Authors:H. Takahashi; A. Okamoto, S. Kitajima, T. Kobayashi, P. Boonyarittipong, T. Miura, D. Nakamura, J. Kon, T. Saikyo, Y. Tanaka, M. Goto Abstract: The influence of electron energy distribution on helium recombining plasma diagnostics is investigated using a helium collisional-radiative model. The population densities of excited helium atoms are calculated for Maxwellian and non-Maxwellian distribution plasma cases. In the case of the Maxwellian distribution plasma, the electron temperature and electron density determined by the Boltzmann plot method agree well with the input plasma parameters. On the other hand, it is indicated that the electron temperature and electron density are significantly underestimated in the bi-Maxwellian distribution plasma case, even though the density of the hot electron components is three orders smaller than that of the bulk electrons. This result indicates that in a non-Maxwellian helium recombining plasma, evaluation of the particle balance based on line emissions from excited helium atoms would be difficult because the reaction rate of atomic and molecular processes is strongly dependent on the electron temperature and density. PubDate: 2017-08-15T01:05:35.803819-05: DOI: 10.1002/ctpp.201700014

Authors:Quan Shi; Shuyu Dai, A. Kirschner, Dezhen Wang Abstract: The impact of rough surface morphology on the angular distribution of eroded impurities has been investigated with the three-dimensional (3D) rough surface code SURO and a newly developed analytic model. The property of the rough surface structure can be described by the shadow angle of the rough surface in SURO, which is defined as the ratio of the horizontal characteristic length to the initial surface roughness. The SURO simulation results show that the influence of the rough surface on the angular distribution of eroded impurities comes into play when the shadow angle is larger than a threshold value. The larger shadow angle of the rough surface leads to a stronger shift of the angular distribution of the eroded impurities. Different rough surface topographies have been used in the SURO code to check the angular distribution of the eroded impurities. It is found that the shift tendency of the angular distribution is similar for different structures of the rough surface. Based on the numerical modelling results, an analytical model has been developed to investigate the impact of the shadow angle on the angular distribution of the eroded impurities, which shows the consistent result as the SURO simulations. PubDate: 2017-08-15T00:41:00.843403-05: DOI: 10.1002/ctpp.201700022

Authors:G.Lj. Majstorović; J. Jovović, N.M. Šišović Abstract: Emission spectroscopy is applied to measure the gas temperature Tg and the vibrational distribution of N2(C3Πu) and N2+(B2Σu+) excited states from a helium microhollow gas discharge (MHGD) at atmospheric pressure. The rotational temperature Trot of N2+ is determined from relative intensity of the R-branch lines of the N2+(B2Σu+–X2Σg+) bands at 427.81 and 419.91 nm and the well-known Boltzmann plot (BP). Using the same diagnostic technique, the rotationally resolved N2(C3Πu–B3Πg) band at 380.49 nm is used to measure Trot. Under our experimental conditions, Tg is equal to Trot = 550–650 K for nitrogen molecules and shows a slight increase with the discharge current in the current range 3–10 mA. From the intensity ratio of two consecutive vibrational bands of the same sequence, the N2(C3Πu) and N2+(B2Σu+) vibrational temperature Tvib = 3,700–4,000 K is determined. It has been found that N2+(B2Σu+) ions have non-Boltzmann distribution in the helium MHGD, while N2(C3Πu) molecules are populated according to the Boltzmann distribution. Following the Franck–Condon principle, the vibrational distribution of the ground state of N2(X1Σg+) molecules has been determined from the N2(C3Πu) distribution using the inversion matrix of elements qXC(ν,ν′). PubDate: 2017-08-08T00:58:23.637271-05: DOI: 10.1002/ctpp.201600092

Authors:Ram Kishor Singh; Subodh Kumar, R.P. Sharma Abstract: We propose a theoretical model for the generation of electromagnetic waves in the terahertz (THz) frequency range by the optical rectification of a Gaussian laser pulse in a plasma with an applied static electric field transverse to the direction of propagation. A Gaussian laser pulse can exert a transverse component of the quasi-static ponderomotive force on the electrons at a frequency in the THz range by a suitable choice of the laser pulse width. This nonlinear force is responsible for the density oscillation. The coupling of this oscillation with the drift velocity acquired by electrons due to the applied static electric field leads to the generation of a nonlinear current density. A spatial Gaussian intensity profile of the laser beam enhances the generated THz yield by many folds as compared to a uniform spatial intensity profile. PubDate: 2017-07-06T03:15:34.966135-05: DOI: 10.1002/ctpp.201700029

