283 results found
Halliday JWD, Crilly A, Chittenden J, et al., 2022, Investigating radiatively driven, magnetized plasmas with a university scale pulsed-power generator, Physics of Plasmas, Vol: 29, Pages: 1-13, ISSN: 1070-664X
We present first results from a novel experimental platform which is able toaccess physics relevant to topics including indirect-drive magnetised ICF;laser energy deposition; various topics in atomic physics; and laboratoryastrophysics (for example the penetration of B-fields into HED plasmas). Thisplatform uses the X-Rays from a wire array Z-Pinch to irradiate a silicontarget, producing an outflow of ablated plasma. The ablated plasma expands intoambient, dynamically significant B-fields (~5 T) which are supported by thecurrent flowing through the Z-Pinch. The outflows have a well-defined(quasi-1D) morphology, enabling the study of fundamental processes typicallyonly available in more complex, integrated schemes. Experiments were fielded onthe MAGPIE pulsed-power generator (1.4 MA, 240 ns rise time). On this machine awire array Z-Pinch produces an X-Ray pulse carrying a total energy of ~15 kJover ~30 ns. This equates to an average brightness temperature of around 10 eVon-target.
McLean KW, Rose SJ, 2022, Multi-group radiation diffusion convergence in low-density foam experiments, Journal of Quantitative Spectroscopy and Radiative Transfer, Vol: 280, Pages: 1-13, ISSN: 0022-4073
We present an in-depth analysis of a Marshak radiation wave moving through an iron-oxide (Fe2O3) foamusing a 1D multigroup diffusive radiation transport model, MDART (Multigroup Diffusion Algorithm forRadiation Transport). We consider the consequences of under-resolving the group structure and addresshow this could lead to incorrect conclusions in the analysis of general supersonic radiation wave experiments. We also provide an analysis of the types of experimental outcome one may incorrectly link tophysical effects but are in fact due to poor simulation practice.
Kettle B, Hollatz D, Gerstmayr E, et al., 2021, A laser-plasma platform for photon-photon physics: the two photon Breit-Wheeler process, New Journal of Physics, Vol: 23, ISSN: 1367-2630
We describe a laser–plasma platform for photon–photon collision experiments to measure fundamental quantum electrodynamic processes. As an example we describe using this platform to attempt to observe the linear Breit–Wheeler process. The platform has been developed using the Gemini laser facility at the Rutherford Appleton Laboratory. A laser Wakefield accelerator and a bremsstrahlung convertor are used to generate a collimated beam of photons with energies of hundreds of MeV, that collide with keV x-ray photons generated by a laser heated plasma target. To detect the pairs generated by the photon–photon collisions, a magnetic transport system has been developed which directs the pairs onto scintillation-based and hybrid silicon pixel single particle detectors (SPDs). We present commissioning results from an experimental campaign using this laser–plasma platform for photon–photon physics, demonstrating successful generation of both photon sources, characterisation of the magnetic transport system and calibration of the SPDs, and discuss the feasibility of this platform for the observation of the Breit–Wheeler process. The design of the platform will also serve as the basis for the investigation of strong-field quantum electrodynamic processes such as the nonlinear Breit–Wheeler and the Trident process, or eventually, photon–photon scattering.
Baggott RA, Rose SJ, Mangles SPD, 2021, Temperature Equilibration Due to Charge State Fluctuations in Dense Plasmas, ISSN: 0730-9244
The charge states of ions in dense plasmas fluctuate due to collisional ionization and recombination. In this work we show how, by modifying the ion interaction potential, these fluctuations can mediate energy exchange between the plasma electrons and ions. We also develop a theoretical framework for this novel electron-ion energy transfer mechanism.
Rose SJ, Hatfield PW, 2021, Astronomy Domine: advancing science with a burning plasma, Contemporary Physics, Vol: 62, Pages: 14-23, ISSN: 0010-7514
Inertial Confinement Fusion (ICF) is a subject that has been studied for decades, because of its potential for clean energy generation. Although thermonuclear fusion has been achieved, the energy out has always been considerably less than the energy in, so high energy gain with a burning thermonuclear plasma is still some way off. A multitude of new science has come from the ICF programme that is relevant outside the field (typically in astrophysics). What we look at in this text is what new science can come from the much more extreme conditions that would be created in the laboratory if a burning ICF plasma could be created -- in terms of energy density the most extreme macroscopic environment ever created. We show that this could impact science from particle physics through astrophysics and on to cosmology. We also believe that the experiments that we propose here are only a small part of the science that will be opened up when a burning thermonuclear plasma is created in the laboratory.
