Robert A. Schill

General relation for the vector magnetic field of a circular current loop: a closer look

This paper presents a general theory for the fields generated by a circular current loop and compares it with existing theories. The existing, general, closed solution for the vector magnetic field may be expressed in a number of seemingly different but equivalent forms. These relations offer alternative closed-form solutions that may find various applications, including the characterization of Helmholtz coils. The paper provides alternative closed forms in both spherical and cylindrical coordinate systems. It employs Gausss magnetic law to show that the alternative closed form is self-consistent and correct and also shows agreement with well-known solutions. Finally, it develops a new (or not readily found) tabulated mathematical identity.

Characterizing and calibrating a large Helmholtz coil at low ac magnetic field levels with peak magnitudes below the earth’s magnetic field

Characterizing and calibrating a low impedance large Helmholtz coil generating 60 Hz magnetic fields with amplitudes well below the earth’s magnetic field is difficult and imprecise when coil shielding is not available and noise is an issue. Parameters influencing the calibration process such as temperature and coil impedance need to be figured in the calibration process. A simple and reliable calibration technique is developed and used to measure low amplitude fields over a spatial grid using a standard Hall effect probe gaussmeter. These low amplitude fields are typically hard or impossible to detect in the presence of background fields when using the gaussmeter in the conventional manner. Standard deviations of two milligauss and less have been achieved over a spatial grid in a uniform field region. Theoretical and measured fields are compared yielding reasonable agreement for a large coil system designed and built for bioelectromagnetic experiments at the University of Nevada at Las Vegas using simple...

A simplistic plasma dust removal model employing radiation pressure

A simple heuristic model is developed to examine the feasibility of using radiation pressure as a means to transport plasma dust out of the path of the forthcoming electron or photon beam. A slow electromagnetic surface wave coupled to a planar target or substrate exerts the required pressure in the removal process. The model is examined using data and parameters from single-shot radiography experiments. Optimal source requirements are identified for a typical radiography experiment. Source energies and powers are a minimum over an optimum band of frequencies where both conduction and plasma oscillation effects are mutually significant. Above the band of frequencies, dissipative losses in the surface supporting the surface wave increases exponentially with frequency. Below the optimal band, the energy concentration over the plume at the surface structure decreases significantly with frequency, thereby requiring higher source energies/powers for plasma removal.

Spatial Distribution of Electron Stimulated Electron Desorption from a Metal Surface

Electron stimulated electron desorption yielding secondary electron emission is to be examined both experimentally and computationally. A novel secondary electron emission test stand at UNLV housing a particle position detector maps the evolution of the spatial distribution of electron beam stimulated secondary electrons emitted from a metal target. Electrons emitted from the target in the drift free region with initial momentum within an approximate 3deg<thetas<40deg conical angle of acceptance relative to the primary beam axis are collected and recorded. This allows for transient surface conditioning and degrading studies on a single pulse basis and a multi-pulse basis over time. In this preliminary study, an approximate analysis of the secondary electron emission test stand is provided with special considerations given to the detector assembly (e.g., grid, micro-channel plate, and particle position detector) and data acquisition electronics. Spatial distribution tendencies and total secondary electron emission counts are provided for a single target under test. The MAGIC modeling tool is employed to study the secondary electron emission process. Comparisons will be made with experiment.

The two-dimensional electrostatic field configuration for parallel plate dust particle guide including end effects

Abstract Microdiameter dust particles in a viscous air medium are to be transported by a bi-planar arrangement of flat parallel plate electrodes. Sequential shifting of the static potentials on the electrodes is employed to guide the particles. The two dimensional configuration of these quasi-electrostatic fields in the semi-infinite guide composed of the parallel plate electrodes is theoretically and numerically examined. Loading effects of a two dimensional external rectangular cavity are incorporated in the theory. Numerical error resulting from the slow convergence properties of the series representing the fields near the boundaries of the guide is corrected. Field plots are presented and discussed in a qualitative manner for particle transport. The analytical fields are also compared with the fields obtained from employing the finite element method. Good agreement is shown.

