ALICE spectrometer polarity reversal

The external crossing-angle bump in IR2 of LHC partly compensates the crossing angle created by the ALICE spectrometer. In view of the spectrometer polarity reversal requested by ALICE during the 2011 Pb-Pb run, a fast procedure consisting in reversing IP2 external crossing angle after ramp and squeeze of beams was envisaged. This requires a passage through zero external crossing angle, exposing beams to beam-beam interactions at parasitic encounters where normalized separation is less than one sigma. A test of the reversal at the end of physics fill no. 2319 showed that beam sizes are large enough at these locations not to have significant effects on intensity or emittance. Consequently, the fast polarity reversal procedure was validated and the full operational cycle was tested with a few bunches before adoption in the future physics fills.