# Experimental Study of Nanosecond Laser-Generated Plasma Channels
## 5 Conclusions  
The use of a nanosecond laser pulse for generation of 5-mm long and tens of microns wide elongated plasma structures in a gas-jet was demonstrated experimentally. Estimation of the significant intensity threshold formation useful for a future symmetrization of the plasma channel is reported for Ar and N. The results indicate that further investigation is needed for the symmetrization of the channels and that such laser-plasma interaction conditions are accessible with joule-class, nanosecond lasers. Moreover, such results establish an optimal energy/intensity working range for future studies focused on symmetrization issues, along with a preliminary assessment on needed adjustment of the focusing optics, as well as use of additional plasma diagnostics. In particular, this experimental technique will require a second probe beam to measure the degree of symmetry of the plasma channel. Additionally, a shorter laser probe pulse will be required to measure the instantaneous density profile inside the plasma channel, as well as monitoring its expansion velocity. This approach allows to use time-modulated energy release (temporally shaped laser pulse) with additional control of the cylindrical hydrodynamic evolution over a wider space of parameters.  
This work represents an important step towards future implementation of such potentially innovative experimental approach for generation of symmetric plasma-channels with required lengths,  
Figure 10: Optical plasma emission signal (blue-channel) vs laser energy for Ar and N gas targets.  
both in a single stage and in a multi-stage LFWA scheme, thus potentially promising for implementation of future compact accelerators aimed to produce high-energy electrons by laser light.  
Conceptualization, T.L.; Data curation, T.L., M.N., M.F.N. and F.G.; Investigation, T.L., M.N., M.F.N., H.Z., J.P., M.H. and M.D.; Project administration, T.L., L.G. and A.L.; Software, T.L., M.N. and M.F.N.; Supervision, T.M. and D.M.; Validation, T.L., L.G., A.L., T.M. and D.M.; Writing--original draft, T.L., M.N. and M.F.N.; Writing--review & editing, L.G., F.G., H.Z., J.P., M.H., M.D., A.L., T.M. and D.M. All authors have read and agreed to the published version of the manuscript.  
This work was carried out within the Particle Acceleration by Laser Research Program (RP3) at ELI-Beamlines. This work was supported by the European Regional Development Fund, by the Ministry of Education, Youth and Sports of the Czech Republic (Project No. LQ1606) and by the project \"Advanced Research Using High Intensity Laser Produced Photons and Particles\" (CZ.02.1.0/0.0/0.0/16_019/0000789). The work was co-financed by the European Regional Development Fund and the state budget of the Czech Republic (project HiLASE CoE: Grant No. CZ.02.1.01/0.0/0.0/15_006/0000674) and also supported by the Ministry of Education, Youth and Sports of the Czech Republic (Programmes NPU I Project No. LO1602 and Large Research Infrastructure Project No. LM2015086).  
The authors gratefully acknowledge [PERSON] for the support provided to improve the readability of the manuscript, along with [PERSON] and [PERSON] for fruitful discussions.  
The authors declare no conflict of interest.