The range of options.

Hundreds of European neutron experts have been responsible for developing a world leading ESS Project for which a detailed design and costing of a facility consisting of a 5 MW SPTS and a 5 MW LPTS, both using liquid Hg has been published. In the final stages of the project , the Scientific Advisory Committee of ESS concluded on the basis of its analysis of the performance of the instruments on the two target stations, that a staged approach starting with the LPTS would offer the most novel and complementary opportunities. It would also be technically less demanding and considerably cheaper to begin with. In the phase of consolidating the results of all the technical work carried out for ESS to initial thoughts on optimising such a LPTS first facility by considering higher power levels. A main conclusion was that the pitting problem in the Hg-target would be considerably less serious compared to the 5 MW SPTS even at energies of 10 MW. H+-ion sources to produce those power levels are state of the art but basically available. Another conclusion was that a later decision to include a SPTS, which involves replacing the H+-ion source by one or more H- sources, would not be inhibitively expensive (of the order of 50 M€).

This defines the range of eventual options for Europe’s next top tier source, if new technical considerations will not point to other possibilities. ESS-I very much concurs with the assessment of Carpenter et.al. that for a facility the construction of which is to start in this decade, no alternatives promising enough exist. Worthwhile prototyping of especially FFAG accelerator technologies for proton accelerators are taking place now, and deserve certainly close attention, but they are generally judged not yet mature enough the next few years as the basis for a high power proton accelerator that must provide a higly reliable beam at a high level of availability on FFAG technologies.

Comparing performance
The ESFRI Neutron WG, reporting early 2003, has slightly enhanced the methodology of assessing instrument performance done by the ESS Project to compare source performance, to arrive at a comparison of four basic options for Europe: the full ESS, a 5 MW LPTS facility, a 1 MW stand-alone option, and an upgrade of ISIS to 1 MW.

Using the 1.4 MW SNS as a yardstick, the performance of the four scenarios were summarised by establishing whether a particular scenario would provide World Lead over, Some Lead over, or be Competitive to the 1.4 MW SNS in eight science areas (Solid State Physics, Material Science and Engineering, Liquids and Glasses, Soft Condensed Matter, Chemical Structure, Kinetics and Dynamics, Biology and Biotechnology, Mineral Science, Earth Science, Environment and Cultural Heritage, and Fundamental Physics) that were linked to crucial European Priority Research Missions.

This analysis still is the best available to identify the performance potential of various sources, by separating source performance and instrument performance, based on the assumption that with a delay of a number of years instrument improvements will find their way to all facilities with sufficient resources to continually refurbish their instruments as well as their infrastructure.

The outcome is that a full ESS that combines the potential of a 5 MW SPTS in particular areas of science with the unique opportunities of a 5 MW LPTS in other areas, would provide World Lead in all areas. A 5 MW LPTS facility would provide World Lead in the important areas of Soft Condensed Matter and Biology and Biotechnology as well as in Fundamental Physics, and some lead in all other areas. The 1 MW scenarios would be competitive, with the addition that the stand-alone scenario that was an theoretical extrapolation of the 10 Hz AUSTRON design, would give some lead in Soft Condensed Matter and in Fundamental Physics.

Considerations underlying the choice of the facility.

ESS-I is of the opinion that three main considerations must guide the choice of the facility Europe should build.

• A new European facility will not produce its first neutrons before 2017 or 2018 given the long lead times. It will trail SNS by 10+ years. The goal must therefore be, as always in science, to be substantially better, say “5x SNS” in many areas.

• Cost-effectiveness dictates that eventually the facility must have as many instruments as possible. Many of them will be unique instruments capable of utilising the full power of ESS stage one, but the facility will inevitably also become the major workhorse for neutron science and technology because of its short throughput times and advanced support facilities. That does not mean that it will replace all existing sources. It has rightly been remarked (McGreevy, informal discussions) that only a network of bigger and smaller sources will support a sustainable community.

• As a staged approach seems the best way forward, for both technical and financial reasons, it makes sense to choose the concept in such a way that the initial areas of excellence of ESS stage one are complementary to what other spallation facilities, which are all SP facilities, offer.