“Boiling the vacuum”: in silico plasmas under extreme conditions in the laboratory and in astrophysics

Abstract

Laser technology has progressed tremendously since Theodore Maiman first demonstrated the laser in 1960. One of the most striking examples of this progress is the focused intensity of lasers. Present day lasers in the near infrared frequency range can deliver intensities in excess of 10 23 W/cm 2 . Imagine that you have a very large and powerful lens that captures the light of the sun hitting the upper atmosphere of the Earth and focus it down to a region with the cross section of a human hair. The corresponding intensity is in the range of the most intense lasers now available.EPN 48/5&6 35 bOILING THE VAcUUM FEATURES start to be important. A single ultra relativistic electron quivering in an ultra intense laser field can radiate x-rays. As the intensity is ramped up, the electron oscillating in the laser field will radiate gamma-rays. These gamma-rays can, in turn, interact with the laser and generate electron-positron pairs which, in the presence of the laser field, can again radiate strongly. An electromagnetic cascade of pairs and gamma-rays is then triggered, generating a quantum electrodynamic (QED) electron-positron plasma.It is also possible to conceive scenarios where a static (or slowly varying) ultra strong electric field is present in a finite region of space. Let us imagine the field is so strong that, as the electron is accelerated over the distance of a Compton wavelength it gains more than 2 m e c 2 , i.e. the rest mass energy of an electron and a positron. In this case, an electron-positron pair is generated, which will then interact with the ultra strong field itself, and a cascading process can also be triggered. The critical electric field for pair creation in vacuum, first identified by Sauter, is called the Schwinger critical field E s = 2 π m e 2 c 3 /e h, where e is the electron charge, m e the electron mass, h is the Planck constant, and c the speed of light. For extreme field physics a common used dimensionless parameter is χ = E/E s , where E represents the electric field in the rest frame of the electron. χ determines the transition from the classical to the quantum dynamics; for χ on the order or higher than unity, QED effects are important and must be considered. Colloquially, when copious amounts of electrons and positrons are produced by ultra strong electromagnetic fields in vacuum, or via a cascade from a low density seed of electrons, the "vacuum is boiling".The prospect to reach in the laboratory these conditions in the near future is triggering many exciting developments [7,8] at the cross roads of nonperturbative QED, plasma physics, and astrophysics. In many extreme astrophysical objects (e.g. pulsars, magnetars, or in the magnetosphere U nder the action of these intense electromagnetic fields, electrons quiver at velocities close to the speed of light with Lorentz gamma factors ~100. The nonlinear effects associated with special relativity determine the evolution of light-plasma interactions at these intensities. These lasers are now being explored to drive plasma acceler...