- Email: [email protected]
The European Spallation Source
PHYSICAB ELSEVIER
Physica B 213&214 (1995) 1037 1041
The European Spallation Source A.D.
Taylor
ISIS Facilio,, Rutherford Appleton Laborator3,, Chilton. Oxon, UK
Abstract The European Spallation Source is a proposed third-generation neutron source designed to meet the anticipated needs of European scientists and technologists in the 21st century. The current status of the project is described together with a discussion of the strategic development of neutron scattering in Europe.
1. Introduction Neutron scattering provides basic microscopic information on the structure and dynamics of materials which underpins our understanding of condensed matter in fields as diverse as biology, materials science, chemistry, the earth sciences and physics. In the Large Facilities Report to the Commission of the European Community (CEC) in 1990, the Neutron Study Panel underlined the continuing need for the neutron scattering as a microscopic probe of the condensed state, and recognised that a major initiative was necessary to secure an effective ongoing neutron science programme in Europe for the year 2000 and beyond. The Panel recommended that a design study be initiated for a next generation neutron source. Subsequently, in a joint initiative by KFA Jfilich and Rutherford Appleton Laboratory, a series of meetings have been held to explore options for such a next generation European Neutron Source. These meetings focused on the provision and utilisation of an advanced high-powered accelerator-driven pulsed spallation source and the scientific opportunities that it would herald. The source specification for the European Spallation Source (ESS) is based on a proton accelerator producing a pulsed beam with an average beam power of 5 MW in ~ 1 ~ts pulses at a repetition rate of 50 Hz. Two target stations are envisaged, one for operation at 10 Hz for high-resolution and long-wavelength instruments and
one operating at 50 Hz for high-intenstiy applications. The neutronic performance anticipated from this specification will make the European Spallation Source some 30 times brighter than ISIS, presently the world's most powerful source of this type. The ESS will have a time averaged thermal flux roughly equivalent to that of the High Flux Reactor at the Institut Laue Langevin (ILL), but because of its sharp pulse structure, the peak flux of the ESS will be more than a factor 40 greater than this average. Accelerator concepts have been identified which will meet this specification using technologies which are considered by Expert Meetings to be within attainable goals. A target design concept to dissipate 5 MW and an appropriate moderator system although challenging was thought to be realisable with present-day technology. The scientific benefit that the ESS would bring to a wide range of scientific disciplines was considered in detail at the Expert Meeting on Instrumentation & Techniques [-1]. It was confidently predicted that the enhanced temporal brightness of the ESS would allow the source to make both a major impact on established fields and substantial contributions to new areas of research. A two year site-independent study was identified which would investigate various aspects of accelerator technology and confirm the feasibility of a target design and neutronic performance of the moderators. Full project costing would be established at the 20% precision
0921-4526/95/$09.50 < 1995 Elsevier Science B.V. All rights reserved SSDI 0 9 2 1 - 4 5 2 6 ( 9 5 ) 0 0 3 5 8 - 4
1038
A.D. Taylor/Physica B 213&214 (1995) I037 1041
level. To achieve this, the early participation of industry would be actively sought. It was estimated that some 40 man-years of effort would be required for this study. The CEC Human Capital and Mobility Programme has provided funding for 50% of this effort with the remainder being found from the internal resources of the participating institutes. Neutron scattering technique development is the subject of a parallel European-wide development programme under the same initiative. Construction of the European Spallation Source would maintain Europe in the pre-eminent position that it has held for the past 20 years in the field of neutron scattering. In addition, it is anticipated that the ESS would have a major impact on the other areas such as neutrino physics and muon science, and would provide a strategic opportunity to develop high-powered accelerator technology with potential applications in nuclear waste management and isotope production.
