Digitally compensated beam current transformer

Abstract

The Spallation Neutron Source (SNS) is being built by a collaboration of six laboratories. Beam current monitors (BCMs) will be used to record the current of H-minus and H-plus beams ranging from 15 mA (tune-up in the Front End and Linac) to over 60A fully accumulated in the Ring and dumped to the load as a single pulse in the Ring to Beam Target (RTBT). The time structure of these beams ranges from 645 ns “mini” bunches at the 1.05 MHz ring revolution rate, to an overall 1 ms long macro-pulse. The requirements for the BCMs will depend upon their location within the system. The need to measure individual mini-pulses, examine the characteristics of the chopper edge, as well as the longer average current pulse of the macropulse, or long duration pulses during Linac tuning place wide requirements upon the response of current transformers. To obtain the desired accuracy and resolution, current transformers must have <1 ns rise time and droops of 0.1%/ms. This places a significant design burden on the current transformer; such a design is almost impossible to achieve. Extremely large expensive cores are needed to meet the low droop, while leakage inductance increases with size, thereby reducing the achievable rise time. In this paper, I discuss a digital compensation approach [M. Kesselman, Spallation neutron source beam current monitor electronics, PAC2001 June 18–22, 2001, Chicago, IL.] that extends the lower cut-off frequency of the current transformer, optimized for high frequency response, so that it can be used in this application with improvements in droop of the order of 1000:1. Transformer saturation (current-time product) is a separate issue and the transformer must be designed to handle the current-time product of the signal to assure it does not saturate.