The gas flow of a disruption mitigation valve (DMV) has been analysed by using a Michelson interferometer. The gas flow follows the JET design where the gas is injected via a 4 m long and 4 cm inner diameter tube. The gas densities of helium, neon and argon are measured at three locations along the tube with high time resolution. At the valve exit, the gas pulse has a rise time of less than 1 ms; the gas pulse rise time is smeared out and delayed on its passage through the tube. For the favoured Ar gas, the rise time and the delay time become about 11 ms each while for He the delay amounts to 4 ms and the rise time 2 ms. The gas velocity depends on the gas species but only weakly on the filling pressure. The output density amplitude increases nearly linearly with the filling pressure of the DMVs.
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