arXiv subjects

U. Shumlak

Publications and source records attributed to U. Shumlak.

7 recordsLinked to original sources

arXiv2025

Effects of transitional orbit magnetization on transport and current in Z pinches

The azimuthal self-magnetic field of the ideal Z pinch contains a central magnetic null. Trajectories around this null govern transport in the core. Particles follow cyclotron orbits when the guiding-center approximation holds. Approaching the field null, where the ordinary guiding-center regime breaks down, particles exhibit trajectories called, in some historical contexts, betatron orbits. We quantify transitional magnetization between cyclotron and betatron orbits by a magnetization parameter that decomposes phase space into these orbit regimes. Considering the distribution of all orbits, this phase-space decomposition reveals a transitional magnetization region wherein both populations coexist. Classical magnetized transport theory fails within this region, where the diamagnetic drift reverses. The drift flux is instead supported by the flux of betatron orbits. Kinematic diffusivity remains approximately constant rather than diverging at the null. These transport modifications are governed solely by the number density per unit length in the ideal pinch.

physics.plasm-ph↗

arXiv2024

Z Pinch Kinetics II -- A Continuum Perspective: Betatron Heating and Self-Generation of Sheared Flows

Adiabatic compression of a self-magnetizing current filament (a Z pinch) is analyzed via the adiabatic invariants of its constituent cyclotron and betatron motions. Chew-Goldberger-Low (CGL) models are recovered for both trajectories but with distinct anisotropy axes, about the magnetic field for cyclotron fluid and about the electric current for betatron fluid. In particular, betatron heating produces agyrotropic anisotropy which balances with gyrophase mixing. A hybrid CGL model is proposed based on the local densities of cyclotron and betatron orbits, then validated by numerical experiments. The relation between anisotropy and shear is explored by constructing the kinetic equilibrium of a flow expanded in the flux function. Flow as a linear flux function is simply bi-Maxwellian, while higher powers display higher-moment deviations. Next, weakly collisional gyroviscosity (magnetized pressure-strain) is considered in a forward process (forced flow) and an inverse process (forced anisotropy). The forward process phase-mixes flow into a simple flux function, freezing flow into flux and inducing anisotropy. In the inverse process, betatron heating-induced anisotropy self-generates a sheared flow to resist changes in flux. This flow, arising from momentum diffusion, is concentrated in the betatron region.

physics.plasm-ph↗

arXiv2024

Time-resolved measurement of neutron energy isotropy in a sheared-flow-stabilized Z pinch

Previous measurements of neutron energy using fast plastic scintillators while operating the Fusion Z Pinch Experiment (FuZE) constrained the energy of any yield-producing deuteron beams to less than $4.65 keV$. FuZE has since been operated at increasingly higher input power, resulting in increased plasma current and larger fusion neutron yields. A detailed experimental study of the neutron energy isotropy in these regimes applies more stringent limits to possible contributions from beam-target fusion. The FuZE device operated at $-25~kV$ charge voltage has resulted in average plasma currents of $370~kA$ and D-D fusion neutron yields of $4\times10^7$ neutrons per discharge. Measurements of the neutron energy isotropy under these operating conditions demonstrates the energy of deuteron beams is less than $7.4 \pm 5.6^\mathrm{(stat)} \pm 3.7^\mathrm{(syst)}~keV$. Characterization of the detector response has reduced the number of free parameters in the fit of the neutron energy distribution, improving the confidence in the forward-fit method. Gamma backgrounds have been measured and the impact of these contributions on the isotropy results have been studied. Additionally, a time dependent measurement of the isotropy has been resolved for the first time, indicating increases to possible deuteron beam energies at late times. This suggests the possible growth of $m$=0 instabilities at the end of the main radiation event but confirms that the majority of the neutron production exhibits isotropy consistent with thermonuclear origin.

