PSI, Villigen PSI, Switzerland
At PSI, a dedicated proton therapy facility, with a superconducting cyclotron, delivers 250 MeV beam energy, pulsed at 72.85 MHz. The measurement of beam currents (0.1-10 nA) is generally performed by ionisation chambers (ICs), but at the expense of reduced beam quality, and scattering issues. There is a strong demand to have an accurate signal with a minimal beam disturbance. A cavity resonator, on fundamental resonance mode, has been built for this purpose. The cavity, coupled to the second harmonic of the pulse rate, provides signals proportional to the beam current. It is installed in a beamline to measure for the energy range 238-70 MeV. Good agreement is reached between the expected and measured sensitivity of the cavity. The cavity delivers information for currents down to 0.15 nA with a resolution of 0.05 nA when integrated over one second. Its application is limited to a machine-safety monitor to trigger inter-locks, within the existing domain of the proton therapy due to the low beam current limits. With new advancements in proton therapy, especially FLASH, the cavity resonator’s application as an online beam-monitoring device is feasible.
*Instruments 2018, 2(4), 24; https://doi.org/10.3390/instruments2040024
PSI, Villigen PSI, Switzerland
Low beam currents (0.1-10 nA) are used for tumour treatment in the proton radiation therapy facility at PSI. The facility houses a superconducting cyclotron with extraction energy of 250 MeV pulsed at 72.85 MHz. Online measurement of the beam position is traditionally performed with the help of ionisation chambers (ICs), however, at the expense of reduced beam quality and scattering issues. There is a strong demand to have this measurement performed with minimal beam disturbance since the beam position is directly associated with the dose-rate applied. A cavity resonator, working on the principle of an electric dipole mode resonance, whose frequency is coupled to the second harmonic of the pulse rate, has been built to measure beam position in a purely non-invasive manner. Followed by a reasonable agreement between the test-bench and the simulation results, the cavity is installed in one of the beamlines. Here, we report on the measurement of the cavity BPM as a function of beam current and position and its shortcomings. The cavity BPM can deliver position information within the accuracy and resolution demands of 0.50 mm, when measured with a spectrum analyzer.