Emittance preserving thin film plasma mirrors for GeV scale laser plasma accelerators

Laser-plasma accelerators (LPAs) now routinely produce electron beams with GeV energies over acceleration lengths on the order of a few centimeters. This capability and the demonstration of multistage coupling make LPAs promising for numerous applications. However, beam quality preservation in multistage accelerators remains an obstacle because of the need to separate the laser pulse from the electron beam. Plasma mirrors can be used to redirect the laser pulse, but their substrate thickness threatens to substantially degrade the electron beam emittance. Ultrathin ($\sim 20\mathrm{nm}$) liquid crystal (LC) plasma mirrors are an excellent candidate to address this challenge. This work investigates the robustness of thin LC plasma mirrors in the presence of capillary discharge plasma and an auxiliary heater laser. We find they can be operated $\sim 10\mathrm{cm}$ from the capillary exit when a heater laser is used. We then performed a normalized emittance measurement enabled using a 20 nm LC plasma mirror to protect electron beam optics after the LPA. The emittance contribution from scattering through the plasma mirror is calculated to be of order 100 nm, much less than the measured emittance of $4.0\mu \mathrm{m}$. Finally, we develop a model to calculate plasma mirror performance based on the laser polarization and intensity, and plasma mirror thickness.

Figure 1

A schematic of the survivability experimental setup, including the heater, capillary, and probe laser is shown.

Figure 2

These are representative probe images of 40 nm films at three different delay values: 0, 1, and $150\mu \mathrm{s}$. An image of an undisturbed film is also displayed for comparison.

Figure 3

Time at which damage occurred vs the heater laser fluence at the surface of the LC film. The blue points are for data collected with a focused heater beam incident on the film without a capillary present. The green point shows data collected at higher energy with a defocused heater. The Red point is a scan done with the heater and capillary discharge both active with the film 6 cm from the capillary exit. A representative scan is provided to show the distortions that appear when the film is damaged. The error bar represents the range of measured heater delays for a set of shots where the delay was known to be constant.

Figure 4

Graph of normalized emittance after transmission through a plasma mirror as a function of initial normalized emittance. All curves are calculated using the method described in Ref. [6]. The effect of scattering through two different types of plasma mirror (20 nm LC and $20\mu \mathrm{m}$ Kapton) is compared for five discrete initial divergence values. Initial divergence is labeled on each curve.

Figure 5

Characteristic “bowtie” measurement of transverse electron distribution vs energy (a). Simulation using particle tracking and assuming a carbon based scatterer between the LPA and APL with thicknesses of 20 nm (b) and $20\mu \mathrm{m}$ (c). Energy dependent beam size vs energy (d) for the measurement in (a) (green line), the simulation in (b) (blue line), and the simulation in (c) (yellow line).

Figure 6

A schematic overview of algorithm used to calculate reflection, transmission, and absorption is shown.

Figure 7

(a) Reflectance vs intensity for s-polarization at 15° incidence as calculated by the model described in this article (blue) is shown in comparison to simulations from Ref. [16] (black). (b) Transmittance vs intensity for both s (solid orange curve) and p (solid blue curve) polarizations at 45° incidence again calculated with the model described here. These curves are compared to the transmission data taken in the experiment in Sec. 3 with a focused (purple circles) and defocused (yellow circles) beam incident on the plasma mirror.