Plasma probing and single-shot crossed-beam energy transfer measurements using a broadband probe.

Precise knowledge of plasma conditions is critical for accurately modeling laser-plasma interactions relevant to inertial confinement fusion. Traditionally, optical Thomson scattering (OTS) has been employed to diagnose these plasmas. However, since the scattered light is seeded by random plasma fluctuations, the measured signals are typically extremely low; signal to noise can therefore be a challenge or require working with high OTS probe intensities which can in turn affect local plasma conditions.

It was recently proposed that a low energy broadband probe laser could be used to measure the properties of plasmas irradiated by a high-power laser, by having the probe couple to the high-power “pump” via plasma waves. The ion-acoustic resonances can be “imprinted” onto the probe spectrum, producing the same spectra as those obtained with OTS off ion waves and revealing the same information (plasma temperature and flow)—but with signal levels that are many orders of magnitude higher.

This concept was demonstrated at the Jupiter Laser Facility, by leveraging the recent upgrade of the Janus laser with the STILETTO system, a high-bandwidth time and frequency pulse-shaping device. The experimental campaign used CH4 and CO2 plasmas across more than 50 shots, spanning a range of pump intensities, background densities, and timing offsets between the pump and probe. Using nonlinear regression models, we extracted various plasma properties with high precision and compared them to independent measurements from the same experimental campaign. Besides its application as a plasma probe, this technique can also generate the gain curve of crossed-beam energy transfer (CBET) in a single shot (instead of multiple shots scanning the pump-probe wavelength separation), and paves the way for new experimental investigations of CBET and other laser-plasma instabilities.