Freeform shape optimization of a compact DC photo-electron gun using isogeometric analysis

TL;DR

This paper presents a novel isogeometric analysis-based shape optimization method for compact high-voltage DC photo-electron guns, reducing maximum electric field strength to improve performance and safety.

Contribution

It introduces an efficient IGA-based optimization framework using open source tools for designing electrode geometries in high-voltage photo-electron guns.

Findings

Maximum field gradient of 9.06 MV/m on electrode surface

Field strength below 3 MV/m on photocathode surface

Framework adaptable to other geometries and quantities of interest

Abstract

Compact DC high-voltage photo-electron guns are able to meet the sophisticated demands of high-current applications such as energy recovery linacs. A main design parameter for such sources is the electric field strength, which depends on the electrode geometry and is limited by the field emission threshold of the electrode material. In order to minimize the maximum field strength for optimal gun operation, isogeometric analysis (IGA) can be used to exploit the axisymmetric geometry and describe its cross section by non-uniform rational B-splines, the control points of which are the parameters to be optimized. This computationally efficient method is capable of describing CAD-generated geometries using open source software (GeoPDEs, NLopt, Octave) and it can simplify the step from design to simulation. We will present the mathematical formulation, the software workflow, and the results of an IGA-based shape optimization for a planned high-voltage upgrade of the DC photogun teststand Photo-CATCH at TU Darmstadt. The software builds on a general framework for isogeometric analysis and allows for easy adaptations to other geometries or quantities of interest. Simulations assuming a bias voltage of -300 kV yielded maximum field gradients of 9.06 MV/m on the surface of an inverted insulator electrode and below 3 MV/m on the surface of the photocathode.