S. Bousson

The ESS spoke cavity cryomodules

The European Spallation Source (ESS) is a multi-disciplinary research centre under design and construction in Lund, Sweden. This new facility is funded by a collaboration of 17 European countries and is expected to be up to 30 times brighter than today’s leading facilities and neutron sources. The ESS will enable new opportunities for researchers in the fields of life sciences, energy, environmental technology, cultural heritage and fundamental physics. A 5 MW long pulse proton accelerator is used to reach this goal. The pulsed length is 2.86 ms, the repetition frequency is 14 Hz (4 % duty cycle), and the beam current is 62.5 mA. It is composed of one string of spoke cavity cryomodule and two strings of elliptical cavity cryomodules. This paper introduces the thermo-mechanical design and expected operation of the ESS spoke cavity cryomodules. These cryomodules contain two double spoke bulk Niobium cavities operating at 2 K and at a frequency of 352.21 MHz. The superconducting section of the Spoke Linac accelerates the beam from 90 MeV to 220 MeV. A Spoke Cavity Cryomodule Technology Demonstrator will be built and tested in order to validate the ESS series production.

Thermal Stability Analysis of Superconducting RF Cavities

Future linear accelerators using superconducting RF cavities (TESLA proposal), requires accelerating gradients Eacc=25 MV/m to be achieved reliably at large scale. This high gradient level is mainly limited by electron emission and thermal instabilities (quench). Impressive improvements have been recently accomplished by pushing further the onset threshold of electron emision using careful surface cleaning techniques. On the other side micron size resistive defects embedded in the niobium walls of the cavity continue to induce thermal breakdowns for Eacc in the range 15 to 25 MV/m. In this paper the thermal stability of SRF cavities is analysed using analytical and numerical simulation models. The effects of the most relevant parameters (i.e. defect size, RF frequency, thermal conductivity, cooling conditions, etc.) having an incidence on the cavity quench are studied. The steady-state and the transient solutions are presented in two cases : the defect free surface, and the micron size defect on the cavity surface. Experimental observations of the thermal events occuring during the quench have been obtained with the help of two diagnostic systems: surface thermometers working in superfluid helium and RF measurements. All the proposed experimental and modelling methods could contribute to get a more complete insight on the thermal effects taking place in the cavity wall.

ESS Superconducting RF Collaboration

The European Spallation Source (ESS) project is a neutron-scattering facility, currently under construction by a partnership of at least 17 European countries, with Sweden and Denmark as host nations. The ESS was designated a European Research Infrastructure Consortium, or ERIC, by the European Commission in October of 2015. Scientists and engineers from 50 different countries are members of the workforce in Lund who participate in the design and construction of the European Spallation Source. In complement to the local workforce, the superconducting RF linear accelerator is being prototyped and will be constructed based on a collaboration with European institutions: CEA-Saclay, CNRS-IPN Orsay, INFN-LASA, STFC-Daresbury, Uppsala and Lund Universities. After a description of the ESS collaborative project and its in-kind model for the SRF linac, this article will introduce the linac component first results.

ESS Spoke Cryomodule and Test Valve Box

The future European Spallation Source is based on a superconducting linac which includes a Spoke section. For prototyping purpose, a Spoke cryomodule and a valve box are being built and will be assembled by the end of 2015 and tested at full power. Design, fabrication and procurement of those components provide an insight of the series phases. The valve box will also be used for the test of the 13 series cryomodules.

Design and test of superconducting RF cavity prototypes for high intensity proton accelerators

High intensity proton beams, in the multi-MW range (typically 1 GeV and a few mA) are considered today for different applications: neutron sources, nuclear waste transmutation, radioactive ion beams and neutrino factories. All the foreseen projects are based on superconducting RF cavities for the high energy part of the linac accelerator between 100 MeV and 1 GeV. In this paper we present conceptual and experimental work made by the French group in the R&D preliminary phase. The aim of this study was to design an optimized cavity prototype integrating the more recent progress on RF superconductivity in terms of fabrication and preparation techniques. To reach high accelerating gradients while keeping safety margins and good reliability imposes careful cavity geometry optimization and detailed study of some important technological issues. The most relevant results obtained with several cavity prototypes (accelerating gradient, multipactor,...) are presented. Some other important components of the cavity (helium tank and cold tuner) are also discussed.