Magnetized Plasma Channel Research Articles

Terahertz field generation in a self-sustained magnetic laser-plasma channel

We theoretically investigate the terahertz field excitation from a laser-driven self-sustained magnetized plasma channel. The expulsion of plasma electrons by the laser ponderomotive force modifies the plasma density, which self-focuses the laser pulse. For the optimized laser parameters, the laser propagates without diverging in plasma and a plasma channel is created. The magnetic field applied along the laser propagation enhances the channel formation efficiency. We utilize this magnetic plasma channel to excite the transverse radiation field by the self-focused laser via wakefield excitation. The magnetic plasma channel maintains the laser intensity over a larger propagation distance, exciting wakefields efficiently. The second order perturbation technique is applied to calculate the wakefield components excited by the laser pulse in a self-sustained magnetic plasma channel. The density perturbation associated with the low-frequency ponderomotive force derives the transverse nonlinear current at terahertz frequency. Our results show that the magnetic field plasma channel can significantly enhance the terahertz conversion efficiency. The tunable terahertz radiation fields of 20 THz frequency with about 10 GV/m may be obtained using one Tesla magnetic field. The efficiency of the process may be optimized and controlled by the laser and plasma parameters. These high-field THz may be useful in various applications such as ultra-fast technology.

Suppression of stimulated Raman scattering in magnetized plasma channel

The current study illustrates the occurrence of Stimulated Raman Scattering (SRS) within a plasma channel characterized by magnetized density conditions. Stimulated forward Raman scattering takes place when a laser beam with a q-Gaussian profile traverses a plasma while being exposed to a wiggler magnetic field. The coupling between the applied magnetic field and the generated Langmuir wave ensures the Raman process generation. As a result, second Langmuir wave is produced which remains strongly damped on the electrons. An external magnetic field is employed for the effective suppression of the SRS. The initial enhancement in the growth rate is observed with the magnetic field and after attaining the maxima, it suppresses with the growth rate.

Generation of tunable terahertz radiation by a laser pulse propagating in a magnetized plasma channel

A study of the generation of terahertz (THz) radiation by propagation of a circularly polarized laser pulse in a parabolic plasma channel is presented. The laser–plasma system is embedded in a uniform axial magnetic field. Transverse electric and magnetic wakefields are evaluated using a perturbation scheme and quasi-static approximation. Nonlinear plasma electron velocities arise along the longitudinal and transverse directions as a result of interaction with the laser pulse. This results in the generation of an electromagnetic wave oscillating at THz frequency. The frequency of the obtained THz radiation can be controlled using the plasma channel parameters and can be tuned by varying the transverse position of observation. The THz radiation amplitude is enhanced in the presence of the parabolic channel compared to the homogeneous plasma configuration. The possibility of obtaining a tunable range of THz frequencies is presented. The analytical results have been validated using three-dimensional particle-in-cell simulations.

Comparison of double layer in argon helicon plasma and magnetized DC discharge plasma

We present in this paper the comparison of an electric double layer (DL) in argon helicon plasma and magnetized direct current (DC) discharge plasma. DL in high-density argon helicon plasma of 13.56 MHz RF discharge was investigated experimentally by a floating electrostatic probe and local optical emission spectroscopy (LOES). The DL characteristics at different operating parameters, including RF power (300–1500 W), tube diameter (8–60 mm), and external magnetic field (0–300 G), were measured. For comparison, DL in magnetized plasma channel of a DC discharge under different conditions was also measured experimentally. The results show that in both cases, DL appears in a divergent magnetic field where the magnetic field gradient is the largest and when the plasma density is sufficiently high. DL strength (or potential drop of DL) increases with the magnetic field in two different structures. It is suggested that the electric DL should be a common phenomenon in dense plasma under a gradient external magnetic field. DL in magnetized plasmas can be controlled properly by magnetic field structure and discharge mode (hence the plasma density).

