Session JI01: Lab Astro and Extreme Physics

Creating observable QED collective plasma effects

To observe in laboratory plasmas the QED plasma regime, it is crucial not just to create a pair plasma – but to create a pair plasma such that collective effects can be observed.   Colliding intense laser beams produces pairs, but collective plasma effects are then notoriously hard to observe [1]. The small pair plasma is only micron-scale.  Compounding the difficulty in observation, the plasma is also moving at relativistic speeds, making the pairs heavy, which diminishes the plasma frequency and the associated collective effects. So, even if produced, collective QED effects would be hard to see. The coupled "production-observation" problem can be approached instead by colliding a relativistic electron beam with a less intense laser.  This creates pairs that have larger plasma frequency, made even larger as they slow down by reversing direction due to the laser pressure [2-5].  Signatures of collective pair plasma effects in the QED cascades then appear in exquisite detail through plasma-induced frequency upshifts in the laser spectrum. Further distinctive features include a chirp to the frequency upshift with parametric dependencies. The electron beam and laser technologies are available, so, if ultra-dense electron beams were to be co-located with multi-PW lasers [6,7], this solution to the coupled production-observation problem means that strong-field quantum and collective pair plasma effects can in fact be explored with existing technology. 

[1] H. Chen, F. Fiuza, Phys. Plasmas 30, 020601 (2023). 

[2] K. Qu, S. Meuren, N. J. Fisch, PRL 127, 095001 (2021).

[3] K. Qu, S. Meuren, N. J. Fisch, Phys. Plasmas 29, 042117 (2022).

[4] A. Griffith, K. Qu, N. J. Fisch, Phys. Plasmas 29, 073104 (2022).

[5] K. Qu, S. Meuren, N. J. Fisch, Plasma Phys. Control. Fusion 65, 034007 (2023). 

[6] S. Meuren et al., arXiv:2002.10051 (2020). 

[7] S. Meuren et al., arXiv:2105.11607 (2021).   

Examining astrophysical gas-cloud collapse using an optical depth-scaled, x-ray-irradiated, carbon-foam sphere

When stellar radiation interacts with a molecular cloud, the cloud fate depends on the strength of the incident radiation and the radiation’s mean-free-path within the cloud [1]. Under the right conditions, the radiation compresses the cloud and star formation may occur. Where and when the stellar formation occurs in the cloud’s collapse are open questions. Direct observation of a cloud implosion with initial conditions is nearly impossible, making laboratory astrophysics a way to investigate.

This work describes laboratory experiments to study the radiation-driven implosion of clouds, using x-rays from a laser-irradiated, thin-gold foil as a surrogate star and a carbon-foam sphere as a surrogate cloud. The system’s optical depth was controlled by the foam density. Gold foil and sphere motions were imaged by x-ray radiography. Radiographic images show the formation of an interface between rarefied gold and carbon plasmas, a shock moving into the sphere, and a blunting of the initial sphere’s shape. Measurements show that the shock moved linearly around 64 um/ns into the sphere, and the gold-carbon interface formed by 2 ns at the sphere edge remained stationary. The deformation of the sphere was driven by the incident radiation and not by mechanical pressures applied by gold plasma. The blunting of the sphere was likely due to the geometric reduction of flux near the sphere’s poles. Higher x-ray flux near the sphere’s equator caused high compression and a faster shock, which flattened the sphere. We will discuss the results and implications of our observations.

[1] F. Bertoldi. ’The photoevaporation of interstellar clouds I Radiation Driven Implosion’. In The Astrophysical Journal 346.2 (1989), pp. 735-755.   

Laboratory tests of astrophysical black hole accretion disk plasma models using the Z-machine at Sandia National Laboratories

The Z-machine at Sandia National Laboratories generates powerful X-ray radiation fluxes. This enables experiments to produce macroscopic quantities of matter at extreme conditions such as those found in accretion powered plasmas around black holes in both active galactic nuclei and x-ray binaries. Complex models for these non-Local-Thermodynamic-Equilibrium (non-LTE) plasmas remain mostly untested with laboratory data. A novel platform developed on the Z-machine for expanding foil photoionized plasma experiments opens a new regime for benchmark measurements of non-LTE plasmas. We use the platform to create plasmas that reach the same photon flux, density, and temperature conditions in black hole accretion disks. The data from these experiments have already shown that an approximation often used for radiation transport in these plasmas was incorrect. They also reveal difficulties in modeling both emission intensities and the level of ionization in the plasma. We will present data of the first ever high S/N iron L-shell x-ray emission spectra from a laboratory photoionized plasma. Such data have been a laboratory astrophysics goal for two decades but are even more critical now because of the "Super-Solar" iron abundance problem (Garcia et al. 2016). Iron abundances in accretion disks inferred from x-ray spectra emitted by photoionized plasma surrounding about a dozen black holes appear to contain 5-20 times more iron than the Sun. This contradicts the widely held expectation that most objects in the universe have the Sun's composition. One prevailing theory is that effects of high electron density are not properly accounted for in the models. Reinterpreting the x-ray spectra with updated models resolved much of that discrepancy. However, a key question still remains: do photoionized plasma spectral models accurately account for x-ray emission? We will describe our progress in using this dataset to evaluate model accuracy and its potential to inform the super-solar iron abundance problem.   

