Recent experiments undertaken at the Rutherford Appleton
Laboratory to produce X-ray lasing over the 5–30 nm
wavelength range are reviewed. The efficiency of lasing is
optimized when the main pumping pulse interacts with a preformed
plasma. Experiments using double 75-ps pulses and picosecond
pulses superimposed on 300-ps background pulses are described.
The use of travelling wave pumping with the approximately
picosecond pulse experiments is necessary as the gain duration
becomes comparable to the time for the X-ray laser pulse to
propagate along the target length. Results from a model taking
account of laser saturation and deviations from the speed of
light c of the travelling wave and X-ray laser group velocity
are presented. We show that X-ray laser pulses as short as
2–3 ps can be produced with optical pumping pulses of
≈1-ps.
Ultrafast X-ray Science at the Sub-Picosecond Pulse Source