Session PO04: Fundamental Plasmas: Antimatter Plasmas and Shock Waves

Live

Antihydrogen, the bound state of a positron and an antiproton, is often synthesized by merging cold clouds of positrons and antiprotons. In the more recent techniques, where this is done carefully to allow magnetic trapping of the nascent antihydrogen, the positron temperature dominates both the rate of this process and the temperature of the nascent antihydrogen as the antiprotons thermalize of the positrons before recombination. We have therefore implemented a setup allowing us to load Beryllium ions into our antihydrogen trap system where a near axial laser can be used to laser-cool them. Merging the Beryllium ions with positrons in this setup, we have demonstrated sympathetic cooling of the positrons to below 8 K. The positron number and density, are commensurable with those used for antihydrogen synthesis, but with a temperature now about a factor of three below the temperatures we normally have, a significant increase in antihydrogen trapping when antiprotons become available again is expected.   

Live

We present a technique to adiabatically cool antihydrogen by trapping in a small magnetic well and subsequently expanding the trapping volume. Lowering~antihydrogen kinetic energy is expected to benefit antihydrogen spectroscopy and gravity experiments. For spectroscopic measurements, kinetic-energy reduction can decrease doppler effects and increase laser interaction-time, thus narrowing spectral linewidth. For gravity experiments, reducing antihydrogen kinetic energy makes it possible to confine in a reduced-strength magnetic field and minimizes effects from field errors. Simulations of the experimental procedure are presented, showing that an ensemble of antihydrogen atoms held in a static well undergo no significant changes in mean total energy, whereas adiabatically cooled populations have mean total energy reduced by about 37{\%}. Simulating gradual removal of the magnetic trap confirms the expectation that adiabatically cooled ensembles tend to annihilate at later times. Simulated annihilation-time distributions are found to resemble experimental data. The presented technique predominantly reduces axial kinetic energy but orbit-mixing between axial and transverse dimensions leads to transverse kinetic-energy reduction.   

Live

As firstly predicted by D'yakov and Kontorovich (DK), an initially disturbed shock front may exhibit different asymptotic behaviours depending on the slope of the Rankine-Hugoniot curve. Adiabatic and isolated planar shocks traveling steadily through ideal gases are stable, in the sense that any perturbation on the shock front decays in time with the power $t^{-3/2}$. In this work, it has been found that unstable conditions are might be found when the gas undergoes an endothermic or exothermic transformation behind the shock. In particular, it is reported that constant-amplitude oscillations can occur when the amount of energy release is positively-correlated to the shock strength and, if this correlation is sufficiently strong, the shock turns to be fully unstable. The opposite highly-damped oscillating regime may occur in negatively-correlated configurations. The mathematical description then adds two independent parameter to the regular adiabatic index and shock Mach number, namely: the total energy added/removed and its sensitivity with the shock strength. The formulation in terms of endothermic or exothermic effects is extended, but not restricted, to include effects associated to ionization, dissociation, thermal radiation, and thermonuclear transformations.   

Live

We are developing a platform that can measure via radiography the interaction of strong and weak shocks. Our first step is to show that we can produce weak shocks via the absorption of radiation as it transitions from supersonic to subsonic transport within a medium. We present the results of an experiment carried out at the Omega laser facility intended to study the supersonic-to-subsonic transition of radiation in low density (32 mg/cc) silica (SiO$_2$) aerogel. The target package was comprised of a silica foam cylinder which was driven by a $\sim$115 eV radiation drive created by a laser-driven halfraum. The drive induced a supersonic radiation front that propagated axially into the cylinder. X-ray radiography showed the creation of a density perturbation in the middle of the silica, consistent with the transition of supersonic-to-subsonic radiation transport. These results will be used to prepare for a future Omega experiment where we will study the interaction of such a shockwave with a strong shock induced through direct laser ablation, replicating a phenomena frequently observed in high-energy-density experiments. The goal of this campaign is to demonstrate that the described shock wave interaction can be accurately simulated with the radiation hydrodynamic code KULL.   

