same velocity and same temperature due to the high col-lision and charge exchange. An electromagnetic force is ignored for simplicity. In addition, the domain of the simulation includes solid and gas regions by using the Cubic-Interpolated Psudoparticle (CIP) method that can calculate simultaneously incompressible and compress-ible regions [2]. The heat flux is carried by the inci-dent electrons which the distribution of energy is taken as l\1axwellian. The code is time dependent, starting from an initial condition of a bare pellet in a bulk plasma of specified electron temperature (Teoo) and number density (ne 00 ). Figure 1 shows special profiles obtained by running up to 8J.Lsec. in 1D spherical symmetry where p 8 , T8 and r 8 are the pressure, temperature and radius at the sonic point, 6.0 (/) 5.0 C1) :c ('(I 'i: 4.0 ('(I > "C 3.0 C1) .!::! -m 2.0 E ... 0 1 .0 z 0.0 0.0 1. 0 cip_ 18iiO_gnu3_x1 / , /TIT ~ s / I / 2.0 3.0 4.0 Normalized Radius 5.0 6.0 Fig. 1: Normalized fluid profiles at 8 J-tSec. A dashed, dot-dashed and solid lines show M, pjp8 , T/Ts as functions of r /rs, respectively. Dot lines show /d and /i· 194 600.0 z (x 0.1 mm) 100.0 200.0 300.0 400.0 > 400.0 ~ 100.0 3 ci ~ ~ E-< 200.0 200.0 0 ~ 1-< 300.0 Fig. 2: Temperature contour on r-z plane at 8 J1 sec. respectively. The parameters used are the pellet radius rp = 2 mm, electron temperature Teoo = 2 keV and number density neoo = 1020 m - 3 in the bulk plasma. M has a jump structure around r frp -:::::::: 2.4 where M on the pellet side is greater than unity and .A1 on the other side out is less than unity. Since the flow goes from the pellet surface to the bulk plasma, an incident flow becomes supersonic and a transmitted flow becomes subsonic in a reference frame moving with this structure. In addition, ionization is dominant in the region as shown by fi. Then, this structure is clear to be a shock wave driven by ionization. Next we evaluate effects of the heat flux propagating into the cloud along the magnetic field ( z). The geometry of the code is cylindrical with symmetry assumed about the magnetic field line that passes through the center of the pellet. Figure 2 shows a temperature contour at 8 J.LSec. where a black region corresponds to the pellet. The parameters used are r P = 5 mm, Teoo = 4 ke V and neoo = 1020 m - 3 . It is found that the pellet radius in the r direction becomes longer than one in the z direc-tion . This comes from nonuniform pressure on the pellet surface. Almost energy of the heat flux through the pole of the pellet is deposited around the pellet surface. On the contrary, a part of the heat flux energy through the equator is deposited around the pellet and another goes through the pellet. Then, since the pressure at the pole is greater than one at the equator of the pellet, the pel-let makes deformation. Effects of atomic process with a shock wave will be evaluated in the pellet deformation. REFERENCES [1] L. R. Baylor. et al., Nucl. Fusion 32, 2177 (1992). [2] T. Yabe and P. Y. Wang, J. Phys. Soc. Jpn 60, 2105 (1991). 

Fluid Simulation on Pellet Ablation with Atomic Process

Abstract

Similar works

Full text

Available Versions

National Institute for Fusion Science (NIFS-Repository)