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PAL-XFEL (Pohang Accelerator Laboratory X-ray Free Electron Laser) started RF conditioning in October 2015 and has been operating reliably for ~ 4 years. The machines LLRF and SSA systems contributed to the stable operation of PAL-XFEL with over 99% availability. The LLRF and SSA systems showed some problems in rare cases. The delay caused by the problem is very small, but PAL-XFEL can stop working. Some issues have been identified and resolved. We want to share the experience.
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At the Ibaraki Neutron Medical Research Center, an accelerator-based neutron source for iBNCT (Ibaraki - Boron Neutron Capture Therapy) is being developed using an 8-MeV proton linac and a beryllium-based neutron production target. The proton linac consists of an RFQ and a DTL, which is almost the same as the front part of J-PARC linac. However, here only one high-power klystron is used as the RF source to drive the two cavities, which have quite different Q-values and responses. From June 2016, a cPCI based digital feedback system was applied to the iBNCT accelerator. It serves not only as a controller for the feedback of acceleration fields, but also as a smart operator for the auto-tuning of the two cavities in the meantime, especially during the RF startup process to the full power. The details will be described in this report.
The superconducting cavities operated at high Q level need to be precisely tuned to the RF frequency. Well tuned cavities assure the good field stability and require a minimum level of RF power to reach the operating gradient level. The TESLA cavities at XFEL accelerator are tuned using slow (step motors) and fast (piezo) tuners driven by the control system. The goal of this control system is to keep the detuning of the cavity as close to zero as possible even in the presence of disturbing effects (LFD - Lorentz Force Detuning and microphonics). The step motor tuners are used to coarse cavity tuning while piezo actuators are used to fine-tuning and disturbance compensation. The crucial part of the piezo control system is the piezo driver. To compensate LFD the piezo driving with relatively high voltage (up to 100V) and high current (up to 1A) is needed. Since the piezo components are susceptible to destruction with overvoltage, overcurrent, and also overtemperature, one has to pay special attention to keep the piezos healthy. What makes things worse and more critical, is that the piezo exchange is not possible after the module is assembled. Therefore the special hardware must be assisting the power amplifier, detecting the dangerous conditions and disabling piezo operation when needed. It must be fail-safe, so even in a case of failure the piezos shall survive. It must be also robust and it must not disturb or disable normal operation. Due to many channels (16 for master/slave RF), the hardware solution must be well scalable. The paper discuss the design of XFELs piezo driver together with PEM (Power and Energy Monitor) supervising the driver operation and preventing piezos from destruction. The achieved results and operation of the complete system are demonstrated.
First beam commissioning of SuperKEKB (Phase-1), which is an asymmetry double ring collider of 7-GeV electron and 4-GeV positron beams, which had started from February, has been successfully accomplished at the end of June 2016, and the desired beam current for Phase-1 was achieved in both rings. This paper summarize the operation results related to low level RF (LLRF) control issues during the Phase-1 commissioning, including the system tuning, the coupled bunch instability and the bunch gap transient effect. RF system of SuperKEKB consists of about thirty klystron stations in both rings. Newly developed LLRF control systems were applied to the nine stations among the thirty for Phase-1. The RF reference signal distribution system has been also upgraded for SuperKEKB. These new systems worked well without serious problem and they contributed to smooth progress of the commissioning. The old existing systems, which had been used in the KEKB operation, were still reused for the most stations, and they also worked as soundly as performed in the KEKB operation.
CEA is committed to the design, construction and commissioning of a Medium Energy Beam Transfer line and a superconducting linac (SCL) for SARAF accelerator in order to accelerate 5mA beam of either protons from 1.3 MeV to 35 MeV or deuterons from 2.6 MeV to 40 MeV. The Low Level RF (LLRF) is a subsystem of the CEA control domain for the SARAF-LINAC instrumentation. The top level requirement of the LLRF system has been presented in the last LLRF conference. The paper shows a simulink model to analyse and determinate the LLRF technical specification. The public bidding for SARAF LLRF is in the last phase: discussion with the selected company. The first prototype test will be performed at the start of 2020.
Maxwell theory is usually treated in the lab frame under the standard time order (light-signal clock synchronization). Particle tracking in the lab frame usually treats time as an independent variable. Then, the evolution of electron beams is treated according to the absolute time convention (non-standard clock synchronization). This point has never received attention in the accelerator community. There are two ways of coupling fields and particles. The first, Lorentzs way, consists in `translating Maxwells electrodynamics to the absolute time world-picture. The second, Einsteins way, consists in `translating particle tracking results to the electromagnetic world-picture. Conventional particle tracking shows that the electron beam direction changes after a transverse kick, while the orientation of the microbunching fronts stays unvaried. We show that under Einsteins time order, in the ultrarelativistic asymptote the orientation of the planes of simultaneity is always perpendicular to the electron beam velocity. This allows for the production of coherent undulator radiation from a modulated electron beam in the kicked direction without suppression. We hold a recent FEL study at the LCLS as experimental evidence of microbunching wavefront readjusting after a large kick. In a previous paper we quantitatively described this result invoking the aberration of light, corresponding to Lorentzs way of coupling fields and particles. Here we give details of the Lorentzs approach used in that paper. We also demonstrate that an `inverse translation of particle tracking results to the standard time order resolves puzzles related with the strong qualitative disagreement between simulations and experiments. Previous simulation results in ultrarelativistic electron beam physics should be reexamined in the light of the difference between particle tracking and electromagnetic world-pictures.
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