arXiv subjects

Thomas C. Underwood

Publications and source records attributed to Thomas C. Underwood.

9 recordsLinked to original sources

arXiv2026

Microwave Resonant Discharges for Spatiotemporally Selective Plasma Breakdown Near Surfaces

Generating non-equilibrium plasmas close to surfaces remains a significant challenge for conventional plasma sources. Existing plasma generation schemes create volumetric discharges with limited spatial selectivity that lead to inefficient energy deposition and poor coupling between reactive intermediates and nearby surfaces. This work establishes tailored resonant field enhancement as a mechanism for prescribing where plasma forms near dielectric surfaces through microwave excitation. In this approach, the geometry, refractive index, and packing configuration of dielectric materials define resonant field structures that interfere constructively and amplify electric fields locally. Plasma forms only within these resonant volumes where the amplified fields exceed the local breakdown threshold, while the surrounding gas remains below breakdown. Microwave pulse shaping then provides dynamic control over these modes and can be used to excite different families of resonances, determine where breakdown occurs, and reconfigure what locations microplasmas occupy from one pulse to the next. We validate this framework through theory, electromagnetic simulations, and experiments using a pair of high-permittivity dielectric resonators. These studies identify multiple resonant mode families, demonstrate dynamic repositioning of microplasmas between prescribed breakdown sites, quantify the ignition characteristics of each mode, and confirm that multiple resonant microplasmas remain confined to localized field-enhancement regions during a microwave pulse. Together, these results establish a framework for designing resonant dielectric materials that localize, reconfigure, and control atmospheric-pressure plasmas near surfaces.

physics.plasm-ph↗

arXiv2026

Electromagnetically Driven Thermal Dissipation Scaling in Plasma Centrifuges for Mass Separation

Electromagnetically driven centrifuges (EMDCs) rotate fluids using the Lorentz force, but their separative performance is limited by thermal dissipation that is coupled to electromagnetic forcing. This follows because local centrifugal strength is characterized by λ=mV_θ^2/2k_B T, which compares directed kinetic energy that drives species separation to thermal energy that smooths concentration gradients and counteracts separation. In this work, we develop a two-temperature magnetohydrodynamic model to determine how radial geometry, current density, and magnetic field strength control the coupled evolution of rotation and heating that dictates λ. The model is benchmarked against Ar velocity, temperature, and pressure measurements spanning current density up to 15 kA/m2, magnetic field up to 0.57 T, feed pressures of 0.5-3 Torr, and annulus sizes of 1-5 cm. The results show that volumetric Lorentz forcing sustains elevated λ, and therefore greater local compositional shifts throughout a larger radial portion of the fluid volume than wall-bounded shear centrifuges, despite producing a lower peak λ. Simulations of a 40Ar/36Ar isotopic mixture demonstrate that, when electromagnetic force and geometry are jointly optimized, EMDCs can approach or match the separative performance of shear-driven centrifuges operating near material speed limits while requiring lower area-averaged values of λ, and can sustain greater radial compositional shifts than the SDC reference over much of the annulus. These results challenge assertions that viscous dissipation constrains weakly ionized plasma centrifuges to λ<1, and indicate that enhanced radial mass separation can be achieved by broadening the radial region over which λ remains elevated rather than maximizing its peak or area-averaged value.

physics.plasm-ph↗

arXiv2026

Pulse Shaping Increases Efficiency in Pulsed Plasma Accelerators

The performance of a gas-fed pulsed electromagnetic thruster is governed by the ability to deposit electrical energy while propellant is available for acceleration. Current pulsed-power systems require tradeoffs between high-current discharges that produce high exhaust velocities and longer pulses that overlap the energy deposition with more of the gas injection. This limited control restricts the specific impulse and mass utilization of these thrusters. This work introduces programmable pulse shaping as a method to increase control over energy deposition and expand the accessible operating space. We use solid-state integrated power modules to vary the discharge delay, pulse width, peak current, and the shape of the current waveform as propellant is injected. Experiments varying pulse widths from 30 to 500 $μ$s and peak currents from 3 kA to 16 kA show that short, high-current pulses produce higher exhaust velocities and greater impulse bits than longer, lower-current pulses at comparable discharge energy. The switches also enable multiple discharges of arbitrary positioning and duration during a single gas injection. This micro-burst operation is shown to increase specific impulse in air by 278\% from 840 to 3177 s through improved propellant utilization. This same pulse shaping ability is found to increase thrust efficiency from 0.2\% in single-shot operation to 3.3\% in micro-burst operation while operating with air.

