Abstract
This paper presents the first results of an in-house developed low-level radio frequency (LLRF) system and a 10 kW solid state power amplifier (SSPA). The design approach for the SSPA is based on eight resonant single-ended kilowatt modules combined using a planar Gysel combiner. Each of the single-ended modules is based on a two-stepped impedance resonant matching, allowing for harmonic suppression, simple design for massive production, and high-performance design. A design methodology to tune SSPA modules for optimum combining efficiency is presented thoroughly in the time domain. We characterize the power droop due to capacitor banks in the time domain. In open loop of compensation, it is about 1 kW within the pulse of peak value 10 kW and a duration of 3.5 ms. This may lead to the beam instability of the accelerator as particles are not provided with the same energy during the pulse. By incorporating our LLRF system, it is demonstrated that the objective of amplitude and phase stability is satisfied, as required in the European Spallation Source proton accelerator. The presented design also offers the advantages of compact form factor, low complexity, and better performance. In closed loop compensation, the variation of amplitude (pulse droop) is measured on the order of 20 W, which is equivalent to 0.2% at 10 kW peak output power.
Highlights
Radio frequency (RF) vacuum tubes, i.e., klystrons, tetrodes, and Inductive Output Tubes (IOTs), are used to power particle accelerators operated in the frequency range from megahertz to gigahertz.1 The continuous innovation of solid-state technology allows foreseeing the replacement of these vacuum tubes in the frequency range up to around 2 GHz
The feedback signal is fed to the level radio frequency (LLRF) system, allowing for compensating the droop of the pulsed RF signal caused by capacitor banks and correcting other disturbances caused by cavities detuning, phase and amplitude variations, etc
In the presented 10 kW system, the single-ended design approach toward compactness, high stability, high efficiency, and easy repeatability is validated at the kilowatt level, each of the power amplifier modules delivering up to about 1.3 kW output power
Summary
Radio frequency (RF) vacuum tubes, i.e., klystrons, tetrodes, and Inductive Output Tubes (IOTs), are used to power particle accelerators operated in the frequency range from megahertz to gigahertz. The continuous innovation of solid-state technology allows foreseeing the replacement of these vacuum tubes in the frequency range up to around 2 GHz. The synchrotron SOLEIL pioneered the development of solid state power amplifiers (SSPAs) at 352 MHz for its boost and storage ring amplifiers Those amplifiers are based on a combination of 330 W modules to deliver up to 190 kW in continuous wave (CW) mode.. When developing solid-state power amplifiers for particle accelerators, one often raised question is related to the stability and flatness of the power source during the pulse duration. To supply the required energy to the accelerated particles, a feedback control loop is implemented, which allows stabilizing the amplitude and phase disturbances of the RF power supplied in order to establish a constant field gradient in the superconducting cavities (see Fig. 1).
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