| Guiding of intense (I ∼ 2 x 10 17W/cm2) 80 fs laser pulses with negligible spectral distortion through 1.5-cm-long pre-formed helium plasma waveguides is demonstrated. Guiding in a fully ionized gas is necessary for laser wakefield acceleration. Waveguides were formed by axicon-focused Nd:YAG laser pulses of either 0.3 J energy, 100 ps duration, after pre-ionizing a 200--700 Torr backfill of He gas to ne ∼ 1016cm -3 with a pulsed electrical discharge; or 0.6--1.1 J energy, 400 ps duration, which required neither pre-ionization nor intentional impurities for seeding. Transverse interferometry showed that He was fully-ionized on the waveguide axis in both cases. Identical short pulses suffered substantial ionization-induced blueshifts after propagating through Ar and N2 waveguides of similar dimensions. Coupling into the waveguide is analyzed for backfilled vacuum chambers and alternatives are discussed. Probe pulses are co-propagated in the guide with the intense pulse to analyze distortions and electron density profiles. Application of the plasma guide for laser wakefield acceleration is discussed. |
