Study of imaging plate detector sensitivity to 5-18 MeV electrons

Laboratory evidence of magnetic reconnection hampered in obliquely interacting flux tubes

Magnetic reconnection can occur when two plasmas, having anti-parallel components of the magnetic field, encounter each other. In the reconnection plane, the anti-parallel component of the field is annihilated and its energy released in the plasma. Here, we investigate through laboratory experiments the reconnection between two flux tubes that are not strictly anti-parallel. Compression of the anti-parallel component of the magnetic field is observed, as well as a decrease of the reconnection efficiency. Concomitantly, we observe delayed plasma heating and enhanced particle acceleration. Three-dimensional hybrid simulations support these observations and highlight the plasma heating inhibition and reconnection efficiency reduction for these obliquely oriented flux tubes.

Magnetic reconnection acts as energy transfer process in plasma and induces processes like plasma heating, particle acceleration. Here the authors demonstrate the variation of magnetic reconnection between two flux tubes in the presence of external magnetic field.

A laser parameter study on enhancing proton generation from microtube foil targets

The interaction of an intense laser with a solid foil target can drive [Formula: see text] TV/m electric fields, accelerating ions to MeV energies. In this study, we experimentally observe that structured targets can dramatically enhance proton acceleration in the target normal sheath acceleration regime. At the Texas Petawatt Laser facility, we compared proton acceleration from a [Formula: see text] flat Ag foil, to a fixed microtube structure 3D printed on the front side of the same foil type. A pulse length (140-450 fs) and intensity ((4-10) [Formula: see text] W/cm[Formula: see text]) study found an optimum laser configuration (140 fs, 4 [Formula: see text] W/cm[Formula: see text]), in which microtube targets increase the proton cutoff energy by 50% and the yield of highly energetic protons ([Formula: see text] MeV) by a factor of 8[Formula: see text]. When the laser intensity reaches [Formula: see text] W/cm[Formula: see text], the prepulse shutters the microtubes with an overcritical plasma, damping their performance. 2D particle-in-cell simulations are performed, with and without the preplasma profile imported, to better understand the coupling of laser energy to the microtube targets. The simulations are in qualitative agreement with the experimental results, and show that the prepulse is necessary to account for when the laser intensity is sufficiently high.

Development of an experimental platform for the investigation of laser-plasma interaction in conditions relevant to shock ignition regime

The shock ignition (SI) approach to inertial confinement fusion is a promising scheme for achieving energy production by nuclear fusion. SI relies on using a high intensity laser pulse (≈10¹⁶ W/cm², with a duration of several hundred ps) at the end of the fuel compression stage. However, during laser-plasma interaction (LPI), several parametric instabilities, such as stimulated Raman scattering and two plasmon decay, nonlinearly generate hot electrons (HEs). The whole behavior of HE under SI conditions, including their generation, transport, and final absorption, is still unclear and needs further experimental investigation. This paper focuses on the development of an experimental platform for SI-related experiments, which simultaneously makes use of multiple diagnostics to characterize LPI and HE generation, transport, and energy deposition. Such diagnostics include optical spectrometers, streaked optical shadowgraph, an x-ray pinhole camera, a two-dimensional x-ray imager, a Cu Kα line spectrometer, two hot-electron spectrometers, a hard x-ray (bremsstrahlung) detector, and a streaked optical pyrometer. Diagnostics successfully operated simultaneously in single-shot mode, revealing the features of HEs under SI-relevant conditions.

Absolute calibration of Fujifilm BAS-TR image plate response to laser driven protons up to 40 MeV

Image plates (IPs) are a popular detector in the field of laser driven ion acceleration, owing to their high dynamic range and reusability. An absolute calibration of these detectors to laser-driven protons in the routinely produced tens of MeV energy range is, therefore, essential. In this paper, the response of Fujifilm BAS-TR IPs to 1-40 MeV protons is calibrated by employing the detectors in high resolution Thomson parabola spectrometers in conjunction with a CR-39 nuclear track detector to determine absolute proton numbers. While CR-39 was placed in front of the image plate for lower energy protons, it was placed behind the image plate for energies above 10 MeV using suitable metal filters sandwiched between the image plate and CR-39 to select specific energies. The measured response agrees well with previously reported calibrations as well as standard models of IP response, providing, for the first time, an absolute calibration over a large range of proton energies of relevance to current experiments.

