The most often used media include
highly ionized plasmas, created in a capillary discharge or when a linearly focused optical pulse hits a solid target. In accordance with the
Saha ionization 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 plasmas usually correspond to energies on the order of hundreds of
electron volts (
eV). laboratory in Prague, where a 1
kJ pulse creates plasma for X-ray generation Common methods for creating plasma X-ray lasers include: •
Capillary plasma-discharge media: In this setup, a several centimeters long capillary made of resistant material (e.g.,
alumina) confines a high-current, submicrosecond electrical pulse in a low-pressure gas. The
Lorentz force causes further compression of the plasma discharge (see
pinch). In addition, a pre-ionization electric or optical pulse is often used. An example is the capillary neon-like Ar8+ laser, generating radiation at 47 nm, which was first demonstrated in 1994. •
Solid-slab target media: After being hit by an ultra-intense optical (laser) pulse, the metal target evaporates and emits highly excited plasma. Again, a pair of pulses is usually used in the so-called "transient pumping" scheme: (1) a longer pulse on the order of nanoseconds (sometimes preceded by one or several smaller "pre-pulses") is often used for plasma creation and (2) a second, shorter (on the order of hundreds of
femtoseconds or a
picosecond) and more energetic pulse is used for further excitation in the plasma volume. For short lifetimes a so-called "travelling wave" has been developed, where the plasma is heated just before the passage of the x-ray photons (so-called "guillotine principle" geometry). In order to increase the efficiency of energy transfer from the heating laser pulse into the active medium (plasma), a sheared excitation pulse is sometimes employed, so-called GRIP -
grazing incidence pump geometry. The
gradient in the
refractive index of the plasma causes the amplified pulse to bend
from the target surface, because at the frequencies above resonance the refractive index decreases with matter density. This can be compensated for by using curved targets or multiple targets in series. •
Plasma excited by optical field: At optical densities high enough to cause effective electron
tunnelling, or even to suppress the potential barrier (> 1016 W/cm2), it is possible to highly ionize gas without contact with any capillary or target. A collinear setup is usually used, enabling the synchronization of pump and signal pulses. An alternative amplifying medium is the relativistic electron beam in a
free-electron laser, which, strictly speaking, uses stimulated
Compton scattering instead of stimulated emission. Other approaches to optically induced coherent X-ray generation are: •
high-harmonic generation • stimulated
Thomson scattering •
Betatron radiation ==Applications==