The increasing demand for high laser powers is placing huge demands on current laser technology. This is now reaching a limit, and to realise the existing new areas of research promised at high intensities, new cost-effective and technically feasible ways of scaling up the laser power will be required. Plasma-based laser amplifiers may represent the required breakthrough to reach powers of tens of petawatt to exawatt, because of the fundamental advantage that amplification and compression can be realised simultaneously in a plasma medium, which is also robust and resistant to damage, unlike conventional amplifying media. Raman amplification is a promising method, where a long pump pulse transfers energy to a lower frequency, short duration counter-propagating seed pulse through resonant excitation of a plasma wave that creates a transient plasma echelon that backscatters the pump into the probe. Here we present the results of an experimental campaign conducted at the Central Laser Facility. Pump pulses with energies up to 100 J have been used to amplify sub-nanojoule seed pulses to near-joule level. An unprecedented gain of eight orders of magnitude, with a gain coefficient of 180 cm−1 has been measured, which exceeds high-power solid-state amplifying media by orders of magnitude. High gain leads to strong competing amplification from noise, which reaches similar levels to the amplified seed. The observation of 640 Jsr−1 directly backscattered from noise, implies potential overall efficiencies greater than 10%.
Gas-filled capillary discharge waveguide is an useful medium for investigating high-power laser-plasma
interactions over extended lengths because guiding can increase the interaction length to many Rayleigh
lengths.
The role of the gas plasma plume at the entrance of a CDW in increasing the laser intensity is under
investigation. Experimentally have been performed different measurements of the plasma density
profiles in the region adjacent to exit plane of capillary. Simulations of laser pulse evolution in this
region, employing simulation codes and analytical functions, show that relativistic self-focusing may
lead to an increase of the pulse intensity compared to the case without the plume. Measurements show
that the on-axis plasma density over this region is close to that inside the waveguide (~1018 cm-3). Here
the laser beam converges to the smallest focal spot and relativistic self-focusing leads to an increase in
the laser intensity. If injection is guaranteed to occur early, the required length would only be of the
order of 3-8 mm for our parameters, with important advantages of a lower discharge voltages for gas
breakdown, and an easier alignment of laser beam
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