2. Neutrons
an ideal probe for condensed matter
Neutrons are an expensive probe: we consistently argue that if a problem can be solved by another technique then there is no need to resort to neutron scattering. This does not mean that neutrons are not cost effective. Indeed, for many problems neutron scattering is the technique of choice. The achievements of steady-state neutron sources such as the ILL or the newer pulsed sources such as ISIS are fully documented in the scientific literature, in review articles, in conference proceedings [2] and the annual reports of the facilities themselves [3]. The 1994 award of the Nobel Prize for physics underlines the importance of neutron scattering and its contribution to our detailed microscopic understanding of technically important materials such as plastics, proteins, polymers, fibres, liquid crystals, ceramics, hard magnets and superconductors as well as our understanding of fundamental phenomena such as phase transitions, quantum fluids and spontaneous ordering. The development of thirdgeneration synchrotron radiation sources, and other new techniques such as scanning tunnelling microscopy, will undoubtedly make an impact in these areas, but so will the enhancement of present day neutron sources and the development of a third-generation source dedicated to neutron scattering.
3. The enhancement of present day sources Should we develop new sources or fully utilise existing sources? The answer to this question is that, as with X-ray synchrotron sources, we should do both. The utilisation of current sources continues to improve: new
techniques are developed (e.g. spin-echo techniques and ~teV backscattering spectroscopy); better components are produced neutron guides, curved focusing monochromators and multi-detectors; more advanced sample environment equipment is realised; and our ability to understand and analyse the resulting data improves through advances in computing and development of new methods such as maximum entropy and reverse MonteCarlo analysis. There has been a long history of improvements to steady state source instrumentation, and more recently an even more rapid development in pulsedsource instrumentation, which fully bears out this approach. There are however limitations: • Instrument improvements are not always universally applicable and often there is a penalty to pay. For example focusing monochromators and guides increase the intensity at the sample at the expense of q-resolution. For some experiments this is acceptable; for others it is not. • Although many detector systems have already achieved reasonable efficiency (greater than 50% thus limiting any future improvement to at most a factor two), there is certainly scope to improve the speed of detectors (particularly for pulsed-source applications) and to reduce the cost. The dream is to produce a cheap pixellated detector which can provide 4~ coverage. But even here we must be cautious. Not all the 4~ is equivalent in a scattering experiment. High-pressure studies at ISIS (which at 300 kbar are an order of magnitude greater than any previous developments) utilise the favourable geometry of scattering a white neutron beam at 90 ° to eliminate background scattering from the massive pressure cell. Relevant present-day instruments at ISIS have the solid angle at 90 ° fully covered by detectors with 80% efficiency. In this case the only improvement that can be made is increasing the primary intensity on the sample. • There are many historical examples which show the new instrumentation and new higher intensity sources go hand in hand. A classical example was the development of the 'backscattering spectrometer' which revolutionised experiments on the dynamics of soft condensed matter at ILL. The principle was developed a generation earlier at a small reactor in Munich, but until the advent of the ILL high flux reactor, data collection times were so long that its potential could not be realised (typical collection times were 500 h). Experimentalists at ISIS have seen their source increase from 10 laA in the mid 1980s to 200 ~A less than a decade later. The result is that experiments which were inconceivable have become possible, and previously one-off 'demonstration' experiments have become routine.
AD. Taylor/ Physica B 213&214 (1995) 1037 1041
The case for developing third-generation sources with higher intensity will be made in the text section. The case for building more sources at the present intensity rather than fully exploiting present sources is less easy to justify since: • ILL currently has spare capacity high quality beams which can be made available at marginal costs; • a second target station at ISIS would double its capacity and improve the source quality for applications in soft condensed matter; • a second guide hall at Orph6e would similarly lead to enhanced capacity at a fraction of the cost of a new reactor; • enhancements of instruments at Riso and HMI would lead to substantial gains. Such full exploitation of the current generation of neutron sources in Europe must be a priority in the short term. The majority of neutron sources available today in Europe will, however, cease to exist by around the year 2000. Prof. R. Com6s predicted in his report to the OECD Megascience Forum [4] that, in addition to the state-of-the-art sources ISIS and ILL, probably only the HMI reactor in Berlin and the Orph6e reactor in Paris will be available at the turn of the century. ILL has recently been fully refurbished at the cost of 50 Mecu and this process can be repeated again. As the AUSTRON project shows, ISIS can be duplicated at the 300 Mecu level; a new reactor in Munich is likely to be constructed and medium flux reactors of the Orph6e type can probably be built again from around 300 Mecu. Full exploitation of the remaining sources is certainly a priority, but this policy alone will not satisfy the demands of the widening community of chemists, biologists, material scientists and physicists who use neutrons as a tool to probe the microscopic structure and dynamics of the condensed state. Some brief arguments for a third generation neutron source are given below. These are based on a preliminary scientific case outlined in Abingdon in 1992 I-1]. A full scientific case is being developed in a series of meetings to be organised in 1995 with support from the European Science Foundation. The cost effectiveness of a third-generation source is also discussed.
