High Intensity Short Pulse Research Articles

Postacceleration of Laser-Generated High-Energy Protons Through Conventional Accelerator Linacs

The postacceleration of laser-generated protons through conventional drift-tube linear accelerators (linacs) (DTLs) has been simulated with the particle-code PARMELA. The proton source is generated on the rear surface of a target irradiated by an high-intensity short-pulse (350 fs) laser and focused by a microlens that allows selecting collimated protons at 7 0.1 MeV with root-mean-square unnormalized transverse emittance of 0.180 mm mrad. The simulations show that protons can be accelerated by one DTL tank to more than 14 MeV with un normalized transverse-emittance growth of 8 and 22.6 in different transverse directions when considering a total proton charge of 0.112 mA. This first numerical study shows that coupling between laser-plasma accelerators with traditional accelerators is possible, allowing a luminosity gain for the final beam.

Experimental evidence and theoretical analysis of photoionized plasma under x-ray radiation produced by an intense laser

Photoionized plasma was studied experimentally under laboratory conditions by means of high intensity short pulse lasers. The experiment consists of a gold cavity filled with nitrogen gas. Six laser beams were focused on the inner surface of the gold cavity, thereby generating an almost black-body radiation having temperature of 80eV inside the cavity. This radiation heats the nitrogen gas mainly by means of photoionization. L-shell emissions from N V to N VII have been observed in the wavelength range between 90 and 200Å. A time-dependent Detailed Configuration Accounting computer program has been developed to analyze the experimental spectra. In contrast to standard analysis of astrophysical observations, the evidence for photoionization is inferred from the spectral lines ratios. Comparison between the experimental and simulated line spectra indicates that the radiation heated nitrogen attains temperature of 20–30eV, much lower than the source radiation temperature. Paradoxically, it is also shown that similar line emissions can be reproduced computationally also when the radiation and plasma temperatures both equal approximately 60eV. This misleading result indicates that experimental simulation in laboratory is sometimes necessary to avoid misinterpretation of astrophysical spectra.

High-resolution 17–75keV backlighters for high energy density experiments

17 – 75 keV one- and two-dimensional high-resolution (<10μm) radiography has been developed using high-intensity short pulse lasers. High energy Kα sources are created by fluorescence from hot electrons interacting in the target material after irradiation by lasers with intensity IL>1017W∕cm2. High-resolution point projection one- and two-dimensional radiography has been achieved using microfoil and microwire targets attached to low-Z substrate materials. The microwire size was 10μm×10μm×300μm on a 300μm×300μm×5μm polystyrene substrate. The radiography experiments were performed using the Titan laser at Lawrence Livermore National Laboratory. The results show that the resolution is dominated by the microwire target size and there is very little degradation from the plasma plume, implying that the high-energy x-ray photons are generated mostly within the microwire volume. There are enough Kα photons created with a 300J, 1-ω, 40ps pulse laser from these small volume targets, and that the signal-to-noise ratio is sufficiently high, for single shot radiography experiments. This unique technique will be used on future high energy density experiments at many new high-power laser facilities.

Quasi-isentropic material property studies at extreme pressures: from Omega to NIF

We are developing an experimental platform that can compress materials quasi-isentropically to very high pressures at ultrahigh strain rates. This laser driven, ramped (shockless) drive is used to study material properties such as strength, equation of state, phase, and phase transition kinetics under extreme conditions. We have achieved a ramped, shockless drive up to 2 Mbar on the Omega laser using both direct laser illumination and indirect x-ray illumination. In order to probe high-Z materials under extreme pressures, we are also developing high energy x-ray backlighters, 17 to 100 keV, created by high intensity (>1018 W/cm2) short pulse lasers (1 to 50 ps) such as the Titan laser at LLNL. Using a micro-wire embedded in a low-Z substrate, we have obtained radiographs with better than 10 ?m spatial resolution. This paper will show designs of isentropic platforms that can reach >10 Mbar on the NIF laser, using both direct and indirect drive configurations.

Laser-acceleration of high-energy protons in small-scale gradients

The parameters of proton beams accelerated by a high-intensity short pulse laser from a thin foil are studied for various small plasma gradients created on the rear-surface of the foil. Compared to the case of an initially cold foil, the proton beam maximum energy as well as the beam spectral slope reduce steadily when increasing the gradient scale length. The measurements are in good agreement with simulations. For future ion-driven Fast Ignition of fusion targets, multi-kJ Peta Watt laser pulses will induce target preheat due to the leading part of the laser pulse. Estimates of such preheat in these conditions suggest that it should modify by less than 10 % the ion beam parameters compared to the case of a unheated target.

