Radio Frequency System (RF-System)

The high-frequency system supplies the accelerating energy for the main accelerator, for the electromagnetic fields of the buncher in the injector and it generates phase-locked all signals required for synchronization in other accelerator components (electron sources, beam diagnostics, experiments).

The RF system has a modular design, with the frequency generating assemblies (fundamental frequency generation) being coupled in a phase-locked manner and derived from a highly stable reference frequency oscillator with very low phase noise.



Base frequency generation

To operate the accelerator, the frequencies 260 MHz and 1300 MHz are generated from a highly stable 13 MHz oscillator with very low phase noise (this corresponds to the oscillation frequency of the ELBE Free Electron Laser (FEL) by means of phase grid oscillators (PLL). The frequency 1300 MHz is the operating frequency of the superconducting cavities. The first buncher resonator in the injector is operated at the fifth subharmonic of the accelerator frequency (260 MHz). The basic frequency of 13 MHz is required for the synchronization of further oscillators in different parts of the system and is transmitted there via phase-stable optical fibers (OFC)


Thermionic injector


It consists of the electronically pulsed thermal electron source (GUN) and the two normal conducting buncher resonators (260 MHz and 1.3 GHz). The GUN is at high voltage potential with a terminal voltage of -250 kV. The grid voltage of the GUN, which is constructed like a triode system of a transmitter tube, is pulsed according to the desired operating regime. The electron bunches exiting the injector with a pulse length of 500 ps are compressed to about 5 ps by the buncher before they reach the first cavity of the main accelerator. The injector is supplied with the required signals via fiber optic cables.


The 500 ps-long electron bunches extracted from the GUN at an energy of 250 keV are shortened to about 5 ps by the energy modulation in both buncher resonators and the subsequent drift distances. For the excitation of the respective electromagnetic fields in the buncher resonators, radio frequency powers of maximum 1kW (260 MHz) and 200 W (1300 MHz) are provided by transistor amplifiers.

RF for the main accelerator


The RF power of maximum 20 kW at a frequency of 1300 MHz is provided for each cavity by two 10 kW transistor amplifiers connected in parallel. The RF system has a modular design, the cavity resonators (9-cell, buncher) have individual RF systems. This ensures a high degree of interchangeability of individual components.


ELBE Lowlevel-ControllerPrinciple of an HF module. The RF (Low level Controller) is used to stabilize the amplitude and phase of the acceleration voltage in the cavity resonators. The RF control module is identical in construction for all RF systems, only the power amplifiers are different. The 20 kW RF systems consist of two 10 kW transistor amplifiers connected in parallel.



ELBE HF-Verstärkerraum


ELBE- amplifier room: Two 10 kW transistor amplifiers are connected in parallel. In the middle of the picture you can see a waveguide hybrid coupler, to which the waveguides of two amplifiers are connected from above.