Mini-Review of Intra-Stark X-ray Spectroscopy of Relativistic Laser–Plasma Interactions

Abstract

Intra-Stark spectroscopy (ISS) is the spectroscopy within the quasi-static Stark profile of a spectral line. The present paper reviews the X-ray ISS-based studies recently advanced for the diagnostics of the relativistic laser–plasma interactions. By improving experiments performed on the Vulcan Petawatt (PW) laser facility at the Rutherford Appleton Laboratory (RAL), the simultaneous production of the Langmuir waves and of the ion acoustic turbulence at the surface of the relativistic critical density gave the first probe by ISS of the parametric decay instability (PDI) predicted by PIC simulations. The reliable reproducibility of the experimental signatures of PDI—i.e., the Langmuir-wave-induced dips—allowed measurements of the fields of the Langmuir and ion acoustic waves. The parallel theoretical study based on a rigorous condition of the dynamic resonance depending on the relative values of the ion acoustic and the Langmuir fields could explain the disappearance of the Langmuir dips as the Langmuir wave field increases. The ISS used for the diagnostic of the PDI process in relativistic laser–plasma interactions has reinforced the reliability of the spectral line shape while allowing for all broadening mechanisms. The results can be used for a better understanding of intense laser–plasma interactions and for laboratory modelling of physical processes in astrophysical objects.

Analysis of Plasma Emission Experiments and ‘Dips’

“…Recent experiments [9,10] using high-powered lasers claim to show dip structures in the Lyman β(Ly − β) and γ(Ly − γ) lines in plasmas that can be used to reliably infer from these both plasmas parameters and information on high and low-frequency turbulence.…”

“…• when the spectra were recorded (how long after the pulse); • whether optical thickness was an issue (Ref. [10] mentions that FLYCHECK-not the analysis performed-accounted for opacity, if necessary); • homogeneity of the target area producing the radiation, although Figure 7 of Ref. [10] shows the electron density varying greatly along the target length.…”

“…In terms of spectral lines, the H-like Si Ly − β line was mostly used, although the papers mention and show Ly − γ lines, also exhibiting "dips". However, no mention was made of whether the parameters inferred from the Ly − β and Ly − γ [10] (for the same shot) are consistent. It is also not clear how many shots there were.…”

“…It is reasoned that the static field could be due to static ionic fields and/or low-frequency turbulence, and the oscillatory fields could be due to an external laser and/or high frequency (such as Langmuir) turbulence. The purpose of the present work is to review relevant experiments and analysis [9,10] using the Lyman β(Ly − β) and γ(Ly − γ) lines of H-like Si in hot, dense plasma experiments. Specifically, the analysis of these experiments claims that the plasma can be reliably diagnosed in terms of observed (although not clearly above the noise level) "dips".…”

X-ray Spectroscopy Based Diagnostic of GigaGauss Magnetic Fields during Relativistic Laser-Plasma Interactions

“…In the present short communication, we propose a method for measuring GG magnetic fields based on the phenomenon of Langmuir-wave-cased dips (L-dips) in X-ray line profiles. The L-dips were observed in several experimental spectroscopic studies of relativistic laser-plasma interactions-see, e.g., papers [3,7] and review [8].…”