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Abstract: We theoretically investigated the temporal profile of a background pedestal in a single-stage optical parametric chirped pulse amplification (OPCPA) system, and showed that the amplified spontaneous emission (ASE) prepulse in the saturation regime degrades the contrast ratio. To improve the contrast ratio in a saturated OPCPA system, a method using pump pulse shaping and the time delay control between pump and seed pulses was utilized. Through numerical simulations, the effect of the rising time and the time delay of a pump pulse on the background pedestal was subsequently investigated in terms of the contrast ratio. From the simulation results, it was determined that an optimal rising time and the time delay of the pump pulse are critical for obtaining a high-contrast broadband laser pulse from a saturated OPCPA system. [Copyright &y& Elsevier]

Abstract: We design a two-stage non-collinear optical parametric chirped pulse amplifier using BBO crystals for a high-power Ti:sapphire laser system, and numerically analyze the output characteristics of the OPCPA system. We find optimal phase-matching conditions and calculate the parametric gains as a function of pump intensity and interaction length. By constructing a two-stage OPCPA system using two BBO crystals, we obtain 12 mJ output energy corresponding to a high gain of 107, a pulse duration of 37 fs, and a contrast ratio of 5×10−8. [Copyright &y& Elsevier]

Petawatt-class lasers can produce multi-GeV electron beams through laser wakefield electron acceleration. As a by-product, the accelerated electron beams can generate synchrotron-like radiation known as betatron radiation. In the present work, we measure the properties of the radiation produced from 2 GeV, 215 pC electron beams, which shows a broad radiation spectrum with a critical energy of 515 keV, extending up to MeV photon energies and 10 mrad divergence. Due to its high energy and flux, such radiation is an ideal candidate for γ -ray radiography of dense objects. We employed a compact betatron radiation setup operated at relatively high-repetition rates (0.1 Hz) and used it to scan cm-sized objects: a DRAM circuit, BNC and SMA connectors, a padlock and a gas jet nozzle. GEANT4 simulations were carried out to reproduce the radiograph of the gas jet. The setup and the radiation source can reveal the interior structure of the objects at the sub-mm level, proving that it can further be applied to diagnose cracks or holes in various components. The radiation source presented here is a valuable tool for non-destructive inspection and for applications in high-energy-density physics such as nuclear fusion. [ABSTRACT FROM AUTHOR]

The absolute response of a real-time proton detector, composed of a microchannel plate (MCP) assembly, an imaging lens, and a charge-coupled device (CCD) camera, is calibrated for the spectral characterization of laser-accelerated protons, using a Thomson parabola spectrometer (TPS). A slotted CR-39 plate was used as an absolute particle-counting detector in the TPS, simultaneously with the MCP–CCD detector to obtain a calibration factor (count/proton). In order to obtain the calibration factor as a function of proton energy for a wide range of proton numbers, the absolute response was investigated for different operation parameters of the MCP–CCD detector, such as MCP voltage, phosphor voltage, and CCD gain. A theoretical calculation for the net response of the MCP was in good agreement with the calibrated response of the MCP–CCD detector, and allows us to extend the response to higher proton energies. The response varies in two orders of magnitude, showing an exponential increase with the MCP voltage and almost linear increase with the phosphor voltage and the CCD gain. The calibrated detector enabled characterization of a proton energy spectrum in a wide dynamic range of proton numbers. Moreover, two MCP assemblies having different structures of MCP, phosphor screen, and optical output window have been calibrated, and the difference in the absolute response was highlighted. The highly-sensitive detector operated with maximum values of the parameters enables measuring a single proton particle and evaluating an absolute spectrum at high proton energies in a single laser shot. The absolute calibrations can be applied for the spectral measurement of protons using different operating voltages and gains for optimized response in a large range of proton energy and number. [ABSTRACT FROM AUTHOR]

In all-optical Compton scattering driven by a multi-petawatt laser, it is critical to have accurate spatiotemporal synchronization between the ultrarelativistic electron bunch and the ultrahigh-intensity laser beam. Such a synchronization was realized by using two complementary optical setups. The first setup, used for the initial synchronization, recorded the spatial interferogram between the two femtosecond lasers used for a GeV electron beam production and an ultrahigh scattering laser beam. The second one, consisting of spatial and spectral interferometers, measured the time delay between the two laser beams in the range of 0–200 fs in real time. These monitoring systems played an essential role in conducting Compton scattering experiments. [ABSTRACT FROM AUTHOR]

