Short Pulse Laser Plasma Research Articles

Ultra-compact x-ray spectrometer for high-repetition-rate laser-plasma experiments.

We present in this work the development of an ultra-compact, multi-channel x-ray spectrometer (UCXS). This diagnostic has been specially built and adapted to perform at high-repetition-rate (>1Hz) for high-intensity, short-pulse laser plasma experiments. X-ray filters of varying materials and thicknesses are chosen to provide spectral resolution up to ΔE ≈ 1keV over the x-ray energy range of 1-30keV. These filters are distributed over a total of 25 channels, where each x-ray filter is coupled to a single scintillator. The UCXS is designed to detect and resolve a large variety of laser-driven x-ray sources such as low energy bremsstrahlung emission, fluorescence, and betatron radiation (up to 30keV). Preliminary results from commissioning experiments at the ABL laser facility at Colorado State University are provided.

On the electromagnetic-electron rings originating from the interaction of high-power short-pulse laser and underdense plasma

We investigate the evolution of radial profile of a high-power short-pulse laser interacting with underdense plasma, and in particular, we concentrate on the transverse electromagnetic rings, which are formed due to the laser radiation defocusing induced by the excitation of Langmuir waves. We illustrate the physical processes involved in the formation of such structures analytically and use the three-dimensional numerical simulations to reveal the relationships among the electromagnetic ring properties and the parameters of laser and plasma. Within the studied parameter range, we find that up to ≈70 % of the total initial driver pulse energy can be carried off by the electromagnetic rings having the opening angles ≈45–115 mrad. Furthermore, we show that the electromagnetic rings can become a source of high-energy ring-shaped electron beams.

Strong surface magnetic field generation in relativistic short pulse laser–plasma interaction with an applied seed magnetic field

While plasma often behaves diamagnetically, we demonstrate that the laser irradiation of a thin opaque target with an embedded target-transverse seed magnetic field Bseed can trigger the generation of an order-of-magnitude stronger magnetic field with opposite sign at the target surface. Strong surface field generation occurs when the laser pulse is relativistically intense and results from the currents associated with the cyclotron rotation of laser-heated electrons transiting through the target and the compensating current of cold electrons. We derive a predictive scaling for this surface field generation, Bgen ∼ −2πBseedΔx/λ0 (in the large spot size limit), where Δx is the target thickness and λ0 is the laser wavelength, and conduct 1D and 2D particle-in-cell simulations to confirm its applicability over a wide range of conditions. We additionally demonstrate that both the seed and surface-generated magnetic fields can have a strong impact on application-relevant plasma dynamics, for example substantially altering the overall expansion and ion acceleration from a μm-thick laser-irradiated target with a kilotesla-level seed magnetic field.

Time-resolved diffraction with an optimized short pulse laser plasma X-ray source.

We present a setup for time-resolved X-ray diffraction based on a short pulse, laser-driven plasma X-ray source. The employed modular design provides high flexibility to adapt the setup to the specific requirements (e.g., X-ray optics and sample environment) of particular applications. The configuration discussed here has been optimized toward high angular/momentum resolution and uses Kα-radiation (4.51 keV) from a Ti wire-target in combination with a toroidally bent crystal for collection, monochromatization, and focusing of the emitted radiation. Ti-Kα1 photons per pulse with relative bandwidth are delivered to the sample at a repetition rate of 10 Hz. This allows for the high dynamic range (104) measurements of transient changes in the rocking curves of materials as for example induced by laser-triggered strain waves.

Abnormal spectral distortion of a silicon sensor-based single photon counting charge coupled device (PIXIS-XB: 1300R) in detecting laser plasma x-ray source of 20–100 keV

Single photon counting using a charge-coupled device (CCD) is a conventional method of measuring the x-ray production of laser plasma. But the spectrum is seriously distorted when measuring a short pulse laser plasma x-ray source. Here we explore this abnormal spectral distortion using a silicon sensor-based CCD (PIXIS-XB: 1300R). We found that the spectral distortion is caused by the continuous spectrum of Compton scattering electrons and the K-α and K-β characteristic lines of Ag elements excited by the harder x-rays of >25 keV. We calibrated the detection efficiency of the incident x-ray, the excitation efficiency of the Ag characteristic lines and the Compton scattering electrons spectrum in the x-ray energy range from 20 keV to 100 keV. We also improve the algorithm of spectrum reconstruction to loosen the exposure level limit markedly. Our effort extends the detecting range of silicon sensor-based CCDs from 0.5–30 keV to 0.5–80 keV. The present analysis can also be used in the spectral extraction of GaAs and CdTe high-Z sensor-based CCDs.

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.

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.

Two-dimensional time-resolved ultra-high speed imaging of K-alpha emission from short-pulse-laser interactions to observe electron recirculation

Time resolved x-ray images with 7 ps resolution are recorded on relativistic short-pulse laser-plasma experiments using the dilation x-ray imager, a high-speed x-ray framing camera, sensitive to x-rays in the range of ≈1−17 keV. This capability enables a series of 2D x-ray images to be recorded at picosecond scales, which allows for the investigation of fast electron transport within the target with unprecedented temporal resolution. An increase in the Kα-emission spot size over time was found for targets thinner than the recirculation limit and is absent for thicker targets. Together with the observed polarization dependence of the spot size increase, this indicates that electron recirculation is relevant for the x-ray production in thin targets.

