Intense Short Laser Pulse Research Articles

Numerical investigation of plasma heating during the entrance of an intense short laser pulse into a density profile

In this paper, plasma heating is numerically investigated using a 1D-3V particle-in-cell simulation code through the interaction between an intense short laser pulse with temporal duration, , and a plasma having a slightly overcritical bulk density and an exponential pre-plasma density profile. The aim is to identify, via suitable parametric analysis, the most relevant nonlinear mechanisms as a function of the system parameters. As key points of our research, simulation results revealed that, for pulse-lengths less than the local nonlinear plasma wavelength, , mechanisms such as vacuum–plasma interface wave-break and longitudinal plasma oscillations are the dominant mechanisms for the initiation of electron acceleration and plasma heating at earlier times. Further, for variable density scale lengths our results have proven that the heating mechanisms mentioned above can be ignored for the laser intensities less than a specific threshold value, , even for short density scale length, . Meanwhile, by increasing the laser intensity to higher values, it is observed that the trend of the time history curve for the mean temperature changed in a way that the total level of temperature increased. Also, by lengthening the scale length, at first the amounts of the mean temperature curve increased, and then smoothly decreased.

Lasing without population inversion in N2+

We experimentally demonstrate the existence of long-lived coherent polarizations coupling simultaneously ground state X (X2Σg+) to excited states A (A2Πu) and B (B2Σu+) of N2+ inside a plasma created by a short intense laser pulse at 800 nm. This three-level V-scheme arrangement is responsible for a strong optical gain without population inversion at the B–X transition at 391.4 nm. Simulations based on Maxwell-Bloch equations reproduce well the kinetics and the pressure dependence of the gain.

Electron–Ion Recombination Effect on Electron Acceleration by an Intense Laser Pulse

Electron–ion recombination effect on electron acceleration by a high-intensity laser pulse propagating through a tunnel ionizing gas is investigated in order to observe the actual electron energy gain during acceleration. An intense short-pulse laser with a Gaussian radial profile propagates through a vacuum followed by gas. The point at which the peak of the pulse interacts with the electron is the initial point of the gas region. Tunnel ionization causes defocusing of the laser pulse due to high-density plasma formation on the propagation axis. The electron experiences an additional acceleration during the trailing part of the pulse and, thus, gains net energy. In the presence of electron–ion recombination, the laser pulse focuses more, and hence, the net energy gain is affected significantly for specific parameters region. A model that self-consistently evolves the laser electron acceleration as it ionizes a neutral gas is presented. The model incorporates the electron–ion recombination effects for multiple ionization stage and for tempospatial variations in the neutral gas density appropriate for studying gas-jet system. The electron energy gain during acceleration is calculated in He gas, where the conditions are appropriate for recombination. It is found that for a given laser intensity, there is always an optimal spot size and focal position with respect to the gas jet, which minimizes the refraction and maximizes the acceleration length for higher energy gain of the electrons. The inclusion of electron–ion recombination is more realistic if the pulse duration is longer in a laser–gas-jet experiment.

Gravitational Hertz experiment in dielectrics, excited by intense laser pulses

The possibility of generating and detection of high-frequency gravitational waves based on parametric optical processes in dielectric media excited by intense laser emission is analyzed. The theory predicts the feasibility of the Hertz gravitational laboratory experiment in dielectric media at the presence of strong constant magnetic field with illumination of this media by intense ultra short laser pulses of visible or ultraviolet ranges. Parametric conversion of ultra short intense laser pulses with frequency ω0 to a gravitational wave with frequency ωg=2ω0 and reconversion process of gravitational radiation to optical radiation at resonance condition for some dielectrics and photonic structures are predicted

Experimental Observation of Ion–Ion Acoustic Instability Associated with Collisionless Shocks in Laser-produced Plasmas

Abstract We report experimental evidence of ion–ion acoustic instability in laser-produced astrophysically relevant plasma. Irradiation of a copper foil with a short (2 ps) intense (1017 W cm−2) laser pulse generates dense hot plasma, whose expansion into the rarefied preplasma generates the electrostatic collisionless shock. A shock-reflected ion beam penetrates through the shock upstream excites the ion–ion acoustic instability. The shock oscillation electric field and the instability filamentary modulation are monitored by proton radiography. Particle-in-cell and particle-tracing simulations reproduce the experimental observation.

