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Plasma-Based Particle Acceleration Technology
Published in Hitendra K. Malik, Laser-Matter Interaction for Radiation and Energy, 2021
In this section, we talk about the interaction of an intense laser beam with underdense plasma to accelerate ions. The ponderomotive force of the laser beam expels the plasma electrons and induces charge separation, which sets up the space-charge field. During such interaction, the focussed laser beam was found to be in a ‘cigar’-like shape, which implies that the ions will experience primarily a radial electric field (Krushelnick et al. 1999; Wei et al. 2004). Hence, the energetic ions were measured along the perpendicular direction to the laser propagation direction. The above technique used to accelerate the ions is termed as ‘Coulomb explosion’ or ‘ponderomotive shock acceleration’. The ions accelerated in the laser-plasma interaction region have been reported to gain energy roughly equal to the quiver energy. The analysis of ion acceleration in laser-plasma interaction is quite essential as it unveils a great deal of important information related to self-focussing and channeling occurred due to relativistic and charge displacement effects. Also, many researchers have successfully related the acceleration of ions with the production of neutrons within hot channel formation (Tabak et al. 1994; Roth et al. 2001).
Light–Matter Interactions (Part 2)
Published in Marcos Dantus, Femtosecond Laser Shaping, 2017
The above examples considered isolated small molecules or atoms interacting with intense light. A different scenario emerges when the molecules are in close proximity, such as in solids or liquids. In these cases, the front part of the pulse generates a few electrons. As the main part of the pulse interacts with the free electrons, it accelerates them and these electrons collide with the nearby atoms and molecules. The process generates more electrons, which can be further accelerated by the pulse. The process (generation, acceleration, high-energy electron-atom collisions releasing more electrons) leads to a process known as avalanche ionization, resulting in a large electron cloud moving very fast as a result of Coulomb repulsion and leaving behind a large number of positively charged atoms that also repel each other. The subsequent explosion is known as a Coulomb explosion, which leads to the ejection of the electrons and, soon after, the positively charged ions.
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Published in Splinter Robert, Illustrated Encyclopedia of Applied and Engineering Physics, 2017
[atomic, nuclear] Variations in electron density for the free-electron distribution in a conductor, semiconductor as well as insulator after the passage of an ion with charge (Ze) traveling with high velocity (v) through the solid-state medium (condensed matter). The wake will be formed with a spatial resonant frequency (ω) with spatial period: d=2πv/ω0, also known as the “wavelength” (λ). The local amplitude of the wake oscillation has a maximum just within half a wavelength from the trajectory amounting to: Vmax=Ze(ω0/4ε0v), rapidly dampened by inelastic collisions of charges. The amplitude of the wake reduces to e−1 within 10λ behind the traveling ion and laterally re−1=v/ω0. For conducting media the resonant frequency (ω0) is the plasma frequency of the metal: ωp=(ρe,0e2/ε0m′e), with ρe,0 the electron density, e = 1.60217657 × 10−19C the electron charge, m′e the effective electron mass, and ε0 = 8.85419 × 10−12 C2/Nm2 the permittivity of free space. At speeds greater than the Bohr speed (i.e., vM=2.2×106m/s which is the velocity of an electron in the hydrogen ground state in the Bohr model) the atoms in the path are stripped of their valence electrons, thus corresponding to an equivalent Mach speed. Under these conditions, the created ions are freely migrating. The ion wake formed under these conditions is subject to interference of traveling ion clusters. The Coulomb forces between the heavy ions creates conditions that support collisions and an “explosion” will be effected (known as a “Coulomb explosion”). In analog-to-fluid dynamic principles the ion will also generate a “bow” wave of electrons in front of the ion location, spreading radially from the ion location on the trajectory.
Coulomb explosion dynamics of methoxycarbonylsulfenyl chloride by 3D multimass imaging
Published in Molecular Physics, 2022
S. Tahereh Alavi, Graham A. Cooper, Arthur G. Suits
Coulomb explosion imaging (CEI) is a powerful technique that is commonly initiated by a high-intensity laser beam [1–6]. In favourable cases, the molecular structure and dissociation dynamics can be inferred from the ionic fragments’ momentum images recorded in a Coulomb explosion (CE) experiment. In order to acquire the full picture of the dynamics, it is necessary to determine the correlation between various fragments’ momenta. This can be done using multi-mass imaging techniques along with coincidence analysis. In the case of an ultrafast laser-induced CE experiment, since multiple molecules dissociate at each laser shot, to get the correlation between ion momenta, we need to use statistical methods. Covariance imaging, first introduced by Hansen et al. [7] and Slater et al. [8], is a useful statistical analysis method that can obtain such correlations. In this method, covariance mapping is coupled to three-dimensional ion momentum images [9] to produce covariance images.
IR laser ablation of high boiling elements (C, Mo, Ta, W and Re)
Published in Radiation Effects and Defects in Solids, 2021
The generation of the multi-charged ions is attributed to the high plasma temperature and density proportional to the Iλ2 parameter (17). The ion acceleration mechanism was attributed to a transient dynamic plasma sheath during the laser interaction process emitting fast electrons from the target and generating a high electric field normal to the target surface between the high mobility electron cloud and the positive charged target. The Coulomb explosion process emits ions which are driven in the direction of the emitted electrons by the highly developed electric field.
Thermo-elasto-plasto-dynamics of ultrafast optical ablation in polycrystalline metals. Part I: Theoretical formulation
Published in Journal of Thermal Stresses, 2021
In response to laser energy absorption, ionization and free-electron emission result in ions being rapidly emitted from the irradiated surface. It remains controversial as to the dominant mechanism that governs ion emission. Thermion and non-thermal emissions are known as the two competing mechanisms in describing ion emission [8]. To initiate non-thermal emission of ions, electrons emit in the local region of the irradiated surface via the photoelectric effect, a strong electrostatic field generates due to the accumulation of positive charges as a result of the rigorous emission, and the positive charges expel from the surface layer via the generated electrical field when the covalence bonds between the ions are severed. This kind of ion emission process is known as Coulomb explosion. Such non-thermal process would take place depending on the generated electric field, and it would be absent from the ablation process when the electric field is lower than the Coulomb explosion threshold at several 108 V/cm [5] and [6]. Thermionic emission takes place on a picosecond scale characterized by thermal relaxation time. It has been observed in experiments that ion thermal emissions or thermally enhanced photoemissions become dominant at high laser intensity [29]; thus, indicating that thermal emission has a strong temperature dependency. However, at low-laser intensity, the probability for the occurrence of thermal emission of ions is low given that the temperature involved is significantly lower. It is noteworthy that in metals, Coulomb explosion is inhibited due to the screening effect in which the residual electrons of high mobility effectively neutralize the positive charges generated by the electron emission in the surface layers [22]. Nevertheless, experimental observations seem to support that Coulomb explosion can take place in irradiated surfaces subject to relatively low laser intensity [31] and [32]. The debate over if ion emission is the dominant mechanism is settled in this article by looking into the resulted electric field, which is a gauging criterion for triggering Coulomb explosion.