physics.plasm-ph↗

arXiv2026

Single-shot spectral-encoded waveform reconstruction through probabilistic inversion

Spectral encoding enables single-shot measurements of ultrafast transients by mapping temporal information onto the spectrum of a chirped probe. This encoding allows dynamics to be recorded that are beyond the response limits of conventional electronic detectors. However, because the measurements record only spectral intensity, the phase of encoded signals is lost, and dispersion in the detection process introduces waveform distortions that complicate reconstruction and quantitative interpretation of spectra. In single-shot terahertz time-domain spectroscopy (THz-TDS), these distortions manifest as a tradeoff between temporal resolution and the measurement window of signals and can produce spectral null frequencies that limit the recoverable THz bandwidth. To address this challenge, a Bayesian inversion framework is developed to recover the underlying waveform from the squared spectral observable by inferring the THz field, the modulation coefficient, and a low-dimensional empirical parameterization of the probe spectrum jointly, while a Gaussian process prior regularizes the waveform. The framework is validated using single-shot THz-TDS experiments spanning two probe spectral profiles and three chirp conditions with $α$ ranging from 14.5 to 40 ps$^{-2}$. Across all cases, the inversion reconstructs both the time-domain waveform and spectral null frequency structure within the credible interval of a delay-line reference measurement. These results establish a pathway to eliminate penalties that are associated with the detection process in spectral encoding methods without adding additional optics or alignment complexity.

physics.optics↗

arXiv2025

Magnetohydrodynamic Operating Regimes of Pulsed Plasma Accelerators for Efficient Propellant Utilization

The presence of magnetohydrodynamic (MHD) acceleration modes in pulsed plasma thrusters has been verified using the magnetic extension of Rankine-Hugoniot theory. However, the impact of initial conditions within the accelerator volume on the formation and structure of these modes remains poorly understood. This work develops a regime map to clarify how key initial conditions - such as propellant gas dynamics, pulse energy, and the timing between propellant injection and discharge initiation - govern transitions between two distinct MHD operating modes, a magneto-detonation and magneto-deflagration, along with an unstable transition regime that connects them. To characterize these regimes, a combination of time-of-flight and thrust stand diagnostics was used to assess their properties, scalability, and structure while operating with air. Time-of-flight measurements reveal that reducing the initial downstream propellant mass ($m_{dwn}$) of air from 120 $μ$g to 60 $μ$g shifts the thruster from the magneto-detonation to the magneto-deflagration regime, increasing exhaust velocity ($v_{ex}$) from 20 km/s to 55 km/s. In this regime, the thruster exhibits improved propellant utilization as less mass is injected. At a constant 8 kA of peak current, specific impulse (Isp) increases from ~100-2000 s as $m_{dwn}$ decreases from 70 to 10 $μ$g, corresponding to an increase in utilization efficiency ($η$util) from 5% to 35%. Thrust-to-power ratios, measured using a thrust stand, also improve with peak current in the magneto-deflagration regime, increasing from 4.5 mN/kW to 8 mN/kW and 6.7 mN/kW for injected mass bits of 25 $μ$g and 50 $μ$g, respectively. This work provides critical insights into how the initial conditions in pulsed plasma thrusters dictate the formation of ionization waves, structure of plumes, and the performance of thrusters.

physics.plasm-ph↗

arXiv2025

Quantitative Single-Shot Supercontinuum-Enhanced Terahertz Spectroscopy (SETS)