Abstract: The evolution of electron beam injection and acceleration in the bubble regime of a laser wakefield is studied experimentally. When a 50TW, 35fs laser pulse is focused to a spot size of ∼22μm (FWHM) in a tenuous 1-cm-long helium gas jet, well-collimated (a few milliradians) electron beams with energies in the sub-GeV level are produced. Multiple electron bunches are observed when the laser–plasma channel (i.e., interaction length) is long (<∼1cm). However, only one bunch is generated when the channel is short (∼2mm). These experimental results confirm the evolution scenarios of the electron bunches observed in 3D PIC simulations [refer to the publications: N. Hafz et al., Nat. Photon. 2 (2008) 571 and S. Kalmykov et al., Phys. Rev. Lett. 103 (2009) 135004], which suggested that multiple electron injections are a characteristic process for a long acceleration length and an evolving plasma bubble. [Copyright &y& Elsevier]

A localized nanoparticle insertion scheme is developed to decouple electron injection from laser evolution in laser wakefield acceleration. Here we report the experimental realization of a controllable electron injection by the nanoparticle insertion method into a plasma medium, where the injection position is localized within the short range of 100 μm. Nanoparticles were generated by the laser ablation process of a copper blade target using a 3-ns 532-nm laser pulse with fluence above 100 J/cm2. The produced electron bunches with a beam charge above 300 pC and divergence of around 12 mrad show the injection probability over 90% after optimizing the ablation laser energy and the temporal delay between the ablation and the main laser pulses. Since this nanoparticle insertion method can avoid the disturbing effects of electron injection process on laser evolution, the stable high-charge injection method can provide a suitable electron injector for multi-GeV electron sources from low-density plasmas. [ABSTRACT FROM AUTHOR]

We present a proof of principle for onsite calibration of a radiochromic film (EBT3) using CR-39 as an absolute proton-counting detector and laser-accelerated protons as a calibration source. A special detector assembly composed of aluminum range filters, an EBT3 film, and a CR-39 detector is used to expose the EBT3 film with protons in an energy range of 3.65 MeV–5.85 MeV. In our design, the proton beam is divided into small beamlets and their projection images are taken on the EBT3 film and the CR-39 detector by maintaining a certain distance between the two detectors. Owing to the geometrical factor of the configuration and scattering inside the EBT3, the areal number density of protons was kept below the saturation level of the CR-39 detector. We also present a method to relate the number of protons detected on the CR-39 in a narrow energy range to protons with a broad energy spectrum that contribute to the dose deposited in the EBT3 film. The energy spectrum of protons emitted along the target normal direction is simultaneously measured using another CR-39 detector installed in a Thomson parabola spectrometer. The calibration curves for the EBT3 film were obtained in the optical density range of 0.01–0.25 for low dose values of 0.1 Gy–3.0 Gy. Our results are in good agreement with the calibrations of the EBT3 film that are traditionally carried out using conventional accelerators. The method presented here can be further extended for onsite calibration of radiochromic films of other types and for a higher range of dose values. [ABSTRACT FROM AUTHOR]

We demonstrated single-shot high-numerical-aperture coherent diffraction imaging (CDI) with a diffraction-limited resolution using table-top x-ray laser at 13.9 nm. In order to achieve the diffraction-limited resolution, a nonlinear phase variation was introduced in a sample so that an asymmetric diffraction pattern with clear visibility could be extended to high spatial frequencies. From the diffraction signal, the sample image was successfully reconstructed with a resolution of 22 nm, which is close to the diffraction-limited resolution of 21 nm, using the phase retrieval procedure. The model calculation and simulations, performed to verify the experimental results, showed that the implementation of the nonlinear phase variation in the sample enhanced the resolution by a factor of about two, as compared to the case of a pure absorption sample. In addition, we examined the CDI with phase controllers introducing nonlinear phase variation illuminated by a table-top femtosecond Kr x-ray laser pulse at 32.8 nm that produced an asymmetric diffraction pattern extended to high spatial frequencies. [ABSTRACT FROM AUTHOR]

