Publications
156 results found
Merlini S, Hare JD, Burdiak GC, et al., 2023, Radiative cooling effects on reverse shocks formed by magnetized supersonic plasma flows, Physics of Plasmas, Vol: 30, ISSN: 1070-664X
We study the structure of reverse shocks formed by the collision of supersonic, magnetized plasma flows driven by an inverse (or exploding) wire array with a planar conducting obstacle. We observe that the structure of these reverse shocks varies dramatically with wire material, despite the similar upstream flow velocities and mass densities. For aluminum wire arrays, the shock is sharp and well-defined, consistent with magneto-hydrodynamic theory. In contrast, we do not observe a well-defined shock using tungsten wires, and instead we see a broad region dominated by density fluctuations on a wide range of spatial scales. We diagnose these two very different interactions using interferometry, Thomson scattering, shadowgraphy, and a newly developed imaging refractometer that is sensitive to small deflections of the probing laser corresponding to small-scale density perturbations. We conclude that the differences in shock structure are most likely due to radiative cooling instabilities, which create small-scale density perturbations elongated along magnetic field lines in the tungsten plasma. These instabilities grow more slowly and are smoothed by thermal conduction in the aluminum plasma.
Russell DR, Burdiak GC, Carroll-Nellenback JJ, et al., 2023, Observation of subcritical shocks in a collisional laboratory plasma: scale dependence near the resistive length, Journal of Plasma Physics, Vol: 89, ISSN: 0022-3778
We present a study of subcritical shocks in a highly collisional laboratory plasma with a dynamically significant magnetic field. Shocks were produced by placing cylindrical obstacles into the supermagnetosonic ( MMS∼1.9 ) outflow from an inverse wire array z-pinch at the MAGPIE pulsed power facility ( ne∼8.5×1017cm−3 , v∼45kms−1 ). We demonstrate the existence of subcritical shocks in this regime and find that secondary stagnation shocks form in the downstream which we infer from interferometry and optical Thomson scattering measurements are hydrodynamic in nature. The subcritical shock width is found to be approximately equal to the resistive diffusion length and we demonstrate the absence of a jump in hydrodynamic parameters. Temperature measurements by collective optical Thomson scattering showed little temperature change across the subcritical shock ( <10% of the ion kinetic energy) which is consistent with a balance between adiabatic and Ohmic heating and radiative cooling. We demonstrate the absence of subcritical shocks when the obstacle diameter is less than the resistive diffusion length due to decoupling of the magnetic field from the plasma. These findings are supported by magnetohydrodynamic simulations using the Gorgon and AstroBEAR codes and discrepancies between the simulations and experiment are discussed.
Munj AH, Smith RA, Kerridge-Johns WR, 2023, Optical levitation using an LG01 vortex beam, ISSN: 0277-786X
We present optical trapping and levitation of silver-coated hollow glass spheres using a LG01 vortex laser. We successfully levitate reflective targets ranging from 53μm to 93μm in diameter with powers ranging from 50mW to 1W. Our system is designed for long working distance trapping (40mm-100mm) which is desirable for in-vacuo alignment and manipulation of mass-limited microtargets for high energy plasma applications, by limiting optical component damage from high-intensity light-matter interactions.
Russell D, Burdiak G, Carroll-Nellenback JJ, et al., 2022, Perpendicular subcritical shock structure in a collisional plasma experiment, Physical Review Letters, Vol: 129, ISSN: 0031-9007
We present a study of perpendicular subcritical shocks in a collisional laboratory plasma. Shocks areproduced by placing obstacles into the supermagnetosonic outflow from an inverse wire array z pinch. Wedemonstrate the existence of subcritical shocks in this regime and find that secondary shocks form in thedownstream. Detailed measurements of the subcritical shock structure confirm the absence of ahydrodynamic jump. We calculate the classical (Spitzer) resistive diffusion length and show that it isapproximately equal to the shock width. We measure little heating across the shock (< 10% of the ionkinetic energy) which is consistent with an absence of viscous dissipation.
