Development of a multichannel dispersion interferometer for measurements of the plasma density distribution after massive gas injection and during the runaway electron phase in TEXTOR

The tokamak performance is limited both by technological and physical reasons. The crossing of operational limits often leads to a global loss of plasma confinement called disruption. Strong heat and electromagnetic loads are produced during the disruption. The applicability of massive injection of impurities for mitigation of thermal and electromagnetic stresses has been shown at many machines. The third issue of disruption is the generation of runaway electrons. A runaway electron beam with energy about 10 MeV and carried current up to 70% of initial plasma current could cause a serious damage in case of localized impact to the wall. It is theoretically predicted that significant increase of electron density could suppress the generation of runaway electron via collision mechanism. However, the consistent proof of this statement has never been done. The use of massive noble gas injection for the suppression of runaway electron generation is investigated in the present work. A reliable measurement of the electron density during disruption characterized by fast and significant change of plasma parameters is crucially important for that purpose. To solve this issue a multi-channel dispersion-interferometer has been developed on TEXTOR. This is a unique diagnostics allowing reliable measurement of line plasma density during disruption and post-disruption stage with high time resolution. The generation of runaway electrons has been investigated in deliberately induced disruption. The successful suppression of runaway electron generation has been found in the case of sufficient gas amount injection with proper timing not later than beginning of current quench. The critical density threshold has been found to be up to one order of magnitude below the theoretically predicted. The possible reasons are discussed in detail. The minimum of critical density threshold dependence on Z impurity number has been found. The different amount of electrons delivered into the plasma during disruption by helium, neon and argon injection has been found. The determining factors of impurity penetration into the plasma are discussed. The influence of gas species and injected gas amount on the onset and duration of thermal and current quenches has been investigated.