S.A. Ramsden

Observation of gain in a possible extreme ultraviolet lasing system

Abstract Measurements of gain by stimulated emission of the Balmer α line of the hydrogenic ion, C VI, at 182 Aring; are reported. The population inversion is generated by recombination in the expansion of a carbon fibre heated by a line focussed laser pulse. The gain is identified by observations using two calibrated spectrographs from the ratio of intensities, and by spectral line modification. Gain/length products of value up to 5 are deduced.

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.

MEASUREMENT AND DETAILED ANALYSIS OF SINGLE PASS GAIN AT 81-A IN A RECOMBINING LASER-PRODUCED FLUORINE PLASMA

Abstract XUV amplication at 81 A was observed in a freely expanding adiabatically cooled plasma produced by laser irradiation of LiF coated carbon fibre targets. Measurements included time resolved on- and off-axis spectroscopy, target length variation and the variation of the absorbed laser energy. Detailed analysis of the experimental data has been compared with numerical simulations.

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.

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.

X-Ray Laser Research at the Rutherford Appleton Laboratory

Recent experiments carried out at the Central Laser Facility clearly show amplification of spontaneous emission at 182Å with a g1-4 in a recombining carbon plasma. Thin 7µm diam. carbon fibres were irradiated with the VULCAN Nd.glass laser system. This scheme was extended to shorter wavelengths by using LiF-coated carbon fibre targets. Initial analysis of the experimental data indicates gain of the Balmer a transition of fluorine at 81Å.

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.

A Direct Observation of Gain in the XUV Spectral Region

Lasers operating at short wavelengths in the XUV and X-ray regions have excited considerable interest over the past decade. However, despite the many imaginative and original approaches suggested, no successful demonstration of gain in a system which can be scaled to yield laser action has yet been described. In this paper we report observation of gain on the Balmer α (Hα) transition of the hydrogen-like carbon ion (CVI) with a gainlength product in the range 1–5.

XUV amplification in cylindrically expanding laser plasmas

This invited paper describes recent experimental results demonstrating amplification of spontaneous emission in the XUV spectral regions. The gain medium is a rapidly expanding recombining cylindrical plasma produced by laser irradiation of thin fiber targets. Moderately larger laser gain between 1–5 cm−1 has been observed in transitions of hydrogenic (C VI and F IX) and Li‐like (AL XI and CI XV) ions. This work hs been carried at the Central Laser Facility of the Rutherford Appleton Laboratory by various groups using the high power VULCAN laser.