4. T h e need for a next generation neutron source
Neutron scattering is still very much an intensity limited technique. As source have developed in strength, the sophistication of the experiments possible has increased. Measurements of total cross-sections in the 1930s and 1940s gave way to differential scattering in the 1940s and 1950s which allowed structure to be explored. The high flux reactors of the 1960s and 1970s allowed dynamics to
1039
be studied and cold sources exploited; the 1980s saw limited use of polarisation analysis and the exploration of high energy and wide dynamic range studies on pulsed sources. All of these developments required increase in source strength as well as investment in instrumentation and techniques. The highest flux sources now allow timedependent variations of the simple experiments to be performed, and more sophisticated techniques such as double difference isotopic substitution experiments to be performed with favourable isotopes, giving previously unobtainable structural information about the liquid state. The expansion in the use of neutrons in recent years has come from chemists, biologists and material scientists seeing the potential of these techniques, in addition to the original physics-based community who developed them. Future user communities are interested in complex systems aqueous solutions, polymers, biological systems, advanced materials such as zeolite catalysts and superconductors, non-invasive measurements of real engineering components and in answers to complex questions related to the kinetics and time-dependent processes in these materials. Future measurements will be performed with higher precision and better resolution in a shorter time opening up the ability to study routinely parametric dependence on temperature, pressure, composition, magnetic or electric field, and following the kinetics in real time rather than simply increasing the throughput of experiments. In summary, a more intense source will allow: • Better Resolution: it is self evident-from astronomy (cf. the Hubble telescope) to microscopy that better resolution lead to better understanding of the system under study. • Smaller samples: either in the geometric sense (new materials are often only available in small quantities or as impure phases), or more dilute systems, allowing for example biological measurements to be performed under in vivo conditions. • More subtle effects: smaller cross-section processes in a wide range of systems will become amenable to study. The extremely powerful technique of isotopic difference has so far been confined to the set of elements with large differences in scattering lengths between isotopes. High intensity will allow this powerful technique to be generalised to include the biologically important atoms such as carbon and nitrogen. • More complex systems: the materials used in our everyday world are becoming increasingly complex- from superconductors to drugs to engineering components. • New technique will be developed, and existing technique such as polarisation analysis which in theory will give an extra dimension to the information in
1040
A.D. Taylor/Physica B 213&214 (1995) 103~1041
a wide range of systems polymers as well as magnetism - could become effective in practice. Polarisation analysis is currently the tool exploited only by physicists for a few limited problems e.g. in magnetism. If it is ever to realise its full potential for chemistry, biology and materials science, then a new generation of instruments on a higher intensity source will be required.
Y
1.334 GeV, 5_0 Hz, 3.8 mA Y 5 MW, H LINAC
II
5. The timeliness of the ESS study ISIS and ILL form a complementary pair: they both have unique features, but there is significant overlap in their capabilities. The development of either of these technologies would be an acceptable route to a third generation neutron source. For historical reasons, the US has already embarked on a 'super-ILL' study the Advanced Neutron Source project at ORNL. A project team has been in existence for more than a decade and a sophisticated, fully costed engineering design has been carried out. The estimated total project cost for this 350 MW reactor is some $ 3B. This may be taken as the reference case for a third generation reactor-based neutron source. The emergence of pulsed sources as a viable contender is more recent. ISIS at 160 kW is more than an order of magnitude more effective than, for example, the IPNS source built a decade earlier. The rapid advance in accelerator technology in the 1980s suggests that progression to a 5 MW pulsed spallation source is possible. Evaluation of the feasibility and costing of such a source is the basis of the recently funded European Spallation Source Study.
