Fast valves for massive gas injection in tokamaks

Disruptions are a major issue for todays Tokamak fusion devices. Plasma instabilities and technical errors can lead to large magnetohydrodynamic modes, resulting in a sudden loss of the plasma thermal energy. This causes large heat loads on the plasma facing components. Additionally, the decaying plasma current produces a changing magnetic flux which induces eddy currents in the plasma vessel. Vertically moving plasmas can generate a large halo current. These eddy and halo currents can give rise to tremendous Lorenz forces when crossed with the stationary magnetic field of the tokamak. Finally, the cooling plasma can generate strong electric fields which accelerate electrons up to relativistic energies. A concentrated beam of these electrons can erode plasma facing components if decon fined. All these disruptive effects, large heat loads, strong forces and highly energetic electrons can cause severe damage to the machine and have to be mitigated. Massive gas injection (MGI) has proven to be an effective tool to mitigate disruptive effects. A strong pulse of noble gas is released onto the plasma, radiating the power and thereby controlling heat loads, forces and highly energetic electrons. High speed gas valves are used to deliver this gas pulse. This thesis describes the development of two types of high speed gas valves and their implementation into the ASDEX Upgrade vacuum vessel. Based on the experimental findings of previous disruption mitigation experiments and on the technical state of the art, a spring-driven and a piezo-driven in-vessel valve have been developed. These valves were designed to withstand the conditions inside a fusion device especially concerning high temperatures, ultra-high vacuum, strong magnetic fields and ionizing radiation. The spring-driven valve holds a maximal gas inventory of 640 Pa m³, has an opening time of 2.7 ms and a peak mass flow rate of 1.1 (kg)/(s). The piezo-driven valve has a maximal gas inventory of 210 Pa m³, an opening time of 3.5 ms and a peak mass flow rate of 0.7 (kg)/(s). The valves were developed in 2015, two spring-driven valves and one piezo-driven valve were build and tested in 2016, installed into the vacuum vessel of ASDEX Upgrade in October 2016 and went into operation in March 2017. The first technical test of the valves was performed using the in-vessel manometers and proved full operability of the three new valves. A measurement of the delay times of the valves revealed delays of 0.63 ms and 0.82 ms for the spring-driven valves and 9.2 ms for the piezo-driven valve. Radiation asymmetry during MGI can be significantly reduced when two toroidally separated valves inject gas with a time delay smaller than 0.1 ms from one another.