arXiv subjects

C. B. Forest

Publications and source records attributed to C. B. Forest.

At least 19 recordsLinked to original sources

arXiv2026

Nonlinear anisotropic equilibrium reconstruction in axisymmetric magnetic mirrors

Magnetic equilibrium reconstruction is a crucial simulation capability for interpreting diagnostic measurements of experimental plasmas. Equilibrium reconstruction has mostly been applied to systems with isotropic pressure and relatively low plasma $β= 2μ_0p/B^2$. This work extends nonlinear equilibrium reconstruction to high-$β$ plasmas with anisotropic pressure and applies it to the Wisconsin High Temperature Superconducting Axisymmetric Magnetic Mirror experiments to infer the presence of sloshing ions. A novel basis set for the plasma profiles and machine learning algorithm using scalable constrained Bayesian optimization allow accurate nonlinear reconstructions with uncertainty quantification to be made more quickly with fewer experimental diagnostics and improves the robustness of reconstructions at high $β$. In addition to WHAM and other mirrors, such reconstruction techniques are potentially attractive in high-performance devices with constrained diagnostic capabilities such as fusion power plants.

physics.plasm-ph↗

arXiv2025

First implementation of AXUV-based analysis and macro-instability diagnostics on WHAM

Absolute extreme ultraviolet (AXUV) diode arrays are widely used in fusion experiments for time-resolved measurements of plasma radiation. We report the first implementation of an AXUV-based analysis framework on the Wisconsin High-Temperature Superconducting (HTS) Axisymmetric Mirror (WHAM). A single, precisely calibrated 20-channel AXUV assembly measures line-integrated plasma emission with $ 100~\mathrm{kHz}$ temporal resolution and $\sim1~\mathrm{cm}$ spatial accuracy across the mid-plane. The data were processed to obtain plasma's statistical moments, yielding time-resolved measurement of the centroid displacement $Φ(t)$ and effective radius $R(t)$. From the joint covariance of these quantities, we define a macroscopic instability parameter $χ(t)$, that quantifies large-scale plasma motion and profile evolution directly from AXUV observables. The parameter $χ$ serves as a compact indicator of global macroscopic instability, decreasing with increasing end-plate bias and exhibiting strong anti-correlation with diamagnetic flux during confinement transitions. These results demonstrate that a single AXUV array can provide quantitative, real-time assessment of macroscopic plasma instabilities, constituting the first demonstration of such capability in a magnetic mirror plasma. Future extensions to multiple arrays will further enhance spatial coverage and enable full-mode tracking in axisymmetric mirror configurations and related fusion devices.

physics.plasm-ph↗

arXiv2025

Confinement performance predictions for a high field axisymmetric tandem mirror

This paper presents Hammir tandem mirror confinement performance analysis based on Realta Fusion's first-of-a-kind model for axisymmetric magnetic mirror fusion performance. This model uses an integrated end plug simulation model including, heating, equilibrium, and transport combined with a new formulation of the plasma operation contours (POPCONs) technique for the tandem mirror central cell. Using this model in concert with machine learning optimization techniques, it is shown that an end plug utilizing high temperature superconducting magnets and modern neutral beams enables a classical tandem mirror pilot plant producing a fusion gain Q > 5. The approach here represents an important advance in tandem mirror design. The high fidelity end plug model enables calculations of heating and transport in the highly non-Maxwellian end plug to be made more accurately. The detailed end plug modelling performed in this work has highlighted the importance of classical radial transport and neutral beam absorption efficiency on end plug viability. The central cell POPCON technique allows consideration of a wide range of parameters in the relatively simple near-Maxwellian central cell, facilitating the selection of more optimal central cell plasmas. These advances make it possible to find more conservative classical tandem mirror fusion pilot plant operating points with lower temperatures, neutral beam energies, and end plug performance requirements than designs in the literature. Despite being more conservative, it is shown that these operating points have sufficient confinement performance to serve as the basis of a viable fusion pilot plant provided that they can be stabilized against MHD and trapped particle modes.

physics.plasm-ph↗

arXiv2023

Characterization of fast magnetosonic waves driven by compact toroid plasma injection along a magnetic field

Magnetosonic waves are low-frequency, linearly polarized magnetohydrodynamic (MHD) waves commonly found in space, responsible for many well-known features, such as heating of the solar corona. In this work, we report observations of interesting wave signatures driven by injecting compact toroid (CT) plasmas into a static Helmholtz magnetic field at the Big Red Ball (BRB) Facility at Wisconsin Plasma Physics Laboratory (WiPPL). By comparing the experimental results with the MHD theory, we identify that these waves are the fast magnetosonic modes propagating perpendicular to the background magnetic field. Additionally, we further investigate how the background field, preapplied poloidal magnetic flux in the CT injector, and the coarse grid placed in the chamber affect the characteristics of the waves. Since this experiment is part of an ongoing effort of creating a target plasma with tangled magnetic fields as a novel fusion fuel for magneto-inertial fusion (MIF), our current results could shed light on future possible paths of forming such a target for MIF.

