Preformed Plasma Research Articles

Plasma scale-length effects on electron energy spectra in high-irradiance laser plasmas.

An analysis of an electron spectrometer used to characterize fast electrons generated by ultraintense (10^{20}Wcm^{-2}) laser interaction with a preformed plasma of scale length measured by shadowgraphy is presented. The effects of fringing magnetic fields on the electron spectral measurements and the accuracy of density scale-length measurements are evaluated. 2D EPOCH PIC code simulations are found to be in agreement with measurements of the electron energy spectra showing that laser filamentation in plasma preformed by a prepulse is important with longer plasma scale lengths (>8μm).

The formation mechanism of the periodic nanograting structure by the Weibel instability

The two-dimensional particle in cell code has been used to demonstrated the formation mechanism for the periodic nanograting structure using 500-fs pulses of an ultra-fast laser with wavelength 800 nm, incidence angle 0°, linearly-polarized, and intensity 1018 W/cm2 µm2 in hydrogen plasma. The periodic nanograting structure has been clearly self-organized at the boundary between the preformed plasma and the dense plasma at t = 250 fs. By time evolution of the magnetic field and the current density in the dense plasma, it has been found that the Weibel instability plays a significant role to form the periodic nanograting structure.

Enhancement of conversion efficiency and spatial coherence of high order harmonics generated from pre-formed plasma plumes using an apertured laser beam

The effect of laser beam aperturing (Ti: sapphire laser; 800 nm, 45 fs) on the conversion efficiency (CE), harmonic yield and spatial coherence of high order harmonics generated from a low excited plasma plume of carbon (neutral and singly charged ions) is studied. The optimization of CE by varying the aperture size in combination with different laser and plasma plume parameters was carried out. The CE of the 15th harmonic is enhanced from ∼10−7 for an unapertured laser beam to ∼10−6 for ∼¼ times the unapertured beam diameter, at same laser energy in both cases. Further, it was found that, at a given aperture size, there is an optimum prepulse intensity for maximum CE. The results are explained by taking into account the theoretical consideration of phase matching issues in plasma plumes. The study shows that tuning the prepulse intensity and aperture size gives an easy handle to increase the harmonic conversion along with its coherence properties. The study will be useful for producing an intense source of harmonics with high spatial coherence from plasmas plumes for applications.

Sharpening of the 6.8 nm peak in an Nd:YAG laser produced Gd plasma by using a pre-formed plasma

For effective use of a laser-produced-plasma (LPP) light source, an LPP is desired to emit a narrow spectral peak because the reflection spectrum of multilayer mirrors for guiding emission from the source is very narrow. While a Gd plasma has been studied extensively as an extreme ultraviolet (EUV) light source at around 6.8 nm, where La/B4C multilayer is reported to have a high reflectivity with a bandwidth of about 0.6 %, all previous works using an Nd:YAG laser reported very broad spectra. This paper reports the first narrowing of the 6.8 nm peak in the case of using an Nd:YAG laser to generate a Gd plasma by using a pre-pulse. The best peak narrowing is observed when a pre-formed plasma is heated by a 1064 nm main laser pulse with a duration of 10 ns at the irradiation density of 4x 1011 W/cm2 at a delay time of 50 ns after the pre-pulse irradiation. The observed spectral width of about 0.3 nm is about one fifth of the value for no pre-formed plasma. The peak wavelength of the 6.8 nm band shifted to a longer wavelength side and the peak was broadened both for lower and higher laser irradiation density. It is discussed that this robustness of the peak position of the 6.8 nm Gd peak against temperature change is suitable to achieve a narrow bandwidth from an LPP generated on solid. The observed spectra are compared with those previously reported in various conditions.