Superradiant resonant Rayleigh scattering of light by the Bose-Einstein condensate of rarified atomic gases

Based on the earlier model, the superradiant Rayleigh scattering of the high-intensity short light pulses by the Bose-Einstein condensate of rubidium vapor is studied. It is demonstrated that only six states with a certain momentum are filled when the resonance detuning is about the Rabi frequency. The first two states describe the atomic motion along the condensate due to absorption, whereas the remaining four states describe the atomic motions at angles of 45° and 135° to the condensate axis owing to the reemission of the photon. The condensate pulsations between the state with zero momentum and the last four states are revealed. The corresponding shape of the coherent Rayleigh pulse is found for various resonance detunings.

Principles of laser micro sintering

PurposeThe purpose of the paper is the elucidation of certain mechanisms of laser material processing in general and laser micro sintering in particular. One major intention is to emphasize the synergism of the various effects of q‐switched laser pulses upon metal and ceramic powder material and to point out the non‐equilibrium character of reaction steps.Design/methodology/approachRecent results and observations, obtained in development of “laser micro sintering,” are surveyed and analyzed. By breaking down the overall process into relevant steps and considering their possible kinetics, an approach is made towards interpreting specific phenomena of laser micro sintering. Thermodynamics upon heating of the material as well as its photo‐electronic response to the incident radiation are considered.FindingsThe findings corroborate a model whereby short pulses of high intensity provide non‐equilibrium pressure conditions at the location of incidence, that allow for the melting of metal powder with an almost immediate expansion of a plasma and/or vapor bulb. Thereby the molten material is condensed and propelled towards the substrate. A final boiling eruption after each pulse is the reason for the morphology of the laser micro‐sintered surfaces and can prevent oxidation when the process is conducted under normal atmosphere. In sintering of ceramics, the short pulsed and intensive radiation increases the chance to excite the material even with photon energies below the bandgap value and it lowers the risk of running into a destructive avalanche.Research limitations/implicationsOwing to the stochastic character of the respective sintering event, that is initiated by each individual pulse, the gathered data are not suitable yet for the formulation of an exact quantitative function between sintering behavior and laser parameters.Practical implicationsThe qualitative findings yield a good rule of thumb for the choice of parameters in laser sintering on a micrometer scale and the model is conducive for advanced interpretation of other phenomena in laser material processing besides sintering.Originality/valueThe kinetics and thermodynamics of laser sintering with q‐switched pulses are approached by a qualitative explanation. The heterogeneous and non‐equilibrium character of the processes is taken into account; this character is often neglected by researchers in the area.

Efficient coupling of high intensity short laser pulses into snow clusters

Measurements of energy absorption of high intensity laser pulses in snow clusters are reported. Targets consisting of sapphire coated with snow nanoparticles were found to absorb more than 95% of the incident light compared to 50% absorption in flat sapphire targets.

Nonlinear Thomson scattering with strong radiation damping

With the advent of high-intensity short pulse lasers it has become possible to achieve extremely high intensities. When such laser pulses interact with plasmas or high-energy electron beams, radiation damping, which is usually small, can become large. We examine the effects of radiation damping on the backscattered Thomson radiation numerically and analytically. Using parameters comparable to the laser and microtron at the Advanced Photon Research Center we find that the overall spectrum is down-shifted and that the overall amplitude of the radiation is smaller than in the case with no damping.

Superradiant Rayleigh scattering of high-intensity short light pulses from a Bose-Einstein condensate of dilute atomic gases

Equations of a semiclassical model of superradiant Rayleigh scattering of high-intensity short light pulses from a Bose-Einstein condensate of dilute atomic gases are solved numerically taking into account the excitation of atoms by coherent Rayleigh radiation and their recoil in the backward direction. The evolution of the populations of coherent atomic states with a particular momentum is studied, and the pulse shape and the structure of the spectrum of such scattering are found in relation to the laser beam intensity and the recoil kinetic energy of atoms.

