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The study of planetary ionospheres dates back thousands of years, to humanity's first attempts to explain the polar aurora. Ionospheric physics is a study of one pathway for energy to enter a planetary atmosphere: transfer from sunlight and solar wind plasma to planetary plasma, and from the planetary plasma to the neutral atmosphere. In the modern era of ionospheric physics, electrostatic analyzers with attached time-of-flight velocity analyzers are excellent tools for the study of planetary ionospheres because they can measure mass-resolved 3-dimensional ion velocity distribution functions over a wide range of energies and fluxes, with large fields-of-view and moderate angular resolution. One example of an electrostatic analyzer with attached time-of-flight section is the SupraThermal And Thermal Ion Composition (STATIC) instrument onboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission. MAVEN has been collecting data since 2014 with the goal of illuminating how Martian climate and habitability have been affected by the escape of the atmosphere to space. To that end, STATIC was designed to measure the main ionospheric and escaping species, operating from deep in the collisional atmosphere out to the tenuous exosphere and magnetotail. STATIC samples ion velocity distribution functions every 4 seconds across a field-of-view covering 2$\pi$ steradians for ions with energies between 0.1 eV and 30 keV and masses between 1 and 60 amu. This work will treat STATIC as a case study in order to examine some of the challenges associated with using electrostatic analyzers in space, including the effects of background counts, spacecraft potential, and supersonic spacecraft motion on the measurement and analysis of distribution functions. Before the discussion of those details, we first provide an introduction to the geometry of electrostatic analyzers and how the measured count rates are related to physical quantities. This work also describes the results of several investigations in which MAVEN-STATIC data were used to examine the beginnings of cold ion outflow at Mars. Models of the Martian ionosphere show that the vast majority of ions are created at altitudes near 120 km, the location of the main ionospheric peak. The peak is located deep in the collisional atmosphere, where collisions are theorized to keep the ions in thermal equilibrium with the cold neutral atmosphere. The vast majority of the ionospheric ions are therefore gravitationally bound to the planet. Most ions that have the potential to escape the planet's gravity are created near a boundary called the exobase, where the mean free path between collisions becomes equal to the scale height and the ions no longer thermalize with the neutrals. The exobase can be considered the top of the collisional atmosphere and varies between 140 and 210 km altitude dependent on local time and season. In the present epoch, 10-20\% of atmospheric loss is attributable to the loss of ions from hundreds or thousands of kilometers above the main peak and the exobase region. The mechanisms by which ions gain enough energy to escape the planet's gravity between the exobase and the altitudes where they escape are not yet understood, but we have used STATIC data to begin analysis of ion distribution functions in the vicinity of the exobase. The analysis of ion velocity distribution functions measured near the exobase is experimentally challenging because the spacecraft travels supersonically with respect to the cold ions, affecting observations of both the energy and angular distributions. The details of how the instrumentation affects measurements of the distribution function must be understood in detail in order to extract accurate plasma parameters. In this work, we describe many of the procedures used to extract accurate plasma parameters from STATIC data. We use these corrections to begin to bridge the gap between studies of ion outflow conducted high above the exobase and studies of the cold, thermal ionosphere near the main peak. Specifically, we report the first measurements of Martian ionospheric ion temperatures since the Viking landers in 1975 and 1976, including the results of both a case study and a statistical study of over 10,000 MAVEN orbits. Unexpectedly, ion temperatures are significantly elevated over neutral gas temperatures many scale heights below the exobase, and none of the obvious mechanisms for ion heating explained the observed temperature difference. This surprising result was noted both in the case study and the study using the majority of the STATIC dataset, suggesting that a fundamental piece of physics is missing from current models of the Martian ionosphere.Finally, we also report the results of a statistical study with the goal of determining where signatures of ion energization are observed in STATIC data. By fitting the measured distribution functions with drifting Maxwell-Boltzmann distributions, we identified distributions in which suprathermal ions produced a significant portion of the measured energy flux. Most distributions are well-described by the Maxwell-Boltzmann distribution below the exobase region---in other words, the neutral atmosphere dominates low-altitude ion dynamics. Suprathermal ions are observed just above the exobase at all solar zenith angles. A comparison of results inside and outside Mars' strong crustal magnetic field regions showed that more thermal plasma is observed inside crustal fields on the dayside, while more suprathermal plasma is observed inside crustal fields on the nightside. These results suggest that crustal fields shield dayside plasma from energization by the solar wind while enhancing energization and outflow on the nightside. The techniques developed in this work provide tools for continued investigation into the physics of initial ion acceleration at Mars using STATIC data, while the results reported here provide context for case studies in which the processes responsible for ion acceleration can be analyzed. The study of initial ion acceleration at Mars is just one context in which electrostatic analyzers in combination with time-of-flight analyzers provide insight into the physics that cannot be matched by any other instrument.

