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Lasers & Sources

Novel geometries reduce pump-energy requirements in x-ray lasers

A station providing an easily-accessible and operable x-ray laser is desirable for a number of laboratory applications. This requires introducing new driver lasers that enable efficient pumping.
1 February 2006, SPIE Newsroom. DOI: 10.1117/2.1200601.0072

The term x-ray laser (XRL) describes a source emitting coherent and very-high-intensity radiation dominantly in the extreme ultraviolet (XUV) region between 4nm and 30nm. Emission of such short wavelengths requires very-high power density to excite (pump) the active medium. The main goal pursued in the development of x-ray lasers has been to reduce the pump requirements, with a view to providing an easily-accessible source for most applications. Ideally the source would be a table-top instrument that could be installed, including the pump laser, on a laboratory table with a surface area of 10–20m2.

The first major breakthrough in developing a compact XRL was demonstrating the transient inversion scheme.1 In this approach, the pump consists of a long (nanosecond) laser pulse to create a plasma, followed by a variably-delayed picosecond laser pulse to rapidly heat this plasma. This scheme takes advantage of the chirped pulse amplification (CPA) technique developed for laser drivers, and many variants have been successfully applied in practice. In particular, the technique has been applied to the elements from the Ni-like iso-electronic sequence.2–4 The active medium is either a plasma that is ablated from metallic slab targets or gases that are ionized by an optical field ionization (OFI) process,5 and contains selected ions with energy levels excited in electron-ion collisions.

Here, we concentrate on collisionally-pumped XRLs using slab targets, which demonstrate suitable output parameters and robustness for many applications. In the traditional lateral geometry (emission occurs in the direction normal to the pump) with double-pulse irradiation, the pump energy has been reduced below 5J.2 Using this irradiation geometry and a single, shaped picosecond laser pulse, we achieved saturation of an XRL in Ni-like Ag at 13.9nm with an energy as low as 2.6J.3 Lasing has already been observed at pump energies lower than 1J. Theoretical analysis has revealed that the energy reduction effect is determined by the pulse profile. However, lateral irradiation has the disadvantage of strong refraction in the area where the energy is incident, hindering efficient lasing.

The advent of the grazing incidence pumping (GRIP) geometry (see Figure 1) is a very important step towards the solution of this problem.6–8 This geometry enables saturation at wavelengths between 13nm and 19nm with a total pump energy of about 1–1.2J. This level can be obtained using commercial repetitive (10Hz) drivers, enabling XRLs with repeating pulses. The GRIP experiment demonstrated that the preforming pulse is crucial. Its intensity determines the hydrodynamic pressure in the plasma plume, and hence determines the state of the active medium when the heating pulse arrives. The intrinsically long (400–500ps) preforming laser pulse in our system gave very-smooth density gradients, but resulted in under-ionized plasma and hindered the saturation regime in a Ni-like molybdenum laser at 18.9nm. In spite of this, repetitive lasing was achieved with pump energies of 150mJ and 300mJ in the long and heating (9ps length) pulses, respectively.

Figure 1. Schematic of the x-ray master oscillator power amplifier (XMOPA) arrangement using an amplifier in the grazing incidence pumping (GRIP) geometry. HHG: high harmonics generation.
However, progress in pulse-energy reduction was achieved at the expense of the total output energy of the XUV signal, since the volume of the active medium was greatly reduced in order to obtain suitable plasma parameters. The conversion efficiency is still (in the beginning of the GRIP phase) at a level of 10-6.

One of the most promising ways to potentially improve performance is to use the master oscillator power amplifier (MOPA) geometry, which was developed for traditional lasers. This arrangement was originally discussed many years ago for XRLs, but the scheme was demonstrated recently using high harmonics generation (HHG) as the source of the injected signal (oscillator) into an OFI amplifier.9 The most important aspect of this experiment was the demonstration of seed amplification. The inherent limitation in the saturation parameter of the OFI medium limits the output pulse energy.

We are following the same development trend but intend to use a GRIP medium as the amplifier (see Figure 1). Due to higher density, this has a significantly higher (one to two orders of magnitude) saturation parameter, so that a higher extracted energy (a few microjoules) is expected. However, this requires higher input signal, as well as a solution to the problem of natural mismatch between the broad bandwidth of efficiently-generated harmonics and the inherently-narrow amplification bandwidth of the XUV amplifiers.

Collisionally-pumped XRLs have reached a technological and physical limit. However, it is expected that the XMOPA arrangement will accelerate and improve the development process. This approach offers more sophisticated, compact, and powerful x-ray lasers that can potentially be used as easily-accessible stations.

Karol Adam Janulewicz and Johannes Tümmler
Max Born Institute
Berlin, Germany
Dr. Karol Adam Janulewicz is the project leader on X-ray lasers at the Max Born Insitute in Berlin, Germany. He was previously at the University of York, UK working on numerical modeling of X-ray lasers. His main interests include strong field physics, laser physics and laser-matter interaction. He has written numerous papers presented at the SPIE conferences on laser physics and technology.
Johannes Tümmler received his Ph.D. degree in physics from Aachen University of Technology in 2000. His doctoral research involved new refractive optics for hard x-rays. In 2000 he joined the Physikalisch-Technische Bundesanstalt (PTB) where he worked on the characterization of optics for EUV lithography. In 2003 he joined the Max Born Institute and works on the development and characterization of x-ray lasers, and their applications.