# Experimental Study of Nanosecond Laser-Generated Plasma Channels
## 1 Introduction  
After the advent of the laser chirped pulse amplification (CPA) technique [1] giving rise to high peak power femtosecond pulses, the laser wake field acceleration (LWFA) concept [2] has become a routinely used experimental technique to generate electron beams by means of such ultrashort laser pulses [3]. The main advantage in LWFA is represented by the presence of ultrahigh electric fields (GV/m-TV/m) generated by a plasma-wave, known as \"wakefield\", which is a result of ponderomotive forces generated by a driving short laser pulse during its propagation into a pre-formed under-dense plasma. Electrons injected in such a wave can easily reach relativistic energies by being in phase with the wakefield and propagating behind the laser pulse, thus be accelerated to multi-GeV energies until the process is sustained. In recent decades, various methods have been proposed and experimentally tested to extend maximum plasma length over which the acceleration process takes place [4; 5; 6; 7; 8; 9]. In fact, as it is well known, a focused laser pulse can sustain its peak intensity only within the \"Rayleigh length\" determined by the given focusing optics, thus such a length limits the maximum distance over which an efficient acceleration occurs and, as a consequence the maximum energy gain of the accelerated electrons. Guiding a short laser pulse well beyond its Rayleigh length is possible by using plasma densities that allow laser self-focusing in the ionized gas. However, such a process is sensitive to plasma instabilities [3]. On the other hand, stable plasma guiding methods have been demonstrated experimentally, and allowed to accelerate electron bunches up to 8 GeV, as described in [10; 11]. This experimental technique is based on the use of a capillary-discharge gas target in combination with a laser heater, which allows the generation of a parabolic plasma-channel used, in turn, to guide an additional driving high peak-power laser pulse over a distance of about 10 cm. Nevertheless, a disadvantage of such a discharge capillary target is represented by the experimental limitation in terms of optical probing of the plasma and bunch dynamics, due to the presence of the capillary walls. Additionally, the capillary input region can rapidly deteriorate due to the incoming high intensity laser pulse, even if such an effect is mitigated by the use of the laser heater. Various studies focus on peculiar aspects of such capillary-discharge based approaches [12; 13; 14]. The basic idea is that, by means of the electric discharge, energy is released on the main cylindrical axis of the capillary-discharge-target to produce a cylindrical expansion that generates a lower density profile in proximity of the plasma axis. A different approach consists in using an axicon lens and a femtosecond laser beam to release energy into the cylindrical plasma axis and produce plasma-channels, as reported in the literature [15]. The advantages of using all-optical techniques for generating the required plasma-channel characteristics are absence of target deterioration effects, and possibility of optical probing of the plasma density evolution during laser-plasma interaction. Moreover, the all-optical approach allows for a more sophisticated release of the energy in time, in principle up to the femtosecond scale, by using different laser pulses or a time-modulated one. The time dependence of the energy release is of high importance for the hydrodynamic plasma evolution to explore new parameter ranges. The experimental approach presented here investigates the possibility of generating plasma-channels in an all-optical geometry by using a cylindrical lens and a nanosecond laser pulse with moderate intensity. In future experiments, time-modulated nanosecond laser pulse may be considered. The cylindrical lens is used to release the energy into the main plasma axis, similarly to the discharge-capillary target.