Fast beam probe development for longitudinal bunch measurements at UC Davis Crocker Nuclear Laboratory Cyclotron

The UC Davis Crocker Nuclear Laboratory (CNL) operates a 76-inch Isochronous Cyclotron dating to the 1960s. Recent experiments have revealed unexplained beam behavior, which cannot be directly measured with the current diagnostics. Direct measurements of the beam in the Cyclotron are challenging due to the harsh environment, including high radiation, strong magnetic fields, RF interference, and spatial constraints. To address this, we are developing a novel beam probe capable of resolving longitudinal bunch structure across 16 positions simultaneously. The fast beam probe consists of a segmented fast plastic scintillator array coupled via fiber optics to external Silicon Photomultipliers (SiPMs), mounted on a radially translating probe. We report on the probe's performance from in-air tests at the general-purpose beamline. The results demonstrate sub-nanosecond resolution, consistent sensitivity across channels, and clear signatures of beam dynamics, establishing the system’s viability for measurements inside the CNL Cyclotron.