A meeting at GSI Darmstadt on 8–10 April provided a first opportunity to formulate a strategy on the laser technology needed to meet the challenge of future accelerators that will use or rely on lasers with very high average power. Hartmut Eickhoff, technical director of GSI, and Wim Leemans of Lawrence Berkeley National Laboratory opened the event. Leemans is chairman of the newly established Joint Task Force on Future Applications of Laser Acceleration, which operates under the umbrella of the International Committee for Future Accelerators (ICFA) and the International Committee on Ultra-High Intensity Lasers (ICUIL). The task force had invited experts on high-power laser technology and accelerator technology and their applications to this first meeting. Altogether, there were 47 participants from countries around the world, including China (1), France (4), Germany (18), Japan (4), Switzerland (2), the UK (4) and the US (14).
The main topics of discussion were the laser performance needed for accelerator technology to support the most challenging present and future needs, as well as questions of laser architecture, laser material and optical components. Representatives from accelerators and light sources outlined the top-level laser requirements for potential laser-based accelerator applications – that is, for colliders, light sources and medical applications.
The biggest challenge for laser technology is a laser-plasma e+e– collider with the goal of a top energy of 10 TeV. The consensus in the global high-energy physics community is that the next large collider after the LHC should be a tera-electron-volt-scale lepton collider. Options currently under study include an International Linear Collider (ILC) at 0.5–1 TeV, a Compact Linear Collider (CLIC) at up to 3 TeV and a muon collider at up to 4 TeV, all using RF technology. On the other hand, the very high gradients of around 10 GeV/m that are possible with laser acceleration, offer new avenues to reach even higher energy and more compact machines.
The workshop investigated the beam and laser parameters of a 1–10 TeV e+e– collider, with a luminosity of 1036 cm–2s–1, based on two different technologies – laser plasma acceleration and direct laser acceleration. The main challenges to the practical achievement of laser acceleration are high average power (around 100 MW), high repetition rate (kilohertz to megahertz), and high efficiency (around 40–60%) at a cost that ideally would be an order of magnitude lower than using technology based on RF. The workshop also studied the laser requirements for a 200 GeV γγ collider, proposed as the first stage of a full-scale ILC or CLIC. The laser systems required for such a collider may be within reach of today’s technology.
For light sources, lasers already play a significant role in existing facilities but they face new challenges with future projects that aim at much higher repetition frequencies. Ultrafast (femtosecond) lasers reaching levels of 1–10 kW will be required for use as “seed lasers” and for user-driven experiments. The third area of application is the use in medicine of laser acceleration of protons or ions and its potential to replace technology currently used in tumour therapy. Such lasers typically have very high peak-power (petawatt class) and require special pulse shapes with very high temporal contrast. Again, compact multi-kilowatt lasers will be needed.
Laser requirements for these applications are often many orders of magnitude beyond the capabilities of the lasers that are used in today’s scientific work, i.e. they require megawatts instead of tens of watts. Representatives from laser science at the meeting discussed and outlined how, with appropriate R&D, emerging 100-kW-class industrial lasers, 10-MW-class laser technologies for fusion energy and megawatt-class laser systems for defence work might be adapted to meet these challenging requirements.
Results from the workshop, including tables of the parameters required for laser technology and the goals, will be compiled in a report and submitted to ICFA and ICUIL for their approval, prior to public release.