Stable collimation of MeV proton beams by self-driven magnetic pinching*

We report the generation of a multi-MeV proton beam from a novel continuously-flowing ambient-temperature liquid water jet target [Treffert et al., Physics of Plasmas 29, 123105 (2022)]. Compared to those generated from a more typical polyimide tape target, proton beams from this water target were less divergent (≤ 20 mrad), higher dosage (55 Gy), stable (peak dose variation of 11% rms), high-energy (4-6 MeV), and could operate reliably at 5 Hz with the potential to scale up to kHz rates. The presence of a low-density vapor surrounding the target aided in the generation of these desirable proton beams. Here, we report on 2D OSIRIS simulations used to study the collimation mechanism. Through proton collisional ionization, the beam was able to maintain an amount of neutrality via the newly ionized electrons that helped to mitigate electrostatic fields that would otherwise cause the beam to expand. It does not, however, fully negate the beam current, which generates an azimuthal magnetic field that acts to pinch the proton bunch much like the ion Weibel instability would. This allows for the self-focusing of a single filament. And while these simulations are inherently simplified, they offer an exciting opportunity to explore experimental conditions to allow for the control of proton beam propagation.