Baggott RA, Rose S, Mangles SPD, 2021, Temperature equilibration due to charge state fluctuations in dense plasmas, Physical Review Letters, Vol: 27, ISSN: 0031-9007
The charge states of ions in dense plasmas fluctuate due to collisionalionization and recombination. Here we show how, by modifying the ioninteraction potential, these fluctuations can mediate energy exchange betweenthe plasma electrons and ions. Moreover, we develop a theory for this novelelectron-ion energy transfer mechanism. Calculations using a random walkapproach for the fluctuations suggest that the energy exchange rate from chargestate fluctuations could be comparable to direct electron-ion collisions. Thismechanism is, however, predicted to exhibit a complex dependence on thetemperature and ionization state of the plasma, which could contribute to ourunderstanding of significant variation in experimental measurements ofequilibration times.
Spencer Kelly R, Hart LJF, Rose SJ, 2021, An investigation of efficient muon production for use in muon catalyzed fusion, Journal of Physics: Energy, Vol: 3, Pages: 1-7, ISSN: 2515-7655
We model the energy cost of producing muons for use in muon catalyzed fusion and show that by careful design the cost can be reduced by a factor of 2.5 below current values. This is done by recapturing the kinetic energy of waste particles and generating heat through tritium breeding. When put together with the modeling of muon catalyzed fusion we estimate that electrical output/electrical input of 14% can be achieved currently.
Hatfield PW, Gaffney JA, Anderson GJ, et al., 2021, The data-driven future of high-energy-density physics, Nature, Vol: 593, Pages: 351-361, ISSN: 0028-0836
High-energy-density physics is the field of physics concerned with studying matter at extremely high temperatures and densities. Such conditions produce highly nonlinear plasmas, in which several phenomena that can normally be treated independently of one another become strongly coupled. The study of these plasmas is important for our understanding of astrophysics, nuclear fusion and fundamental physics—however, the nonlinearities and strong couplings present in these extreme physical systems makes them very difficult to understand theoretically or to optimize experimentally. Here we argue that machine learning models and data-driven methods are in the process of reshaping our exploration of these extreme systems that have hitherto proved far too nonlinear for human researchers. From a fundamental perspective, our understanding can be improved by the way in which machine learning models can rapidly discover complex interactions in large datasets. From a practical point of view, the newest generation of extreme physics facilities can perform experiments multiple times a second (as opposed to approximately daily), thus moving away from human-based control towards automatic control based on real-time interpretation of diagnostic data and updates of the physics model. To make the most of these emerging opportunities, we suggest proposals for the community in terms of research design, training, best practice and support for synthetic diagnostics and data analysis.
Perez-Callejo G, Marley E, Liedahl DA, et al., 2021, Demonstration of geometric effects and resonant scattering in the X-Ray spectra of high-energy-density plasmas, Physical Review Letters, Vol: 126, Pages: 1-7, ISSN: 0031-9007
In a plasma of sufficient size and density, photons emitted within the system have a probability of being reabsorbed and reemitted multiple times—a phenomenon known in astrophysics as resonant scattering. This effect alters the ratio of optically thick to optically thin lines, depending on the plasma geometry and viewing angle, and has significant implications for the spectra observed in a number of astrophysical scenarios, but has not previously been studied in a controlled laboratory plasma. We demonstrate the effect in the x-ray spectra emitted by cylindrical plasmas generated by high power laser irradiation, and the results confirm the geometrical interpretation of resonant scattering.
Rose S, Hatfield P, Scott R, 2020, Modelling burning thermonuclear plasma, Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, Vol: 378, Pages: 1-8, ISSN: 1364-503X
Considerable progress towards the achievement ofthermonuclear burn using inertial confinement fusion has beenachieved at the National Ignition Facility (NIF) in the USA inthe last few years. Other drivers, such as the Z-machine atSandia, are also making progress towards this goal. A burningthermonuclear plasma would provide a unique and extremeplasma environment; in this paper we discuss a) differenttheoretical challenges involved in modelling burning plasmasnot currently considered, b) the use of novel machine learningbased methods that might help large facilities reach ignition,and c) the connections that a burning plasma might have tofundamental physics, including QED studies, and the replicationand exploration of conditions that last occurred in the first fewminutes after the Big Bang.
