ATLAS Silicon Microstrip Tracker Operation and Performance

In December 2009 the ATLAS experiment at the CERN Large Hadron Collider (LHC) recorded the first proton-proton collisions at a centre-of-mass energy of 900 GeV. The SemiConductor Tracker (SCT) is the key precision tracking device in ATLAS, made up from silicon micro-strip detectors processed in the planar p-in-n technology. The completed SCT has been installed inside the ATLAS experimental hall. After the commissioning phase it arrived to the first LHC pp collision runs in very good shape: 99.3% of the SCT strips are operational, noise occupancy and hit efficiency exceed the design specifications, the alignment is already close enough to the ideal one to allow on-line track reconstruction and invariant mass determination. In the talk the current status of the SCT will be reviewed, including results from the latest data- taking periods at autumn 2009 and spring 2010, and from the detector alignment. We will report on the operation of the detector and observed problems. The main emphasis will be given to the performance of the SCT with the LHC in collison mode in a comparison with the expected parameters and with the Monte Carlo simulations