Laser Stripping for High Intensity Synchrotrons

The standard method for producing intense, time structured proton beams for HEP applications is through H- charge exchange injection from a linac into a synchrotron. While in principal this approach can yield extremely high proton beam densities, in practice the achievable densities are limited by the stripping mechanism. The conventional implementation of charge exchange relies on a thin carbon foil to strip the electrons from the incoming H- beam after merger with the circulating proton beam. The presence of the foil in the beamline introduces significant performance issues, including high levels of activation due to foil scattering and vulnerabilities in the survivability of the foils. The likely increases in beam power densities in future HEP accelerators will exacerbate these issues, possibly beyond tolerable levels. Currently, there is no viable alternative for foil-based charge exchange injection once this threshold is reached. Overcoming this technological barrier falls squarely into the HEPAP Accelerator R&D Subpanel Report recommendation to “Perform the necessary exploratory research to bring new concepts into practice,” for multi-MW proton accelerators and beyond. The laser stripping concept offers an attractive alternative that does not suffer from the same performance limitations and is scalable to arbitrarily high beam power densities. In this scenario, the foil mechanism is replaced with a laser and dipole ensemble. An initial proof-of-principle experiment validated the concept for a 6 ns, 1 GeV H- beam, but was not scalable to realistic duty factors due to the extraordinary laser power requirement. Advancing this method requires the implementation of ion and laser beam manipulations to reduce the required laser power to achievable levels. This is thrust of an ongoing experiment to demonstrate stripping of a 10 μs, 1 GeV H- beam using available laser technology. This experiment is a three-orders-of-magnitude improvement on the initial demonstration. To move beyond 10 us, power recycling to achieve 1ms high peak power UV pulses is required. The primary goal of this project is to develop the recycling cavity and employ it to demonstrate millisecond-capable H- stripping, thus advancing the laser stripping method into operational parameter regimes. The required infrastructure is complex but has been largely developed as part of 10 μs stripping experiment. A second, complementary goal is to explore design concepts for an operational laser stripping system.