Plasma Propulsion Dynamics
Dr. Mitchell L.R. Walker II
DANIEL GUGGENHEIM SCHOOL OF AEROSPACE ENGINEERING, GEORGIA TECH, GA, USA
Background
Dr. Mitchell Walker's research focuses on experimental and theoretical studies of advanced plasma propulsion concepts which have applications in the development of spacecraft technologies, working with Newell-Rubbermaid, Lockheed Martin, and NASA.
One of Dr. Walker's recent projects uses an innovative experimental setup to measure the properties of plasma at small scales using spectroscopy. This is primarily used to anchor the theoretical models but has promising leads towards understanding the physical mechanisms underlying plasma creation, by studying specific parameters such as electron mobility within magnetic fields. This could be a way of leveraging the development of more efficient propulsion technologies, as Dr. Walker mentions, "We measure what's going on in the plasma, which allows us to anchor calibrate our models and then, when we're more sophisticated, use that to make a better device."
Figure 1: Raw Thomson scattering spectrum in the near field of a 9-kW Hall effect thruster, captured with the Pl -MAX 4 PM4-1024i -HB- FG -18- P46. The raw Thomson scattering spectrum provides a direct measurement of the electron velocity distribution, and a theoretical fit on the data is shown for comparison.
Challenge
Dr. Walker outlines some of the experimental challenges this project faces, "It's a very low-density plasma, so we are effectively counting photons." The low sampling rate requires the spectroscopy equipment to provide a high sensitivity, paired to high spectral resolutions.
Dr. Walker mentions, "The most important thing is having the spectral resolution and sensitivity to detect what you're looking at in your small sample area".
With the [lsoPlane] spectrometer and your [Pl-MAX 4]'s signal to noise, we can now measure electron temperature and density within a few percent of uncertainty, which is just unheard of for our community!
Dr. Mitchell Walker
Solution
Dr. Walker uses various Teledyne Princeton Instruments devices for his spectroscopy experiments, which serve as ideal solutions. For instance, the setup used for the experiments described above consists of a Pl-MAX 4 emlCCD with an lsoPlane 320 spectrograph, controlled by our proprietary LightField software. The high sensitivity of the Pl-MAX 4 removes an experimental bottleneck in the spectral characterization of the plasma. as Dr. Walker mentions. "The noise from our process is greater than any noise from the camera sensor, and [with the Pl-MAX 4 we don't need long exposures to capture things. this is much nicer."
The sensitivity of the PI-MAX4 is ideal for amplifying signals originating down to only a few photons, while minimizing noise during a comparatively short acquisition time. This sensitivity, combined with the high spectral resolution provided by the lsoPlane 320 spectrograph allowed Dr. Walker to perform experiments that were previously not feasible in this field and to innovate in the development of propulsion.
"The hardware is robust which is nice, even though it's incredibly sensitive, my students can learn on it, and it works!"