Particle Acceleration and Ion Acoustic Waves during Magnetically Driven Reconnection using Laser-Powered Capacitor Coils

Magnetic reconnection is a ubiquitous phenomenon in astrophysical plasmas that rapidly converts magnetic energy into some combination of plasma flow energy, thermal energy, and non-thermal energetic particles in the presence of a magnetic topology change. Over the past decade, our team has developed a new experimental platform to study magnetically driven reconnection using strong coil currents powered by high power lasers [1] at low plasma beta, typical conditions under which reconnection is energetically important. KJ-class lasers were used to drive parallel currents to reconnect MG-level magnetic fields in a quasi-axisymmetric geometry, resemble to the Magnetic Reconnection Experiment or MRX [2], and thus this platform is termed micro-MRX. In this presentation, we report two major findings of our work on direct measurement of particle acceleration [3] and observation of ion acoustic waves [4] during anti-parallel reconnection in the micro-MRX. For the first time, we have successfully measured the energetic electrons generated by magnetic reconnection using particle spectrometers in a laboratory plasma. The angular dependence of the measured electron energy spectrum and the resulting accelerated energies, supported by particle-in-cell simulations, indicate that direct electric field acceleration by the out-of-plane reconnection electric field is at work [3]. Furthermore, we observe a sudden onset of ion acoustic bursts measured by collective Thomson scattering in the exhaust of magnetic reconnection, which are followed by electron acoustic bursts with electron heating and bulk acceleration [4]. These results demonstrate that the micro-MRX platform offers a novel and unique approach to study magnetic reconnection in the laboratory beyond the capabilities provided by typical magnetized plasma experiments such as MRX and the upcoming FLARE or Facility for Laboratory Reconnection Experiments [5]. Implications of these laboratory findings to space and astrophysical scenarios and future work on studying other particle acceleration mechanisms and ion acoustic waves will be discussed.   

Laboratory study of the initial stages of quasi-parallel collisionless shocks at high Alfvén Mach number

Collisionless shocks are ubiquitous in astrophysics and a possible source of the highest-energy cosmic rays in our universe. Recent experimental and numerical efforts have shown that ion-Weibel instability (IWI) is a leading candidate mechanism for collisionless shock formation in unmagnetized astrophysical objects. In a magnetized environment, ion beam instabilities, such as the right-hand instability or the non-resonant instability (NRI), can dominate the dynamics and mediate the development of collisionless shocks. This mediation occurs in a quasi-parallel configuration, meaning that the plasma flow is parallel to the ambient magnetic field.

We present an experimental and simulations investigation of the formation stage of a quasi-parallel collisionless shock at a high-Alfvén Mach number, M_A ~ 200. In experiments, a laser-driven super-Alfvénic plasma flow interpenetrates a premagnetized background plasma at a high velocity, v_flow ~ 1800 km/s. The background magnetic field (10 T) is aligned with the bulk ion velocity. As these kinetic ions interact with the background plasma and magnetic field, streaming instabilities develop and produce magnetic fields, which can eventually mediate collisionless shocks. Proton deflectometry was used to visualize the electromagnetic fluctuations at different times during the evolution, allowing the determination of the streaming instabilities involved in collisionless dissipative processes. A thorough analysis of the proton images indicates the development of the IWI followed by the non-resonant parallel streaming instability, also known as the Bell instability, after an ion gyroperiod.  3D hybrid PIC simulations were performed and support the growth of the non-resonant instability in the experimental conditions.  Simulations suggest that NRI provides an efficient source of dissipation for the formation of a shock, especially in the non-linear regime reached after eight ion gyroperiods.   

Electronic structure of Fe2O3 above 700 GPa

Characterizing chemical bonding and the behavior of valence electrons in solids has historically been difficult at high-energy-density (HED) conditions. X-ray absorption fine structure (XAFS) spectroscopy was performed on Fe₂O₃ (hematite) to determine the interactions between the valence electrons of iron and oxygen under compression. At the OMEGA-60 Laser Facility, Fe₂O₃ was ramp compressed to above 700 GPa and probed with an implosion x-ray source. A new x-ray spectrometer with improved spectral resolution and energy calibration measured the absorption spectrum, allowing x-ray absorption near edge spectroscopy (XANES) features of Fe₂O₃ to be measured at these extreme conditions. Fe₂O₃ undergoes a structural and insulator-to-metal transition when compressed to above 150 GPa, resulting in a negative shift in the K-edge absorption energy of 3 eV. When further compressed to above 700 GPa, the K-edge shifted to higher energy with increasing density. Analysis of the XANES spectrum revealed the iron 3d electron orbitals, which are bonded to the oxygen 2p electron orbitals, spread in energy as the oxygen atoms were compressed closer to the absorbing iron atom. The persistence of this 1s to 3d transition peak is an indication that iron remains bonded to oxygen above 700 GPa demonstrating the ability for iron to capture oxygen deep into the cores of super-Earth planets. In this talk, these techniques for characterizing chemistry at HED conditions will be presented and discussed as well as how to extend these techniques to characterize other complex changes in electron structure at extreme conditions.