physics.plasm-ph↗

arXiv2022

Off-axis runaway-electron seed formation, growth and suppression

Novel x-ray detection technology enabled the first profile measurements of the birth and growth dynamics of runaway electrons (REs) at the edge of tokamaks during quiescent RE studies at the Madison Symmetric Torus. The formation of an off-axis RE seed with linear growth rates has been resolved for low energies, a hollow streaming parameter and large electric fields ($E_{\parallel}/E_{D}$) in agreement with theory and simulations. Secondary exponential growth rates have also been spatially resolved for the first time and are consistent with a convective transport of the order of the Ware pinch and energies up to $10^3\times T_{e,0}$. Numerical simulations are shown to reproduce the experimental observations including the off-axis runaway electron generation, radial transport and exponential growth at the core, as well as suppression due to $m=3$ resonant magnetic perturbations.

physics.plasm-ph↗

arXiv2021

Regulation of the Normalized Rate of Driven Magnetic Reconnection through Shocked Flux Pileup

Magnetic reconnection is explored on the Terrestrial Reconnection Experiment (TREX) for asymmetric inflow conditions and in a configuration where the absolute rate of reconnection is set by an external drive. Magnetic pileup enhances the upstream magnetic field of the high density inflow, leading to an increased upstream Alfven speed and helping to lower the normalized reconnection rate to values expected from theoretical consideration. In addition, a shock interface between the far upstream supersonic plasma inflow and the region of magnetic flux pileup is observed, important to the overall force balance of the system, hereby demonstrating the role of shock formation for configurations including a supersonically driven inflow. Despite the specialised geometry where a strong reconnection drive is applied from only one side of the reconnection layer, previous numerical and theoretical results remain robust and are shown to accurately predict the normalized rate of reconnection for the range of system sizes considered. This experimental rate of reconnection is dependent on system size, reaching values as high as 0.8 at the smallest normalized system size applied.

physics.plasm-ph↗

arXiv2021

Ion heating and flow driven by an Instability found in Plasma Couette Flow

We present the first observation of instability in weakly magnetized, pressure dominated plasma Couette flow firmly in the Hall regime. Strong Hall currents couple to a low frequency electromagnetic mode that is driven by high-$β$ ($>1$) pressure profiles. Spectroscopic measurements show heating (factor of 3) of the cold, unmagnetized ions via a resonant Landau damping process. A linear theory of this instability is derived that predicts positive growth rates at finite $β$ and shows the stabilizing effect of very large $β$, in line with observations.

physics.plasm-ph↗

arXiv2020

Formation of Transient High-$β$ Plasmas in a Magnetized, Weakly Collisional Regime

We present experimental data providing evidence for the formation of transient ($\sim 20~μ$s) plasmas that are simultaneously weakly magnetized (i.e., Hall magnetization parameter $ωτ> 1$) and dominated by thermal pressure (i.e., ratio of thermal-to-magnetic pressure $β> 1$). Particle collisional mean free paths are an appreciable fraction of the overall system size. These plasmas are formed via the head-on merging of two plasmas launched by magnetized coaxial guns. The ratio $λ_{gun}=μ_0 I_{gun}/ψ_{gun}$ of gun current $I_{gun}$ to applied magnetic flux $ψ_{gun}$ is an experimental knob for exploring the parameter space of $β$ and $ωτ$. These experiments were conducted on the Big Red Ball at the Wisconsin Plasma Physics Laboratory. The transient formation of such plasmas can potentially open up new regimes for the laboratory study of weakly collisional, magnetized, high-$β$ plasma physics; processes relevant to astrophysical objects and phenomena; and novel magnetized plasma targets for magneto-inertial fusion.

physics.plasm-ph↗

arXiv2020

Weakly magnetized, Hall dominated plasma Couette flow

A novel plasma equilibrium in the high-$β$, Hall regime that produces centrally-peaked, high Mach number Couette flow is described. Flow is driven using a weak, uniform magnetic field and large, cross field currents. Large magnetic field amplification (factor 20) due to the Hall effect is observed when electrons are flowing radially inward, and near perfect field expulsion is observed when the flow is reversed. A dynamic equilibrium is reached between the amplified (removed) field and extended density gradients.

physics.plasm-ph↗

arXiv2019

[Plasma 2020 Decadal] The Material Properties of Weakly Collisional, High-Beta Plasmas

