Note: Real-time monitoring via second-harmonic interferometry of a flow gas cell for laser wakefield acceleration

High-sensitivity optical tomography of instabilities in supersonic gas flow

Characterization of gas targets relies largely on conventional optical techniques, providing millisecond time resolution, which not only overlooks the fluctuations occurring at shorter time scales but also often challenges the sensitivity limits of optical probing as their refractive index is close to unity. Hence, the ability to resolve these fluctuations needs to be addressed as it is paramount for accurate gas jet characterization for their applications, including laser-matter interaction in laser wakefield electron acceleration or plasma x ray sources. In this Letter, we introduce an advanced gas jet characterization system capable of visualizing fast density fluctuations by Schlieren imaging, combined with density characterization by interferometric tomography, both with increased sensitivity due to the four-pass probing configuration. We demonstrate that combining the two modalities provides a substantial advancement in achieving a comprehensive, both quantitative and qualitative, characterization of gas jets.

Experimental study on the performances of second-harmonic dispersion interferometers at 10.6 µm and 1064 nm for plasma density measurements

Two common-path interferometers based on CO₂ and Nd:Y₃Al₅O₁₂ (Nd:YAG) lasers are benchmarked with a two-arm microwave interferometer on a hydrogen plasma produced by an RF discharge and injected into a large magnetic-confinement vessel. The ∼10¹⁹m^-2 line-integrated electron density is clearly measured in agreement by the interferometers. The frequency spectrum of the measured data is analyzed on the 20 kHz range for all interferometers and up to 600 kHz for the Nd:YAG laser-based interferometer. Mechanical vibration measurements performed on the components of the two common-path interferometers result in a peak-to-peak displacement up to about one and twenty wavelengths for the CO₂ and Nd:YAG laser-based interferometers, respectively. Such results set for the first time, to the best of our knowledge, a quantitative limit for the displacement that the two second-harmonic dispersion interferometers can sustain while still providing a high sensitivity for accurate plasma density measurements.

Widefield quantitative phase imaging by second-harmonic dispersion interferometry

Widefield optical characterization of transparent samples is of great importance for gas flow and plasma diagnostics, for example, as well as label-free imaging of biological samples. An optically transparent medium, however, cannot be imaged by techniques based on intensity contrast imaging. Very well-known qualitative phase-contrast imaging methodologies are routinely used to overcome this limitation, and quantitative phase-imaging approaches have also been developed. Here we report the demonstration of, to the best of our knowledge, a novel widefield quantitative phase-imaging technique, based on fully common-path second-harmonic dispersion interferometry that is combined with pixel-by-pixel homodyne dual-channel polarization-dependent phase detection. The device is tested in a harsh environment reaching sub-10 mrad harmonic phase dispersion sensitivity and a spatial resolution of several tens of microns with an optical configuration that is very stable and easy to implement. The time resolution of the demonstrated device is 600 ps, set by the laser-pulse time duration.

Density characterization of discharged gas-filled capillaries through common-path two-color spectral-domain interferometry

Electrically discharged plasma structures, typically several centimeters in length and sub-millimeter in diameter, have been applied to guide laser pulses in laser plasma accelerators and to focus ion and relativistic electron beams in compact, radially symmetric transport configurations. Knowledge of the on-axis plasma density is critical. Traditional density interferometry has been ineffective for these laser-machined structures, while group velocity delay (GVD) techniques involve combining two laser paths with corresponding alignment complexities and stability sensitivities. Here the GVD technique is advanced to a common-path two-color interferometer configuration performed in the spectral domain of a broad-bandwidth femtosecond laser. Multi-shot tracking of the phase is not required, and the common path assures improved stability. This in situ technique was validated on 15 mm long plasma structures, measuring electron densities of 10¹⁷-10¹⁸  cm^-3 for various fill pressures.

Novel gas target for laser wakefield accelerators

A novel gas target for interactions between high power lasers and gaseous medium, especially for laser wakefield accelerators, has been designed, manufactured, and characterized. The gas target has been designed to provide a uniform density profile along the central gas cell axis by combining a gas cell and slit nozzle. The gas density has been tuned from ∼10¹⁷ atoms/cm³ to ∼10¹⁹ atoms/cm³ and the gas target length can be varied from 0 to 10 cm; both changes can be made simultaneously while keeping the uniform gas profile. The gas density profile inside the gas cell has been measured using interferometry and validated using computational fluid dynamics.