Comparison of On-Orbit and Ground Based Hollow Cathode Operation

The Plasma Contactor Unit (PCU) was developed by the Rocketdyne division of Boeing to control charging of the International Space Station (ISS). Each PCU contains a Hollow Cathode Assembly (HCA), which emits the charge control electrons. The HCAs were designed and fabricated at NASA’s Glenn Research Center (GRC). GRC’s HCA development program included manufacture of engineering, qualification, and flight model HCAs as well as wear tests and qualification tests. GRC is currently tracking the on-orbit data for the flight HCAs. This data will be discussed with comparison to operating parameters verified by ground based HCA tests. The flight HCAs continue to operate flawlessly. The first unit has accumulated more than 3650 hours of on-orbit operation and the second unit has accumulated over 5550 hours. INTRODUCTION The International Space Station (ISS) high voltage solar arrays deliver output voltages of 140 V to 160 V. The electrical configuration of the ISS and the plasma current balance could cause the station to float at voltages predicted to be as much as 120 V below the ambient space plasma if no charge control device is used. If large negative floating potentials are present, the ISS could interact with ambient space plasma. These interactions could include arcing and sputter erosion.