Characteristics of a cylindrical collector mirror for laser-produced xenon plasma soft X-rays and improvement of mirror lifetime by buffer gas.

The focusing characteristics of a ruthenium-coated cylindrical mirror were investigated on the basis of its ability to collect and focus broadband 5-17-nm soft X-rays emitted from a laser-produced plasma. Based on the plasmas spectral intensity distribution and the reflectivity function of the mirror, we defined the optimum position of the integrated cylindrical mirror at which the X-ray energy flux transported and focused through the mirror was maximum. A minimum spot diameter of 22 mm at a distance of approximately 200 mm from a soft X-ray source was confirmed. The maximum intensity of the collected soft X-rays was 1.3 mJ/cm(2) at the center of the irradiation zone. Thus, the irradiation intensity was improved by approximately 27 times when compared to that of 47 µJ/cm(2) without the mirror. The debris sputtering rate on the reflection surface of the mirror can be reduced to 1/110 by argon gas at 11 Pa, while the attenuation rate of the soft X-rays due to absorption by the buffer gas can be suppressed to less than 10% at the focal point. The focusing property of the mirror is expected to be maintained for 3000 h or longer without significant degradation for a 100 W/320 pps laser shot if the ruthenium layer is thicker than 10 µm. These results suggest that a stand-alone broadband soft X-ray processing system can be realized by using laser-produced plasma soft X-rays.

Laser-plasma debris from a rotating cryogenic-solid-Xe target.

We investigate the characteristics of laser plasma debris that is responsible for damaging optics. The debris is composed of fast ions, neutral particles, and fragments, and originates from a solid Xe target on a rotating drum that we developed as an extreme ultraviolet (EUV) source. The ice fragments appear to be a problem most notably with solid Xe targets; however, we find that the damage induced by Xe ice fragments can be avoided by simply reducing the laser pulse energy. We find the number of fast neutral particles to be an order of magnitude less than the number of ions, and we clarify that the plasma debris is primarily composed of fast ions. In addition, we find that the number of fast ions having a few dozen keV of energy decreases when using the rotating target compared with the rest target. We attribute this to a gas curtain effect from the Xe gas localized at the rotating target surface. We estimate the sputtering rate of the Mo/Si mirror, which is caused primarily by the fast ions, to be 104 nm/1x10(6) shots at 190 mm from the source plasma and at an 11.25 degree angle from the incident laser beam. Up to the 1x10(6) shots exposure, remarkable degradation of the mirror reflectivity is not observed though the sputtering damages the mirror. Mitigation of the ions by using gas and/or magnetic fields will further improve the mirror lifetime. By comparing with a liquid jet Xe target, we conclude that the sputtering rate per conversion efficiency when using the solid Xe targets on the rotating drum is the same as that when using the liquid Xe targets. The high conversion efficiency of 0.9% in the rotating drum solid Xe target makes this technique useful for developing laser plasma EUV sources.

Xe capillary target for laser-plasma extreme ultraviolet source.

A cryogenic Xe jet system with an annular nozzle has been developed in order to continuously fast supply a Xe capillary target for generating a laser-plasma extreme ultraviolet (EUV) source. The cooling power of the system was evaluated to be 54 W, and the temperature stability was +/-0.5 K at a cooling temperature of about 180 K. We investigated experimentally the influence of pressure loss inside an annular nozzle on target formation by shortening the nozzle length. Spraying caused by cavitation was mostly suppressed by mitigating the pressure loss, and a focused jet was formed. Around a liquid-solid boundary, a solid-Xe capillary target (10070 microm phi) was formed with a velocity of < or =0.01 ms. Laser-plasma EUV generation was tested by focusing a Nd:YAG laser beam on the target. The results suggested that an even thinner-walled capillary target is required to realize the inertial confinement effect.

Highly time-resolved correlation measurement between laser and synchrotron radiation pulses without synchronization control.

A mode-locked laser has been introduced in combination with synchrotron radiation to establish a versatile technique for highly time-resolved correlation measurements utilizing the short-pulse and high-pulse frequency characteristics of both photon sources. Successive pulse timing delay detected by nonlinear optical mixing between the two sources yields a cross-correlation profile capable of accurate measurement of the picosecond pulse profile of the synchrotron radiation without any synchronization control. Although the experiment was performed in the visible spectral domain, the present technique opens up a methodology for time-resolved spectroscopy in femtosecond and higher-energy domains by introducing a suitable nonlinear process that informs of the pulse coincidence between the two radiation sources.

Propagation characteristics of a diffracted M2 beam.

The propagation characteristics of a beam diffracted by a circular aperture are investigated. The beam-quality factor M2 defined by an 86.5% power-content radius is given theoretically and experimentally as a function of the truncation ratio. It is found that the theoretical limit of M2 is 2.37 times as great as that of an incident beam as the truncation ratio approaches 0. For a weakly diffracted beam a simple formula giving M2 is derived. Although M2 does not increase much with diffraction, the influence of diffraction should be taken into account in beam brightness.