RF reference distribution system for J-PARC linac

Abstract

The linac used by the Japan Proton Accelerator Research Complex (J-PARC) is a high-intensity proton linac with a peak current of 50 mA. The error in the accelerating field needs to be maintained within ±1% in amplitude and ±1° in phase. Thus, high phase stability is required for the RF reference distribution system. Our objective is to maintain the phase stability of the reference to less than ±0.3°. The beam quality or intensity of a proton linac strongly depends on the stability of the accelerating fields of each cavity; therefore, the stability of the RF reference distribution system is essential for the performance of the J-PARC linac. A highly stable and unique RF reference distribution system was developed for the J-PARC linac. A 12-MHz RF reference signal is converted into an optical signal and amplified by an optical amplifier. Then it is distributed through optical fiber links to 60 low-level RF control systems comprising klystron and solid-state amplifier driving systems. Phase-stabilized optical fiber (PSOF) is employed in the optical transfer line. The thermal coefficient of the PSOF transmission time was measured: its value was 0.4 ppm/°C. This property is insufficient for the required stability; accordingly, the temperature change in the PSOF should be controlled to be within ±0.5 °C by means of a cooling water system. New stable and low-jitter optical-to-electric (O/E) and electric-to-optical (E/O) converters were developed for this linac. The O/E converter is so compact that is can be mounted on a compact PCI board: its temperature is maintained constant by a Peltie device. A pulse driver (limiting amplifier) is introduced in the E/O and O/E converters for signal modulation to reduce the detection jitters. It transforms the 12-MHz signal into a rectangular wave: its the rise time is shorter than 200 ps. Then, a low-transfer jitter of 0.8 ps (rms) was obtained in the optical link of the E/O and O/E converters. The installation of the RF reference distribution system was completed. The phase stability of the distributed signal was evaluated, and a phase stability of the ±0.2° was obtained for a frequency of 927 MHz; consequently, the required system stability was achieved. The beam acceleration to a design energy of 181 MeV for the first phase (Phase I) was successfully performed in February 2007 and the centroid momentum fluctuation of the beam pulse was within ±0.015%, while the required stability was within ±0.025%. Now the beam commissioning has been steadily continued.