Experimental Evidence of Plasmoids in High-β Magnetic Reconnection

Magnetic reconnection is a ubiquitous and fundamental process in plasmas by which magnetic fields change their topology and release magnetic energy. Despite decades of research, the physics governing the reconnection process in many parameter regimes remains controversial. Contemporary reconnection theories predict that long, narrow current sheets are susceptible to the tearing instability and split into isolated magnetic islands (or plasmoids), resulting in an enhanced reconnection rate. While several experimental observations of plasmoids in the regime of low-to-intermediate β (where β is the ratio of plasma thermal pressure to magnetic pressure) have been made, there is a relative lack of experimental evidence for plasmoids in the high-β reconnection environments which are typical in many space and astrophysical contexts. Here, we report strong experimental evidence for plasmoid formation in laser-driven high-β reconnection experiments.

X-ray-imaging spectrometer (XRIS) for studies of residual kinetic energy and low-mode asymmetries in inertial confinement fusion implosions at OMEGA (invited)

A system of x-ray imaging spectrometer (XRIS) has been implemented at the OMEGA Laser Facility and is capable of spatially and spectrally resolving x-ray self-emission from 5 to 40 keV. The system consists of three independent imagers with nearly orthogonal lines of sight for 3D reconstructions of the x-ray emission region. The distinct advantage of the XRIS system is its large dynamic range, which is enabled by the use of tantalum apertures with radii ranging from 50 μm to 1 mm, magnifications of 4 to 35×, and image plates with any filtration level. In addition, XRIS is capable of recording 1-100's images along a single line of sight, facilitating advanced statistical inference on the detailed structure of the x-ray emitting regions. Properties such as P0 and P2 of an implosion are measured to 1% and 10% precision, respectively. Furthermore, T_e can be determined with 5% accuracy.

Measurements of ion-electron energy-transfer cross section in high-energy-density plasmas

We report on measurements of the ion-electron energy-transfer cross section utilizing low-velocity ion stopping in high-energy-density plasmas at the OMEGA laser facility. These measurements utilize a technique that leverages the close relationship between low-velocity ion stopping and ion-electron equilibration. Shock-driven implosions of capsules filled with D^{3}He gas doped with a trace amount of argon are used to generate densities and temperatures in ranges from 1×10^{23} to 2×10^{24} cm^{-3} and from 1.4 to 2.5 keV, respectively. The energy loss of 1-MeV DD tritons and 3.7-MeV D^{3}He alphas that have velocities lower than the average velocity of the thermal electrons is measured. The energy loss of these ions is used to determine the ion-electron energy-transfer cross section, which is found to be in excellent agreement with quantum-mechanical calculations in the first Born approximation. This result provides an experimental constraint on ion-electron energy transfer in high-energy-density plasmas, which impacts the modeling of alpha heating in inertial confinement fusion implosions, magnetic-field advection in stellar atmospheres, and energy balance in supernova shocks.

In situ calibration of charged particle spectrometers on the OMEGA Laser Facility using 241Am and 226Ra sources

Charged particle spectrometry is a critical diagnostic to study inertial-confinement-fusion plasmas and high energy density plasmas. The OMEGA Laser Facility has two fixed magnetic charged particle spectrometers (CPSs) to measure MeV-ions. In situ calibration of these spectrometers was carried out using ²⁴¹Am and ²²⁶Ra alpha emitters. The alpha emission spectrum from the sources was measured independently using surface-barrier detectors (SBDs). The energy dispersion and broadening of the CPS systems were determined by comparing the CPS measured alpha spectrum to that of the SBD. The calibration method significantly constrains the energy dispersion, which was previously obtained through the measurement of charged particle fusion products. Overall, a small shift of 100 keV was observed between previous and the calibration done in this work.

Experiments on the dynamics and scaling of spontaneous-magnetic-field saturation in laser-produced plasmas

In laser-produced high-energy-density plasmas, large-scale strong magnetic fields are spontaneously generated by the Biermann battery effects when temperature and density gradients are misaligned. Saturation of the magnetic field takes place when convection and dissipation balance field generation. While theoretical and numerical modeling provide useful insight into the saturation mechanisms, experimental demonstration remains elusive. In this letter, we report an experiment on the saturation dynamics and scaling of Biermann battery magnetic field in the regime where plasma convection dominates. With time-gated charged-particle radiography and time-resolved Thomson scattering, the field structure and evolution as well as corresponding plasma conditions are measured. In these conditions, the spatially resolved magnetic fields are reconstructed, leading to a picture of field saturation with a scaling of B∼1/L_{T} for a convectively dominated plasma, a regime where the temperature gradient scale (L_{T}) exceeds the ion skin depth.