Live

Magnetized collisionless shocks are ubiquitous in astrophysics, and are believed to be the source of nonthermal spectra inferred from numerous observations. Experimental platforms are capable of studying relevant physics in laboratories. Within the accessible parameter space of OMEGA EP, perpendicular shocks in hydrogen and neon gas are studied by 2-D particle-in-cell simulations with real ion-electron mass ratios. A modified two-stream instability was proposed to be the main dissipation mechanism for the shock formation by previous research [J. Park et al., Phys. Plasmas 19, 062904 (2012)], and is used to estimate the optimal parameter settings for the simulations and upcoming experiments. Simulation results show that the magnetized collisionless shocks can be readily formed within a few tenths of a nanosecond, or a few hundreds of microns in both hydrogen and neon gas, with a background magnetic field of 50 T, which is achievable on OMEGA EP. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856, Department of Energy Award Number DE-SC0020431, and the resources of NERSC. The authors thank the OSIRIS consortium for the use of OSIRIS code.   

Live

Self-organization processes are considered to have an important role in well confined plasmas produced by present day experiments where the heating source is externally applied. The observation of ``Profile Consistency'' [1] is viewed as a manifestation of the presence of these processes. In the case of fusion burning plasmas close to self-sustainment (ignition) most of the heating due to fusion products is strongly dependent on the evolution of both the plasma temperature and density profiles. Therefore, self-organization is expected to be of considerably greater importance than in the case of non-reacting plasmas. This fact involves a significant degree of unpredictability on the outcome of envisioned experiments on burning plasmas that has to be added to the complexity of the collective modes that are expected to emerge. Thus, one of the motivations for the Ignitor program is to shed light on these issues and minimize the uncertainties for the design of more ambitious undertakings such as a Compact Pilot Plant. *Sponsored in part by CNR of Italy. [1] B. Coppi, Comm. Plasma Phys. Cont. Fusion \textbf{5}, 261 (1980).   

Live

Lateral interaction between two geometrically modified laser produced plasma (LPP) plumes in the presence of transverse magnetic field has been investigated in vacuum (i.e. $5\times10^{-7}$ mbar). Characteristic behaviour and expansion dynamics of both the seed plumes and interaction region in the presence of field is compared with those for field free case. A well defined sharp interaction region is formed between the seed plumes in absence of field. Contrary to the field free case, no sharp interaction zone is observed in the presence of field, rather large enhancement in emission intensities of ionic lines in both seed as well as interaction regions is observed in case of magnetic field. The modification in plume geometry and emission intensity enhancement become more prominent with the increase of magnetic field. The observed results are explained on the basis of atomic analysis of the spectral lines from the interaction region of the interacting plumes. The physical processes responsible for higher electron temperature and increased ionic line emission from singly as well as doubly ionized aluminium will be discussed in this presentation.   

Live

We present simulations of magnetized laser-plasama interactions using the one-dimensional kinetic code SAPRISTI [S. Brunner, E. J. Valeo, Phys. Rev. Lett. 93, 145003 (2004)]. This code utilizes finite-difference methods to solve the Vlasov-Maxwell system of equations, including the full non-linear kinetic dynamics of electrons and ions in the longitudinal direction, and a cold-fluid model of transverse dynamics. The code has been updated to include an external magnetic field in the longitudinal direction. We model stimulated Raman scattering with a background B field, and discuss ``stimulated whistler scattering'', where a pump light wave decays to a Langmuir wave and an electromagnetic whistler wave. This talk will discuss the algorithm development and results for various simulations.   

Live

The most prolific star formers in cosmological history lie in a regime where dense filament structures carried substantial mass into the galaxy to sustain star formation without producing a shock. However, hydrodynamic instabilities present on the filament surface limit the ability of such structures to deliver dense matter deeply enough to sustain star formation. Cosmological-scale simulations lack the finite resolution necessary to allow fair treatment of the instabilities present at the stream boundary. Therefore, hydrodynamic scaling analysis is established between the cosmological system and an experimental analogue. Then, using the Omega EP laser, we create this mode of galaxy formation with a cold, dense, filament structure within a hotter, subsonic flow and observe the interface evolution. Machined surface perturbations stimulate the development of the Kelvin-Helmholtz (KH) instability due to the resultant shear between the two media. A spherical crystal imaging system produces high-resolution radiographs of the KH structures along the filament surface. The results from this series of experiments using a rod with single-mode, long-wavelength modulations, will be discussed.