Laser-Plasma Accelerator Workshop 2019

Estimation of the preplasma scale length via time-resolved spectroscopy of back-reflected light

Nicht eingeplant

20m

MedILS

Poster Contribution Laser-driven ion acceleration

Sprecher

Johannes Hornung (GSI Helmholtzzentrum für Schwerionenforschung)

Beschreibung

Many of the underlying mechanisms for laser-driven ion acceleration exploit the interaction of an ultra-intense laser pulse with sub-micrometer thin foils. These mechanisms rely on well-defined plasma conditions at the time of the maximum laser intensity. These conditions, especially the preplasma scale length, are extremely hard to measure and remain mostly not known, which prevents a detailed study and an efficient use of these acceleration mechanisms.

During this interaction, a part of the laser pulse is reflected back at the critical plasma density and carries important information about the interaction process itself. The spectrum is modulated due to effects such as relativistic self-phase-modulation and is additionally Doppler-shifted by the moving critical density occurring during hole boring or plasma expansion. The interplay between these effects is intimately related to the plasma density gradient in the vicinity of the reflection point, as a shallow plasma gradient will favor hole boring, leading to a red Doppler-shift or instance, whereas a steep plasma gradient will impose a strong electron-pressure, counteracting the laser pressure.

To study these effects and corresponding time scales, a diagnostic for back-reflected light based on frequency resolved optical gating (FROG) has been commissioned at the PHELIX facility at GSI Helmholtzzentrum für Schwerionenforschung GmbH, where intensities above 10${}^{20}$ W/cm² and pulses with ultra-low temporal pedestal are available. We have conducted measurements for different plasma conditions: at first with the standard temporal profile of the laser pulse, then with a double plasma mirror setup that dramatically steepens the pulse. A supplementary characterization of the target state was made by measuring the transmitted pulse, showing that the plasma mirror enables 50- nm thick targets to remain opaque throughout the interaction.

As a support to the experimental data, we have performed 1D and 2D simulations using the particle-in-cell code EPOCH, with parameters as close as possible to the experiment, including a pre-expanded target. We varied the scale length of the plasma and monitored its effect on the spectrum of the back-reflected pulse. With decreasing scale-length around or below 1 µm, a transition from a red shifted spectrum to a blue shifted one at even higher gradients is visible, as observed experimentally.

We believe that this method can deliver some estimates on the preplasma expansion on a sub-micrometer scale, a spatial range which can be hardy covered by other experimental methods like shadowgraphy or interferometry (though more complex Frequency Domain Interferometry can access similar ranges). This result is of particular interest for the understanding of experiments aiming at laser-driven ion acceleration, which mostly rely on unexpended foils to maximize the acceleration process.