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.
Patrick Anderson, Florian Wiegandt, Daniel Treacher, Matthias Mang, Ilaria Gianani, Andrea Schiavi, David Lloyd, Kevin O'Keeffe, Simon Hooker, Ian Walmsley
A blind variant of digital holographic microscopy is presented that removes the requirement for a well-characterized, highly divergent reference beam. This is achieved by adopting an off-axis recording geometry where a sequence of holograms is recorded as the reference is tilted, and an iterative algorithm that estimates the amplitudes and phases of both beams while simultaneously enhancing the numerical aperture. Numerical simulations have demonstrated the accuracy and robustness of this approach when applied to the coherent imaging problem.
We report the operation of an optical parametric amplifier (OPA) capable of producing gigawatt peak-power laser pulses with tunable wavelength in either the visible or near-infrared spectrum. The OPA has two distinct operation modes (i) generation of > 350 μJ, sub 100 fs pulses, tunable between 1250 - 1550 nm; (ii) generation of > 170 μJ, sub 150 fs pulses tunable between 490 - 530 nm. We have recorded high-order harmonic spectra over a wide range of driving wavelengths. This flexible source of femtosecond pulses presents a useful tool for exploring the wavelength-dependence of strong-field phenomena, in both the multi-photon and tunnel ionization regimes.
We describe the first demonstration of a collisionally-excited optical field ionisation laser driven within a gas-filled capillary waveguide. Lasing on the 4d95d - 4d95p transition at 41.8 nm in Xe8+ was observed to be closely-correlated to conditions under which the pump laser pulses were guided well by the waveguide. Simulations of the propagation of the pump laser radiation show that gain was achieved over essentially the whole 30 mm length of the waveguide.
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