Measurement and analysis of field emission electrons in the LCLS gun

The field emission was measured during the high- power testing of the LCLS photocathode RF gun. A careful study and analysis of the field emission electrons, or dark current is important in assessing the gun´s internal surface quality in actual operation, especially those surfaces with high fields. The first indication of a good RF gun design and fabrication is short processing time to the required fields and low electron emission at high- fields. The charge per 2 microsecond long RF pulse (the dark charge) was measured as a function of the peak cathode field for the 1.6 cell, 2.856 GHz LCLS RF gun. Faraday cup data was taken for cathode peak RF fields up to 120 MV/m producing a maximum of 0.6 nC/RF pulse for a diamond-turned polycrystalline copper cathode installed in the gun. Digitized images of the dark charge were taken using a 100 micron thick YAG crystal for a range of solenoid fields to determine the location and angular distribution of the field emitters. The FN plots and emitter image analysis will be described in this paper.