Potentially relevant FCC complaint from November 2018: https://ecfsapi.fcc.gov/file/111998371507/11_19_18_FCC_compl...
"Apollo tested several fuel types and found that if the corrosive effects of mercury on the spacecraft could be controlled, and they could keep the use of mercury secret, the ACE could seriously outperform its competitors indefinitely. To date, no other electric propulsion thruster can match the efficiency or power of the ACE, and leading experts in the plasma and fusion fields are convinced that Apollo has made a miraculous breakthrough because they all assume that the ACE uses xenon as a fuel source."