Dennis J. Chornay

Valence orbital momentum distributions in s‐triazine

Valence orbital momentum distributions, ρ(q), are obtained for s‐triazine, C3H3N3, from (e,2e) spectroscopy and split valence SCFMO calculations. The separation energy spectrum simulated using the calculated ρ(q) and ionization energies from Green’s function calculations is in good agreement with experiment from 9–20 eV, but at higher energy the simulated spectrum has higher intensity than experiment, indicating that the Green’s function calculation underestimates the complexity of the inner valence region. The calculated momentum distributions have their maxima at higher values than observed experimentally for the outermost valence orbitals, but the difference in momentum distributions between the antisymmetric 4e’ N2p lone pair orbital of s‐triazine and the N2p lone pair orbital of ammonia is reproduced well by the calculations. Similarly, calculated and experimental differences of Fourier transformed ρ(q) for the 4e’ and 3a’1 N2p lone pair orbitals of triazine are in agreement and can be interpreted us...

The parameterization of microchannel-plate-based detection systems

The most common instrument for low energy plasmas consists of a top-hat electrostatic analyzer (ESA) geometry coupled with a microchannel-plate (MCP)-based detection system. While the electrostatic optics for such sensors are readily simulated and parameterized during the laboratory calibration process, the detection system is often less well characterized. Here we develop a comprehensive mathematical description of particle detection systems. As a function of instrument azimuthal angle, we parameterize (1) particle scattering within the ESA and at the surface of the MCP, (2) the probability distribution of MCP gain for an incident particle, (3) electron charge cloud spreading between the MCP and anode board, and (4) capacitive coupling between adjacent discrete anodes. Using the Dual Electron Spectrometers on the Fast Plasma Investigation on NASAs Magnetospheric Multiscale mission as an example, we demonstrate a method for extracting these fundamental detection system parameters from laboratory calibration. We further show that parameters that will evolve in flight, namely MCP gain, can be determined through application of this model to specifically tailored in-flight calibration activities. This methodology provides a robust characterization of sensor suite performance throughout mission lifetime. The model developed in this work is not only applicable to existing sensors but can be used as an analytical design tool for future particle instrumentation.

Extending the dynamic range of microchannel plate detectors using charge-integration-based counting

Microchannel plate (MCP) detectors provide a mechanism to produce a measureable current pulse (∼0.1 mA over several nanoseconds) when stimulated by a single incident particle or photon. Reductions of the devices amplification factor (i.e., gain) due to high incident particle flux can lead to significant degradation of detection system performance. Here we develop a parameterized model for the variation of MCP gain with incident flux. This model provides a framework with which to quantify the limits of high-flux MCP operation. We then compare the predictions of this model to laboratory measurements of an MCPs response to a pulsed charged particle beam. Finally, we demonstrate that through integration of the MCP output current in pulsed operation, effective count rates up to ∼1 GHz can be achieved, more than an order of magnitude increase over conventional counting techniques used for spaceflight applications.

The 2π charged particles analyzer: All-sky camera concept and development for space missions

Increasing the temporal resolution and instant coverage of velocity space of space plasma measurements is one of the key issues for experimentalists. Today the top-hat plasma analyzer appears to be the favorite solution due to its relative simplicity and the possibility to extend its application by adding a mass-analysis section and an electrostatic angular scanner. Similarly, great success has been achieved in MMS mission using such multiple top-hat analyzers to achieve unprecedented temporal resolution. An instantaneous angular coverage of charged particles measurements is an alternative approach to pursuing the goal of high time resolution. This was done with FONEMA 4-D and, to a lesser extent, by DYMIO instruments for Mars-96 and with the FIPS instrument for MESSENGER mission. In this paper we describe, along with precursors, a plasma analyzer with a 2π electrostatic mirror that was developed originally for the Phobos-Soil mission with a follow-up in the frame of the BepiColombo mission, and is under development for future Russian missions. Different versions of instrument are discussed along with their advantages and drawbacks.

Tomographic imaging of electron distributions: Leveraging computing power advances to produce inexpensive, low-power, lightweight, and robust instrumentation

We describe a prototype of a novel electron spectrometer that employs a two-dimensional tomographic method to obtain the electron velocity distribution in a space plasma. The spectrometer consists of a magnetic field cavity with a single entrance aperture and a one-dimensional position sensitive microchannel plate/anode assembly electron detector. Electrons entering the instrument through the aperture which have the same component of velocity normal to the aperture plane will all strike the electron detector at the same location regardless of the magnitude of the other velocity component. Thus, the instrument performs line integrals in velocity space. By placing the instrument on a spinning spacecraft, it will provide a complete set of these integrals that may be deconvoluted using standard tomographic techniques. In many cases, basic plasma parameters can be inferred directly from the raw data without deconvolution. This instrument has advantages over conventional electrostatic analyzers: it does not require power consuming voltage stepping, makes efficient use of volume, and operates with a high duty cycle. A modified version of the instrument that is insensitive to photons and can determine three-dimensional distribution functions is also described.

Fourier Transform of Spherically Averaged Momentum Densities

The first (e,2e) experiments had as their objective the demonstration of the feasibility of the technique1. Energy resolution was several eV and the signal rate was low, however, the early measurements clearly showed that the electron knock-out coincidence technique could be used for obtaining single electron momentum densities. Once the feasibility of the technique was established new instruments were constructed with better energy resolution and greater efficiency.2,3,4,5 For structure measurements the noncoplanar symmetric geometry was preferred because of the ease with which the data could be converted to momentum densities within the context of the plane wave impulse approximation. During this phase a great deal of data was accumulated on atoms and small molecules. The emphasis was on the assignment of electronic states, the investigation of satellite structure and the comparison between momentum densities calculated from atomic and molecular wave-functions and the corresponding experimental measurements.6 In this second phase there was a dramatic increase in the quality of the experimental data, however, the data were not of sufficient precision to make more than qualitative comparisons with theory. An important criticism of the technique was that all the experimental data looked qualitatively similar and most analyses were essentially descriptive.