Vulcan upgrade: power to the people

17 Jun 2002

Within the next few weeks, Europe will become home to the world's most intense laser. Some 25 years after its inauguration, scientists at the UK's Central Laser Facility are putting the finishing touches to the Vulcan laser's new petawatt beam. Michael Hatcher visited the new Vulcan building, where optical components are moved with cranes.

From Opto & Laser Europe May 2002

Scientists and engineers at the UK's Rutherford-Appleton Laboratory (RAL) are nearing completion of the Vulcan laser upgrade to produce a petawatt source. The first pulses from the new beam are expected around the end of next month, and by November, the facility is scheduled for use by external researchers for the first time.

Vulcan is already capable of delivering a 100 TW beam, so why go to the trouble of building a petawatt laser? According to Henry Hutchinson, who is director of the central laser facility (CLF) at RAL, the reason goes to the very heart of science: "The reason is scientific curiosity - the interaction between matter and light of such a high intensity is of great interest for both science and technological applications. What happens at these high intensities? We know some of the answers, and can predict some of the phenomena, but unanswered questions remain.

"This new beam will be a unique opportunity for scientists to do physics at the frontiers of their subjects. It will be the highest intensity laser in the world, and is a great opportunity for the UK community," Hutchinson added.

To say that the upgrade has been a big job would be a considerable understatement. The compression system and target area alone has required an entirely new building, complete with walls 60 cm thick for radiation protection and individual foundations to support half-tonne optics.

The thick walls are needed because using such high-power pulses means that the laser induces nuclear physics, accompanied by gamma rays, X-rays and neutrons.

The £5 m (EURO 8.1 m) project, which is funded primarily by the UK's Engineering and Physical Sciences Research Council, has benefited significantly from the decommissioning of the Nova laser at Lawrence Livermore National Laboratory (LLNL). Vulcan group leader Colin Danson said: "They decided to close down Nova, which was the largest laser in the world at the time, in order to concentrate on the National Ignition Facility (NIF)."

While LLNL concentrated on producing high-energy pulses for fusion research, the CLF decided to pursue higher irradiance by using shorter pulses. In a deal with the US Department of Energy, roughly £1 m worth of metre-scale optics and other equipment needed for the new petawatt configuration was traded in exchange for beam-time on the existing Vulcan systems.Along with upgrade project manager Chris Edwards, Danson's brief is to deliver a petawatt pulse, and he is confident that this will be achieved by the end of June. The upgrade uses the optical parametric chirped-pulse amplification method, which was conceived and developed a few years ago at RAL. Essentially, this technique takes an ultrashort pulse and stretches and amplifies it to produce a high-intensity broadband emission of a few nanoseconds' duration. This amplified pulse is then compressed back to the ultrashort regime, resulting in phenomenally high power levels.

"We have established the beamline and the front end of the laser, and proved that we can generate enough energy. The next strand is the pulse length," said Danson. Compressing the pulse requires two huge gratings, which are set 13 m apart in the new building and must be parallel to a precision of around 5 µrad for the technique to work. With such high accuracies needed, the Vulcan upgrade team has had to design a novel high-precision alignment technique for this purpose alone.

Another in-house development that the upgrade has demanded is adaptive optics (AO) to shape the beam. To achieve the very high irradiances of 10²¹ W/cm², it is crucial to maintain a high-quality beamshape near to the diffraction limit so that a sufficiently small spot can be produced at the beam's focus. Unlike astronomical AO systems, which convert starlight that has been highly distorted by the Earth's atmosphere, with the Vulcan laser it is more a case of fine-tuning a beam that is already of high quality. "We want to improve beam quality to the 'nth' degree, to make it almost diffraction-limited," said Danson.

He says that the sheer size and weight of the optical components needed to generate and manipulate the petawatt beam has been the upgrade team's greatest challenge. "Taking the beam size from 150 mm to 600 mm means that suddenly we're using 940 nm optics that weigh half a tonne, so the infrastructure has had to change - dramatically." The increase in size has had a considerable impact on the way in which things are physically done in the laboratory - for example, a crane is required simply to mount the optics.

As if the challenge of moving enormous lenses and mirrors around were not enough to contend with, there is the additional problem of doing all this in a vacuum and cleanroom environment. "Because this is a petawatt pulse, you can't put it through anything, not even air. Air's nonlinear refractive index would destroy the phase front. Putting it through a crystal for harmonic generation is also impossible. For one thing, the technology doesn't exist to frequency-double a 600 mm beam with a very thin crystal, and it's dubious whether anybody could produce such a crystal. We'd like to provide a frequency-doubled [petawatt] source, but the technology isn't in place to do that at the moment."

Because of the inherent complications introduced by reaching petawatt powers, all optics after the beam's production are necessarily reflective - otherwise they would be damaged. A reflective parabolic mirror focuses the compressed 500 fs beam to a 10 µm spot in the target chamber.

Unusually, the centre wavelength of the output beam is still not accurately known at this late stage of the upgrade. According to Danson, it will be somewhere between 1064 and 1053 nm - probably about 1057 nm. The reason for the uncertainty is that two different types of glass are used at the amplifying stage - silicate, which has a 1064 nm centre, and phosphate, which lases at the slightly shorter wavelength.

The building housing the new compressor unit and target area is certainly a striking sight, with the high-vacuum modules not so much resembling optical components as submarine capsules. The target area is big enough for a "walk-in" style door.

At the petawatt facility's inauguration last month, former CLF director Mike Key - now head of petawatt science at LLNL - hailed the facility as "the best short-pulse, high-intensity facility in the world".

Precisely what Vulcan users will be able to do with a petawatt pulse that cannot already be done with the 100 TW beam remains to be seen, but nuclear physics is likely to be the primary area under investigation. In going to 100 TW, Vulcan users have certainly witnessed new phenomena: the light-matter interactions, for example, started to occur in the nucleus rather than with electrons, as is normally the case.When the pulse hits its target, gamma rays are produced. These interact with neutrons in the target nuclei, change the atomic state and produce radioactive nuclei. With the 100 TW beam, physicists only just have a window onto this new world, and the extra order of magnitude of power in the petawatt pulse should give them a better view.

"[At the petawatt level] the physics will become more and more extreme," Danson told OLE. "We're going to be able to look into areas that nobody's ever looked into before. Things like stellar interiors, where the interactions are currently only theorized about. We will actually be able to see them in the laboratory."

Beyond physics, it's difficult to predict at this stage what the benefits for branches such as medicine, biology and chemistry are likely to be, but there are sure to be spin-offs in all of these areas.

Vulcan's petawatt beamline is expected to be available to external users in November. UK research teams have been allocated 90% of beam-time, with the remaining 10% allocated to European projects under the "large facility" programme.

And it is ultimately the satisfaction of Vulcan's users that is all-important to the CLF team. "We don't want the plaudits for building a new facility," said Danson. "We want the plaudits for getting new physics from the interactions it produces, and for creating the best facility possible to provide a petawatt for our users."

Central Laser Facility www.clf.rl.ac.uk