J. Krishnan

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.

The influence of prepulse level on the 3p-3s XUV laser output from Ne-like ions of Zn, Cu and Ni

Abstract We have studied the effect of prepulses in enhancing the efficiency of generating ASE beams in soft X-ray laser plasma amplifiers based on pumping Ne-like ions. Slab targets were irradiated with a weak prepulse followed by a main plasma heating pulse of nanosecond duration. Time-integrated: time and spectrally resolved and time and angularly resolved lasing emissions on the 3p-3s ( J = 0–1) XUV lasing lines of Ne-like Ni, Cu and Zn at wavelengths 232 A, 221 A and 212 A respectively have been monitored. Measurements were made for pre-pulse/main-pulse intensity ratios from 10 −5 –10 −1 and for pump delay times of 2 ns and 4.5 ns. Zinc is shown to exhibit a peak in output intensity at ∼ 2 × 10 −3 pre-pulse fraction for a 4.5 ns pump delay, with a main pulse pump intensity of ∼ 1.3 × 10 13 W cm −2 on a 20 mm target. The Zn lasing emission had a duration of ∼ 240 ps and this was insensitive to prepulse fraction. The J = 0–1 XUV laser output for nickel and copper increased monotonically with prepulse fraction, with copper targets showing least sensitivity to either prepulse level or prepulse to main pulse delay. Under the conditions of the study, the pre-pulse level was observed to have no significant influence on the output intensity of the 3p-3s ( J = 2−1) lines of any of the elements investigated.

Film calibration for soft x-ray wavelengths

The response of photographic film to X-rays from laser- plasma is of practical interest. Film is often used for the ultimate detection of x-rays in crystal and grating spectrometers and in imaging instruments such as pinhole cameras largely because of its high spatial resolution (approximately 1 - 10 microns). Characteristic curves for wavelengths--3 nm and 23 nm are presented for eight x-ray films (Kodak 101-01, 101-07, 104-02, Kodak Industrex CX, Russian UF-SH4, UF-VR2, Ilford Q plates and Shanghai 5F film). The calibrations were obtained from the emission of laser-produced carbon plasmas and a Ne-like Ge X-ray laser.

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.

212-Angstrom neonlike zinc laser of LULI

The main feature of x-ray laser research at LULI is the development of a saturated laser at 212 angstrom with a relatively small pump laser of 0.4 kJ in 600 ps. The laser works with the 3p- 3s J equals O yields 1 transition of neon-like zinc, by using the double-pass of amplified radiation in the active medium. Plasma parameters (temperature, density, homogeneity), and x-ray laser emission properties (intensity, pointing angle, divergence, and coherence) have been studied. Lasing action needs the main laser pulse to be preceded by a ten-prepulse train (contrast ratio less than 103) due to the remnant oscillator. The effect of a single prepulse was investigated as a function of contrast ratio and delay between the prepulse and the main pulse.

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.

Studies of ∼ps laser driven plasmas in line focus geometry

Measurements of X‐ray emission along linear plasmas produced in short pulse (2–12 ps) experiments using the Rutherford Appleton Laboratory glass (1.06 μm) and KrF (0.268 μm) lasers are interpreted to provide information about the uniformity and lateral and axial energy transport of X‐ray laser gain media. For fiber targets, the difficulties of achieving uniform irradiation and accurate plasma length measurements are illustrated and discussed. For slab targets, it is shown that the ratio of the distance between the critical density surface and the ablation surface to the laser focal width controls lateral transport in a similar manner as for spot focus experiments.

Study of beam aberrations in a germanium XXIII XUV laser amplifier

Abstract A beam of amplified spontaneous emission at 23.2 23.6 nm from a GeXXIII XUV laser has been injected into a separate amplifier plasma and the astigmatic aberrations introduced by plasma density gradients in the amplifier have been estimated from analysis of images of the amplified beam.

Improving the efficiency of x-ray lasers

Current successful approaches for achieving soft x-ray lasing typically require pumping laser pulses of duration approximately ns and energy approximately kJ (collisionally pumped schemes) or approximately ps pulses and powers of approximately several TW (recombination-pumped schemes). For applications, it is important to improve the efficiency of soft x-ray lasers and so reduce the required power of pumping lasers. The effect of pre- pulse on neon-like collisionally pumped lasers has been investigated using the LULI laser (Ecole Polytechnique, France). A small pre-pulse level approximately 10-3 of the main pulse energy was found to increase the J equals 0 minus 1 neon-like zinc laser output at 21 nm by an order-of-magnitude with a comparable increase in efficiency. A double pumping laser pulse on neon-like yttrium lasing output at 15 nm obtained with the VULCAN laser (Rutherford Appleton Laboratory, England) was also found to increase the x-ray lasing efficiency. With adiabatically cooled recombination lasing, it is shown that approximately 2 ps pulses are optimum for achieving the desired ionization balance for lasing output. The possibility of achieving recombination lasing at short wavelengths on lithium-like ions with longer pulse lasers has been investigated using the ASTERIX laser (Max-Planck Quantenoptik, Germany). These results are presented and interpreted to provide possible directions for improving the efficiency of x-ray lasers.

Development of injector/amplifier XUV lasers and initial studies of ultrashort pulse UV multiphoton ionization

An injector-amplifier architecture for XUV lasers has been developed and demonstrated using the Ge XXIII collisional laser. Results are described for injection into single and double plasma amplifiers. Prismatic lens-like and higher order aberrations in the amplifier are considered. Limitations on ultimate brightness are discussed and also scaling to operation at shorter wavelengths. A preliminary study has been made of UV multiphoton ionization using 300 fs pulses at high intensity.