This white paper, submitted for the Plasma 2020 Decadal Survey, concerns the physics of weakly collisional, high-beta plasmas -- plasmas in which the thermal pressure dominates over the magnetic pressure and in which the inter-particle collision time is comparable to the characteristic timescales of bulk motions. This state of matter, although widespread in the Universe, remains poorly understood: we lack a predictive theory for how it responds to perturbations, how it transports momentum and energy, and how it generates and amplifies magnetic fields. Such topics are foundational to the scientific study of plasmas, and are of intrinsic interest to those who regard plasma physics as a fundamental physics discipline. But these topics are also of extrinsic interest: addressing them directly informs upon our understanding of a wide variety of space and astrophysical systems, including accretion flows around supermassive black holes, the intracluster medium (ICM) between galaxies in clusters, and regions of the near-Earth solar wind. Specific recommendations to advance this field of study are discussed.

physics.plasm-ph↗

arXiv2017

Laboratory evidence of dynamo amplification of magnetic fields in a turbulent plasma

Magnetic fields are ubiquitous in the Universe. Extragalactic disks, halos and clusters have consistently been shown, via diffuse radio-synchrotron emission and Faraday rotation measurements, to exhibit magnetic field strengths ranging from a few nG to tens of $μ$G. The energy density of these fields is typically comparable to the energy density of the fluid motions of the plasma in which they are embedded, making magnetic fields essential players in the dynamics of the luminous matter. The standard theoretical model for the origin of these strong magnetic fields is through the amplification of tiny seed fields via turbulent dynamo to the level consistent with current observations. Here we demonstrate, using laser-produced colliding plasma flows, that turbulence is indeed capable of rapidly amplifying seed fields to near equipartition with the turbulent fluid motions. These results support the notion that turbulent dynamo is a viable mechanism responsible for the observed present-day magnetization of the Universe.

physics.plasm-ph↗

arXiv2017

Numerical modeling of laser-driven experiments aiming to demonstrate magnetic field amplification via turbulent dynamo

The universe is permeated by magnetic fields, with strengths ranging from a femtogauss in the voids between the filaments of galaxy clusters to several teragauss in black holes and neutron stars. The standard model behind cosmological magnetic fields is the nonlinear amplification of seed fields via turbulent dynamo to the values observed. We have conceived experiments that aim to demonstrate and study the turbulent dynamo mechanism in the laboratory. Here we describe the design of these experiments through simulation campaigns using FLASH, a highly capable radiation magnetohydrodynamics code that we have developed, and large-scale three-dimensional simulations on the Mira supercomputer at Argonne National Laboratory. The simulation results indicate that the experimental platform may be capable of reaching a turbulent plasma state and study dynamo amplification. We validate and compare our numerical results with a small subset of experimental data using synthetic diagnostics.

physics.plasm-ph↗

arXiv2015

The Wisconsin Plasma Astrophysics Laboratory

The Wisconsin Plasma Astrophysics Laboratory (WiPAL) is a flexible user facility designed to study a range of astrophysically relevant plasma processes as well as novel geometries that mimic astrophysical systems. A multi-cusp magnetic bucket constructed from strong samarium cobalt permanent magnets now confines a 10 m$^3$, fully ionized, magnetic-field free plasma in a spherical geometry. Plasma parameters of $ T_{e}\approx5$ to $20$ eV and $n_{e}\approx10^{11}$ to $5\times10^{12}$ cm$^{-3}$ provide an ideal testbed for a range of astrophysical experiments including self-exciting dynamos, collisionless magnetic reconnection, jet stability, stellar winds, and more. This article describes the capabilities of WiPAL along with several experiments, in both operating and planning stages, that illustrate the range of possibilities for future users.

physics.plasm-ph↗

arXiv2015

Prospects for observing the magnetorotational instability in the Plasma Couette Experiment

Many astrophysical disks, such as protoplanetary disks, are in a regime where non-ideal, plasma-specific magnetohydrodynamic (MHD) effects can significantly influence the behavior of the magnetorotational instability (MRI). The possibility of studying these effects in the Plasma Couette Experiment (PCX) is discussed. An incompressible, dissipative global stability analysis is developed to include plasma-specific two-fluid effects and neutral collisions, which are inherently absent in analyses of Taylor-Couette flows (TCFs) in liquid metal experiments. It is shown that with boundary driven flows, a ion-neutral collision drag body force significantly affects the azimuthal velocity profile, thus limiting the flows to regime where the MRI is not present. Electrically driven flow (EDF) is proposed as an alternative body force flow drive in which the MRI can destabilize at more easily achievable plasma parameters. Scenarios for reaching MRI relevant parameter space and necessary hardware upgrades are described.

