High energy density physics with intense ion and laser beams

A transient collisionally pumped X-ray laser (XRL) driven by the infrared pulses from the PHELIX laser preamplifier at GSI has successfully been put into operation. Strong lasing at 22 nm has been observed in nickel-like zirconium. Experimental data from the optimization of the XRL energy output support the conclusion that inverse Bremsstrahlung plays a key role in the pumping mechanism. Laser induced fluorescence spectroscopy in lithium-like heavy ions can be performed using narrow bandwidth light pulses with a few μJ of energy in the XUV spectral region [1]. Such pulses are routinely produced in the scheme of transient collisional excitation (TCE) x-ray laser , which was first demonstrated by Nickles et al. [2]. In this scheme, saturated output can be obtained with a few J of pulse energy from the pumping laser (see e.g. [3]). A laser pulse with an energy of a few J and ns duration is focused to a line onto a solid target, creating a line-shaped plasma with a high abundance of nickel-like ions. A high intensity short (∼ ps) infrared laser pulse produced by the technique of ,,chirped pulse amplification” (CPA) heats the plasma rapidly to a high temperature which leads to a short lived (transient) population inversion. A bright, partly coherent XUV-pulse is emitted from the end of the plasma column by amplified spontaneous emission. Due to the short life time of the transient gain [4], the pumped region has to travel with the amplified radiation, which is achieved by so called travelling wave excitation [5]. At GSI, a high intensity/high energy laser system (PHELIX) is under construction [6]. The CPA front-end together with the preamplifier and pulse compressor are well suited for pumping a TCE XRL. The front-end delivers stretched pulses of > 50 mJ with a bandwidth of ∼ 7 nm. The preamplifier consists of three flashlamp pumped Nd:Glass rods (with 2x 19 mm and 45 mm diameter) which amplify the pulses further to an energy of several J. The pulse compressor, in a double folded single grating arrangement, is entirely housed in a vacuum chamber and is designed to compress pulses of up to 15 J to durations below 400 fs. Pulses up to 10 TW peak power have been achieved. Without the use of adaptive optics for the wave front correction the repetition rate is limited by the cooling time of the three heads to one shot every 6 minutes . For pumping the XRL we have used 5-6 Joule pulses; 20 % of the pulse energy are split off by a beam splitter and remain uncompressed to form the prepulse. After being delayed in an optical delay line the prepulse is injected Figure 1: Line focus image and longitudinal intensity distribution for the infrared laser, obtained with the parabolic mirror into the target chamber and focused by means of a cylindrical lens to a line focus (80μm× 10mm) to create the preplasma. The remaining 80 % of the pulse are injected into the pulse compressor. After compression the pulse is transported under vacuum into the target chamber. A single on-axis parabola, tilted at an incidence angle of 22 degrees, is used to generate a line focus of 30μm 100μm width and over a length of 10 mm (fig. 1). The off-axis geometry intrinsically leads to a travelling wave excitation along the line focus with a close to the optimum speed of 1.4 c where c is the speed of light. The delay between the prepulse and the compressed pulse was set to 0.7 ns. Figure 2: Typical spectrum of the Ni-like zirconium laser, corrected for the Al-filter transmission The main diagnostics was an XUV flat-field Hitachi