Generation of extreme high laser intensities in plasma

Abstract

Studies of nonlinear processes of self-focusing of a laser beam in a plasma were performed using the PALS TW iodine laser system. The leading part of the 400 ps PALS laser pulse generated a plasma with which the main part of the pulse interacted. Changing a position of the optical focus with regard to the target surface, the interaction conditions were also significantly changed. Due to the self-focusing processes, high charge states and ion energies were recorded, which could not be obtained without the intervention of nonlinear processes. Laser intensities of the order of 10 19 W cm −2 in plasma were deduced from ion measurements for some conditions. A threshold intensity of 2 × 10 14 W cm −2 for nonlinear processes in plasma was confirmed.PACS: 52.38 Hb, 52.38 Kd

The Effect of Pre-plasma and Self-focusing on Characteristics of Laser Produced Ions

“…Knowing the shape of the laser caustic [15] it is possible to recalculate the variation with the position into a dependence on the intensity. However, a dramatic increase of z max was recorded at a laser intensity of I L ≈ 2 × 10 14 W/cm 2 if P f < 0 [10]. This implies a significant participation of new processes in the generation and the acceleration of the ions.…”

“…A schematic view of the experimental arrangement can be seen in, e.g., [7]. Changing the focus position from P f = −1730 µm to 0 changes nominal laser intensities from 1 × 10 14 up to 3 × 10 16 W/cm 2 (further change to P f = +582 µm results again in a decrease of the nominal laser intensity) [10]. Considering that the maximum velocity of the emitted ions at PALS is 6 × 10 8 cm/s (see below), the front edge of the expanding plasma plume (of low density) attains a maximum distance of 2.4 mm from the target surface after a 400 ps irradiation.…”

“…Conversely, the variation of z max with P f shows a flat maximum over a broad range of about 900 µm, but asymmetric with respect to P f = 0, where the laser intensity has a maximum [10]. Knowing the shape of the laser caustic [15] it is possible to recalculate the variation with the position into a dependence on the intensity.…”

“…Results of laser beam interaction with a target differ, if realized with pre-formed plasma than without it [10,11]. Such pre-plasma can be produced using a separte laser pre-pulse preceeding the main pulse [12][13][14].…”

“…Highly charged heavy ions with charge states above 50+ can be generated, either by using a sufficiently high laser intensity (estimated to be about ≈ 10 19 W/cm 2 ), or using lower laser intensities (above ≈ 1 × 10 14 W/cm 2 ) if the non-linear processes (self-focusing) contracting the beam are present [10,11]. With these processes, ions with the highest charge states and energies can be obtained in an "optimum" laser focus position, which is always in front of the target surface [1,2,5,9,15,16].…”