6. Reference parameters for the ESS study The proposed 5 MW beam power can be obtained by a combination of a linac with an accumulator ring or a rapid cycling synchrotron [5]. In the first option, shown in Fig. 1, an energy of 1.334 GeV has been chosen, based on the requirements of low-loss H - injection into the accumulator rings. Two 60 mA H ion sources feed two independent 175 MHz RFQs. These two beam lines are funnelled at 5 7 MeV into a 350 MHz drift-tube linac for acceleration to 100-150 MeV. Further acceleration to 1.334 GeV will be achieved in a normal or superconducting coupled-cavity linac. High efficiency H - injection of 1000 turns into each of the two accumulator rings is achieved using an elegant painting scheme in the longitudinal and both transverse planes. Compression to 390 ns is followed by sequential extraction giving the desired 1 ~s pulse to either of the two targets. A 50 Hz highpower (5 MW) high-intensity target design based on stationary, rotating and liquid metal target concepts is
HIGH POWER BEAM DUMP L ej MUON AREA 60 ~JA FOR OTHER USERS~ ~HHo~
]3NO 1.334 GeV, 50 Hz 1.9 mA ACCUMULATORS 50 Hz TARGET
0.75 mA
10 Hz TARGET
being considered as well as a 10 Hz stationary low power (1 MW) target for high-resolution and cold-neutron instrumentation. Problem areas identified so far are the ion source, losses in the linac and ring (to allow hands-on maintenance) and thermal shock and radiation damage problems associated with target materials, A second more expensive option using a linac and two rapid cycling synchrotrons to accelerate from 0.8 to 3.0 GeV will be considered if anticipated damage in the target suggests a lower current solution. As the ANS experience has shown, the time scale for realising a next generation neutron source is long. Beginning the ESS study now is consistent with having the ESS fully available for science in the year 2010: Feasibility study R & D phase Design and funding Construction Commissioning
1994~1996 1996-1998 1998-2000 200~2007 2007 2010.
A.D. Tavlor/Physica B 213&214 (1995) 1037 1041
If the capital and operating cost of ESS were to scale with its power, then the project would indeed be unrealisable. Advances in accelerator technology have led to much more realisable designs, and the target station costs are dominated by shielding considerations which only scale logarithmically with beam power. It is possible to contemplate 5 M W accelerator (30 × ISIS) together with two target stations for three times the replacement cost of ISIS. The wall-plug efficiency of such an accelerator has also much improved over ISIS. ISIS requires 8 M W to produce a 160 kW beam; the ESS is estimated to require only three times the wall plug power to realise 30 times the beam power. In addition, the overall operating costs of such large user facilities (e.g. ISIS and ILL) are dominated by staff costs which again do not scale with increased intensity.
7. Conclusion Neutron scattering is a vibrant and vital technique which contributes greatly to our understanding of an
1041
increasingly diverse set of problems in physics, chemistry, materials science, earth science and biology. Scientific advances in the short term depend on the full exploitation of present sources and the development of current instrumentation. In the long term the development of a next generation source is essential. A good balance between these short term needs and long term goals is needed for the health of European condensed matter research.
References [1] A.D. Taylor (ed.), Instrumentation and Techniques for the European Spallation Source, RAL-92-040. [2] See, for example, these proceedings. [3] ISIS 94 or ILL Annual Report for 1993. [4] R. Com6s, Synchrotron Radiation and Neutron Beams, OECD Megascience Forum, Riso 1993. [5] H. Lengeler, 4th European Particle Accelerator Conference, EPAC 94.
