Beam Phase Detection for Proton Therapy Accelerators

The industrial application of proton cyclotrons has become one of the important contributions of accelerator physics to medical therapy during the last years. Beam phase, energy, and intensity of the accelerated proton bunches can be detected by non-intercepting capacitive cylindrical probes. For the read out of the detected pulsed signals an advanced phase detection system using vector demodulating technique was developed. It has a very large dynamic range allow measurements over a beam current range of up to three orders of magnitude. In order to avoid interference from the fundamental cyclotron frequency of 72 MHz, the phase detection is performed at the second harmonic frequency. A phase detection range of 180° was achieved at the fundamental frequency. To improve accuracy a digital low pass filter with adjustable bandwidth and steepness is implemented. With an estimated sensitivity of the capacitive pickup in the beam line of 30 nV per nA proton beam at 250 MeV, accurate phase and intensity measurements are expected to be possible down to beam currents of 3.3 nA. First measurements at the cyclotrons will show how far the resolution will be limited to higher values by RF disturbance at the fundamental frequency.