Snapshots of laser wakefields

Tabletop plasma devices can now produce electron beams in the GeV range. In such machines, electrons experience enormous accelerating gradients as they surf on electric fields generated in the wake of a high-power pulse of either laser light or charged particles traversing the plasma. (See the article on plasma accelerators in Physics Today June 2003, page 47 .) The precise nature of idealized wakefields has been calculated and simulated, but actual wakefields have been difficult to see for two reasons: They are very small and they move very fast. Now, however, physicists from the University of Texas and the University of Michigan have found a way to take snapshots of wakefields in the laboratory. In a technique called frequency-domain holography, the researchers use a pair of long, wide laser pulses. The leading “reference” pulse arrives at the gas just before the high-power laser pulse turns the gas into a plasma. Meanwhile, the trailing “object” pulse rides with the ionizing pulse and overlaps both the ionization front and the wake oscillations. The interfering reference and object pulses generate a hologram that can be reconstructed and displayed in a few seconds. The image shown here is of a strongly driven wake whose trailing wavefronts are successively more curved. The physicists say that such images will help them understand, for example, how the wave curvature relates to injection of electrons from the plasma into the waves. In addition, the images provide a means for active feedback control and optimization of a laser-plasma accelerator. (N. H. Matlis et al., Nat. Phys. 2 , 749, 2006 http://dx.doi.org/10.1038/nphys442 .)