Electromagnetic (EM) waves/disturbances are typically the best means to understand and analyse an ionized medium like plasma. However, the propagation of EM waves with a frequency lower than the ...plasma frequency is prohibited by the freely moving charges of the plasma. In dense plasmas, though the plasma frequency can be typically quite high, EM sources at such higher frequency are not easily available. It is, therefore, of interest to seek possibilities wherein a low frequency (lower than the plasma frequency) EM disturbance propagates inside a plasma. This is possible in the context of magnetized plasmas. However, in order to have a magnetized plasma response in high‐density plasmas, one requires an extremely strong external magnetic field. In this manuscript, it is demonstrated that the nonlinearity of the plasma medium can aid the propagation of a slow (effective frequency lower than the plasma frequency) EM wave inside an overdense plasma. A possible mechanism of guiding, collimating, and trapping of the EM pulse or electron current pulses by appropriate tailoring of the local plasma density profile is also shown. Certain interesting applications of the propagation of such slow EM pulse through the inhomogeneous plasma is also discussed.
Atmospheric pressure nonequilibrium plasma jets (APNP-Js), which generate plasma in open space rather than in a confined discharge gap, are recently a topic of great interest. However, researchers ...actually know little about the bright plasma plume in the beginning. Fortunately, after about one decade of researches, the main active species concentrations of APNP-Js are measured. Although the measurements are carried out for different plasma jet devices and under different experimental conditions, it still gives us some insight information about the plasma plumes. In this paper, the measurements on the concentrations of the main active species such as electron, O atom, OH, {\rm O}_{2}(^{1}\Delta_{g}) , metastable state He and Ar, nitric oxide (NO), metastable state {\rm N}_{2}({\rm A}^{3}\Sigma_{u}^{+}) , and N atom for different experimental conditions are reviewed. The peak electron density of a microplasma plume could reach in the order of 10^{14}~{\rm cm}^{-3} . The O atom density is in the order 10^{14}~{\rm cm}^{-3} in the plasma plume but it is in the order 10^{16}~{\rm cm}^{-3} inside the discharge gap. The OH and {\rm O}_{2}(^{1}\Delta_{g}) densities are in the order of 10^{15}~{\rm cm}^{-3} . The metastable state He and Ar concentrations could reach the order of 10^{11} and 10^{12}~{\rm cm}^{-3} , respectively. The NO density is in the order of 10^{15}~{\rm cm}^{-3} in the plasma plume. It should be emphasized that all the measurements are carried out under different experimental conditions and for different experimental setups.
We study the stability of current filaments produced by the Weibel, or current filamentation, instability in weakly magnetized counterstreaming plasmas. It is shown that a resonance exists between ...the current-carrying ions and a longitudinal drift-kink mode that strongly deforms and eventually breaks the current filaments. Analytical estimates of the wavelength, growth rate, and saturation level of the resonant mode are derived and validated by three-dimensional particle-in-cell simulations. Furthermore, self-consistent simulations of counterstreaming plasmas indicate that this drift-kink mode is dominant in the slow down of the flows and in the isotropization of the magnetic field, playing an important role in the formation of collisionless shocks.
Availability of relativistically intense, single-cycle, tunable infrared sources will open up new areas of relativistic nonlinear optics of plasmas, impulse IR spectroscopy and pump-probe experiments ...in the molecular fingerprint region. However, generation of such pulses is still a challenge by current methods. Recently, it has been proposed that time dependent refractive index associated with laser-produced nonlinear wakes in a suitably designed plasma density structure rapidly frequency down-converts photons. The longest wavelength photons slip backwards relative to the evolving laser pulse to form a single-cycle pulse within the nearly evacuated wake cavity. This process is called photon deceleration. Here, we demonstrate this scheme for generating high-power (~100 GW), near single-cycle, wavelength tunable (3-20 µm), infrared pulses using an 810 nm drive laser by tuning the density profile of the plasma. We also demonstrate that these pulses can be used to in-situ probe the transient and nonlinear wakes themselves.
Understanding magnetised laser–plasma interactions is important for controlling magneto-inertial fusion experiments and developing magnetically assisted radiation and particle sources. For nanosecond ...pulses at non-relativistic intensities, interactions are dominated by coherent three-wave interactions, whose nonlinear coupling coefficients became known only recently when waves propagate at oblique angles with the magnetic field. In this paper, backscattering coupling coefficients predicted by warm-fluid theory are benchmarked using particle-in-cell simulations in one spatial dimension, and excellent agreements are found for a wide range of plasma temperatures, magnetic field strengths and laser propagation angles, when the interactions are mediated by electron-dominant hybrid waves. Systematic comparisons between theory and simulations are made possible by a rigorous protocol. On the theory side, the initial boundary value problem of linearised three-wave equations is solved, and the transient-time solutions allow the effects of growth and damping to be distinguished. On the simulation side, parameters are carefully chosen and calibration runs are performed to ensure that comparisons are well controlled. Fitting simulation data to analytical solutions yields numerical growth rates that match theory predictions within error bars. Although warm-fluid theory is found to be valid for a wide parameter range, genuine kinetic effects have also been observed.
For revealing the effects of gas components on the plasma jet characteristics, the corresponding jet characteristics of a home-made thermal plasma torch working with various gas components were ...experimentally studied by using the orthogonal test method. The experimental results showed that: 1) When nitrogen (N<inline-formula> <tex-math notation="LaTeX">_{2})</tex-math> </inline-formula> is defined as the main working gas, the arc voltage, working power, thermal efficiency, and mean specific enthalpy all decrease with increasing argon (Ar). With the increase of helium (He), the mean specific enthalpy first decreases and then increases, while other three jet characteristics decrease. With the increase of hydrogen (H<inline-formula> <tex-math notation="LaTeX">_{2})</tex-math> </inline-formula>, the arc voltage and working power increase, while the thermal efficiency and mean specific enthalpy decrease. 2) When Ar is defined as the main working gas, the mentioned four jet characteristics all increase with N<inline-formula> <tex-math notation="LaTeX">_{2}</tex-math> </inline-formula>. With the increase of He, the thermal efficiency decreases, while other three jet characteristics increase. With the increase of H<inline-formula> <tex-math notation="LaTeX">_{2}</tex-math> </inline-formula>, the arc voltage and working power increases, while the mean specific enthalpy varies little and the thermal efficiency decreases. 3) Besides, the gas volume percentage of the gas component is the uppermost influence factor of the jet characteristics.
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