No Arabic abstract
We consider the reflection of relativistically strong radiation from plasma and identify the physical origin of the electrons tendency to form a thin sheet, which maintains its localisation throughout its motion. Thereby we justify the principle of the relativistic electronic spring (RES) proposed in [A. Gonoskov et al. PRE 84, 046403 (2011)]. Using the RES principle we derive a closed set of differential equations that describe the reflection of radiation with arbitrary variation of polarization and intensity from plasma with arbitrary density profile for arbitrary angle of incidence. PIC simulations show that the theory captures the essence of the plasma dynamics. In particular, it can be applied for the studies of plasma heating and surface high-harmonic generation with intense lasers.
Relativistic mirrors can be realized with strongly nonlinear Langmuir waves excited by intense laser pulses in underdense plasma. On reflection from the relativistic mirror the incident light affects the mirror motion. The corresponding recoil effects are investigated analytically and with particle-in-cell simulations. It is found that if the fluence of the incident electromagnetic wave exceeds a certain threshold, the relativistic mirror undergoes a significant back reaction and splits into multiple electron layers. The reflection coefficient of the relativistic mirror as well as the factors of electric field amplification and frequency upshift of the electromagnetic wave are obtained.
When a fs duration and hundreds of kA peak current electron beam traverses the vacuum and high-density plasma interface a new process, that we call relativistic transition radiation (R-TR) generates an intense $sim100$ as pulse containing $sim$ TW power of coherent VUV radiation accompanied by several smaller fs duration satellite pulses. This pulse inherits the radial polarization of the incident beam field and has a ring intensity distribution. This R-TR is emitted when the beam density is comparable to the plasma density and the spot size much larger than the plasma skin depth. Physically, it arises from the return current or backward relativistic motion of electrons starting just inside the plasma that Doppler up-shifts the emitted photons. The number of R-TR pulses is determined by the number of groups of plasma electrons that originate at different depths within the first plasma wake period and emit coherently before phase mixing.
On the base of the quantum kinetic equation for density matrix in plasma at the stimulated bremsstrahlung of electrons on ions, the nonlinear absorption rate for high-intensity shortwave radiation in plasma has been obtained within relativistic quantum theory. Both classical Maxwellian and degenerate quantum plasma are considered for x-ray lasers of high intensities. Essentially different dependences of nonlinear absorption rate on polarization of strong laser radiation is stated.
Relativistic flying mirrors in plasmas are realized as thin dense electron (or electron-ion) layers accelerated by high-intensity electromagnetic waves to velocities close to the speed of light in vacuum. The reflection of an electromagnetic wave from the relativistic mirror results in its energy and frequency changing. In a counter-propagation configuration, the frequency of the reflected wave is multiplied by the factor proportional to the Lorentz factor squared. This scientific area promises the development of sources of ultrashort X-ray pulses in the attosecond range. The expected intensity will reach the level at which the effects predicted by nonlinear quantum electrodynamics start to play a key role.
Upon combining Northrops picture of charged particle motion with modern liquid crystal theories, this paper provides a new description of guiding center dynamics (to lowest order). This new perspective is based on a rotation gauge field (gyrogauge) that encodes rotations around the magnetic field. In liquid crystal theory, an analogue rotation field is used to encode the rotational state of rod-like molecules. Instead of resorting to sophisticated tools (e.g. Hamiltonian perturbation theory and Lie series expansions) that still remain essential in higher-order gyrokinetics, the present approach combines the WKB method with a simple kinematical ansatz, which is then replaced into the charged particle Lagrangian. The latter is eventually averaged over the gyrophase to produce Littlejohns guiding-center equations. A crucial role is played by the vector potential for the gyrogauge field. A similar vector potential is related to liquid crystal defects and is known as `wryness tensor in Eringens micropolar theory.