# Experimental Study of Nanosecond Laser-Generated Plasma Channels
## 2 Concept of Elongated Focus for Plasma-Channel Generation
Additionally, considering laser intensity distribution and phase front fluctuations, even an ideal cylindrical laser field geometry could fail to generate shot-to-shot reproducible symmetric plasma channels. In principle, all these unwanted effects can be mitigated by a proper control of the nanosecond laser intensity range (upper and lower limits) within a confined radial region around the cylindrical axis, which is smaller than the desired channel diameter resulting from the hydrodynamic cylindrical expansion. The main goal of the experiment was to investigate such laser intensity threshold and proper range (upper and lower limits) for the future generation and control of symmetric plasma channels in gas targets of interest for LWFA. In the following, we will refer to the term significant-intensity-threshold. Such threshold can be estimated generating simpler elongated plasma structures (asymmetric channels). This experimental threshold depends critically on the gas atomic species and on the operational backing pressure of the gas target (i.e., gas density). The present experimental work was focused on investigation of the conditions for reproducible elongated plasma structures using various gases and pressures. The laser significant-intensity-threshold was estimated without any attempt to generate symmetric plasma channels. Figure 3: Measured focal distribution, (**bottom**) y-axis profile averaged over 100 \(\upmu\)m on the x-axis and (**top**) 2-mm portion of the x-axis profile averaged over the full y-axis. The laser significant-intensity-threshold required to generate a shot-to-shot reproducible plasma from a gas target is a key parameter, also if one aims to extend the plasma channel length over tens of centimeters and beyond. From this point of view, in our experimental conditions, gases with higher atomic species are favorable due to a required lower laser energy per unit length of the channel and to a lower plasma radial expansion velocity. Consequently, we have used Ar and N having the same specific heat ratio with respect to He and H gases (key parameter from a gas dynamic point of view), which are more frequently used in LWFA. It is worth mentioning that estimation of the significant-intensity-thresholds for given gas species and pressure conditions provide a rough indication of the behavior of different gas species based on ionization potentials and laser absorption processes, if the ultimate goal remains to confine the nanosecond laser energy within a certain volume, thus leading to the generation of a symmetric cylindrical plasma expansion. Standard laser-target experimental conditions were used in the experiment: 10 ns, 5-J laser pulse with gas target pressure in the range 20-60 bar. A relatively high pressure was used for two reasons. First, the hydrodynamic expansion generates a density drop of 1-2 order of magnitudes in proximity of the axis. Second, a low gas neutral density would lead to an excessively high plasma expansion velocity, thus limiting the future scalability of this experimental concept to longer plasma channels. In fact, future availability of the 100-J beamline of the Bivoj laser at the HiLASE (High average power pulsed LASErs) research center is considered to extend this work to formation of plasma channels with lengths of several tens of cm [19,20]. This study is limited to the generation of ns-plasma elongated structures and no investigation of fs-laser pulse propagation is presented.