Calorimeter with Bayesian unfolding of spectra of high-flux broadband x rays

We report the development of a multipurpose differential x-ray calorimeter with a broad energy bandwidth. The absorber architecture is combined with a Bayesian unfolding algorithm to unfold high energy x-ray spectra generated in high-intensity laser-matter interactions. Particularly, we show how to extract absolute energy spectra and how our unfolding algorithm can reconstruct features not included in the initial guess. The performance of the calorimeter is evaluated via Monte Carlo generated data. The method accuracy to reconstruct electron temperatures from bremsstrahlung is shown to be 5% for electron temperatures from 1 to 50 MeV. We study bremsstrahlung generated in solid target interaction showing an electron temperature of 0.56 ± 0.04 MeV for a 700 μm Ti titanium target and 0.53 ± 0.03 MeV for a 50 μm target. We investigate bremsstrahlung from a target irradiated by laser-wakefield accelerated electrons showing an endpoint energy of 551 ± 5 MeV, inverse Compton generated x rays with a peak energy of 1.1 MeV, and calibrated radioactive sources. The total energy range covered by all these sources ranges from 10 keV to 551 MeV.

Microstructured layered targets for improved laser-induced x-ray backlighters

We present the usage of two-layer targets with laser-illuminated front-side microstructures for x-ray backlighter applications. The targets consisted of a silicon front layer and copper back side layer. The structured layer was irradiated by the 500-fs PHELIX laser with an intensity above 10^{20}Wcm^{-2}. The total emission and one-dimensional extent of the copper Kα x-ray emission as well as a wide spectral range between 7.9 and 9.0 keV were recorded with an array of crystal spectrometers. The measurements show that the front-side modifications of the silicon in the form of conical microstructures maintain the same peak brightness of the Kα emission as flat copper foils while suppressing the thermal emission background significantly. The observed Kα source sizes can be influenced by tilting the conical microstructures with respect to the laser axis. Overall, the recorded copper Kα photon yields were in the range of 10^{11}sr^{-1}, demonstrating the suitability of these targets for probing applications without subjecting the probed material to additional heating from thermal line emission.

Spectral characterization of flash and high flux x-ray radiographic sources with a magnetic Compton spectrometer

In this work, we present a new analysis method applied to revitalize permanent magnet Compton spectrometers used to measure photon energy spectra in the MeV range. The inversion of the measured electron distribution to determine the original photon distribution is achieved via a method of consistent coupled radiation transport and magnetic field mapping of the input photon spectra to the measured electron distribution. The method of linear least squares was used to perform the unfolding of the electron distribution to the initial photon spectra, without any assumptions made regarding the electron distribution. We present an application of this method to data from a nominal 19.4 MeV flash radiographic source (the first axis of the Dual Axis Radiographic Hydro-Test Facility) capable of generating 500 R @ 1 m in ∼60 ns and a medical therapy source (a Scanditronix M22, Microtron) capable of variable energies with nominal endpoints of 6, 10, 15, and 20 MeV and an output of ∼1000-2000 R/min @ 1 m. The results provide agreement between the modeled and unfolded experimentally measured photon spectra as quantified by statistical tests, from 1.5 to 20 MeV. Experimental results are presented as well as a discussion of the novel MCNP6-based simulations and methods for reconstruction of the spectra.

Microcoulomb (0.7 ± [Formula: see text] μC) laser plasma accelerator on OMEGA EP

Laser-plasma accelerators (LPAs) driven by picosecond-scale, kilojoule-class lasers can generate particle beams and x-ray sources that could be utilized in experiments driven by multi-kilojoule, high-energy-density science (HEDS) drivers such as the OMEGA laser at the Laboratory for Laser Energetics (LLE) or the National Ignition Facility at Lawrence Livermore National Laboratory. This paper reports on the development of the first LPA driven by a short-pulse, kilojoule-class laser (OMEGA EP) connected to a multi-kilojoule HEDS driver (OMEGA). In experiments, electron beams were produced with electron energies greater than 200 MeV, divergences as low as 32 mrad, charge greater than 700 nC, and conversion efficiencies from laser energy to electron energy up to 11%. The electron beam charge scales with both the normalized vector potential and plasma density. These electron beams show promise as a method to generate MeV-class radiography sources and improved-flux broadband x-ray sources at HEDS drivers.