Superradiant Rayleigh scattering of light by the Bose-Einstein condensate of rarefied atomic gases

Based on the earlier model, the superradiant Rayleigh scattering of the high-intensity short light pulses by the Bose-Einstein condensate of rubidium vapor is studied. It is demonstrated that only six states with a certain momentum are filled when the resonance detuning is about the Rabi frequency. The first two states describe the atomic motion along the condensate due to absorption, whereas the remaining four states describe the atomic motions at angles of 45° and 135° to the condensate axis owing to the reemission of the photon. The condensate pulsations between the state with zero momentum and the last four states are revealed. The corresponding shape of the coherent Rayleigh pulse is found for various resonance detunings.

Pulsed Cavitational Ultrasound: A Noninvasive Technology for Controlled Tissue Ablation (Histotripsy) in the Rabbit Kidney

The optimal minimally invasive treatment for small renal masses continues to evolve. Current ablative technologies rely on thermal mechanisms for tissue destruction. However, the creation of precise lesions is limited by inhomogeneous heating/cooling due to tissue variability, perfusion effects and tissue charring. We hypothesized that nonthermal mechanical effects of ultrasound (cavitation) can be used to progressively homogenize tissue in controlled fashion with predictable results. We developed a focused annular array ultrasound system capable of delivering high intensity (greater than 20 kW/cm) short pulses (20 microseconds) of energy to a target volume. This system operates at a repetition frequency of 100 Hz, resulting in a low time averaged power output (approximately 5 W total acoustic output). Following approval from the institutional animal care committee a series of transcutaneous ablations were performed in the normal kidneys of 10 rabbits. Lesions created with a small number of pulses (10 or 100) produced scattered areas of damage characterized by focal hemorrhage and small areas of cellular injury in the targeted volume. Lesions created with greater numbers of pulses (1,000 or 10,000) demonstrated complete destruction of the targeted volume. Gross examination revealed that lesions contained a liquefied core with smooth walls and sharply demarcated boundaries. Histological examination demonstrated extensive areas of acellular debris surrounded by a narrow margin of cellular injury. This pulsed cavitational ultrasound system is capable of transcutaneous nonthermal destruction of renal tissue. Refinement of this technology for noninvasive ablation of small renal masses is currently under way.

Measurements of plasma-wave generation using a short-pulse high-intensity laser beat wave

Experiments to examine the generation of relativistic plasma waves via a high-intensity short-pulse beat-wave scheme are described in detail. The pulse stretcher of the Vulcan chirped-pulse amplification (CPA) laser system was modified to produce two frequency, 3ps pulses focusable to intensities up to 1018Wcm−2. Short high-intensity pulses were used to avoid limitations to the plasma-wave amplitude due to the modulational instability. Two experiments were undertaken, at 3 and 10TW, with the generation of plasma waves diagnosed by measuring the sidebands produced in the spectrum of the forward scattered beam. A resonance in the sideband signal was observed for an initial plasma density higher than expected for the given beat frequency. This resonance shift can be attributed to transverse ponderomotive expulsion of plasma electrons from the laser focal region. A monotonically increasing background was also observed, which was due to nonresonant cross-phase modulation.

Spectral and dynamical features of the electron bunch accelerated by a short-pulse high intensity laser in an underdense plasma

The results of the theoretical consideration and two-dimensional particle in cell simulation of electron acceleration with a short-pulse intense laser propagating through a finite length underdense plasma layer are presented. The fast electron energy spectrum and emittance are analyzed for moderate to high intensity and for different plasma density. It is shown that for laser pulse lengths above the plasma wake wavelength the wakefield accelerated electrons are further accelerated by the electromagnetic wave.

Vlasov Models for Laser‐Plasma Interaction

Abstract In this paper we discuss Vlasov models and Vlasov simulations to study short pulse high intensity laser‐plasma interaction. For these studies analytical calculations are performed from more simple fluid models: nonlinear dispersion relations, growth rates, envelope models, Manley‐Rowe partition, etc. For a full kinetic description numerical codes are necessary; among different methods particle in cell or PIC codes play a central role. On the other hand, Vlasov codes solve the Vlasov equation as a partial differential equation (PDE) in phase space. Due to their excellent resolution, they have proven to be a useful tool to study the nonlinear resonant wave‐particle interaction, which may be important in laser‐plasma studies. A disadvantage is the need of large memory to ensure correct discretization in phase space. The goal of the paper is to give some insight into the question of choosing between PIC and Vlasov codes, both from a numerical and a physical point of view. Some pedagogical examples wi...

