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For experiments that need high peak currents in the bunches, the pulsed injector will be used. It provides a pulsed electron beam that - after having been accelerated by the superconducting cavities - is required, e.g., for the successful operation of the FEL's.
The pulsed electron beam delivered by the injector has an energy of 250 keV, a bunch charge of 77 pC and a repetition rate of 13 MHz, what corresponds to an average beam current of 1 mA. The repetition frequency turns out to be the 100th fraction of the working frequency of the accelerator of 1.3 GHz. The beam can additionally be modulated by a macro-pulse mode. At the entrance of the first superconducting cavity (= exit of the injector), the transversal emittance amounts to about 13 p · mm · mrad whereas the longitudinal emittance runs up to about 50 p · keV · deg.
The injector consists of an electronically pulsed thermionic electron source (triode) with subsequent electrostatic acceleration to an energy of 250 keV. The electron current is extracted from a thermionic cathode. The electron bunches are generated by a subsequent grid modulated with short HV pulses. The electron source delivers bunches 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 several steerer magnets are required for the beam optics. A macropulse generator delivers macro pulses above 0.1 s length at repetition rates of up to 100 Hz. Diagnostic devices are installed for beam alignment as well as for optimization and control during accelerator operation.
For applications requiring the smallest possible transversal emittance and average beam currents less than < 120 /uA, an alternative injector operating according to the principle of an RF gun is available. Thereby, the continuous electron current from a thermionic cathode with a very small diameter is modulated by an alternating electric field in the RF of the main accelerator. A subsequent buncher compresses the just generated electron buches. This method enables the filling of each possible bunch according to the fundamental frequency of 1.3 GHz.
A superconducting photo injector with low emittance at high average electron current is being developed within the scope of a separate project. This injector is based on a photo cathode RF gun combined with a superconducting 1/2-cell cavity.