Modulation of terahertz generation in dual-color filaments by an external electric field and preformed plasma**Project supported by the National Key Scientific Instrument Project, China (Grant No. 2012YQ150092), the National Basic Research Program of China (Grant No. 2011CB808105), the National Natural Science Foundation of China (Grant No. 11434005), the China Postdoctoral Science Foundation (Grant No. 2014M560348), and the Fund from the

Terahertz generation driven by dual-color filaments in air is demonstrated to be remarkably enhanced by applying an external electric field to the filaments. As terahertz generation is sensitive to the dual-color phase difference, a preformed plasma is verified efficiently in modulating terahertz radiation from linear to elliptical polarization. In the presence of preformed plasma, a dual-color filament generates terahertz pulses of elliptical polarization and the corresponding ellipse rotates regularly with the change of the preformed plasma density. The observed terahertz modulation with the external electric field and the preformed plasma provides a simple way to estimate the plasma density and evaluate the photocurrent dynamics of the dual-color filaments. It provides further experimental evidence of the photo-current model in governing the dual-color filament driven terahertz generation processes.

Multistage coupling of independent laser-plasma accelerators.

Laser-plasma accelerators (LPAs) are capable of accelerating charged particles to very high energies in very compact structures. In theory, therefore, they offer advantages over conventional, large-scale particle accelerators. However, the energy gain in a single-stage LPA can be limited by laser diffraction, dephasing, electron-beam loading and laser-energy depletion. The problem of laser diffraction can be addressed by using laser-pulse guiding and preformed plasma waveguides to maintain the required laser intensity over distances of many Rayleigh lengths; dephasing can be mitigated by longitudinal tailoring of the plasma density; and beam loading can be controlled by proper shaping of the electron beam. To increase the beam energy further, it is necessary to tackle the problem of the depletion of laser energy, by sequencing the accelerator into stages, each powered by a separate laser pulse. Here, we present results from an experiment that demonstrates such staging. Two LPA stages were coupled over a short distance (as is needed to preserve the average acceleration gradient) by a plasma mirror. Stable electron beams from a first LPA were focused to a twenty-micrometre radius--by a discharge capillary-based active plasma lens--into a second LPA, such that the beams interacted with the wakefield excited by a separate laser. Staged acceleration by the wakefield of the second stage is detected via an energy gain of 100 megaelectronvolts for a subset of the electron beam. Changing the arrival time of the electron beam with respect to the second-stage laser pulse allowed us to reconstruct the temporal wakefield structure and to determine the plasma density. Our results indicate that the fundamental limitation to energy gain presented by laser depletion can be overcome by using staged acceleration, suggesting a way of reaching the electron energies required for collider applications.

Electron acceleration at grazing incidence of a subpicosecond intense laser pulse onto a plane solid target

AbstractGeneration of hot electrons at grazing incidence of a subpicosecond relativistic-intense laser pulse onto a plane solid target is analyzed for the parameters of petawatt class laser systems. We study preplasma formation on the surface of solid aluminum targets produced by laser prepulses with a different time structure. For modeling of the preplasma dynamics, we use a wide-range two-temperature hydrodynamic model. As a result of simulations, the preplasma expansion under the action of the laser prepulse and the plasma density profiles for different contrast ratios of the nanosecond pedestal are found. These density profiles are used as the initial density distributions in three-dimensional particle-in-cell simulations of electron acceleration by the main P-polarized laser pulse. Results of modeling demonstrate a substantial increase of the characteristic energy and number of accelerated electrons for the grazing incidence of a subpicosecond intense laser pulse in comparison with the ponderomotive scaling of laser–target interaction.

Second harmonic generation by self focused q-Gaussian laser beam in preformed collisional parabolic plasma channel

Abstract This paper presents an investigation of self-focusing of a q-Gaussian laser beam and its effect on second generation in a preformed collisional parabolic plasma channel. In the presence of a q-Gaussian laser beam redistribution of carriers takes place on account of nonuniform heating produced due to collisional nonlinearity, as a result of which, a transverse density gradient is established in the channel which in turn generates plasma wave at pump frequency. Generated plasma wave interacts with the incident laser beam and generate second harmonics of the incident laser beam. Moment theory has been used to derive differential equation governing the evolution of spot size of laser beam propagating through the channel. The differential equation so obtained has been solved numerically. The effect of the intensity of laser beam, deviation of intensity distribution of laser beam along its wavefront from Gaussian distribution, plasma density and depth of channel on beam width of laser beam as well as second harmonic yield has been investigated.