Laser beat frequency heating of a rippled density plasma

Two collinear laser beams propagating through a rippled density plasma, with their frequency difference close to plasma frequency, resonantly excite a large amplitude plasma wave. The density ripple of suitable wavenumber slows down the plasma wave very significantly, leading to strong electron heating via the Landau damping of the plasma wave. An analytical framework of the process is developed and the electron temperature scaling with plasma density, laser power and laser frequency have been obtained. Its relevance to recent experiments on intense short pulse laser plasma interaction has been discussed.

Effect of pulse slippage on density transition-based resonant third-harmonic generation of short-pulse laser in plasma

The resonant third-harmonic generation of a self-focusing laser in plasma with a density transition was investigated. Because of self-focusing of the fundamental laser pulse, a transverse intensity gradient was created, which generated a plasma wave at the fundamental wave frequency. Phase matching was satisfied by using a Wiggler magnetic field, which provided additional angular momentum to the third-harmonic photon to make the process resonant. An enhancement was observed in the resonant third-harmonic generation of an intense short-pulse laser in plasma embedded with a magnetic Wiggler with a density transition. A plasma density ramp played an important role in the self-focusing, enhancing the third-harmonic generation in plasma. We also examined the effect of the Wiggler magnetic field on the pulse slippage of the third-harmonic pulse in plasma. The pulse slippage was due to the group-velocity mismatch between the fundamental and third-harmonic pulses.

The National Ignition Facility: Transition to a User Facility

The National Ignition Facility (NIF) at Lawrence Livermore National Laboratory (LLNL) has been operational since March 2009 and has been transitioning to a user facility supporting ignition science, high energy density science (HEDS), national security applications, and fundamental science. The facility has achieved its design goal of 1.8 MJ and 500 TW of 3ω light on target, and has performed target experiments with 1.9 MJ at peak powers of 410 TW. The facility is on track to perform over 200 target shots this year in support of all of its user communities. The facility has nearly 60 diagnostic systems operational and has shown flexibility in laser pulse shape and performance to meet the requirements of its multiple users. Progress continues on its goal of demonstrating thermonuclear burn in the laboratory. It has performed over 40 indirect-drive experiments with cryogenic-layered capsules. New platforms are being developed for HEDS and fundamental science. Equation-of-state and material strength experiments have been done on a number of materials with pressures of over 50 MBars obtained in diamond, conditions never previously encountered in the laboratory and similar to those found in planetary interiors. Experiments are also in progress investigating radiation transport, hydrodynamic instabilities, and direct drive implosions. NIF continues to develop as an experimental facility. Advanced Radiographic Capability (ARC) is now being installed on NIF for producing high-energy radiographs of the imploded cores of ignition targets and for short pulse laser-plasma interaction experiments. One NIF beam is planned for conversion to two picosecond beams in 2014. Other new diagnostics such as x-ray Thomson scattering, low energy neutron spectrometer, and multi-layer reflecting x-ray optics are also planned. Incremental improvements in laser performance such as improved optics damage performance, beam balance, and back reflection control are being pursued.

Self-consistent particle-in-cell modelling of short pulse absorption and transport for high energy density physics experiments

In order for detailed, solid density particle-in-cell (PIC) simulations to run within a reasonable time frame, novel approaches to modelling high density material must be employed. For the purposes of modelling high intensity, short pulse laser-plasma interactions, however, these approaches must be consistent with retaining a full PIC model in the low-density laser interaction region. By replacing the standard Maxwell field solver with an electric field update based on a simplified Ohm's law in regions of high electron density, it is possible to access densities at and above solid without being subject to the standard grid and time step constraints. Such a model has recently been implemented in the PIC code EPOCH. We present the initial results of a detailed two-dimensional simulation performed to compare the adapted version of the code with recent experimental results from the Orion laser facility.

Transient self-focusing of an intense laser pulse in magnetized plasmas under non-paraxial approximation

AbstractNon-paraxial approximation based study of transient self-focusing of an intense short-pulse laser in plasma has been investigated by considering the effect of a transverse magnetic field. The laser with non-uniform distribution of intensity exerts a ponderomotive force on electrons and sets in an ambi-polar diffusion of the plasma. The ambient magnetic field, however, strongly inhibits the process, when the electron Larmor radius is comparable to or shorter than the laser spot size. As the plasma density is depleted, the laser beam becomes more self-focused. This study addresses a significant enhancement in the laser self-focusing rate by including the correction terms due the off-axis approximation.

