Single shot, temporally and spatially resolved measurements of fast electron dynamics using a chirped optical probe

J S Green,L Wilson,R J Dance,R J Gray,D A Maclellan,P Mckenna,C D Murphy,D Rusby,N Booth

doi:10.1088/1748-0221/9/03/p03003

J S Green, L Wilson + Show 7 more

Open Access

https://doi.org/10.1088/1748-0221/9/03/p03003

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Abstract

A new approach to rear surface optical probing is presented that permits multiple, time-resolved 2D measurements to be made during a single, ultra-intense ( > 1018 W cm−2) laser-plasma interaction. The diagnostic is capable of resolving rapid changes in target reflectivity which can be used to infer valuable information on fast electron transport and plasma formation at the target rear surface. Initial results from the Astra-Gemini laser are presented, with rapid radial sheath expansion together with detailed filamentary features being observed to evolve during single shots.

Highlights

In the case of a perfect, step-like interface between the Al slab and the vacuum, or a very steep scalelengh l λ the fraction of laser light reflected can still be calculated from the Fresnel equations, with the electrical conductivity, σ (ω), largely determining the level of reflectivity: σ (ω) ωp2 4π(1/τei − iω) where ωp is the plasma frequency and τei is the electron-ion collision frequency
In the region of the laser focal spot, where filamentary-type structure is observed, a greater flux of high energy electrons on axis would be expected to seed instabilities that would grow rapidly during the laser pulse length, leading to some break-up of the beam inside the target. Such instabilities would lead to strong modulations in the electron density at the target rear surface, which can be seen imprinted into the sheath profile in figure 4
A drop in reflectivity of almost 20% can be seen over 300 fs, demonstrating a rapid change in the plasma density profile at the target rear surface

Summary

Target reflectivity

The characteristic formation of a plasma at the target rear surface during TNSA, on the order of the Debye length, is mainly a result of field-induced barrier suppression (FIBS) ionisation that occurs rapidly under the influence of intense TV/m electric fields [8]. In the case of a perfect, step-like interface between the Al slab and the vacuum, or a very steep scalelengh l λ (where λ is the wavelength of the probe laser) the fraction of laser light reflected can still be calculated from the Fresnel equations, with the electrical conductivity, σ (ω), largely determining the level of reflectivity:. Calculating the fraction of laser light absorbed and reflected requires solving Maxwell’s equations for light propagating through a known plasma density gradient at a defined angle of incidence. Celliers et al [9] used the standard Drude model together with Maxwell’s equations to calculate the reflectivity of an optical probe propagating through a range of shock-heated plasma gradients. The authors probed the target rear surface with ∼ 4 ps resolution, making 1D spatial measurements of the probe’s phase in order to extract data on the shape of the plasma expansion and fast electron distribution. By studying changes in reflectivity across the whole target surface, the structure and evolution of the electron distribution can be studied for a range of laser and target parameters, even for cases where non-uniform or highly asymmetric structures are expected

Diagnostic outline

Initial results

Findings

Conclusion