Physics of the expanding plasma ejected from a small spot illumined by an ultraviolet pulsed laser

We present the results concerning the physics of the expanding plasma produced by a laser ion source. An efficient source of multiple charged ions was realized by means of an excimer laser. The analysis of the generated plasma was performed for three different laser spot sizes, determining the threshold conditions of the ablation process for a Cu target. Two typologies of Faraday cups were developed in order to detect the plasma current and the ion current along the propagation tube. The time-of-flight measurements were performed inserting in front of the cup an adjustable voltage electrostatic barrier that allowed us to get quantitative information about the ion flux and the kinetic energy of the produced ions. To study the plasma characteristics we measured the total etched material per pulse, 0.25 mg, and the fractional ionization, 12%. The ablated material distribution was monitored by optical transmission analysis of a deposited film. Applying a high voltage to the extraction gap, an ion beam containing Cu11 ~0.44 mA!, Cu12 ~0.34 mA!, Cu13 ~0.09 mA!, and Cu14 ~0.01 mA! ions was obtained.