Integrated calculations of short-pulse laser interactions with matter

A complete description of high energy density physics experiments on laser systems incorporating short, picosecond, beamlines requires a daunting breadth of physics, which cannot reasonably be included in a single model. Modern particle in cell (PIC) codes, Vlasov–Fokker–Planck models and radiation hydrodynamic codes together constitute the tools required to model key aspects of short-pulse laser–plasma interaction in such experiments. A predictive modelling capability in this area requires that all these tools are applied to a given problem. Our approach is to develop an integrated modelling capability in which these detailed models for different aspects of the problem are linked together. We outline our methodology and demonstrate the approach taken to link PIC models of laser–plasma interaction into hybrid models of electron transport within a radiation hydrodynamics code. This integrated model is used to study the absorption and transport of short-pulse delivered energy in a solid diamond target, highlighting distinct differences between the integrated results and modelling which relies on an assumed hot electron spectrum. We extend this work to consider structured targets which show the promise of providing an additional element of control over target heating as well as presenting an opportunity to test the transport models employed. Finally we demonstrate the survivability of such targets to the pressures generated by isochoric heating over the timescales of the laser–plasma interaction.

Thomson scattering in short pulse laser experiments

Thomson scattering is well used as a diagnostic in many areas of high energy density physics. In this paper, we quantitatively demonstrate the practicality of using Thomson scattering as a diagnostic of short-pulse laser-plasma experiments in the regime, where the plasmas probed are at solid density and have temperatures of many hundreds of eV using a backlighter produced with an optical laser. This method allows a diagnosis both spatially and temporally of the density and temperature distributions in high energy density laser-plasma interactions which is independent from, and would act as a useful complement to, the existing spectroscopic methods.

Effect of substrate temperature in the synthesis of BN nanostructures

Boron nitride (BN) nanostructures were grown on molybdenum discs at different substrate temperatures using the short-pulse laser plasma deposition technique. Large numbers of randomly oriented nanorods of fiber-like structures were obtained. The variation in the length and diameter of the nanorods as a function of the substrate temperature was systematically studied. The surface morphologies of the samples were studied using scanning electron microscopy. Energy dispersive x-ray spectroscopy confirmed that both the elements boron and nitrogen are dominant in the nanostructure. The x-ray diffraction (XRD) technique was used to analyse BN phases. The XRD peak that appeared at 26° showed the presence of hexagonal BN phase, whereas the peak at 44° was related to cubic BN content in the samples. Raman spectroscopic analysis showed vibrational modes of sp2- and sp3-type bonding in the sample. The Raman spectra agreed well with XRD results.

Resonant third harmonic generation of a short pulse laser in plasma by applying a wiggler magnetic field

We examine the effect of wiggler magnetic field on pulse slippage of short pulse laser-induced third harmonic generation in plasma. The process of third harmonic generation of an intense short pulse laser in plasma is resonantly enhanced by the application of a magnetic wiggler. The laser exerts a ponderomotive force at second harmonic driving density oscillations. The second harmonic oscillations coupled with electron velocity at the laser frequency, produces a non-linear current, driving the third harmonic. Third harmonic pulse generates in the fundamental pulse domain. However, the group velocity of the third harmonic wave is greater than the fundamental wave. Hence, the third harmonic pulse saturates strongly and moves forward from the fundamental pulse at shorter distance than the second harmonic pulse.

Properties of nanostructured cubic boron nitride films prepared by the short-pulse laser plasma deposition techniques

Abstract Nanostructured cubic boron nitride (BN) films were synthesized on molybdenum substrates by using the short-pulse laser plasma deposition techniques. The surface morphologies, chemical compositions, bond structures, and mechanical properties of the obtained BN thin films have been investigated by scanning electron microscopy, Raman scattering, Fourier transform infrared spectroscopy (FTIR), energy dispersive spectra, and hardness measurements. High power density laser deposition yielded boron-rich BN nanorod arrays where tBN component dominates. Reducing power density down to 8 × 10 7 W/cm 2 during laser plasma deposition produced flat cBN thin films. Typical TO and LO bands in the Raman and FTIR spectra of the cBN samples were identified. The cBN sample with hardness up to 40 GPa was obtained.

Weibel instability of relativistic electron flows in a laser produced plasma

The strong flows of relativistic electrons, encountered in high power short pulse laser plasma interaction, have the potential to drive Weibel instability producing short wavelength magnetic fields. The presence of a Langmuir wave couples the Weibel instability to two electrostatic wave sidebands. The density perturbations due to the sidebands couple with the oscillatory electron velocity due to the pump to produce a current that enhances the growth rate of Weibel instability. The thermal spread of the energetic (drifting) electrons, however, reduces the growth rate.

Raman sidescatter in numerical models of short pulse laser plasma interactions

Certain reduced models of a plasma are subject to a phenomenon known as the ultraviolet catastrophe, whereby the Raman growth rate diverges with increasing transverse wave number. Numerical solutions that make use of such models cannot account for large angle scattering. Several of these models are evaluated in terms of their ability to reproduce the growth rates given by the two-dimensional Raman dispersion relation. The quasistatic approximation causes a much more severe ultraviolet catastrophe than does the paraxial approximation alone. Removal of the quasistatic approximation eliminates the ultraviolet catastrophe provided nonparaxial terms are retained. Fully explicit particle-in-cell simulations show that large angle scattering leads to a fine structure which perturbs the pump wave and heats the plasma.