Monte Carlo Study of Imaging Plate Response to Laser-Driven Aluminum Ion Beams

We measured the response of BAS-TR imaging plate (IP) to energetic aluminum ions up to 222 MeV, and compared it with predictions from a Monte Carlo simulation code using two different IP response models. Energetic aluminum ions were produced with an intense laser pulse, and the response was evaluated from cross-calibration between CR-39 track detector and IP energy spectrometer. For the first time, we obtained the response function of the BAS-TR IP for aluminum ions with a kinetic energy as high as 222 MeV. On close examination of the two IP response models, we confirm that the exponential model fits our experimental data better. Moreover, we find that the IP sensitivity in the exponential model is nearly constant in this energy range, suggesting that the response function can be determined even with little experimental data.

Bremsstrahlung cannon design for shock ignition relevant regime

We report on the optimization of a BremsStrahlung Cannon (BSC) design for the investigation of laser-driven fast electron populations in a shock ignition relevant experimental campaign at the Laser Megajoule-PETawatt Aquitaine Laser facility. In this regime with laser intensities of 10¹⁵ W/cm²-10¹⁶ W/cm², fast electrons with energies ≤100 keV are expected to be generated through Stimulated Raman Scattering (SRS) and Two Plasmon Decay (TPD) instabilities. The main purpose of the BSC in our experiment is to identify the contribution to x-ray emission from bremsstrahlung of fast electrons originating from SRS and TPD, with expected temperatures of 40 keV and 95 keV, respectively. Data analysis and reconstruction of the distributions of x-ray photons incident on the BSC are described.

Quantitative phase contrast imaging of a shock-wave with a laser-plasma based X-ray source

X-ray phase contrast imaging (XPCI) is more sensitive to density variations than X-ray absorption radiography, which is a crucial advantage when imaging weakly-absorbing, low-Z materials, or steep density gradients in matter under extreme conditions. Here, we describe the application of a polychromatic X-ray laser-plasma source (duration ~0.5 ps, photon energy >1 keV) to the study of a laser-driven shock travelling in plastic material. The XPCI technique allows for a clear identification of the shock front as well as of small-scale features present during the interaction. Quantitative analysis of the compressed object is achieved using a density map reconstructed from the experimental data.

Compact high energy x-ray spectrometer based on forward Compton scattering for high intensity laser plasma experiments

This article describes the design and presents recent results from testing and calibration of a forward Compton scattering high energy X-ray spectrometer. The calibration was performed using a bremsstrahlung source on the photon scattering facility at the γ Electron linac for beams with high brilliance and low emittance accelerator at Helmholtz-Zentrum Dresden-Rossendorf, which provides high energy X-ray photons with energies up to 18 MeV. The calibration was conducted at different bremsstrahlung end point energies-10.5, 13, 15, and 18 MeV. Experimental spectra show a systematic increase in the maximum energy, photon temperature, and flux. The spectrometer is effective for an energy range of 4-20 MeV with 20%-30% energy resolution. The spectrometer operates in low vacuum with pressure less than 0.1 mbar. Experimental tests showed that operating such a spectrometer in air causes a spuriously enhanced high energy signal due to Compton scattering of photons within air. The article also describes the design and shielding considerations which helped to achieve a dynamic range greater than 30 with this spectrometer. The comparison between the experimental results and Monte Carlo simulations are also presented.

Readout models for BaFBr0.85I0.15:Eu image plates

The linearity of the photostimulated luminescence process makes repeated image-plate scanning a viable technique to extract a more dynamic range. In order to obtain a response estimate, two semi-empirical models for the readout fading of an image plate are introduced; they relate the depth distribution of activated photostimulated luminescence centers within an image plate to the recorded signal. Model parameters are estimated from image-plate scan series with BAS-MS image plates and the Typhoon FLA 7000 scanner for the hard x-ray image-plate diagnostic over a collection of experiments providing x-ray energy spectra whose approximate shape is a double exponential.

