A Toroidal DC Beam Current Transformer with High Resolution

Beam current is measured with a system combining a second harmonic magnetic modulator with an active current transformer in an operational feedback loop, to obtain wide band response down to dc. A beam current monitor using this principle has been in operation in the CERN Intersecting Storage Rings (ISR) for over 10 years. The original design concept and performance has since been improved. The magnetic modulator-demodulator circuit has two modes of operation: low gain and wide band for initial search until tracking (with the beam current signal) and high gain, narrow band for optimum dc stability during normal operation. A second active L/R integrator loop is added to remove any residual modulator noise from the output signal to improve signal-to-noise ratio in the mid-frequency range. The performance of the system depends very much on the magnetic characteristics of the toroidal cores. The choice of material, measurement methods and core selection is discussed. More recent applications are beam intensity monitors for the CERN Antiproton Accumulator Ring (AA) and the CERN Proton Synchrotron (CPS).