BELLA, the Berkeley Laboratory Laser Accelerator – an experimental facility for advancing the development of laser-driven plasma acceleration. Image: Roy Kaltschmidt, Lawrence Berkeley Nat’l Lab
California, officially known as the “Golden State”, has recently been the scene of two key meetings of the wider linear-collider community. The first, from 1 to 4 December at SLAC National Laboratory, was the latest in the long-standing series of meetings of the Tesla Technology Collaboration (TTC), which explores the latest developments in the superconducting radio-frequency acceleration technique at the heart of the ILC. The second meeting, last week, was a one-off meeting organised by Wim Leemans of Lawrence Berkeley National Laboratory, which brought together many of the key players in plasma-wakefield acceleration (PWA) techniques to discuss a roadmap to a realistic design for a PWA linear collider. I was lucky enough to attend (most of!) both.
The TTC meeting saw 130 participants gather at SLAC National Laboratory. It was a pleasure to see many old friends among the participants and to listen to a remarkable array of talks detailing the latest advances in superconducting radiofrequency technology. As usual there were some highly enjoyable social encounters, including the conference banquet in the splendour of the new SLAC administrative building which now towers over the entrance to the lab. I have been coming to SLAC since the late 1970s and although much remains reassuringly recognisable, there have also been great changes. In particular the imposing presence of the new building from Sand Hill Road is an interesting contrast to the early laboratory, which almost hid itself away from the gaze of the general public. Any why not be prominently in the public eye? After all, few labs have as much to be proud about as SLAC does.
The defining characteristic to me of the TTC meetings is certainly the excellent overview it gives of what is going on in superconductivity worldwide but also the very detailed reports and opportunities for discussion that exist in the parallel sessions. The first morning was devoted to plenary talks on cavity and cryomodule production, which have been greatly informed by the European XFEL procurement and testing programme. The afternoon saw the first parallel session meetings. There were four working groups: on issues relating to high-quality-factor cavities; on cavity construction and production; on cryomodule design and construction; and finally on couplers and tuners. The issues covered were bewilderingly broad, from lessons learnt from industrial mass production to the optimum design of clean rooms. Prominent but perhaps not so prominent as at the previous meeting was nitrogen doping: its effects, the “magic recipe” and progress in theoretical understanding. The current chair of TTC, Hasan Padamsee from Cornell, not only introduced the meeting but also gave us a rousing farewell with a closing speech that contained a passionate appeal to make political progress with building the ILC. As demonstrated yet again by this meeting, no particle physics project has ever been so well understood both technologically and from the industrial mass production side; now is the time to exploit that and realise the ILC.
The other workshop that I attended from 6 to 8 January was entitled “Plasma-Based Accelerator Concepts for Colliders” and held at Lawrence Berkeley National Laboratory across the bay from SLAC. Constructing “back of the envelope” designs for a PWA linear collider has been a popular pastime in the margins and bars of accelerator and particle physics conferences for a number of years. Recently the envelopes have given way to several sheets of paper and several interesting ideas have appeared in journals and conference proceedings. The recent “P5” strategic review contained a number of recommendations for future accelerator R&D in the US. To follow these up, in spring last year a panel chaired by Don Hartill produced a report called “Accelerating discovery”, in which PWA activities of various kinds played a prominent role. In particular, recommendation 10 advised: “Convene the university and laboratory proponents of advanced acceleration concepts to develop R&D roadmaps with a series of milestones and common down-selection criteria towards the goal of constructing a multi-TeV e+e– collider.” This, together with the continued rapid pace of advances in plasma acceleration, have led the US Department of Energy to convene a meeting next month to discuss such a roadmap. The Berkeley meeting was intended to gather and process the inputs necessary to inform this meeting.
The workshop began with a call to arms from Wim, whose first slide stated that the purpose of the meeting was to identify the key physics and technology R&D needed to realise a plasma-based collider, and to formulate a nationally and internationally coordinated roadmap for carrying out this research over the next two decades. Undaunted by the magnitude of the task, the around 50 participants buckled down to the job, hearing on the first day a series of excellent talks summarising particle physics requirements and the accelerator physics constraints on a PWA linear collider. The subsequent talks summarised the status and challenges for two distinct areas of current activity, which nevertheless have much in common: laser-driven and beam-driven. Laser-driven devices use a powerful laser to “blow away” the plasma electrons, forming a “bubble” of electron depletion whose electric fields can be utilised to produce enormous accelerating gradients. Beam-driven accelerators produce the same effect via the electric field of a beam of particles. The potential development of commercial lasers is a vital input to plans for laser-driven devices, so this was covered in a further talk, these three areas forming the topics for the working groups that convened for the remaining one and a half days of the workshop. Finally on the first afternoon, reports on the relevant US facilities at Argonne, Brookhaven, Fermi, Lawrence Berkeley and SLAC National Labs were given, together with an overview of activity in Europe.
I attended the beam-driven working group, as my main research interests are in FLASHForward, the beamline currently under construction at DESY to utilise the unique advantages of the superconducting technology of the FLASH-II accelerator by using its bunches to drive plasma-acceleration experiments. The working-group discussion was highly productive, with a clear path forward emerging from the first day of concentrated work. Perhaps surprisingly, the conclusions on priorities, directions and timescales from the laser-driven working group presented on the final morning of the workshop were both compatible and rather similar to those of our group; these were complemented by the conclusions of the laser-technology working group on the critical milestones towards developing the lasers running at tens of kilohertz and producing average powers of 300 kilowatts needed for a viable application to linear colliders. For comparison, the current state of the art is the BELLA facility at Lawrence Berkeley National Laboratory with 40W average power running at 1 Hz; the next planned step, K-BELLA, would increase this power to 3kW.
The final picture emerging from the workshop envisages a road map stretching to 2040, with the next fifteen years or so dedicated to addressing the fundamental questions that need to be resolved to make a plasma-based linear collider a reality. For example, the crucial question for a linear collider of positron acceleration, despite exciting recent advances at the FACET facility at SLAC, is still far from solved; others, such as the required tolerances for a PWA collider, have not even begun to be seriously considered. Many of the key issues of a conceptual design will be addressed in this 15-year R&D phase, giving the hope that, by the end of the road-map period, a technical design for a multi-TeV linear collider may be possible.
The “Golden” state refers to the great California Gold Rush, but also no doubt to the glorious sunny weather with which it is usually blessed. A particularly strong “El Niño” this year however meant that both meetings brought fairly torrential and much needed rain to a state in a five-year drought. Let’s hope that this refreshment is mirrored by new impetus given to two vital but very different areas of our linear-collider technology. In fact, it turns out that in a beam-driven PWA linear collider, superconducting RF acceleration will still play a vital role: the only way to get sufficient beam power to give the required luminosity is to use a (beyond state of the art) superconducting accelerator. Whatever linear collider the future brings, both communities have a great deal of exciting work in front of them!
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