RF Stations Research Articles

General automation of LLRF control for superconducting accelerators

Future and present superconducting linear accelerator projects based on superconducting resonators have tight requirements on field stability that vary with their application. The vacuum ultra violet free electron laser (VUV–FEL) that is currently commissioned at DESY, Hamburg, requires a stability of 10 −3 in amplitude and 0.1° in phase. The upcoming X-ray free electron laser (XFEL) is even more demanding by one order of magnitude. Additionally, these machines need to provide a high reliability and availability, since light sources serve as user facilities. Facing the large number of RF stations as for the case of an international linear collider, this is even more challenging. Therefore, a high degree of automation is mandatory for the low level RF (LLRF) control in order to accomplish for these demands. At the VUV–FEL, an automation framework based on the techniques of finite state machines has been developed and tested. It provides already a number of automated procedures that improve the operation of the VUV–FEL. It is a design goal to develop a framework that is general enough to be applied to future accelerator projects.

RF system of the SPring-8 storage ring.

Construction of three RF stations in the storage ring of SPring-8 has been completed. The design concept concentrates on avoiding a coupled-bunch instability which limits the stored current or makes the synchrotron radiation beam unstable. The cavity is bell-shaped to reduce the coupling impedance of the higher-order modes. The cavity dimensions are trimmed systematically to distribute the higher-order mode frequencies. Each cavity has two movable tuners. The temperature of the cavity cooling water is controlled within 0.02 K and the water flow is kept constant. The construction and commissioning of the SPring-8 storage ring RF system is reported.

Simulation of instabilities in the presence of beam feedback

The effect of longitudinal and transverse instabilities in electron storage rings is simulated by tracking many superparticles for many turns through a model of a machine lattice. This lattice model is defined by a series of machine elements such as RF stations (including longitudinal and transverse wake fields), beam pick-ups, feedback kicker magnets, etc. The machine elements may be interconnected in any specified way so as to produce for example feedback on the longitudinal or transverse beam motion. Each superparticle is treated in six-dimensional phase space and the effects of quantum excitation and radiation damping are included. Insofar as possible the program has been structured to allow study of all known single-beam effects (such as synchro-betatron resonances, transverse mode coupling etc.) in the presence or the absence of some form of beam feedback. The primary goal of the program was to study the effect of a reactive beam feedback system on the threshold for transverse mode coupling.

Long-Pulse Applications of Pulse-Forming Lines for High-Power Linac Application

The ever present demands for high efficiency in the RF power stations for particle accelerators has caused increased interest in longer RF pulses (ten's of microseconds) for linacs such as the Pion Generator for Medical Irradiation (PIGMI) and Free Electron Laser (FEL). For either RF power station, a fundamental decision is whether to use a modulating anode/hard-tube driver or pulsed cathode/line-type pulser configuration. The choices in the extremes of low power for very long pulses or for very-high-power, short pulses are, respectively, a modulated anode/hard tube modulator and pulsed cathode/pulse forming line. However, the demarcation between these two extremes is not clearcut. The criteria (cost, flexibility performance, reliability, efficiency) that resulted in the RF station definition of these two specific systems will be described.

Control Electronics of the PEP RF System

The operation of the major components used for controlling the phase and field level of the PEP RF cavities is described. The control electronics of one RF station is composed of several control loops: each cavity has a tuners' servo loop which maintains the frequency constant and also keeps the fields of each cavity balanced; the total gap voltage developed by a pair of cavities is regulated by a gap voltage controller; finally, the phase variation along the amplification chain, the klystron and the cavities are compensated by a phase lock loop. The design criteria of each loop are set forth and the circuit implementation and test results are presented.

Longitudinal Behavior of the Beam in KEK Booster

The measurements of the longitudinal beam properties and high intensity effects of the KEK booster are presented. The capture efficiency has been improved by using two kinds of cure for beam loading instabilities appeared just after injection. Operation of the second RF station in cooperation with the existing one reduces the beam loss during acceleration. Severe longitudinal beam instabilities observed at the end of acceleration have been cured by incorporating a 2fs feedback loop.

Booster and Main Accelerator Phase Detector System for Cavity Tuning

Each NAL booster and main accelerator rf station contains a phase lock control loop to resonate the beam-line cavities at the applied excitation (reference) frequency. The phase sensor in the loop is a dual channel phase detector having an octave bandwidth, an amplitude dynamic range of 60 dB, a phase balance vs. frequency better than ±2° and a zero crossover phase sensitivity of 0.18 V/deg. The phase detector develops a feedback error signal proportional to the phase difference between the rf reference signal and an rf signal representing the rf station beam line voltage. This paper discusses the rf cavity tuning feedback loop configuration and describes the details of the phase detector circuitry.