The RFQ (Radio Frequency Quadrupole) is an Accelerating Structure used as the first accelerating stage or in the injector of many proton and ion accelerators. This device has a length of several meters and typically accelerates particles from the range of tens of keV to several MeV per nucleon. These devices fulfill three tasks in an accelerator in just one device: it accelerates particles, keep them focused as they travel along it without needing extra magnetic focusing and finally groups particles in the so-called “bunches”, which is the optimum way of further accelerating them in later stages.

In ESS-Bilbao, an RFQ is planned to be built. A first design of the device, planned to operate at an input energy of 45 keV was completed [link to the TDR document]. The design was evaluated by a panel of international experts. Based on their recommendations and adapting to the new general plan in ESS-Bilbao, a new design has been proposed with enhanced characteristics.

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National and International colaborations

- ISIS Spallation Neutron Source.


An RFQ is a very complex device, and its design, fabrication and operation involved different technical groups in ESS-Bilbao.

- From a design point of view, the RFQ needs extensive beam dynamics calculations to define the adequate vane profile, electromagnetic modeling to define the geometry of the cavity, RF engineering to provide the adequate power to operate it, thermo-mechanical calculations to validate the cooling strategy selected, etc.

- The fabrication is also very complex, as it involves machining of oxygen free copper with very high accuracy and in complex shapes. The assembly of the different components in the final device (whether it is by welding them or by using another method) is also not easy, and is an usual source of trouble with this kind of devices.

- The operation of the device is again not trivial. First, the cavity must be made to have the resonant frequency corresponding to the input RF power, but the RFQ frequency changes continuously due to thermo-mechanical efforts driven by the same RF power and by other sources. An adequate strategy to keep the frequency constant must be used (the so-called 'tuning strategy').

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