Impementation of a multipass laser system on a free-free apparatus

A free-free experiment investigates the emission or absorption of photons when an electron scatters from an atom in a laser field. For pulsed lasers of repetition rates of tens of hertz, and pulse durations of tens of nanoseconds, the experimental live-time is a few microseconds per year; typical experiments can take well over a week of continuous data taking. We have therefore developed and installed a multipass laser system on our free-free apparatus. The principal of the system is to use a Pockels cell to rotate the laser polarization $90^\circ$ as the beam enters a circuit which passes through the electron-scattering interaction region and then returns to a polarizing beamsplitter cube (PBS) placed just before the (now deactivated) Pockels cell. The orientation of the PBS is such that the beam is reflected through $90^\circ$ and therefore trapped in the circuit of path length 20~ns. Preliminary results are encouraging: the multipass system results in an increase of the free-free signal by a factor of 6.5, with a corresponding improvement in statistics from 3.6$\sigma$ to 8.1$\sigma$, over a single pass system. We will present a progress report on this system and plans to install a similar one on a second free-free apparatus.