ISINGLASS Sounding Rocket Mission
I joined the Lynch Rocket Laboratory at Dartmouth College as a postdoctoral researcher in 2015. Our group studies auroral plasma physics, primarily with diagnostics flown on sounding rockets. Studying the aurora gives us a better understanding of the coupled magnetospheric-ionospheric system. This is important for a variety of reasons, including better protection of our orbital assets (satellites, telescopes, astronauts, etc).
At Dartmouth I’ve worked on the design and analysis techniques for a new ionospheric measurement system where a sounding rocket deploys four small spacecraft and multipoint measurements of plasma parameters are made:
A video animating this can be found here. The size of this system (~1 km) will allow for exploration of a difficult scale-length in auroral plasma measurements.
The ejected spacecraft are called “Bobs”:
and they carry two plasma sensors (retarding potential analyzers, RPA), an inertial measurement unit (for attitude and position estimation), and a radio for communication back to the main payload. These were designed and fabricated in-house between Dartmouth College and University of Alaska Fairbanks.
I’ve been lucky enough to be involved in almost all aspects of the project, but my focus has been on the telemetry system and sensor configuration, development of a Bob-specific attitude determination algorithm, and interpretation of the measurements made by the RPAs. These measurements are different from traditional RPA measurements due to the subsonic payload velocity, and require careful interpretation. Here is a diagram of the current RPA:
RPAs collect ions as a function of energy by applying a retarding electric field with a screen in front of the collection surface. The relation established between the voltage applied to the screen and the collected current allows us to measure parameters of the plasma.
A test flight out of NASA Wallops Flight Facility in October 2015 proved the mechanical and communication aspects of this measurement system, and the ISINGLASS science mission successfully demonstrated the full system in March 2017:
Laboratory Plasma Automation
Our lab at Dartmouth has a large vacuum chamber for simulation of a variety of ionospheric conditions:
Inside the chamber is a two degree-of-freedom motion table, allowing external adjustment of the orientation of a sensor relative to the particles sources, which include a microwave plasma source and ion/electron gun. Here’s an image of the gun in operation:
With this system we can test and calibrate our flight plasma diagnostics. Part of my work at Dartmouth has been to automate this laboratory system. This involved integrating multiple data acquisition system, current/voltage supplies, the motion table, and various diagnostics into one simple interface. The end results, described in the Projects section of this page, was a GUI that can perform fully automated runs where the configuration and orientation (relative to the plasma) of the diagnostics under test can be scanned. Sensors for the RENU2 and ISINGLASS missions were tested and calibrated using this system.
Turbulence in Dipole-Confined Plasmas
My graduate research in the Columbia University Plasma Laboratory focused on the laboratory plasmas confined by dipole magnetic fields, similar to that of the Earth’s magnetosphere. These plasmas are characterized by interchange instabilities (analogous to the well known Rayleigh-Taylor instability) which drive turbulent dynamics. There are currently two groups working in this specific field, CTX / LDX, a Columbia-MIT collaboration, and RT-1 at the University of Tokyo. The below figure highlights some of the key particle/plasma motions which characterize these plasmas:
The experiment at Columbia is called the Collisionless Terrella Experiment, or CTX. The dipole magnetic field is produced by a mechanically supported coil pack, and plasmas are created through electron cyclotron resonance heating with an injected microwaves. I made an animated fly-through of the machine in SolidWorks. With sufficient microwave heating and neutral fueling, a high density, turbulent plasma state develops. This video shows what this turbulence looks like by measuring the plasma lost to the poles of the magnet. Quasi-coherent structures are observed to form and exist for a duration of time before becoming decorrelated.
My thesis work involved the application of broadband current injection feedback to these fluctuations, which we found to strongly modify the turbulent spectrum. Potential fluctuations measured in the plasma are phase shifted, amplified, and re-applied (below, left), resulting in amplification or suppression of the measured fluctuations, depending on the phase (below, right).
The top right shows two measurements of the plasma potential fluctuations. The red lines depict when positive feedback is triggered on and the blue lines show when negative feedback is triggered on. We can directly see that the fluctuation amplitude is enhanced or reduced by the feedback. Measurement of the spectrum during feedback shows broadband amplification or suppression depending on the phase (lower right).
High Speed Imaging in Plasmas
In many confined plasmas, there are regions which are inaccessible to Langmuir probes and other inserted diagnostics. The probe will either be too perturbative or may be damaged by the plasma. Often, these regions are studied with non-invasive diagnostics like photodiodes or cameras, or parameters are inferred from measurements made outside of the region. These non-local estimates often cannot explore the smaller-scale structure in the plasma. We developed a method to make local measurements in a non-invasive manner by dropping small (300 micron) particles though the plasma, and imaging the visible fluctuations in the ablating gas around them. On the left is the experimental configuration, and the right shows particles falling through the plasma:
Tracking the particles as they move though different regions of plasma, the local brightness fluctuation are extracted:
The fluctuation spectrum measured by this technique matches that measured by traditional probes, and these measurements are strongly correlated when the particles are local to the probe. Using multiple particles, spatial structure can be studied in a multipoint manner. This technique allowed us to make measurements in a previously unexplored region of the plasma.
More information on my past and current research can be found via ResearchGate.