Effects of External Magnetic Fields on the Photoelectron Emission from a Copper Beam Chamber

Applying magnetic fields around a beam chamber seems to be one of the effective methods to suppress the generation of electron cloud due to photoelectrons and, therefore, the beam size blow up of the positron beam observed at the KEKB. In order to investigate the effective magnetic field configuration and to estimate the necessary strength of it, the photoelectron current at the center of a test chamber is measured applying external magnetic fields. The test chamber is made of copper as the real beam chamber. The external magnetic fields are applied by a solenoid or small pieces of permanent magnets changing the magnetic field configuration, the structure of irradiated surface and the bias voltage of electrodes. The uniform magnetic field larger than about 50 G is found to be useful to reduce the photoelectrons at the beam position.