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-resolved 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. The x-ray absorber of this calorimeter is an aluminum disk, attached to a two-dimensional thermopile and surrounded by an isoperibol shroud. The time integral and the maximum of the thermopile voltage signal are both proportional to the x-ray energy deposited. Data are collected for 90 s, and the instrument has, thus far, been used in the 1–25 mJ range. A wider dynamic measuring range for x-ray fluence (energy/area) can be achieved by varying the area of the defining aperture. The calorimeter is calibrated by an electrical substitution method. Calibrations are performed before and after each x-ray experiment on the Z facility. The calibration of the time integral of the thermopile voltage versus energy deposited (or the peak of thermopile voltage versus energy deposited) is linear with zero intercept at the 95% confidence level. The irreproducibility of the calibration is <2%, and the imprecision in the measurement of the incident x-ray energy (inferred from signal noise and the calibration) is estimated to be ∼0.9 mJ (95% confidence level). The systematic uncertainty (inaccuracy), due to correctable baseline shifts, is estimated at ±10%. Comparisons have been made of the calorimeter to time-resolved x-ray diagnostics, e.g., bolometers and x-ray diode arrays, by integrating the flux measured by such instruments over time.