Direct electron acceleration with a linearly polarized laser beam in a two-dimensional magnetized plasma channel

AbstractWe examine the electron acceleration induced by an ultra-relativistic intensity laser–plasma interaction in a two-dimensional plasma channel in the presence of a self-generated transverse magnetic field. We find that the electron dynamics is strongly affected by the laser pulse polarization angle, plasma density, and magnetic field strength. We investigate in detail, the dependencies of the electron acceleration in terms of different parameters and find excellent agreement with non-magnetized plasma in the absence of the magnetic field. The numerical results show that the self-generated magnetic field plays a constructive role in the electron acceleration process. It is shown that electron acceleration is more affected by self-generated magnetic field for the laser radiations with large polarization angles. The numerical results show the maximum enhancement for electron acceleration for a laser radiation with polarization angle θ = π/2.

Evolution of a circularly polarized laser beam in an obliquely magnetized plasma channel

AbstractThe evolution of the spot size and amplitude of a circularly polarized laser beam propagating in a plasma channel embedded in an obliquely applied magnetic field has been investigated. The wave equation describing the evolution of the radiation field is set up and a variational technique is used to obtain the equations governing the evolution of the spot size and amplitude. Numerical methods are used to analyze the evolution of the laser beam spot size and amplitude. It is seen that the amplitudes of the two transverse components of the electric field of the laser beam evolve differently, since they are driven by unequal current densities. This leads to the conversion of a circularly polarized laser beam into an elliptically polarized beam, under appropriate conditions.

Raman scattering of circularly polarized laser beam in homogeneous and inhomogeneous magnetized plasma channel

Raman scattering of circularly polarized laser beams in a magnetized plasma channel is investigated. The scattering is considered as parametric instability. Dispersion relations of backward and forward scattered waves in a magnetized plasma are derived in a weakly relativistic regime. Growth rates of the corresponding instabilities are calculated. The effects of laser intensity and its polarization as well as the strength of the magnetic field and corresponding cyclotron frequency along with plasma density and its inhomogeneity on the growth rate of Raman scattering are examined. The study shows that the left-handed circularly polarized laser beam has different behaviors in comparison to the right-handed beam, and their growth rates are different due to the anisotropic properties of the magnetized plasma. In addition, Raman scattering in an inhomogeneous plasma with a linear density profile is investigated. The comparison between homogeneous and inhomogeneous plasmas has indicated that inhomogeneity reduces the growth rate. The frequency shift of scattered waves, when laser intensity is high, is studied in the magnetized plasma. The findings indicate that the shift depends on laser intensity and its polarization as well as plasma density and dc magnetic field. The frequency shift can be used as a diagnostic tool for density measurement in laser-plasma interactions.

Interaction of a relativistic electron beam with a laser pulse in the presence of a magnetized plasma medium

Significant progress has been made using plasma channels to guide relativistic e-beams in magnetostatic wiggler free-electron lasers (FELs). In this regard, we study the interaction of an intense laser pulse (as a laser wiggler) with a relativistic electron beam embedded in a background of magnetized plasma. The short wavelength of the laser wiggler ( ∼ µm) allows a higher radiation frequency to be obtained than from conventional magnetostatic wigglers. Laser-plasma interaction in a magnetized plasma channel has been studied. Numerical results reveal that the laser self-focusing and self-compression can be enhanced with increasing the external axial magnetic field strength, which leads to a decrease in the spot size of the laser beam. Hence, we employ the laser beam as a suitable laser wiggler in a FEL-device. Injection of plasma in the interaction region of a laser-wiggler-FEL suggests the possibility of producing short wavelengths ( ∼ x-ray) using lower energy beams ( ∼ O(10) MeV). Furthermore, this configuration has a higher tunability by controlling the plasma density in addition to the e-beam energy tunability of the conventional FELs. Effects of the external axial guide magnetic field and plasma medium on the gain of the FEL have been studied. It is found that by increasing the plasma frequency the FEL-gain increases. The effects of e-beam self-electric and self-magnetic fields on the laser gain have also been investigated in the presence of various plasma frequencies. The results indicate that in the presence of beam self-fields, the sensitivity of the gain increases in the vicinity of resonance regions. Besides, the normalized wiggler-induced self-magnetic field has been obtained which reveals that it is a positive parameter, i.e., it can act as a paramagnetic correction to the wiggler magnetic field, therefore, the gain enhancement can be due to the paramagnetic effect of the self-magnetic field. This concept opens a path toward a new source of soft x-ray pulse based on table-top plasma loaded laser wigglers.

