Understanding turbulence and associated density fluctuations in high-energy-density plasmas is crucial for inertial confinement fusion, astrophysical flows, and laboratory plasma physics. However, direct experimental characterization of the wavenumber-resolved density fluctuation spectrum—particularly from the inertial range down to dissipation scales—remains a challenging task. Here we report on the development and application of collective Thomson scattering (CTS) in a novel platform based on grid-structured foil-foil collision.
In the experiment, two CH foils are irradiated by a high-power laser, driving the unablated portions to form counter-propagating plasma slabs. The slabs pass through a metallic grid, which imprints a density modulation onto the plasma, and subsequently collide head-on, generating a stagnation layer where strong shear, compression, and turbulence develop. The plasma parameters in the stagnation region are n_e∼10¹⁹-10²⁰  cm^-3 and T_e∼10-100 eV. A frequency-tripled probe beam (λ₀=351 nm) is used to perform CTS with a fixed scattering geometry, corresponding to a well-defined wavenumber k. The scattered spectra provide direct access to the density fluctuation power ∣δn_e(k)∣² via the low-frequency resonance near ω≈0.
During the collision, we observe a transient burst of scattered signal lasting approximately 1 ns, occurring at a well-defined time window after the onset of stagnation. The signal intensity exceeds the thermal fluctuation level by more than two orders of magnitude. Notably, the spectral width reaches several nanometers—far exceeding the expected ion-acoustic shift (∼0.2 nm)—implying a characteristic velocity ∼3×10⁸ cm/s, comparable to the thermal velocity of 100 eV electrons.
This unexpectedly broad and intense signal may originate from either a superthermal electron population or an extreme level of turbulent velocity fluctuations (δv∼10⁸ cm/s). Possible interpretations include non-collective scattering from a hot electron tail, strong Doppler broadening due to large-amplitude turbulence, or transient electrostatic shock formation in the stagnation layer. Distinguishing among these scenarios will require systematic wavenumber scanning (by varying the scattering angle) and correlated measurements with optical imaging and interferometry.
This work demonstrates the capability of collective Thomson scattering to diagnose non-thermal density fluctuations in a grid-structured foil-foil collision platform, providing an experimental basis for further studies of turbulence, particle acceleration, and energy dissipation in high-energy-density plasmas.Understanding turbulence and associated density fluctuations in high-energy-density plasmas is crucial for inertial confinement fusion, astrophysical flows, and laboratory plasma physics. However, direct experimental characterization of the wavenumber-resolved density fluctuation spectrum—particularly from the inertial range down to dissipation scales—remains a challenging task. Here we report on the development and application of collective Thomson scattering (CTS) in a novel platform based on grid-structured foil-foil collision.
In the experiment, two CH foils are irradiated by a high-power laser, driving the unablated portions to form counter-propagating plasma slabs. The slabs pass through a metallic grid, which imprints a density modulation onto the plasma, and subsequently collide head-on, generating a stagnation layer where strong shear, compression, and turbulence develop. The plasma parameters in the stagnation region are n_e∼10¹⁹-10²⁰  cm^-3 and T_e∼10-100 eV. A frequency-tripled probe beam (λ₀=351 nm) is used to perform CTS with a fixed scattering geometry, corresponding to a well-defined wavenumber k. The scattered spectra provide direct access to the density fluctuation power ∣δn_e(k)∣² via the low-frequency resonance near ω≈0.
During the collision, we observe a transient burst of scattered signal lasting approximately 1 ns, occurring at a well-defined time window after the onset of stagnation. The signal intensity exceeds the thermal fluctuation level by more than two orders of magnitude. Notably, the spectral width reaches several nanometers—far exceeding the expected ion-acoustic shift (∼0.2 nm)—implying a characteristic velocity ∼3×10⁸ cm/s, comparable to the thermal velocity of 100 eV electrons.
This unexpectedly broad and intense signal may originate from either a superthermal electron population or an extreme level of turbulent velocity fluctuations (δv∼10⁸ cm/s). Possible interpretations include non-collective scattering from a hot electron tail, strong Doppler broadening due to large-amplitude turbulence, or transient electrostatic shock formation in the stagnation layer. Distinguishing among these scenarios will require systematic wavenumber scanning (by varying the scattering angle) and correlated measurements with optical imaging and interferometry.
This work demonstrates the capability of collective Thomson scattering to diagnose non-thermal density fluctuations in a grid-structured foil-foil collision platform, providing an experimental basis for further studies of turbulence, particle acceleration, and energy dissipation in high-energy-density plasmas.

Thomson scattering,turbulence,plasma,collision