physics.plasm-ph↗

arXiv2024

Phase space eigenfunctions with applications to continuum kinetic simulations

Continuum kinetic simulations are increasingly capable of resolving high-dimensional phase space with advances in computing. These capabilities can be more fully explored by using linear kinetic theory to initialize the self-consistent field and phase space perturbations of kinetic instabilities. The phase space perturbation of a kinetic eigenfunction in unmagnetized plasma has a simple analytic form, and in magnetized plasma may be well approximated by truncation of a cyclotron-harmonic expansion. We catalogue the most common use cases with a historical discussion of kinetic eigenfunctions and by conducting nonlinear Vlasov-Poisson and Vlasov-Maxwell simulations of single- and multi-mode two-stream, loss-cone, and Weibel instabilities in unmagnetized and magnetized plasmas with one- and two-dimensional geometries. Applications to quasilinear kinetic theory are discussed and applied to the bump-on-tail instability. In order to compute eigenvalues we present novel representations of the dielectric function for ring distributions in magnetized plasmas with power series, hypergeometric, and trigonometric integral forms. Eigenfunction phase space fluctuations are visualized for prototypical cases such as the Bernstein modes to build intuition. In addition, phase portraits are presented for the magnetic well associated with nonlinear saturation of the Weibel instability, distinguishing current-density-generating trapping structures from charge-density-generating ones.

physics.plasm-ph↗

arXiv2024

Whole Device Modeling of the FuZE Sheared-Flow-Stabilized Z Pinch

The FuZE sheared-flow-stabilized Z pinch at Zap Energy is simulated using whole-device modeling employing an axisymmetric resistive magnetohydrodynamic formulation implemented within the discontinuous Galerkin WARPXM framework. Simulations show formation of Z pinches with densities of approximately 10^22 m^-3 and total DD fusion neutron rate of 10^7 per μs for approximately 2 μs. Simulation-derived synthetic diagnostics show peak currents and voltages within 10% and total yield within approximately 30% of experiment for similar plasma mass. The simulations provide insight into the plasma dynamics in the experiment and enable a predictive capability for exploring design changes on devices built at Zap Energy.

physics.plasm-ph↗

arXiv2022

On the validity of quasilinear theory applied to the electron bump-on-tail instability

The accuracy of quasilinear theory applied to the electron bump-on-tail instability, a classic model problem, is explored with conservative high-order discontinuous Galerkin methods applied to both the quasilinear equations and to a direct simulation of the Vlasov-Poisson equations. The initial condition is chosen in the regime of beam parameters for which quasilinear theory should be applicable. Quasilinear diffusion is initially in good agreement with the direct simulation but later underestimates the turbulent momentum flux. The direct simulation corrects from quasilinear evolution by quenching the instability in a finite time and producing a robust state of oscillation. Flux enhancement above quasilinear levels occurs as the phase space eddy turnover time in the largest amplitude wavepackets becomes comparable to the transit time of resonant phase fluid through wavepacket potentials. In this regime eddies effectively turn over during wavepacket transit so that phase fluid predominantly disperses by eddy phase mixing rather than by randomly phased waves. The enhanced turbulent flux of resonant phase fluid leads in turn, through energy conservation, to an increase in non-resonant turbulent flux and thus to an enhanced heating of the main thermal body above quasilinear predictions. These findings shed light on the kinetic turbulence fluctuation spectrum and support the theory that collisionless momentum diffusion beyond the quasilinear approximation can be understood through the dynamics of phase space eddies (or clumps and granulations).

physics.plasm-ph↗

arXiv2019

Sustained neutron production from a sheared-flow stabilized Z-pinch

The sheared-flow stabilized $Z$-pinch has demonstrated long-lived plasmas with fusion-relevant parameters. This Letter presents the first experimental results demonstrating sustained, quasi-steady-state neutron production from the Fusion $Z$-pinch Experiment (FuZE), operated with a mixture of 20% deuterium/80% hydrogen by volume. Neutron emissions lasting approximately $5~μ$s are reproducibly observed with pinch currents of approximately $200$ kA during an approximately $16~μ$s period of plasma quiescence. The average neutron yield is estimated to be $\left ( 1.25\pm 0.45 \right )\times 10^{5}$ neutrons/pulse and scales with the square of the deuterium concentration. Coincident with the neutron signal, plasma temperatures of $1-2$ keV, and densities of approximately $10^{17}$ cm$^{-3}$ with $0.3$ cm pinch radii are measured with fully-integrated diagnostics.

physics.plasm-ph↗