arXiv subjects

K. J. McCollam

Publications and source records attributed to K. J. McCollam.

5 recordsLinked to original sources

arXiv2025

Three-dimensional unmagnetized Mach probe analysis and initial flow measurements in reversed-field pinch experiments

A novel matrix method of analyzing ion saturation current data from a general three-dimensional (3D) array of unmagnetized Mach probe tips is developed and used with data sets from two 3D Mach probes to make initial measurements of local plasma flow velocity in reversed-field pinch (RFP) experiments in the Madison Symmetric Torus (MST). The two 3D Mach probes are composed of regular polyhedral arrays of six and four tips, respectively, with the six-tip array composed of three orthogonal pairs of mutually opposite tips at the vertices of a regular octahedron and the four-tip array composed of non-opposite tips at the vertices of a regular tetrahedron, the analysis of which is specifically facilitated by the matrix method. Velocity measurement uncertainties for the Mach probes are derived based on uncertainties in probe machining and ion saturation current measurements, and typical relative uncertainties for the probes are estimated to be of order several percent, likely smaller than systematic uncertainties related to the Mach probe calibration constant and experimental uncertainties related to plasma and probe conditioning. Initial results for the octahedral probe show flow speeds of roughly the expected magnitudes based on previous MST measurements but with somes differences in flow direction, while those for the tetrahedron probe show similar flow directions to some previous measurements but also some larger than expected speeds. We consider possible causes for the unexpected results of these initial tests, with a focus on probe conditioning and fast electron issues.

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↗

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

Simultaneous feedback control of toroidal magnetic field and plasma current on MST using advanced programmable power supplies

Programmable control of the inductive electric field enables advanced operations of reversed-field pinch (RFP) plasmas in the Madison Symmetric Torus (MST) device and further develops the technical basis for ohmically heated fusion RFP plasmas. MST's poloidal and toroidal magnetic fields ($B_\text{p}$ and $B_\text{t}$) can be sourced by programmable power supplies (PPSs) based on integrated-gate bipolar transistors (IGBT). In order to provide real-time simultaneous control of both $B_\text{p}$ and $B_\text{t}$ circuits, a time-independent integrated model is developed. The actuators considered for the control are the $B_\text{p}$ and $B_\text{t}$ primary currents produced by the PPSs. The control system goal will be tracking two particular demand quantities that can be measured at the plasma surface ($r=a$): the plasma current, $I_\text{p} \sim B_\text{p}(a)$, and the RFP reversal parameter, $F\sim B_\text{t}(a)/Φ$, where $Φ$ is the toroidal flux in the plasma. The edge safety factor, $q(a)\propto B_t(a)$, tends to track $F$ but not identically. To understand the responses of $I_\text{p}$ and $F$ to the actuators and to enable systematic design of control algorithms, dedicated experiments are run in which the actuators are modulated, and a linearized dynamic data-driven model is generated using a system identification method. We perform a series of initial real-time experiments to test the designed feedback controllers and validate the derived model predictions. The feedback controllers show systematic improvements over simpler feedforward controllers.

physics.plasm-ph↗