8-1	Efficient Soft X-ray Amplification Using Traveling-Wave Pumping

Fig. 8-1 Schematic of a traveling-wave generator Uniform wavefront of the ultra-short pulse is divided, and each component reaches the target with an appropriate delay.

Fig. 8-2 Effect of traveling-wave pumping Soft X-ray laser strength (photoelectron counts on the detector) is plotted against the plasma length (or gain medium length). The circles represent traveling-wave pumping. Compared with the case without traveling-wave pumping (triangles), the efficiency of the amplification clearly is enhanced. By use of traveling-wave pumping, we can achieve gain-saturation or a plasma length longer than 0.43 cm.

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We developed a new pumping method to realize more efficient soft X-ray amplification of a nickel-like tin laser at a wavelength of 12 nm (nm=10-9 m) in which substantial amplification already has been observed. In typical X-ray laser experiments, highly charged ions in a high-temperature dense plasma column having a 5-6 mm length are used as a gain medium. This plasma column is generated by a few ps (ps=10-12 s) pumping laser pulse irradiating a slab target. However, with a decrease in electron temperature, amplification is reduced. When the gain medium is composed of nickel-like tin ions, the lifetime of the amplification gain is around 5 ps, which corresponds to a light propagation length of 1.5 mm. Thus, when the amplified X-rays propagates in the gain medium from one edge to the other, the amplification gain of the gain medium is lost before the amplified X-rays reach the other edge, and strong amplification is obstructed. To overcome this difficulty, we have employed a step mirror in the focusing system to tilt the wavefront of the pumping laser light (Fig. 8-1). This pumping method is called traveling-wave pumping. The step mirror, specially made for this experiment, consisted of six blocks, each 25 mm wide. All blocks are mounted on a 150x150 mm base-plate. This step mirror divides the pumping laser pulse, and each laser pulse component reaches the target with the appropriate temporal delay. By optimizing the steps of each mirror component, we can adjust the temporal delays. This makes it possible to amplify the X-rays propagating the gain medium under optimum conditions. Fig. 8-2 shows the amplification curve of the nickel-like tin lasers for both the traveling-wave pumping case and the simultaneous pumping case. By use of traveling-wave pumping, the gain coefficient was improved from 14 to 30 cm-1. We observed gain-saturation at a gain medium length of 4.3 mm. A 15 mu J/pulse output energy with an output intensity of 4x1010 W/cm2 under the gain-saturation condition provides the best performance, at the present time, in laboratory-size soft X-ray lasers.

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Reference T. Kawachi et al., X-ray Laser Research in JAERI, IEEE/LEOS Annual Conference Proceedings 2001, 457 (2002).

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Persistent Quest Research Activities 2002 Copyright(c) Japan Atomic Energy Research Institute