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Graduate Student Seminar - 03/01/2013

"Modelling of Microprotrusions on the Surface of High-Gradient Accelerating Structures"

by Aydin Keser

Friday, March 1, 2013 -- 12:00 p.m.
Large Conference Room, 1207 Energy Research Facility

Advisors:  Prof. Thomas Antonsen and Dr. Gregory Nusinovich

Microprotrusions on the accelerator surface constitute one of the factors that limit the high-gradient operation of the metallic accelerator structures. These micron-sized emitting spots are both a source of dark current and breakdown. The electron emission is a function of the electric field gradient and the temperature of the emitting surface. In this study, a model that combines electrical and associated thermal processes in a unified framework is built and simulated. These processes include the field and thermal electron emission, in conjunction with Joule heating, the Nottingham effect, and the space charge effect. Firstly, an image charge model is used for the specification of protrusion geometry and the external electric field. The validity of the Fowler-Nordheim emission equation is discussed and the general thermal field emission is calculaled numerically. A phenomenological scheme is devised to estimate the effect of space charge on the electric field. Secondly, the boundary value problem for the current density and the temperature in the metallic body of protrusion is stated. The finite elements method is used with an ODE solver to numerically solve the boundary problem. The effects of the Nottingham heating and space charge are discussed along with possible conditions for runaway scenarios.

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