3. ACCELERATOR CHARACTERISTICS, CLASSIFICATION AND
3.8. New facilities and technology
The technologies and facilities listed in the previous sections should not be taken as an exhaustive list of the complete range of accelerator facilities. New facilities and technologies are emerging at the time of writing, possibly creating new challenges at the time of their decommissioning.
A short, non-exhaustive overview of the emerging facilities and technologies is given in the following subsections, first listing facilities under construction and for which a detailed technical design is available, followed by the current trends in accelerator studies.
3.8.1. Emerging technologies
Plasma acceleration is a technique for accelerating charged particles, such as electrons, positrons and ions, using an electrical field associated with an electron plasma wave. The wave is created either through electron pulses or through the passage of very brief laser pulses [35]. The plasma accelerator is a relatively new technique of particle acceleration in which particles ‘surf’ on a wave of plasma. The plasma consists of a fluid of positively and negatively charged particles, generally created by heating a dilute gas.
Laser plasma acceleration is based on different physical principles than typical particle acceleration, which to date has relied on electric fields generated by radiowaves to accelerate electrons and other particles close to the speed of light. A variety of approaches has been proposed to accelerate particles by laser fields; of the other approaches considered, the laser wakefield acceleration in low density plasma is one of the most promising. It potentially allows the achievement of energies up to approximately 1 GeV in very small accelerator chambers with a dimension of centimetres, instead of hundreds of metres. Such a system could use a laser pulse to create a charge ‘wake’ in a low density plasma medium, where particles would ride the plasma wake (like a surfer who has caught a good wave) to achieve ever increasing speeds. Many technical challenges require resolution before tabletop accelerators and plasma driven turbochargers, as part of larger accelerators, are likely to become a reality.
FIG. 10. View of the 3.2 km long accelerator at SLAC National Accelerator Laboratory (courtesy of SLAC National Accelerator Laboratory).
Plasma based accelerators have been successfully used to accelerate electrons, protons, deuterons and other ions. The acceleration of electrons is obtained by hitting a gas jet with a very powerful laser beam. Electrons can be accelerated to energies of hundreds of mega-electronvolts and, after the interaction with the experimental chamber walls and surrounding material, will generate prompt radiation, as bremsstrahlung photons, neutrons and other particles (charged particles, ions, nuclear fragments and delayed radiation). The resulting prompt radiation is more complex the higher the energy of the accelerated particles [36, 37].
Rapid progress in the development of high intensity laser–matter interactions into the relativistic domain has provided several applications by generating intense particle sources [38]. Proton energies in the order of tens of mega-electronvolts have been obtained, and the possibility of producing medical radionuclides has been proven [39]. Some initial reports have been published on radiation protection issues with this type of accelerator and on the activation aspects that may be relevant to decommissioning [40, 41]. These estimates indicate limited activation of the system itself and of surrounding materials.
However, at the time of writing this publication, the amount of information available is very limited and mostly based on estimates rather than on experience collected in the use of the first prototypes. As research regarding this type of accelerator is still in development, and as technology for applications has not yet reached maturity, the decommissioning of the first experimental sites is far from being on the agenda. Nevertheless, the first experimental sites could undergo upgrades or partial modification that might lead to dismantling or recycling of at least some components. Accelerators of this type cannot yet be assigned to the classification system used in this publication (Classes 1–4) owing to the immaturity of design and the paucity of information that exists.
Nevertheless, initial tests and plans involving powerful lasers hint that giga-electronvolt energy range is within reach. The Extreme Light Infrastructure Beamlines facility, which is under construction in Romania, plans to accelerate electrons to 1 GeV and protons to 100 MeV.
In the design of new facilities, a precautionary approach should be considered, which would take into account the options to facilitate future decommissioning given in this publication for other types of accelerator.
3.8.2. New facilities under construction
Other facilities under construction include:
(a) The Facility for Antiproton and Ion Research (FAIR). This is a new, unique international accelerator facility specifically designed for research with antiprotons and ions. It is already under construction near Darmstadt, Germany. Its core, a double ring accelerator (SIS100 heavy ion synchrotron) with a circumference of 1100 m, will be associated with a complex system of cooler and storage rings and experimental set-ups.
The synchrotron will deliver ion beams of unprecedented intensities and energies. Thus, intensive secondary beams can be produced, providing antiprotons and exotic nuclei for groundbreaking experiments. It will provide unique accelerator and experimental facilities, allowing for a large variety of unprecedented forefront research in hadron, nuclear, atomic and plasma physics as well as applied sciences [42]. FAIR is expected to deliver beams for science experiments by 2025. Figure 11 is a schematic layout of the facility.
(b) Kō Enerugī Kasokuki Kenkyū Kikō (KEK), High Energy Accelerator Research Organization, Japan. The Super KEKB is under construction after replacing the KEKB. Two synchrotrons are being constructed for 7 GeV electron and 4 GeV positron colliding experiments. The luminosity of the Super KEKB will be 40 times greater than that of the KEKB.
(c) The International Fusion Materials Irradiation Facility. This is a joint nuclear fusion project between Japan, the Russian Federation, the USA and the European Union (EU) (Fig. 12), which is under construction in Aomori Prefecture in Japan [43]. The key objective of the facility is to study the behaviour of materials and components subjected to irradiation under conditions typically found in a nuclear fusion reactor. To achieve this, deuterium with a high beam current will be accelerated using two accelerators for 14 MeV neutron sources from a lithium target.
