T. Mastorides

Selecting rf amplifiers for impedance controlled LLRF systems - nonlinear effects and system implications

Several high-current accelerators use feedback techniques in the accelerating RF systems to control the impedances seen by the circulating beam. [1, 2] These Direct and Comb Loop architectures put the high power klystron and LLRF signal processing components inside feedback loops, and the ultimate behavior of the systems depends on the individual sub-component properties. Imperfections and non-idealities in the signal processing leads to reduced effectiveness in the impedance controlled loops. In the PEP-II LLRF systems non-linear effects have been shown to reduce the achievable beam currents, increase low-mode longitudinal growth rates and reduce the margins and stability of the LLRF control loops. We present measurements of the driver amplifiers used in the PEP-II systems, and present measurement techniques needed to quantify the small-signal gain, linearity, transient response and image frequency generation of these amplifiers.

Klystron Linearizer for Use with 1.2 mW 476 MHz Klystrons in PEP-II RF Systems

The direct and comb loop feedback around the RF cavities in PEP-II is critical in reducing longitudinal instabilities driven by the cavity impedance. The non-linear 1.2 MW klystron is in the signal path for these feedback loops. As a result, the effective small-signal gain of the klystron at 85% saturation reduces the impedance control by factors of 5 to 20 as compared to a linear power amplifier. A klystron linearizer circuit has been developed which operates in series with the power amplifier and acts to equalize the small and large signal gains through the combination. The technique must implement a 1 MHz linear control bandwidth over roughly 15 dB of RF signal level variation. The dynamics of this system is operating point dependent, and the channel must have dynamic gain compensation to keep the linearity compensation loop stable over changes in operating point. The design of this non-linear signal processing channel (incorporating RF and DSP techniques) and measured results from full-power klystron testing are presented.

Analysis of the longitudinal low-order mode beam dynamics in PEP-II rings at high current beams

PEP-II plans to achieve the final goal in luminosity will require an increase of the beam currents to 4 A for LER and 2.2 A for HER. These magnitudes are challenging in part because they will push the longitudinal low-order mode (LOM) beam stability and the station stability to the limit. To analyze the behavior of both rings at high currents and to understand the limits in the longitudinal feedback systems, a simulation tool has been developed at SLAC. This tool is based on a reduced model of the longitudinal LOM dynamics of the beam interacting with the effective impedance presented by the RF stations. Simulations and measurements of the longitudinal beam behavior in both rings have been performed to understand the ultimate limits of the systems. These studies have defined the impact of control loop parameters in the longitudinal beam dynamics, identified the limiting behavior of RF devices affecting the optimal performance of the RF stations and quantified the behavior of the longitudinal LOM beam dynamics. Results of sensitivity to parameter variations in the beam dynamics and limits in the maximum current that LER/HER can achieve based on the longitudinal beam stability are reported in this paper.