Authors:Scott D. Baalrud; Jérôme Daligault Abstract: The relaxation rate of a Maxwellian velocity distribution function that has an initially anisotropic temperature (T‖≠T⊥) is an important physical process in space and laboratory plasmas. It is also a canonical example of an energy transport process that can be used to test theory. Here, this rate is evaluated using molecular dynamics simulations of the one-component plasma. Results are compared with the predictions of four kinetic theories; two treating the weakly coupled regime, namely (a) the Landau equation, and (b) the Lenard–Balescu equation, and two that attempt to extend the theory into the strongly coupled regime, namely (c) the effective potential theory and (d) the generalized Lenard–Balescu theory. The role of dynamic screening is studied, and is found to have a negligible influence on this transport rate. Oscillations and a delayed relaxation onset in the temperature profiles are observed at strong coupling, which are not described by the kinetic theories. PubDate: 2017-06-02T03:35:40.267866-05: DOI: 10.1002/ctpp.201700028

Pages: 233 - 233 Abstract: Change in the normalized terahertz (THz) amplitude with normalized distance and normalized plasma frequency. Figures 3b, 3d of the paper by R.K Singh et al. PubDate: 2017-08-11T05:32:12.744733-05: DOI: 10.1002/ctpp.201790017

Authors:Naveen Gupta; Arvinder Singh Pages: 258 - 271 Abstract: An analysis of dynamics of a quadruple laser pulse propagating through an under-dense plasma is presented. The Drude model is used to derive the dielectric function of the plasma for relativistic non-linearity in the electron mass. An approximate numerical solution of the nonlinear Schrödinger wave equation for the field of the laser beam is obtained with the help of the moment theory approach in the Wentzel–Kramers–Brillouin (WKB) approximation. Particular emphases are placed on the variations of spot size, pulse width, and longitudinal phase delay with the distance of propagation through the plasma. Self-trapping of the laser pulse is also investigated. PubDate: 2017-08-11T05:32:12.435356-05: DOI: 10.1002/ctpp.201600075

Authors:Rokhsare Jaafarian; Alireza Ganjovi, Gholam Reza Etaati Pages: 272 - 281 Abstract: In this work, a kinetic model is developed to study the effects of the radio frequency antenna wavenumber, helicon plasma electron density, as well as their drift velocity and temperature on the instability increment rate of the helicon wave in both longitudinal and transverse directions. The ion acoustic (IA) wave frequencies and wavenumbers of the helicon waves are obtained when the maximum wave energy is deposited on the plasma ions. Moreover, it is shown that, at the IA wavenumber and frequencies, while the longitudinal instability increment rates for both the helicon and IA waves are ignorable, the transverse instability increment rate for both the helicon and IA wave increases. Besides, the longitudinal instability increment rate for the helicon or IA wave has non-zero resonant frequencies. On the other hand, the transverse instability increment rate of helicon or IA wave can be neglected. Furthermore, it is observed that, while both the imaginary part of longitudinal permittivity and longitudinal instability increment rate are not influenced by the electron temperature, their transverse component increases linearly with the electron temperature. In addition, the imaginary part of transverse permittivity increases almost linearly with the drift velocity of the plasma electrons. PubDate: 2017-08-11T05:32:11.486199-05: DOI: 10.1002/ctpp.201700041

Authors:M. S. Soltani Gishini; A. Ganjovi Pages: 293 - 310 Abstract: In this work, using a two-dimensional particle-in-cell Monte Carlo collision computation method, terahertz (THz) radiation generation via the interaction of two-colour, ultra-short, high-power laser pulses with the polyatomic molecular gases sulphur dioxide (SO2) and ammonia (NH3) is examined. The influence of SO2 and NH3 pressures and two-colour laser pulse parameters, i.e., pulse shape, pulse duration, and beam waist, on the THz radiation generation is studied. It is shown that the THz signal generation from SO2 and NH3 increases with the background gas pressure. It is seen that the THz emission intensity for both gases at higher laser pulse durations is higher. Moreover, for these polyatomic gases, the plasma current density increases with increase in the laser pulse beam waist. A more powerful THz radiation intensity with a larger time to peak of the plasma current density is observed for SO2 compared to NH3. In addition, many THz signals with small intensities are observed for both polyatomic gases. It is seen that for both SO2 and NH3 the generated THz spectral intensity is higher at higher gas pressures. PubDate: 2017-08-11T05:32:13.255689-05: DOI: 10.1002/ctpp.201700018