Induced Nonstationary Transition Process

Abstract

How will the nonstationarity of a medium reflect on the process of charged particle interaction with strong laser radiation? In the current laser fields of ultrashort pulse duration and relativistic intensities, any medium turns instantaneously (on a time span much smaller than one wave cycle) into a plasma, that is, abrupt change of the medium properties, particularly the dielectric permittivity, occurs in time. On the other hand, with the abrupt change in time of the dielectric permittivity of a medium, charged particle radiation occurs similar to transition radiation on the boundary of two media with different dielectric permittivity. In the presence of an external EM radiation field, this nonstationary transition process acquires induced character and the inverse process of radiation absorption by a charged particle is actualized, particularly in plasmas where in the stationary states the radiation or absorption of quanta of a transverse EM radiation field (monochromatic radiation such as a laser one) by a free particle cannot proceed. With the abrupt change in time of the medium dielectric permittivity, the production of hard quanta of relativistic energies from the laser radiation is possible and, consequently, electron–positron pair creation in nonstationary plasma of common densities is available. Meanwhile, for electron–positron pair production in a stationary plasma (a medium should be plasmalike for this process) by a \(\gamma \)—quantum, a superdense plasma with electron densities greater than \(10^{34}\,\text {cm}^{-3}\) is necessary. Such superdense matter exists in astrophysical objects (in the core of neutron stars—pulsars), leading to special interest in the processes of electron–positron pair production and annihilation in superdense plasma. On the other hand, the matter in the astrophysical objects may also be in a strongly nonstationary state. Hence, it is important to study the induced nonstationary transition process in the strong EM radiation field in a medium with an arbitrary dielectric permittivity changing abruptly in time.