[Display omitted] • Nanosecond laser irradiation with gas flow through a supersonic nozzle is a new surface modification technique. • The surface properties of the samples were systematically studied for different laser energies and environments. • Gas flow during nanosecond laser irradiation led to significant changes on the surface structure of the samples, including the growth of periodic surface structure. • The microhardness increased due to effect of gas flow and laser irradiation by a factor of 3.40 and 4.90 for Al and Ag, respectively. This study investigates the surface modification of a material irradiated by a nanosecond Nd:YAG green laser while exposed to a gas flow. Scanning electron microscopy images and X-ray diffraction profiles illustrate the effect of laser irradiation in different environments, especially under backing gas pressures of 0.5, 1, and 1.5 bar applied through a supersonic nozzle. After irradiation, the microhardness obtained through the Vickers hardness increased by a factor of 3.40 and 4.90 for Al and Ag, respectively. This enhancement can be attributed to the surface and structural modification of the irradiated sample under different environments. Furthermore, the microhardness was found to be associated with the microstrain. [ABSTRACT FROM AUTHOR]

This paper demonstrates the design and development of a synchronized femtosecond laser pulse switching system and its applications in nano-patterning of transparent materials. Due to synchronization, we are able to control the location of each irradiated laser pulse in any kind of substrate. The control over the scanning speed and scanning step of the laser beam enables us to pattern periodic micro/nano-metric holes, voids, and/or lines in various materials. Using the synchronized laser system, we pattern synchronized nano-holes on the surface of and inside various transparent materials including fused silica glass and polymethyl methacrylate to replicate any image or pattern on the surface of or inside (transparent) materials. We also investigate the application areas of the proposed synchronized femtosecond laser pulse switching system in a diverse field of science and technology, especially in optical memory, color marking, and synchronized micro/nano-scale patterning of materials. [ABSTRACT FROM AUTHOR]

An amendment to this paper has been published and can be accessed via a link at the top of the paper. [ABSTRACT FROM AUTHOR]

With the recent advances in ultrahigh intensity lasers, exotic astrophysical phenomena can be investigated in laboratory environments. Collisionless shock in a plasma, prevalent in astrophysical events, is produced when a strong electric or electromagnetic force induces a shock structure in a time scale shorter than the collision time of charged particles. A near-critical-density (NCD) plasma, generated with an intense femtosecond laser, can be utilized to excite a collisionless shock due to its efficient and rapid energy absorption. We present electrostatic shock acceleration (ESA) in experiments performed with a high-density helium gas jet, containing a small fraction of hydrogen, irradiated with a 30 fs, petawatt laser. The onset of ESA exhibited a strong dependence on plasma density, consistent with the result of particle-in-cell simulations on relativistic plasma dynamics. The mass-dependent ESA in the NCD plasma, confirmed by the preferential reflection of only protons with two times the shock velocity, opens a new possibility of selective acceleration of ions by electrostatic shock. [ABSTRACT FROM AUTHOR]

We developed a two-stage optical parametric chirped-pulse amplification (OPCPA) amplifier with a double-sliced pump pulse. To utilize a single-longitudinal laser pulse with an 8-ns duration and a 532-nm central wavelength as a high-energy pump source efficiently, two 2-ns quasi-rectangular pump pulses were generated by double-slicing the green pulse with two Pockels cells. The energy and spectral width of the amplified laser pulse with an 820-nm central wavelength were maximized by optimizing the phase matching conditions in the OPCPA amplifier via a 3-D simulation. The amplified laser pulse had a broad bandwidth of 120 nm (FWHM) and a high energy of 75 mJ. And the amplified pulse had an energy stability of 2.14% (RMS) on account of the temporal jittering between the single-mode green pulse and the Pockels cells while the single-mode green pump laser had an energy stability of 1.1% (RMS). [ABSTRACT FROM AUTHOR]

The phase velocity of the wakefield of a laser wakefield accelerator can, theoretically, be manipulated by shaping the longitudinal plasma density profile, thus controlling the parameters of the generated electron beam. We present an experimental method where using a series of shaped longitudinal plasma density profiles we increased the mean electron peak energy more than 50%, from 175 ± 1 MeV to 262 ± 10 MeV and the maximum peak energy from 182 MeV to 363 MeV. The divergence follows closely the change of mean energy and decreases from 58.9 ± 0.45 mrad to 12.6 ± 1.2 mrad along the horizontal axis and from 35 ± 0.3 mrad to 8.3 ± 0.69 mrad along the vertical axis. Particle-in-cell simulations show that a ramp in a plasma density profile can affect the evolution of the wakefield, thus qualitatively confirming the experimental results. The presented method can increase the electron energy for a fixed laser power and at the same time offer an energy tunable source of electrons. [ABSTRACT FROM AUTHOR]