The perfect wave.  In wakefield acceleration, electrons “ride” an electromagnetic wave and are accelerated.  A new technique may provide electrons with the right speed and timing to get them onto that wave. 

In the next generation of accelerators, electrons may “ride a wave” of electromagnetic fields in a plasma, just as a surfer rides a wave of water.  But as any surfer knows, riding the wave is only part of the challenge: You have to get on it first.  In the 22 February PRL, a group of physicists proposes a new way to produce the tightly focused, exquisitely timed electron beams that would be needed to make the scheme work.  They showed that blasting a sample of krypton gas with extremely intense laser pulses produces a beam of electrons with the right properties. 

Christopher Moore of the Naval Research Laboratory in Washington, DC, along with his colleagues, zapped a vacuum chamber filled with krypton gas with an ultra-short, intense burst of infrared laser light.  At 3×1018W/cm2, the intensity of the burst was equivalent to momentarily focusing the entire U.S. power output onto a region the size of a human cell.  When it struck the krypton gas, the laser pulse stripped off up to 18 electrons from each atom, removing them not only from the outer orbital but the inner orbitals as well–in effect, ripping off the atom’s shirt at the same time as its coat. “ The weakly-bound electrons fly out of the laser pulse before they can gain much energy,” says Moore, but the inner ones absorb the full force of the laser beam.  By the time they get away, they have enough energy to move at four-fifths of the speed of light. 

“We knew that the electrons would be ejected,” says Moore. “ The surprise was the high degree of directionality.”  Theory predicted that the electrons should emerge from the krypton atoms at all angles perpendicular to the laser light, with a slight preference for the laser’s direction of polarization.  But in fact, they came out in two oppositely-directed beams, moving only along the polarization direction.  While the discrepancy is up to the theorists to explain, it makes Moore’s experiment an attractive way to generate electron beams for accelerators. 

Over the last decade, physicists have demonstrated an idea called wakefield acceleration, in which a laser pulse creates a traveling electric field in a plasma.  If an electron is caught in the wave, the electric field pulls it along and causes it to accelerate–in the same way that gravity pulls a surfer down the front of a wave.  Physicists believe the idea may enable them to build “tabletop accelerators” that will replace the mile-long behemoths of today.  But such accelerators would need a source of electrons that are traveling at just the right speed and at the right time.  Other methods have been proposed, in which the electrons are jolted out of the plasma itself, but Moore’s is the first scheme that would inject them from outside of the plasma. 

“It’s like an action movie where the hero jumps onto the moving car,” says Howard Milchberg, a laser physicist at the University of Maryland.  He emphasizes that Moore and his colleagues have not demonstrated a working model for an injector yet, but “They have shown they can get high brightness with femtosecond timing.”