Focusing shocks are created underwater by exploding 10- μ m-thick copper foils with circular and polygonal geometries. Their symmetry and trajectory are characterized to assess this technique’s potential contributions to fundamental and applied investigations of nonlinear wave propagation and high-energy-density phenomena. The foils are exploded using a pulsed power generator which delivers kiloamp currents in microseconds. Current and voltage time traces of the explosions are recorded concurrently with high-speed shadowgraph images of the shocks. The electric waveforms of the explosions of different foil geometries resemble each other, showing peak resistive voltages, currents, and powers around 10 kV, 300 kA, and 2.5 GW, respectively. By extracting the shocks’ trajectories through statistical analysis of the shadowgraph images, it is found that circular foils, whether free standing or attached to the inside of a plastic shell, create shocks which accelerate up to Mach 1.7. Comparable Mach numbers are achieved by exploding a circular wire array of 32 100- μ m-diameter copper wires, indicating that foil designs perform similarly to this traditional design. In contrast, free-standing polygonal foils create shocks which travel at a constant near-sonic speed, seemingly behaving as non-interacting weak planar shocks. This contradicts the theoretically predicted reshaping and acceleration of such shocks; manufacturing imperfections are suspected to cause this unexpected behavior. Alternate designs in which foils are attached to polygonal plastic shells are tested and found to create shocks which do reshape and accelerate.

The plasma package Analyzer of Space Plasma and Energetic Atoms (ASPERA-3) onboard Mars Express has observed ions and electrons accelerated by the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) radar when it operates in its active ionospheric sounding mode. To better study the processes involved, new operational modes for MARSIS have been developed. In the first, a fixed frequency mode, the transmitter does not sweep over a range of frequencies, as normal, but instead transmits pulses at a fixed frequency. This frequency has been chosen to be close to the fundamental frequency of the local ionosphere around the spacecraft, which in all cases is < 350 kHz. Also, an alternating mode has been introduced, where observations in the fixed frequency mode are interleaved with observations that sweep over frequencies in order to investigate how ions are accelerated. Here we describe the new operational modes and present the results of several tests performed in the years 2020, 2021, and 2023.

Spacecraft discharge time constants are calculated from measurements of electron differential flux before and during operation of an ionospheric sounding radar. Determining these time constants provides insight into how the operation of a sounding radar affects the surrounding plasma's interaction with the spacecraft. The analysis is enabled by the fixed-frequency operation mode of a sounding radar which enhances resonant interaction with the ambient plasma. This mode's effect on measured energy spectra of ion and electron fluxes is described. Measurements of electron fluxes disturbed by radar operation serve as input to a model of spacecraft discharge for calculating capacitive discharge time constants. A case study using electron fluxes measured at Mars yields discharge time constants in the range 0.6–0.8 ms and reveals that a residual potential around (Formula presented.) V remains on the spacecraft long after radar operation ceases. The minimum spacecraft potential cannot be determined with these data and model due to the narrow energy range of electrons in the ambient plasma.

Convergence of cylindrical shock in argon is studied both experimentally and numerically. Shock tube experiments are conducted, where a planar shock is first transformed to a cylindrical shape and then converged to its focal axis. Numerical simulations of the converging shock using equations of state for an ideal gas and a real gas (SESAME 5173 model) are conducted and compared. High temporal resolution data of cylindrical shock convergence is presented. When comparing the trajectories of the converging shock of initial shock Mach number (M-S) of 4.63, the convergence exponent (alpha) in experiments is found to be 0.833. This alpha value in experiments is higher than the value obtained from computations with argon treated as an ideal gas but agrees well with the real gas computations. It is revealed that the form of convergence varies with different M-S. An asymptotic approach of alpha toward the self-similar solution for high M-S is attributed to an earlier transition of shock motion to self-similarity, while a significantly higher alpha observed at lower M-S is attributed to the negative influence of upstream nonuniformities and weaker initiation of the shock. It is found that even before the shock reflection, real gas effects are significant enough to affect the convergence of the shock and limit the extreme conditions predicted by the ideal gas computations. For an M-S of 4.63, the maximum temperature reached is 9250 K before reflection, leading to 0.12% of the argon gas undergoing the first stage of ionization.

Proton plasma asymmetries with respect to the convective electric field (E) are characterized in Venus' dayside magnetosheath using measurements taken by an ion mass-energy spectrometer and a magnetometer. Investigating the spatial structure of the magnetosheath plasma in this manner provides insight into the coupling between solar-wind protons and planetary ions. A previously developed methodology for statistically quantifying asymmetries is further developed and applied to an existing database of proton bulk-parameter measurements in the dayside magnetosheath. The density and speed exhibit mild asymmetries favoring the hemisphere in which E points towards the planet, while the magnetic-field-strength asymmetry favors the opposite hemisphere. The temperature perpendicular to the background magnetic field has a mild asymmetry favoring the hemisphere in which E points away from the planet; the temperature parallel to the background magnetic field and the temperature anisotropy present no significant asymmetries. Deflection of the solar wind due to momentum exchange with planetary ions is revealed by the O+ Larmor-radius trends of the asymmetries of the bulk-velocity components perpendicular to the upstream solar-wind flow. This interpretation is enabled by comparisons to experimental and numerical studies of solar-wind deflection at Mars, highlighting the benefits of comparative planetology studies.