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A laser-plasma accelerator driven by two-color relativistic femtosecond laser pulses

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Science Advances  22 Nov 2019:
Vol. 5, no. 11, eaav7940
DOI: 10.1126/sciadv.aav7940
  • Fig. 1 Schematic of the experimental setup for a laser-plasma electron accelerator using the CTLP scheme.

    The 4-inch, 2.1-J, 800-nm, and 30-fs p-polarized laser pulses are split into two laser beams: 1.9-J FL infrared (800 nm) beam and 100-mJ SH ultraviolet (400 nm) beam. The former is focused onto 4-mm-long slit-shaped gas jet using an (F/20) off-axis parabolic mirror (OAP1) [see (A)]. The SH 1-inch laser pulses are generated in a BBO crystal and filtered by a DBS. After a variable time delay of τ1, they are focused collinearly with the 1.9-J FL beam onto the gas jet by using a short focal length (F/4) optic with a small hole (OAP2). The polarization of the SH beam is controlled via half- or quarter-wave plates (WP) (see text). The remnant FL laser beam after the BBO serves as an ultrafast probe laser beam (~30-fs pulse duration). After a proper time delay of τ2, the probe laser is used for a precise temporal-overlapping of the two ultrashort laser pulses via plasma shadowgraphy (see Materials and Methods and the Supplementary Materials). (B) The probe laser beam was also used for measuring the electron density profile via interferometry. (C) The two laser foci are monitored at lower intensities via a movable forward optical imaging system, which helps in simultaneously monitoring the spatial overlapping of the two laser beams just before shooting the CTLP lasers at high energy. These monitoring and optimization of the two laser spots and their spatial overlapping are repeated every five laser shots (roughly 1 shot/min) during the whole experimental campaigns. The accelerated electron beams exit the vacuum chamber through a thin Be window, propagate through an integrating current transformer (ICT) for measuring the charge, and then enter a 1-T permanent magnet spectrometer. (D) The electron energy spectra are recorded on an Al foil–shielded image plate (IP). (E) The electron spectra are also simultaneously recorded by a fluorescent (DRZ) screen coupled with an ICCD. (F) After the interaction, a portion of the scattered SH spectra is collected by an imaging optical spectrometer. a.u., arbitrary units.

  • Fig. 2 Electron energy spectra from the LWFA in a helium gas jet for four different cases and the effect of the time delay between the CTLP on the electron beam energy and charge.

    Each spectrum is obtained with an accumulation of data on the IPs from five laser shots (A) using the full FL (800 nm) laser pulse (E = 2.1 J) and (B to D) using the CTLP scheme with the clipped FL (800 nm, 1.9 J) and the SH (400 nm, 100 mJ) laser pulses of various polarizations for the SH pulse at a relative time delay of T0 ≈ 0 fs. The plasma density, inferred from the interferometry, is ne = 8 × 1018 cm−3. The signals shown at the ∞ direction are x-rays emitted spontaneously by the electrons as a result of their transverse betatron oscillations in the ion channel. (E) Vertically integrated electron spectra in logarithmic scale for the shots in (A) to (D). The horizontal error bars correspond to the resolution [estimated according to a method given in (29)] of the spectrometer, which are ±4.5, ±9, and ±19% at 150, 300, and 600 MeV, respectively. (F) Cutoff energies (red squares) and highest monoenergetic peak energy (blue circles) as a function of the time delay τ1 between the two laser pulses, measured using the IPs. (G) Total electron beam charge as a function of τ1, measured using the ICT. Each data point in (G) represents the result from a single laser shot. The data in (F) and (G) are obtained for the p-polarized SH laser pulses case. Positive time delays indicate that the SH pulse leads the FL pulse.

  • Fig. 3 Typical electron energy spectra (left column) recorded on the fluorescent DRZ screen and the vertically integrated spectra in linear scale (right column) from 2.1-J, 800-nm full laser pulses.

    The plasma medium is composed of mixed gases of 99.5% He and 0.5% N2, and the plasma density is ne = 4 × 1018 cm−3. The blue arrows indicate the cutoff energy Emax in the integrated spectra. The horizontal error bars in the right column correspond to the resolution of the spectrometer, which is ±5% at 300 MeV, applicable to each spectrum in this figure.

