Extreme ultraviolet (EUV) spectra from laser-produced tin plasmas have been recorded as a function of time
using an ISAN grazing incidence spectrograph to study the temporal evolution of the tin unresolved transition
array (UTA) responsible for the peak EUV emission. This paper reports the experimental as well as simulated
results for a 10 ns gate width with 2 ns time steps which confirm that the development and collapse of the
UTA follow the temporal behavior of the laser pulse. The self-absorption features at longer wavelengths are
observed particularly during plasma cooling and arise from lower ion stages ranging from Sn VI to Sn XI.
The aim of this study is to investigate ways to maximise the efficiency of tin based laser produced plasmas as sources of EUV radiation in the 2% band centered on 13.5 nm. It has been found that targets containing below 15% tin atoms by number emit more brightly in the spectral region around 13.5 nm than pure tin targets. Furthermore, if the remaining material in the target is composed on primarily low-Z atoms, then both plasma continuum radiation and Bremsstrahlung radiation are greatly reduced. In addition, if the target is illuminated with a prepulse, the conversion efficiency shows a distinct increase. The third parameter under examination is the laser power density, which controls the ion distribution in the plasma. The influence of low-Z atoms on the tin ion distribution in the plasma has been investigated and found to be of little consequence.
Measurements were made in the region from 9-17 nm on an absolutely calibrated 0.25-m flat field grazing incidence spectrograph, and on two 2-m grazing incidence spectrographs. Spectra and conversion efficiency data from a range of target materials and illumination regimes are presented.
The emission spectra of laser produced plasmas of pure tin targets are dominated by recombination continuum emission throughout the entire EUV spectral region with intense structure due to line emission dominating the spectra in the 13 - 14 nm region. This feature arises from resonant 4p64dn - 4p54dn+1 + 4p64dn-14f emission lines that are generally concentrated in a narrow band, 5 - 10 eV wide, which overlaps considerably in adjacent ion stages to form an intense unresolved transition array (UTA). Such plasmas are optically thick; the strongest lines are attenuated and frequently appear in absorption. However, if tin comprises a few percent of a predominantly low-Z matrix, the recombination is suppressed and the plasmas can become optically thin to resonance radiation. Under these conditions, resonance line emission can dominate the spectra. The application of a collisional radiative (CR) model, combined with ab initio atomic structure calculations, allows one to estimate the laser plasma parameters that will optimize the UTA as efficient narrow bandwidth emitters of EUV radiation. The dependence on laser power density of both in-band emission and debris generation from pure tin targets is presented. The influence of a pre-pulse on the plasma output is also investigated.