Collapse of cycloidal electron flows induced by misalignments in a magnetically insulated diode

The effect of a slight misalignment in the magnetic field on a magnetically insulated diode is investigated. It is found that a slight tilt in the magnetic field, with a minute component along the dc electric field, completely destabilizes the cycloidal electron flow in the crossed-field gap. The final state consists of the classical Brillouin flow superimposed by a turbulent background, together with a slow electron drift across the gap. This disruption of the cycloidal flow is quite insensitive to the emission current density, and is due to the accumulation of space charge in the gap caused by the magnetic misalignment. This result was obtained from a one-dimensional simulation code. It reinforces the notion that the turbulent, near Brillouin-like states are generic in ALL vacuum crossed-field devices. © 1998 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69778/2/PHPAEN-5-6-2447-1.pd

Incorporating collisions and resistance into the transition from field emission to the space charge regime

Advancements in microelectromechanical systems (MEMS) and microplasmas, particularly with respect to applications in combustion and biotechnology, motivate studies into microscale gas breakdown to enable safe system design and implementation. Breakdown at microscale deviates from that predicted by Paschen’s law due to field emission—the stripping of electrons from the cathode in the presence of strong surface field—and follows the Fowler-Nordheim (FN) law. As injected current increases at this length scale, electrons accumulate in the gap and FN electron emission becomes space charge limited, leading to the Child-Langmuir (CL) law at vacuum and the Mott-Gurney (MG) law at high pressure. While theoretical studies link CL to FN and CL to MG, none links all three to simultaneously assess the importance of pressure and external resistance (perturbation) on electron emission. This study extends existing theory to elucidate the transition between these regimes as a function of applied voltage, gap distance, electron mobility, and external resistance, and in particular, derives asymptotic equations illustrating the transitions between the three. It also demonstrates the presence of a triple point, where one theoretically encounters FN, CL, and MG at once, and characterizes the importance of gap pressure and distance on these regimes, especially when MG dominates at non-vacuum pressures. The sensitivity of the triple point to external resistance, representative of the effects of perturbations in system parameters on electron emission, receives special attention

Optimizing Neutron Yield for Active Interrogation

Neutrons are commonly used for many applications, including active interrogation and cancer therapy. One critical aspect for active interrogation efficiency is neutron yield, which is more important for successful resolution than the energy spectrum. The typical approach for improving neutron yield entails producing more neutrons, which has motivated multiple studies using the interaction of increasingly more powerful tabletop lasers with plastic targets to generate protons or deuterons that are absorbed by another target to create neutrons [1]. Alternatively, one may use lenses to focus the neutrons to increase yield rather than simply generating more neutrons with more powerful lasers [2]. Assessing either approach requires a comprehensive model simulating neutron generation and transport to optimize the target material, system geometry, and neutron yield. A complete model from laser source to neutron generation is beyond the scope of the current study, so this project focuses on simulating the interaction of deuterons with typical target materials, such as lithium or beryllium. We use the neutron transport code Monte Carlo N-Particles (MCNP), which applies the Monte Carlo method to track particles [3]. The simulations accurately reflected experimental results from several groups [4]. Future analyses will assess improvements in neutron yield and directionality through strategically incorporating neutron lenses

Permutation combinatorics of worldsheet moduli space

52 pages, 21 figures52 pages, 21 figures; minor corrections, "On the" dropped from title, matches published version52 pages, 21 figures; minor corrections, "On the" dropped from title, matches published versio

Resistive destabilization of cycloidal electron flow and universality of (near‐) Brillouin flow in a crossed‐field gap

It is shown that a small amount of dissipation, caused by current flow in a lossy external circuit, can produce a disruption of steady‐state cycloidal electron flow in a crossed‐field gap, leading to the establishment of a turbulent steady state that is close to, but not exactly, Brillouin flow. This disruption, which has nothing to do with a diocotron or cyclotron instability, is fundamentally caused by the failure of a subset of the emitted electrons to return to the cathode surface as a result of resistive dissipation. This mechanism was revealed in particle simulations, and was confirmed by an analytic theory. These near‐Brillouin states differ in several interesting respects from classic Brillouin flow, the most important of which is the presence of a microsheath and a time‐varying potential minimum very close to the cathode surface. They are essentially identical to that produced when (i) injected current exceeds a certain critical value [P. J. Christenson and Y. Y. Lau, Phys. Plasmas 1, 3725 (1994)] or (ii) a small rf electric field is applied to the gap [P. J. Christenson and Y. Y. Lau, Phys. Rev. Lett. 76, 3324 (1996)]. It is speculated that such near‐Brillouin states are generic in vacuum crossed‐field devices, due to the ease with which the cycloidal equilibrium can be disrupted. Another novel aspect of this paper is the introduction of transformations by which the nonlinear, coupled partial differential equations in the Eulerian description (equation of motion, continuity equation, Poisson equation, and the circuit equation) are reduced to an equivalent system of very simple linear ordinary differential equations. © 1996 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71350/2/PHPAEN-3-12-4455-1.pd

