Refraction-enhanced backlit imaging of axially symmetric inertial confinement fusion plasmas.

X-ray backlit radiographs of dense plasma shells can be significantly altered by refraction of x rays that would otherwise travel straight-ray paths, and this effect can be a powerful tool for diagnosing the spatial structure of the plasma being radiographed. We explore the conditions under which refraction effects may be observed, and we use analytical and numerical approaches to quantify these effects for one-dimensional radial opacity and density profiles characteristic of inertial-confinement fusion (ICF) implosions. We also show how analytical and numerical approaches allow approximate radial plasma opacity and density profiles to be inferred from point-projection refraction-enhanced radiography data. This imaging technique can provide unique data on electron density profiles in ICF plasmas that cannot be obtained using other techniques, and the uniform illumination provided by point-like x-ray backlighters eliminates a significant source of uncertainty in inferences of plasma opacity profiles from area-backlit pinhole imaging data when the backlight spatial profile cannot be independently characterized. The technique is particularly suited to in-flight radiography of imploding low-opacity shells surrounding hydrogen ice, because refraction is sensitive to the electron density of the hydrogen plasma even when it is invisible to absorption radiography. It may also provide an alternative approach to timing shockwaves created by the implosion drive, that are currently invisible to absorption radiography.

Abel solution to a bremsstrahlung [corrected] inverse problem.

Bremsstrahlung is correlated with high-energy electrons in laser-heated plasma [K. Brueckner, Phys. Rev. Lett. 36, 677 (1976)]. Since the result is important to the National Ignition Campaign (NIC) we reconsider the derivation, and the energy dependence of the Gaunt factor(s). We find an expression for bremsstrahlung we can Abel invert, and we demonstrate the accuracy of the transform with a simple numeric exercise.

Nonintrusive characterization of the azimuthal drift current in a coaxial ExB discharge plasma.

A diagnostic is developed for the nonintrusive study of the azimuthal drift current in the coaxial ExB discharge of a Hall plasma accelerator. The technique of fast current interruption is used to generate a signal on several loop antenna that circle the outer wall of the discharge channel. The signal on the antenna is recorded, and used to determine the spatial distribution of the azimuthal drift at the moment of current interruption. The results of the experiment are compared to estimates derived via prior intrusive measurements, and the intrusive estimates are found to predict the spatial characteristics of the azimuthal drift, but underestimate its total magnitude. The self-induced magnetic field is then calculated and added to the applied magnetic field. The peak total magnetic field is seen to shift 2-5mm towards the anode due to self-induction, and suffer a reduction in magnitude of 10%-15%. The peak in the total magnetic field is then found to more closely coincide with the peak of the measured electric field than the peak of the vacuum magnetic field. It is concluded that the self-induced magnetic field could be important to anomalous electron mobility in the Hall-effect thruster, and simulation efforts should try to include its impact.