K. Tanaka

M. Shoji

National Institute for Fusion Science (NIFS-Repository)

 The particle transport are studied in hydrogen and helium plasma and its characteristics are compared in LHD. Helium glow discharge cleaning was done to keep helium rich wall condition for helium plasma experiments. Figure 1 shows comparison of the time trace. In 118352 hydrogen plasma, neutral beam was injected till t=2.5sec, then, plasma is sustained by 77 and 154GHz ECRH. In 119049 helium plasma, plasma is produced and sustained by 77 and 154GHz ECRH only. ECRH power of both cases are 0.9MW in total. Density was modulated at 1.25Hz for particle transport study. In ECRH sustainment phase, line averaged density was kept almost constant in hydrogen plasma, while it increases in time in helium plasma. The increase of the density in helium plasma is likely due to the high recycling rate of helium. The fuelling ratio, which is H/(H+He) are around 85-90% in hydrogen plasma and 10-20% in helium plasma as shown in Fig. 1(a). Electron temperature decreased at t=1.6sec in hydrogen plasma and 4sec in helium plasma. This is due to the back transition from electron internal transport barrier to L mode. Ion temperature is kept almost constant in both cases through discharge. The comparison of particle transport was studied by density modulation in L mode phase after back transition.  Figure 2 shows comparison of the temporary averaged profiles at 6-6.8 sec, which is one modulation period. Electron temperature in slightly higher in helium plasma at almost same density. Electron density profile is flat in hydrogen plasma and hollow in helium plasma. In NBI heated plasma, hollow density profiles are widely observed in low collisionality regime of hydrogen plasma[1]. But the peak of the density in NBI plasma is at reff/a99 = 0.7-0.8 and is more outwardly located[1]. This suggests that particle transport in ECRH heating plasma is different from those in NBI heating plasma.  Ionization rate was estimated by 3D Monte Carlo simulation code EIRINE [2]. Particle source penetrates deeper toward core in hydrogen plasma. This is due to that hydrogen atoms penetrates deeper by the charge exchange with high temperature hydrogen ions. Diffusion coefficient (D) and convection velocity (V) are estimated by density modulation experiments [1] taking into account of difference of ionization rate. D and V are determined to fit both equilibrium profile and modulation profile [1]. Estimation was done at t=3.6-9sec in hydrogen plasma and t=4.5-10sec in helium plasma. As shown in Fig.3 (a), core diffusion coefficient are higher than edge values in both cases. Due to the uncertainty of fitting, the difference of core diffusion (at reff/a99 < 0.7) are within fitting error. While in edge region (at reff/a99 >0.7), diffusion coefficients in helium plasma is higher beyond the fitting error. As shown in Fig.3(b), convection velocity is outwardly at reff/a99 < 0.6 in helium plasma but close to zero at reff/a99 < 0.4 in hydrogen plasma. In edge region (reff/a99 > 0/7), higher inwardly directed convection velocity is observed. However, total particle flux is dominated by diffusive flux in this region. Thus, edge particle transport is larger due to larger edge diffusion coefficients in helium plasma. These results will be useful to consider mass effect on particle transport in LHD. Fig.1 Time trace of hydrogen and helium plasma (a) line averaged density, fueling ration (b) Te and TiFig2 Comparison of profiles (a) Te  (b) ne and ionization rate. Te profiles are from Thomson scattering and ne profiles are from FIR interferometer Fig.3 Comparison of (a) diffusion coefficients and (b) convection velocities. Thin lines are upper and lower bounds of fitting error 1) Tanaka. K. et al.: Fusion Sci, Tech. 58, 70 (2010) 2) Shoji, M, et al., this annual report   The radial profile of neutral hydrogen/helium in pure hydrogen/helium plasmas are investigated using a neutral particle transport simulation code (EIRENE) with a fully three-dimensional grid model of a LHD geometry for Rax=3.60m. In the simulation code, large amount of test particles representing neutral hydrogen atoms/molecules and helium atoms are released from the positions of the strike points on the divertor plates with an energy corresponding to the room temperature. By summing up the weight of the test particles passing each grids, the three-dimensional profiles of neutral densities are calculated. For neutral hydrogen transport simulation in pure hydrogen plasma, the following eight atomic and molecular processes are included in the simulation: e + H → e + H+ + e   (p1), H+ + H → H + H+    (p2), e + H2 → e + H2+ + e   (p3), e + H2 → e + H + H   (p4), e + H2 → e + H+ + H + e   (p5), e + H2+ → e + H+ + H   (p6), e + H2+ → e + H+ + H+ + e   (p7), e + H2+ → H + H    (p8). For neutral helium transport simulation in pure helium plasma, the following two situations are assumed. First one is purely singly ionized helium (He+) plasma case, and the second one is purely doubly ionized helium (He++) plasma case. In the first case, the following three atomic and molecular processes are considered in the simulation: e + He → e + He+ + e   (p9), He+ + He → He+ + He+ + e   (p10), He+ + He → He + He+   (p11). In the second case, included atomic and molecular processes in the simulation are the following three ones: e + He → e + He+ + e   (p12), He++ + He → He++ + He+ + e  (p13), He++ + He → He + He++   (p14). Figure 1 is the observed radial profiles of the electron temperature Te and the plasma density ne which are used for the neutral hydrogen/helium transport simulations. Figure 2 indicates the calculated radial profile of neutral hydrogen atoms and molecules. Hydrogen atoms can penetrate deeply into the plasma due to the effect of the charge exchange (p2).  Figure 3 is the simulations in helium plasmas for the first and the second cases, showing that the penetration of the neutral helium in the peripheral plasma is weaker than that of the hydrogen atoms. The reason for this is that while the average penetration speed of the helium atoms corresponds to that of the room temperature (~0.026eV), the penetration speed of hydrogen atoms is based on the Franck-Condon energy (~3eV) produced by molecular dissociation (p3~p8). In addition, the charge exchange also contributes to the penetration of neutral particles. This is because low energy neutrals are changed to high energy neutrals. The cross-section of this process in hydrogen plasmas (p2) is larger than that in helium plasmas (p11, p14) by more than a factor of two in the range of ion temperatures in LHD. In Figure 3, the density of the neutral helium in the plasma center is slightly reduced in the second (He++ plasma) case compared to that in the first case (He+ plasma). The reason for this is that the cross-section of the charge exchange process in the second case (p14) is less than that s in the first case (p11) by more than a factor of two. Fig. 1. The observed radial profiles of the electron temperature and the plasma density in pure hydrogen (left) and helium (right) plasmas for the simulation. Fig. 2. Simulations of the radial profiles of the neutral hydrogen atoms and molecules in the pure hydrogen plasma case. Fig. 3. Simulations of the radial profiles of the neutral helium atoms in the first (pure He+) and the second (pure He++) plasma cases. 97§6. Simulations of the Radial Profiles of Neutral Particle Density in Hydrogen and Helium Plasmas in the LHDShoji, M., Tanaka, K.

Simulations of the Radial Profiles of Neutral Particle Density in Hydrogen and Helium Plasmas in the LHD

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Simulations of the Radial Profiles of Neutral Particle Density in Hydrogen and Helium Plasmas in the LHD

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