Mapping of induced electric field in FSA configuration

A recent method to produce accelerated protons and ions is the Front Surface Acceleration (FSA) realized by Laser Ion Source (LIS). We analyzed a LIS implemented to justify the ion acceleration. The plasma is reached by a KrF excimer laser operating at 248 nm. The beam was focused on a solid aluminum target mounted inside a vacuum chamber in order to obtain warm particles. The laser energy was varied from 28 to 56 mJ/pulse and focused onto the target by a 15 cm focal lens forming a spot of 0.05 cm in diameter. A high impedance resistive probe was utilized to detect the potential value inside the chamber, around the target. In order to avoid that the plasma particles invest the probe, a PVC shield containing the probe was placed perpendicularly to the main axis. Particles inevitably streaked the shield but their influence on the probe was irrelevant trivial. By the above consideration we detected potential values from 4.7 cm to 6.2 cm with respect to target axis, while the distance of the shield from the axis is about 3 cm. The electric field, determined by the potential measurement, can be very important to find the phenomenon responsible of the accelerating field creation. The behavior of the field on distance x assumes the dependence of 1/$x^1.85$ with 28 mJ laser energy, 1/$x^1.83$ with 49 mJ laser energy and 1/$x^1.84$ with 56 mJ laser energy. The power degree is between 1÷2. So, it is possible to hypothesize that the electric strength is the contribution of an electrostatic and of an induced field The extension of the experimental behavior just to x = 0.02 cm the electric strength is of the order of tens kV/m which delivers ions up to 1keV. These values were justified by measurement performed with the electrostatic barrier. Considering exclusively the induced field and applying the Larmor formula the field strength results to be maximum at the center. For an electronic density of ρ = 1 × $10^18$ $m^{-3}$, we estimated a field of E = 1.97 * $10^6$ V/m at the center, and an filed of E = 1.26 * $10^6$ V/m at beam borderline.