Mixed Metal based Hollow Cathode Neutralizer Technology for HEMP based Ion Propulsion Systems

Thales Deutschland - Electron Devices - is current ly developing an electric propulsion system for the SmallGEO Hispasat AG1 mission. Main part of the system is a High Efficiency Multistage Plasma Thruster (HEMPT) which self-consistently generates and accelerates xenon ions to provide thrust. To prevent electrical charging of the satellite the positive ion current has to be compensated with an equal negative electron current. A hollow cathode is used for this purpose as an electron source. For the emissive cat hode material a barium impregnated tungsten-osmium sintered body is used. Thales employs this technology since a long period in travelling wave tubes (TWT) and has gathered expansive operating experience with more than 300 millions of hours of flight heritage. The work function of the emissive material amounts to 1.98 eV. The cathode is especially desig ned to provide a long-term stable Ba reservoir resulting in a stable keeper discharge an d stable coupling voltages and in particular in very low ignition voltages of the kee per discharge. The design of the neutralizer combines robustness a nd low weight with emphasis on the thermo-mechanical stability of welded and brazed junctions faced with high thermal loads. All temperature critical linkages are successfully tested and verified prior to qualification of the HEMP Thruster Module (HTM). 4800 firing hours in continuous operation are gathered during the ET1 endurance test on HTM level. Environmental verification including mechanical and thermal vacuum loads on qualification level has been performed successfully both on component and on HTM level.