Danson CN, White M, Barr JRM, et al., 2021, A history of high-power laser research and development in the United Kingdom, High Power Laser Science and Engineering, Vol: 9, Pages: 1-86, ISSN: 2095-4719
The first demonstration of laser action in ruby was made in 1960 by T. H. Maiman of Hughes Research Laboratories, USA. Many laboratories worldwide began the search for lasers using different materials, operating at different wavelengths. In the UK, academia, industry and the central laboratories took up the challenge from the earliest days to develop these systems for a broad range of applications. This historical review looks at the contribution the UK has made to the advancement of the technology, the development of systems and components and their exploitation over the last 60 years.
Hare J, Burdiak G, Merlini S, et al., 2021, An imaging refractometer for density fluctuation measurements in high energy density plasmas, Review of Scientific Instruments, Vol: 92, ISSN: 0034-6748
We report on a recently developed laser-probing diagnostic which allows direct measurements of ray-deflection anglesin one axis, whilst retaining imaging capabilities in the other axis. This allows us to measure the spectrum of angulardeflections from a laser beam which passes though a turbulent high-energy-density plasma. This spectrum containsinformation about the density fluctuations within the plasma, which deflect the probing laser over a range of angles. Wecreate synthetic diagnostics using ray-tracing to compare this new diagnostic with standard shadowgraphy and schlierenimaging approaches, which demonstrates the enhanced sensitivity of this new diagnostic over standard techniques. Wepresent experimental data from turbulence behind a reverse shock in a plasma and demonstrate that this technique canmeasure angular deflections between 0.06 and 34 mrad, corresponding to a dynamic range of over 500.
Consoli F, Andreoli PL, Cipriani M, et al., 2021, Sources and space-time distribution of the electromagnetic pulses in experiments on inertial confinement fusion and laser-plasma acceleration, PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES, Vol: 379, ISSN: 1364-503X
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- Citations: 11
Hare JD, Burdiak GC, Merlini S, et al., 2020, An imaging refractometer for density fluctuation measurements in high energy density plasmas, Publisher: arXiv
We report on a recently developed laser-based diagnostic which allows directmeasurements of ray-deflection angles in one axis, whilst retaining imagingcapabilities in the other axis. This allows us to measure the spectrum ofangular deflections from a laser beam which passes though a turbulenthigh-energy-density plasma. This spectrum contains information about thedensity fluctuations within the plasma, which deflect the probing laser over arange of angles. The principle of this diagnostic is described, along with ourspecific experimental realisation. We create synthetic diagnostics usingray-tracing to compare this new diagnostic with standard shadowgraphy andschlieren imaging approaches, which demonstrates the enhanced sensitivity ofthis new diagnostic over standard techniques. We present experimental data fromturbulence behind a reverse shock in a plasma and demonstrate that thistechnique can measure angular deflections between 0.05 and 34 mrad,corresponding to a dynamic range of over 500.
Scott RHH, Booth N, Hawkes SJ, et al., 2020, Modeling radiative-shocks created by laser-cluster interactions, PHYSICS OF PLASMAS, Vol: 27, ISSN: 1070-664X
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- Citations: 3
Consoli F, Tikhonchuk VT, Bardon M, et al., 2020, Laser produced electromagnetic pulses: generation, detection and mitigation, HIGH POWER LASER SCIENCE AND ENGINEERING, Vol: 8, ISSN: 2095-4719
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- Citations: 53
Consoli F, De Angelis R, Robinson TS, et al., 2019, Generation of intense quasi-electrostatic fields due to deposition of particles accelerated by petawatt-range laser-matter interactions, Scientific Reports, Vol: 9, ISSN: 2045-2322
We demonstrate here for the first time that charge emitted by laser-target interactions at petawatt peak-powers can be efficiently deposited on a capacitor-collector structure far away from the target and lead to the rapid (tens of nanoseconds) generation of large quasi-static electric fields over wide (tens-of-centimeters scale-length) regions, with intensities much higher than common ElectroMagnetic Pulses (EMPs) generated by the same experiment in the same position. A good agreement was obtained between measurements from a classical field-probe and calculations based on particle-flux measurements from a Thomson spectrometer. Proof-of-principle particle-in-cell simulations reproduced the measurements of field evolution in time, giving a useful insight into the charging process, generation and distribution of fields. The understanding of this charging phenomenon and of the related intense fields, which can reach the MV/m order and in specific configurations might also exceed it, is very important for present and future facilities studying laser-plasma-acceleration and inertial-confinement-fusion, but also for application to the conditioning of accelerated charged-particles, the generation of intense electric and magnetic fields and many other multidisciplinary high-power laser-driven processes.
