Summarized are properties of 10 MW ESS H- SC Reference Linac, as described in ESS Update Report 2003. Particles are either injected into 2 accumulator rings, resulting in 1.4 μs pulses sent to a short pulse target station, or 2 ms long pulses directed to a long pulse target station. The 10 MW, 1.334 GeV ESS H-linac is 570m in length, modular cryomodules are used above 400 MeV with separated warm quads. After winding up the ESS Council and the Technical Team by the end of 2003, the neutron scattering community wanted to keep ESS on the political table and demonstrate to young scientist that the struggle for ESS goes on. For this purpose, the ESS-Initiative ( ESS-I ) was formed to include the European Neutron Scattering Association (ENSA), various consortia for site candidatures and some key European laboratories. A major highlight was the successful promotion of ESS to be as a high maturity project on the European Road Map of Research Infrastructures, published in October 2006 by the European Strategy Forum on Research Infrastructures (ESFRI). The ESS Preparatory Phase project has been accepted in the meanwhile after a very positive assessment and has been allocated 5 M€ EU support to investigate site independent technical items and answer legal, financial and governance questions, starting January 2008. ESS 07 will be a 5 MW LP source, the worlds first one to offer the unique capabilities of novel long pulse instruments for almost all of the most interesting areas of science and technology. ESS will be built in such a way that governments can decide later on during its lifetime to increase power and add more target stations, as the future scientific need would arise. The 5 MW, 1 GeV ESS H+ linac is similar to the ESS H- SC Reference Linac, but only 464m in length, as more intense H+ sources are available. ESS’s world lead would become even more pronounced, even when SNS would upgrade its power to, say, 3 MW, if the ESS beam power would be increased to 7.5 MW. This goal is achievable by enlarging the linac tunnel by 115m, which allows to add later on 16 cyromodules, and increasing the accelerating gradient in all SC cavities by 15 %. The H+ linac length of 579 m in this case would still not be larger than the ESS H- SC Reference Linac. Synergy with the CERN plans would require to avoid the funnel section at 20 MeV, leading to a 5 MW LP power at 2 GeV.Linac length is unchanged by using accelerating high gradient in SC cavities. SPL frequencies can be used, but components will differ, as ESS LP source requires high current and long pulses, whereas SPL linac is designed for high energy and high rep. rate
