D.M. Oneill

Characterisation of soft X-ray amplification observed in Ne-like germanium

Abstract Soft X-ray amplification has been observed on five 3p → 3s transitions of Ne-like germanium at 19.6, 23,2, 23.6, 24.7 and 28.6 nm using flat, massive targets irradiated in 50 and 150 μm wide line foci up to 3.2 cm long at intensities of 0.3 → 14 × 10 13 W/cm 2 by 1.06 μm laser light in 1 ns and 0.5 ns pulses. Gain lengths up to gl ≈ 12 were measured for the J =2 → 1 transition at 23.6 nm and the temporal behaviour of four of the Ne-like germanium lasing lines has been investigated for the first time. The duration of the gain was 60 → 80% of the fwhm of the laser pulse, with the gain turning off at the trailing half-maximum of the incident laser intensity. Output powers in excess of 10 kW per line have been observed.

Gain scaling relationships for Ne-like Ge slab targets

Abstract The gain coefficient of the strongest 3p→3s, J = 2→1 lasing transition at 23.6 nm in the Ne-like Ge collisional excitation scheme has been measured, using the fundamental wavelength from a Nd: glass laser (1.06 μm), for a range of incident intensities on massive stripe targets up to 2.2 cm in length. From a threshold incident laser intensity of ∼ 6 x 10 12 W/cm 2 , the gain coefficient rises to ∼ 4.5 cm -1 for an irradiation intensity of ∼ 2.5 x 10 13 W/cm 2 , tending towards still higher gain coefficients at higher incident intensities. For targets of maximum length, a gain-length product gL ≈ 10 was reached with a resultant output power at 23.6 nm estimated to be at the ∼ kW level. The beam divergence decreased with length to a minimum of ∼ 7 mrad but no significant trend in beam pointing with plasma length was observed. From the trend in the gain coefficient, it appears that for a fixed energy laser irradiating a ∼ 100 μm wide slab targets, an incident incident of I i ∼ 1.2 x 10 13 W/cm 2 represents an optimum working level, assuming that plasma length is not limited by refractive effects. In addition to the usual valence electron excited 3p→3s transitions, the gain coefficient for the core excited 1s 2 2s2p 6 3d→1s 2 2s2p 6 3p transition at 19.9 nm has been measured to be ∼ 1.5 cm -1 for an incident irradiance of ∼2.5 x 10 13 W/cm 2 .

Soft X-ray amplification in aluminium recombining plasma produced from a thin coated fibre

Thin carbon fibres, coated with aluminium, are irradiated with approximately=120 ps, 0.53 mu m laser pulses yielding a specific absorbed energy of approximately=4 J cm-1 and time-resolved gain measurements on the n=4-3 and n=5-3 transitions of lithium-like Al XI ions in the recombining plasma are made. In particular, gain on the 4f-3d line at 15.4 nm is measured with alpha max approximately=2-3 cm-1 using both single-shot and multi-shot techniques. Gain measurements from multi-shot data on the 5f-3d line at 10.6 nm indicate alpha max approximately=1.5 cm-1. Observations are compared with theoretical calculations, discrepancies are noted and the need for reproducible plasma conditions in low gain-length product data sets is discussed.

STATUS OF SOFT-X-RAY LASER RESEARCH AT THE RUTHERFORD APPLETON LABORATORY

This review describes experimental results demonstrating amplification of XUV radiation at discreet wavelengths between 8.1 nm and 18.2 nm. The amplifying medium is generated by recombination in expanding, laser-heated thin fibre targets. Gain coefficients between 1-5 cm-1 have been observed in transitions in hydrogenic (CVI and FIX) and Li-like (AlXI) ions. The work has been carried out using the VULCAN laser of the SERC Central Laser Facility at the Rutherford-Appleton Laboratory and is the product of national and international collaborations.

