Periodic features modifying the Heβ line profile from an aluminium plasma

scale length plasmas. These experiments offer the opportun ity to study time-dependent atomic kinetics effects since the pulse duration is comparable to the atomic processes time scale. Xray spectroscopy is an extremely valuable tool to infer t he properties of these high-density states 1 . In this work we report the observation and analysis of X-ray emission produced by picosecond laser irradiation of a solid Al target. The us e of a high-luminosity, high-resolution spectrometer has enabled the measurement of unusual intensi ty modulations on He� transitions. The hydrodynamic and thermodynamic behavior of the expanding plasma is calculated using one-dimensional hydrodynamic simulations. EXPERIMENT In the experiment 200 �m diameter 8 cm long aluminium fibre targets were irradia ted using the Gaussian laser pulses produced by the RAL Astra facility (wavelength 800 nm, FWHM pulse duration 3.4 ps). An off axis parabolic mirror OAP was used to focus the laser pulse onto the target with an angle of 10� to the vertical axis. By defocusing the OAP, the on target focal spot varied to give intensities between 7 � 10 14 W cm ‐2 and 3 � 10 16 W cm -2 . By translating the aluminium fibre through the focal spot a nd firing the laser at a repetition rate of 2 Hz, up to 100 shots were integrated to make a single reco rding. The high-resolution, highly dispersive toroidally bent crystal spectrometer (HDTS) 2 was positioned to record spectral emission parallel to the target surface and spatially resolved perpendicular to the surface. The spectral measurements were recorded with 3 �m spatial resolution having a magnification of the crystal of about 4. The energy dispersed X-ray emissi on from the plasma was recorded onto a cooled, large area scientific-grade CCD camera . The experimental setup is shown from above in Fig 1a. Typical data integration times were ap proximately 1 minute. The spectrometer was positioned to disperse an energy range of 1830 eV to 1872 eV containing the