# Experimental Study of Nanosecond Laser-Generated Plasma Channels
## 4 Experimental Results and Discussion  
Various high-aspect-ratio plasma structures were produced. Figure 6b shows the structure of the elongated plasma in the shadowgraphy image. Figure 6a shows the top-view of the optical plasma emission for the same shot in Ar at a baking pressure of 51.2 bar and a laser energy of about 2.5 J. In these experimental conditions, a fully continuous elongated plasma structure and optical plasma emission is clearly visible along the entire long focus. Figure 7 summarizes results obtained in Ar at 51.2 bar and different laser energies. Figure 7a shows the shadowgraphy profiles along the y-axis, averaged over 1 mm of the structure in the x-axis. Figure 7b shows the optical plasma emission averaged along the entire z-axis. At 0.9 J, no plasma is generated. In the 1.67-J case a plasma is generated, however the entire elongated plasma structure is not uniform, but roughly reproduces the gas-jet density profile, as detected by optical plasma emission. In this specific case, a similar profile is present in the shadowgraphy. At the value of 2.5 J an entirely formed elongated plasma structure, with an improved degree of uniformity, is present both in the shadowgraphy and in the optical plasma emission. Figure 6: Experimental measurement of plasma formation in Ar gas target at 51.2 bar backing pressure and 2.5-J laser energy. (**a**) Top-view image of the optical plasma emission (the gas nozzle is visible in the background); (**b**) shadowgraphy image of the elongated plasma structure and inner diameter measurement on the x-y plane. Figure 8b shows the structure of the elongated plasma in the shadowgraphy image. Figure 8a shows the top-view of the optical plasma emission for the same shot in N at a backing pressure of 64 bar and a laser energy of about 5 J. In these experimental conditions, a fully continuous elongated plasma structure and optical plasma emission is clearly visible along the entire long focus. Figure 9 summarizes the results obtained with Nitrogen at a backing pressure of 64 bar for different laser energy. Figure 9a shows the shadowgraphy profiles along the y-axis, averaged over 1 mm of the structure in the x-axis. Figure 9b shows optical plasma emission averaged along the entire z-axis. In the 4.2-J case a plasma channel is created, but this presents a low degree of uniformity through its entire length, as detected by optical plasma emission. In the 3.3-J case, no plasma is generated, as confirmed by the absence of optical plasma emission. Based on shadowgraphy images and plasma optical emission, various kinds of uniformities can be seen. This information is taken into account for an estimation of the laser-significant intensity-threshold--and for considerations of the best laser  
Figure 8: Experimental measurements obtained with N at 64 bar and 5 J. (**a**) Top view of the gas-nozzles and optical plasma emission; (**b**) shadowgraphy of the plasma channel and inner diameter measurement. Figure 7: Experimental measurements obtained with an Ar target at 51.2 bar for different laser energy values; (**a**) Shadowgraphy profiles of the elongated plasma structures in the y-axis, averaged over 1 mm in the x-axis; (**b**) top-view profiles of the optical plasma emission along the x-axis, averaged along the entire z-axis. focusing conditions--with the aim to localize the incoming laser energy within a region that evolves symmetrically during the hydrodynamic expansion. In summary, elongated plasma structures of about 5-mm long and 50-\(\upmu\)m wide were measured in Argon, while 3.7-mm long and 90-\(\upmu\)m wide were estimated in the Nitrogen case. Such quantities were measured after 10 ns from the arrival of the laser pulse by using a probe beam which was 10 ns long and using an integration time of 10 \(\upmu\)s for the cameras. The significant-intensity-threshold estimation is a conservative estimation (upper threshold) of the laser intensity for localized plasma formation.