High energetic electron bunches from lasernear critical density layer interaction

In this paper, we report our results from interactions between sub-picosecond laser and relativistic near-critical density plasma layer. To create the near-critical density plasma layer, low density foam targets are utilized in our experiments. The foam is comprised of tri-cellulose acetate. Their average densities vary from 1 mg/cm3 to 5 mg/cm3, corresponding to full ionization densities ranging from 0.6nc to 3nc. When laser pulse is incident on the near-critical density plasma, some energetic bunches with a large quantity of charges are measured in most of the shots. The maximum charge quantity reaches to 6.1 nC/sr. Furthermore, the observed electron energy spectrum is Boltzmann-like with a wide plateau at the tail of the energy spectrum, rather than a Maxwell-like. The concept of average temperature is not available any more, and we define average effective temperature instead, namely the slope temperature. Fitting the Boltzmann-like spectrum exponentially, we find that the average effective temperature even exceeds 8 MeV at 7.51019 W/cm2, far beyond the ponderomotive limit. Aiming at analyzing the implication of physics, several two-dimensional particle-in-cell (PIC) simulations are performed. The PIC simulations indicate that the hole-boring effect and relativistic self-transparency play an important role in the electrons heating process. At the earlier stage of heating process, a short plasma channel is created by the hole-boring effect and relativistic self-transparency. The length and the width of the plasma channel are about tens of micrometers and several micrometers respectively. Around the plasma channel, there is an intensive azimuthal magnetic field. The magnitude of the azimuthal magnetic field is 100 MGs. However, the radical electrostatic field is not seen. The possible reason is that the plasma channel would be cavitated by the hole-boring effect. As a result, the electrons will experience Betatron resonance in the magnetized plasma channel. The traverse momentum of the electron would be converted into forward momentum. Assisted by the Betatron resonance, the electrons gain energies from the laser directly and efficiently. Thus, the average effective temperatures of the electron bunches are much higher than predicted by the ponderomotive scaling law. Besides, we also conducte another simulation to instigate the differences by adopting different laser polarizations. Within our expectation, the electron spectrum of the P-polarization accords well with the experimental result, while the electron spectrum of the S-polarization obviously deviates from the experimental result. It also demonstrates that the Betatron resonance heating dominates the electron acceleration process. This research paves the way to generating the highly energetic bunches with a large quantity of charges, and wound also be helpful for producing the high-bright laser bremsstrahlung sources in future.

External magnetic field effect on the growth rate of a plasma-loaded free-electron laser

In order to extend the production of intense coherent radiation to angstrom wavelengths, a laser wave is employed as a laser wiggler which propagates through a magnetized plasma channel. The plasma-loaded laser wigglers increase the ability of laser guidance and electron bunching process compared to the counterpropagating laser wigglers in vacuum. The presence of the plasma medium can make it possible to propagate the laser wiggler and the electron beam parallel to each other so that the focusing of the pulse will be saved. In addition, employing an external guide magnetic field can confine both the ambient plasma and the transverse motions of the electron beam, therefore, improving the free-electron lasers’ efficiency, properly. Electron trajectories have been obtained by solving the steady state equations of motion for a single particle and the fourth-order Runge-Kutta method has been used to simulate the electron orbits. To study the growth rate of a laser-pumped free-electron laser in the presence of a plasma medium, perturbation analysis has been performed to combine the momentum transfer, continuity, and wave equations, respectively. Numerical calculations indicate that by increasing the guide magnetic field frequency, the growth rate for group I orbits increases, while for group II and III orbits decreases.