Generating keV ion distributions for nuclear reactions at near solid-density using intense short-pulse lasers

Our understanding of a large range of astrophysical phenomena depends on a precise knowledge of charged particle nuclear reactions that occur at very low rates, which are difficult to measure under relevant plasma conditions. Here, we describe a method for generating dense plasmas at effective ion temperatures >20 keV, sufficient to induce measurable charged particle nuclear reactions. Our approach uses ultra-intense lasers to drive micron-sized, encapsulated nanofoam targets. Energetic electrons generated in the intense laser interaction pass through the foam, inducing a rapid expansion of the foam ions; this results in a hot, near-solid density plasma. We present the laser and target conditions necessary to achieve these conditions and illustrate the system performance using three-dimensional particle-in-cell simulations, outline potential applications and calculate expected nuclear reaction rates in the D(d,n) and 12C(p,γ) systems assuming CD, or CH aerogel foams.

Electron Acceleration by a Chirped Short Intense Laser Pulse in Presence of an External Axial Magnetic Field in Vacuum With Different Phase Values

We investigated electron acceleration by a chirped short intense laser pulse in presence of an external magnetic field. The retained electron energy is very high with frequency chirp on increasing the value of chirp parameter and constant value of laser intensity parameter. Also, the retained electron energy increases on increment of laser intensity parameter. A linear frequency chirp \(\omega (t ) =\omega_0 (1-\alpha t)t\) was considered, here \(\omega _0\) is the laser frequency at \(z=0\) and \(\alpha\) is the frequency chirp parameter. On increasing the chirp parameters corresponding to the magnetic field with phase then the retained electron energy become so high. Also, we study the variation of the relativistic factor gamma \((\gamma)\) and the laser intensity parameter \((a _0 )\); also the variation of the relativistic factor gamma \(( \gamma )\) and the magnetic field \((b_0)\) with different values of the phase, \(\phi=0\), \(\pi /4\) and \(\pi/2\), respectively. As the time duration is increased the energy gain increased.

Effect of linear absorption on self focusing of Hermite Gaussian laser beam in plasma in relativistic and ponderomotive regime

Effect of linear absorption, intensity of fundamental laser pulse and plasma frequency on beam width parameter is investigated to study the self-focusing of Hermite-Gaussian(HG) laser beam in plasma under relativistic and ponderomotive regime. Intense short pulse laser propagates through plasma imparts oscillatory velocity to electrons and non linearity arises. Using the paraxial approach we have derived the second order differential equation for beam width parameter and the equation for normalized radius of incident beam is also obtained using the critical condition. The outcome of our study reveals that light absorption and intensity of radiation enhance the self focusing of Hermite-Gaussiam laser beam. It is observed that the beam width parameter shows significant decrease for higher values of linear absorption coefficient and plasma frequency. We also observe that radius of the incident beam decreases sharply with increasing intensity of incident pulse for higher values of plasma frequency.

Space and time resolved measurement of surface magnetic field in high intensity short pulse laser matter interactions

Magnetic fields produced by the interaction of intense laser beams on the surface of flat solid targets have been characterized. Laser probe polarimetry diagnosed the spatial and temporal evolution of the magnetic field by measuring the changes in the probe beam polarization due to Cotton-Mouton and Faraday effects at different times with respect to a pump laser pulse. The results show that 1 ps after the interaction of the pump laser with the target, a magnetic field of the order of a few MG is already present over a region ≈150 μm in diameter centered around the interaction spot. From the spatial and temporal evolutions of the magnetic field, we infer information on the resistivity of the material, showing evidence of a strongly magnetized resistivity.

Self modulation and scattering instability of a relativistic short laser pulse in an underdense plasma

In this paper, we present an analysis framework for description of nonlinear, self-consistent laser-plasma evolutions during propagation of a short intense laser pulse in a high-density sub-critical plasma (the pulse length exceeds the plasma wavelength). In this context, the pulse evolutions are attributed to the wakefield induced self-modulation and destabilization via parametric exponentiation of the initial noise content. The self-consistent plasma evolutions are formulated in terms of used-to-be motion constants in the absence of pulse evolutions. This proves very useful both in uncovering important plasma dynamics during self-modulation and also in facilitating the instability studies in the strongly nonlinear regime through refinement of unstable plasma perturbations. General analytical solutions, at arbitrary pulse conditions, are derived for self-modulation, indicating that the envelope evolutions are driven by the induced spatial frequency-chirp. Also, these results state that the envelope attains fine modulations which produce long wavelength low-frequency modes via beating the carrier mode. The plasma wave variations are found to convect and amplify away from the pulse front. Regarding parametric instability, we assess different scattering regimes at different pulse shapes and peak intensities, manifesting anomalous behaviors ranging from wild positioning of the Stokes wave in dispersion plane to broadening in the scattered spectrum and halting the instability. Our analyses are assisted and verified by numerous fluid and particle-in-cell simulations. Based on our results, we discuss phenomena like the pulse breakup and its different regimes, and assisted particle acceleration in the presence of pulse evolutions.