Effect of Strongly Magnetized Electrons and Ions on Heat Flow and Symmetry of Inertial Fusion Implosions

This Letter presents the first observation on how a strong, 500 kG, externally applied B field increases the mode-two asymmetry in shock-heated inertial fusion implosions. Using a direct-drive implosion with polar illumination and imposed field, we observed that magnetization produces a significant increase in the implosion oblateness (a 2.5× larger P2 amplitude in x-ray self-emission images) compared with reference experiments with identical drive but with no field applied. The implosions produce strongly magnetized electrons (ω_{e}τ_{e}≫1) and ions (ω_{i}τ_{i}>1) that, as shown using simulations, restrict the cross field heat flow necessary for lateral distribution of the laser and shock heating from the implosion pole to the waist, causing the enhanced mode-two shape.

Proton deflectometry with in situ x-ray reference for absolute measurement of electromagnetic fields in high-energy-density plasmas

We report a technique of proton deflectometry that uses a grid and an in situ reference x-ray grid image for precise measurements of magnetic fields in high-energy-density plasmas. A D³He fusion implosion provides a bright point source of both protons and x-rays, which is split into beamlets by a grid. The protons undergo deflections as they propagate through the plasma region of interest, whereas the x-rays travel along straight lines. The x-ray image, therefore, provides a zero-deflection reference image. The line-integrated magnetic fields are inferred from the shifts of beamlets between the deflected (proton) and reference (x-ray) images. We developed a system for analysis of these data, including automatic algorithms to find beamlet locations and to calculate their deflections from the reference image. The technique is verified in an experiment performed at OMEGA to measure a nonuniform magnetic field in vacuum and then applied to observe the interaction of an expanding plasma plume with the magnetic field.

Thermal decoupling of deuterium and tritium during the inertial confinement fusion shock-convergence phase

A series of thin glass-shell shock-driven DT gas-filled capsule implosions was conducted at the OMEGA laser facility. These experiments generate conditions relevant to the central plasma during the shock-convergence phase of ablatively driven inertial confinement fusion (ICF) implosions. The spectral temperatures inferred from the DTn and DDn spectra are most consistent with a two-ion-temperature plasma, where the initial apparent temperature ratio, T_{T}/T_{D}, is 1.5. This is an experimental confirmation of the long-standing conjecture that plasma shocks couple energy directly proportional to the species mass in multi-ion plasmas. The apparent temperature ratio trend with equilibration time matches expected thermal equilibration described by hydrodynamic theory. This indicates that deuterium and tritium ions have different energy distributions for the time period surrounding shock convergence in ignition-relevant ICF implosions.

An x-ray penumbral imager for measurements of electron-temperature profiles in inertial confinement fusion implosions at OMEGA

Hot-spot shape and electron temperature (T_e) are key performance metrics used to assess the efficiency of converting shell kinetic energy into hot-spot thermal energy in inertial confinement fusion implosions. X-ray penumbral imaging offers a means to diagnose hot-spot shape and T_e, where the latter can be used as a surrogate measure of the ion temperature (T_i) in sufficiently equilibrated hot spots. We have implemented a new x-ray penumbral imager on OMEGA. We demonstrate minimal line-of-sight variations in the inferred T_e for a set of implosions. Furthermore, we demonstrate spatially resolved T_e measurements with an average uncertainty of 10% with 6 μm spatial resolution.

Yield degradation due to laser drive asymmetry in D3He backlit proton radiography experiments at OMEGA

Mono-energetic proton radiography is a vital diagnostic for numerous high-energy-density-physics, inertial-confinement-fusion, and laboratory-astrophysics experiments at OMEGA. With a large number of campaigns executing hundreds of shots, general trends in D³He backlighter performance are statistically observed. Each experimental configuration uses a different number of beams and drive symmetry, causing the backlighter to perform differently. Here, we analyze the impact of these variables on the overall performance of the D³He backlighter for proton-radiography studies. This study finds that increasing laser drive asymmetry can degrade the performance of the D³He backlighter. The results of this study can be used to help experimental designs that use proton radiography.

A novel method to recover DD fusion proton CR-39 data corrupted by fast ablator ions at OMEGA and the National Ignition Facility

CR-39 detectors are used routinely in inertial confinement fusion (ICF) experiments as a part of nuclear diagnostics. CR-39 is filtered to stop fast ablator ions which have been accelerated from an ICF implosion due to electric fields caused by laser-plasma interactions. In some experiments, the filtering is insufficient to block these ions and the fusion-product signal tracks are lost in the large background of accelerated ion tracks. A technique for recovering signal in these scenarios has been developed, tested, and implemented successfully. The technique involves removing material from the surface of the CR-39 to a depth beyond the endpoint of the ablator ion tracks. The technique preserves signal magnitude (yield) as well as structure in radiograph images. The technique is effective when signal particle range is at least 10 μm deeper than the necessary bulk material removal.