Impact of pre-plasma on fast electron generation and transport from short pulse, high intensity lasers

Previous experiments and modeling examining the impact of an underdense, pre-formed plasma in laser-plasma interactions have shown that the fast electrons are generated with energies higher than predicted by ponderomotive scaling [, –]. We report on experiments using the Texas Petawatt high intensity (150 fs, 1.5 × 1020 W cm−2) laser pulse, which were conducted to examine the mechanism for accelerating these high energy electrons. These experiments gauge the impact a controlled low density pre-formed plasma has on electron generation with a shorter time scale than previous experiments, 150–180 fs. Electron temperatures measured via magnetic spectrometer on experiment were found to be independent of preformed plasma. Supplemental computational results using 1D PIC simulations predict that super-ponderomotive electrons are generated inside a potential well in the pre-plasma []. However, while the potential well is established around 150 fs, the electrons require at least an additional 50 fs to be trapped and heated inside it.

Second harmonic generation of q-Gaussian laser beam in preformed collisional plasma channel with nonlinear absorption

This paper presents a scheme for second harmonic generation of an intense q-Gaussian laser beam in a preformed parabolic plasma channel, where collisional nonlinearity is operative with nonlinear absorption. Due to nonuniform irradiance of intensity along the wavefront of the laser beam, nonuniform Ohmic heating of plasma electrons takes place. Due to this nonuniform heating of plasma, the laser beam gets self-focused and produces strong density gradients in the transverse direction. The generated density gradients excite an electron plasma wave at pump frequency that interacts with the pump beam to produce its second harmonics. The formulation is based on a numerical solution of the nonlinear Schrodinger wave equation in WKB approximation followed by moment theory approach. A second order nonlinear differential equation governing the propagation dynamics of the laser beam with distance of propagation has been obtained and is solved numerically by Runge Kutta fourth order technique. The effect of nonlinear absorption on self-focusing of the laser beam and conversion efficiency of its second harmonics has been investigated.

Terahertz radiation by self-focused amplitude-modulated Gaussian laser beam in magnetized ripple density plasma

AbstractThis paper presents a theoretical model for efficient terahertz (THz) radiation by self-focused amplitude-modulated laser beam in preformed ripple density plasma. The density of plasma is modified due to ponderomotive nonlinearity which arises because of the nonuniform spatial profile of the laser beam in magnetized plasma and leads to the self-focusing of the laser beam. The rate of self-focusing depends on the intensity of the amplitude-modulated beam as well as on the externally applied magnetic field strength. The electron also experiences time-dependent ponderomotive force by the laser beam at modulated frequency. A nonlinear current at THz frequency arises on account of the coupling between the ripple density plasma and nonlinear oscillatory velocity of the electrons. The yield of the generated THz radiation enhances with enhancement in self-focusing of the laser beam and applied magnetic field.

Laser electron acceleration in the prepulse produced plasma corona

The generation of hot electrons at grazing incidence of a subpicosecond relativistic-intense laser pulse onto the plane solid target is analyzed for the parameters of the petawatt class laser systems. We study the preplasma formation on the surface of solid Al target produced by the laser prepulses with different time structure. For modeling of the preplasma dynamics we use a wide-range two-temperature hydrodynamic model. As a result of simulations, the preplasma expansion under the action of the laser prepulse and the plasma density profiles for different contrast ratios of the nanosecond pedestal are found. These density profiles were used as the initial density distributions in 3-D PIC simulations of electron acceleration by the main P-polarized laser pulse. Results of modeling demonstrate the substantial increase of the characteristic energy and number of accelerated electrons for the grazing incidence of a subpicosecond intense laser pulse in comparison with the laser-target interaction at normal incidence.