Nonlinear Compton scattering of an ultraintense laser pulse in a plasma.

Laser pulses traveling through a plasma can feature group velocities significantly differing from the speed of light in vacuum. This modifies the well-known Volkov states of an electron inside a strong laser-field from the vacuum case and, consequently, all quantum electrodynamical effects triggered by the electron. Here we present an in-depth study of the basic process of photon emission by an electron scattered from an intense short laser pulse inside a plasma, labeled nonlinear Compton scattering, based on modified Volkov solutions derived from first principles. Consequences of the nonlinear, plasma-dressed laser dispersion on the Compton spectra of emitted photons and implications for high-intensity laser-plasma experiments are pointed out. From a quantitative numerical evaluation we find the plasma to effectively suppress emission of low-frequency photons, whereas the emission of high-frequency photons is enhanced. The emission's angular distribution, on the other hand, is found to remain qualitatively unchanged with respect to the vacuum case.

High-order Particle-In-Cell simulations of laser-plasma interaction

The simulation of laser-plasma interaction via three- dimensional Particle-In-Cell methods based on higher-order schemes is considered. High-order methods allow for drastically reducing the required number of degrees of freedom while still capturing the complex physical nature of non-linear processes. The suitability of high-order methods for the acceleration of protons from thin films by intense short-pulse lasers in terms of accuracy and efficiency is shown.

Experimental evidence for the enhanced and reduced stopping regimes for protons propagating through hot plasmas

Our understanding of the dynamics of ion collisional energy loss in a plasma is still not complete, in part due to the difficulty and lack of high-quality experimental measurements. These measurements are crucial to benchmark existing models. Here, we show that such a measurement is possible using high-flux proton beams accelerated by high intensity short pulse lasers, where there is a high number of particles in a picosecond pulse, which is ideal for measurements in quickly expanding plasmas. By reducing the energy bandwidth of the protons using a passive selector, we have made proton stopping measurements in partially ionized Argon and fully ionized Hydrogen plasmas with electron temperatures of hundreds of eV and densities in the range 1020–1021 cm−3. In the first case, we have observed, consistently with previous reports, enhanced stopping of protons when compared to stopping power in non-ionized gas. In the second case, we have observed for the first time the regime of reduced stopping, which is theoretically predicted in such hot and fully ionized plasma. The versatility of these tunable short-pulse laser based ion sources, where the ion type and energy can be changed at will, could open up the possibility for a variety of ion stopping power measurements in plasmas so long as they are well characterized in terms of temperature and density. In turn, these measurements will allow tests of the validity of existing theoretical models.

Observation of Ultrafast Solid-Density Plasma Dynamics Using Femtosecond X-Ray Pulses from a Free-Electron Laser

The complex physics of the interaction between short pulse high intensity lasers and solids is so far hardly accessible by experiments. As a result of missing experimental capabilities to probe the complex electron dynamics and competing instabilities, this impedes the development of compact laser-based next generation secondary radiation sources, e.g. for tumor therapy [Bulanov2002,ledingham2007], laboratory-astrophysics [Remington1999,Bulanov2015], and fusion [Tabak2014]. At present, the fundamental plasma dynamics that occur at the nanometer and femtosecond scales during the laser-solid interaction can only be elucidated by simulations. Here we show experimentally that small angle X-ray scattering of femtosecond X-ray free-electron laser pulses facilitates new capabilities for direct in-situ characterization of intense short-pulse laser plasma interaction at solid density that allows simultaneous nanometer spatial and femtosecond temporal resolution, directly verifying numerical simulations of the electron density dynamics during the short pulse high intensity laser irradiation of a solid density target. For laser-driven grating targets, we measure the solid density plasma expansion and observe the generation of a transient grating structure in front of the pre-inscribed grating, due to plasma expansion, which is an hitherto unknown effect. We expect that our results will pave the way for novel time-resolved studies, guiding the development of future laser-driven particle and photon sources from solid targets.