Physica B 213&214 (1995) 1037 1041
The European Spallation Source A.D.
Taylor
ISIS Facilio,, Rutherford Appleton Laborator3,, Chilton. Oxon, UK
Abstract The European Spallation Source is a proposed third-generation neutron source designed to meet the anticipated needs of European scientists and technologists in the 21st century. The current status of the project is described together with a discussion of the strategic development of neutron scattering in Europe.
1. Introduction Neutron scattering provides basic microscopic information on the structure and dynamics of materials which underpins our understanding of condensed matter in fields as diverse as biology, materials science, chemistry, the earth sciences and physics. In the Large Facilities Report to the Commission of the European Community (CEC) in 1990, the Neutron Study Panel underlined the continuing need for the neutron scattering as a microscopic probe of the condensed state, and recognised that a major initiative was necessary to secure an effective ongoing neutron science programme in Europe for the year 2000 and beyond. The Panel recommended that a design study be initiated for a next generation neutron source. Subsequently, in a joint initiative by KFA Jfilich and Rutherford Appleton Laboratory, a series of meetings have been held to explore options for such a next generation European Neutron Source. These meetings focused on the provision and utilisation of an advanced high-powered accelerator-driven pulsed spallation source and the scientific opportunities that it would herald. The source specification for the European Spallation Source (ESS) is based on a proton accelerator producing a pulsed beam with an average beam power of 5 MW in ~ 1 ~ts pulses at a repetition rate of 50 Hz. Two target stations are envisaged, one for operation at 10 Hz for high-resolution and long-wavelength instruments and
one operating at 50 Hz for high-intenstiy applications. The neutronic performance anticipated from this specification will make the European Spallation Source some 30 times brighter than ISIS, presently the world's most powerful source of this type. The ESS will have a time averaged thermal flux roughly equivalent to that of the High Flux Reactor at the Institut Laue Langevin (ILL), but because of its sharp pulse structure, the peak flux of the ESS will be more than a factor 40 greater than this average. Accelerator concepts have been identified which will meet this specification using technologies which are considered by Expert Meetings to be within attainable goals. A target design concept to dissipate 5 MW and an appropriate moderator system although challenging was thought to be realisable with present-day technology. The scientific benefit that the ESS would bring to a wide range of scientific disciplines was considered in detail at the Expert Meeting on Instrumentation & Techniques [-1]. It was confidently predicted that the enhanced temporal brightness of the ESS would allow the source to make both a major impact on established fields and substantial contributions to new areas of research. A two year site-independent study was identified which would investigate various aspects of accelerator technology and confirm the feasibility of a target design and neutronic performance of the moderators. Full project costing would be established at the 20% precision
0921-4526/95/$09.50 < 1995 Elsevier Science B.V. All rights reserved SSDI 0 9 2 1 - 4 5 2 6 ( 9 5 ) 0 0 3 5 8 - 4
1038
A.D. Taylor/Physica B 213&214 (1995) I037 1041
level. To achieve this, the early participation of industry would be actively sought. It was estimated that some 40 man-years of effort would be required for this study. The CEC Human Capital and Mobility Programme has provided funding for 50% of this effort with the remainder being found from the internal resources of the participating institutes. Neutron scattering technique development is the subject of a parallel European-wide development programme under the same initiative. Construction of the European Spallation Source would maintain Europe in the pre-eminent position that it has held for the past 20 years in the field of neutron scattering. In addition, it is anticipated that the ESS would have a major impact on the other areas such as neutrino physics and muon science, and would provide a strategic opportunity to develop high-powered accelerator technology with potential applications in nuclear waste management and isotope production.