Characterization of intense laser-produced fast electrons using hard x-rays via bremsstrahlung

Energy distribution of high-power, short-pulse laser produced fast electrons was experimentally and numerically studied using high-energy bremsstrahlung x-rays. The hard x-ray photons and escaping electrons from various metal foils, irradiated by the 50 TW Leopard laser at Nevada Terawatt Facility, were recorded with a differential filter stack spectrometer that is sensitive to photons produced by mainly 0.5–2 MeV electrons and an electron spectrometer measuring >2 MeV electrons. The experimental bremsstrahlung and the slope of the measured escaped electrons were compared with an analytic calculation using an input electron spectrum estimated with the ponderomotive scaling. The result shows that the electron spectrum entering a Cu foil could be continuous single slope with the slope temperature of ∼1.5 MeV in the detector range. The experiment and analytic calculation were then compared with a 2D particle-in-cell code, PICLS, including a newly developed radiation transport module. The simulation shows that a two-temperature electron distribution is generated at the laser interaction region, but only the hot component of the fast electrons flow into the target during the interaction because the low energy electron component is trapped by self-generated magnetic field in the preformed plasma. A significant amount of the photons less than 100 keV observed in the experiment could be attributed to the low energy electrons entering the foil a few picoseconds later after the gating field disappears.

Pulse evolution and plasma-wave phase velocity in channel-guided laser-plasma accelerators.

The self-consistent laser evolution of an intense, short-pulse laser exciting a plasma wave and propagating in a preformed plasma channel is investigated, including the effects of pulse steepening and energy depletion. In the weakly relativistic laser intensity regime, analytical expressions for the laser energy depletion, pulse self-steepening rate, laser intensity centroid velocity, and phase velocity of the plasma wave are derived and validated numerically.

Chirped pulse Raman amplification in warm plasma: towards controlling saturation.

Stimulated Raman backscattering in plasma is potentially an efficient method of amplifying laser pulses to reach exawatt powers because plasma is fully broken down and withstands extremely high electric fields. Plasma also has unique nonlinear optical properties that allow simultaneous compression of optical pulses to ultra-short durations. However, current measured efficiencies are limited to several percent. Here we investigate Raman amplification of short duration seed pulses with different chirp rates using a chirped pump pulse in a preformed plasma waveguide. We identify electron trapping and wavebreaking as the main saturation mechanisms, which lead to spectral broadening and gain saturation when the seed reaches several millijoules for durations of 10’s – 100’s fs for 250 ps, 800 nm chirped pump pulses. We show that this prevents access to the nonlinear regime and limits the efficiency, and interpret the experimental results using slowly-varying-amplitude, current-averaged particle-in-cell simulations. We also propose methods for achieving higher efficiencies.

Self-focusing and self-channelling of short circularly laser pulses propagating in a magnetized plasma channel

In this study, non-paraxial nonlinear propagation of a relativistic laser pulse with linear and circular polarization in a preformed plasma channel having a parabolic density profile is investigated. The circular laser spot size variation in the absence/presence of an external longitudinal magnetic field is separately analysed and the results are compared. The amplitude of oscillation is decreased in the relativistic case, but it is changed in the presence of an external field. The right-turn circular pulse behaviour is completely opposite to that of the left-turn circular pulse in the presence of an external field, because the right-turn pulse is focused later than the left-turn pulse. The effect of nonlinearities on oscillations of spot size is analysed and the nonlinear critical channel depth required for propagation of a self-trapped laser pulse is evaluated. The laser field amplitude is derived for a matched pulse and it is shown that the group velocity dispersion effect causes broadening of the pulse length.

