Bifurcation mode of relativistic and charge-displacement self-channelling

Stable self-channelling of ultra-powerful (P0 ∼ 1 TW-1 PW) laser pulses in dense plasmas is a key process for many applications requiring the controlled compression of power at high levels. Theoretical computations predict that the transition zone between the stable and highly unstable regimes of relativistic/charge-displacement self-channelling is well characterized by a form of weak instability that involves bifurcation of the propagating energy into two channels. Recent observations of unstable behaviour with femtosecond 248 nm pulses reveal a mode of bifurcation that corresponds well to these theoretical predictions. It is further experimentally shown that the use of a suitable longitudinal gradient in the plasma density can eliminate this unstable response and restore the efficient formation of single stable channels.