Time-resolved measurements of the angular distribution of lasing at 23.6 nm in Ne-like germanium

Abstract The time dependence of the angular distribution of soft X-ray lasing at 23.6 nm in Ne-like germanium has been measured using a streak camera. Slabs of germanium have been irradiated over ≈ 22 mm length × 100 μm width with three line focussed beams of the SERC Rutherford Appleton Laboratory VULCAN laser at 1.06 μm wavelength. The laser beam sweeps in time towards the target surface plane and the divergence broadens with time. The change of the peak intensity pointing and the broadening of the profile with time are consistent with expectations of the time dependence of refraction and divergence due to density gradients in the plasma.

Development of soft x-ray lasers at the Institute of Laser Engineering: recent results on Ge soft x-ray laser (Invited Paper)

Basic characteristics of soft x-ray lasers generated as amplified spontaneous emission are described. Experimental results on soft x-ray amplification in neon-like germanium ions, obtained recently at the Institute of Laser Engineering, are reported. By comparing the experimental results with a simplified model on amplified spontaneous emission, basic parameters such as coherence and brightness of the Ge soft x-ray laser are evaluated.

An investigation of the X-ray point-source brightness for a short-pulse laser plasma

Abstract The production of X-rays is examined for photon energies in the range ∼3–5 KeV Created when a laser of 0.53 μm wavelength, 100 psec pulse duration and pulse energy ∼15 J is focussed onto various targets of dimension ≲ 10 μm. The aim is to produce a point source (⩽15 μm) of duration ≲ 100 psec suitable, for example, as a backlighter for laser-fusion experiments. We found that tin (Sn) is the best emitter in the 3–5 keV range of the materials studied (Ca, Ti, Sn, Te, CsI) and that stripe targets are better emitters than fibre targets.

Collisionally excited X-ray laser schemes: progress at Rutherford Appleton Laboratory (Invited Paper)

Systematic study of the collisionally pumped Ne-like germanium 3p-3s laser system has led to the observation of saturated output on the J = 2-1 lines at 23.2 and 23.6 nm when a double slab target is double passed using a normal incidence concave mirror. The about-1-MW output power on the same lines from a simple double plasma has been used to record preliminary images with a Schwarzchild condenser and zone plate microscope arrangement. Initial analysis of measurements on the 4d-4p J = 1-0 line at 7.3 nm from nickel-like samarium indicate a gain coefficient of between 0.4/cm and 0.8/cm for an incident irradiance of about 2 x 10 exp 13 W/sq cm using a l.06-micron heating laser on slab targets of SmF3.

Measurement of the photo-pump strength of the 3d-5f transitions in the automatically line matched Ni-like Sm photo-pumped X-ray laser

The photo-pump strengths of both the ((3d4)0(3d6)0)0 - (((3d3)3/2(3d6)0)3/2(5f)5/2)1 and the ((3d4)0(3d6)0)0 - (((3d4)0(3d5)5/2)5/2(5f)7/2)1 transitions in Ni-like Sm34+ have been measured as 2.4*10(-4) and 2.4*10(-4) photons/mode respectively. The implications of the measurement are briefly discussed in a comparison of the merits of automatically line matched photo-pump scheme to those of the collisional excitation Ni-like Sm(+34) scheme.

Radiation damage of multilayer mirrors for soft x-ray lasers

We have successfully demonstrated double pass enhancement of amplified spontaneous emission of soft x rays, 23.2 and 23.6 nm of 3p - 3s transitions in Ne-like Ge, using an x-ray multilayer mirror. In this paper, we report on the fabrication of the mirror and analysis of its damage suffered during the experiments. The mirror used was a Mo-Si multilayer mirror with the reflectivity of 35% at the wavelength of 23.6 nm, deposited by an rf-sputtering system. In the damaged area of the mirror, only the multilayer was locally evaporated and the bare substrate underneath appeared. The size of the damaged area corresponded to the aperture size. We carried out the simulation on the spatial and temporal distribution of the mirror temperature during the experiment. Assuming that thermal x rays enter the mirror with the largest amount of energy among all the fluxes at the early stage of the enhancement, the result of the simulation can explain the damage feature and the temporal profile of the intensity of the amplified spontaneous emissions.