First direct observation of a wakefield generated with structured light

Abstract

The use of structured light to control the phase velocity of the wake in laser-wakefield accelerators has generated significant interest for its ability to mitigate electron dephasing. Combining the diffraction-free properties of Bessel beams with spatio-temporal shaping of the pulse promises to enable acceleration with an unprecedented combination of long acceleration lengths and high gradients. This would facilitate the acceleration of electrons to energies above 100 GeV in existing laser facilities. In-depth understanding of the physical mechanisms involved is critical to achieving dephasing-free electron acceleration. Here we present the first experimental observation of wakefields generated by beams that were spatio-temporally sculpted and then focused with a long-focal-depth mirror, known as an axiparabola, which generates a quasi-Bessel beam. The resulting wakefield was imaged using femtosecond relativistic electron microscopy. Novel insights into this minimally explored regime include mapping the wakefield development over the focal depth and studying the effects of spatio-temporal manipulations of the beam on the structure and phase velocity of the wakefield. Such insights pave the way towards realizing the potential of structured-light based solutions to dephasing in laser-wakefield acceleration.

Figures (9)

• Figure 1: (a) Schematic of the FREM experimental setup. Beam 1 is focused by the axiparabola onto jet 1, generating the novel laser wakefield. Beam 2 is focused by a parabola onto jet 2, generating a second laser wakefield which accelerates a femtosecond duration electron bunch, known as the "probe electron bunch". This probe bunch is then allowed to expand and impinges onto the axiparabola-generated wakefield, which imposes momentum changes onto the probe electrons. After further propagation, these momentum changes turn into density modulations which are then imaged when the electrons hit a YAG screen. (b) 2D axiparabola focal spots, measured in vacuum, at specified points along the focal depth relative to the beginning of the focal line 480mm away from the axiparabola.
• Figure 2: (a--c) Experimentally obtained FREM images at different relative points $\Delta L$ along the focal depth showing the development of the V-shaped structure. (d--f) Simulated FREM images for similar experimental conditions and at similar points in the nozzle. Image (d) corresponds to the depth of 3.95 mm inside the nozzle (0.95 mm after the beginning of the focal line) in the PIC simulations. (g--i) Relative electron density distribution $n_\mathrm{e} / n_0$ in the simulated wakefields that correspond to the FREM images in (d--f). The color scale in (a--f) shows the relative intensity of the signal on the screen where $0$ corresponds to the intensity of the unperturbed probe beam. The vertical lines in (d--i) show the plasma period corresponding to $n_0 = 5e17cm^{-3}$. The red color in (g--i) shows the intensity of the axiparabola laser field.
• Figure 3: (a--b) Experimentally obtained FREM images at the same point along the focal depth for PFC values of $-0.002$ and 0.020fs/mm^2, respectively. (c) Simulated FREM images for PFC values -0.002fs/mm^2 (top) and 0.020fs/mm^2 (bottom) and the (d) corresponding relative electron density distribution $n_\mathrm{e}/n_0$ in PIC simulations. The color scale in (a--c) shows the relative intensity of the signal where $0$ is the unperturbed beam intensity. The red color in (d) shows the intensity of the axiparabola laser field.
• Figure 4: Distributions of the laser field intensity of the axiparabola-focused pulse and relative electron density $n_\mathrm{e} / n_0$ in the excited wake at the propagating distance of 5.5mm (a) with and (b) without plasma. The dashed line shows the dependence of the position of the peak intensity on the transverse coordinate.
• Figure 5: Comparison of the spatial distribution of the transverse electric field $E_x$ between (a) the PIC simulation and (b) the calculated linear solution. (c) Transverse distribution of $E_x$ for a slice $z - ct = 114\um$ shown with vertical lines in (a--b). Comparison of the longitudinal electric field $E_z$ for (d) the PIC simulation and (e) the linear solution. (f) Longitudinal distributions of $E_z$ at the off-axis coordinate $x = 23\um$ for the PIC (solid blue) and linear (dashed orange) and at the center of the wake for the PIC (green dotted) and linear (purple dashed-dotted). The red color in (a--b, d--e) shows the distribution of the laser pulse intensity.