In a multi-GeV laser-driven plasma accelerator the driving laser pulse must remain focused as it propagates through tens of centimetres of plasma of density 1017 cm-3. This distance is orders of magnitude greater than the Rayleigh range, and hence the laser pulse must be guided with low losses. Since many applications of laser-plasma accelerators will require that the pulse repetition rate is in the kilohertz range, methods for guiding relativistically-intense laser pulses at high repetition rates must be developed.
We describe the development of hydrodynamic optical-field-ionized (HOFI) plasma channels and conditioned HOFI channels, which can meet all of these challenging requirements. We present experiments and numerical simulations that show that hydrodynamic expansion of optical-field-ionized plasma columns can generate channels at low plasma densities. We show that guiding a conditioning pulse in a HOFI channel leads to the formation of long, very low loss plasma channels via ionization of the collar of neutral gas which surrounds the HOFI channel.
We describe proof-of-principle experiments in which we generated conditioned HOFI (CHOFI) waveguides with axial electron densities of ne0 ≈ 1×1017 cm−3 and a matched spot size of approximately 30 μm. We present hydrodynamic and particle-in-cell simulations which demonstrate that meter-scale, low-loss CHOFI waveguides could be generated with a total laser pulse energy of about 1 J per meter of channel.
Laser-driven plasma accelerators operating in a quasi-linear regime require external guiding of the laser driver. We describe our work to develop hydrodynamic optical-field-ionized (HOFI) channels with properties which are well suited to all types of laser-driven plasma accelerator. In this approach a plasma channel is formed by hydrodynamic expansion of a plasma column formed by OFI with elliptically-polarized laser pulses; since the electron energies generated with OFI are independent of the gas density, channels can be formed with much lower axial densities than is possible with collisional heating. An attractive feature of HOFI channels is that they are free-standing, and hence they could operate at high-repetition rates for extended periods.
We present simulations which demonstrate the possibility of forming plasma channels 100s of millimetres long, with axial densities of order 10^{17} cm^{-3} and lowest-order modes of spot size of order 40 um. We also present recent experimental results which confirm the formation of HOFI channels with properties similar to those predicted by simulations.
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