RF focussing – an instrument for beam quality improvement in superconducting RF guns

Abstract

Rf electron guns require an external magnetic field focussing to produce a low emittance electron beam. A dc magnetic field focuses electrons in the beam already inside the cavity as they leave the photocathode. In superconducting rf guns (srf) the possibility of external magnetic focussing is quite limited. In this paper we investigate an effect of rf focussing in the first half cell of a 3+1/2 cell srf gun cavity without a dc magnetic focussing. The shape of the first half cell is optimized. The calculations were done to fulfil the beam requirements of the injector for the ELBE project [1]. Besides normalized transversal emittances of the order of 1 π mm·mrad for bunch charges up to 1 nC were obtained. In the paper a graphic explanation of the rf focusing effect is represented.

Radial bunch compression: Path-length compensation in an rf photoinjector with a curved cathode

“…Furthermore, a curved cathode reduces space-charge induced energy spread, which in turn makes the spot-size less sensitive to the chromatic aberrations of the focusing system. Finally, the focusing properties of a curved cathode relax the need for a focusing solenoid and aid the emittance compensation process, see [21] describing the beneficial effects of initial radial focusing in a similar structure.…”

Electron beam diagnostics for a superconducting radio frequency photoelectron injector

“…Further, other designs for a SRF electron injector 6 also use a solenoid in close proximity to the gun. Other schemes to achieve low emittances, such as rf focusing 7 with a retracted cathode and a shaped cathode visor, will be tried within the framework of this project.…”

Beam dynamics studies for a 312 cell superconducting RF photo injector operating at low and high bunch charge

“…A laser at 263 nm wavelength with a radial flat top distribution illuminates the Cs 2 Te photo layer at the cathode. To obtain a focusing field at the tip, the cathode is retracted a few millimeters from the half-cell back wall [9]. In order to optimize the focusing field with respect to the transverse emittance, a visor with a proper shape at the cathode tip is used as shown in Fig.…”