Current monitor for the isis synchrotron RF cavity bias regulator

The ISIS facility at the Rutherford Appleton Laboratory in the UK is currently the world\´s most intense pulsed neutron source. The accelerator consists of a 70 MeV H" linac and an 800 MeV, 50 Hz, proton synchrotron. The synchrotron beam is accelerated using six, fundamental harmonic, ferrite loaded, RF cavities each having its own high voltage RF drive amplifier and bias system. Each of these RF cavities is driven as a high Q tuned RF circuit; the resonant frequency being controlled by passing a current through a bias winding. This current comes from the cavity\´s own Bias Regulator system which consists in part of eight banks of 10 transistors. The machine is currently being upgraded by adding four second harmonic cavities, each having its own drive amplifier and bias system. The second harmonic bias regulators have eight banks of 40 transistors (i.e. four transistors in parallel to replace each of the 10 single ones used in the original system). This paper describes the design of a modular system which will use digital techniques to monitor and display the current of each of the 80 transistors in the Fundamental Bias Regulator system, or 320 transistors in the Second Harmonic System.