When an intense electromagnetic wave is incident obliquely on a sharply bounded overdense plasma, strong energy absorption can be accounted for by the electrons that are dragged into the vacuum and sent back into the plasma with velocities v=v, . This mechanism is more efficient than usual resonant absorption for v,+co) L, with L being the density gradient length. In the very high-intensity CO2-laser-target interaction, this mechanism may account for most of the energy absorption. PACS numbers: 52.50.Jm, 52.25.Wz, 52.75.Di When a very intense laser field is incident obliquely on a metallic surface or a sharply bounded overdense plasma, a large absorption rate can be accounted for by electrons that are pulled into the vacuum and sent back into the plasma with v=v. ""with v",=eE/m, co being the quiver velocity. This mechanism is eA'ective when the overdense plasma density is well over the critical density and when a strong density gradient or discontinuity is present; the presence of an underdense plasma is not necessary. Those conditions are met in the laser-grating accelerator concept, ' where an intense laser field incident on a grating is used to accelerate electrons. For this purpose, high-power CO2 lasers are now available that can deliver up to 100 mJ in a time scale of 1 ps: A short enough time scale is necessary if we want to maximize the electric field without damaging the grating surface. I find that the present mechanism has a strong limiting eAect on the magnitude that the electric field normal to a grating can reach without significant absorption of the wave energy. In the laser inertial fusion context in the very-high intensity regime (i.e. , Ik~) 10'6 W pm /cm ), a strong density steepening with an overdense region where the density may be as large as 40no (no being the critical density) arises in the region around critical density.Under those conditions, the present mecha-E(xt =0) =E, ", (tt)+AEt =0 or AEt = -E,",(tt), nism may become more efficient than resonant absorption to absorb energy. Also, some laser fusion schemes involving a magnetic field may become more attractive since, with this new mechanism, the energy absorption takes place directly at the focal spot (the dragged electron goes back to the overdense plasma in about one cycle).To describe the energy absorption mechanism, let us consider a one-dimensional capacitor model where we have at x~0 a perfect conductor which can freely emit electrons. For x (0 I assume a vacuum region where a uniform electric field E "t = E p sin cot is present in the x direction. As the field builds up for t )0, electrons will be pulled out in order to maintain a zero field on the conductor surface at x =0. The lth particle that is emitted at time t =tI will see a field E(xi) =E, ",+hE(, where t Xl(t) AEt = -4treJ I & ndx(2) comes from Poisson's equation with n being the electron density and x 1 (t) and xt(t) are the positions of the first (left-most) particle and lth particle, respectively. Since the particles cannot overtake one another, AFI is constant...
In the long-wavelength limit, above-threshold ionization (ATI) is primarily the result of the interaction of a newly freed electron with the laser field. Classical physics requires that linearly and circularly polarized light produce very diff'erent ATI spectra. Measurements performed using both linearly and circularly polarized, picosecond, 10-pm pulses confirm these conclusions.PACS numbers: 32.80.t Above-threshold ionization (ATI) provides a new controllable heating mechanism for plasmas. It will be particularly important at low densities where other heating mechanisms are ineAective.This paper provides new understanding
Both electron thermal conductivity and thermal exchange with the lattice can cool an electron distribution initially heated on a metallic surface with an ultrashort laser pulse. The interplay between the two processes allows the electron-lattice coupling parameter to be determined. We report measurements of optical damage to molybdenum and copper. Damage caused by pulses have a duration TL ^ 1 nsec can be understood only with a two-temperature model of metals.PACS numbers: 72.15.Eb, 78.47.+p A reliable value of the electron-phonon coupling constant g in metals ^ has wide implications, perhaps even in the field of superconductivity.^ Recent investigations^'^ have deduced or inferred experimental values of g. However, a large uncertainty remains because of the difficulty in firmly relating observations to theory. The experimental approach has been to diagnose the electron temperature either by photon-assisted electron emission^ or by the change in reflection^'^ (or transmission^) of visible light near the rf-state resonance. These processes are all limited to relatively weak excitation, the former because of space-charge effects and the latter because of nonlinear saturation processes. ^ Moreover, in the case of thick samples, the initial reduction of the electron temperature that is measured at the surface is mainly due to the fast electron diff*usion process,^ and unless correctly treated leads to an incorrect value of g. Thin samples, on the other hand, can have anomalously fast electron cooling due to electron-impurity and electron-surface scattering.This paper reports a new method of determining g. It relies on competition between two fast processes: electron-lattice energy exchange and electron thermal conduction. The former cools electrons heated with an ultrashort 10-/im pulse (penetration depth -200 A) by transferring their heat to the lattice. The latter removes energy from the surface.The paper has a second purpose. A theory is developed to describe the pulse-duration dependence of optical damage to metals, and experimental results are presented in support of the theory.The heat transport inside the metal can be described with the following one-dimensional, two-temperature model ^:Ce-l-Te-^^K^Te-g(Te-Ti) +A(x,t) (l) at ox ox andQ^Ti=g(Te-Ti),(2) ot where x is the direction perpendicular to the surface.The electron heat capacity Ce is given by Ce =CeTe, Ce being a constant,^ and K is the heat conductivity. According to Sommerfeld's model,^ KozTe/v, where v = Vee + Vei and Vee and Vei are the electron-electron and electron-phonon collision frequencies, respectively. Te and Ti are, respectively, the electron and lattice temperatures and d is the lattice heat capacity. A{xj) is a source term and g is the electron-phonon coupling constant.As a guide, we will derive some approximate scaling laws for the response of a metal to an ultrashort pulse. These scaling laws have two essential roles. First, they allow quantities that are necessary to understand the following measurements to be introduced. Second, they...
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