The drift velocity of electrons in the ALICE TPC is for the selected gas mixture (NeCO2, 90/10) and at the nominal drift field (400 V/cm) very sensitive on the gas composition. Therefore the relative change of the drift velocity with CO2 changes was investigated to get an idea about the necessary stability of the gas mixture. For the ALICE Experiment a stability of 0.1 % for the drift velocity is compulsory. Due to the use of a recirculating gas system the effect of added N2 is also of intrest and was measured. wire chamber the drift velocity by about 0.77 % and agrees well with the calculation. Aiming for a drift velocity stability of 0.1 % the stability of the gas mixture has to be better than 0.013% field cage optics + apertures laser Figure 2 Drift velocity change relative to a CO2 change around 10 % CO2 as a function of the electric field. The triangular markers show a GARFIELD calculation. Figure 1 Setup with laser and Test TPC to measure the electron drift velocity. We have set up a test bench, which allows to determine the dependence of the electron drift velocity on various parameters with a precision better than 0.1%. The setup is depicted in Figure 1: A 266 nm beam of a NdYAG-laser is, after being adjusted in height by a set of mirrors, widened by a telescope to a size of about 10 mm. Thereafter the beam is split and sent through a set of precision apertures of typically 0.3-1.0 mm diameter, which define the position of the two beams inside the drift volume with an accuracy of about 10 µm. The arrival time of the electrons from the two beams is measured in the wire chamber and from the difference the drift velocity is inferred. Three temperature probes (PT1000) are installed inside the gas volume and allow to record the temperature of the gas with a precision of ∆T = 0.03 K. In addition, the pressure, the O2- and H2O-content as well as the CO2 admixture are monitored Figure 2 depicts a measurement of the drift velocity v−vref change relative to CO2 changes ∆
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