Plasma disruptions state a serious threat for next step fusion devices, therefore, strategies for disruption avoidance or at least the mitigation of their consequences are mandatory. One method of disruption mitigation is the sudden injection of large quantities of noble gases. This technique of massive gas injection (MGI) requires gas valves that release a large gas quantity within short time. On ASDEX Upgrade, several types of fast valves for MGI have been installed in the recent years [1]

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... . First, two electromagnetically operated valves were mounted on a vessel port at a distance of 1.5 meters from the low-field side separatrix of the plasma. Later, a fast piezo valve was installed inside the vacuum vessel which is located at the low-field side and close to the plasma. This valve was found to provide higher fueling efficiency thanks to its location close to the plasma. During the 2010 vessel opening, an additional piezo valve was installed on the high-field side of ASDEX Upgrade, whose fueling efficiency turned out to be roughly twice as high as that of the low-field side valve [2] . At the end of 2011, a second piezo valve was mounted on the high-field side. With increasing fueling efficiency of the valves and increasing gas quantity released by them, the measurement of the electron density of the plasma during MGI becomes more and more challenging. The two-color CO 2 /HeNe interferometer on ASDEX Upgrade, which operates at wavelengths of 10.6 µm and 633 nm, can be used to measure the density characteristics during MGI [3] . The standard readout electronics of this interferometer, however, has a limited dynamic range which ends at a line-integrated density of 5.26 · 10 20 m −2 , and an internal 40 kHz low-pass filter, which limits the time resolution. The density limit is a factor of 5 higher than the typical flat-top density of ASDEX Upgrade discharges. However, the peak densities obtained during MGI meanwhile exceed this limit by far. Therefore, an alternative approach to the acquisition of interferometry data had to be made, which is presented in the following. When a laser beam passes through a plasma, it experiences a phase shift ϕ which is proportional to the line-integrated electron density along the path of the beam, and proportional to the wavelength λ . Any mechanical displacement ∆L of mirrors or other optical components in an interferometer, e.g. due to vibration, causes a phase shift as well which is proportional to 1/λ . When operating at short wavelengths, vibration compensation is required. In two-color interferometers, this is achieved by sending two laser beams of different wavelength through the plasma along the same path. Accordingly, two phase shifts are measured and the two quanities (plasma density and mechanical displacement ∆L) can be calculated. On ASDEX Upgrade, the measurement of the phase shift is based on the heterodyne method: The beam of each of the two lasers (an infrared CO 2 and a visible HeNe laser) is sent to an acousto-optic modulator