We demonstrate the efficient generation of 4 mm and 8 mm long plasma waveguides in hydrogen and helium. These waveguides have matching spots sizes for 13 to 34 lm laser beams. The plasma waveguides are created by ultra-short laser pulses (sub-picosecond) of moderate intensities, $10 15 -10 16 W cm À2 , that heat the plasma to initial temperatures of tens of eV in order to create a hot plasma column that will expand into a plasma waveguide. We have determined that the main heating mechanism when using fs laser pulses and plasma densities $10 18-19 cm À3 is Above Threshold Ionization. Detailed time and space electron density measurements are presented for the laser produced plasma waveguides. V C 2013 AIP Publishing LLC. [http://dx.

A multi-terawatt, ultra-short laser pulse was used to create a large, well-defined relativistic ionization front, and the collision of a probe laser pulse with it was studied. Due to the good definition of the ionization front it was also possible to isolate the probe frequency upshift due to the collision, unlike in previous experiments. A good agreement with the theoretical model for photon acceleration was found. Furthermore, we report novel measurements of ionization front velocities, based on the photon acceleration effect, in excess of 0.99c.

We experimentally demonstrate that plasma waveguides produced with ultra-short laser pulses (sub-picosecond) in gas jets are capable of guiding high intensity laser pulses. This scheme has the unique ability of guiding a high-intensity laser pulse in a plasma waveguide created by the same laser system in the very simple and stable experimental setup. A hot plasma column was created by a femtosecond class laser that expands into an on-axis parabolic low density profile suitable to act as a waveguide for high intensity laser beams. We have successfully guided ~1015 W cm−2 laser pulses in a 8 mm long hydrogen plasma waveguide with a 35% guiding efficiency.

We experimentally demonstrate that a column of hydrogen plasma generated by an ultra-short (sub-picosecond), moderate intensity (∼1015–16 W.cm–2) laser, radially expands at a higher velocity when using a circularly polarized laser beam instead of a linearly polarized beam. Interferometry shows that after 1 ns there is a clear shock structure formed, that can be approximated to a cylindrical blast wave. The shock velocity was measured for plasmas created with linearly and circularly polarized laser beams, indicating an approximately 20% higher velocity for plasmas generated with a circularly polarized laser beam, thus implying a higher plasma electron temperature. The heating mechanism was determined to be the Above Threshold Ionization effect. The calculated electrum energy spectrum for a circularly polarized laser beam was broader when compared to the one generated by a linearly polarized laser beam, leading to a higher plasma temperature.

A plasma waveguide scheme for high-intensity laser guiding with densities and lengths suitable for laser-plasma particle accelerators is presented. This scheme uses a laser-triggered high-voltage discharge, presents negligible jitter, allows full access to the plasma, and can be scaled to large distances. Experimental results showing the feasibility of this scheme are presented.