A number of liquid argon time projection chambers (LAr TPC's) are being buildor are proposed for neutrino experiments on long- and short baseline beams. Forthese detectors a distortion in the drift field due to geometrical or physicsreasons can affect the reconstruction of the events. Depending on the TPCgeometry and electric drift field intensity this distortion could be of thesame magnitude as the drift field itself. Recently, we presented a method tocalibrate the drift field and correct for these possible distortions. Whilestraight cosmic ray muon tracks could be used for calibration, multiple coulombscattering and momentum uncertainties allow only a limited resolution. A UVlaser instead can create straight ionization tracks in liquid argon, and allowsone to map the drift field along different paths in the TPC inner volume. Herewe present a UV laser feed-through design with a steerable UV mirror immersedin liquid argon that can point the laser beam at many locations through theTPC. The straight ionization paths are sensitive to drift field distortions, afit of these distortion to the linear optical path allows to extract the driftfield, by using these laser tracks along the whole TPC volume one can obtain a3D drift field map. The UV laser feed-through assembly is a prototype of thesystem that will be used for the MicroBooNE experiment at the Fermi NationalAccelerator Laboratory (FNAL).
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