Pérez-Callejo G, Jarrott LC, Liedahl DA, et al., 2020, Measuring the oscillator strength of intercombination lines of helium-like V ions in a laser-produced-plasma, Journal of Quantitative Spectroscopy and Radiative Transfer, Vol: 256, ISSN: 0022-4073
We present results of measurements of the oscillator strength of intercombination lines of He-like Vanadium ions in high energy density (HED) laser-produced-plasmas and compare them with the simulations from commonly used codes and data from the NIST database. Whilst not yet sufficiently accurate to constrain different trusted atomic-physics models for the particular system studied, our results are in agreement with the available data within experimental error bars, yet differ from cruder approximations of the oscillator strength used in certain atomic-kinetics packages, suggesting that this general method could be further extended to be used as a measurement of the oscillator strength of additional atomic transitions under the extreme conditions that are achieved in HED experiments.
Pérez-Callejo G, Barrios MA, Liedahl DA, et al., 2020, A novel method to measure ion density in ICF experiments using x-ray spectroscopy of cylindrical tracers, Physics of Plasmas, Vol: 27, Pages: 112714-1-112714-11, ISSN: 1070-664X
The indirect drive approach to inertial confinement fusion has undergone important advances in the past few years. Improvements in temperature and density diagnostic methods are leading to more accurate measurements of the plasma conditions inside the Hohlraum and therefore to more efficient experimental designs. The implementation of dot spectroscopy has proven to be a versatile approach to extracting space- and time-dependent electron temperatures. In this method, a microdot of a mid-Z material is placed inside the Hohlraum and its K-shell emission spectrum is used to determine the plasma temperature. However, radiation transport of optically thick lines acting within the cylindrical dot geometry influences the outgoing spectral distribution in a manner that depends on the viewing angle. This angular dependence has recently been studied in the high energy density regime at the OMEGA laser facility, which allowed us to design and benchmark appropriate radiative transfer models that can replicate these geometric effects. By combining these models with the measurements from the dot spectroscopy experiments at the National Ignition Facility, we demonstrate here a novel technique that exploits the transport effects to obtain time-resolved measurements of the ion density of the tracer dots, without the need for additional diagnostics. We find excellent agreement between experiment and simulation, opening the possibility of using these geometric effects as a density diagnostic in future experiments.
White S, Irwin R, Warwick R, et al., 2020, Generation of photoionized plasmas in the laboratory: Analogues to astrophysical sources, Laboratory Astrophysics fFrom Observation to Interpretation, Publisher: Cambridge University Press, Pages: 321-325, ISSN: 1743-9213
Implementation of a novel experimental approach using a bright source of narrowband x-ray emission has enabled the production of a photoionized argon plasma of relevance to astrophysical modelling codes such as Cloudy. We present results showing that the photoionization parameter ζ = 4πF/ne generated using the VULCAN laser was ≈ 50 erg cm s−1, higher than those obtained previously with more powerful facilities. Comparison of our argon emission-line spectra in the 4.15 - 4.25 Å range at varying initial gas pressures with predictions from the Cloudy code and a simple time-dependent code are also presented. Finally we briefly discuss how this proof-of-principle experiment may be scaled to larger facilities such as ORION to produce the closest laboratory analogue to a photoionized plasma.
Baggott R, Rose S, Mangles S, 2020, Calculating opacity in hot, dense matter using second-order electron-photon and two-photon transitions to approximate line broadening, Physical Review Letters, Vol: 125, Pages: 145002 – 1-145002 – 5, ISSN: 0031-9007
Calculations of the opacity of hot, dense matter require models for plasma line broadening. How-ever, the most general theories are too complex to calculate directly and some approximation is inevitably required. The most widely-used approaches focus on the line centre, where a Lorentzian shape is obtained. Here, we demonstrate that in the opposite limit, far from the line centre, the opacity can be expressed in terms of second-order transitions, such as electron-photon and two-photon processes. We suggest that this insight could form the basis for a new approach to improve calculations of opacity in hot, dense matter. Preliminary calculations suggest that this approach could yield increased opacity away from absorption lines.