Thin target charging in short laser pulse interactions

A model of target charging in interaction with an intense short laser pulse is revised. It provides estimates of the current of energetic electrons escaping the target and the total accumulated electric charge as a function of the laser pulse characteristics and target thickness. Comparisons with experimental data show the model performance, in particular, in the case of targets thinner than the mean-free path of energetic electrons.

Pulse Slippage of Resonant Third Harmonic Generation in Electron-Hole Plasma**Supported by a financial grant from CSIR, New Delhi, India, under Project No. 03(1438)/18/EMR-II

Effect of pulse slippage on resonant third harmonic generation of a short pulse laser in electron-hole plasma in the presence of wiggler magnetic field has been investigated. The group velocity mismatch of the third harmonic pulse and the fundamental pulse is significant in electron hole plasma. As the third harmonic pulse has higher group velocity than that of fundamental pulse, therefore, it moves faster than the fundamental pulse. It gets slipped out of the domain of fundamental pulse and its amplitude saturates. Phase matching condition is satisfied by applying wiggler magnetic field, which provides additional angular momentum to the third harmonic photon to make the process resonant. Enhancement in the efficiency of third harmonic generation of an intense short pulse laser in electron-hole plasma embedded with a magnetic wiggler is seen.

Multi-electron beam generation using co-propagating, parallel laser beams

High intensity short pulse laser plasma interaction experiments were performed to investigate laser wakefield acceleration (LWFA) in the ‘bubble’ regime. Using a specially designed phase plate, two high intensity laser focal spots were generated adjacent to each other with a transverse spacing of 70 μm and were focused onto a low density plasma target. We found that this configuration generated two simultaneous relativistic electron beams from LWFA (with low divergence) and that these beams often interact strongly with each other for longer propagation distances in the plasma thus reducing beam quality. In addition, it was observed that the existence of an adjacent laser driven wakefield significantly reduced the self-trapping threshold for injection of electrons. Numerical modeling of these interactions demonstrated similar phenomena and also showed that electron beam properties can be affected through precise control of the phase and polarization of the incident laser beam.

Influence of density ripple on pulse slippage of third harmonic generation in plasma

Influence of density ripple on pulse slippage of third harmonic generation has been investigated. When highly intense short range laser pulse incident on plasma, it imparts oscillatory velocity to electrons and exert a ponderomotive force at twice the frequency of incident laser beam. It induces oscillatory velocity at second harmonic frequency 2ω which further couples with density ripple resulting in density oscillations at 2ω. Density oscillations at 2ω, couple with the oscillatory velocity at ω, to produce non-linear current density at 3ω which drives the third harmonic generation. In order to provide phase matching condition between fundamental and third harmonic pulses, density ripple in plasma is introduced which offers the required momentum to the third harmonic photon to make the process resonant. At resonance, amplitude of the third harmonic pulse is found to be maximum for optimized values of density ripple factor. It is observed that the peak value of third harmonic pulse shifted towards higher value of normalized distance z' for higher values of normalized time t'. It is also seen that on account of the group velocity mismatch between pump and third harmonic pulses, the third harmonic pulse slips out of the domain of pump pulse.

Making pions with laser light

The interaction of high intensity short pulse laser beams with plasmas can accelerate electrons to energies in excess of a GeV. These electron beams can subsequently be used to generate short-lived particles such as positrons, muons, and pions. In recent experiments, we have made the first measurements of pion production using ‘all optical’ methods. In particular, we have demonstrated that the interaction of bremsstrahlung generated by laser driven electron beams with aluminum atoms can produce the long lived isotope of magnesium (27Mg) which is a signature for pion (π+) production and subsequent muon decay. Using a 300 TW laser pulse, we have measured the generation of 150 ± 50 pions per shot. We also show that the energetic electron beam is a source of an intense, highly directional neutron beam resulting from (γ, n) reactions which contributes to the 27Mg measurement as background via the (n, p) process.

Photoelectron circular dichroism in the multiphoton ionization by short laser pulses. II. Three- and four-photon ionization of fenchone and camphor.

Angle-resolved multiphoton ionization of fenchone and camphor by short intense laser pulses is computed by the time-dependent single center method. Thereby, the photoelectron circular dichroism (PECD) in the three-photon resonance enhanced ionization and four-photon above-threshold ionization of these molecules is investigated in detail. The computational results are in satisfactory agreement with the available experimental data, measured for randomly oriented fenchone and camphor molecules at different wavelengths of the exciting pulses. We predict a significant enhancement of the multiphoton PECD for uniaxially oriented fenchone and camphor.