Single-shot terahertz (THz) spectroscopy probes sub-picosecond, non-repetitive events by combining the advantages of laser absorption techniques with the phase-sensitive detection of interferometric methods. However, its usage as a quantitative tool is hindered by the chirp penalty of the spectral encoding scheme, where a narrow probe bandwidth distorts the THz signal, limiting the bandwidth and spectral resolution of THz measurements. In this work, we introduce Supercontinuum-Enhanced Terahertz Spectroscopy (SETS), a method that leverages a broadband supercontinuum probe to overcome these challenges. With SETS, we show an increase in usable THz bandwidth from 1.5 THz to 2.3 THz, reducing signal distortion by 50% and offering a scalable pathway to extend bandwidth further. Numerical models of spectral encoding highlight the flexibility of SETS, achieving high spectral resolution (< 4 GHz with a 250 ps supercontinuum pulse) while maintaining a usable bandwidth > 1 THz for amplitude and phase spectra. Experiments on argon plasma and water vapor, complemented by theoretical validation, generalization, and extrapolation, show the capability of SETS to measure electron density, collision frequency, and absorption spectra simultaneously with tailored measurement accuracy and resolution (as low as 5x10^15 m^-3 for electron density). By addressing a critical limitation in single-shot diagnostics, SETS enables high-resolution, non-intrusive, quantitative measurements for complex reactive flows and dynamic refractive media.

physics.optics↗

arXiv2022

The Soft Compiler: A Web-Based Tool for the Design of Modular Pneumatic Circuits for Soft Robots

Developing soft circuits from individual soft logic gates poses a unique challenge: with increasing numbers of logic gates, the design and implementation of circuits leads to inefficiencies due to mathematically unoptimized circuits and wiring mistakes during assembly. It is therefore practically important to introduce design tools that support the development of soft circuits. We developed a web-based graphical user interface, the Soft Compiler, that accepts a user-defined robot behavior as a truth table to generate a mathematically optimized circuit diagram that guides the assembly of a soft fluidic circuit. We describe the design and experimental verification of three soft circuits of increasing complexity, using the Soft Compiler as a design tool and a novel pneumatic glove as an input interface. In one example, we reduce the size of a soft circuit from the original 11 logic gates to 4 logic gates while maintaining circuit functionality. The Soft Compiler is a web-based design tool for fluidic, soft circuits and published under open-source MIT License.

cs.RO↗

arXiv2018

Effects of Flow Collisionality on ELM Replication in Plasma Guns

Degradation of first wall materials due to plasma disturbances severely limit both the lifetime and longevity of fusion reactors. Among the various kinds of disturbances, type I edge localized modes (ELMs) in particular present significant design challenges due to their expected heat loading and relative frequency in next step fusion reactors. Plasma gun devices have been used extensively to replicate ELM conditions in the laboratory, however feature higher density, lower temperatures, and thus higher flow collisionality than those expected in fusion conditions. This work presents experimental visualizations that indicate strong shocks form in gun devices over spatial and temporal scales that precede ablation dynamics. These measurements are used to validate detailed magnetohydrodynamic simulations that capture the production of plasma jets and the shielding effect collisionality plays in particle transport to material surfaces. Simulations show that self-shielding effects in plasma guns reduce the free streaming heat flux by up to 90% and further reduce the incoming particle kinetic energy impinging on material surfaces. These simulations are performed over a range of operating conditions for gun devices and a discussion is provided regarding how existing experimental measurements can be interpreted when extrapolating to fusion conditions.

physics.plasm-ph↗

arXiv2018

Gaseous Plasmonic Resonators for Metamaterial Applications

We examine the properties of a gaseous plasma resonator generated by focusing a high-energy laser pulse through a lens and into a gas. An analytical model is presented describing the scattering resonance of these near-ellipsoidal plasmas and its dependence on their eccentricity and intrinsic plasma properties. This dependence is investigated through Ku band transmission experiments of a waveguide with an embedded single plasma element and through optical diagnostics of the laser-induced plasma. The described resonator has the potential to be used as the building block in a new class of metamaterials with fully three-dimensional structural flexibility.

physics.app-ph↗