Efficiency of massive gas injection for increase of plasma density in TEXTOR experiments on disruption mitigation

A disruption mitigation system is essential for the reliable ITER operation. The most critical issue to be dealt with by such a system is the suppression of runaway (high-energy) electrons. To suppress the runaway production the total electron density must be increased to about 1023 m−3 in a few milliseconds. Massive gas injection (MGI) is considered as a candidate to provide this density. However, experiments in TEXTOR [1, 2] and JET [3] demonstrated that only a small fraction of atoms released by the fast injection system reaches the plasma core before possible runaway generation. In this contribution we analyze gas flow from the reservoir to plasma and show that unsteady flow in the delivery tube can explain the observed M-dependence of the mixing efficiency in TEXTOR. Model of gas flow in a delivery tube. Initially (t < 0) the high pressure reservoir of volume V , i.e. a MGI valve, is sealed with a piston (figure 1). The initial conditions in the reservoir are: density ρ0, sound speed c0, adiabatic exponent γ and velocity u0 = 0. On a trigger the orifice is quickly opened and the gas is delivered into plasma via a vacuum tube of length L. The delivery process is essentially non-stationary, as a consequence the gas pulse is considerably flattened along the tube. For the case of equal diameters of the plenum, orifice and tube, and of an instantaneous opening, a centered rarefaction wave runs into the reservoir from x = 0 until it is reflected at the closed end. The gas-vacuum contact surface runs to the right with the velocity of 2 · c0/(γ −1). The solution of this classical problem is [4]: