An active magnetic field compensation system for the HKS aerogel Cherenkov detectors

Three segmented aerogel Cerenkov detectors are employed in the Jefferson Lab High Resolution Kaon Spectrometer (HKS). Cerenkov light is detected with 5" PMTs. At the detector location a stray field from the magnet of $\sim$5 gauss significantly reduces the photoelectron collection efficiency of the PMTs, despite the use of magnetic shielding. Therefore, we propose to compensate the stray field with bucking coils. In a test setup, a Helmholtz Coil was used to generate external magnetic fields up to 15 gauss. In order to counteract this B field 20 turns of bucking coil (gauge: 12 AWG) were mounted around the cathode sides of the PMT. A recovery of roughly 87.5\% was determined for an external B field of 5 Gauss. The experimental procedure and results of both this and new studies on the installed detector will be presented in detail.