On the node of a wave

A compact electron accelerator can be made by the cunning use of laser pulses to let electrons 'surf' on a plasma wave. The problem has been controlling exactly how much the electrons are accelerated.
Jerome Faure and Victor Malka at the ENSTA/CNRS laboratory near Paris injected electrons into a plasma wave created by a single intense laser pulse.

a, In Faure and colleagues' experimental scheme, the primary laser wakefield pulse ionizes helium gas to a plasma. If the parameters of pulse and plasma are chosen appropriately, the electrons of the plasma oscillate about a fixed spot. b, If a second 'tow-in' pulse travelling in the opposite direction crosses the first, a standing wave forms. Electrons are pushed left and right in the standing wave from the antinodes to the nodes. c, Some electrons — those pushed to the right — gain enough speed to get caught up in the following wave crest and are accelerated forwards. The energy gain of the electrons is determined by how far they have to surf through the plasma, and so by where exactly along the plasma the two laser pulses cross.

From Nature 444, 688-689 (7 December 2006)