Dual-harmonic broadband low-level RF system for HIAF booster

Purpose The high-intensity heavy-ion accelerator facility (HIAF) is currently being built by the Institute of Modern Physics, Chinese Academy of Sciences in Huizhou, Guangdong Province. The booster ring (BRing) radio-frequency (RF) system operates in the sweeping mode within the frequency range of 0.3–2.1MHz, at a peak voltage of 60 kV per cavity and five cavities. The BRing can accelerate most types of beams, from protons to uranium. For example, a proton beam can be accelerated to 9.3 GeV/u with an intensity of 2.0 × 10¹² particles per pulse (ppp), and 238U³⁵⁺ can be accelerated to 0.835 GeV/u with an intensity of 1.0 × 10¹¹ ppp. To improve the beam intensity, the BRing will use a dual-harmonic-mode RF system for longitudinal acceleration. The RF acceleration unit of the BRing is a broadband high-gradient magnetic alloy (MA)-loaded cavity; therefore, it can be adapted to the multi-harmonic acceleration mode. A fully digitalized dual-harmonic broadband low-level system based on the MA-loaded cavity RF system is investigated.
Methods The system has a standardized CompactPCI -architecture chassis. A field-programmable gate array (FPGA; XCKU060 series) and 16-bit high-speed analog-to-digital/digital-to-analog converters are used as the core hardware units. The control algorithms for the fundamental and harmonic amplitudes and phases in the sweeping mode are designed and implemented within the FPGA.
Results The dual-harmonic low-level RF system is verified by using the synchrotron at the proton radiation effects facility, which is a radiation damage simulation test facility for space devices. The experimental results show that the dual-harmonic low-level system can control the amplitude and phase of the fundamental and harmonic waves in the sweeping mode in real time and that the phase shift between the fundamental and harmonic waves can be precisely controlled. After the amplitude and phase proportional–integral loop closes, the amplitude error of the fundamental and harmonic waves is 0.5 and 0.75%, respectively, and the phase error of the fundamental and harmonic waves is 1◦ and 1.2◦, respectively.
Conclusion Compared with that for the fundamental operation mode, the beam intensity of the synchrotron DCCT can be increased by 27.6% for the dual-harmonic operation mode. This result confirms the functionality of the HIAF wideband low-level RF system with dual-harmonic operation mode. The amplitude and phase control accuracy of the prototype satisfies the requirements of the HIAF low-level RF system.