A coplanar multipass laser system for 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.

Last year we presented a progress report on the installation of a non-coplanar multipass laser system

on a free-free apparatus using a 10Hz Surelite laser. The system used a Pockels cell to convert initially  horizontal laser polarization to vertical polarization. Periscopes before and after the interaction region in the non-coplanar circuit then produced horizontal polarization before the electron-scattering interaction

region, and vertical polarization afterwards so that the beam was turned through 90$^o$ by

a polarizing beamsplitter cube (PBS) just before the (now deactivated) Pockels cell and trapped in the circuit.  

This year we will present a progress report on a second multipass arrangement that uses a 30Hz Powerlite laser in a coplanar laser path that includes the interaction region of a second free-free apparatus. The laser is modified to emit {\em vertically} polarized light by removing a half-wave plate from the original laser optics. The beam then enters a circuit by being turned through 90$^o$ by a PBS, is converted to horizontal polarization by a Pockels cell, and passes through the coplanar circuit containing the electron-scattering interaction region, before being returned to the transmission face of the PBS and re-entering the (now deactivated) Pockels cell, and being trapped in the coplanar circuit. We will discuss the merits of the two systems.