A baffle plate set in the closed-divertor reminds us of the concept of gas conductance. If substantial dense SOL plasma passes through the slot or the opening of the baffle, the impurity flow toward the main plasma can collide with the SOL plasma flow at the slot or opening of the baffle, which may induce the local enhancement of the friction, and which leads to the reduction in the gas conductance. This improves impurity screening called plasma-plugging.[1] The plasma-plugging has been clearly found in the linear machines with baffle.[1,2] The value of conductance for the impurity gas injected into the simulated divertor region was found to be reduced by a factor of 8 by compared with the value in the case of the vacuum.
Here, the effective formula that fairly expresses the relationship between conductance and the plasma flow will be presented based on the experimental results obtained by using the linear machine TPD-II (Test Plasma produced by a Dc discharge) with baffle.[2] The conductance was evaluated by measuring the neutral pressure difference between the inlet (divertor region) and outlet (edge plasma region), ΔP, as a function of flow rate of the helium gas injection into the divertor region, Q_D, for various flux intensities of the steady-state plasma flow (typically, n_e=6×10¹⁸m^-3, T_e=6eV for discharge current I_d of 100A with helium gas and B~0.2 T).
As the value of Q_D is increased, ΔP increases and follows an offset-linear scaling: ΔP=Q_D/C+ΔP₀, where the 1/C and ΔP₀ are the slope (effective conductance) and intercept, respectively. It is remarkable that C decreases with increasing n_i of the plasma flow, which can be represented by C=C₀/(1+ζ n_i C₀), where C₀ is the conductance for vacuum, and ζ is a constant. The experimentally obtained ζ is comparable within an order of magnitude to the value calculated from the force balance between the friction and the neutral pressure gradient in one-dimensional fluid equations for the neutral gas.

References