2. Neutrons
an ideal probe for condensed matter
Neutrons are an expensive probe: we consistently argue that if a problem can be solved by another technique then there is no need to resort to neutron scattering. This does not mean that neutrons are not cost effective. Indeed, for many problems neutron scattering is the technique of choice. The achievements of steady-state neutron sources such as the ILL or the newer pulsed sources such as ISIS are fully documented in the scientific literature, in review articles, in conference proceedings [2] and the annual reports of the facilities themselves [3]. The 1994 award of the Nobel Prize for physics underlines the importance of neutron scattering and its contribution to our detailed microscopic understanding of technically important materials such as plastics, proteins, polymers, fibres, liquid crystals, ceramics, hard magnets and superconductors as well as our understanding of fundamental phenomena such as phase transitions, quantum fluids and spontaneous ordering. The development of thirdgeneration synchrotron radiation sources, and other new techniques such as scanning tunnelling microscopy, will undoubtedly make an impact in these areas, but so will the enhancement of present day neutron sources and the development of a third-generation source dedicated to neutron scattering.
3. The enhancement of present day sources Should we develop new sources or fully utilise existing sources? The answer to this question is that, as with X-ray synchrotron sources, we should do both. The utilisation of current sources continues to improve: new
techniques are developed (e.g. spin-echo techniques and ~teV backscattering spectroscopy); better components are produced neutron guides, curved focusing monochromators and multi-detectors; more advanced sample environment equipment is realised; and our ability to understand and analyse the resulting data improves through advances in computing and development of new methods such as maximum entropy and reverse MonteCarlo analysis. There has been a long history of improvements to steady state source instrumentation, and more recently an even more rapid development in pulsedsource instrumentation, which fully bears out this approach. There are however limitations: • Instrument improvements are not always universally applicable and often there is a penalty to pay. For example focusing monochromators and guides increase the intensity at the sample at the expense of q-resolution. For some experiments this is acceptable; for others it is not. • Although many detector systems have already achieved reasonable efficiency (greater than 50% thus limiting any future improvement to at most a factor two), there is certainly scope to improve the speed of detectors (particularly for pulsed-source applications) and to reduce the cost. The dream is to produce a cheap pixellated detector which can provide 4~ coverage. But even here we must be cautious. Not all the 4~ is equivalent in a scattering experiment. High-pressure studies at ISIS (which at 300 kbar are an order of magnitude greater than any previous developments) utilise the favourable geometry of scattering a white neutron beam at 90 ° to eliminate background scattering from the massive pressure cell. Relevant present-day instruments at ISIS have the solid angle at 90 ° fully covered by detectors with 80% efficiency. In this case the only improvement that can be made is increasing the primary intensity on the sample. • There are many historical examples which show the new instrumentation and new higher intensity sources go hand in hand. A classical example was the development of the 'backscattering spectrometer' which revolutionised experiments on the dynamics of soft condensed matter at ILL. The principle was developed a generation earlier at a small reactor in Munich, but until the advent of the ILL high flux reactor, data collection times were so long that its potential could not be realised (typical collection times were 500 h). Experimentalists at ISIS have seen their source increase from 10 laA in the mid 1980s to 200 ~A less than a decade later. The result is that experiments which were inconceivable have become possible, and previously one-off 'demonstration' experiments have become routine.
AD. Taylor/ Physica B 213&214 (1995) 1037 1041
The case for developing third-generation sources with higher intensity will be made in the text section. The case for building more sources at the present intensity rather than fully exploiting present sources is less easy to justify since: • ILL currently has spare capacity high quality beams which can be made available at marginal costs; • a second target station at ISIS would double its capacity and improve the source quality for applications in soft condensed matter; • a second guide hall at Orph6e would similarly lead to enhanced capacity at a fraction of the cost of a new reactor; • enhancements of instruments at Riso and HMI would lead to substantial gains. Such full exploitation of the current generation of neutron sources in Europe must be a priority in the short term. The majority of neutron sources available today in Europe will, however, cease to exist by around the year 2000. Prof. R. Com6s predicted in his report to the OECD Megascience Forum [4] that, in addition to the state-of-the-art sources ISIS and ILL, probably only the HMI reactor in Berlin and the Orph6e reactor in Paris will be available at the turn of the century. ILL has recently been fully refurbished at the cost of 50 Mecu and this process can be repeated again. As the AUSTRON project shows, ISIS can be duplicated at the 300 Mecu level; a new reactor in Munich is likely to be constructed and medium flux reactors of the Orph6e type can probably be built again from around 300 Mecu. Full exploitation of the remaining sources is certainly a priority, but this policy alone will not satisfy the demands of the widening community of chemists, biologists, material scientists and physicists who use neutrons as a tool to probe the microscopic structure and dynamics of the condensed state. Some brief arguments for a third generation neutron source are given below. These are based on a preliminary scientific case outlined in Abingdon in 1992 I-1]. A full scientific case is being developed in a series of meetings to be organised in 1995 with support from the European Science Foundation. The cost effectiveness of a third-generation source is also discussed.
