Measurement of HO HPD noise rates during October 2008 re-commissioning of the CMS magnet at 3.8 Tesla and experience of operation of HO HPDs during CRAFT

A major problem for HCAL subsystem was discovered during second part of Magnet Test Cosmic Challenge (MTCC) in summer/fall of 2006 MTCC. HPD discharges were observed in HB and HE HPDs (axial B field, with CMS magnet set at 2T) and HO (misaligned E and B fields, CMS magnet set at 3.8T). In this note we concentrate on behavior of HPDs in HO RBXes. Separate note describing behavior of HB and HE RBXes in magnetic field is under preparation. Note that when CMS magnet is set at 3.8 Tesla, HO HPDs on the central wheel (YB0) operate in the fringe field of 0.02T (equivalent to 200 Gauss) while HO HPDs on YB+2/YB+1/YB-1/Y-2 wheels experience magnetic field above 0.2 Tesla (equaivalent to 2000 Gauss).