Terahertz radiation finds applications in medical science, spectroscopy, security screening, communication, etc. There are several methods that focus on the interaction of lasers with plasma, and laser beating process is an effective scheme for the Terahertz radiation generation. In this scheme, a resonance between the beating frequency and plasma frequency needs to be achieved. In general, only the plasma electrons respond to the lasers field. However, achieving a Terahertz source with variable frequency and power is very difficult, even with the application of an external magnetic field. In this article, we give an analytical analysis of how employing an external magnetic field, a laser-induced transient current in an array of GaAs structures can be used to increase the power of the emitted radiation. The magnetic field has the benefit of not stopping the carriers since these gyrate before they reache the corner, which causes the frequency to affect the current and helps lasting this interaction for a longer time. As a result, 2-3 cycles of the higher frequency pulse are generated, increasing the power by 100 times. پرونده مقاله

Laser-plasma interaction is a fascinating subject in view of its various applications in wave generation, particle acceleration, radiation generation, etc. The laser beam gets reflected at the vacuum-plasma interface if the plasma density is equal or larger than the critical density. However, in the presence of strong magnetic field in the order of kilo Tesla, the beam can travel some distance through over-dense plasma. Here E×B heating and pondermotive force play role for the laser beams to propagate through the over-dense plasma. In the present article, taking the external magnetic field along the propagation direction of the laser beam we have observed the R- and L- waves to be excited. The applied magnetic field is chosen in such way that the laser frequency 𝜔𝑙 falls between the electron cyclotron frequency 𝜔𝑐𝑒 and ion cyclotron frequency 𝜔𝑐𝑖. Under this situation, it generates only an R-wave. If the laser frequency is considered to be less than the ion cyclotron frequency ( 𝜔𝑙𝑎𝑠𝑒𝑟<𝜔𝑐𝑖) then an L-wave is additionally generated. In both the cases, an electrostatic disturbance is also formed with different but significant electric field amplitudes. We simulate these R- and L-waves in 1-D by using Particle-in-Cell (PIC) simulation using the EPOCH-4.17.10. Specifically, the electric and magnetic fields are studied that are associated with these waves, and the waves are verified based on the dispersion relation and the polarization studies. پرونده مقاله

An E ⃗×B ⃗ plasma is important for various applications including Hall thrusters and magnetic nozzle for long-lasting space propulsion. Such a cross field arrangement in inductively coupled plasma plays vital role in film deposition and etching that are the basic ingredients in semiconductor industries; though in these applications, only the electrons are magnetized which enhance the plasma production and hence, ultimately control the etching aspect ratio and film quality. In the present work, an E ⃗×B ⃗ plasma is considered where ionization takes place and finite temperature gradient also exists. Specifically, a theoretical model is developed for analysing the effect of magnetic field on the density gradient driven instability. The growth rate of the instability is evaluated as a function of plasma background density, scale length of density gradient, ionization frequency, charge on ions, ion temperature gradient, temperatures of plasma species and magnetic field. To generalize the situation, case of different masses of the ions is also reviewed by considering both the electrons and the ions to be magnetized. پرونده مقاله

Plasma-material interaction has been a subject of interest for the past several decades due to its importance in various fields of research such as film deposition, surface nitriding, plasma etching, plasma reactors at low-pressure conditions etc. During this interaction, the presence of negative ions further plays a vital role to ease defect-free analysis of soft substrates. The response of plasma through the sheath formation, when a metallic plate or probe is inserted into it, depends on the plasma characteristics / parameters and the bias voltage used on the plate or probe. The Bohm’s criterion decides such kind of interaction. The present work theoretically demonstrates a modified Bohm’s criterion in an electronegative plasma which is collisional and has two-temperature non-extensive electrons (hot and cold electrons). The behaviour of positive ions is considered through their fluid equations, whereas the negative ions are taken to follow Boltzmann distribution. While writing the basic equations, ion source term and ionization rate are retained and Sagdeev’s potential approach is employed to evaluate the Bohm’s criterion which reveals a band for the positive ion velocity, means maximum and minimum values for the ion velocity. This modified Bohm’s criterion is studied under the effects of various plasma parameters. پرونده مقاله

