PSI - Paul Scherrer Institute

RF-based Measurement of Ultra Low charges

Trainee: Sudharsan Srinivasan 
Supervisor: Pierre-André Duperrex

Optimal patient treatment is one of the main goals of the Center for Proton Therapy, CPT. Another goal equally is continuous research and development of innovative treatment concepts. The achievement of these goals is facilitated by the unique position of CPT, embedded within the infrastructure of PSI Beam diagnostics plays an important role at accelerator facilities. At PROSCAN, it helps optimize medical treatment schemes. For medical facilities, invasive diagnostics are predominantly used due to low intensities involved. This causes scattering issues and consequently reduces the beam quality delivered for patient treatment.

Under OMA, a non-invasive monitor to measure beam current information has been developed. This monitor is built as a dielectric-filled reentrant cavity resonator with its fundamental resonance frequency as 145.7 MHz, the second harmonic of the PSI proton beam bunch repetition rate.

Using the resonator, the beam intensity was measured to be lower than 100 pA for proton beam energies of 230 MeV, and 200 MeV. As the beam energy is lowered, the sensitivity of the reentrant cavity resonator is also reduced. For energies of 170 MeV and 140 MeV, the beam intensity is as low as 1nA. We evolved the design of reentrant cavity to measure beam position information. For this, we built a Quadrated Dielectric Filled Reentrant Cavity Resonator tuned to the dipole mode of resonance as its working mode. The dipole mode resonance frequency for this position monitor is matched to 145.7 MHz. Initial measurements of S-parameter i.e. transmission coefficient on the dedicated test-bench provided promising results in comparison to the simulated results. The prototype was validated through beam line characterization in June 2019.