In-depth study of intra-Stark spectroscopy in the x-ray range in relativistic laser–plasma interactions

Abstract

Intra-Stark Spectroscopy (ISS) is the spectroscopy within the quasistatic Stark profile of a spectral line. In the ISS some local depressions ('dips') occur at certain locations of the quasistatic Stark profile of a spectral line. This phenomenon arises when radiating atoms/ions are subjected simultaneously to a quasistatic field F and to a quasimonochromatic electric field E(t) at the characteristic frequency. The present paper advances the study of the relativistic laserplasma interaction from our previous paper (Oks et al, Optics Express 25 (2017) 1958). First, by improving the experimental conditions and the diagnostics, it provides a systematic spectroscopic study of the simultaneous production of the Langmuir waves and of the ion acoustic turbulence at the surface of the relativistic critical density. It demonstrates a reliable reproducibility of the Langmuir-wave-induced dips at the same locations in the experimental profiles of Si XIV Lybeta line, as well as of the deduced parameters (fields) of the Langmuir waves and ion acoustic turbulence in different laser shots. Second, this study employs for the first time the most rigorous condition of the dynamic resonance, on which the ISS phenomenon is based, compared to all previous studies in all kinds of plasmas in a wide range of electron densities. It shows how different interplays between the Langmuir wave field and the field of the ion acoustic turbulence lead to distinct spectral line profiles, including the disappearance of the Langmuir-wave-induced dips.

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

“…The experiments were performed at Vulcan Petawatt (PW) facility at the Rutherford Appleton Laboratory [12,13], which provides a beam using optical Parametric, Chirped Pulse Amplification (OPCPA) technology with a central wavelength of 1054 nm and a pulse Full-Width-Half-Maximum (FWHM) duration, which could be up to 1500 fs. The OPCPA approach associated with a plasma mirror enables an amplified spontaneous emission to the peak-intensity contrast ratio exceeding 10 −11 several nanoseconds before the peak of the laser pulse [15][16][17].…”

“…In the profiles from shots A, B, and C, there are clearly seen 'bump-dip-bump' structures (both in the red and blue parts of the profiles) typical for the L-dips phenomenon. The L-dips are not observed in shot D. The following parameters provided the best fit: (a) N e = 2.2 × 10 22 cm −3 , T = 600 eV, F t,rms = 4.8 GV/cm, E 0 = 0.7 GV/cm; (b) N e = 2.2 × 10 22 cm −3 , T = 550 eV, F t,rms = 4.4 GV/cm, E 0 = 0.5 GV/cm; (c) N e = 2.2 × 10 22 cm −3 , T = 600 eV, F t,rms = 4.9 GV/cm, E 0 = 0.6 GV/cm; (d) N e = 6.6 × 10 21 cm −3 , T = 550 eV, F t,rms = 2.0 GV/cm, E 0 = 2.0 GV/cm [13]. Figure 6.…”

“…Up to now, it seems to be the first experimental profile diagnosed in relativistic laser-plasma Figure 6. Experimental profiles of the Si XIV Ly-beta line in shots A, B, C (blue color in the online version) and their comparison with simulations using code FLYCHK (red color in the online version) at the electron density N e = 6 × 10 23 cm −3 and the temperature T = 500 eV [13].…”

“…In these experiments the L-dips observed were caused by Langmuir waves resulting from the two-plasmon decay instability. The present review is totally dedicated to experimental studies performed with incident laser intensities as high as 10 20 -10 21 Wcm −2 and generating strongly relativistic laser-plasma interactions [12,13]. It advances the significant improvements developed at RAL, using the Optical Parametric Chirped Pulse Amplification technology (OPCPA) for the laser pulse generation and a plasma mirror for increasing the laser contrast and as a consequence diminishing the importance of the laser pre-plasma.…”

“…The reproducibility of the interpretation of spectroscopic results obtained from different shots of about the same laser intensity allowed the demonstration of the simultaneous production of the Langmuir high frequency waves and, for the first time, the ion acoustic low frequency waves, in high-density plasmas. Both kinds of waves have been predicted and attributed to the development of the non-linear physical process Parametric Decay Instability (PDI) developing at the surface of the relativistic critical density [13,14].…”