arXiv subjects

K. A. Rahman

Publications and source records attributed to K. A. Rahman.

7 recordsLinked to original sources

arXiv2018

Direct measurement of electron numbers created at infrared laser-induced ionization of various gases

In this work, we present temporally resolved measurements of electron numbers created at photoionization of various gases by femtosecond laser pulse at 800 nm wavelength. The experiments were conducted in $O_2$, $Xe$, $Ar$, $N_2$, $Kr$ and $CO$ at room temperature and atmospheric pressure. Elastic microwave scattering was used to directly measure the electron numbers. Numbers of electrons in the range $3*10^8$ to $3*10^{12}$ electrons were produced by the laser pulse energies 100-700 uJ. After the laser pulse, plasma decayed on the time scale varied from 1 to 40 ns depending on the gas type and governed by two competing processes, namely, the creation of new electrons from ionization of the metastable atoms and loss of the electrons due to dissociative recombination and attachment to oxygen.

physics.plasm-ph↗

arXiv2017

Counting the electrons in a multiphoton ionization by elastic scattering of microwaves

Laser induced plasmas have found numerous applications including plasma-assisted combustion, combustion diagnostics, laser induced breakdown spectroscopy, light detection and ranging techniques (LIDAR), microwave guiding, reconfigurable plasma antennae etc. Multiphoton ionization (MPI) is a fundamental first step in high-energy laser-matter interaction and is important for understanding of the mechanism of plasma formation. With the discovery of MPI more than 50 years ago, there were numerous attempts to determine basic physical constants of this process in the direct experiments, namely photoionization rates and cross-sections of the MPI, however, no reliable data is available until today and spread in the literature values often reaches 2-3 orders of magnitude. This is due to inability to conduct absolute measurements of plasma electron numbers generated by MPI which leads to uncertainties and, sometimes, contradictions between the MPI cross-section values utilized by different researchers across the field. Here we report first direct measurement of absolute plasma electron numbers generated at MPI of air and subsequently we precisely determine ionization rate and cross-section of eight-photon ionization of oxygen molecule by 800 nm photons $σ_8=(3.32{\pm}0.3)*10^{-130} W^{-8}m^{16}s^{-1}$. Method is based on the absolute measurement of electron number created by MPI using elastic scattering of microwaves off the plasma volume in Rayleigh regime and establishes a general approach to directly measure and tabulate basic constants of the MPI process for various gases and photon energies.

physics.plasm-ph↗

arXiv2009

Wormholes supported by Scalar Fields with Negative Kinetic Energy

We provide a new model of higher dimensional wormholes supported by phantom energy derived from a scalar field with negative kinetic term. We have shown that the Averaged Null Energy Condition (ANEC) violating phantom energy can be reduced as desired. We have also noticed that dimension of the spacetime plays a crucial role for measuring the ANEC violating matter needed.

gr-qc↗

arXiv2008

Can electro-magnetic field, anisotropic source and varying $Λ$ be sufficient to produce wormhole spacetime ?

It is well known that solutions of general relativity which allow for traversable wormholes require the existence of exotic matter (matter that violates weak or null energy conditions [WEC or NEC]). In this article, we provide a class of exact solution for Einstein-Maxwell field equations describing wormholes assuming the erstwhile cosmological term $Λ$ to be space variable, viz., $Λ= Λ(r)$. The source considered here not only a matter entirely but a sum of matters i.e. anisotropic matter distribution, electromagnetic field and cosmological constant whose effective parts obey all energy conditions out side the wormhole throat. Here violation of energy conditions can be compensated by varying cosmological constant. The important feature of this article is that one can get wormhole structure, at least theoretically, comprising with physically acceptable matters.

gr-qc↗

arXiv2008

Conical thin shell wormhole from global monopole: A theoretical construction

By applying 'Darmois-Israel formalism', we establish a new class of thin shell wormhole in the context of global monopole resulting from the breaking of a global O(3) symmetry. Since global monopole is asymptotically conical (no longer asymptotically flat), we call it as conical thin shell wormhole. Different characteristics of this conical thin shell wormhole, namely, time evolution of the throat, stability, total amount of exotic matter have been discussed.

gr-qc↗

arXiv2008

Wormhole geometry from real feasible matter sources

We provide a prescription of real feasible sources that supply fuel to construct a traversable wormhole. A class of exact solutions for Einstein-Maxwell field equations describing wormhole with an anisotropic matter distribution has been presented. The anisotropy plays a crucial role for the existence of the spacetime comprising wormhole geometry.

gr-qc↗

arXiv2007

Some new class of Chaplygin Wormholes

Some new class of Chaplygin wormholes are investigated in the framework of a Chaplygin gas with equation of state $ p = - \frac{A}ρ$, $A>0$. Since empty spacetime ($ p = ρ= 0 $) does not follow Chaplygin gas, so the interior Chaplygin wormhole solutions will never asymptotically flat. For this reason, we have to match our interior wormhole solution with an exterior vacuum solution i.e. Schwarzschild solution at some junction interface, say $ r = a $. We also discuss the total amount of matter characterized by Chaplygin gas that supplies fuel to construct a wormhole.

gr-qc↗