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Electromagnetic Waves and Lasers
Published in Hitendra K. Malik, Laser-Matter Interaction for Radiation and Energy, 2021
Ponderomotive force is a nonlinear electrorestrictive force experienced by a charged particle in an inhomogeneous oscillating EM field. The mechanism of the ponderomotive force can be explained by considering the motion of a charge in an oscillating electric field. In a homogenous field, after one cycle of oscillation, the charge returns to its initial position, whereas in case of an inhomogeneous field, the force exerted on the charged particle is largely spent in a region with higher amplitude points in the direction where the field is weaker.
Accelerators
Published in Shalom Eliezer, Kunioki Mima, Applications of Laser–Plasma Interactions, 2008
Another way net acceleration can occur is by having a frequency variation (chirp) in the laser pulse (Khachatryan et al., 2004). As can be seen from the ponderomotive force, it is inversely proportional to the square of the laser frequency. Intuitively, we can see that if the frequency is properly varied along the pulse then there can be a net force on the electron. Figure 2.3 shows the interaction of an electron with Gaussian pulse with a linear chirp where
Time-Domain Solutions
Published in Dikshitulu K. Kalluri, Principles of Electromagnetic Waves and Materials, 2017
Photon in vacuum has zero mass and zero charge. However, one can consider it having an effective mass meff and equivalent charge qph in a medium [20]. For example, Mendonca [20] arrives at the following values for these in an isotropic cold electron plasma of plasma frequency ωp : meff=ωpℏc2qph=−ℏekp2meffω0kp=ωpvg, where meff and e are the mass and absolute value of the charge, respectively, of an electron. ω0 is the central frequency and vg is the group velocity of the wave packet. These concepts help explain [20] the “ponderomotive force,” photon-beam plasma instabilities, etc. Ponderomotive force is due to the radiation pressure of the photon gas exerted on the electrons of the plasma.
Terahertz radiation generation through nonlinear interaction of frequency chirped laser pulses with hot inhomogeneous plasmas
Published in Waves in Random and Complex Media, 2022
It is mentioned to note that the inhomogeneity of temperature and density of electrons and the effect of frequency chirped laser pulse can play an important role in the laser–plasma interactions. However, the generation of THz radiation and effects of these parameters have been less studied. In this paper, it is assumed that two laser pulses with different frequencies obliquely incident on a plasma medium. The plasma is unmagnetized, collisionless with rippled density and linear inhomogeneity of temperature. Two incident lasers are with linearly chirped frequencies. These high power lasers exert an axial ponderomotive force on electrons. This force in the presence of inhomogeneity and chirped frequency can produce a transverse current with strong spectral component at a specific frequency [22]. This frequency is the difference frequency between two laser beams which is of the order of some wavelengths at THz and sub-THz frequencies. In other words, the nonlinear current density produces the THz radiation.
Dynamics of quadruple laser beams in collisionless plasmas
Published in Waves in Random and Complex Media, 2019
In laser fusion applications, self-focusing of the laser beams is of particular importance because it can guide the laser beam to the core of the fuel pellet without being diffracted. Significant self-focusing alters the usual picture of the linear absorption of laser energy in fusion plasma, because as self-focusing progresses the laser beam digs a duct in the plasma where the density is depressed [11]. In laser–plasma interactions, there are mainly three mechanisms leading to self-focusing of the laser beams. These mechanisms are (1) Relativistic [12,13] (2) Thermal [14–16] (3) Ponderomotive [17,18]. Relativistic nonlinearity occurs due to the change in electron mass when its quiver velocity in the field of laser beam becomes comparable to that of light in vacuum. This nonlinearity does not show any transient behavior. It arises instantaneously when the incident laser power is grater than the threshold power required for self-focusing. The other two mechanisms take finite time (relaxation time) to settle down, as in these cases, physical displacement of plasma electrons is involved. The ponderomotive force mechanism changes the optical properties of plasma by expelling the electrons from the high-intensity regions to low-intensity regions. Since the refractive index of plasma depends on the local plasma density, the density perturbation due to ponderomotive force reduces the local plasma frequency and consequently, increases the plasma refractive index.
Nonlinear effect of microwave longitudinal ponderomotive force on the dynamics and energy of an externally injected electron in an inhomogeneous plasma-filled circular and elliptical cylinder waveguides
Published in Waves in Random and Complex Media, 2021
The ponderomotive force is a nonlinear phenomenon that can arise in plasma. When high-powered microwaves or laser beams are used to heat or confine plasmas, the radiation pressure can reach several hundred thousand atmospheres. When applied to plasma, this force is coupled to the particles in a somewhat subtle way and is called the ponderomotive force [1]. A simple example and a direct effect of the ponderomotive force is the self-focusing of laser light in a plasma. It is seen that a laser beam of finite diameter causes a radially directed ponderomotive force in a plasma. By considering this effect, plasma then acts as a convex lens, focusing the beam to a smaller diameter. However, the ponderomotive force on the plasma electrons is exerted by the spatial inhomogeneity of the microwave field [2], and in a high-power microwave interaction with plasma, if this force is comparable to the electron pressure gradient force, hence the electromagnetic field profile, electron density distribution and dielectric permittivity are modified [3–8]. In accelerators, an electromagnetic wave interacts with a charged particle and it increases the particle kinetic energy is increased to nearly light speed. Furthermore, electron accelerators have many applications and so it is clear that the importance of electron accelerators is dominated by the achievable accelerating gradient. Recently, extensive researches in the field of electron acceleration in types of various waveguides are done [9–19]. In research done in this field, the various effects such as magnetic field effect [9, 10], thermal effects [11], collision effect [12, 13], the effect of density inhomogeneity [14] and etc. on the injected electron acceleration in a waveguide have been investigated. Furthermore, the effect of a longitudinal density gradient on electron plasma wakefield acceleration has investigated.