Plasma and cyclotron frequency effects on output power of the plasma wave-pumped free-electron lasers

Significant progress has been made employing plasmas in the free-electron lasers (FELs) interaction region. In this regard, we study the output power and saturation length of the plasma whistler wave-pumped FEL in a magnetized plasma channel. The small wavelength of the whistler wave (in sub-μm range) in plasma allows obtaining higher radiation frequency than conventional wiggler FELs. This configuration has a higher tunability by adjusting the plasma density relative to the conventional ones. A set of coupled nonlinear differential equations is employed which governs on the self-consistent evolution of an electromagnetic wave. The electron bunching process of the whistler-pumped FEL has been investigated numerically. The result reveals that for a long wiggler length, the bunching factor can appreciably change as the electron beam propagates through the wiggler. The effects of plasma frequency (or plasma density) and cyclotron frequency on the output power and saturation length have been studied. Simulation results indicate that with increasing the plasma frequency, the power increases and the saturation length decreases. In addition, when density of background plasma is higher than the electron beam density (i.e., for a dense plasma channel), the plasma effects are more pronounced and the FEL-power is significantly high. It is also found that with increasing the strength of the external magnetic field frequency, the power decreases and the saturation length increases, noticeably.

Laser beat wave resonant terahertz generation in a magnetized plasma channel

Resonant excitation of terahertz (THz) radiation by nonlinear mixing of two lasers in a ripple-free self created plasma channel is investigated. The channel has a transverse static magnetic field and supports a THz X-mode with phase velocity close to the speed of light in vacuum when the frequency of the mode is close to plasma frequency on the channel axis and its value decreases with the intensity of lasers. The THz is resonantly driven by the laser beat wave ponderomotive force. The THz amplitude scales almost three half power of the intensity of lasers as the width of the THz eigen mode shrinks with laser intensity.

Experimental study of the communication performance of electromagnetic wave in time-varying and magnetized plasma channel

In this paper the influences of the time-varying plasma and magnetized time-varying plasma on the communication performance are investigated. Using a 5.8 GHz microwave source, the electron density and collision frequency of the time-varying glow discharge plasma are measured. An experimental platform is set up to test the bit error rates (BERs) of a variety of the modulation signals after going though the time-varying plasma channel. The experimental results show that the binary phase shift keying (BPSK) modulation signal has a minimal communication BER. Meanwhile, the variations of L-band BPSK and S-band QPSK (quadrature phase shift keying) signal's eye diagram and the constellation diagram, and the variation of energy after a magnetized plasma are observed. Compared with the un-magnetized situation, the magnetized plasma communication performance is greatly improved and the BER becomes much lower. The results prove that the magnetic field can effectively relieve the amplitude modulation and phase modulation caused by the plasma channel.

Self-focusing and self-channelling of short circularly laser pulses propagating in a magnetized plasma channel

In this study, non-paraxial nonlinear propagation of a relativistic laser pulse with linear and circular polarization in a preformed plasma channel having a parabolic density profile is investigated. The circular laser spot size variation in the absence/presence of an external longitudinal magnetic field is separately analysed and the results are compared. The amplitude of oscillation is decreased in the relativistic case, but it is changed in the presence of an external field. The right-turn circular pulse behaviour is completely opposite to that of the left-turn circular pulse in the presence of an external field, because the right-turn pulse is focused later than the left-turn pulse. The effect of nonlinearities on oscillations of spot size is analysed and the nonlinear critical channel depth required for propagation of a self-trapped laser pulse is evaluated. The laser field amplitude is derived for a matched pulse and it is shown that the group velocity dispersion effect causes broadening of the pulse length.

Low-frequency wiggler modes in the free-electron laser with a dusty magnetoplasma medium

An advanced incremental scheme for generating tunable coherent radiation in a free-electron laser has been presented: the basic concept is the use of a relativistic electron beam propagating through a magnetized dusty plasma channel where dust helicon, dust Alfven and coupled dust cyclotron-Alfven waves can play a role as a low-frequency wiggler, triggering coherent emissions. The wiggler wavelength at the sub-mm level allows one to reach the wavelength range from a few nm down to a few Å with moderately relativistic electrons of kinetic energies of a few tens/hundreds of MeV. The laser gain and the effects of beam self-electric and self-magnetic fields on the gain have been estimated and compared with findings of the helical magnetic and electromagnetic wigglers in vacuum. To study the chaotic regions of the electron motion in the dusty plasma wave wiggler, a time independent Hamiltonian has been obtained. The Poincare surface of a section map has been used numerically to analyze the nonintegrable system where chaotic regions in phase-space emerge. This concept opens a path toward a new generation of synchrotron sources based on compact plasma structures.

