In order to obtain the required high intensity of 10¹⁷ W/cm², this has to be done in a vacuum chamber. Any gas atmosphere would be ionized at this intensity level. The ionization of the copper tape itself triggers "micro-explosions", generating tiny holes in the target. The erupting metal would coat the lens within several seconds. To prevent this, a moving and highly transparent plastic band protects the lens. If the copper target is thinner than 20 mu, the copper-debris is also emitted in the forward direction, creating the nose of the smiley face on the nearby vacuum flange. The eyes are beryllium windows, through which the generated X-ray are extracted from the chamber.

The laser parameters (focusing, chirp) are optimized to obtain maximum efficiency for the prduction of Kα characteristic line emission and at the same time minimize the high energetic background due to Bremsstrahlung. A typical spectrum of the x-ray emission is shown on the left. The two peaks correspond to the characteristic Kα and Kβ lines of Cu. The total flux of X-ray's is of ~8x10¹⁰ photons/sec, the Ka-line lines contain ~5x10¹⁰ photons/sec. The efficiency of laser energy to X-ray photon energy conversion is estimated to be 1.3x10⁵. For the extrapolation to much higher repetition rates we have tested a vertically streaming Ga-jet as a metal target to generate hard x-rays. The performance concerning x-ray flux is similar to the metal band targets, however, for very long operation times of several hours it suffers from hydrodynamic instabilities.