Behm K, Hussein AE, Zhao TZ, et al., 2020, Demonstration of femtosecond broadband X-rays from laser wakefield acceleration as a source for pump-probe X-ray absorption studies, High Energy Density Physics, Vol: 35, Pages: 1-5, ISSN: 1574-1818
We present X-ray absorption measurements near the K-edge of laser heated aluminum in a pump-probe configuration using X-rays generated in a laser wakefield accelerator. A 30 fs duration laser pulse from the Herculeslaser system was split into two beamlines, with one used to heat a 4 µm thickness Al foil and the second to drive a laser wakefield accelerator. The laser-heated plasma was probed at various pump-probe delays using the femtosecond duration X-rays generated by betatron oscillations of the electrons in the wakefield. We observe an apparent blue-shift of the K-edge occurring on a sub-picosecond timescale in the transmission spectra.
McLean KW, Rose SJ, 2020, Corrections to weighted opacities and energy exchange rate in 3-T radiation-hydrodynamics, High Energy Density Physics, Vol: 35, Pages: 1-5, ISSN: 1574-1818
It is often the case that high energy density systems can be well described and simulated in the 3T approximation, where electrons, ions and the radiation field are defined at unique temperatures given by Te, Ti, Tr. The difference in temperature between the electrons and radiation field is important when calculating weighted opacities and electron-radiation energy exchange rates. Often, it is assumed that Tr ≈ Te, meaning the quantities can be calculated as functions of Te only. This paper explores the consequences that arise when one uses this assumption in regions where Tr ≠ Te. Mutliplicative correction factors are derived for the Rosseland and Planckian mean opacities (κR and κP) and for the electron-radiation energy exchange rate. We find that there exists a very small region of parameter space where the corrections are negligible. However, for the majority of parameter space explored, numerical corrections vary from factors of 2 to multiple orders of magnitude.
Hobbs LMR, Burridge D, Hill MP, et al., 2020, X-ray-line coincidence photopumping in a potassium-chlorine mixed plasma, Physical Review A, Vol: 101, Pages: 053431-1-053431-8, ISSN: 2469-9926
Exploiting the multiple long pulse capability and suite of x-ray diagnostics of the Orion laser, we have set out to explore line coincidence photopuming—the enhancement in population of an atomic level brought on by resonant absorption of x rays from a different emitting ion. Unlike previous work, the two ions are in the same plasma and so the experiment is an x-ray analog of the well-known Bowen resonance fluorescence mechanism that operates in astrophysical situations in the optical region. Our measurements have shown enhanced fluorescence in a chlorine plasma, attributable to line coincident photopumping from co-mixed potassium ions. To detect this relatively low signal-to-noise phenomenon, the data from multiple shots are combined, and the statistical method of bootstrapping is used to assign a confidence value to the measured enhancement, resulting in an estimate of the enhancement of 39±1618% compared to the null case, where no pumping occurs. The experimental results have been compared to coupled radiation-transport and radiation hydrodynamics simulations using the cretin code together with the nym radiation hydrodynamics model and agreement has been found, with the simulations also predicting modest enhancement.
Nilsen J, Burridge D, Hobbs LMR, et al., 2020, Enhanced fluorescence from X-Ray line coincidence pumping, 16th International Conference on X-ray Lasers, Publisher: Springer International Publishing, Pages: 29-35, ISSN: 0930-8989
Many resonant photo-pumped X-ray laser schemes that use a strong pump line such as Ly-α or He-α to populate the upper laser state of a separate lasing material have been proposed over the last four decades but none have been demonstrated. As a first step to creating a photo-pumped X-ray laser we have decided to reinvestigate some of these schemes at the Orion laser facility with the goal to show enhanced fluorescence. In particular we look at using the Ly-α or He-α K lines to pump the 1s–3p and 4p transitions in H-like Cl and see fluorescence on the 4f–3d line at 65 Å and the 3d–2p line at 23 Å. Preliminary experiments are presented that show a modest enhancement. As an alternative we also look at enhancing the 2p–2s line in Ne-like Ge at 65 Å using the Ly-α Mg line to photo-pump the 2s–3p line of Ne-like Ge. Calculations are presented that suggest modest enhancements of 2.5.
