First electron beam generated by superconducting gun at FZD.
On November 12th 2007, the first superconducting electron gun was put in operation at the Forschungszentrum Dresden-Rossendorf. This kind of gun will also be part of the BERLinPro. The gun consists of a photocathode, which emits electrons, and a cavity to accelerate electrons and to form the electron beam. It is this cavity, which is made of superconducting material (Niobium). For this reason, the gun can be run continuously, whereas a normal-conducting cavity has to be operated in a pulsed mode. Running a ERL by a superconducting gun allows to generate laser pulses in a continuous wave mode.
A group at BESSY II has developed and installed the diagnostics for the electron beam of the superconducting gun at FZD. It consists of screens, which can be moved into the electron beam, instruments for complete characterisation of the beam, and the software used for diagnostics. The project is a collaboration between BESSY/HZB, DESY, FZD and MBI, and supported by the BMBF.
Injector and Booster
Electrons are the main actors in a lightsource, as they emit synchrotron radiation while travelling through a dedicated magnetic structure. For this the electrons must be grouped in small bunches, such that they appear from the outside as one particle radiating. In order to achieve this the electrons must be created from the start in the form of small, short bunches, dense clouds of electrons. This can be achieved by using a superconducting radio-frequency photoinjector. For this reason the electron injector is of fundamental importance for the light source.
In the figure the schematic setup of an electron source is sketched. One can see a superconducting acceleration section of the same kind as the ones used in the linear accelerator. The structure for the electron source is just a little bit shorter. On the left side of the structure a semiconductor photocathode is embedded. This photocathode will be illuminated with light pulses from a high power laser system. The light pulses are converted by the cathode with the photoeffect into electrons, hence the name photoinjector. There are 10 Billion electrons in a volume of 1 mm diameter and 10 mm length. As all these electrons have the same charge, they are repelling each other. To counteract these repelling forces, the electron bunches are accelerated rapidly and subsequently focused by special magnetic lens.
Crucial beam parameters must be measured and controlled before the electron bunches from the source can be injected into the main linac and accelerated to high energy. For this the electrons will be send through a dedicated diagnostics beamline. Viewscreens are inserted into the beam path and luminate due to the energy deposition by the electrons. These luminescent images can be analysed as they directly reflect the homogenity and quality of the electron bunches. The electron energy and energy distribution is measured with a spectrometer dipole magnet. The spectrometer works similar as a light prism. In a prism Light (here electrons) of different wavelength (or energy) is deflected with a different angle. These angles can be measured using a viewscreen and from there the electron energy can be calculated. Electrons with the wrong beam energy a dumped in a heat sink, only electrons with the right set of beam parameters are allowed to enter the linear accelerator.
HZB participates actively at two leading R&D projects on electron sources: In the framework of the PITZ collaboration the electron source for the European XFEL is developed. The SRF/ELBE injector collaboration is concerned with the development of an electron injector employing a superconducting acceleration structure as described above. For both projects HZB is developing advanced solutions and components for electron beam diagnostics.For t he future light source at HZB conceptual R&D we investigate options for a high brightness, high average power electron source.
