Guiding of positive streamers in nitrogen, argon and N$_{2}$-O$_{2}$   mixtures by very low $n_{e}$ laser-induced pre-ionization trails
  In previous work we have shown that positive streamers in pure nitrogen can be guided by a laser-induced trail of low electron density. Here we show more detailed results from such measurements. We show the sensitivity of this laser-guiding on pressure p and found that the maximum delay between the laser pulse and voltage pulse for guiding scales with something between $1/p$ and $1/p^{2}$. We also show that when we use a narrower laser beam the laser guiding occurs less frequent and that when we move the laser beam away from the symmetry axis, guiding hardly is observed. Finally we show that laser guiding can also occur in pure argon. 
S. Nijdam, E. Takahashi
physics.plasm-ph