Fourth Harmonic Cavity Research Articles

Development of a control system for the fourth-harmonic cavity of the HLS storage ring

Harmonic RF cavities are commonly used in storage rings to lengthen the bunches and thus suppress the beam’s instabilities and increase its Touschek lifetime. The voltage and phase of the electromagnetic fields in the harmonic cavity are of great importance for stretching the bunch. In the Hefei Light Source storage ring, a passive fourth-harmonic cavity is installed, and the cavity is monitored and controlled by an analog control module provided by its manufacturer. To vary and maintain the voltage of the harmonic cavity in a more effective way, a digital proportional, integral, and derivative feedback system based on the Experimental Physics and Industrial Control System is developed on top of the analog control module. This paper reports the details of the development of this voltage control system. Some test and operational results are also presented.

Analytic modeling of instabilities driven by higher-order modes in the HLS II RF system with a higher-harmonic cavity

The utility of a passive fourth-harmonic cavity plays a key role in suppressing longitudinal beam instabilities in the electron storage ring and lengthens the bunch by a factor of 2.6 for the phase II project of the Hefei Light Source (HLS II). Meanwhile, instabilities driven by higher-order modes (HOM) may limit the performance of the higher-harmonic cavity. In this paper, the parasitic coupled-bunch instability, which is driven by narrow band parasitic modes, and the microwave instability, which is driven by broadband HOM, are both modeled analytically. The analytic modeling results are in good agreement with those of our previous simulation study and indicate that the passive fourth-harmonic cavity suppresses parasitic coupled-bunch instabilities and microwave instability. The modeling suggests that a fourth-harmonic cavity may be successfully used at the HLS II.

Analysis of quadrature hybrids for accelerator applications

The operation of a quadrature hybrid as a power combiner is analyzed using four-port scattering parameters model. The analytical results are compared with data measured experimentally using a 211 MHz cavity. Graphical solutions of the measured cases for different cavity phases are in good agreement with analytical predictions. The use of the 90°-hybrid as an RF circulator is also analyzed. The active operation of the harmonic cavity in the NSLS VUV-ring is used as an example for this application. This fourth-harmonic cavity is used to change the shape of the bucket potential to lengthen a stored bunch. Thus, a longer stored-beam lifetime can be achieved without compromising the high brightness of the VUV photon beam. If operated actively, the harmonic cavity would present a mismatched load to an RF generator. A circulator would allow for better operation of the RF generator. It would also minimize the generator-accelerating cavity-beam-harmonic cavity-generator coupling. In this paper, we use the harmonic cavity case as an example for the use of the 90°-hybrid as a circulator. The advantages and disadvantages of the use of the 90°-hybrid as a circulator are discussed.