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Register a new userLaser Absorption Measurements of Electron Density in Nanosecond-ScaleAtmospheric Pressure Pulsed Plasmas
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We report on time-resolved measurements of electron number density by continuous-wave laser absorption in a low-energy nanosecond-scale laser-produced spark in atmospheric pressure air. Laser absorption is a result of free-free and bound-free electron excitation, with the absorption coefficient modeled and evaluated using estimates of the time-variation in electron temperature and probe laser absorption path length. Plasma electron number densities are determined to be as high as $n_text{e}=7times10^{19}$ cm$^{-3}$, and decay to $1/e$ of their peak values over a period of about 50 ns following plasma formation using a 20 mJ, 10 ns pulse width frequency-doubled Nd:YAG laser. The measured plasma densities at later times are shown to be in reasonable agreement with Stark broadening measurements of the 3s[$^5S{^o}$]-3p[$^5P$] electronic transition in atomic oxygen at 777 nm. This study provides support for the use of such continuous wave laser absorption for time resolved electron density measurements in low energy spark discharges in air, provided that an estimate of the electron temperature and laser path length can be made by accompanying diagnostics.
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In this work, Rayleigh microwave scattering was utilized to measure the electron number density produced by nanosecond high voltage breakdown in air between two electrodes in a pin-to-pin configuration (peak voltage 26 kV and pulse duration 55 ns). The peak electron density decreased from 1*10^17 cm^-3 down to 7*10^14 cm^-3 when increasing the gap distance from 2 to 8 mm (total electron number decreased from 2*10^13 down to 5*10^11 respectively). Electron number density decayed on the timescale of about several microseconds due to dissociative recombination.
This work proposes a novel method of Thomson microwave scattering for electron number density measurements of miniature plasmas at pressures < 10 Torr. This method is applied to determine electron number density in a positive column of glow discharge initiated at 5 Torr in air with a plasma column diameter of 3.4 mm. The Thomson Microwave Scattering(TMS) system measured the electron number density to be 3.36*10^10 cm^-3. The result obtained using the TMS system was validated against the measurements made using the well-known technique of microwave quarter-wave hairpin resonator. Measurements with the hairpin resonator yielded an electron number density of 2.07*10^10 cm^-3 providing adequate agreement with the TMS system.
Betatron x-ray sources from laser-plasma accelerators combine compactness, high peak brightness, femtosecond pulse duration and broadband spectrum. However, when produced with Terawatt class lasers, their energy was so far restricted to a few kilo-electronvolt (keV), limiting the range of possible applications. Here we present a simple method to increase the energy and the flux by an order of magnitude without increasing the laser energy. The orbits of the relativistic electrons emitting the radiation were controlled using density tailored plasmas so that the efficiency of the Betatron source is significantly improved.
We carry out simulations of laser plasmas generated during UV nanosecond pulsed laser ablation of the chalcogens selenium (Se) and tellurium (Te), and compare the results to experiments. We take advantage of a 2D-axisymmetric, adaptive Cartesian Mesh (ACM) framework that enables plume simulations out to centimeter distances over tens of microseconds. Our model and computational technique enable comparison to laser-plasma applications where the long-term behavior of the plume is of primary interest, such as pulsed laser synthesis and modification of materials. An effective plasma absorption term is introduced in the model, allowing the simulation to be constrained by experimental time-of-flight kinetic energy distributions. We show that the effective simulation qualitatively captures the key characteristics of the observed laser plasma, including the effect of laser spot size. Predictions of full-scale experimentally-constrained Se and Te plasmas for 4.0 J/cm$^2$ laser fluence and 1.8 mm$^2$ circular laser spot area show distinct behavior compared to more commonly studied copper (Cu) plumes. The chalcogen plumes have spatial gradients of plasma density that are steeper than those for Cu by up to three orders of magnitude. Their spatial ion distributions have central bulges, in contrast to the edge-only ionization of Cu. For the irradiation conditions explored, the range of plasma temperatures for Se and Te is predicted to be higher than for Cu by more than 0.50 eV.
The production of polarized proton beams with multi-GeV energies in ultra-intense laser interaction with targets is studied with three-dimensional Particle-In-Cell simulations. A near-critical density plasma target with pre-polarized proton and tritium ions is considered for the proton acceleration. The pre-polarized protons are initially accelerated by laser radiation pressure before injection and further acceleration in a bubble-like wakefield. The temporal dynamics of proton polarization is tracked via the T-BMT equation, and it is found that the proton polarization state can be altered both by the laser field and the magnetic component of the wakefield. The dependence of the proton acceleration and polarization on the ratio of the ion species is determined, and it is found that the protons can be efficiently accelerated as long as their relative fraction is less than 20%, in which case the bubble size is large enough for the protons to obtain sufficient energy to overcome the bubble injection threshold.
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