We demonstrate optical guiding of high-intensity laser pulses in long, low density hydrodynamic optical-field-ionized (HOFI) plasma channels. An axicon lens is used to generate HOFI plasma channels with on-axis electron densities as low as ne(0) = 1.5 × 10 17 cm −3 and matched spot sizes in the range 20 µm WM 40 µm. Control of these channel parameters via adjustment of the initial cell pressure and the delay after the arrival of the channel-forming pulse is demonstrated. For laser pulses with a peak axial intensity of 4 × 10 17 W cm −2 , highly reproducible, high-quality guiding over more than 14 Rayleigh ranges is achieved at a pulse repetition rate of 5 Hz, limited by the available channel-forming laser and vacuum pumping system. Plasma channels of this type would seem to be well suited to multi-GeV laser wakefield accelerators operating in the quasi-linear regime.Many applications of high-intensity laser-plasma interactions require the propagation of high-intensity laser pulses through plasmas which are orders of magnitude longer than the Rayleigh range. One example of particular current interest is the laser wakefield accelerator (LWFA) [1], in which a laser pulse with an intensity of order 10 18 W cm −2 propagates though a plasma, driving a trailing density wave. The electric fields generated within this plasma wave are of the order of the wave-breaking field E 0 = m e ω p c/e, where ω p = (n e e 2 /m e ǫ 0 ) 1/2 and n e is the electron density [2,3]. For plasma densities in the range n e = 10 17 −10 18 cm −3 , E 0 ≈ 30 − 100 GV m −1 , which is several orders of magnitude higher than the fields generated in radio-frequency machines.Plasma accelerators can drive compact sources of femtosecond-duration [4-6] radiation via betatron emission [7,8], undulator radiation [9, 10], and Thomson scattering [11][12][13], with many potential applications in ultrafast science. In the longer term they could provide a building block for future high-energy particle colliders [14].For LWFAs operating in the quasilinear regime [2], the energy gain per stage varies as W ∝ E 0 L acc ∝ 1/n e , and the required length of the stage varies as L acc ∝ 1/n 3/2 e . Hence reaching higher energy gains requires the drive laser to propagate over longer lengths of lower density plasma.To date, the highest reported electron energy generated in a LWFA is 7.8 GeV, which was achieved by guiding intense laser pulses through a 200-mm-long plasma channel with an axial electron density of 2.7 × 10 17 cm 3 [15].Laser-plasma accelerators providing 10 GeV energy gain per stage will require laser guiding through 100s of millimetres of plasma of electron density n e ≈ 10 17 cm −3 [16,17]. Further, for many of the applications identified above it will be necessary to operate at pulse repetition rates, f rep , several orders of magnitude above the few hertz typical of today's GeV-scale LWFAs.To date, the workhorse waveguide for LWFAs has been the capillary discharge waveguide [18,19]. Capillary discharge waveguides have generated plasma channels up to 150...

We demonstrate through experiments and numerical simulations that low-density, low-loss, meter-scale plasma channels can be generated by employing a conditioning laser pulse to ionize the neutral gas collar surrounding a hydrodynamic optical-field-ionized (HOFI) plasma channel. We use particle-in-cell simulations to show that the leading edge of the conditioning pulse ionizes the neutral gas collar to generate a deep, low-loss plasma channel which guides the bulk of the conditioning pulse itself as well as any subsequently injected pulses. In proof-of-principle experiments, we generate conditioned HOFI (CHOFI) waveguides with axial electron densities of n e0 ≈ 1×10 17 cm −3 and a matched spot size of 26 μm. The power attenuation length of these CHOFI channels was calculated to be L att = (21 ± 3) m, more than two orders of magnitude longer than achieved by HOFI channels. Hydrodynamic and particle-in-cell simulations demonstrate that meter-scale CHOFI waveguides with attenuation lengths exceeding 1 m could be generated with a total laser pulse energy of only 1.2 J per meter of channel. The properties of CHOFI channels are ideally suited to many applications in high-intensity light-matter interactions, including multi-GeV plasma accelerator stages operating at high pulse repetition rates.

Hydrodynamic optically-field-ionized (HOFI) plasma channels up to 100 mm long are investigated. Optical guiding is demonstrated of laser pulses with a peak input intensity of 6 × 10 17 W cm −2 through 100 mm long plasma channels with on-axis densities measured interferometrically to be as low as n e0 ¼ ð1.0 AE 0.3Þ × 10 17 cm −3. Guiding is also observed at lower axial densities, which are inferred from magneto-hydrodynamic simulations to be approximately 7 × 10 16 cm −3. Measurements of the power attenuation lengths of the channels are shown to be in good agreement with those calculated from the measured transverse electron density profiles. To our knowledge, the plasma channels investigated in this work are the longest, and have the lowest on-axis density, of any free-standing waveguide demonstrated to guide laser pulses with intensities above >10 17 W cm −2 .

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