Optimisation of Transient Gain X-ray Laser for 20-32 nm

The development of X-ray lasers is marked by the progression toward shorter wavelengths using less drive energy. A recent X-ray laser experiment at the Rutherford Appleton Laboratory demonstrated saturated X-ray laser operation in the Ne-like Ti and Ge X-ray laser schemes at 32.6 and 19.6 nm respectively, with a drive energy of only a few joules, a significant step in the progression to table top systems. In this report we describe the laser development necessary to generate suitable travelling wave line-focus laser drive pulses and the transient population inversion X-ray laser results obtained. BACKGROUND REVIEW The brightest and most robust X-ray lasers to date rely on electron collisional excitation to pump electrons into the upper laser state, a quasi-steady state population inversion being formed due to the rapid radiative decay of the lower laser level with respect to the upper one. The most efficient schemes still require 100’s of Joules of drive laser energy on target i) and in order to become practical and accessible for applications, X-ray lasers must become more compact and efficient. One problem with quasi-steady state schemes is that the regime in which Ne-like ions dominate is of too low an electron temperature to support optimum excitation rates. A novel scheme, utilising a transient population inversion was proposed theoretically a number of years ago . It involves a combination of ns and ps duration optical pulses. The ns pulse is low intensity and preforms a column of plasma, heated sufficiently to produce an abundance of Ne-like ions. Once the Ne-like fraction is optimised, the high intensity ps pulse rapidly heats the plasma in a time-scale shorter than that of the relaxation processes of the excited states, and before any significant further ionisation can occur. Due to the ps pulse excitation, the transient inversion which occurs, is related to the different population rates of the levels via collisions and not determined by the slower relaxation rates of the excited levels relevant in the quasi-steady state quasi-steady state regime. This transient inversion is characterised by a short life time and theoretically yields much higher small signal gain values (10-100 cm) as obtained in the quasi-steady state regime (several cm) with long pulse pumping. Recently a collisionally excited transient gain X-ray laser in Ne-like Ti with a gain value of 19 cm on the 3p-3s 32.6 nm line at a low pump level of only a few Joules was demonstrated by the MBI group. By comparison, the most efficient quasi-steady state (long pulse driven) gain coefficient yet measured for Ti is 3.3 cm , requiring substantially higher drive energy. Simulations of the Ge X-ray laser transient population inversion scheme have also shown the possibility of ultra-high gains on a number of transitions . A peak local gain coefficient of 140 cm has been predicted for the J = 0-1, 196 Å transition, which when ray traced to take account of refractive effects gives a ray averaged gain of ~30 cm. However, due to the restricted experimental conditions at the MBI, neither gain saturation nor significant shifting to shorter wavelengths could be demonstrated. Therefore, a joint experiment was carried out based on the high energy resources of the Vulcan CPA laser and the well proven RAL diagnostics to investigate some key parameters of this new transient excitation scheme for Nelike ions. The aims were: • Achieve saturation and characterise a Ti X-ray laser under the higher irradiance conditions available using Vulcan, utilising a travelling wave pump (see later) to sample longer target lengths. • Demonstrating the practicality of scaling the scheme to higher Z materials, in order to reduce the XRL wavelength. INTRODUCTION The X-ray laser experiment was carried out on the VULCAN high power laser system, delivering synchronised nanosecond and subpicosecond pulses in a multi-beam configuration. Short pulse (ps) generation at ultra-high intensities is achieved using the technique of chirped pulse amplification (CPA). For this experiment both pulses were configured in line focus geometry and overlapped on target. The travelling wave was optimised by inserting a gold coated diffraction grating, and associated mirrors, at the output of the rod amplifier stage of the VULCAN laser chain. The first order diffraction imposes a tilt on the laser wave front in one dimension due to the path length difference across the beam. The near-field is imaged through the system and the tilt is preserved through the focusing system and onto target generating a travelling line focus. The short pulse was synchronised to the falling edge of the nanosecond long pulse allowing optimised plasma conditions to be formed. In this report, the amplification characteristics of the 3p-3s (J=0-1) transition at λ= 32.6 nm and the 3d-3p (J=1-1) transition at λ= 30 nm were studied with plasma column lengths up to 10 mm. Also, the dependence of the X-ray lasing signal on the optical laser pump energy was measured, the X-ray beam divergence was determined and the total output energy of the X-ray laser pulse was estimated. For the first time saturation of the low pump energy X-ray laser utilising transient gain on the 3p-3s transition in Ne-like Ti at 32.6 nm and Ge at 19.6 nm was demonstrated. EXPERIMENTAL SET-UP The experimental set-up for this experiment involved reconfiguring the laser to provide a travelling wave, constructing a suitable line focus illumination system and diagnosing the X-ray laser and plasma conditions. These will be described in the following two sections 1. The Line Focus and Travelling wave 2. X-ray laser interaction and diagnostics 1. THE LINE FOCUS AND TRAVELLING WAVE The creation of both the short and long pulse line focii relies on the optical aberration that is introduced by spherical mirrors operated at an angle relative to the optic axis. A schematic of the target chamber optical configuration is shown in figure 1. For the ultra-short CPA pulse the incoming laser beam is focused to a point (surrogate focus) using an off-axis parabolic (OAP) mirror. The spherical mirror then images this point source to the target plane. The angled nature of the spherical mirror introduces a large astigmatism into the beam producing two, one dimensional ‘images’ which are line focii. The second of the line focii which lies in the horizontal plane is the one that is used. The line focus always points toward the surrogate focus as can be seen from the geometry shown in Figure 1. The long pulse pre-ionising beam is delivered to target in a very similar way to the short pulse. The beam is brought to a surrogate focus using a 275 mm focal length lens and the line focus generated using a 315 mm focal length spherical mirror used off-axis. The geometry leads to an inherent optical path difference between rays arriving on one side of the line focus with respect to the other. This optical path difference can be significant for the ultra-short pulses used. This means that the incident laser pulse will propagate from one side of the target to the other with a finite velocity that is normally several times the speed of light. This propagation velocity increases rapidly with reduced incidence angle onto the spherical mirror. The target lengths are typically 10 mm which means that the transit time for an X-ray laser photon from one end of the line focus to the other is typically tens of picoseconds. The X-ray laser upper state lifetime (gain) only lasts for a few picoseconds so if the plasma were to be formed at the same time everywhere along the line length then the gain at the end of the line will have dissipated by the time the X-ray photons arrive. The travelling wave effect can therefore significantly assist the X-ray laser process. One consequence is that the X-ray laser becomes directional with significantly higher output in the plasma propagation direction. The typical phase velocities encountered in this scheme are of the order 2-3 times the speed of light (c). Ideally, the phase velocity should be reduced to exactly c to ensure that that the X-ray photons always see the peak of the available gain throughout the whole length of the line. The input beam dimensions to the target interaction chamber were 88 mm horizontally and 130 mm vertically. The off-axis parabola had a focal length of 450 mm producing a F5.1 surrogate cone. The spherical mirror, located 350 mm from the surrogate had a 350 mm focal length and was operated at an incidence angle of 15.5°. This gives a distance of 195 mm from the surrogate to line focus centre. This focusing system was modelled by the optical design program ZEEMAX which gave a propagation time of 15.9 ps over a 12.0 mm length line focus which gives a phase velocity of 2.5 c. FIGURE 1. Optical layout of the target interaction chamber for the X-ray laser experiment. 2 ps 30 J 450 mm fl. OAP CPA Diagnostics 1ns 100 J