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Control of Superconductor Cavities in a Linear Particle Accelerator

As part of an integrated effort at European level to promote investigation into strategic areas, Project EUROTRANS (European Research Programme for the TRANSmutation of High Level Nuclear Waste in an Accelerator Driven System) was initiated as part of the FP6 program. This project is aimed at determining the technological viability of transmutation. One of the principal results of EUROTRANS was the preliminary design of a transmutation reactor driven by an accelerator, concept which is referred to as “Accelerator Driven System (ADS)”. The accelerator is a key element of ADS technology. Its main function is the generation of a beam of protons at high energy which impacts a Lead-Bismuth target in order to produce fast neutrons able to cause the transmutation of minor actinides and other nuclear waste products which are generated in fission plants, breaking them down into radioactive elements with smaller half-lives. The EUROTRANS results became the starting point for the MAX project (MYRRHA Accelerator eXperiment, research & development program) of the FP7 program whose objective was to carry out detail design and guarantee the technical viability of the linear proton accelerator (LINAC) for MYRRHA. MYRRHA will be the first transmutation reactor whose construction is planned for completion before 2023.

ADEX is one of the participants in project MAX, in conjunction with CNRS, (Centre National de la Recherche Scientific, France), SKC•CEN (Centre d’etude de l’Energie Nucléaire, Belgium), IAP (Institut für Angewandte Physik, Germany) and INFN (Istituto Nazionale di Fisica Nucleare, Italy), among other institutions and companies. ADEX is in charge of task 3.2 “Experimental evaluation of ADEX control on a 700 MHz prototypical criomodule” which holds an elliptical superconducting cavity designed for the purpose of carrying out the beam acceleration. The objective of this task is to evaluate the performance of adaptive predictive expert control solutions when applied to processes as complex as those of the elliptical cavity, and overcome the problems encountered with conventional control. Both CNRS and INFN are collaborating with ADEX S.L. in carrying out this task.

At the moment, ADEX has completed the design of the Optimized Adaptive Control System ADEX (OACS ADEX) within a simulation of the cryogenic module provided by CNRS. In this context, the results in simulation have demonstrated that the OACS reduced the oscillations of the resonance frequency around the setpoint of the cavity by 38% with respect to other strategies based on conventional control, minimizing the energy consumption required to accelerate the proton beam.

Elliptic Cavity

The upper part of the figure above shows the elliptical cavity while the lower part shows a section through the cavity in which the proton beam is shown in yellow and the radio frequency field (RF) used to accelerate the beam. The RF field is induced in the cavity by an emitter antenna (power coupler) and is measured inside the cavity by a receiver antenna (pick-up antenna). Paradoxically, the RF field measured by the receiver antenna is not the same as that emitted by the emitter antenna. The amplitude and phase of the field inside the cavity are different from those of the field emitted by the emitter antenna due to the resonance effect generated inside the cavity. In view of this, in order to accelerate the beam adequately, the amplitude and phase of the RF field inside the cavity must be maintained closely bound to the set points by modifying the field emitted using the emitting antenna.

Nevertheless, it is not enough to precisely control the amplitude and phase of the field in order to achieve adequate beam acceleration. The electromagnetic resonance frequency of the cavity, which is a property closely related to the geometry of the cavity, will vary due to external perturbations such as noise, seismic vibrations or Lorenz’s Forces. The Lorenz’s Forces are caused by the high intensity of the electromagnetic field in the cavity and they cause small deformations in the cavity walls, modifying the resonance frequency of the cavity. Deviations in the resonance frequency with respect to the optimum value produce drops in the field´s amplitude and deviations in its phase which have to be compensated by acting on the emitted field, raising thus the power consumed to generate it. For this reason, it is necessary to control the resonance frequency of the cavity in order to maintain energy consumption within reasonable limits. 

System of cold tuning


To control the resonance frequency of the cavity, a Cold Tuning System (CTS) as shown in the figure above is used. The CTS consists of a metallic structure which, driven by a motor or the piezoelectric actuators (shown in detail to the right of the figure) compresses or expands the walls of the cavity, modifying its resonance frequency. The motor is a slow actuator which maintains a constant pressure during the normal operation of the cavity. At the same time, fast piezoelectric actuators control the resonance frequency of the cavity in real time.

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