Generation of uniform plasmas for beat wave experiments

The laser plasma ``beat wave'' mechanism for the generation of ultra-high electric fields requires plasmas of several metres length with density uniformity of about 1%. Multiphoton ionization of molecular hydrogen gas at a pressure of a few torr provides a scaleable mechanism for generating these plasmas. We describe measurements of electron density, temperature and uniformity of plasmas generated by a frequency doubled Neodymium glass laser, at an irradiance of about 10¹⁴Wcm^-2. The plasma density corresponds to 100% ionization and is measured to be uniform to within the measurement errors over a length of 8mm.