4. T h e need for a next generation neutron source
Neutron scattering is still very much an intensity limited technique. As source have developed in strength, the sophistication of the experiments possible has increased. Measurements of total cross-sections in the 1930s and 1940s gave way to differential scattering in the 1940s and 1950s which allowed structure to be explored. The high flux reactors of the 1960s and 1970s allowed dynamics to
1039
be studied and cold sources exploited; the 1980s saw limited use of polarisation analysis and the exploration of high energy and wide dynamic range studies on pulsed sources. All of these developments required increase in source strength as well as investment in instrumentation and techniques. The highest flux sources now allow timedependent variations of the simple experiments to be performed, and more sophisticated techniques such as double difference isotopic substitution experiments to be performed with favourable isotopes, giving previously unobtainable structural information about the liquid state. The expansion in the use of neutrons in recent years has come from chemists, biologists and material scientists seeing the potential of these techniques, in addition to the original physics-based community who developed them. Future user communities are interested in complex systems aqueous solutions, polymers, biological systems, advanced materials such as zeolite catalysts and superconductors, non-invasive measurements of real engineering components and in answers to complex questions related to the kinetics and time-dependent processes in these materials. Future measurements will be performed with higher precision and better resolution in a shorter time opening up the ability to study routinely parametric dependence on temperature, pressure, composition, magnetic or electric field, and following the kinetics in real time rather than simply increasing the throughput of experiments. In summary, a more intense source will allow: • Better Resolution: it is self evident-from astronomy (cf. the Hubble telescope) to microscopy that better resolution lead to better understanding of the system under study. • Smaller samples: either in the geometric sense (new materials are often only available in small quantities or as impure phases), or more dilute systems, allowing for example biological measurements to be performed under in vivo conditions. • More subtle effects: smaller cross-section processes in a wide range of systems will become amenable to study. The extremely powerful technique of isotopic difference has so far been confined to the set of elements with large differences in scattering lengths between isotopes. High intensity will allow this powerful technique to be generalised to include the biologically important atoms such as carbon and nitrogen. • More complex systems: the materials used in our everyday world are becoming increasingly complex- from superconductors to drugs to engineering components. • New technique will be developed, and existing technique such as polarisation analysis which in theory will give an extra dimension to the information in
1040
A.D. Taylor/Physica B 213&214 (1995) 103~1041
a wide range of systems polymers as well as magnetism - could become effective in practice. Polarisation analysis is currently the tool exploited only by physicists for a few limited problems e.g. in magnetism. If it is ever to realise its full potential for chemistry, biology and materials science, then a new generation of instruments on a higher intensity source will be required.
Y
1.334 GeV, 5_0 Hz, 3.8 mA Y 5 MW, H LINAC
II
5. The timeliness of the ESS study ISIS and ILL form a complementary pair: they both have unique features, but there is significant overlap in their capabilities. The development of either of these technologies would be an acceptable route to a third generation neutron source. For historical reasons, the US has already embarked on a 'super-ILL' study the Advanced Neutron Source project at ORNL. A project team has been in existence for more than a decade and a sophisticated, fully costed engineering design has been carried out. The estimated total project cost for this 350 MW reactor is some $ 3B. This may be taken as the reference case for a third generation reactor-based neutron source. The emergence of pulsed sources as a viable contender is more recent. ISIS at 160 kW is more than an order of magnitude more effective than, for example, the IPNS source built a decade earlier. The rapid advance in accelerator technology in the 1980s suggests that progression to a 5 MW pulsed spallation source is possible. Evaluation of the feasibility and costing of such a source is the basis of the recently funded European Spallation Source Study.