physics.plasm-ph↗

arXiv2014

Taylor-Couette Flow of Unmagnetized Plasma

Differentially rotating flows of unmagnetized, highly conducting plasmas have been created in the Plasma Couette Experiment. Previously, hot-cathodes have been used to control plasma rotation by a stirring technique [C. Collins et al., Phys. Rev. Lett. 108, 115001(2012)] on the outer cylindrical boundary---these plasmas were nearly rigid rotors, modified only by the presence of a neutral particle drag. Experiments have now been extended to include stirring from an inner boundary, allowing for generalized circular Couette flow and opening a path for both hydrodynamic and magnetohydrodynamic experiments, as well as fundamental studies of plasma viscosity. Plasma is confined in a cylindrical, axisymmetric, multicusp magnetic field, with $T_e< 10$ eV, $T_i<1$ eV, and $n_e<10^{11}$ cm$^{-3}$. Azimuthal flows (up to 12 km/s, $M=V/c_s\sim 0.7$) are driven by edge ${\bf J \times B}$ torques in helium, neon, argon, and xenon plasmas, and the experiment has already achieved $Rm\sim 65$ and $Pm\sim 0.2 - 12$. We present measurements of a self-consistent, rotation-induced, species-dependent radial electric field, which acts together with pressure gradient to provide the centripetal acceleration for the ions. The maximum flow speeds scale with the Alfvén critical ionization velocity, which occurs in partially ionized plasma. A hydrodynamic stability analysis in the context of the experimental geometry and achievable parameters is also explored.

physics.plasm-ph↗

arXiv2014

The Madison plasma dynamo experiment: a facility for studying laboratory plasma astrophysics

The Madison plasma dynamo experiment (MPDX) is a novel, versatile, basic plasma research device designed to investigate flow driven magnetohydrodynamic (MHD) instabilities and other high-$β$ phenomena with astrophysically relevant parameters. A 3 m diameter vacuum vessel is lined with 36 rings of alternately oriented 4000 G samarium cobalt magnets which create an axisymmetric multicusp that contains $\sim$14 m$^{3}$ of nearly magnetic field free plasma that is well confined and highly ionized $(>50\%)$. At present, 8 lanthanum hexaboride (LaB$_6$) cathodes and 10 molybdenum anodes are inserted into the vessel and biased up to 500 V, drawing 40 A each cathode, ionizing a low pressure Ar or He fill gas and heating it. Up to 100 kW of electron cyclotron heating (ECH) power is planned for additional electron heating. The LaB$_6$ cathodes are positioned in the magnetized edge to drive toroidal rotation through ${\bf J}\times{\bf B}$ torques that propagate into the unmagnetized core plasma. Dynamo studies on MPDX require a high magnetic Reynolds number $Rm > 1000$, and an adjustable fluid Reynolds number $10< Re <1000$, in the regime where the kinetic energy of the flow exceeds the magnetic energy ($M_A^2=($v$/$v$_A)^2 > 1$). Initial results from MPDX are presented along with a 0-dimensional power and particle balance model to predict the viscosity and resistivity to achieve dynamo action.

physics.plasm-ph↗

arXiv2013

Fast dynamos in spherical boundary-driven flows

We numerically demonstrate the feasibility of kinematic fast dynamos for a class of time-periodic axisymmetric flows of conducting fluid confined inside a sphere. The novelty of our work is in considering the realistic flows, which are self-consistently determined from the Navier-Stokes equation with specified boundary driving. Such flows can be achieved in a new plasma experiment, whose spherical boundary is capable of differential driving of plasma flows in the azimuthal direction. We show that magnetic fields are self-excited over a range of flow parameters such as amplitude and frequency of flow oscillations, fluid Reynolds (Re) and magnetic Reynolds (Rm) numbers. In the limit of large Rm, the growth rates of the excited magnetic fields are of the order of the advective time scales and practically independent of Rm, which is an indication of the fast dynamo.

physics.plasm-ph↗

arXiv2012

Optimized boundary driven flows for dynamos in a sphere

We perform numerical optimization of the axisymmetric flows in a sphere to minimize the critical magnetic Reynolds number Rm_cr required for dynamo onset. The optimization is done for the class of laminar incompressible flows of von Karman type satisfying the steady-state Navier-Stokes equation. Such flows are determined by equatorially antisymmetric profiles of driving azimuthal (toroidal) velocity specified at the spherical boundary. The model is relevant to the Madison plasma dynamo experiment (MPDX), whose spherical boundary is capable of differential driving of plasma in the azimuthal direction. We show that the dynamo onset in this system depends strongly on details of the driving velocity profile and the fluid Reynolds number Re. It is found that the overall lowest Rm_cr~200 is achieved at Re~240 for the flow, which is hydrodynamically marginally stable. We also show that the optimized flows can sustain dynamos only in the range Rm_cr<Rm<Rm_cr2, where Rm_cr2 is the second critical magnetic Reynolds number, above which the dynamo is quenched. Samples of the optimized flows and the corresponding dynamo fields are presented.

physics.plasm-ph↗