X-ray laser active media

Most often used media include highly ionized plasma created in a capillary discharge or when a linearly focused optical pulse hits a solid target. In accordance to the Sah equation, the most stable electron configurations are neon-like with 10 electrons remaining and nickel-like with 28 electrons remaining. The electron transitions in highly ionized plasma usually correspond to energies in the order of 100s eV.
• Capillary plasma discharge medium: In this setup, a several centimeters long capillary made of resistant material (e. g. alumina) confines a high current, sub microsecond electrical pulse in low-pressure gas. The Lorentz force causes further compression of the plasma discharge (see pinch). A pre-ionisation electric pulse and/or optical pulse is often used. An example is the capillary neon-like Ar8+ laser (generating at 47 nm).


• Solid slab target medium: After being hit by optical pulse, the target emits highly excited plasma. Again, a longer prepulse is often used for the plasma creation and a second, shorter and more energetic pulse is used for further excitation in the plasma volume. For short lifetimes, a sheared excitation pulse may be needed (GRIP - grazing incidence pump). The refractive index gradient causes the amplified pulse to bend from the target surface. This can be compensated using curved target or multiple targets in series.

• Plasma excited by optical field: At optical densities high enough to cause effective electron tunelling or even to suppress the potential barrier (> 1016 W/cm2), it is possible to highly ionize the gas without contact with any capillary or target. Usually a collinear setup is used, enabling to synchronize pump and signal pulses.

Alternative amplifying medium is the relativistic electron beam in free electron laser.

A completely different approach to X-ray generation is the high-harmonic generation.