FRIB, East Lansing, Michigan, USA
Chopper in the low energy beam line is a key ele-ment to control beam power in FRIB. As appropriate functioning of chopper is critical for machine protec-tion for FRIB, an FPGA-based chopper monitoring system was developed to monitor the beam gated pulse at logic level, deflection high voltage level, and in-duced charge/discharge current levels, and shut off beam promptly at detection of a deviation outside tolerance. Once FRIB beam power reaches a certain level, a cold start beam ramp mode in which the pulse repetition frequency and pulse width are linearly ramped up becomes required to mitigate heat shock to the target at beam restart. Chopper also needs to gen-erate a notch in every machine cycle of 10 ms that is used for beam diagnostics. To overcome the challeng-es of monitoring such a ramping process and meeting the response time requirement of shutting off beam, two types of process checkers, namely, monitoring at the pulse level and monitoring at the machine cycle level, have been implemented. A pulse look ahead algorithm to calculate the expected range of frequency dips and rises was developed, and a simplified mathe-matical model suitable for multiple ramp stages was built to calculate expected time parameters of accumu-lated pulse on time within a given machine cycle. Both will be discussed in detail in this paper, followed by simulation results with FPGA test bench and actual instrument test results with the beam ramp process.
