Dielectric Screening In The Thomas-Fermi Model

In this paper, we investigate the dielectric screening process in the Thomas-Fermi model with several variations such as the thermal effects, relativistic effects, motion of the test charge, and the effect of a very strong magnetic field. Comparisons are made with the corresponding results in a classical plasma. Some apparently anomalous results are pointed out and explained

Collisionless absorption, hot electron generation, and energy scaling in intense laser-target interaction

Among the various attempts to understand collisionless absorption of intense ultrashort laser pulses a variety of models has been invented to describe the laser beam target interaction. In terms of basic physics collisionless absorption is understood now as the interplay of the oscillating laser field with the space charge field produced in the plasma. A first approach to this idea is realized in Brunel's model the essence of which consists in the formation of an oscillating charge cloud in the vacuum in front of the target. The investigation of statistical ensembles of orbits shows that the absorption process is localized at the ion-vacuum interface and in the skin layer: Single electrons enter into resonance with the laser field thereby undergoing a phase shift which causes orbit crossing and braking of Brunel's laminar flow. This anharmonic resonance acts like an attractor for the electrons and leads to the formation of a Maxwellian tail in the electron energy spectrum. Most remarkable results of our investigations are the Brunel-like hot electron distribution at the relativistic threshold; the minimum of absorption at $I\lambda^2 \cong (0.3-1.2)\times 10^{21}$ W/cm$^2\mu$m$^2$, in the plasma target with the electron density of $n_e \lambda^2\sim 10^{23}$cm$^{-3}\mu$m$^2;$ the drastic reduction of the number of hot electrons in this domain and their reappearance in the highly relativistic domain; strong coupling of the fast electron jets with the return current through Cherenkov emission of plasmons. The hot electron energy scaling shows a strong dependence on intensity in the moderately relativistic domain $I\lambda^2 \cong (10^{18} - 10^{20})$ W/cm$^2\mu$m$^2$, a scaling in vague accordance with current published estimates in the range $I\lambda^2 \cong (0.14-3.5)\times 10^{21}$ W/cm$^2\mu$m$^2$, and a distinct power increase beyond $I=3.5\times 10^{21}$ W/cm$^2\mu$m$^2$.Comment: 11 pages, 10 figure

Exact field ionization rates in the barrier suppression-regime from numerical TDSE calculations

Numerically determined ionization rates for the field ionization of atomic hydrogen in strong and short laser pulses are presented. The laser pulse intensity reaches the so-called "barrier suppression ionization" regime where field ionization occurs within a few half laser cycles. Comparison of our numerical results with analytical theories frequently used shows poor agreement. An empirical formula for the "barrier suppression ionization"-rate is presented. This rate reproduces very well the course of the numerically determined ground state populations for laser pulses with different length, shape, amplitude, and frequency. Number(s): 32.80.RmComment: Enlarged and newly revised version, 22 pages (REVTeX) + 8 figures in ps-format, submitted for publication to Physical Review A, WWW: http://www.physik.tu-darmstadt.de/tqe

Absorption of Ultrashort Laser Pulses in Strongly Overdense Targets

We report on the first absorption experiments of sub-10 fs high-contrast Ti:Sa laser pulses incident on solid targets. The very good contrast of the laser pulse assures the formation of a very small pre-plasma and the pulse interacts with the matter close to solid density. Experimental results indicate that p-polarized laser pulses are absorbed up to 80 percent at 80 degrees incidence angle. The simulation results of PSC PIC code clearly confirm the observations and show that the collisionless absorption works efficiently in steep density profiles

Heating mechanisms in radio frequency driven ultracold plasmas

Several mechanisms by which an external electromagnetic field influences the temperature of a plasma are studied analytically and specialized to the system of an ultracold plasma (UCP) driven by a uniform radio frequency (RF) field. Heating through collisional absorption is reviewed and applied to UCPs. Furthermore, it is shown that the RF field modifies the three body recombination process by ionizing electrons from intermediate high-lying Rydberg states and upshifting the continuum threshold, resulting in a suppression of three body recombination. Heating through collisionless absorption associated with the finite plasma size is calculated in detail, revealing a temperature threshold below which collisionless absorption is ineffective.Comment: 14 pages, 7 figure