Overview of Plasma-Material Interactions in Electric Propulsion Program

The Plasma Surface Interactions Science Center (PSISC) and Space Propulsion Laboratory (SPL) at MIT are currently collaborating with Stanford University Plasma Physics Laboratory (SPPL) in a Air Force Office of Scientific Research funded program to investigate plasma-material interactions in electric propulsion. Although the SPPL has collaborated in the past with the MIT SPL on electric propulsion research (specifcally, the testing of the diverging-cusped field thruster), the PSISC introduces unique facilities and expertise towards surface characterization. With these resources and MIT SPL and Stanford PPL's expertise and test facilities in electric propulsion, this program aims to provide new diagnostic methods to the propulsion community, new plasma exposure scenarios to the fusion community, and greater insight on plasma-material interactions from the cross-fertilization of the two communities.

Motivation -- This cross-collaboration research between the fusion and plasma propulsion fields is predicated on the growing overlaps in plasma conditions. While baseline comparisons of plasma conditions between magnetically-confined fusion plasma and thruster plasma may seem invalid (magnetically-confined fusion plasma: ne ~ 1x10^20 m^-3, Te ~ 104 eV; typical thruster plasma: ne ~ 10^17-10^18 m^-3, Te ~ 1-50 eV), there are regions in a Tokamak which feature far more similar conditions. For instance, fusion, divertor & wall conditions (ne ~10^18-10^20 m^-3, Te ~ 1-10 eV) has far more commonality with thruster plasma. With the development of thrusters with higher temp./ density plasmas in propulsion, we may see even greater overlap in the near future.

The objectives of this research program are to:
(1) Generate detailed understanding of plasma sheaths over electrodes and stressed wall sections in plasma thrusters.

(2) Predict and measure space and time-‐resolved plasma particle bombardment in these stressed areas.

(3) Couple this with precise data and first‐principles calculations of wall response, including thermal,sputtering, evaporation in The presence of re‐deposition and secondary electron emission and, in the case of electrodes, high current densities and associated phenomena.

(4) Support and validate these calculations with materials science experiments and modeling.

(5) Integrate the above into multi‐scale codes for simulation of joint plasma/solid behavior at the device level. 6) Use the resulting insight to formulate candidate revolutionary concepts for novel wall inserts and electrodes.

(6) Use these codes for optimization of complex devices, including novel wall inserts and electrodes for stressed areas.

(7) Design and test devices with novel inserts, and electrodes to verify the resulting improvements in performance and lifetime, and iterate as needed