Nonlinear Theory Modeling Electron Beam Constriction in a Pulsed Power Discharge

A simple nonlinear theory is developed to examine the process of electron channeling and constriction for an energetic electron beam passing through a cool thermalized plasma discharge. A number of different perturbations characterize and categorize the physical process of the three-fluid system. In this model, the electron species is separated into two different species of the same type but with very different initial particle states. A normalized set of curves parameterize the conditions most probable leading to electron channeling and pinch (beam constriction) in a pulsed power glow discharge. The theory suggests that energetic secondary electron emission at the cathode is responsible for glow discharge constriction in a pulsed power discharge.

Electron stimulated secondary electron emission from a warm metal surface

Heat removal in metal structures is often the limiting factor in high power, high frequency microwave tube devices. As higher critical heat flux cooling techniques are found, the operating power of the microwave device may be increased if other limiting factors are not of importance. Larger output powers may be realized for brief periods in time at the expense of operating in a pulsed power and/or pulsed cooling mode. During these brief periods, temporary temperature fluctuations on the walls of the microwave tube will be realized. It is hypothesized that the temperature fluctuations and to a much lesser extent temperature gradients may influence secondary electron emission resulting from primary electrons colliding with the microwave wall. Low energy primary electrons, that penetrate the microwave wall, experience the collective effects of the electrons (plasmons) in the conduction band of the metal. It is anticipated that the surface and bulk temperature of the metal wall influences the ability for plasmons to interact with the primary electron in the scattering process. Further, aging effects resulting from heat stress cycling of the metal cavity may alter the state of the cavity walls leading to changes in the secondary electron emission properties. Pulsed, low energy (1 keV), low beam currents are used to examine collectively the change in the spatial distribution profile of secondary electrons as the sample cools without significantly modifying or stressing the surface of the sample under test. SEE distribution profiles will be presented for a metal target at room temperature and in transient cooling mode with peak temperatures on the order of 390° K.

Electromagnetic dot sensor - calibration

The recently patented UNLV Electromagnetic (EM) dot sensor measures the rate of change of the electric flux density and the magnetic flux density at the same point in space simultaneously over time. This single device performs the function of two to four sensors distributed in space. The calibration setup of the EM-dot is presented ultimately leading to a pair of calibration factors.

Secondary Electron Emission from Niobium

Summary form only given. A novel ultra high vacuum secondary electron emission test stand supporting an electron gun, a charged particle position detector with controlling grid, and a cryostat assembly is used to conduct electron emission studies. The controlling grid aids in collecting the true secondary electrons. The particle position grid provides an incomplete picture of the final state of the secondary electron collected. With the aid of particle tracking codes, initial states of a family of secondary electrons leading to the final measured state are identified. A sample calculation will be presented and compared against results from a secondary electron emission Monte Carlo code originally developed by Dr. David Joy at the University of Tennessee and significantly modified at UNLV.

Status of the Nevada shocker at the University of Nevada Las Vegas

The Nevada shocker is a 540 kV, 7 /spl Omega/, 50 ns pulsed power device based on Marx bank and Blumlein technologies. The Marx bank is composed of nine 60 kV capacitors charged in series with a gamma high voltage source connected by means of Ross relays in an air environment. A trigatron switch energized with an isolated mini capacitor bank is used to erect the Marx bank. The trigatron switch and erecting electrodes are contained in a gas manifold pressurized to 20 /spl plusmn/ 1 psi with dry air. The energy is released sequentially through an inductor and a water filled charging transmission line to the Blumlein immersed in deionized water. The Blumlein shapes and compresses the energy into a 50 ns pulse upon discharge. A self-breaking water switch initiates the release of energy in the Blumlein. The energy flows through a water filled discharging transmission line to the diode end of the Nevada Shocker. The current diode end of the Blumlein supports vacuum pressures as low as 6.5 /spl times/ 10/sup -6/ Torr. The chamber is pumped with the aid of a roughing pump and a cryogenic vacuum pump. The vacuum section of the Nevada Shocker is currently being rebuilt to incorporate mechanical and thermal loading capabilities with sensors located at the experiment. A number of diagnostic developments are currently underway to support flashover studies on plastics. Resistive probe and differential B-dot diagnostics with the aid of a 6 GHz 20 GS/s TDS 6604 real time scope is documented demonstrating the capability of the machine.