Dann SJD, Baird CD, Bourgeois N, et al., 2019, Laser wakefield acceleration with active feedback at 5 Hz, Physical Review Accelerators and Beams, Vol: 22, ISSN: 2469-9888
We describe the use of a genetic algorithm to apply active feedback to a laser wakefield accelerator at a higher power (10 TW) and a lower repetition rate (5 Hz) than previous work. The temporal shape of the drive laser pulse was adjusted automatically to optimize the properties of the electron beam. By changing the software configuration, different properties could be improved. This included the total accelerated charge per bunch, which was doubled, and the average electron energy, which was increased from 22 to 27 MeV. Using experimental measurements directly to provide feedback allows the system to work even when the underlying acceleration mechanisms are not fully understood, and, in fact, studying the optimized pulse shape might reveal new insights into the physical processes responsible. Our work suggests that this technique, which has already been applied with low-power lasers, can be extended to work with petawatt-class laser systems.
Streeter M, Dann S, Scott JDE, et al., 2018, Temporal feedback control of high-intensity laser pulses to optimize ultrafast heating of atomic clusters, Applied Physics Letters, Vol: 112, Pages: 244101-1-244101-5, ISSN: 1077-3118
We describe how active feedback routines can be applied at limited repetition rate (5 Hz) to optimize high-power (P>10 TW) laser interactions with clustered gases. Optimization of x-ray production from anargon cluster jet, using a genetic algorithm, approximately doubled the measured energy through temporalmodification of the 150 mJ driving laser pulse. This approach achieved an increased radiation yield throughexploration of a multi-dimensional parameter space, without requiring detailedaprioriknowledge of thecomplex cluster dynamics. The optimized laser pulses exhibited a slow rising edge to the intensity profile,which enhanced the laser energy coupling into the cluster medium, compared to the optimally compressedFWHM pulse (40 fs). Our work suggests that this technique can be more widely utilized for control of intensepulsed secondary radiation from petawatt-class laser systems.
Suttle LG, Hare JD, Lebedev SV, et al., 2018, Ion heating and magnetic flux pile-up in a magnetic reconnection experiment with super-Alfvenic plasma inflows, Physics of Plasmas, Vol: 25, ISSN: 1070-664X
This work presents a magnetic reconnection experiment in which the kinetic, magnetic, and thermal properties of the plasma each play an important role in the overall energy balance and structure of the generated reconnection layer. Magnetic reconnection occurs during the interaction of continuous and steady flows of super-Alfvénic, magnetized, aluminum plasma, which collide in a geometry with two-dimensional symmetry, producing a stable and long-lasting reconnection layer. Optical Thomson scattering measurements show that when the layer forms, ions inside the layer are more strongly heated than electrons, reaching temperatures of Ti∼Z⎯⎯⎯Te≳300 eV—much greater than can be expected from strong shock and viscous heating alone. Later in time, as the plasma density in the layer increases, the electron and ion temperatures are found to equilibrate, and a constant plasma temperature is achieved through a balance of the heating mechanisms and radiative losses of the plasma. Measurements from Faraday rotation polarimetry also indicate the presence of significant magnetic field pile-up occurring at the boundary of the reconnection region, which is consistent with the super-Alfvénic velocity of the inflows.
Hare J, Suttle L, Lebedev S, et al., 2018, An experimental platform for pulsed-power driven magnetic reconnection, Physics of Plasmas, Vol: 25, ISSN: 1070-664X
We describe a versatile pulsed-power driven platform for magnetic reconnection experiments, based on the exploding wire arrays driven in parallel [Suttle et al., Phys. Rev. Lett. 116, 225001 (2016)]. This platform produces inherently magnetised plasma flows for the duration of the generator current pulse (250 ns), resulting in a long-lasting reconnection layer. The layer exists for long enough to allow the evolution of complex processes such as plasmoid formation and movement to be diagnosed by a suite of high spatial and temporal resolution laser-based diagnostics. We can access a wide range of magnetic reconnection regimes by changing the wire material or moving the electrodes inside the wire arrays. We present results with aluminium and carbon wires, in which the parameters of the inflows and the layer that forms are significantly different. By moving the electrodes inside the wire arrays, we change how strongly the inflows are driven. This enables us to study both symmetric reconnection in a range of different regimes and asymmetric reconnection.