Evolution of chirped laser pulses in a magnetized plasma channel

The propagation of intense, short, sinusoidal laser pulses in a magnetized plasma channel has been studied. The wave equation governing the evolution of the radiation field is set up and a variational technique is used to obtain the equations describing the evolution of the laser spot size, pulse length and chirp parameter. Numerical methods are used to analyze the simultaneous evolution of these parameters. The effect of the external magnetic field on initially chirped as well as unchirped laser pulses on the spot size, pulse length and chirping has been analyzed.

Relativistic Third-Harmonic Generation of a Laser in a Self-Sustained Magnetized Plasma Channel

Relativistic third-harmonics of a laser in a magnetized plasma channel is studied. Due to relativistic self-focusing in plasma, a high-intensity laser pulse pushes the plasma electrons radially outward to create a plasma channel. The relativistic mass effect and the magnetic field contribute to the nonlinear dielectric response of plasma and create a plasma channel due to the laser self-focusing. Self-sustained plasma channel is very important and may affect the efficiency of harmonic generation of the interacting laser beam. The velocity and density perturbation associated with the self-focused laser beam can generate a nonlinear current at triple fold frequency of the fundamental laser. Our results show the significant effect of the self-sustained magnetized plasma channel on the efficiency of third-harmonic generation of the laser beam.

Modulation instabilities and group velocity dispersion in partially stripped magnetoplasma channels

The nonlinear theory of propagation of an intense laser beam in a partially stripped magnetized plasma channel is investigated. An external magnetic field is considered, along the direction of propagation of the laser pulse. A three-dimensional envelope equation for the evolution of the laser field is derived. Using the source dependent expansion technique, an expression for the laser spot evolution is derived. The effect of the external magnetic field on the transverse oscillation of the laser spot is analyzed analytically and numerically. A formulation for the amplitude modulation instability is derived and the results are compared against the fully stripped plasma case. The excitation condition and the growth rate of the modulation instability are discussed. It is observed that increased positive chirp in the laser frequency enhances the modulation instability significantly. The group velocity dispersion is also calculated and the results are compared with those for the case of a fully stripped magnetized plasma channel. The presence of an external magnetic field and that of bound electrons have a significant effect on the dephasing length, as compared to that for a fully stripped plasma. The separability of the plasma frequency and laser frequency (ωp and ω0 respectively) is considered, such that it satisfies the criterion ωp/ω0 < 1.

Free-electron laser with a plasma wave wiggler propagating through a magnetized plasma channel

A plasma eigenmode has been employed as a wiggler in a magnetized plasma channel for the generation of laser radiation in a free-electron laser. The short wavelength of the plasma wave allows a higher radiation frequency to be obtained than from conventional wiggler free-electron lasers. The plasma can significantly slow down the radiation mode, thereby relaxing the beam energy requirement considerably. In addition, it allows a beam current in excess of the vacuum current limit via charge neutralization. This configuration has a higher tunability by controlling the plasma density in addition to the γ-tunability of the standard FEL. The laser gain has been calculated and numerical computations of the electron trajectories and gain are presented. Four groups (I–IV) of electron orbits have been found. It has been shown that by increasing the cyclotron frequency, the gain for orbits of group I and group III increases, while a decrease in gain has been obtained for orbits of group II and group IV. Similarly, the effect of plasma density on gain has been exhibited. The results indicate that with increasing plasma density, the orbits of all groups shift to higher cyclotron frequencies. The effects of beam self-fields on gain have also been demonstrated. It has been found that in the presence of beam self-fields the sensitivity of the gain increases substantially in the vicinity of gyroresonance. Here, the gain enhancement and reduction are due to the paramagnetic and diamagnetic effects of the self-magnetic field, respectively.

Nonlinear electromagnetic Eigen modes of a self created magnetized plasma channel and its stimulated Raman scattering

AbstractA theoretical formalism is developed to obtain the mode structure of right circularly polarized nonlinear laser Eigen mode in a self created plasma channel in the presence of an axial magnetic field. The nonlinearity in electron response arises due to relativistic mass effect and ponderomotive force induced density redistribution. The Eigen mode is seen to be unstable to stimulated Raman backscattering involving an electrostatic quasi-mode and a scattered electromagnetic wave. The growth rate increases with ambient magnetic field.