Bailie D, Hyland C, Singh R, et al., 2020, An investigation of the L-shell X-ray conversion efficiency for laser-irradiated tin foils, Plasma Science and Technology, Vol: 22, Pages: 1-7, ISSN: 1009-0630
We have used the Shenguang II laser in third harmonic (351 nm) to investigate the emission ofL-shell radiation in the 3.3 to 4.4 keV range generated using thin foils of Sn coated onto a parylenesubstrate with irradiation of order 1015 Wcm−2 and nanosecond pulse duration. In our experiment,we have concentrated on assessing the emission on the non-laser irradiated side as this allows anexperimental geometry relevant to experiments on photo-ionised plasmas where a secondary targetmust be placed close to the source, to achieve X-ray fluxes appropriate to astrophysical objects.Overall L-shell conversion efficiencies are estimated to be of order 1%, with little dependence onSn thickness between 400 and 800 nm.
Hatfield P, Rose S, Scott R, et al., 2020, Using sparse Gaussian processes for predicting robust inertial confinement fusion implosion yields, IEEE Transactions on Plasma Science, Vol: 48, Pages: 14-21, ISSN: 0093-3813
Here, we present the application of an advanced sparse Gaussian process-based machine learning algorithm to the challenge of predicting the yields of inertial confinement fusion (ICF) experiments. The algorithm is used to investigate the parameter space of an extremely robust ICF design for the National Ignition Facility, the ``Simplest Design''; deuterium-tritium gas in a plastic ablator with a Gaussian, Planckian drive. In particular, we show that: 1) GPz has the potential to decompose uncertainty on predictions into uncertainty from lack of data and shot-to-shot variation; 2) it permits the incorporation of science-goal-specific cost-sensitive learning (CSL), e.g., focusing on the high-yield parts of parameter space; and 3) it is very fast and effective in high dimensions.
Kettle B, Gerstmayr E, Streeter MJV, et al., 2019, Single-shot multi-keV X-ray absorption spectroscopy using an ultrashort laser-wakefield accelerator source, Physical Review Letters, Vol: 123, Pages: 254801-1-254801-6, ISSN: 0031-9007
Single-shot absorption measurements have been performed using the multi-keV x rays generated by a laser-wakefield accelerator. A 200 TW laser was used to drive a laser-wakefield accelerator in a mode which produced broadband electron beams with a maximum energy above 1 GeV and a broad divergence of ≈15 mrad FWHM. Betatron oscillations of these electrons generated 1.2±0.2×106 photons/eV in the 5 keV region, with a signal-to-noise ratio of approximately 300∶1. This was sufficient to allow high-resolution x-ray absorption near-edge structure measurements at the K edge of a titanium sample in a single shot. We demonstrate that this source is capable of single-shot, simultaneous measurements of both the electron and ion distributions in matter heated to eV temperatures by comparison with density functional theory simulations. The unique combination of a high-flux, large bandwidth, few femtosecond duration x-ray pulse synchronized to a high-power laser will enable key advances in the study of ultrafast energetic processes such as electron-ion equilibration.
Beesley JJ, Rose SJ, 2019, Free electron relativistic correction factors to collisional excitation and ionisation rates in a plasma, High Energy Density Physics, Vol: 33, ISSN: 1574-1818
This paper presents approximate correction factors to collisional excitation and ionisation rates which account for special relativity in the free electron motion as a function of free electron temperature and the threshold energy of the reaction. These are calculated by taking the ratio of collisional rates derived from simple empirical cross sections using relativistic and non-relativistic mechanics. These results are extended to de-excitation and three body recombination rates using detailed balance. It is found that the relativistic correction is significant in regimes potentially important to galactic intracluster media and diagnostic dopants in burning ICF plasmas.
