Enhanced Material Assimilation in a Toroidal Plasma Using Mixed ${\mathrm{H}}_2+\mathrm{Ne}$ Pellet Injection and Implications to ITER

The ablation and assimilation of cryogenic pure ${\mathrm{H}}_2$ and mixed ${\mathrm{H}}_2+\mathrm{Ne}$ pellets, which are foreseen to be used by the ITER tokamak for mitigating thermal and electromagnetic loads of major disruptions, are observed by spatially and temporally resolved measurements. It is experimentally demonstrated that a small fraction (here $\approx 5\%$) of neon added to hydrogenic pellets enhances the core density assimilation with reduced outward transport for the low magnetic-field side injection. This is consistent with theoretical expectations that line radiation increased by doped neon in dense plasmoids suppresses the plasmoid pressure and reduces the $\overrightarrow{E}\times \overrightarrow{B}$ transport of the ablated material.

Figure 1

(a) Schematic of the pellet injector in the Large Helical Device (LHD); (b) top view showing the layout of heating and diagnostic systems.

Figure 2

Single and double pellet injection into ECH discharges. The time evolution of the stored energy $W_p$ and the line-averaged $n_e$ are plotted as cyan and black curves. The red and blue curves are the ablation light emission measured by a photodiode for pure ${\mathrm{H}}_2$ and mixed ${\mathrm{H}}_2+\mathrm{Ne}$ (${\mathrm{H}}_2:\mathrm{Ne}=20:1$) pellets, respectively.

Figure 3

(a) and (d) Mapping of ablation light along the major radius $R$. For the sequential injection, the ablation light from the first pellet is shown by the dashed curves. (b) and (e) $n_e$ increment profiles observed after the pellet injection. (c) and (f) Pre- and postinjection $T_e$ profiles.

Figure 4

Radial profiles of the preinjection $T_e$ observed by the main TS (black, left axis) and transient $n_e$ (colors, right axis) observed by fast Thomson scattering (FTS) during the sequential injection of (a) two pure ${\mathrm{H}}_2$ pellets, and (b) a pure ${\mathrm{H}}_2$ pellet followed and a mixed ${\mathrm{H}}_2+\mathrm{Ne}$ pellet. The ablation light emission zones that indicate the radial pellet penetration (obtained from the pellet time of flight assuming $\pm 1\%$ errors in the measured velocities) are overlaid in the same plot as gray- and blue-shaded areas for the first and second pellet, respectively. The relative time after the onset of ablation light emissions from the first or second pellets ($t_{\text{arriv}}$) is 0.3–0.4 ms as specified in the legends.

Figure 5

(a) Radial depths ${\rho }_{\mathrm{dep}}^{10\%}$ (defined in the text) of the observed electron density rise versus the measured pellet burnout position ${\rho }_{\text{burnout}}^{\exp }$. (b) Increment of the ion saturation current $\mathrm{\Delta }{I}_{\mathrm{sat}}$ after the pellet injection.

Figure 6

Comparison of the $\mathrm{\Delta }{n}_e$ profiles from the main TS measurements and model calculations. The electron density from the ablation calculation [36] is computed assuming that the ablated material is deposited uniformly over the magnetic surfaces at the ablation position, and the contribution of neon to $\mathrm{\Delta }{n}_e$ (dashed curve) is obtained from a collisional-radiative equilibrium. For comparison, the ablation light emission (the gray-shaded profiles) observed at the upstream toroidal position is mapped to the normalized minor radius $\rho$.