Kα Cold-Target Imaging and Preheat Measurement Using a Pinhole-Array X-Ray Spectrometer

182 LLE Review, Volume 75 Two-dimensional (2-D) monochromatic imaging of laser-imploded targets1–6 is useful for diagnosing target compression and stability. Here we present a simple method for such imaging, using an array of about 300 pinholes placed in front of a flat-crystal x-ray spectrometer. The main advantage of this method (in addition to its simplicity) is the ability to simultaneously obtain a large number of images over a wide range of photon energies. This is particularly useful for imaging the emission region of a single spectral line from a doped target, where images around the wavelength of the line can be simultaneously obtained and subtracted from the image at the line. Imaging a spectral line of a dopant can be useful for studying mixing of target layers. Here we use the array to image Kα fluorescence from a titanium-doped target (excited by core radiation) and thereby obtain an image of the cold layer at peak compression. This image can otherwise be obtained only through backlighting. Using a flat crystal limits the field of view, but this limitation is shown not to be severe when imaging the compressed target core. On the other hand, the narrow field of view translates into improved spectral resolution. We show that sufficient intensity can be obtained in monochromatic imaging even without the gain in intensity when using a focusing crystal. In addition, the array provides spectra of high spectral resolution because of the reduction in the effective source size. Finally, we show that, in addition to the corepumped Kα emission, a second Kα-emitting zone of a larger radius appears in the image. This Kα emission is pumped during the laser-irradiation pulse, indicating preheat by suprathermal electrons.