Avalanche initiating electron produced by laser-induced tunneling

We have been able to eliminate all extrinsic sources of avalanche initiating electrons in ultrapure N
2
and He gas. Plasma formation is therefore inhibited until the laser intensity glows high enough to produce tunnel ionization of a molecule in the focal volume. This permits a record CO
2
laser intensity, as high as 10
¹³
W/cm
²
in a high-pressure gaseous target. For the first time the electron tunneling limit has been approached in a dense medium.