(d) JT-60SA. This is a fusion experiment designed to support the operation of the International Thermonuclear Experimental Reactor (ITER) (Fig. 13). ‘SA’ stands for ‘super, advanced’, since the experiment will have superconducting coils and study advanced modes of plasma operation [44]. The purpose of JT-60SA is to optimize the operation of fusion power plants that are built after ITER. It is a joint international research and
development project involving Japan and the EU and is to be built in Naka, Japan, using the infrastructure of the existing JT-60 Upgrade experiment.
In addition to the above large facilities, many accelerator projects are planned in Japan, such as an energy recovery linac for future light source, high f1ux neutron source for boron neutron capture therapy, plasma acceleration, and so on.
3.8.3. Facilities for which a conceptual design report is already available
Conceptual design reports are available for the following facilities:
(a) The European Spallation Source, which will be the most powerful long pulse source of neutrons at 5 MW. It is a co-hosted Swedish and Danish project, built in Lund, Sweden, with a data analysis centre in
FIG. 12. Layout of the International Fusion Materials Irradiation Facility (reproduced from Ref. [43]).
FIG. 11. Facility for Antiproton and Ion Research complex 2017 (courtesy of Facility for Antiproton and Ion Research/GSI Helmholtzzentrum für Schwerionenforschung).
Copenhagen and a laboratory test facility and accelerator component factory in Bilbao, Spain. Preliminary decommissioning studies have been conducted and can be found in Refs [45-47]. The preliminary estimate of the decommissioning costs amounts to €300 million [48].
(b) A future multitera-electronvolt e+ e– collider based on the Compact Linear Collider (CLIC) technology, which is under study. The CLIC concept is based on high gradient, normal-conducting accelerating structures, in which the radiofrequency power for the acceleration of the colliding beams is extracted from a high current drive beam that runs parallel with the main linac. The focus of CLIC research and development over recent years has been on addressing a set of key feasibility issues that are essential for proving the fundamental validity of the CLIC concept. The CLIC accelerator study is organized as an international collaboration with 43 partners in 22 countries. Several larger system tests have been performed to validate the two beam scheme;
of particular importance are the results from the CLIC test facility at CERN (CTF3). Both the machine and the detector and physics studies for CLIC have primarily focused on the 3 TeV implementation of CLIC as a benchmark for CLIC feasibility. Specific studies for an initial 500 GeV machine and some discussion of possible intermediate energy stages have also been performed. The performance and operational issues related to operation at reduced energy, compared with the nominal, and considerations of a staged construction programme are included in the conceptual design report [49].
(c) The International Linear Collider, for which the schematic layout is shown in Fig. 14. This is a proposed high luminosity linac, based on a 1.3 GHz superconducting radiofrequency accelerating technique, for which the technical design report was prepared in 2012 [50]. It is planned to produce, initially, collision energy of 500 GeV, with the possibility for a later upgrade to 1000 GeV (1 TeV). The host country for the accelerator has not yet been chosen, and proposed locations are Japan, the USA (Fermilab) and Europe (CERN). The
FIG. 13. The JT-60SA device (reproduced from Ref. [44]).
FIG. 14. Schematic layout of the International Linear Collider, not to scale (courtesy of the International Linear Collider project).
site specific design assumes a Japanese site, as a pre-project phase, to start operation around 2030. The International Linear Collider will make electrons collide with positrons. It will be between 30 and 50 km long, more than 10 times as long as the 50 GeV Stanford Linear Accelerator, the longest existing linac. This will be a Class 4 accelerator facility.
(d) MYRRHA, which is a multipurpose irradiation facility under study at the Belgian Nuclear Research Centre.
It is a flexible, fast spectrum research reactor (50–100 MW(th)), conceived as an accelerator driven system and able to operate in subcritical and critical modes. It contains a proton accelerator of 600 MeV, a spallation target and a multiplying core with mixed oxide fuel, cooled by liquid lead–bismuth. It is the first prototype in the world of a nuclear reactor driven by a particle accelerator. This type of facility will pose challenges at the time of decommissioning as it combines both accelerator and reactor specific problems [51].
(e) The International Fusion Materials Irradiation Facility, which is an accelerator based neutron source that will use deuterium–lithium stripping reactions to simulate 14 MeV neutrons from deuterium–tritium fusion reactions [43].
3.8.4. Installations currently under study
The following two installations are currently under study:
(a) CERN has initiated a global Future Circular Collider study [52] as a direct response to the recommendation made in the Update of the European Strategy for Particle Physics [53], adopted by the CERN Council at a special session in Brussels on 30 May 2013. It addresses both approaches within a single, worldwide scientific project:
— Study of a circular hadron collider (protons, ions) with a centre of mass energy of 100 TeV at a luminosity of 5 to 10 × 1034 cm2/s per interaction point;
— Study of a circular e+ e– collider with centre of mass energies up to 350 GeV and luminosities ranging from 1.8 (tt) to 28 (z pole) × 1034 cm2/s as a potential intermediary step towards an energy frontier hadron collider;
— Study of a high energy LHC with centre of mass collision energies up to 33 TeV in the LHC tunnel;
— Study of hadron–electron collider options, including an energy recovery linac at a collider in the LHC tunnel or an extension based on a new 100 km hadron collider.
Implementation schedules are oriented towards 2035 as the starting date for a stepwise launch of operation.
Although the collider studies are considered site independent, geology, civil engineering, infrastructure and operation studies (including health and safety aspects) build on the experience gathered at CERN.
(b) The Chinese Institute of High Energy Physics announced plans for a domestic circular collider in September 2012. The envisaged facility would host an e+ e– collider (Circular Electron Positron Collider with a centre of mass energy in the order of 250 GeV, acting as a Higgs factory). It could also include a proton–
proton collider with a centre of mass energy in the order of 50–70 TeV [54].