Editing Electron gun (section)

== Design ==
[[file:Kathodestraalbuis2.jpg|thumb|left|Electron gun from an [[oscilloscope]] CRT]]
[[file:Electron Gun with Wehnelt Cylinder.svg|thumb|Setup of an electron gun. 1. [[Hot cathode]]. 2. [[Wehnelt cylinder]]. 3. Anode]]

A direct current, electrostatic thermionic electron gun is formed from several parts: a [[hot cathode]], which is heated to create a stream of [[electrons]] via [[thermionic emission]]; electrodes generating an [[electric field]] to focus the electron beam (such as a [[Wehnelt cylinder]]); and one or more [[anode]] electrodes which accelerate and further focus the beam. A large voltage difference between the cathode and anode accelerates the electrons away from the cathode. A repulsive ring placed between the electrodes focuses the electrons onto a small spot on the anode, at the expense of a lower extraction field strength on the cathode surface. There is often a hole through the anode at this small spot, through which the electrons pass to form a collimated beam before reaching a second anode, called the collector. This arrangement is similar to an [[Einzel lens]].

An RF electron gun<ref name="Kirk">H.G. Kirk, R. Miller, D. Yeremian, ''Electron guns and pre-injectors'', pp. 99-103, in A. W. Chao and M. Tigner, Editors, "Accelerator Physics and Engineering" World Scientific, Singapore, 1998 </ref> consists of a [[Microwave cavity]], either single cell or multi-cell, and a [[cathode]]. In order to obtain a smaller [[beam emittance]] at a given beam current, a [[photocathode]] is used.<ref>I. Ben-Zvi,  ''photoinjectors'', pp. 158-175, in A. W. Chao, H.O. Moser and Z. Zhao, Editors, "Accelerator Physics and Technology Applications" World Scientific, Singapore, 2004 </ref> An RF electron gun with a photocathode is called a [[photoinjector]]. 

[[File:Photoinjector operating at 2856 MHz.jpg|thumb|right|A photoinjector based on a "one and a half cells" microwave cavity at a frequency of 2856 MHz.]]

Photoinjectors play a leading role in X-ray [[Free-electron laser]]s and small [[beam emittance]] [[accelerator physics]] facilities.