Optical Guiding in Meter-Scale Plasma Waveguides

We demonstrate a new highly tunable technique for generating meter-scale low density plasma waveguides. Such guides can enable laser-driven electron acceleration to tens of GeV in a single stage. Plasma waveguides are imprinted in hydrogen gas by optical field ionization induced by two time-separated Bessel beam pulses: The first pulse, a $J_0$ beam, generates the core of the waveguide, while the delayed second pulse, here a $J_8$ or $J_{16}$ beam, generates the waveguide cladding, enabling wide control of the guide’s density, depth, and mode confinement. We demonstrate guiding of intense laser pulses over hundreds of Rayleigh lengths with on-axis plasma densities as low as $N_{e0}\sim 5\times {10}^{16}{\mathrm{cm}}^{-3}$.

Figure 1

Experimental setup. The $\sim 200\mathrm{ps}$ uncompressed pulse from a Ti:Sapphire amplifier is split by a nonpolarizing beam splitter (BS). The transmitted pulse is converted to a $(0,q)$ beam by a segmented $q^{\mathrm{th}}$ order spiral phase plate (SPP). After beam expansion and relay imaging, phase front correction using a deformable mirror, and recompression, the beam is focused by a reflective axicon in either backfill hydrogen gas or over a supersonic gas jet. The counterpropagating guided beam is injected into the waveguide through a hole in the axicon and imaged after guiding. Also shown frequency-doubled ($\lambda =400\mathrm{nm}$, 70 fs) interferometric probe pulse. Left bottom inset: images of $J_0$ and $J_{16}$ beams in focus. Right bottom inset: $J_0$-pulse-induced plasma fluorescence 3.5 mm above the 5 cm supersonic ${\mathrm{H}}_2$ jet.

Figure 2

(a) Optical axis scan of $J_0$ focal spot profiles with planar cut shown in Fig. 1 inset. (b) Optical axis scan of combined $J_0$ and $J_{16}$ beam, with planar cut shown in Fig. 1 inset. (c) Azimuthal average of (b). (d) Combined $J_0+J_{16}$ beam profile. $\gamma =3°$.

Figure 3

(a) Radius of blast wave expansion of $J_0$ plasma (best fit $n=0.56\pm 0.11$) and extracted electron temperature vs time. Hydrogen backfill 50 torr. (b) Interferometric phase shift images of $J_0$ and $J_{16}$ plasmas 10 ps after generation. Hydrogen backfill 100 torr. (c) Time evolution of $J_{16}$-generated annular electron density profile. Pulse energy 100 mJ, hydrogen backfill 100 torr. The plasma pressure gradient mainly drives filling of the central hole. The radial integral of these curves (total charge) is constant to within $\sim 5\%$. (d) Pulse energy dependence of $J_{16}$-generated annular electron density profiles, with saturation for $>50\mathrm{mJ}$. Hydrogen backfill 100 torr. The modulations at larger radius are from ionization by outer rings of the $J_{16}$ pulse.

Figure 4

Transverse electron density profiles in backfill and gas jet hydrogen plasmas. Where shown, the magenta dashed lines indicate maxima of $J_q$ rings. (a) $J_0$-generated core electron density profiles vs time delay. (b) $J_{16}$ pulse propagation simulations of plasma profile generation with and without $J_0$-generated core plasma present. $\alpha =1.5°$. (c)–(f) Electron density profiles measured $\sim 10\mathrm{ps}$ after the $J_0$ pulse (red curves) and 10 ps before and after the $J_q$ pulse (green and blue curves). (c) 100 mJ ($J_0$) $+235\mathrm{mJ}$ ($J_{16}$), $\alpha =3°$, ${\mathrm{H}}_2$ backfill pressure $P=50\mathrm{torr}$. Waveguide length $L=15\mathrm{cm}$; (d) 78 mJ ($J_0$) $+250\mathrm{mJ}$ ($J_{16}$), $\alpha =1.5°$, ${\mathrm{H}}_2$ backfill pressure $P=10\mathrm{torr}$. Waveguide length $L=30\mathrm{cm}$; (e) 100 mJ ($J_0$) $+235\mathrm{mJ}$ ($J_8$), $\alpha =1.5°$, ${\mathrm{H}}_2$ backfill pressure $P=50\mathrm{torr}$. Waveguide length $L=30\mathrm{cm}$; (f) ${\mathrm{H}}_2$ gas jet, $P\approx 30\mathrm{torr}$: 87 mJ ($J_0$) $+235\mathrm{mJ}$ ($J_{16}$), $\alpha =1.5°$, $q=16$. Waveguide length $L=5\mathrm{cm}$.

Figure 5

(a)–(c) Low intensity exit modes from $L=15$, 30, and 30 cm long plasma waveguides [number of Rayleigh ranges $L/z_0\sim 110$, 100, and 260 (for $\lambda =800\mathrm{nm}$)] under conditions of Figs. 4, for injection of $\sim 100\mu \mathrm{J}$, $\lambda =400\mathrm{nm}$ pulse. (d) Gas jet experiment: high intensity exit mode of a 5 cm long waveguide generated in ${\mathrm{H}}_2$ gas jet. Injected pulse 40 mJ, $\lambda =800\mathrm{nm}$, 50 fs FWHM, with $\sim 50\%$ coupling efficiency, giving guided intensity $\sim 6\times {10}^{16}\mathrm{W}/{\mathrm{cm}}^2$ over $L/z_0\sim 25$ Rayleigh ranges. (e) Guided modes from 20 successive gas jet shots for conditions of (d). Variations are mainly from injected beam pointing fluctuations.