Calibration of the low-energy channel Thomson parabola of the LMJ-PETAL diagnostic SEPAGE with protons and carbon ions

The SEPAGE diagnostic will detect charged particles (electrons, protons, and ions) accelerated in the interaction of the PETAL (PETawatt Aquitaine Laser) laser with its targets on the LMJ (Laser MegaJoule)-PETAL laser facility. SEPAGE will be equipped with a proton-radiography front detector and two Thomson parabolas (TP), corresponding to different ranges of the particle energy spectra: Above 0.1 MeV for electrons and protons in the low-energy channel, with a separation capability between protons and ¹²C⁶⁺ up to 20 MeV proton energy and above 8 MeV for the high-energy channel, with a separation capability between protons and ¹²C⁶⁺ up to 200 MeV proton kinetic energy. This paper presents the calibration of the SEPAGE's low-energy channel TP at the Tandem facility of Orsay (France) with proton beams between 3 and 22 MeV and carbon-ion beams from 5.8 to 84 MeV. The magnetic and electric fields' integrals were determined with an accuracy of 10^-3 by combining the deflections measured at different energies with different target thicknesses and materials, providing different in-target energy losses of the beam particles and hence different detected energies for given beam energies.

Calibration of imaging plate detectors to mono-energetic protons in the range 1-200 MeV

Responses of Fuji Imaging Plates (IPs) to proton have been measured in the range 1-200 MeV. Mono-energetic protons were produced with the 15 MV ALTO-Tandem accelerator of the Institute of Nuclear Physics (Orsay, France) and, at higher energies, with the 200-MeV isochronous cyclotron of the Institut Curie-Centre de Protonthérapie d'Orsay (Orsay, France). The experimental setups are described and the measured photo-stimulated luminescence responses for MS, SR, and TR IPs are presented and compared to existing data. For the interpretation of the results, a sensitivity model based on the Monte Carlo GEANT4 code has been developed. It enables the calculation of the response functions in a large energy range, from 0.1 to 200 MeV. Finally, we show that our model reproduces accurately the response of more complex detectors, i.e., stack of high-Z filters and IPs, which could be of great interest for diagnostics of Petawatt laser accelerated particles.

Absolute calibration of imaging plate detectors for electron kinetic energies between 150 keV and 1.75 MeV

This paper presents the calibration of two different kinds of image plates (IPs) for detecting electrons with kinetic energy in the range of 150 keV-1.75 MeV. The calibration was performed using a ⁹⁰Sr β source. The paper also provides the measured fading response for the IPs in the time range from 12 min to 18 h. Calibration results are compared to Monte Carlo simulations of energy deposited by the electrons in the sensitive layer of the IPs. It was found that within this energy range a linear relation between simulated energy deposited by the electron in the phosphor layer and the measured photo stimulated luminescence in the IP is adequate to model the response of the IP.

Calibration of imaging plates to electrons between 40 and 180 MeV

This paper presents the response calibration of Imaging Plates (IPs) for electrons in the 40-180 MeV range using laser-accelerated electrons at Laboratoire d'Optique Appliquée (LOA), Palaiseau, France. In the calibration process, the energy spectrum and charge of electron beams are measured by an independent system composed of a magnetic spectrometer and a Lanex scintillator screen used as a calibrated reference detector. It is possible to insert IPs of different types or stacks of IPs in this spectrometer in order to detect dispersed electrons simultaneously. The response values are inferred from the signal on the IPs, due to an appropriate charge calibration of the reference detector. The effect of thin layers of tungsten in front and/or behind IPs is studied in detail. GEANT4 simulations are used in order to analyze our measurements.

Validation of modelled imaging plates sensitivity to 1-100 keV x-rays and spatial resolution characterisation for diagnostics for the "PETawatt Aquitaine Laser"

Thanks to their high dynamic range and ability to withstand electromagnetic pulse, imaging plates (IPs) are commonly used as passive detectors in laser-plasma experiments. In the framework of the development of the diagnostics for the Petawatt Aquitaine Laser facility, we present an absolute calibration and spatial resolution study of five different available types of IP (namely, MS-SR-TR-MP-ND) performed by using laser-induced K-shell X-rays emitted by a solid silver target irradiated by the laser ECLIPSE at CEntre Lasers Intenses et Applications. In addition, IP sensitivity measurements were performed with a 160 kV X-ray generator at CEA DAM DIF, where the absolute response of IP SR and TR has been calibrated to X-rays in the energy range 8-75 keV with uncertainties of about 15%. Finally, the response functions have been modeled in Monte Carlo GEANT4 simulations in order to reproduce experimental data. Simulations enable extrapolation of the IP response functions to photon energies from 1 keV to 1 GeV, of interest, e.g., for laser-driven radiography.