Electro-optic beam position and pulsed power monitors for the second axis of DARHT

The second axis of the Dual Axis Radiographic Hydro-Test (DARHT) facility utilizes a long pulse electron beam having a duration in excess of two microseconds. This time scale poses problems for many conventional diagnostics that rely upon electrical cables to transmit signals between the accelerator and recording equipment. Recognizing that transit time isolation is not readily achieved for the long pulse regime, difficulties resulting from ground loops are anticipated. An electro-optic (EO) voltage sensor technology has been developed to address this issue. The EO sensor exploits the Pockels effect in Bi₄Ge₃O₁₂ (BGO) to provide linear modulation of laser light in response to the voltage induced on a pickup electrode. Fiber coupling between the light source, Pockels cell and receiver ensures complete galvanic isolation with improved cost and performance as compared to conventional sensors fitted with fiber optic links. Furthermore, the EO approach requires that only the passive sensor element be located near the accelerator while the light source and receiver can be installed in remote locations. This paper describes the design and development of EO sensors for electron beam and pulsed power monitoring on the second axis of DARHT. Typical calibration and testing data for the sensors is also presented