Applying next-generation plasma acceleration techniques to high-energy physics requires a global effort.
Progress in experimental particle physics is driven by advances in accelerators. The conversion of storage rings into colliders in the 1970s is one example, another is the use of superconducting magnets and RF structures that allow higher energies to be reached. CERN’s Large Hadron Collider (LHC) is halfway through its second run at an energy of 13 TeV, and its high-luminosity upgrade is expected to operate until the mid-2030s. Several machines are under consideration for the post-LHC era and many will be weighed up during the European Strategy for Particle Physics beginning in 2019. All are large facilities based on advanced but essentially existing accelerator technologies.
A completely different breed of accelerator based on novel accelerating technologies is also under intense study. Capable of operating with an accelerating gradient larger than 1 GV/m, advanced and novel accelerators (ANAs) could reach energies in the 1–10 TeV range in much more compact and efficient ways. The technological challenge is huge and the timescales are long, but the eventual goal is to have a linear electron–positron or an electron–proton collider at the energy frontier. Such a machine would have a smaller footprint than conventional collider designs and promises energies that otherwise are technologically extremely difficult and expensive to reach.
The first Advanced and Novel Accelerators for High Energy Physics Roadmap (ANAR) workshop took place at CERN in April, focusing on the application of ANAs to high-energy physics (CERN Courier June 2017 p7). The workshop was organised under the umbrella of the International Committee for Future Accelerators as a step towards an international ANA scientific roadmap for an advanced linear collider, with the aim of delivering a technical design report by 2035. The first task towards this goal is to take stock of the scientific landscape by outlining global priorities and identifying necessary facilities and existing programmes.
The ANA landscape
The first idea to accelerate particles in a plasma came as long ago as 1979, with a seminal publication by Tajima and Dawson. It involved the use of wakefields – accelerating longitudinal electric fields generated in a plasma in the wake of a driving laser pulse or a particle bunch – to accelerate and focus a relativistic bunch of particles. In ANAs using plasma as a medium, the wakefields are sustained by a charge separation in the plasma driven by a laser pulse or a particle beam. Large energy gains over short distances can also be reached in ANAs using dielectric material structures that can sustain maximum accelerating fields larger than is possible in metallic structures. These ANAs can accelerate electrons as well as positrons and can also be driven by laser pulses or particle bunches.
Initial experiments took place with electrons at SLAC and elsewhere in the 1990s, demonstrating the principles of the technique, but the advent of high-power lasers as wakefield drivers led to increased activity. After the first demonstration of peaked electron spectra in millimetre-scale plasmas in 2004, GeV electron beams were obtained with 40 TW laser pulses in 2006 and subsequently electron beams with multi-GeV energies have been reported with PW-class laser systems and few-centimetre-long plasmas. Advanced and novel technologies for accelerators have made remarkable progress over the past two decades. They are now capable of bringing electrons to energies of a few GeV over a distance of a few centimetres, compared to 0.1 MeV per centimetre for the Large Electron–Positron (LEP) collider. Reaching such energies with ANAs has therefore sparked interest for high-energy physics applications, in addition to their potential for industry, security or health sectors.
Several challenges must be addressed before proposing a technical design for an advanced linear collider (ALC), requiring the sustained efforts of a diverse community that currently includes more than 62 laboratories in more than 20 countries. The key challenges are either related to fundamental components of ANAs – such as the injectors, accelerating structures, staging of components and their reliability – or to beam dynamics at high energy and the preservation of energy spread, emittance and efficiency.
A major component necessary for the application of an ANA to high-energy physics is a wakefield driver. In practice, this could be an efficient and reliable laser pulse with a peak power topping 100 TW, or a particle bunch with an energy higher than 1 GeV. In both cases, however, the duration of the pulse must be shorter than 100 fs.
The plasma medium, separated into successive stages, is another key component. Assuming accelerating gradients in the region 10–50 GeV/m and energy gains of 10–20 GeV per stage, plasma media 20–200 cm long are required. The main challenges for the plasma medium are the reproducibility, density uniformity, density ramps at their entrance and exit, and the high repetition rate required for collider operation. Tailoring the density ramps is important to mitigate the usually large mismatch between the small transverse size of the accelerated beam inside the plasma and the relatively large beam size that inter-stage optics must handle between plasma modules.
Staging successive accelerator modules is a further challenge in itself. Staging is necessary because the energy carried by most drivers is much smaller than the final energy desired for the accelerated bunch, e.g. 1.6 kJ for 2 × 1010 electrons or positrons at an energy of 500 GeV. Since state-of-the-art femtosecond laser pulses and relativistic electron bunches carry less than 100 J, multiple drivers and multiple stages are needed. Staging has to achieve, in a compact way, coupling of the accelerated bunch out of one plasma module into the next one, while preserving all bunch properties, and evacuating the exhausted driver and bringing the fresh driver before entering the next stage. Staging has been demonstrated, although with low-energy beams (< 200 MeV), in a number of schemes, the most recent being the one performed at the BELLA Center at LBNL. Injection of electrons from a laser plasma injector into a plasma module providing acceleration to 5–10 GeV is one of the goals of the French APOLLON CILEX laser facility starting operation in 2018, and of the baseline explored in the design study EuPRAXIA (see panel on right). The AWAKE experiment at CERN, meanwhile, aims to use protons to drive a plasma wakefield in a single plasma section with the long-term goal of accelerating electrons to TeV energies.