Quasi-static particle simulation of short pulse laser-plasma interaction

We have successfully developed the two-dimensional quasi-static particle simulation code MLPIC2D, which is an effective numerical tool to be used to describe the long-time plasma behavior under the action of an ultra high intensity short pulse laser. Simple physical modeling, numerical realization and code framework are discussed in detail. Three nonlinear phenomena of laser-plasma interaction:1) production of laser wake-fields, 2)relativistic focusing for short laser pulses, 3)quasi-static magnetic fields, are correctly simulated and analysed.

Characteristics of high energy Kα and Bremsstrahlung sources generated by short pulse petawatt lasers

We have measured the characteristics of high energy Kα sources created with the Vulcan Petawatt laser at RAL and the JanUSP laser at Lawrence Livermore National Laboratory. High energy x-ray backlighters will be essential for radiographing high energy-density experimental science targets for NIF projects especially to probe implosions and high areal density planar samples. Hard Kα x-ray photons are created through relativistic electron plasma interactions in the target material after irradiated by short pulse high intensity lasers. For our Vulcan experiment, we employed a CsI scintillator charge coupled device (CCD) camera for imaging and a CCD camera for single photon counting. We have directly measured the 22 keV Ag Kα source size using the RAL petawatt laser and performed knife-edge measurements of a 40 keV Sm Kα source using the JanUSP laser. The measured source sizes are both ∼60 μm full width half maximum. We have also measured the Ag Kα conversion efficiencies. At laser intensities of 1×1018 W/cm2 range, the conversion efficiency at 22 keV is ∼1×10−4.

Pulsed laser deposition of hard coatings in atmospheric air

A new laser plasma technique for non-vacuum deposition of thin films has been proposed and experimentally realized. It is based on the fact that the plasma plume, which occurs under ablation of a target in air by high-intensity short laser pulses, can penetrate through a dense gas environment without significant cooling at the distance of about 1 mm. The technique has been applied to deposit diamond-like carbon (DLC) coatings on stainless steel substrates using four different values of pulse duration: 10 ns, 300 ps, 5 ps and 130 fs. Optimization of different experimental parameters including distance between the target and the substrate, laser intensity and gases (He, Ar, N2, compressed air) blown in the deposition zone, has been performed. The deposition rate in the experiments was estimated as 2–5×10-4 nm/(cm2 pulse) for the pulse energy of 1–4 mJ. The deposited amorphous carbon films with thickness of several hundred nanometers have shown high average nanohardness (10–25 GPa depending on the irradiation conditions) and good adhesion to substrates (≥60 MPa). According to X-ray electron spectroscopy analysis the films consist of both sp2- and sp3-bonded carbon and contain 3–7% of free oxygen in bulk. The mechanisms of DLC non-vacuum laser deposition are discussed. To demonstrate the large potential of this technique, the first results on deposition of titanium nitride using ablation of titanium in air with nitrogen jet assistance are also presented.

Overview of future directions in high energy-density and high-field science using ultra-intense lasers

The increasing proliferation of 100 TW class ultrashort pulse lasers and the near completion of a number of petawatt class lasers world wide is opening many frontiers in laser science. Some of the most exciting frontiers rest in high energy-density science and high field physics. A multi-TW laser can create heated matter with pressure in excess of a Gbar and can create electric fields of ten to one hundred atomic units. In this paper some of the recent advances in high energy density science and high field physics made using high intensity short pulse lasers will be reviewed with illustrative examples from work performed at the University of Texas and Lawrence Livermore National Laboratory.

Mechanism of high-energy electron production in a laser plasma

A mechanism of high-energy electron production in the interaction of high-intensity short laser pulses with a solid target is proposed and analysed. The theoretical dependences of fast-electron kinetic energy on the parameters of laser radiation and target material are given. The effect of ionisation of the target material is considered. The generation of ultrastrong magnetic fields in the laser plasma is shown to play the key part in the formation, transfer, and acceleration of electron beams. This results in the production of vortex electric fields accelerating electrons. The theoretical dependences yield well-proved limits for the electron energy and are in good agreement with the results of experiments performed on high-intensity laser setups, including the results obtained with participation of the author.