Session VM10: Mini-Conference on Recent Advances in Magnetic Fields in High Energy Density Plasmas III

Live

Magnetized ICF and Magneto-Inertial Fusion schemes include a magnetic field in order to reduce electron thermal conduction losses from hot fuel during the implosion phase, thereby reducing the velocity required to reach ignition, and to aid confinement of alpha particles during thermonuclear burn. This work studies the dynamics of the magnetic field during thermonuclear burn in high beta plasmas in which thermal diffusivity is much larger than magnetic diffusivity. Magnetic field advection, the Nernst effect and the transverse current induced by the flux of alpha particles can all play a role in the magnetic field transport. The Nernst effect will transport the field from hot to cold fuel while ablation of the cold fuel can advect plasma in the opposite direction. However, the electron Hall parameter, on which transport coefficients depend, can vary significantly in time due to alpha heating and in space due to large temperature and density differences between hot and cold fuel. This further complicates the field transport. Numerical solutions of the induction, fuel energy and alpha energy equations show that feedback between the field transport, thermal conduction and alpha flux can significantly change the dynamics of thermonuclear burn.   

Live

The fundamental physics of the plasma rotation in plasma implosion with a pre-embedded magnetic field is investigated within an oxygen gas-puff Z-pinch (0.3-MA, 1-$\mu $s long current pulse). Time and space resolved spectroscopy of the polarized Zeeman effect is used to measure, for the first time, simultaneously all three components of the magnetic field together with the plasma rotation velocity obtained from Doppler shifts of spectral lines. The measurements show that an application of an axial magnetic field makes the imploding plasma rotate. The angular velocity of rotation $\omega $ is antiparallel to the applied axial magnetic field, Bz. The plasma does not rotate as a solid body. The measured rotational velocity (1-5)\textbullet 10$^{\mathrm{6}}$ cm/s is comparable to the peak implosion velocity. The self-generated rotation plays a significant role in both the pressure and energy balance. Spectroscopic measurements of all three components of the magnetic field help elucidate the mechanisms of the plasma rotation, both the j\texttimes B force and the E\texttimes B drift. The improved stability of the imploding plasma demonstrates the effect of the plasma rotation on mitigation of plasma instabilities.   

Live

We present the first results of a laboratory-astrophysics experiment with the goal of characterizing magnetized plasma jets on the Michigan Accelerator for Inductive Z-Pinch Experiments (MAIZE) in the Plasma, Pulsed Power, and Microwave Laboratory at the University of Michigan. We aim to explore the interactions of magnetized plasma flows with external magnetic fields and the behavior of the different plasma flows created by conical wire-arrays (hot coronal plasma and radiatively cooled jets). In these first preliminary results, we focus on the structure and development of shock instabilities. To generate the magnetized plasma flows, we used MAIZE to ablate 100-micron, aluminum wire arrays with currents in the order of 500 Kilo-Amp with a rise time of 250 ns. We use a conical array to drive an axial plasma jet, while a Helmholtz coil provides a uniform 5-T axial magnetic field. Our first images come from visible self-emission and shadowgraphy (532 nm), captured by a fast-frame camera, showing the structure and evolution of the plasma jet.   

Live

Tangled magnetic fields are prevalent in astrophysical scenarios, including molecular clouds and the solar corona, and can play an important role in the thermal energy transport and pressure balance of such systems. In 2009, Ryutov [1] proposed the use of such a tangled magnetic field for insulation of a plasma target for magneto-inertial fusion. Such a target has the advantage that it could preserve its insulating role in spherical compression geometry, as opposed to more regular magnetic field structures that are best suited to cylindrical compression. Results in the literature find differing results on the effective thermal transport in such plasmas, dependent on the magnetic spectrum. We are currently investigating this concept for use with the plasma-jet-driven magneto-inertial fusion architecture [2], due to its suitability for spherical compression, and will present updates on our investigations to date, and outlook for future development.\\ 1 Ryutov, D. D., Fusion Sci. Tech. 56.4 (2009): 1489-1494.\\ 2 Hsu, Scott C., et al., IEEE Trans. Plasma Sci. 40.5 (2012): 1287-1298.   

Live

Rayleigh-Taylor (RT) instabilities can occur during both the acceleration and deceleration phase of the implosion in inertial confinement fusion (ICF) settings leading to the undesirable mixing of hot and cold plasmas. Understanding the mechanism of such instabilities for experimentally relevant parameter space provides potential means to mitigate its growth and achieve ignition-grade hot-spots. Here, we numerically investigate the RT and magneto-RT instability using experimental parameters given by Sauppe et al. [\textit{MRE 4, 065403 (2019)}] to study the impact of external magnetic fields in cylindrical implosions. The studies incorporate self-consistent effects of viscosity and resistivity. The simulations have been carried out using plasma fluid models in PHORCE (Package for High ORder simulations of Convection-diffusion Equations) based on the unstructured discontinuous Galerkin finite element method. Using magnetohydrodynamic depiction without and with external magnetic fields, it has been shown that the presence of self-consistent dissipation in the system drastically changes the nature of the instability within the deceleration phase ($\lesssim 7 $ns ). We also provide useful insight into the RT induced turbulence in such high energy density plasmas.   

Live

We have designed and built an~0.5 Tesla LTS~superconducting coil testbed as part of our PFRC research program. The PFRC-3 would be a steady-state FRC fusion-oriented plasma experiment with about 1 Tesla magnetic field. Similar systems can provide the bias field of pre-magnetized HED plasmas, and to demonstrate diagnostics, principles, and scaling laws at weaker fields. Our coil comprises a split pair of winding packs (ID $=$ 25 cm, length 25 cm) to mimic a subset of the PFRC-3 magnets. A separate pulsed copper coil, inserted into the bore of the LTS coil, simulates the plasma, enabling us to study the impact of plasma startup, termination, and instabilities and equipment failures. FRC formation in a PFRC will occur in a fraction of a second and result in rapid increases in magnetic field at the windings, ameliorated, in part, by a conducting bobbin on which the LTS is wound. Depending on Covid restrictions, we plan to drive 365 A in the pulsed coil with a rise time of 15 ms, inducing a flux of 6 mVs and a current of 7 A in the LTS, to be measured with diamagnetic loops and Gauss meters. Access ports through the LTS magnet case allows radial profile measurement.   

On properties of QED plasmas

Current advances in laser-plasma and astrophysical observations of magnetar emission demand better understanding of how quantum electrodynamics (QED) effects that are present in strong-field environments affect plasma dynamics. Interestingly, astrophysical systems such as extremely strongly magnetized neutrons stars, called `magnetars' possess magnetic fields far in excess of the Schwinger (critical) field, so these effects can no longer be ignored. In particular, Maxwell's equations become nonlinear in the strong-QED regime. This effect has not, so far, been incorporated in plasma codes; systemic theoretical studies of QED-plasmas are also absent. Here we present the derivation of the general equation of linear plasma modes in QED-plasma with arbitrarily strong magnetic field. We discuss general trends and some properties of the low-frequency modes. These results can be of interest for understanding of electron-positron plasma in a magnetar magnetosphere, as well as future lab experiments.