Robinson T, Giltrap S, Eardley S, et al., 2018, Electro-optic analysis of the influence of target geometry on electromagnetic pulses generated by petawatt laser-matter interactions, EPJ Web of Conferences, Vol: 167, ISSN: 2100-014X
We present an analysis of strong laser-driven electromagnetic pulses using novel electro-optic diagnostic techniques. A range of targets were considered, including thin plastic foils (20-550 nm) and mass-limited, optically-levitated micro-targets. Results from foils indicate a dependence of EMP on target thickness, with larger peak electric fields observed with thinner targets. Spectral analysis suggests high repeatability between shots, with identified spectral features consistently detected with <1 MHz standard deviations of the peak position. This deviation is reduced for shots taken on the same day, suggesting that local conditions, such as movement of metal objects within the target chamber, are more likely to lead to minor spectral modifications, highlighting the role of the local environment in determining the details of EMP production. Levitated targets are electrically isolated from their environment, hence these targets should be unable to draw a neutralization current from the earth following ejection of hot electrons from the plasma, in contrast to predictions for pin-mounted foils in the Poyé EMP generation model. With levitated targets, no EMP was measurable above the noise threshold of any diagnostic, despite observation of protons accelerated to >30 MeV energies, suggesting the discharge current contribution to EMP is dominant.
Robinson TS, Patankar S, Floyd E, et al., 2017, Spectral characterization of a supercontinuum sourcebased on nonlinear broadening in an aqueous K₂ZnCl₄salt solution, Applied Optics, Vol: 56, Pages: 9837-9845, ISSN: 0003-6935
We report on investigations concerning the shot-to-shot spectral stability properties of a supercontinuum source based on nonlinear processes such as self-phase modulation and optical wave-breaking in a highly concentrated K2ZnCl4K2ZnCl4 double salt solution. The use of a liquid medium offers both damage resistance and high third-order optical nonlinearity. Approximately 40 μJ pulses spanning a spectral range between 390 and 960 nm were produced with 3.8% RMS energy stability, using infrared input pulses of 500±50 fs500±50 fs FWHM durations and 2.42±0.04 mJ2.42±0.04 mJ energies with an RMS stability of 2%. The spectral stability was quantified via acquiring single-shot spectra and studying shot-to-shot variation across a spectral range of 200–1100 nm, as well as by considering spectral correlations. The regional spectral correlation variations were indicative of nonlinear processes leading to sideband generation. Spectral stability and efficiency of energy transfer into the supercontinuum were found to weakly improve with increasing driver pulse energy, suggesting that the nonlinear broadening processes are more stable when driven more strongly, or that self-guiding effects in a filament help to stabilize the supercontinuum generation.
Hare JD, Lebedev SV, Suttle LG, et al., 2017, Formation and structure of a current sheet in pulsed-power driven magnetic reconnection experiments, Physics of Plasmas, Vol: 24, ISSN: 1070-664X
We describe magnetic reconnection experiments using a new, pulsed-powerdriven experimental platform in which the inflows are super-sonic butsub-Alfv\'enic.The intrinsically magnetised plasma flows are long lasting,producing a well-defined reconnection layer that persists over manyhydrodynamic time scales.The layer is diagnosed using a suite of highresolution laser based diagnostics which provide measurements of the electrondensity, reconnecting magnetic field, inflow and outflow velocities and theelectron and ion temperatures.Using these measurements we observe a balancebetween the power flow into and out of the layer, and we find that the heatingrates for the electrons and ions are significantly in excess of the classicalpredictions. The formation of plasmoids is observed in laser interferometry andoptical self-emission, and the magnetic O-point structure of these plasmoids isconfirmed using magnetic probes.
