A diffraction-limited laser interacts with a plasma whose conditions are uniform on the scale of the focused laser spot. Two distinct, narrow waves are observed in the backscattered spectrum with phase velocities of $v_{\phi }{/v}_e=1.4\mathrm{}\pm 0.08$ and $4.2\pm 0.1$, where $v_e$ is the electron thermal speed. The high-velocity wave is ordinary stimulated Raman scattering (SRS) from a Langmuir wave. The low-velocity wave corresponds to stimulated scattering from an electron-acoustic wave (SEAS), and implies strong electron trapping. Previous SRS data from low-density plasmas are reinterpreted in terms of SEAS.

We have demonstrated efficient coupling of 0.35 $\mu \mathrm{m}$ laser light for radiation production in inertial confinement fusion (ICF) cavity targets. Temperatures of 270 eV are measured in cavities used for implosions and 300 eV in smaller cavities, significantly extending the temperature range attained in the laboratory to those required for high-gain indirect drive ICF. High-contrast, shaped drive pulses required for implosion experiments have also been demonstrated for the first time. Low levels of scattered light and fast electrons are observed, indicating that plasma instability production is not significant.

Our understanding of laser-plasma instability (LPI) physics has improved dramatically over the past two decades through advancements in experimental techniques, diagnostics, and theoretical and modeling approaches. We have progressed from single-beam experiments—ns pulses with ∼kJ energy incident on hundred-micron-scale target plasmas with ∼keV electron temperatures—to ones involving nearly 2 MJ energy in 192 beams onto multi-mm-scale plasmas with temperatures ∼4 keV. At the same time, we have also been able to use smaller-scale laser facilities to substantially improve our understanding of LPI physics and evaluate novel approaches to their control. These efforts have led to a change in paradigm for LPI research, ushering in an era of engineering LPI to accomplish specific objectives, from tuning capsule implosion symmetry to fixing nonlinear saturation of LPI processes at acceptable levels to enable the exploration of high energy density physics in novel plasma regimes. A tutorial is provided that reviews the progress in the field from the vantage of the foundational LPI experimental results. The pedagogical framework of the simplest models of LPI will be employed, but attention will also be paid to settings where more sophisticated models are needed to understand the observations. Prospects for the application of our improved understanding for inertial fusion (both indirect- and direct-drive) and other applications will also be discussed.

Single hot spot experiments offer several unique opportunities for developing a quantitative understanding of laser-plasma instabilities. These include the ability to perform direct numerical simulations of the experiment due to the finite interaction volume, isolation of instabilities due to the nearly ideal laser intensity distribution, and observation of fine structure due to the homogeneous plasma initial conditions. Experiments performed at Trident in the single hot spot regime have focused on the following issues. First, the intensity scaling of stimulated Raman scattering (SRS) for classically large damping regimes ${\mathrm{(k\lambda }}_D\text{=0.35)}$ was examined, and compared to classical SRS theory. SRS onset was observed at intensities much lower than expected ${\text{(2×10}}^{15}\hspace{0.17em}{\mathrm{W/cm}}^2\mathrm{),}$ from which nonclassical damping is inferred. Second, Thomson scattering was used to probe plasma waves driven by SRS, and structure was observed in the scattered spectra consistent with multiple steps of the Langmuir decay instability. Finally, scattering from a plasma wave was observed whose frequency and phase velocity are between an ion acoustic wave and an electron plasma wave. The presence of this wave cannot be explained by linear Landau theory, and it is shown to be consistent with a BGK-type mode due to electron trapping.

The observation of energy transfer between two frequency-mismatched laser beams crossing in a low-density, large-scale plasma is described. The beams cross at an angle of 53° from parallel, with the average intensity in the blue beam equal to $2\times {10}^{15}$ W/c${\mathrm{m}}^2$. A redshifted probe beam is amplified by interaction with the blue beam and a stimulated ion acoustic wave. Significant energy is transferred to the probe when the difference in the frequencies of the two beams is comparable to the frequency of the acoustic wave. As much as 52% of the pump energy is transferred to the probe.

Thomson scattering is used to measure Langmuir waves (LW) driven by stimulated Raman scattering (SRS) in a diffraction limited laser focal spot. For SRS at wave numbers $k{\lambda }_D\lesssim 0.29$, where $k$ is the LW number and ${\lambda }_D$ is the Debye length, multiple waves are detected and are attributed to the Langmuir decay instability (LDI) driven by the primary LW. At $k{\lambda }_D\gtrsim 0.29$, a single wave, frequency-broadened spectrum is observed. The transition from the fluid to the kinetic regime is qualitatively consistent with particle-in-cell simulations and crossing of the LDI amplitude threshold above that for LW self-focusing.