Studies of ablated plasma and shocks produced in a planar target by a sub-nanosecond laser pulse of intensity relevant to shock ignition

AbstractThe effect of laser intensity on characteristics of the plasma ablated from a low-Z(CH) planar target irradiated by a 250 ps, 0.438 µm laser pulse with the intensity of up to 1016W/cm2as well as on parameters of the laser-driven shock generated in the target for various scale-lengths of preformed plasma was investigated at the kilojoule Prague Asterix Laser System (PALS) laser facility. Characteristics of the plasma were measured with the use of 3-frame interferometry, ion diagnostics, an X-ray spectrometer, andKαimaging. Parameters of the shock generated in a Cl doped CH target by the intense 3ω laser pulse were inferred by numerical hydrodynamic simulations from the measurements of craters produced by the shock in the massive Cu target behind the CH layer. It was found that the pressure of the shock generated in the plastic layer is relatively weakly influenced by the preplasma (the pressure drop due to the preplasma presence is ~10–20%) and at the pulse intensity of ~1016W/cm2the maximum pressure reaches ~80–90 Mbar. However, an increase in pressure of the shock with the laser intensity is slower than predicted by theory for a planar shock and the maximum pressure achieved in the experiment is by a factor of ~2 lower than predicted by the theory. Both at the preplasma absence and presence, the laser-to-hot electrons energy conversion efficiency is small, ~1% or below, and the influence of hot electrons on the generated shock is expected to be weak.

Generation and pointing stabilization of multi-GeV electron beams from a laser plasma accelerator driven in a pre-formed plasma waveguidea)

Laser pulses with peak power 0.3 PW were used to generate electron beams with energy >4 GeV within a 9 cm-long capillary discharge waveguide operated with a plasma density of ≈7×1017 cm−3. Simulations showed that the super-Gaussian near-field laser profile that is typical of high-power femtosecond laser systems reduces the efficacy of guiding in parabolic plasma channels compared with the Gaussian laser pulses that are typically simulated. In the experiments, this was mitigated by increasing the plasma density and hence the contribution of self-guiding. This allowed for the generation of multi-GeV electron beams, but these had angular fluctuation ≳2 mrad rms. Mitigation of capillary damage and more accurate alignment allowed for stable beams to be produced with energy 2.7±0.1 GeV. The pointing fluctuation was 0.6 mrad rms, which was less than the beam divergence of ≲1 mrad full-width-half-maximum.

Second harmonic generation by relativistic self-focusing of q-Gaussian laser beam in preformed parabolic plasma channel

This paper presents an investigation of relativistic self-focusing effect of a q-Gaussian laser beam on second harmonic generation in a preformed parabolic plasma channel. An expression has been derived for density perturbation associated with the plasma wave excited by the laser beam. This in turn acts as a source of second harmonic generation. The moment theory approach has been used to derive a differential equation that governs the evolution of spot size of the laser beam with the distance of propagation. The detailed effects of intensity distribution deviation from Gaussian distribution, intensity of laser beam, density, and depth of the channel have been studied on self-focusing as well as on second harmonic generation.

Periodic surface structures on titanium self-organized upon double femtosecond pulse exposures

Laser induced periodic surface structures (LIPSS) self-organized on Ti surface after irradiations by femtosecond laser beam composed by double pulses with a fixed time delay of 160fs. The fluence of the first pulse (FPP), responsible for surface plasma formation, was varied in the range 10–50mJcm−2 and always kept below the LIPSS formation threshold fluence (FLIPSS) on Ti for 50-single-shots exposure. The fluence of the delayed pulse (FLP), responsible for LIPSS self-organization, was varied in the range 60–150mJcm−2 and always kept above FLIPSS. Regardless the specific fluence FLP of the delayed pulse, the interspace of the grating structures increases with the increase of FPP, that is an increase of the surface plasma density. This tendency suggests that a variation of the surface plasma density, due to a variation of FPP, actually leads to a modification of the grating features. Moreover, we observed that the LIPSS periodicities after double pulse exposures are in quite good agreement with data on LIPSS periodicities after single 160fs pulse irradiations on Ti surface and with the curve predicted by the parametric decay model. This experimental result suggests that the preformed plasma might be produced in the rising edge of the temporal profile of the laser pulse.