Photo currents in the vertical electrostatic separators at CESR

At CESR a pair of vertical separators are used to separate the two beams vertically at the symmetry point, in the north, of the interaction point in the south. They are 2.7 meter long and each one is providing a 1.0 mrad kick for a differential voltage of 100 KV between the electrodes. One side of each separator is exposed to the high flux of synchrotron radiation from a soft bend, 140 meter radius, magnet and a hard bend, 34.8 meter radius, magnet. The radiation generates large amount of photoelectrons which are converted to a large photocurrent. For a beam current of 150 ma the photocurrent is 1.2-1.5 ma. It caused,a few times a day, a partial drop in the high voltage which caused a vertical beam blow up and beam loss. In addition the photocurrent was increasing the heat load of the positive electrode by 25 percent. In the paper we will show how we were able to suppress the photocurrent almost to zero. As a result we have seen an increase in the life time of the beams and as well an increase in the luminosity.