Keigo Hara

Lowlevel RF Control System of J-PARC Synchrotrons

We present the concept and the design of the low level RF (LLRF) control system of the J-PARC synchrotrons. The J-PARC synchrotrons are the rapid cycling 3-GeV synchrotron (RCS) and the 50-GeV main ring (MR) which require very precise and stable LLRF control systems to accelerate the ultra-high proton beam current. The LLRF system of the synchrotron is a full-digital system based on direct digital synthesis (DDS). The functions of the system are (1) the multi-harmonic RF generation for the acceleration and the longitudinal bunch shaping, (2) the feedbacks for stabilizing the beam, (3) the feedforward for compensating the heavy beam loading, and (4) other miscellaneous functions such as the synchronization and chopper timing. The LLRF system of the RCS is now under construction. We present the details of the system. Also, we show preliminary results of performance tests of the control modules.

Dual Harmonic Operation with Broadband MA Cavities in J-PARC RCS

In the J-PARC RCS RF system, the fundamental rf acceleration voltage and the 2nd higher harmonic one are applied to each cavity. This is possible, because the magnetic alloy loaded cavities have a broadband characteristic and require no resonant frequency tuning. The tube amplifier provides both rf components. We calculate the operation of the tube under the condition of the dual harmonic, the non-pure resistive load and the class AB push-pull mode. We also describe about the single harmonic operation from the view point of the higher harmonic push-push mode.

Present Status and Future Upgrades of the J-PARC Ring RF Systems

J-PARC facility is the multipurpose research institutes. 10 years have passed since the user operation started. We have been considering the accelerator upgrades for the future and the target beam powers for 3 GeV rapid cycling synchrotron (RCS) and 30GeV Main ring (MR) are 1.5MW and 1.3MW, respectively. To achieve a 1.5MW of RCS output beam power, increasing the number of Linac proton particles is necessary. For accelerating such higher beam current, the ring rf systems in the RCS need to upgrade an accelerating voltage and to take into account for heavier beam loading compensation. In case of the MR, increasing the number of proton particles is not appropriate from the viewpoint of space charge effects. We choose to shorten the MR cycle time to increase an output beam power. The required accelerating voltage becomes almost double. All nine systems were replaced to realize the required voltages with the higher accelerating gradient RF systems using a newly developed magnetic alloy material. At present, the proton beam of 470 kW is being delivered to the T2K experiment with a cycle time of 2.48 s. Beam powers of MR will plan to aim first at 750 KW after replacing the magnet power supplies. But, to realize a 1.3 MW of the target beam power, the upgrade of RF power sources will be necessary. We report the present status of the ring RF systems and the upgrades for the future.

Baseband simulation model of the vector rf voltage control system for the J-PARC RCS

Vector rf voltage feedback control for the wideband magnetic alloy cavity of the J-PARC RCS is considered to be employed to compensate the heavy beam loading caused by high intensity proton beams. A prototype system of multiharmonic rf vector voltage control has been developed and is under testing. To characterize the system performance, full rf simulations could be performed by software like Simulink, while the software is proprietary and expensive. Also, it requires much computing power and time. We performed the simplified baseband simulations of the system in z-domain by using free software, the Python control library. It seems to be beneficial for searching the parameters that the baseband simulation can be performed quickly. In this presentation, we present the setup and results of the simulations. The simulations well reproduce the open and closed loop responses of the prototype system.

The Second Harmonic RF System for J-PARC MR Upgrade

Power upgrade scenario of J-PARC Main Ring includes replacement of RF cavities with higher field gradient using magnetic alloy, Finemet ®-FT3L, cores than the present ones. It also needs to install the second harmonic RF cavity in the other section where dedicated water system for RF cavities is not available. Installation scenario of the second harmonic RF will be presented.

Simulation of Phase Modulation for Longitudinal Emittance Blow-Up in J-PARC MR

The J-PARC MR provides a coasting proton beam for nuclear physics experiments by slow extraction. The longitudinal emittance should be enlarged until the MR flat top to mitigate the microwave instability. We have investigated a Phase Modulation (PM) method[1,2,3,4] by using a High Frequency Cavity (HFC) to increase the emittance. We have performed extensive simulation studies to find the appropriate parameters of the PM through the particle tracking simulation. We found that the effective HFC frequency has linear dependence with the PM frequency as shown in Fig. 1, where the emittance is smoothly enlarged. Furthermore, we found that the required HFC voltage is inverse proportional to the square root of the duration time of the PM as shown in Fig. 2. These PM properties will be used for the design of the HFC. We describe the particle tracking simulation results of controlled emittance blow-up by the PM.

Simulation of Debunching for Slow Extraction in J-PARC MR

The J-PARC MR delivers a proton beam for nuclear physics experiments with slow extraction. The beam is debunched at flat top to obtain a coasting beam by turning off the rf voltage. The beam loading effect can disturb the uniformity of the debunching at the flat top. We describe the results of the particle tracking simulation including the beam loading effect.