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HISTORICAL ARCHIVE

Europe's big lasers: the exawatt roadmap

02 Jun 2009

Researchers and industry will need to work closely together if Europe is to maximize the returns on its investment in a planned new generation of high-power laser facilities being built over the next decade. Caryl Richards reports on the challenges and opportunities of big science.

Europe is thinking big – big lasers, big science, big budgets. Over the next decade, a trio of planned pan-European research facilities will give scientists access to unprecedented laser powers and intensities, opening the door to exotic science that will shed light on the origins of the universe and, it is hoped, provide the foundations for a sustainable energy future.

The overall construction cost for this new generation of "super lasers" is in excess of €2 bn, with operational budgets running to several hundred million euros per year. That's a price worth paying, says Christian Kurrer, research programme officer at the European Commission.

"International infrastructures attract the best research scientists," Kurrer told delegates attending the "Emerging European Laser Facilities: Beyond Petawatt" workshop at the recent SPIE Europe conference in Prague, Czech Republic. "The infrastructures are well beyond the man-power and financial resources on a national level. This is why we need more collaborative efforts."

Into the unknown

One of those collaborations is the High Power Laser Energy Research (HiPER) facility. Headed up by the UK Science and Technology Facilities Council (STFC), a research funding body, HiPER's mission is to carry out proof-of-principle research into energy generation from laser-driven inertial-confinement fusion. The grand challenge: to initiate and study nuclear-fusion reactions via laser heating of a millimetre-sized fuel pellet (containing a mixture of deuterium and tritium) to temperatures greater than 100 million °C.

Although construction of HiPER is not slated to begin until 2014, the process of whipping existing laser technology into shape to deliver a light source with the requisite capabilities is already under way. "Current laser capability has reached its culmination in the petawatt (1015 W) scale," observed Mike Dunne, project director of HiPER and a senior scientist at the Rutherford Appleton Laboratory (RAL), UK. "We're looking at how to take it [the technology] to the next generation."

This is the purpose of the three-year preparatory phase on HiPER, which is running alongside initial experiments at the US Department of Energy's $4 bn (€3.1 bn) National Ignition Facility (NIF) in California. (As with HiPER, the end-game for NIF, a huge facility consisting of 192 pulsed laser beams with a total energy of 1.8 MJ, is the creation of nuclear fusion in the laboratory.)

Dunne anticipates that progress at NIF over the next few years will shape the future of HiPER. "[NIF] will prove the scientific principle [of laser fusion]: you get more energy out than you put in. We believe we now need to back that up with a whole bunch of technology that can exploit that science." What he's alluding to here is big science that could one day, after decades of endeavour, open the way to a fundamental reinvention of large-scale power generation.

Another major European laser facility in the works is the Extreme Light Infrastructure (ELI), a project that's being led by scientists at the Laboratoire d'Optique Appliquée (LOA) at the Ecole Polytechnique, Palaiseau, France. Scheduled to fire up in 2015, ELI will enable fundamental science to be carried out at the very highest laser powers (in the exawatt regime, 1018 W) and intensities (1024  W/cm2).

"The idea with ELI would be to develop something of the order of the 200 PW level," said John Collier, science and technology leader on the project. "That would serve a number of purposes. One would be to push the frontiers of the science that's already been done with the 1 PW systems [e.g. the Central Laser Facility at RAL] today and will be done with [future] 10 PW systems. It's also likely to take us into new regimes."

Like HiPER, ELI will allow academic researchers to explore fundamental science at the extremes (stuff like photon–photon scattering and other nonlinear quantum vacuum effects). Other missions outlined in the ELI project include attosecond science (e.g. the study of the ultrafast motion of electrons inside atoms over timescales of the order of 10–18 s) and generating a secondary source of electron beamlines from the light-matter interaction. HiPER, meanwhile, will also enable scientists to study laser–plasma interactions and "laboratory astrophysics" (e.g. the creation of conditions in the lab that could yield insights into supernovae evolution).

At the same time, Collier believes that there may be all sorts of spin-off applications resulting from Europe's new generation of big-laser facilities, in areas as diverse as radiation oncology and homeland security. "You've seen the impact that lasers have had on the photonics industry," he added. "I'm sure that there'll be plenty of applications if we can develop diode-pumped technology at the several-hundred-joule level."

Rising to the challenge

Getting projects like HiPER and ELI off the ground is a massive undertaking, one that will depend on exhaustive definition of the laser designs. One potential route for HiPER is a two-stage process called fast ignition. Here, several driving laser beams would initially compress the fuel pellet to extremely high densities, followed by ignition with an electron beam generated by a multikilojoule laser. Should HiPER take this course then it will require high-repetition-rate technology based on diode-pumped solid-state lasers.

