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. 2017 Jan 25;4(4):044005. doi: 10.1063/1.4974779

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© 2017 Author(s).

2329-7778/2017/4(4)/044005/8

All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

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FIG. 1. — CAD of the electron source concept. The main components are a parabolically shaped photocathode head with a small flat region and a double magnetic lens system with a conical inner form. This cathode shape has been carefully selected in order to confer an on-axis geometrical surface electric field magnitude of 50 MV/m without exceeding a maximum of 60 MV/m in other parts of the head. The in-lens system ensures B_z = 0 at the electron birth to avoid magnetic emittance growth. The conical anode shape helps to maintain the surface electric field at the anode $| E_{a} | ≲ 5 MV / m$ and brings the magnetic poles closer to the cathode head. The magnetic lens system has been optimized in order to obtain, within practical constraints, a reasonable electron spot size at the sample position and low power consumption to avoid water-cooling. Inlet: black dotted line corresponds to the symmetry z-axis; green curves depict two parabolas displaced from the propagation axis by 0.5 mm in the radial direction and which follow the equation $f (r) = 1.6 {cm}^{- 1} r^{2}$ ; blue segment highlights the flat region of 1 mm in diameter.