Nilsen J, Burridge D, Hobbs LMR, et al., 2019, Enhanced fluorescence from x-ray line coincidence pumping of K-pumped Cl and Mg-pumped Ge plasmas, X-Ray Lasers and Coherent X-Ray Sources: Development and Applications XIII, Publisher: SPIE, Pages: 1111105-1-1111105-6
Many resonant photo-pumped X-ray laser schemes that use a strong pump line such as Ly-α or He-α to populate the upper laser state of a separate lasing material have been proposed over the last four decades but none have been demonstrated. As a first step to creating a photo-pumped X-ray laser we decided to reinvestigate some of these schemes at the Orion laser facility with the goal to demonstrate enhanced fluorescence as a first step toward creating a laser. In particular we look at using the Ly-α or He-α K lines to pump the 1s – 3p and 4p transitions in H-like Cl and see fluorescence on the 4f -3d line at 65 Å and the 3d – 2p line at 23 Å. Preliminary experiments are presented that show a modest enhancement of 40% on the 3d-2p line. As an alternative we also look at enhancing the 2p – 2s line in Ne-like Ge at 65Å using the Ly-α Mg line to photo-pump the 2s – 3p line of Ne-like Ge. Calculations are presented that suggest modest enhancements of 2.5 and recent experiments will be presented that show emission from a Ge plasma pumped by the Ly-α Mg line.
Pérez-Callejo G, Wark JS, Rose SJ, 2019, Radiation transfer in cylindrical, toroidal and hemi-ellipsoidal plasmas, Journal of Quantitative Spectroscopy and Radiative Transfer, Vol: 235, Pages: 24-30, ISSN: 0022-4073
We present solutions of the radiative transfer equation for cylinders, hollow hemi-ellipsoidal shells and tori for a uniform plasma of fixed geometry. The radiative transfer equation is explicitly solved for two directions of emission, parallel and perpendicular to the axis of symmetry. The ratio between the fluxes in these two directions is also calculated and its use in measuring the frequency resolved opacity of the plasma is discussed. We find that the optimal geometry to use this ratio as an opacity measurement is a planar geometry.
Weller ME, Beiersdorfer P, Lockard TE, et al., 2019, Observation of He-like satellite lines of the H-like potassium K XIX emission, Astrophysical Journal, Vol: 881, ISSN: 0004-637X
We present measurements of the H-like potassium (K XIX) X-ray spectrum and its Helike (K XVIII) satellite lines, which are situated in the wavelength region between 3.34 and 3.39Å, which has been of interest for the detection of dark matter. The measurements were takenwith a high-resolution X-ray spectrometer from targets irradiated by a long-pulse (2 ns) beamfrom the Orion laser facility. We obtain experimental wavelength values of dielectronicrecombination satellite lines and show that the ratio of the Lyα lines and their dielectronicsatellite lines can be used to estimate the electron temperature, which in our case was about1.5±0.3 keV.
Hoarty DJ, Morton J, Jeffery M, et al., 2019, A proposal to measure iron opacity at conditions close to the solar convective zone-radiative zone boundary, High Energy Density Physics, Vol: 32, Pages: 70-76, ISSN: 1574-1818
A major problem in stellar modelling is the discrepancy between solar models and helioseismology data in the position of the convective zone-radiative zone boundary in the sun. This could be explained by a large uncertainty in the calculated opacity data and recent experimental data on iron using the Sandia National Laboratory Z facility have shown large differences, up to a factor of 4, between measurement and prediction at plasma conditions close to the convective zone-radiative zone boundary. This paper describes a proposal for a radiative burn-through experiment to be fielded on NIF to observe if a radiation wave transit through a Fe2O3 sample is consistent with the factor of 2-4 change in the iron opacity seen in the Z experiments. A target design and the diagnostic method are described. A detailed radiation-hydrodynamic model has been used to generate synthetic results and explore the sensitivities and experimental accuracy needed for the proposed measurement.
