Disruption mitigation studies at TEXTOR

The operation of tokamaks is prone to fast accidental losses of the stored energy. These losses referred to as disruptions can cause a severe damage to the machine. The problem becomes especially tough in the future experiment ITER. A massive gas puff is helpful in protecting the machine during a disruption. An impurity decreases the heat flux density by reradiating the energy over a larger surface area. The high particle density provides the collision frequency high enough to hinder acceleration of runaway electrons. The resulting high plasma resistivity reduces the halo currents and consequently forces on the machine. The work reports on results of the disruption mitigation experiments conducted at TEXTOR. The injections were performed at a predefined time moment into a stable plasma. The addressed gas mixtures are: He, Ar, D2+ Ar (5%, 10%, 20%). A mixture of Ar and D2 constitutes a compromise between cooling capability and particle influx rate. The mixtures were also found to better suppress RE production. To give an h insight into the dynamics of the process the ultra-fast framing camera was used. The neutral gas jet is not observed to penetrate to the center of the discharge. The discussion of typical timescales of the process is given. The scan of the gas pressure allows the qualitative tendencies in the penetration depth to be revealed. A detailed comparative analysis of the mitigating efficiency of different gases is also presented