Stability in the response of HFPMT as a function of magnetic field at the HF position, when CMS solenoid field is ramped up to 4 T

In the fall of 2008 CMS performed a system test, involving most of the sub-detectors, which aimed at operating its solenoidal magnet up to 4 Tesla. During this CRAFT tests, we have operated Forward Hadron Calorimeter system (HF) using LED signals. This was the first opportunity to check that the HF PMT were sufficiently shielded with respect to the CMS solenoid field at the position of HF. In this talk we present the comparison between the data collected at HF in presence of the magnetic field and with no magnetic field. The mean values and the RMSs of the LED signals for each HF PMTs remained unchanged in conditions with solenoid magnetic field up to 3.8 Tesla. At 4 T, HF PMTs are starting to be sensitive. Meanwhile, the fringe field around the HF PMTs are measured and will be presented.