Galinis G, Strucka J, Barnard JCT, et al., 2017, Micrometer-thickness liquid sheet jets flowing in vacuum, Review of Scientific Instruments, Vol: 88, ISSN: 0034-6748
Thin liquid sheet jet flows in vacuum provide a new platform for performing experiments in the liquid phase, for example X-ray spectroscopy. Micrometer thickness, high stability, and optical flatness are the key characteristics required for successful exploitation of these targets. A novel strategy for generating sheet jets in vacuum is presented in this article. Precision nozzles were designed and fabricated using high resolution (0.2 μm) 2-photon 3D printing and generated 1.49 ±± 0.04 μm thickness, stable, and <λλ/20-flat jets in isopropanol under normal atmosphere and under vacuum at 5 ×× 10−1 mbar. The thin sheet technology also holds great promise for advancing the fields of high harmonic generation in liquids, laser acceleration of ions as well as other fields requiring precision and high repetition rate targets.
Patankar S, Gumbrell ET, Robinson TS, et al., 2017, Absolute calibration of optical streak cameras on picosecond time scales using supercontinuum generation, APPLIED OPTICS, Vol: 56, Pages: 6982-6987, ISSN: 1559-128X
Robinson TS, Consoli F, Giltrap S, et al., 2017, Low-noise time-resolved optical sensing of electromagnetic pulses from petawatt laser-matter interactions, Scientific Reports, Vol: 7, ISSN: 2045-2322
We report on the development and deployment of an optical diagnostic for single-shot measurement of the electric-field components of electromagnetic pulses from high-intensity laser-matter interactions in a high-noise environment. The electro-optic Pockels effect in KDP crystals was used to measure transient electric fields using a geometry easily modifiable for magnetic field detection via Faraday rotation. Using dielectric sensors and an optical fibre-based readout ensures minimal field perturbations compared to conductive probes and greatly limits unwanted electrical pickup between probe and recording system. The device was tested at the Vulcan Petawatt facility with 1020 W cm−2 peak intensities, the first time such a diagnostic has been used in this regime. The probe crystals were located ~1.25 m from target and did not require direct view of the source plasma. The measured signals compare favourably with previously reported studies from Vulcan, in terms of the maximum measured intra-crystal field of 10.9 kV/m, signal duration and detected frequency content which was found to match the interaction chamber’s horizontal-plane fundamental harmonics of 76 and 101 MHz. Methods for improving the diagnostic for future use are also discussed in detail. Orthogonal optical probes offer a low-noise alternative for direct simultaneous measurement of each vector field component.
Hare JD, Suttle L, Lebedev SV, et al., 2017, Anomalous heating and plasmoid formation in a driven magnetic reconnection experiment, Physical Review Letters, Vol: 118, ISSN: 0031-9007
We present a detailed study of magnetic reconnection in a quasi-two-dimensional pulsed-power driven laboratory experiment. Oppositely directed magnetic fields (B=3 T), advected by supersonic, sub-Alfvénic carbon plasma flows (Vin=50 km/s), are brought together and mutually annihilate inside a thin current layer (δ=0.6 mm). Temporally and spatially resolved optical diagnostics, including interferometry, Faraday rotation imaging, and Thomson scattering, allow us to determine the structure and dynamics of this layer, the nature of the inflows and outflows, and the detailed energy partition during the reconnection process. We measure high electron and ion temperatures (Te=100 eV, Ti=600 eV), far in excess of what can be attributed to classical (Spitzer) resistive and viscous dissipation. We observe the repeated formation and ejection of plasmoids, consistent with the predictions from semicollisional plasmoid theory.
Sharba AB, Nersisyan G, Zepf M, et al., 2016, Generation of high contrast and high spatial quality idler from a low-gain optical parametric amplifier, Applied Optics, Vol: 55, Pages: 9341-9346, ISSN: 1559-128X
The temporal contrast of a regeneratively amplified, sub-picosecond pulse is enhanced by employing a low-gain optical parametric amplification stage self-pumped by the second harmonic of the pulse. Through careful characterization of the two related nonlinear processes and optimization of the non-collinear geometry, a robust high-contrast idler pulse has been generated, with excellent spatial quality in both the near and far field. The overall energy conversion efficiency exceeds 14%, with 33% intensity conversion efficiency. The temporal cleaning is implemented without any bandwidth losses or spectral shift and produces approximately 20% temporal shortening. These experimental findings are in excellent agreement with numerical calculations.