  • Fig. 4 Electron energy spectra from the two-color LWFA in a helium-nitrogen gas jet.

    Typical electron energy spectra (left column) recorded on the fluorescent DRZ screen and the vertically integrated spectra in linear scale (middle column, electron spectra in the range of 50 to 200 MeV; right column, electron spectra in the range of 200 to 700 MeV) from the two-color LWFA (CTLP) scheme using (A) 1.9-J, 800-nm laser and 100-mJ p-polarized 400-nm laser; (B) 1.9-J, 800-nm laser and 100-mJ s-polarized 400-nm laser; and (C) 1.9-J, 800-nm laser and 100-mJ c-polarized 400-nm laser. The plasma medium is formed from the gas mixture of 99.5% He and 0.5% N2, and the electron density is ne = 4 × 1018 cm−3. The blue arrows indicate the cutoff energy Emax in the integrated spectra. The horizontal error bars (applicable to each spectrum in this figure) in the middle and right columns correspond to the resolution of the spectrometer, which are ±2.5 and ±8% at 150 and 500 MeV, respectively.

  • Fig. 5 Experimental setup for the generation of positron beams based on the LWFA and energy spectra of electron-positron pair beams as recorded by the IPs for 5 mm of a Pb bremsstrahlung target.

    (A) Schematic of the experimental setup. The incident electron beams into the Pb target are produced from an LWFA driven by (B) full 2.1-J, 800-nm laser pulses alone and (C) two-color (CTLP) scheme where the SH pulses are in s-polarized state, respectively. The white dashed circles depict the bremsstrahlung γ-rays, whereas the red dashed lines depict the projection of the magnet gap. Because of the clear presence and collimation of the γ-rays, it is easy to distinguish and resolve the secondary electron beam spectra and the positron beam spectra. (D and E) Vertically integrated positron beam spectra in linear scale from (B) and (C) for two cases. The insets in (D) and (E) show the secondary electron beam spectra in linear scale. The horizontal error bars correspond to the resolution of the spectrometer, which are ±7, ±10, ±11, and ±15% at 150, 200, 220, and 300 MeV, respectively.

  • Fig. 6 2D-PIC simulation results for an LWFA using the two-color (CTLP) scheme with p-polarized 1.9-J, 800-nm FL pulse and s-polarized 100-mJ, 400-nm SH pulse.

    (A to E) Snapshots of the laser E-fields (Ex for the FL pulse; Ey for the SH pulse) and electron density distribution in the x-z plane at different times. (F to J) Energy-space distribution of the accelerated electrons with energies ≥50 MeV at respective times. a.u., arbitrary units. The red dashed plot in (A) is the initial laser normalized vector potential (a0) distribution of FL pulse. The green and blue plots in (A) to (E) are the evolution of a0 for the FL and SH pulses, respectively. The orange lines in (A) to (E) show the evolution of the on-axis longitudinal electric field (Ez) at different times. The red solid lines in (F) to (J) represent energy spectra of the electrons at respective times, showing a cutoff energy around 450 MeV at 2.7 mm; then, it drops to 420 MeV after 3.8 mm of propagation.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/11/eaav7940/DC1

    Fig. S1. A top-view photograph of the experimental setup used for the generation of positron beams based on the LWFA.

    Fig. S2. 2D-PIC simulation results for an LWFA driven by full 2.1-J, 800-nm, and 30-fs laser pulse with p-polarization.

    Fig. S3. Simulation of the SHG process.

    Fig. S4. Side-view plasma shadowgraph for the relative time zero (T0 ~ 0 fs) between the main FL and SH pulses. A 30-fs probe laser beam (800 nm) was used as a back lighter.

  • Supplementary Materials

    This PDF file includes:

    • Fig. S1. A top-view photograph of the experimental setup used for the generation of positron beams based on the LWFA.
    • Fig. S2. 2D-PIC simulation results for an LWFA driven by full 2.1-J, 800-nm, and 30-fs laser pulse with p-polarization.
    • Fig. S3. Simulation of the SHG process.
    • Fig. S4. Side-view plasma shadowgraph for the relative time zero (T0 ~ 0 fs) between the main FL and SH pulses. A 30-fs probe laser beam (800 nm) was used as a back lighter.

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