Alan Mankofsky

Analysis of field emission from three‐dimensional structures

An analysis of the field emission from emitter tips with the geometry of a prolate ellipsoid of revolution indicates that the field enhancement factor, β, and effective emission area, α, are not constant but instead depend on the applied field. The added complexity of a materials related limit on the minimum time for transition of an electron from the solid into the vacuum is also examined in the analysis. The calculated variations of α and β are as large as 35%, and in some instances could result in erroneous interpretation of measured current‐voltage (I‐V) characteristic data.

High brightness electron beam sources for FEL applications

Abstract A new generation of field emitter array (FEA) cathode materials is under development at SAIC, in collaboration with NRL and GTE Laboratories. The emitter structures under consideration consist of large area ( ∼ 1 cm 2 ) arrays of large numbers ( ∼ 10 6 ) of microscopic field emitting tips. The structures can be fabricated so as to choose an emitter tip microstructure that is a solid cone, a hollow cylinder, or a variety of other shapes. These microstructures evidence very high local field enhancement factors, controllable from a factor of ∼ 200 to > 2000 . This large local field enhancement allows quantum field emission of significant current from the large area array while the applied macroscopic electric field is still quite low ( ∼ 20 kV/cm ). Single-tip, noninteracting particle, multigrid simulations indicate that the beam brightnesses B n = I/π 2 ϵ n 2 >10 10 A/cm 2 rad 2 may be possible. Beams with such high brightness allow for a greatly expanded field of FEL applications, including high gain and harmonic operation in the FIR wavelength regime. Experiments have so far demonstrated DC average current densities > 1 A/cm 2 , uniform emission, and improved characteristics when run for long periods of time ( > 100 h, DC ). Our present efforts are concentrated on optimizing the available cathode current density, measuring the actual beam brightness, and including self-field and 3-D effects in the numerical simulations.

Numerical simulation of nonneutral plasmas

We discuss particle‐in‐cell simulation techniques and their applicability to problems involving nonneutral plasmas. An overview of the essential elements of a particle simulation is presented, followed by a series of examples of present‐day code capabilities.

Three-Dimensional Particle-in-Cell and Electromagnetic Simulations

In computational plasma physics the development of simulation techniques and their application has followed an evolution which has been determined, in part, by the cost, speed, and availability of computers. The ever increasing power of modern supercomputers has allowed a progression from modeling of one-dimensional simple problems to two-dimensional simulations which involve complicated geometry and multiple physical processes. One- and two-dimensional PIC codes have become standard research tools and have been applied to an extremely broad set of basic physics and engineering problems. Fully three-dimensional plasma and field models have the obvious attraction that they can deal with problems that are inherently three-dimensional and cannot be analyzed in lower dimensionality, problems in which the dimensionality is suspected to have a role, and design problems in which three-dimensional concepts are a possible option if risk can be assessed through computation or analysis. Until recently the use of general three-dimensional plasma codes, while conceptually attractive, was simply not affordable or highly impractical, requiring very long running times and excessive memory or auxiliary storage.

Development of 3D simulations for heavy-ion fusion beam transport and compression problems

Abstract Longitudinal beam compression and the possibility of concurrent pulse shaping have been the subject of previous studies using analytical methods and 1D