Floyd E, Gumbrell ET, Fyrth J, et al., 2016, A high spatio-temporal resolution optical pyrometer at the ORION laser facility, Review of Scientific Instruments, Vol: 87, ISSN: 0034-6748
A streaked pyrometer has been designed to measure the temperature of ≈100 µm diameter heatedtargets in the warm dense matter region. The diagnostic has picosecond time resolution. Spatialresolution is limited by the streak camera to 4 µm in one dimension; the imaging system hassuperior resolution of 1 µm. High light collection efficiency means that the diagnostic can transmita measurable quantity of thermal emission at temperatures as low as 1 eV to the detector. This isachieved through the use of an f/1.4 objective, and a minimum number of reflecting and refractingsurfaces to relay the image over 8 m with no vignetting over a 0.4 mm field of view with12.5× magnification. All the system optics are highly corrected, to allow imaging with minimalaberrations over a broad spectral range. The detector is a highly sensitive Axis Photonique streakcamera with a P820PSU streak tube. For the first time, two of these cameras have been absolutelycalibrated at 1 ns and 2 ns sweep speeds under full operational conditions and over 8 spectral bandsbetween 425 nm and 650 nm using a high-stability picosecond white light source. Over this range thecameras had a response which varied between 47 ± 8 and 14 ± 4 photons/count. The calibration ofthe optical imaging system makes absolute temperature measurements possible. Color temperaturemeasurements are also possible due to the wide spectral range over which the system is calibrated;two different spectral bands can be imaged onto different parts of the photocathode of the same streakcamera.
Stuart NH, Robinson TS, Hillier D, et al., 2016, Comparative study on the temporal contrast of femtosecond mode-locked laser oscillators, Optics Letters, Vol: 41, Pages: 3221-3224, ISSN: 1539-4794
We have investigated the temporal intensity contrast characteristics from a broad range of mode-locked short-pulse oscillators used for seeding high-power terawatt and petawatt class laser systems. Saturable absorber (SESAM), Kerr-lens (KLM), nonlinear polarization evolution (NPE) in optical fibers and synchronously pumped optical parametric oscillator (OPO) mode-locked sources have been measured using a third-order autocorrelator with up to 1010 dynamic range. We restricted the temporal characterization to features <30ps about the laser pulse that reflect fundamental mode-locking processes. We find additional nonlinear terms and residual higher-order dispersion limits the performance of KLM and NPE sourcesup to the 105 contrast level, whilst >108 contrast was observed from the SESAM and OPO laser pulse trains.
Suttle LG, Hare JD, Lebedev SV, et al., 2016, Structure of a Magnetic Flux Annihilation Layer Formed by the Collision of Supersonic, Magnetized Plasma Flows, Physical Review Letters, Vol: 116, ISSN: 1079-7114
We present experiments characterizing the detailed structure of a current layer, generated by the collision of two counterstreaming, supersonic and magnetized aluminum plasma flows. The antiparallel magnetic fields advected by the flows are found to be mutually annihilated inside the layer, giving rise to a bifurcated current structure—two narrow current sheets running along the outside surfaces of the layer. Measurements with Thomson scattering show a fast outflow of plasma along the layer and a high ion temperature (Ti∼Z¯Te, with average ionization Z¯=7). Analysis of the spatially resolved plasma parameters indicates that the advection and subsequent annihilation of the inflowing magnetic flux determines the structure of the layer, while the ion heating could be due to the development of kinetic, current-driven instabilities.