Existing lasers at the Central Laser Facility provide possible templates for the development roadmap to higher-power regimes. The titanium:sapphire-driven Astra-Gemini, for example, is a high-power, high-repetition-rate laser capable of firing once every 20 s. The Central Laser Facility is also home to the flashlamp-driven Vulcan, one of the world's highest-power laser systems. Vulcan is currently undergoing an upgrade from 1 PW to the 10 PW regime by utilizing optical parametric chirped-pulse amplification, a technique that could also find application in the HiPER and ELI projects.

According to Collier, though, the jury is still out on the best approach to realize a high-repetition-rate 200 PW laser. "For a certain class of experiments, you want this power to be focused as if it were a single beam in order to get maximum intensity. Obviously we can't deliver that amount of energy in a single beamline."

One possible contender is a titanium:sapphire single-laser beamline called ILE, a work-in-progress at LOA. Even when fully optimized, however, the peak output power from ILE falls way short, at around 20 PW. The solution could be to coherently lock multiple beams into a single output, though this is a daunting challenge in its own right.

While none of these lasers will be a direct blueprint for a 200 PW system, Dunne takes the view that, as world-class research facilities, they will provide a huge talent pool for the work in hand. "It's a clustering of people that gives you the capability to move forward," he said.

X-rated

Ready access to such talent is certainly a feature of the European X-ray Free Electron Laser (European XFEL). This large-scale facility is dedicated to generating ultrashort, hard X-ray flashes for a range of basic and applied research, including atomic-scale metrology and time-resolved studies of chemical reactions down to the 100 fs regime. Construction began on the 3.4 km long laser facility at DESY, an established particle physics and photonics research laboratory in Hamburg, Germany, at the beginning of the year.

"DESY has strong accelerator and synchrotron-radiation departments, and there have been contributions from the groups in this department," explained Thomas Tschentscher, director of user operations with the European XFEL project team. "The photon diagnostics group, for example, was selected because it has world-class expertise in this field."

DESY has taken the technology and knowhow from an existing pilot facility, FLASH, which is optimized for the extreme UV and soft X-ray range. "The European XFEL is based on the knowledge that has been accumulated at FLASH," said Tschentscher. "DESY will continue to operate FLASH as a user facility for the 6–60 nm regime and the European XFEL will basically build a new machine covering the wavelength range from below 0.1 nm up to 6 nm."

As with FLASH, the underlying principle of the European XFEL is a process known as self-amplified spontaneous emission. In this case, lasing occurs in a single pass of a relativistic electron "bunch" accelerated through a long magnet structure known as an undulator. Sufficient gain is achieved by using an undulator of suitable length and several of these electron bunches. Provided that there is overlap between the spontaneous radiation from the first part of the undulator and the electron beam, interaction between the electromagnetic radiation and the electron bunch leads to coherent radiation with enhanced power.

Upon completion, the European XFEL is intended to provide the brightest source of hard X-ray pulses at the highest repetition rate (30,000 flashes per second). In an initial version electron bunches will be separated into three beamlines delivering coherent pulses to six different experimental stations tuned to specific wavelengths.

Getting it done

At the operational level, Europe's new generation of big-laser facilities are going to require technology push on a grand scale as they transition off the drawing board into full-scale working laboratories.

High-performance ceramics, thinner KDP crystals with better damage thresholds, and cheaper laser diodes are among the "must-haves" at the component level. Larger optics at the half-metre scale are also needed with coatings capable of withstanding hostile environments. The list goes on. "In a technical sense, we're essentially just conceptualizing and identifying where the areas are that need development," explained Collier.

Put simply, the success of HiPER, ELI and the European XFEL will depend, to a large degree, on research-industry partnerships that outsource some of the technology challenges to companies across the optics supply chain. What's more, says Dunne, the time for industry to engage is now, since the projects are funded to enable the preparatory stage of the work. "If I was in their [the industry vendors'] shoes I would start planning now as to how we are going to engage in that activity in the early part of the next decade. It's a good time to start that conversation."

Equally important is ensuring that the necessary skills base is in place to run these advanced laser facilities to their full potential. This means drumming up awareness of the projects in the physics and engineering departments of Europe's leading universities. "There is a very large capability base out there," said Dunne. "What we would like to see is part of that capability and capacity turn to this problem."

While Collier agrees that Europe already has a strong pool of talent, there are still areas of laser science and interaction physics where he sees room for improvement. "One of the key aspects, particularly for HiPER to go forward, is making sure that we grow these communities over this coming decade," he concluded.

• This article originally appeared in the June 2009 issue of Optics & Laser Europe magazine.

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