Shikha Misra

Photoelectric sheath formation around small spherical objects in space

The formation of a photoelectron sheath around positively charged small (∼cm) spherical objects roaming in near earth space due to the solar radiation (with continuous spectrum) and the solar wind plasma has been investigated. The sheath structure has been derived, taking into account anisotropic photoelectron flux with the Poisson equation, spherical geometry of the object, and half Fermi Dirac distribution of photoelectron velocities. Two cases, viz., when the object is illuminated by (i) isotropic or (ii) unidirectional (parallel beam) radiation, have been analyzed. The analysis predicts a spherically symmetric sheath in case of isotropic illumination, while a symmetry in sheath about a θ=π/4 is seen in case of parallel beam illumination; θ is the angle of incidence which is the angle made by the normal to a surface element with the direction of incidence of solar radiation. The radial and angular profiles of the electric potential and electron density in the photoelectron sheath have been evaluated an...

Effect of electron-ion recombination on self-focusing/defocusing of a laser pulse in tunnel ionized plasmas

A formalism for investigation of the propagation characteristics of various order short duration (pico second) Gaussian/dark hollow Gaussian laser pulse (DHGP) in a tunnel ionized plasma has been developed, which takes into account the electron-ion recombination. Utilizing the paraxial like approach, a nonlinear Schrodinger wave equation characterizing the beam spot size in space and time has been derived and solved numerically to investigate the transverse focusing (in space) and longitudinal compression (in time) of the laser pulse; the associated energy localization as the pulse advances in the plasma has also been analyzed. It is seen that in the absence of recombination the DHGP and Gaussian pulse undergo oscillatory and steady defocusing respectively. With the inclusion of recombination, the DHGP and Gaussian pulse both undergo periodic self-focusing for specific parameters. The DHGPs promise to be suitable for enhancement of energy transport inside the plasma.

Electron density modification in ionospheric E layer by inserting fine dust particles

In this paper, we have developed the kinetics of E-region ionospheric plasma comprising of fine dust grains and shown that the electron density in E-layer can purposely be reduced/enhanced up to desired level by inserting fine dust particles of appropriate physical/material properties; this may certainly be promising for preferred rf-signal processing through these layers. The analytical formulation is based on average charge theory and includes the number and energy balance of the plasma constituents along with charge balance over dust particles. The effect of varying number density, work function, and photo-efficiency of dust particles on ionospheric plasma density at different altitude in E-layer has been critically examined and presented graphically.

Statistical charge distribution over dust particles in a non-Maxwellian Lorentzian plasma

On the basis of statistical mechanics and charging kinetics, the charge distribution over uniform size spherical dust particles in a non-Maxwellian Lorentzian plasma is investigated. Two specific situations, viz., (i) the plasma in thermal equilibrium and (ii) non-equilibrium state where the plasma is dark (no emission) or irradiated by laser light (including photoemission) are taken into account. The formulation includes the population balance equation for the charged particles along with number and energy balance of the complex plasma constituents. The departure of the results for the Lorentzian plasma, from that in case of Maxwellian plasma, is graphically illustrated and discussed; it is shown that the charge distribution tends to results corresponding to Maxwellian plasma for large spectral index. The charge distribution predicts the opposite charging of the dust particles in certain cases.

Kinetics of dust particles around the scrape off layer in fusion devices

A kinetic model based on the balance of charge and energy over the dust particle surface around the scrape off layer (SOL) region in fusion devices has been developed; for describing the dust mass diminution, its temperature evolution and phase change process have been taken into account. The formulation has been utilized to determine the lifetime of cylindrical and spherical dust particles. A realistic situation in fusion devices, when the plasma exhibits meso-thermal flow, has been taken into account; for this purpose a rigorous approach, pioneered by Mott-Smith and Langmuir (1926 Phys. Rev. 28 727), has been adopted to derive the general expressions for the electron (ion) current on cylindrical dust surfaces and the corresponding mean energy of accreting electrons/ions in a flowing plasma. On the basis of analytical modelling the numerical results for the dust electric potential energy and the lifetime of the dust particles corresponding to a typical plasma environment near the SOL region of Mega Ampere Spherical tokamak (MAST)/Joint European Torus (JET) fusion devices have been evaluated for graphite and tungsten dust particles. The results are graphically illustrated as functions of particle size, electron/ion temperature and plasma ionization. It is seen that a large dust particle immersed in low temperature plasma can survive for long time; as an important outcome it is also noticed that the cylindrical particles of tungsten last longer than spherical particles. The findings are of relevance in characterizing and simulating the effects of a variety of dusts for experimental campaigns in large scale (ITER/Demo-like) fusion devices.

Statistical mechanics of dust charging in a multi-ion plasma with negative and positive ionic species

On the basis of statistical mechanics and charging kinetics, the charge distribution over uniform size spherical dust particles in a multi-ion plasma comprising of multiple charged negative and positive ions is investigated. Two specific situations where the complex plasma is viz., (i) dark (no emission from dust) and (ii) irradiated by laser light (causing photoemission from dust) have been taken into account. The analytical formulation includes the population balance equation for the charged dust particles along with number and energy balance of the complex plasma constituents. The departure of the results for multi-ion plasma from that in case of usual singly charged positive ion plasma is graphically illustrated and discussed. In contrast to electron-ion plasma, significant number of particles is seen to acquire opposite charge in case of pure positive-negative ion plasma, even in the absence of electron emission from the dust grains. The effects of various plasma parameters viz., number density, particle size, and work function of dust on charge distribution have also been examined.