Perez-Callejo G, Jarrott LC, Liedahl DA, et al., 2019, Laboratory measurements of geometrical effects in the x-ray emission of optically thick lines for ICF diagnostics, Physics of Plasmas, Vol: 26, ISSN: 1070-664X
Understanding the effects of radiative transfer in High Energy Density Physics experiments is critical for the characterization of the thermodynamic properties of highly ionized matter, in particular in Inertial Confinement Fusion (ICF). We report on non-Local Thermodynamic Equilibrium experiments on cylindrical targets carried out at the Omega Laser Facility at the Laboratory for Laser Energetics, Rochester NY, which aim to characterize these effects. In these experiments, a 50/50 mixture of iron and vanadium, with a thickness of 2000 Å and a diameter of 250 μm, is contained within a beryllium tamper, with a thickness of 10 μm and a diameter of 1000 μm. Each side of the beryllium tamper is then irradiated using 18 of the 60 Omega beams with an intensity of roughly 3 × 1014 W cm−2 per side, over a duration of 3 ns. Spectroscopic measurements show that a plasma temperature on the order of 2 keV was produced. Imaging data show that the plasma remains cylindrical, with geometrical aspect ratios (quotient between the height and the radius of the cylinder) from 0.4 to 2.0. The temperatures in this experiment were kept sufficiently low (∼1–2 keV) so that the optically thin Li-like satellite emission could be used for temperature diagnosis. This allowed for the characterization of optical-depth-dependent geometric effects in the vanadium line emission. Simulations present good agreement with the data, which allows this study to benchmark these effects in order to take them into account to deduce temperature and density in future ICF experiments, such as those performed at the National Ignition Facility.
Hatfield P, Rose S, Scott R, 2019, The blind implosion-maker: Automated inertial confinement fusion experiment design, Physics of Plasmas, Vol: 26, ISSN: 1070-664X
The design of inertial confinement fusion (ICF) experiments, alongside improving the development of energy density physics theory and experimental methods, is one of the key challenges in the quest for nuclear fusion as a viable energy source [O. A. Hurricane, J. Phys.: Conf. Ser. 717, 012005 (2016)]. Recent challenges in achieving a high-yield implosion at the National Ignition Facility (NIF) have led to new interest in considering a much wider design parameter space than normally studied [J. L. Peterson et al., Phys. Plasmas 24, 032702 (2017)]. Here, we report an algorithmic approach that can produce reasonable ICF designs with minimal assumptions. In particular, we use the genetic algorithm metaheuristic, in which “populations” of implosions are simulated, the design of the capsule is described by a “genome,” natural selection removes poor designs, high quality designs are “mated” with each other based on their yield, and designs undergo “mutations” to introduce new ideas. We show that it takes ∼5 × 104 simulations for the algorithm to find an original NIF design. We also link this method to other parts of the design process and look toward a completely automated ICF experiment design process—changing ICF from an experiment design problem to an algorithm design problem.
We study the possibility of observing Schwinger pair production enhanced by a thermal bath of photons. We consider the full range of temperatures and electric field intensities from pure Schwinger production to pure thermal production, and identify the most promising and interesting regimes. In particular, we identify temperatures of ∼ 20 keV/kB and field intensities of ∼ 10²³ W cm¯² where pair production would be observable. In this case, the thermal enhancement over the Schwinger rate is exponentially large and due to effects which are not visible at any finite order in the loop expansion. Pair production in this regime can thus be described as more nonperturbative than the usual Schwinger process, which appears at one loop. Unfortunately, such high temperatures appear to be out of reach of foreseeable technologies, though nonthermal photon distributions with comparable energy densities are possible. We suggest the possibility that similar nonperturbative enhancements may extend out of equilibrium and propose an experimental scheme to test this.
Perez-Callejo G, Liedahl DA, Schneider MB, et al., 2019, The use of geometric effects in diagnosing ion density in ICF-related dot spectroscopy experiments, High Energy Density Physics, Vol: 30, Pages: 45-51, ISSN: 1574-1818
We describe a method to calculate the ion density of High Energy Density (HED) cylindrical plasmas used in dot spectroscopy experiments. This method requires only spectroscopic measurements of the Heα region obtained from two views (Face-on and Side-on). We make use of the fact that the geometry of the plasma affects the observed flux of optically thick lines. The ion density can be derived from the aspect ratio (height-to-radius) of the cylinder and the optical depth of the Heα-y line (1s2p 3P1 → 1s2 1S0). The aspect ratio and the optical depth of the y line are obtained from the spectra using ratios measured from the two directions of emission of the optically thick Heα-w line (1s2p 1P1 → 1s2 1S0) and the ratio of the optically thick to thin lines. The method can be applied to mid-Z elements at ion densities of 1019– and temperatures of a the order of keV, which is a relevant regime for Inertial Confinement Fusion (ICF) experiments.
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