Short-pulse, laser–plasma interaction including collisions and ionization

Wallace, Jessica; Forslund, D. W.; Kindel, J. M.; Olson, G. L.; Comly, J. C.

doi:10.1063/1.859601

Cited by 17 publications

(2 citation statements)

References 13 publications

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“…To understand the origin of the initial T\\ and its further increase with time, simulations were carried out using the 2D fully relativistic, electromagnetic particle-incell code WAVE which includes collisional and tunneling ionization [19]. Simulations were designed to isolate the roles of parametric instabilities, space-charge effects and refraction, and the Weibel instability.…”

Section: Plasma Physics Aspects Of Tunnel-ionized Gasesmentioning

confidence: 99%

Plasma physics aspects of tunnel-ionized gases

et al. 1992

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Tunnel-ionized plasmas have been studied through experiments and particle simulations. Experimentally, x-ray measurements show that the plasma temperature is higher for a circularly polarized laserproduced plasma compared to when linear polarization is used. A higher parallel temperature than expected from the single-particle tunneling model was observed through Compton scattering fluctuation spectra. Simulations indicate that stochastic heating and the Weibel instability play an important role in plasma heating in all directions and isotropization. PACS numbers: 52.40.Nk, 52.50Jm The ionization of atoms by strong electromagnetic fields can in the high-intensity and/or long-wavelength limit be modeled as a process in which an electron tunnels through the Coulomb barrier suppressed by the electric field [1]. This model is valid when 7 = (£ , ion /20 A) ) l/2 « 1, where E\ on is the ionization potential of the charge state under consideration and <£> p is the ponderomotive potential of the laser. Although tunneling ionization of single atoms has been studied with both 10-and 1-^m laser pulses [2,3], no detailed study of macroscopic plasmas produced using this mechanism has been made. These plasmas may be unique because the laser intensity profile /(r,/) and polarization could be used to determine the initial parallel and perpendicular temperatures (T\\,T ± ) of the electrons, density «, and ionization state Z. Such plasmas have applications in the areas of recombination x-ray lasers [4] and various collective accelerator schemes [5]. Moreover, the possibility of tailoring the initial 3D distribution functions may allow the study of basic kinetic and parametric instability theory issues in plasma physics. In this Letter we explore the plasma physics aspects of gases ionized via tunneling ionization through experiments and supporting particle-in-cell computer simulations. Our experimental work shows that in the "plasma regime" T\\ is higher than expected from the singleparticle tunneling model and that ionization-induced refraction clamps the density to n < 10 ~3n c . Here n ( is the critical density. Simulations indicate that stochastic heating [6] and the Weibel [7] instability play a crucial role in plasma heating and isotropization.In the experiment, a CO2 laser beam (up to 100 J contained in an approximately triangular pulse having a 150-ps rise time and a 350-ps fall time) was focused to a spot size 2w>o of 340 fum into a vacuum chamber containing up to 5 Torr of Ar or H2 gas, using an //9 parabolic mirror. The peak laser intensity in vacuum was around 3xl0' 4 W/cm 2 . At this intensity, an estimate based on Gauss' law shows that, for fill pressures P exceeding 1 mTorr, the space-charge potential is large enough to confine most of the electrons against the ponderomotive potential of the laser. The space-charge-dominated plasma was produced over approximately two Rayleigh lengths, 2zo, and was diagnosed by (a) viewing the for-ward laser harmonic emission, (b) collective Thomson scattering of a 0.5-/im beam to...

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Section: Plasma Physics Aspects Of Tunnel-ionized Gasesmentioning

confidence: 99%

Plasma physics aspects of tunnel-ionized gases

et al. 1992

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“…22 Collisional particle in cell ͑PIC͒ codes have also been used to simulate short pulse interaction. Wallace et al 24 performed simulations which included ionization, but using tabulated effective rates depending on the density and temperature determined from the local distribution. The more recent work by Zhidkov et al 25,26 did include ionization in line with the simulation, but did not include excitation.…”

Section: Introductionmentioning

confidence: 99%

Electron kinetic simulations of solid density Al plasmas produced by intense subpicosecond laser pulses. I. Ionization dynamics in 30 femtosecond pulses

Ethier

Matte

2001

Physics of Plasmas

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The interaction of a 1018 W/cm2, 30 fs laser pulse with solid Al was simulated with the electron kinetic code “FPI” [J. P. Matte et al., Phys. Rev. Lett. 72, 1208 (1994)] in which an improved average ion module was fully coupled to the electron kinetics. It includes electron impact ionization and excitation and their inverse processes: collisional recombination and de-excitation; as well as radiative decay and pressure ionization. We compare to runs without the inverse processes, and also without atomic physics (with 〈Z〉 set to 11). Atomic physics strongly affects the energy balance and the shape of the distribution function. Line radiation is mostly due to three body recombination into excited states after the peak of the pulse, as the plasma cools down. Despite the atomic processes and the high density, strongly non-Maxwellian distribution functions were obtained due to very steep temperature gradients and strong collisional heating, at the peak of the pulse. However, after the pulse, there is a very rapid thermalization of the electron distribution to which inverse processes strongly contribute.

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