Swadling GF, Lebedev SV, Hall GN, et al., 2016, Experimental investigations of ablation stream interaction dynamics in tungsten wire arrays: interpenetration, magnetic field advection, and ion deflection, Physics of Plasmas, Vol: 23, ISSN: 1089-7674
Experiments have been carried out to investigate the collisional dynamics of ablation streams produced by cylindrical wire array z-pinches. A combination of laser interferometric imaging, Thomson scattering, and Faraday rotationimaging has been used to make a range of measurements of the temporal evolution of various plasma and flow parameters. This paper presents a summary of previously published data, drawing together a range of different measurements in order to give an overview of the key results. The paper focuses mainly on the results of experiments with tungsten wire arrays. Early interferometric imagingmeasurements are reviewed, then more recent Thomson scattering measurements are discussed; these measurements provided the first direct evidence of ablation stream interpenetration in a wire array experiment. Combining the data from these experiments gives a view of the temporal evolution of the tungsten stream collisional dynamics. In the final part of the paper, we present new experimental measurements made using an imagingFaraday rotationdiagnostic. These experiments investigated the structure of magnetic fields near the array axis directly; the presence of a magnetic field has previously been inferred based on Thomson scattering measurements of ion deflection near the array axis. Although the Thomson and Faradaymeasurements are not in full quantitative agreement, the Faraday data do qualitatively supports the conjecture that the observed deflections are induced by a static toroidal magnetic field, which has been advected to the array axis by the ablation streams. It is likely that detailed modeling will be needed in order to fully understand the dynamics observed in the experiment.
Haerendel G, Suttle L, Lebedev SV, et al., 2016, Stop layer: a flow braking mechanism in space and support from a lab experiment, Plasma Physics and Controlled Fusion, Vol: 58, ISSN: 1361-6587
The paper presents short summaries and a synopsis of two completely independent discoveries of a fast flow braking process, one realized by a laboratory experiment (Lebedev et al 2014 Phys. Plasmas 21 056305), the other by theoretical reasoning stimulated by auroral observation (Haerendel 2015a J. Geophys. Res. Space Phys. 120 1697–714). The first has been described as a magnetically mediated sub-shock forming when a supersonic plasma flow meets a wall. The second tried to describe what happens when a high-beta plasma flow from the central magnetic tail meets the strong near-dipolar field of the magnetosphere. The term stop layer signals that flow momentum and energy are directly coupled to a magnetic perturbation field generated by a Hall current within a layer of the width of c/ω pi and immediately propagated out of the layer by kinetic Alfvén waves. As the laboratory situation is not completely collision-free, energy transfer from ions to electrons and subsequent radiative losses are likely to contribute. A synopsis of the two situations identifies and discusses six points of commonality between the two situations. It is pointed out that the stop layer mechanism can be regarded as a direct reversal of the reconnection process.
Patankar S, Gumbrell ET, Robinson TS, et al., 2015, Multiwavelength interferometry system for the Orion laser facility, Applied Optics, Vol: 54, Pages: 10592-10598, ISSN: 1559-128X
We report on the design and testing of a multiwavelength interferometry system for the Orion laser facility basedupon the use of self-path matching Wollaston prisms. The use of UV corrected achromatic optics allows for botheasy alignment with an eye-safe light source and small (∼ millimeter) offsets to the focal lengths between differentoperational wavelengths. Interferograms are demonstrated at wavelengths corresponding to first, second, andfourth harmonics of a 1054 nm Nd:glass probe beam. Example data confirms the broadband achromatic capabilityof the imaging system with operation from the UV (263 nm) to visible (527 nm) and demonstrates that featuresas small as 5 μm can be resolved for object sizes of 15 by 10 mm. Results are also shown for an off-harmonicwavelength that will underpin a future capability. The primary optics package is accommodated inside the footprintof a ten-inch manipulator to allow the system to be deployed from a multitude of viewing angles inside the4 m diameter Orion target chamber
Rodriguez R, Espinosa G, Gil JM, et al., 2015, Time-dependent and radiation field effects on collisional-radiative simulations of radiative properties of blast waves launched in clusters of xenon, High Energy Density Physics, Vol: 17, Pages: 119-128, ISSN: 1878-0563
Radiative shock waves are ubiquitous throughout the universe and play a crucial role in the transport of energy into the interstellar medium. This fact has led to many efforts to scale the astrophysical phenomenato accessible conditions. In some laboratory experiments radiative blast waves are launched in clusters of gases by means of the direct deposition of the laser energy. In this work, by using a ollisionalradiativemodel, we perform an analysis of the plasma level populations and radiative properties of a blast wave launched in a xenon cluster. In particular, for both the shocked and unshocked material, westudy the influence of different effects such as LTE, steady-state or time-dependent NLTE simulations, plasma self-absorption or external radiation field in the determination of those properties and also in thediagnosis of the electron temperature of the blast wave.
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