The Physics of Spallation Processes

2.3. THE EUROPEAN SPALLATION NEUTRON SOURCE ESS 17Figure 2.5: ACCEL-ModulThe compressor-ringsThe negatively charged hydrogen (H − ) ions are passed through a foil, which strips offeach ion’s two electrons, converting it into a proton. The protons likewise pass into tworings where they accumulate in bunches. Space charge problems and low injection lossesrequire the use of two compressor storage rings, one on top of the other. Approximately1000 turns are accumulated and all 2×10 14 protons are kicked out at once, producinga pulse less than 1µs in duration. Powers in the 5 megawatt range (time average) arebeyond the current technology limits for a single ring. Of several possible options for thelinac/ring combination, a solution was favored with a supra-conducting linac deliveringthe full energy of 1.334 GeV and two compressor rings working in parallel rather than alower-energy linac and a rapid cycling synchrotron.The target stationsThe science advisory group has reassessed the ESS proposal from 1996 and at the workshopin Engelberg [Eng01] the need was clearly demonstrated to have both a short pulse targetstation (SPTS, 50 Hz, 1.4µs, 5 MW) and a long pulse target station (LPTS, 16 2 Hz, 2 ms,35MW). Thus the ESS linac is required to deliver additionally 2 ms long 1.334 GeV beamsof about 110 mA peak current interleaved between the 50 Hz pulses drawn directly intothe target station. Compared to the short pulses, in this case the number of neutronsper pulse increases by a factor of 3. It is conceivable that following an optimization theneutron flux available from the moderators could increase by factors of 2-3 for the LPTS.The two target stations could be of essentially identical design, as far as the targetsand their handling systems are concerned. Differences may result in the moderator layoutand in the beam lines. Good cryogenic moderators are, therefore, at a premium, and aninitiative has been started, to develop an advanced system based on solid pellets of highslowing down power which are annealed periodically to limit radiation damage and torelease stored energy at regular intervals.The horizontal beam injection for both systems (SPTS and LPTS) and the liquid stateof Mercury at room temperature (melting point -38 ◦ C, boiling point at ambient pressure