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Radio Frequency System (RF-System)
The RF-System provides the energy being transferred to the electron beam by electromagnetic fields in the superconducting cavities of the ELBE LINAC. Very low rf - dissipation in the superconducting cavities makes it possible to transfer nearly all of the rf- power of 8.5 kW to the beam.
An overview of the rf-system is given in the block diagram.
The phase-locked-loop (PLL) oscillators for 1,3 GHz (for the accelerator and fundamental buncher) as well as for 260 MHz (for the subharmonic buncher) are phased locked to a 13 MHz quartz oven. The 13 MHz was chosen because of the ELBE- FEL-frequency of 13 MHz. Fibre optics are used to distribute the 13 MHz clock signal within the entire laboratory.
ELBE is operated with two optional injectors.
Pulsed Injector: Parts of the injector are the electronically pulsed thermionic electron souce (Gun), the subharmonic buncher (260 MHz) and the fundamental buncher (1.3 GHz). The gun with associated electronics is assembled on a –250 kV terminal. The grid of the triode-gun is pulsed with 13 MHz, corresponding to a bunch charge of 77 pC at a beam current of 1mA. Variable pulse trains are possible by switching the remote controlled synchrounous divider. For experiments requiring a very low beam emittance the grid is pulsed with 260 MHz. Optical fibres are used for data links as well as for the synchronous signals between the terminal and ground. The gate pulsing signals are amplified and clipped by avalanche-diodes.
Two bunchers are used to compress the 500 ps long bunches, extracted from the GUN to about 10 ps. Both bunchers are normal-conducting and require rf - power of max. 1000 W at 260 MHz and 200W at 1,3 GHz respectively. The amplifiers used are solid state.
The cryomodule contains a pair of superconducting 9 - cell TESLA cavities (DESY). The cavities are individually driven by a pair of 10kW solid state RF-power amplifiers. The RF - power is delivered to the cryomodule by waveguides and fed into the superconducting cavities by - coaxial-couplers with fixed antenna coupling. Amplitude- and phase stabilisation are realized by individual controllers using direct converting techniques at 1.3 GHz (designed at HZDR).