Radio Frequency System

The main task of  the 500 MHz systems of the storage ring is to restore the electron beam losses due to the radiation, keeping stable the electron’s motion.
The task of the single 500 MHz system of the booster is to capture the 100 MeV electron’s beam, to ramp its energy up to 2.5 GeV and to ensure enough lifetime for a high efficiency extraction.
Both tasks are fulfilled using the radio frequency (RF) energy: a continuous sinusoidal wave amplified to a level of several kW by means of RF power source. This electromagnetic energy is stored in resonant cavities with high efficiency (Q-factor) achieving an electromagnetic field suitable to accelerate the electron beam.
The electron beam, crossing the cavities, sees the electric line of the stored field, the so-called accelerating voltage. This voltage should have the proper phase, the synchronous one, and the suitable level. When these requirements are met, the electron beam gains energy, according to the Newton-Lorentz force law.
The RF amplitude and phase feedback loops implement the required accelerating voltage parameters, keeping the voltage amplitude stability within ±1% and the phase stability within ± 0.5°. The frequency tuning’s system as well as the temperature control’s loop keep the volume of the cavities at the resonance working point.

The booster RF power source is a 60 kW klystron transmitter. A 50 ohm 6 1∕8” EIA rigid coaxial line run feeds the five cells resonant cavity (DORIS type). A high power circulator shields the transmitter from any reflected power. Frequency, amplitude and phase feedback loops keeps the proper accelerating voltage along the cavity. At the repetition rate of 2 Hz, the electromagnetic field level in the cavity is raised in a linear ramp from the low voltage, 40 kVolt to the high one, 600 kVolt. The ramp’s timing depends on the electron beam energy: the low 40 kVolt is the minimum field level obtainable in the cavity and it is required to capture the beam at the booster injection. Once the beam is captured, this field shall be raised to match the increasing energy need of the beam, up to the final extraction level.

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The storage ring RF systems shall replace the energy lost by the electron beam turn by turn. Because of the full energy injection and the top up operation, the energy delivered by the RF systems is almost constant.   2 GeV 310 mA 2.4 GeV 140 mAv   Harmonic number 432    432   Revolution frequency 1,16 1,16 MHz Available RF power 315  315  kW Accelerating Voltage 1680   1680   kV Synchrotron frequency 11  11  kHz Total Losses 112 112 kW Power to the beam 72 62 kW Four ELETTRA type cavities are installed in the short straight section of the storage ring. The ELETTRA type cavity is a single cell, made of oxygen free high conductivity (OFHC) copper. Elettra Cavity Resonant frequency 499,654 MHz Unloaded Q    40000     Shunt impedance (transit time included)        > 3.2 MW Wall losses     65 kW Accelerating voltage     650 kV Tuning range        ± 200 kHz Frequency set      ±1 MHz

Each cavity is powered by one RF source plant. Three RF power source are the 60 kW klystron transmitters and the RF power line is a coaxial one. Last transmitter is a 150 kW Inductive Output Tube one: waveguide WR1800 run is then used to deal with such power level. All the plants have high power circulator to decouple the transmitter from the load. Frequency, amplitude and phase feedback loops on the RF chain keeps the proper accelerating voltage along each cavity, according to the stored electron beam need. The RF phase shift among the RF plants can be precisely arranged. The longitudinal High Order Modes (HOM) of the cavity are not damped. Their high quality factors allow to tune them far away from the beam resonances by mean of the accurate cavity cooling temperature set  (temperature accuracy = 0.1° C) in order to avoid any dangerous beam-HOM interactions.