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physicsworld.comNews & Analysisaccelerator. As the laser pro pagatesthrough the plasma, the electrons areexpelled around the laser pulse – justas a boat displaces water around it asit moves forward. As the electronsthen rush back in behind the laserpulse, they set up a trailing wave-likestructure known as a “wakefield” –like a water wave travelling behindthe boat. Other plasma electronstrap ped by these waves “surf” onthem behind the laser pulse pickingup energy and accelerating.This technique allows laser lightto accelerate electrons over a muchsmaller distance than conventionalparticle accelerators, which can betens of kilometres long. “Typically, wethink we can achieve electron energiesof about 10–20 GeV,” says Mou -rou. “So instead of building a 1 kmlin ear accelerator, we can instead usesomething that is only 1 m long.”Mourou says that the Prague ELIcentre, which could also accelerateprotons for use in hadron therapy,will complement, rather than replace,other facilities that generate shortpulses of X-rays, such as the LinacCoherent Light Source (LCLS) at theSLAC National Accelerator Labor at -ory. But while LCLS, which is an X-rayfree-electron laser, can only producemonochromic radiation with pulsedurations of the order of 100 fs, ELIcould produce polychromatic ra di -ation of the order of a femtosecond orless, making it possible to take imagesof chemical reactions in real time.“ELI is pushing the boundaries interms of testing this technology toprovide a range of applications,” saysJohn Collier, head of the high-powerlasersdivision at the Central LaserFacility at the Ruther ford lab.Ripping atomsELI’s nuclear-physics facility in Ro -mania is set to be built in Ma gu rele,20 km south of Bucharest. The facilitywill produce 10 PW beams that areshone directly onto a nucleus to studyhow the pulse affects nuclear energylevels. Researchers expect that thelaser pulse should be able to depositabout 1–10 keV on the nuc leus –enough to modify energy levels andforcing it to release a gamma ray. De -tecting this radiation would be proofthat researchers have affected thenucleus directly with laser light, thusallowing them to study nuclear trans -itions in more detail.As for the Hungarian “attosecond”facility, it will use a 5 fs pulse with alaser beam of a few joules to generatepowers of the order of a petawatt. Thefacility, to be built in Szeged, 100 kmPhysics World May 2010south of Budapest, will generate pulsesevery 1 ms that will be used to takesnap-shots on the attosecond scale(10 –18 s) of electron dynamics in atoms,plasmas and solids. It will do this byshooting a femtosecond pulse of lightat a dense plasma target. In a processknown as “relativistic harmonic generation”,the ionized plasma then givesoff so-called phase-locked radiation inthe ultraviolet and soft X-ray regimeat multiples of the frequency of theoriginal femtosecond pulse. Research -ers at ELI will then select the pulsesthat are generated in the atto secondregime with a filter and send them toexperimental stations to study materialson the atomic scale.Boiling the vacuumThe host for what is dubbed the“heart” of ELI – the “ultrahigh peakpower” facility – will not be knownuntil 2012, after some initial testingof technology for the three main fa -cilities is carried out. With an ex -pected completion date of 2018, thefacility will attempt to generate a100– 200 PW beam and use mirrors tofocus it onto an area of 1 µm 2 in thehope of ripping open the fabric of thevacuum to produce particle and anti -particle pairs. “The vacuum is notsomething empty, but is full of activityof particles being created and de -stroyed,” says Mourou. “It defines allthe constants of physics.”Quantum field theory states these“virtual particles” continually pop inand out of existence. It is predictedthat paired virtual particles could be -come real as they are torn apart bythe pulse’s extremely strong electromagneticfields. How ever, this happenstoo quickly to leave a trace andrequires light with an in tensity ofabout 10 29 Wcm –2 . Known as the“Schwinger limit”, it is seven orders ofmagnitude larger than any currentlaser can achieve.In its current design, ELI’s highintensityfacility will only be able toreach 10 25 Wcm 2 ; however, MannuelParticle test-bedThe LASERIX laserat the UniversitéParis-Sud II has beentesting whether laserscan produce X-rays,as the Czech ELIfacility hopes to do.ELI will createnew scientificcommunitiesand it willbe a magnetfor hi-techcompaniesCNRS Photothèque/Alexis CheziereHege lich, project leader of shortpulseexperiments and lasers at theLos Alamos National Laboratory inNew Mexico, says there are some newtheories being proposed that couldbring the Schwinger limit within ELI’sreach. “The vacuum has energy levelsand it would be great if we couldsomehow manage to modify them,”says Hegelich. Mourou also say theSchwinger limit could be matched bycolliding electron beams created bytwo lasers.One of the technical challenges ofthe ultrahigh-peak-power facility willbe producing the vacuum itself. “Evenultrahigh-vacuum environments producedby highly efficient pumps stillhave a few atoms floating around,”says Hegelich. One method would beto first shoot a laser pulse into thehigh-vacuum environment that wouldexpel all the particles and then quicklyfollow that up with a second highpoweredpulse. “Technically, there isnothing that can’t be overcome withthe ultrahigh-peak-power facility,”says Hegelich. “It is more an engin -eering challenge that a physics one.”One phenomenon that ELI shouldbe able to detect, which is predictedto happen at about 10 23 Wcm –2 , is thevacuum becoming polarized and ex -hibiting optical phenomenon suchas birefringence. Some theorists arealready proposing experiments for theultrahigh-peak-power facility such asa “matterless” double-slit experimentwhere the photons generated fromelectron and positron pairs annihil -ating form a double-slit diffractionpattern (Nature Physics 4 92).As well as being a revolutionaryphys ics project that will test fundamentaltheories and show how laserscould become the next particle acceleratoror collider, ELI is also tippingthe scales of Europe’s portfolio ofmajor infrastructures slightly moreeastwards. The presence of threemajor facilities in the Czech Repub -lic, Hungary and Romania will allowthese nations to attract researchersfrom abroad, as well as inspiring fu -ture generations of researchers.“ELI will create new scientific communitiesand it will be a magnet forhi-tech companies,” says Sander, whonotes that for every euro spent on alarge infrastructure, 74 is given backto the economy. Yet for most physicists,it is ELI’s ultrahigh-power facility,which will provide laser power farbeyond any existing today, that is themost exciting and eagerly awaited.“Within the next decade,” says Mou -rou, “we will be en tering a new paradigmin physics.”13