Measurement of amplified spontaneous emission at 200 A (invited; abstract)

Abstract

Recent interest in the production of laser radiation at soft x-ray wavelengths makes appropriate the discussion of diagnostic considerations and techniques for the measurement of same. A source of soft x-ray ASE has a number of characteristics which drive the design of diagnostic instruments: (1) The anisotropy of the ASE makes target alignment a critical part of the diagnosis, and couples collection solid angle to S/N considerations in the measurement. (2) The narrow linewidth of the amplified emission and its long wavelength put a high S/N premium on spectroscopic instrumentation of high spectral resolution and good higher-order discrimination. (3) The specialized plasma conditions required to produce gain are typically short lived, requiring time-resolved or at least time-discriminating spectroscopy. (4) The nonlinear nature of the threshold processes involved in ASE requires instrumentation of large dynamic range, broad angular acceptance, and large field of view and depth of focus. Of the many possible methods for gain verification of the x-ray source, five are discussed: (1) Probe amplification; (2) spatial coherence measurement (as a function of gain length); (3) output intensity measurement (absolute measurement and nonlinear variation with gain length); (4) divergence measurement; and (5) cavity formation. In addition, recent soft x-ray laser experiments at LLNL are discussed along with descriptions of the instruments used to measure the ASE. Diagnostic design suggestions for future soft x-ray laser experiments are also presented.