John William Luginsland

A virtual prototyping environment for directed-energy concepts

Enhancements in computation hardware and the development of novel software have enabled virtual prototyping in several areas of science and engineering. In particular, the authors discuss directed energy devices that generate high-power microwave pulses.

Emission uniformity and emission area of explosive field emission cathodes

Explosive field emission cathodes have been used extensively in high power microwave tubes. These cathodes emit electrons without the use of cathode heaters. Recently, some theoretical and simulation work has been performed to gain further understanding of the physics of these cathodes. The purpose of this letter is to provide the experimental background and justification for the theoretical work. The general idea of how explosive field emission cathodes operate is that plasma is rapidly formed, which provides the sea of electrons for space charge limited flow. However, recent theoretical and experimental work suggests edge effects, rather than plasma formation across the entire emission area, can also provide the same effect. In this letter we review three types of cathodes which have been tested. We provide optical data on the cathode emission uniformity as well as the electrical data for the same devices. In particular, we find that a large percentage of the cathode can fail to take part in the emissio...

Current and current density of a finite-width, space-charge-limited electron beam in two-dimensional, parallel-plate geometry

The current carried by a steady-state, finite-width beam produced by space-charge-limited emission in two-dimensional parallel plate geometry is known to diverge strongly from estimates based on the classic one-dimensional Child–Langmuir problem. The two-dimensional problem presents formidable analytic difficulties, but a numerical approach to this problem has been developed. The approach simultaneously determines the electrostatic potential and the unknown current density profile. Calculations show that the total current is a function of the dimensionless ratio of beam width (w) to anode–cathode gap (d), but that the current density profile varies with both w and d separately.

Effects of anode materials on the performance of explosive field emission diodes

Explosive field-emission cathodes have been the electron emitter of choice, and often necessity, for high-power microwave (HPM) tubes for many years. The materials typically used for these cathodes range from polymer and cotton velvets, to metals such as stainless steel, and to carbon materials such as bulk carbon and carbon fibers. With several notable exceptions, the issues of the anode and its composition have been largely ignored. Generally, the diode performance, such as current levels, impedance collapse, and out-gassing, have been attributed to the cathode alone rather than to the combination of the cathode and anode. In this paper, we investigate the affects of various anode materials on the performance of explosive field emission cathodes. We show that bipolar flow significantly and rapidly alter diode performance at lower voltage and energy densities than usually observed. We show also the effects of anode material choice on out-gassing, and diode conditioning. Experiments have shown that bipolar flow is a significant issue in diode performance for even short pulses. The theoretical aspects of the diodes are discussed, with a comparison of experiment to theory.

Comprehensive diagnostic suite for a magnetically insulated transmission line oscillator

The magnetically insulated transmission line oscillator (MILO) is a gigawatt-class cross-field microwave tube that requires no external magnetic field due to inherent self-magnetic insulation. The tube operates with a 500 kV, 60 kA electron beam which, along with high rf fields, poses quite a challenge for diagnosing the device. We report on the comprehensive set of experimental diagnostics (both beam and microwave) employed in the MILO experiment, and show how these diagnostics, teamed with particle-in-cell computer simulations, have been instrumental in discovering problems with the tube.

Emission uniformity and shot-to-shot variation in cold field emission cathodes

High-power microwave tubes require currents and voltages generally in excess of 1 kA and 100 kV. In the past, these system requirements led to the use of single shot machines, with repetition rates well under 1 Hz. With advances in pulsed power, the Air Force Research Laboratory recently began to investigate the performance of field emission diodes at repetition rate operation. Greater numbers of shots allowed better accuracy in measurements and the application of better statistics to experimental data. In this paper, we report on new measurements that, with better experimental accuracy, show the statistical correlation between emission uniformity and the shot-to-shot variation in diode current. We report on a comparison to particle-in-cell simulations. These comparisons show the importance of randomly occurring nonemission regions on the cathode surface in dictating the spread in current data. These results imply that uniformity, in addition to playing an important role in any electron interaction with radiation, also affects the current stability for any device using these cathodes. Finally, these experiments show that for repetition rate machines, shot-to-shot variation quantified in terms of Gaussian distributions characterized by a standard deviation and skewness, provide a diagnostic capable of inferring beam uniformity in situations where direct uniformity diagnostics prove extremely difficult or impractical.

Design and implementation of a new UHV threshold cathode test facility

In support of cathode development at the Air Force Research Laboratory, a new ultra-high vacuum cathode test facility is being assembled to complement the existing repetition-rate test pulser. The existing test bed is a 500 kV, 100 Ohm, 1 microsecond(s) duration pulser capable of firing at up to 1 Hz. The new facility is designed to operate at lower voltages (20 - 200 kv), lower impedance (50 - 75 Ohm), and variable pulse lengths (200 - 800 ns) in a single-shot mode. This Threshold Cathode Test Facility (TCTF) will be used to generate data regarding emission turn-on field strengths, outgassing volumes and constituents, vacuum level effects, and anode effects for a variety of field-emitting and explosive- emitting cathode materials. Presented herein are the design parameters of TCTF including diagnostic capabilities and electrostatic simulations of the diode region both with and without beam current.

Virtual prototyping of directed energy weapons

This paper gives an overview of how RF systems, from pulsed power to antennas, may be virtually prototyped with the improved concurrent electromagnetic particle-in-cell (ICEPIC) code. ICEPIC simulates from first principles (Maxwells equations and Lorenzs force law) the electrodynamics and charged particle dynamics of the RF-producing part of the system. Our simulations focus on gigawatt-class sources; the relativistic magnetron is shown as an example. Such simulations require enormous computational resources. These simulations successfully expose undesirable features of these sources and help us to suggest improvements

VIRTUAL PROTOTYPING OF MICROWAVE DEVICES USING MHD, PIC, AND CEM CODES

The Directed Energy Directorate of the Air Force Research Laboratory has the Air Force Responsibility for the development of high power microwave (HPM) weapons. HPM devices tend to fall within the category of ultrawideband or narrowband, each having advantages and difficulties in development and application. The Center for Plasma Theory and Computation has developed a suite of scientific software to aid in the development of the components of such devices. A typical narrowband high power microwave device is made up of 3 components: a source of pulsed power which releases stored energy in the form of a fast (~nsec to msec) applied voltage, a beam/cavity interaction region in which the kinetic energy of a beam is transformed to microwave radiation, and an antenna which is used to direct the microwave radiation. These components may be categorized by their density of charged particles; the pulsed power devices often involve high-density plasmas, the beam/cavity sources have a low density of charged particles, and it is desired that the antennas have no charged particles. These regimes of charged particle density are most efficiently simulated with magnetohydrodynamic (MHD), particle-in-cell (PIC), and computational electromagnetic (CEM) techniques, respectively.

Cathode testing at the Air Force Research Laboratory

An integral part of any vacuum rf device is the cathode. Many rf and microwave tubes utilize thermionic cathodes. However, these cathodes are generally limited to current densities less than 100 A/cm2, a limitation too great for the majority of High Power Microwave tubes. At the Air Force Research Laboratory, Directed Energy Directorate, we have to study a variety of explosive emission cathodes. This paper presents results on studies of several types of cathodes tested in a simple circular geometry. We also present results of research on different types of anode material. The data includes measurements of current, voltage, cathode lifetime, and cathode/anode out-gassing.15