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Accelerator physics

Surf's up at SLAC

A 'plasma afterburner' just 30 centimetres long accelerates electrons hundreds of times faster than giant conventional accelerators. The result may ultimately open up a low-cost technology for particle colliders. See Letter p.92

In November 2012, Guinness World Records reported that 120 surfers in Australia rode the same wave simultaneously for more than 5 seconds1. “The trick was to get them all to do the same thing at the same time,” said group leader Wes Smith. “It was an operation of military-like precision and we finally got there.” Now Litos and colleagues, in work at the SLAC National Accelerator Laboratory in Menlo Park, California, reported on page 92 of this issue2, have 'got there', too, by surfing half a billion 20-billion-electronvolt electrons on a steep charge-density wave about the size of a marine phytoplankton, travelling through ionized gas (plasma). The wave was driven by a companion electron bunch as it raced at nearly the speed of light through a 30-centimetre-long chamber filled with plasma (Fig. 1).

Figure 1: Ramping up the energy.

SLAC Natl Accelerator Lab

SLAC's main accelerator, shown in aerial view, accelerates electron bunches from 0 to 20 GeV energy over 2 km, which amounts to adding 0.01 GeV to each electron every metre. The new Facility for Accelerator Science and Experimental Tests (FACET) used by Litos et al.2 then splits each 20-GeV bunch into two independently controlled tandem bunches. The leading bunch creates a new micro-accelerator inside a 30-cm chamber (top), in which it drives a charge-density wave in ionized gas, much as a boat drives a wake in water. The trailing bunch rides the lead bunch's wake and, when optimally positioned, extracts up to 30% of its energy, boosting each electron's energy by 1.6 GeV in only 30 cm.

Although this inaugural experiment lost about 90% of its 'surfers' along the way, the surviving electrons gained 1.6 billion electronvolts, or 1.6 gigaelectonvolts (GeV), in energy with unparalleled uniformity, maintaining roughly 1% energy spread throughout their wild ride, while sucking away an unprecedented fraction (up to 30%) of the wave's energy. Such uniform, efficient acceleration required the researchers to inject the surfing electron bunch into the wave, and to adjust the bunch's charge and shape, with a military-like precision made possible by SLAC's recently commissioned US$15-million Facility for Accelerator Science and Experimental Tests (FACET)3. Because the plasma wave accelerated electrons 500 times faster than SLAC's main particle accelerator, the result might herald a new generation of compact 'plasma afterburners' that could boost the energy of conventional particle accelerators and potentially reduce the skyrocketing cost of high-energy physics machinery4.

Seven years ago, before FACET was even proposed, the same team had used single bunches of about 10 billion 42-GeV electrons and accelerated them over the full 3.2-kilometre length of SLAC's main machine to drive a similar plasma wave5. A handful of electrons from the tail of the drive bunch were caught in the drive bunch's wake and were accelerated up to 84 GeV, twice the energy of the electrons in the original drive bunch, within a metre-long plasma chamber. However, the electrons emerging from this first-generation plasma afterburner ranged in energy from less than about 35 GeV to 84 GeV, more electrons were decelerated than accelerated, and most of the energy of the plasma wave was left untapped. FACET — which now shares SLAC with the Linac Coherent Light Source, and thus starts with 20-GeV electrons accelerated over part of SLAC's length — was designed to correct these shortcomings. The facility exploits new particle-beam technology to split the SLAC bunches into two tandem bunches whose time separation, charge and shape are, with some limits, independently controllable.

In the new experiments, the researchers used a little over half of the 20-GeV SLAC bunch to drive a plasma wave, and then timed its nearly equally charged twin to surf just a hair's breadth behind, where its core rode the enormous electrostatic field of the drive bunch's wake. Without the trailing surfing bunch, this field would be far from uniform, varying from 3 billion to 10 billion volts per metre (fields stronger than ordinary, non-plasma matter can withstand) just over the tiny region in which the surfing bunch was so painstakingly positioned.

Had the researchers injected a lower-charged surfing bunch, it would have suffered the same fate as in the earlier experiment by broadening in energy. This would render it useless for high-energy physics applications, which require particle energy to be tuned precisely to create and identify new particles, such as the Higgs boson. However, Litos et al. took advantage of physics learned from computer simulations6 showing that a high-charge surfing bunch could 'load' the plasma wake, flattening its electrostatic fields locally. It is as if the 120 Australian surfers had sufficient collective weight to flatten the curved ocean wave into an inclined plane so that they could all accelerate at the same rate. This trick solved two problems simultaneously: it enabled a high-charge bunch to accelerate nearly monoenergetically while maximizing energy extraction from the plasma wake.

Can plasma surfing meet future needs of high-energy physics research, which include electron bunches with sufficiently high energy, charge, repetition rate and focusability that they can create detectable amounts of new particles that may be lurking in the cosmic underworld? The jury is still out. The present 1.6-GeV energy gain (starting from 20 GeV) is no greater than that achieved by plasma accelerators driven by light pulses from lasers (starting from zero)7, which are much smaller and less-expensive instruments than SLAC. Nevertheless, electron-driven plasma accelerators scale more readily to gains of tens of gigaelectronvolts than do their laser-driven counterparts, as demonstrated in previous work5.

Improved bunch-shaping technology will better match surfing bunch to plasma wave, increasing electron-survival rate and thus the number of accelerated electrons. Yet the Higgs boson has a mass equivalent to 126 GeV, and physical theories such as supersymmetry predict additional particles that have even greater mass than the Higgs and may be the source of the elusive 'dark matter' that seems to comprise about 25% of the Universe. Creating and identifying these new denizens of the Universe could set the next energy frontier at many thousands of gigaelectronvolts. Reaching these energies will probably require synchronized, multi-staged plasma accelerators — a daunting, and largely unexplored, technical challenge in view of the micrometre dimensions of plasma waves.

An interesting alternative proposal is to drive plasma waves with very energetic proton bunches, which because of their greater mass can push plasma waves for hundreds of metres, potentially accelerating electrons to the energy frontier in a single stage8. In either case, plasma acceleration of positrons (antielectrons) lags far behind electron acceleration because plasma waves shaped like those in the current experiment defocus surfing positron bunches, degrading their usefulness. Positron acceleration is important because high-energy collisions of electrons and positrons, a natural matter–antimatter pair, create a richer collection of products with higher efficiency than, say, electron–electron collisions, and thus offer one of the most promising routes to particle discovery. FACET, with access to SLAC's companion positron beam, is uniquely positioned to explore new ways to shape plasma waves in order to advance plasma-based positron acceleration.

Finally, even if the energies and charges required for an electron–positron collider are achieved, debate rages over whether focused, plasma-surfed particle beams can yield particle-discovery events at rates competitive with those achieved with conventional accelerator technology9,10,11, which underlies proposed tens-of-kilometres-long machines such as the International Linear Collider and the Compact Linear Collider. These uncertainties notwithstanding, Litos et al. have overcome one of the most difficult challenges so far in the long quest for small, affordable accelerators, and have given the plasma-surfing community every reason to surge ahead.


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Correspondence to Mike Downer.

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Downer, M., Zgadzaj, R. Surf's up at SLAC. Nature 515, 40–41 (2014).

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