The effect of collisions, ionisation and non-extensivity is studied numerically in an electronegative warm plasma associated with electron emission from the wall. Electrostatic potential, space charge density, net negative charge density and emitted electron beam density are plotted with the normalised distance to see the effects of aforesaid parameters The negative ion is described with fluid equations to see it’s effect of the mass ratio (negative ion to positive ion) on emitted electron beam density inside the sheath. The three types of electronegative plasma taken are CF_4, O_2 and C_60. The emitted beam electron density is more in number at the wall for higher collisional and ionisation case and less in higher mass ratio. For super-extensive case the emitted beam electrons is lesser than compared to the Boltzmann distributed electrons. The sheath thickness is found to be more in higher mass ratio, emitted beam electrons and super -extensive case while for higher collisional and ionisation case the sheath thickness is less. پرونده مقاله

Charged particle acceleration is the subject of great interest because of its applications in various fields such as thermonuclear fusion, nuclear physics, radiation generation, coherent harmonic generation, probing materials, medical science, food preservation, etc. The particle acceleration is usually done by interacting the particles with strong electric fields. Since the magnetic field diverts the particle motion, this also plays a vital role in the particle interaction with the electromagnetic fields. In the present work, a combined configuration of helical wiggler and solenoidal magnetic fields has been used to optimize the trajectory of the electron for effective particle acceleration. In this concept, the solenoidal field controls the transverse components of the electron velocities and wiggler field confines the helical motion of the electron. The optimized values of magnitudes of solenoidal field and wiggler magnetic field and its period / wavelength make this configuration useful for particle acceleration in waveguide. پرونده مقاله

The Bohm sheath criterion is formulated by using Sagdeev potential approach for electronegative warm plasma under oblique magnetic field and secondary electron emission from the wall. In this model the effect of collisions between positive ions and neutral, ionization, electronegativity, non-extensivity and temperature ratio (positive/negative ion to electron) are also considered to get the exact behavior of velocity of positive ions at the edge of the sheath. Results are compared with the cases of collision-less, ionization-less and absence of electron emission from the wall and it is found that the Bohm velocity for this case is decreased faster with the increase of non-extensivity and the angle of inclination of the applied magnetic field. Also, Bohm velocity gets increased when the electric field at the edge of the sheath increases. Although Bohm velocity does not depend on the strength of the magnetic field, but the slope in the electrostatic potential gets increased and the sheath thickness decreased on raising the strength of the magnetic field. پرونده مقاله

Electron-self-injection in bubble wakefield acceleration is the new concept for acceleration of electrons inside bubble. In this technique, self-injected plasma electrons have been used for acceleration whose advantage is that there is no need of external source of electrons. In our case, we have carried out numerical investigations of self-injected plasma electrons in different shaped bubbles such as spherical, longitudinal ellipsoid and transverse ellipsoid bubble. For these numerical investigations, by carrying out relativistic Hamiltonian analysis of plasma electrons, we have used 4th order Runge-Kutta (RK) method by employing MATLAB ode45, a nonstiff differential equations solver. We have discussed different parameters such as impact parameter, radius of bubble, bubble velocity for their effect on the formation of bubbles with different shapes and self-injection of the electrons. پرونده مقاله

Near the critical layer, a p-polarized laser that impinges on a dense plasma having density gradient at an angle to the density gradient transforms into a plasma wave. Even after the laser pulse has stopped, the plasma wave survives. The electrons close to the crucial layer are heated by the plasma wave. Increased plasma pressure and the production of ion-acoustic waves at frequencies lower than those of the ion plasma are the results of a sudden increase in plasma electron temperature. It appears to be a mechanism for ion-acoustic waves observed in an experiment (Adak et al. Phys. Rev. Lett. 114 (2015) 115001), where alternatively the plasma wave can excite ion acoustic waves in Terahertz frequency range via the decay instability. In the present work, we have theoretically demonstrated these observations. پرونده مقاله