Development of one meter-long lithium plasma source and excimer mode reduction for plasma wakefield applications

A one meter long plasma source has been developed for studies at the Stanford Linear Accelerator Center (SLAC) for plasma wakefield excitation by a 30 GeV electron beam in an extended plasma column. The plasma is formed by ionization of a Li-vapor with an ArF excimer laser (193 nm). The Li-vapor is produced in a heat pipe oven which will be installed in the electron beam transport line. Through control of the oven temperature, neutral vapor densities reaching 2×10¹⁵ cm^-3 are produced. We report the details of the oven construction and temperature profile measurements with and without Li vapor. In the experiment, the excimer laser will be located about 15 m away from the plasma source. Beams produced by excimer lasers operating with a stable resonator cavity are unsuitable due to the large number of modes present in such beams. A significant reduction of the modal content has been obtained through the use of an unstable resonator design. Results of the implementation of this cavity configuration on the propagation characteristics of the excimer beam will be presented, including the design of the final telescope for spot size reduction