6. Reference parameters for the ESS study The proposed 5 MW beam power can be obtained by a combination of a linac with an accumulator ring or a rapid cycling synchrotron [5]. In the first option, shown in Fig. 1, an energy of 1.334 GeV has been chosen, based on the requirements of low-loss H - injection into the accumulator rings. Two 60 mA H ion sources feed two independent 175 MHz RFQs. These two beam lines are funnelled at 5 7 MeV into a 350 MHz drift-tube linac for acceleration to 100-150 MeV. Further acceleration to 1.334 GeV will be achieved in a normal or superconducting coupled-cavity linac. High efficiency H - injection of 1000 turns into each of the two accumulator rings is achieved using an elegant painting scheme in the longitudinal and both transverse planes. Compression to 390 ns is followed by sequential extraction giving the desired 1 ~s pulse to either of the two targets. A 50 Hz highpower (5 MW) high-intensity target design based on stationary, rotating and liquid metal target concepts is
HIGH POWER BEAM DUMP L ej MUON AREA 60 ~JA FOR OTHER USERS~ ~HHo~
]3NO 1.334 GeV, 50 Hz 1.9 mA ACCUMULATORS 50 Hz TARGET
0.75 mA
10 Hz TARGET
being considered as well as a 10 Hz stationary low power (1 MW) target for high-resolution and cold-neutron instrumentation. Problem areas identified so far are the ion source, losses in the linac and ring (to allow hands-on maintenance) and thermal shock and radiation damage problems associated with target materials, A second more expensive option using a linac and two rapid cycling synchrotrons to accelerate from 0.8 to 3.0 GeV will be considered if anticipated damage in the target suggests a lower current solution. As the ANS experience has shown, the time scale for realising a next generation neutron source is long. Beginning the ESS study now is consistent with having the ESS fully available for science in the year 2010: Feasibility study R & D phase Design and funding Construction Commissioning
1994~1996 1996-1998 1998-2000 200~2007 2007 2010.
A.D. Tavlor/Physica B 213&214 (1995) 1037 1041
If the capital and operating cost of ESS were to scale with its power, then the project would indeed be unrealisable. Advances in accelerator technology have led to much more realisable designs, and the target station costs are dominated by shielding considerations which only scale logarithmically with beam power. It is possible to contemplate 5 M W accelerator (30 × ISIS) together with two target stations for three times the replacement cost of ISIS. The wall-plug efficiency of such an accelerator has also much improved over ISIS. ISIS requires 8 M W to produce a 160 kW beam; the ESS is estimated to require only three times the wall plug power to realise 30 times the beam power. In addition, the overall operating costs of such large user facilities (e.g. ISIS and ILL) are dominated by staff costs which again do not scale with increased intensity.
7. Conclusion Neutron scattering is a vibrant and vital technique which contributes greatly to our understanding of an
1041
increasingly diverse set of problems in physics, chemistry, materials science, earth science and biology. Scientific advances in the short term depend on the full exploitation of present sources and the development of current instrumentation. In the long term the development of a next generation source is essential. A good balance between these short term needs and long term goals is needed for the health of European condensed matter research.
References [1] A.D. Taylor (ed.), Instrumentation and Techniques for the European Spallation Source, RAL-92-040. [2] See, for example, these proceedings. [3] ISIS 94 or ILL Annual Report for 1993. [4] R. Com6s, Synchrotron Radiation and Neutron Beams, OECD Megascience Forum, Riso 1993. [5] H. Lengeler, 4th European Particle Accelerator Conference, EPAC 94.