Fluorocarbon-aluminium compounds. V. Reactions of some fluorotoluene compounds with lithium tetrahydroaluminate

In tetrahydrofuran at -45� and in diethyl ether at 20�, the reaction of α-bromo- 2,3,4,5,6-pentafluorotoluene with lithium tetrahydroaluminate yields 2,3,4,5,6-penta-fluorotoluene. In tetrahydrofuran at 20�, the same reaction occurs initially but the pentafluorotoluene then reacts further with LiAlH4. LiAlH4 attacks the para-fluorine atom of 2,3,4,5,6- pentafluorotoluene to form 2,3,5,6-tetrafluorotoluene and the fluoro- carbon-alane complex LiAl(C6F4CH3)H2F. The reaction between LiAlH4 and C6F5CH3 is complete after 0.5 hr at 65�. In turn, 2,3,5,6- tetrafluorotoluene reacts with LiAlH4 in refluxing tetrahydrofuran to form the fluorocarbon-alane complexes LiAl(C6F4CH3)nH4-n, n = 1-3. ��� The fluorocarbon-aluminium compounds have not been isolated from solution but were characterized by quantitative determination of the products of hydrolysis and by 1H and 19F N.M.R. spectroscopy.

Fluorocarbon-aluminium compounds. IV. Reactions of some chloropolyfluorobenzenes with lithium tetrahydroaluminate

The reaction of lithium tetrahydroaluminate with chloropentafluorobenzene yields mainly p-chlorotetrafluorobenzene. The nature of the solvent influences the formation of other fluorocarbon compounds in the reaction. p-Chlorotetrafluoro-benzene reacts further with LiAlH4 in tetrahydrofuran, but not in diethyl ether, to give predominantly 1-chloro-2,3,5-trifluorobenzene and 1-chloro-2,3,6- trifluorobenzene. In turn, the trifluorobenzene compounds react with LiAlH4 in tetrahydrofuran to give mainly 1-chloro-2,5-difluorobenzene. The major products are formed by a combination of (i) direct nucleophilic displacement of fluorine in the chloropolyfluorobenzene compounds by hydride, and (ii) hydrolysis of the compounds LiAl(C6F4Cl)H2F, LiAl(C6F3HCl)H2F, and LiAl(C6F2H2Cl)H2F. The fluorocarbon- aluminium compounds have not been isolated from solution but they were characterized by quantitative determination of the products of hydrolysis and by 1H and 19F N.M.R. spectroscopy.

Fluorocarbon-aluminium compounds. III. Reactions of Heptafluoro-3-iodopropane, 1,1,1,2,2-Pentafluoro-3-iodopropane, and Heptafluoro-2-iodopropane with lithium tetrahydroaluminate

The complex LiA1(CFzCF2CF3)H21 which is formed from heptafluoro-3- iodopropane and lithium tetrahydroaluminate has been studied in solution. 1,1,1,2,2- Pentafluoro-3-iodopropane and heptafluoro-2-iodopropane also react with lithium tetrahydroaluminate in diethyl ether or tetrahydrofuran solution to form the unstable complexes LiA1(CH2CF2CF3)H21 and LiA1[CF(CF3)2]H21 respectively. The formula- tion of each fluorocarbon-alane compound is indicated by n.m.r. spectroscopic studies and by a quantitative investigation of the products of (i) thermal degradation and (ii) hydrolysis. Of the three fluoroalkyl-alane complexes, LiAl(CHzCF2CF3)Hd is the least stable thermally. I t decomposes readily in solution to give 2,3,3,3-tetra- fluoropropene in high yield. The perfluoroalkyl-alane complexes LiAl(CFzCF2CFs)HnI and LiAl[CF(CF3)2]H21 both decompose in solution to form fluoro-alkane and -alkene compounds.

Fluorocarbon-aluminium compounds. II. Reactions of hexafluorobenzene, pentafluorobenzene, and 1,2,4,5-tetrafluorobenzene with lithium tetrahydroaluminate

Reaction of lithium tetrahydroaluminate with hexafluorobenzene yields pentafluorobenzene. Pentafluorobenzene reacts further with LiAlH4 to give 1,2,4,5-tetrafluorobenzene. The products are formed by a combination of (i) reduction (hydride for fluoride replacement) of the fluoroaromatic compound, and (ii) hydrolysis of one of the compounds LiAl(C6F4H)H2F or LiAl(C6F5)H2F that are formed in the reactions. These competing reactions take place to a similar extent and are much faster in tetrahydrofuran than in diethyl ether. ��� In tetrahydrofuran, 1,2,4,5-tetrafluorobenzene is metalated by LiAlH4. The compounds LiAl(C6F4H)H3, LiAl(C6F4H)2H2, and LiAl(C6F4H)3H are proposed as products of the reaction. ��� The fluorocarbon-aluminium compounds have been characterized by quantitative determination of the products of hydrolysis and by 1H and 19F N.M.R. spectroscopy.