The thermionic injector is the standard injector for user operation at ELBE. It provides a pulsed electron beam for the superconducting main accelerator. The pulsed electron beam delivered by the injector has an energy of 235 keV, a bunch charge up to 100 pC and an average beam current up to 1.6 mA. The repetition frequency of the pulses can be flexibly chosen between 26 MHz and 101.6 kHz. For FEL operation the standard pulse repetition rate is 13 MHz. The beam can additionally be modulated by a macro-pulse mode. At the entrance of the first superconducting cavity (exit of the injector), the transverse emittance amounts to about 20 mm mrad whereas the longitudinal emittance achieve about 100 keV ps at maximum bunch charge.
The injector consists of a DC high voltage electron source with a gridded thermionic cathode at high-voltage potential and an anode on ground with an electrostatic acceleration potential of -250 kV. The emitted electron current is modulated by the grid voltage forming pulses of about 500 ps length. These electron bunches enter the subharmonic buncher that runs at a frequency of 260 MHz (one fifth of the working frequency). The bunches are first compressed in the subsequent drift path by the energy modulation produced in the buncher. They are further compressed in the following 1.3 GHz fundamental buncher before entering the first superconducting cavity. Five magnetic lenses and some steering magnets belong to the beam optics. A macropulse generator and deflector coils allow to cut out pulse trains with lengths between 0.1 and 35 ms at repetition rates up to 25 Hz. Diagnostic devices are installed for beam characterization as well as for optimization control during accelerator operation.
At the ELBE accelerator a second electron injector is installed. It contains a photocathode and a laser beam generates the electron bunches. The photocathode is placed in a superconducting RF cavity. The strong acceleration of the electron in the RF field delivers electron bunch of high charge with low transverse emittance.