Stability, reproducibility and reliability are trademarks of accelerators used for particle physics. Results obtained with ANAs often appear of lower stability and reproducibility than those obtained with conventional accelerators. However, it is important to note that these ANAs are run mostly as experiments and research tools, with limited resources put towards feedback and control systems – which are one of the major features of conventional accelerators. A strong effort therefore has to be put into developing proper tools and devices, for instance by exploiting synergies with the RF-accelerator community to develop more reliable technologies.
Testing the components for an eventual ALC requires major facilities, most likely located at national or international laboratories. ANA technology might be more compact than that of conventional accelerators, but the environment for producing even 10–100 GeV range prototypes is beyond the capability of university labs, requiring multiple engineering skills to demonstrate reliable operation in a safe environment. The size and cost of these facilities are better justified in a collaborative environment, in line with the development of accelerators relevant for high-energy physics.
Four-phase roadmap
Co-ordination of the advanced accelerators field is at different levels of advancement around the world. In the US, roadmaps were drawn up in 2016 for plasma- and structure-based ANAs with application to high-energy physics and the construction of a linear collider in the 2040s. One outcome of the ANAR workshop this year was a first attempt at an international scientific roadmap. Arranged into four distinct phases, the roadmap describes the stages deemed scientifically necessary to elaborate a design for a multi-TeV linear collider.
The first is a five-year-long period in which to develop injectors and accelerating structures with controlled parameters, such as an injector–accelerator unit producing GeV-range electron and positron beams with high-quality bunches, low emittance and low relative energy spread. A second five-year phase will lead to improved bunch quality at higher energy, with the staging of two accelerating structures and first proposals of conceptual ALC designs. The third phase, also lasting five years, will focus on the reliability of the acceleration process, while the fourth phase will be dedicated to technical design reports for an ALC by 2035, following selection of the most promising options.
Community effort
Many very important challenges remain, such as improving the quality, stability and efficiency of the accelerated beams with ANAs, but no show-stopper has been identified to date. However, the proposed time frame is achievable only if there is an intensive and co-ordinated R&D effort supported by sufficient funding for ANA technology with particle-physics applications. The preparation of an eventual technical design report for an ALC at the energy frontier should therefore be undertaken by the ANA community with significant contributions from the whole accelerator community.
From the current state of wakefield acceleration in plasmas and dielectrics, it is clear that advanced concepts offer several promising options for energy frontier electron–positron and electron–proton colliders. In view of the significant cost of intense R&D for an ALC, an international programme, with some level of international co-ordination, is more suitable than a regional approach. Following the April ANAR workshop, a study group towards advanced linear colliders, named ALEGRO for Advanced LinEar collider study GROup, has been set up to co-ordinate the preparation of a proposal for an ALC in the multi-TeV energy range. ALEGRO consists of scientists with expertise in advanced accelerator concepts or accelerator physics and technology, drawn from national institutions or universities in Asia, Europe and the US. The group will organise a series of workshops on relevant topics to engage the scientific community. Its first objective is to prepare and deliver, by the end of 2018, a document detailing the international roadmap and strategy of ANAs with clear priorities as input for the European Strategy Group. Another objective for ALEGRO is to provide a framework to amplify international co-ordination on this topic at the scientific level and to foster worldwide collaboration towards an ALC, and possibly broaden the community. After all, ANA technology represents the next-generation of colliders and could potentially define particle physics into the 22nd century.
EAAC workshop showcases advanced accelerator progress
The 3rd European Advanced Accelerator Concept (EAAC) workshop, held every two years, took place from 24 to 30 September on the Island of Elba, Italy. Around 300 scientists attended, with advanced linear colliders at the centre of discussions. Specialists from accelerator physics, RF technology, plasma physics, instrumentation and the laser field discussed ideas and directions towards a new generation of ultra-compact and cost-effective accelerators with novel applications in science, medicine and industry.
Among the many outstanding presentations at EAAC 2017, at which 70 PhD students presented their work, were reports on: laser-driven kHz generation of MeV beams at LOA/TU Vienna; dielectric acceleration results from PSI/DESY/Cockcroft; first results from the AWAKE experiment at CERN; 7 GeV electrons in laser plasma acceleration from LBNL; 0.5 nC electron bunches from HZDR; new R&D directions towards high-power lasers at LLNL; controllable electron beams from Osaka and LLNL; undulator X-ray generation after laser plasma accelerators from DESY/University of Hamburg/SOLEIL/LOA; important progress in hadron beams from plasma accelerators from Belfast/HZDR/GSI; and future collider plans from CERN.
A special session was devoted to the Horizon2020 design study EuPRAXIA (European Plasma Research Accelerator with eXcellence In Applications). EuPRAXIA is a consortium of 38 institutes, co-ordinated by DESY, which aims to design a European plasma accelerator facility. This future research infrastructure will deliver high-brightness electron beams of up to 5 GeV for pilot users interested in free-electron laser applications, tabletop test beams for high-energy physics, medical imaging and other applications. This study, conceived at the EAAC meeting in 2013, is strongly supported by the European laser industry.
The EAAC was founded by the European Network for Novel Accelerators in 2013 and has grown in its third edition into a meeting with worldwide visibility, rapidly catching up with the long tradition of the Advanced Accelerator Concepts workshop (AAC) in the US. The EAAC2017 workshop was supported by the EuroNNAc3 network through the EU project ARIES, INFN as the host organisation, DESY and the Helmholtz association, CERN and the industrial sponsors Amplitude, Vacuum FAB and Laser Optronic.
• Ralph Assmann, DESY, Massimo Ferrario, INFN and Edda Gschwendtner, CERN.
