John Watrous

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.

3-D ICEPIC simulations of the relativistic klystron oscillator

Improved concurrent electromagnetic particle-in-cell (ICEPIC), developed at the US Air Force Research Laboratory, is a three-dimensional (3-D) particle-in-cell (PIC) code specifically designed for parallel high-performance computing resources. ICEPIC simulates collisionless plasma physics phenomena on a Cartesian grid. ICEPIC has several novel features that allow efficient use of parallel architectures, including automated partitioning, dynamic load balancing, and an advanced parallel PIC algorithm. Even though ICEPIC is still in development, it has successfully simulated real-world high-power microwave (HPM) experimental devices, such as the magnetically insulated line oscillator (MILO) and the relativistic klystron oscillator (RKO), and the results from recent RKO simulations will be presented.

An improved space-charge-limited emission algorithm for use in particle-in-cell codes

An improved space-charge-limited emission algorithm has been developed and tested for use in particle-in-cell codes. Comparisons to established and accepted techniques of modeling space-charge-limited emission demonstrate the new technique to be accurate and efficient. Moreover, the new technique is very insensitive to mesh resolution.

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.

Virtual Prototyping of Directed Energy Weapons

This paper reports on radio frequency (RF) sources that have been 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 several proposed variants of the L-band A66 half height (A66-hh) relativistic magnetron HPM source. The variations of the A66-hh include higher frequency designs (C-band, S-band and X-band), and a design that uses permanent magnets at L-band instead of electromagnets. Such simulations require enormous computational resources. Simulations exposed undesirable features in the first iteration of these designs and by the visualization and analysis of the simulation solutions became apparent. In the next iteration of the design, the simulations did not have these undesirable features.

Computational assessment of the effect of nozzle geometry on the performance of gas-puff plasma radiation sources

A computational assessment of the influence of nozzle geometry on radiative performance of double-shell, gas-puff argon implosions has been carried out. Nozzle geometries fielded in experiments on Double Eagle, Decade Quad-1, and Sandias Z-Machine have been considered, as have nozzle designs that have not been tested experimentally. An important element of this work was the effort to benchmark a computed result against existing experimental data. The influence of grid resolution and random perturbations has also been investigated.