Optical Emission Spectroscopy Diagnostics of Cold Plasmas for Food Sterilization

There is a growing need for economical, effective, and safe methods of sterilizing fresh produce. The most common method is a chlorine wash, which is expensive and may introduce carcinogens. High voltage cold atmospheric pressure plasmas are a promising solution that has demonstrated a germicidal effect; however, the responsible chemical mechanisms and reaction pathways are not fully understood. To elucidate this chemistry, we used optical emission spectroscopy to measure the species produced in the plasma generated by a 60 Hz pulsed dielectric barrier discharge in a plastic box containing various fill gases (He, N2, CO2, dry air, or humid air). In addition to estimating chemical species concentrations, we performed preliminary calculations of electronic, vibrational, rotational, and translational temperatures

Cold Atmospheric Pressure Plasmas for Food Applications

Successfully distributing shelf food requires treatment to eliminate microorganisms. Current chemical methods, such as chlorine wash, can alter food quality while only being effective for a limited time. Cold atmospheric pressure plasmas (CAPs) can eradicate the microorganisms responsible for food spoilage and foodborne illness. Optimizing CAP treatments requires understanding the reactive species generated and relating them to eradication efficiency. Recent studies have used optical emission spectroscopy (OES) to determine the species generated in a sealed package that would hold food. In this study,we supplement the OES results with optical absorption spectroscopy (OAS) using the same gases (helium, nitrogen, compressed air, humid air) to elucidate plasma chemistry and temperature. We first reproduce previous results using a new setup while assessing the impact of the package and surrounding box on the plasma spectrum. A UV-Vis light lightsource is emitted through a series of lenses placed next to the plasma. Analysis using SpecAir software allows the identification of absorbed peaks and the calculation of rotational, vibrational, and electron temperatures. Results show that the air plasma produces a primary absorbance peak at a wavelength of ~260 nm, demonstrating the diagnostic capability of this technique . Species generation declined dramatically during the first two minutes of treatment with the effect leveling off thereafter. These findings elucidate reactive species generation within the plasma to optimize CAP systems for microorganism decontamination

Space-charge-limited current density for nonplanar diodes with monoenergetic emission using Lie-point symmetries

Understanding space-charge limited current density (SCLCD) is fundamentally and practically important for characterizing many high-power and high-current vacuum devices. Despite this, no analytic equations for SCLCD with nonzero monoenergetic initial velocity have been derived for nonplanar diodes from first principles. Obtaining analytic equations for SCLCD for nonplanar geometries is often complicated by the nonlinearity of the problem and over constrained boundary conditions. In this letter, we use the canonical coordinates obtained by identifying Lie-point symmetries to linearize the governing differential equations to derive SCLCD for any orthogonal diode. Using this method, we derive exact analytic equations for SCLCD with a monoenergetic injection velocity for one-dimensional cylindrical, spherical, tip-to-tip (t-t), and tip-to-plate (t-p) diodes. We specifically demonstrate that the correction factor from zero initial velocity to monoenergetic emission depends only on the initial kinetic and electric potential energies and not on the diode geometry and that SCLCD is universal when plotted as a function of the canonical gap size. We also show that SCLCD for a t-p diode is a factor of four larger than a t-t diode independent of injection velocity. The results reduce to previously derived results for zero initial velocity using variational calculus and conformal mapping.Comment: 18 pages, 3 figure

Electron Trajectories and Critical Current in a Two-Dimensional Planar Magnetically Insulated Crossed-Field Gap

The critical current in a one-dimensional (1D) crossed-field gap is defined by the transition from a cycloidal flow to a near-Brillouin (nB) state characterized by electron flow orthogonal to both the electric and magnetic fields and uniform virtual cathode formation. Motivated by recent studies on space-charge-limited current in non-magnetic diodes, we assess the meaning of critical current in a magentically insulated two-dimensional (2D) planar crossed-field geometry. Particle-in-cell (PIC) simulations demonstrate that binary behavior between a laminar and turbulent state does not occur in 2D because the virtual cathode is nonuniform. Rather than a distinct nB state above the critical current as in 1D, there is an increase in Brillouin contribution with the presence of cycloidal components and noise even at low currents. To evaluate the electron flows in a 2D crossed-field gap in the absence of a binary transition, we developed two metrics to assess the Brillouin and cycloidal components in a 2D planar crossed-field gap for various emission widths and injection current densities by comparing the phase space plots from PIC simulations to analytical solutions for cycloidal and Brillouin flow. For a smaller emission width, less Brillouin contribution occurs for a given injection current, while maximizing the cycloidal noise requires a larger injection current. Once the virtual cathode starts to form and expand with increasing injection current, the cycloidal noise reaches its peak and then decreases while the Brillouin components become significant and increase

Incorporating spatial dependence into a multicellular tumor spheroid growth model

Recent models for organism and tumor growth yield simple scaling laws based on conservation of energy. Here, we extend such a model to include spatial dependence to model necrotic core formation. We adopt the allometric equation for tumor volume with a reaction-diffusion equation for nutrient concentration. In addition, we assume that the total metabolic energy and average cellular metabolic rate depend on nutrient concentration in a Michaelis-Menten-like manner. From experimental results, we relate the necrotic volume to nutrient consumption and estimate both the time and nutrient concentration at necrotic core formation. Based on experimental results, we demand that the necrotic core radius varies linearly with tumor radius after core formation and extend the equations for tumor volume and nutrient concentration to the postnecrotic core regime. In particular, we obtain excellent agreement with experimental data and the final steady-state viable rim thickness.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87333/2/124701_1.pd