Absolute, soft x-ray calorimetry on the Z facility at Sandia National Laboratories (invited)

Simple and reliable x-ray fluence measurements, in addition to time-resolve d diagnostics, are needed to understand the physics of hot Z-pinch plasmas. A commercially available laser calorimeter has been modified for measuring soft x-ray fluence from the Z facility at Sandia National Laboratories. Th e x-ray absorber of this calorimeter is an aluminum disk, attached to a two -dimensional thermopile and surrounded by an isoperibol shroud. The time in tegral and the maximum of the thermopile voltage signal are both proportion al to the x-ray energy deposited. Data are collected for 90 s, and the inst rument has, thus far, been used in the 1-25 mJ range. A wider dynamic measu ring range for x-ray fluence (energy/area) can be achieved by varying the a rea of the defining aperture. The calorimeter is calibrated by an electrica l substitution method. Calibrations are performed before and after each x-r ay experiment on the Z facility. The calibration of the time integral of th e thermopile voltage versus energy deposited (or the peak of thermopile vol tage versus energy deposited) is linear with zero intercept at the 95% conf idence level. The irreproducibility of the calibration is < 2%, and the imp recision in the measurement of the incident x-ray energy (inferred from sig nal noise and the calibration) is estimated to be similar to 0.9 mJ (95% co nfidence level). The systematic uncertainty (inaccuracy), due to correctabl e baseline shifts, is estimated at +/- 10%. Comparisons have been made of t he calorimeter to time-resolved x-ray diagnostics, e.g., bolometers and x-r ay diode arrays, by integrating the flux measured by such instruments over time. (C) 1999 American Institute of Physics. [S0034-6748(99)70101-2].