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Optical Thomson scattering is now a mature diagnostic tool for precisely measuring local plasma density and temperature. These measurements typically take advantage of a simplified analytical model of the scattered spectrum, which is built upon the assumption that each plasma species is in thermal equilibrium. However, this assumption fails for most laboratory plasmas of interest, which are often produced through high field ionization of atoms via ultrashort laser pulses and vulnerable to several kinetic instabilities. While it is possible to analytically model the Thomson scattered spectrum for some non-Maxwellian distribution functions, it is often not practical to do so for laboratory plasmas with highly complex and unstable distribution functions. We present a new method for predicting the Thomson scattered spectrum from any plasma directly from fully kinetic particle-in-cell simulations. This approach allows us to model the contributions of kinetic instabilities to the Thomson spectrum that aren’t taken into account in Maxwellian theory. We demonstrate this method’s capability to capture nonthermal features in the Thomson spectrum by simulating a simple bumpon- tail plasma as well as a more complex laser-ionized plasma. The versatility of this approach makes it an effective aid in the experimental design of Thomson diagnostics to directly characterize kinetic instabilities in laboratory plasmas. Index Terms—plasma measurement, low-temperature plasmas, plasma diagnostics, plasma simulation, plasma stability, plasma density, plasma temperature 

Observation of lasing signals at multiple wavelengths in argon atoms pumped by a 3-photon resonance with an intense 260 nm laser were used to observe the dynamics of electronic excited states. 

Absolute density measurements of low-ionization-degree or low-density plasmas ionized by lasers are very important for understanding strong-field physics, atmospheric propagation of intense laser pulses, Lidar etc. A cross-polarized common-path temporal interferometer using balanced detection was developed for measuring plasma density with a sensitivity of ∼0.6 mrad, equivalent to a plasma density-length product of ∼2.6 × 10¹³cm⁻²if using an 800 nm probe laser. By using this interferometer, we have investigated strong-field ionization yield versus intensity for various noble gases (Ar, Kr, and Xe) using 800 nm, 55 fs laser pulses with both linear (LP) and circular (CP) polarization. The experimental results were compared to the theoretical models of Ammosov-Delone-Krainov (ADK) and Perelomov-Popov-Terent’ev (PPT). We find that the measured phase change induced by plasma formation can be explained by the ADK theory in the adiabatic tunneling ionization regime, while PPT model can be applied to all different regimes. We have also measured the photoionization and fractional photodissociation of molecular (MO) hydrogen. By comparing our experimental results with PPT and MO-PPT models, we have determined the likely ionization pathways when using three different pump laser wavelengths of 800 nm, 400 nm, and 267 nm.