The energization of ions in a dynamic system containing two colliding magnetic flux ropes is investigated in an experimental study presented in this dissertation. Two kink-unstable flux ropes on the Large Plasma Device (LAPD) are made to collide in order to trigger magnetic reconnection, a process in which magnetic energy stored in the fields is dissipated into thermal and kinetic energy that is picked up by surrounding ions. The local energy distribution of the ions is measured by a four-grid ion retarding field energy analyzer that was constructed specifically for this study. The average energy ⟨2E⟩, a quantity equivalent to the Maxwellian temperature for non-Maxwellian distribution functions, was plotted as a function of time for two different flux rope conditions. In both cases, ⟨2E⟩ spikes indicated the presence of an ion beam with a sub-Alfv�nic drift velocity of 9 to 15�eV. The beam does not appear to be heated and the ion temperature of the ropes is estimated to be between 4 to 6�eV. This is found to be consistent with a spectrometer’s line-of-sight, volume-averaged measurement involving Doppler broadening of the 320.3�nm He�II spectral line. Using polar plots as a visualization tool for the two-dimensional ion distribution function, the beam appears to travel primarily in the +z direction along the magnetic field and out of the reconnection plane. The presence of the beam is also correlated with magnetic reconnection events that were identified by plotting line contour planes of the magnetic vector potential A_(z). This means that the ions are likely to be accelerated by -dA_(z)/dt, the inductive electric fields created during magnetic reconnection. Furthermore, the energy density produced by the induced electric fields (0.5�J/m�) is comparable to the energy per unit volume required to heat the ions from 6 to 10�eV (0.3�J/m�) after accounting for a less than 50% efficiency of energy transferred to the ions. Overall, the evidence presented in this study strongly suggests that the ions in the beam are produced by magnetic reconnection. To the author’s knowledge, this is the first experimental observation of a field-aligned ion beam generated in a reconnection experiment.

A forward analytic model is required to rapidly simulate the neutron time-of-flight (nToF) signals that result from magnetized liner inertial fusion (MagLIF) experiments at Sandia’s Z Pulsed Power Facility. Various experimental parameters, such as the burn-weighted fuel-ion temperature and liner areal density, determine the shape of the nToF signal and are important for characterizing any given MagLIF experiment. Extracting these parameters from measured nToF signals requires an appropriate analytic model that includes the primary DD neutron peak, once-scattered neutrons in the beryllium liner of the MagLIF target, and direct beamline attenuation. Mathematical expressions for this model were derived from the general geometry time- and energy-dependent neutron transport equation with anisotropic scattering. Assumptions consistent with the time-of-flight technique were used to simplify this linear Boltzmann transport equation into a more tractable form. Models of the un-collided and once-collided neutron scalar fluxes were developed for one of the five nToF detector locations at the Z Machine. Numerical results from these models were produced for a representative MagLIF problem and found to be in good agreement with similar radiation transport simulations. Twenty experimental MagLIF data sets were analyzed using the forward models, which were determined to only be sensitive to the ion temperature. The results of this work were found to be in good agreement with values obtained separately using other low and high fidelity models.