|
We have built an x-ray streaked crystal spectrograph for making time-resolved x-ray spectral measurements. This instrument can access Bragg angles from 110 to 38° and x-ray spectra from 200 eV to greater than 10 keV. We have demonstrated resolving powers, E/▵E > 200 at 1 keV and time resolution less than 20 psec. A description of the instrument and an example of the data is given. We have combined a Bragg diffraction crystal with an x-ray streak camera in order to time resolve the x-ray spectra from laser-produced plasmas. The resolution is sufficient to resolve individual lines in the x-ray spectra and measure their time history. These x-ray spectral measurements can be an important plasma diag nostic. Relative intensities of various x-ray lines can be used to infer plasma temperatures and densities.1 Also, if there is sufficient energy resolution, stark-broadened individual line profiles are a plasma density diagnostic.2 The streaked spectrograph can also assist in developing a short-pulsed x-ray line source. Such a short-pulsed x-ray line source is needed for making time-resolved short-pulsed absorption measurements, such as those proposed for measuring the core size of an ICE pellet.3 The instrument discussed here has been developed at LLNL for use on our Novette laser system. It uses an elliptical curved crystal focusing design to concentrate the x-ray spectrum onto the streak camera s1it.4 With such a design we have obtained moderately high resolving powers of E/▵E > 200 at x-ray energies around 1 keV. The instrume,lt is versatile, being able to cover spectral regions from 200 eV to 10 keV. This is accomplished by usin9 various geometries which change the Bragg angle in the range from 110 to 38° and by using various dificacting crystals. For the first application, whose results we present here, we have measured spectra in the 1 keV range using a KAP crystal (2d=26.632Å). The final and most important feature of the instrument is that it uses an existing LLNL streak camera with little modification. The spectrograph design takes advantage of the point-to-point focusing properties of an ellipse. X rays emitted from the laser-produced plasma placed at one of the foci of the ellipse will be reflected from the elliptical surface converging on the other focus of the ellipse. This is shown schematically in Fig. 1. A Bragg diffraction crystal is elastically bent to conform to the elliptical surface. Each ray is reflected from the surface at their unique Bragg angle, providing wavelength selection. The streak camera is then placed in the far field behind the focal spot and measures the dispersed spectrum. The dispersion of the system depends on the geometry of the ellipse and the distancP of the streak camera slit from the focal spot.
|
