Electron Gun-Ion Gun

Description

This experiment (SC4) consisted of an electron-beam system (EBS) and a positive-ion-beam system (PIBS), which were flown to control the ejection, respectively, of negative charge (electrons) and positive charge (xenon ions) from the spacecraft. The EBS consisted of a control grid and an indirectly heated oxide-covered cathode, which was kept at a controlled negative potential with respect to the space vehicle. The controlled negative potential determined the energy of ejected electrons and varied in steps as follows: 50, 150, 300, 500, 1500, and 3000 V. The control grid was normally kept negative with respect to the cathode and was pulsed positively to allow electron ejection current. The duration and electron-current level of the pulse were controlled by ground command. A focusing element between the control grid and the grounded exit anode served to reduce the beam divergence. The magnitude of the beam current could vary over six steps (0.001, 0.01, 0.10, 1.0, 6.0, and 13 mA). The maximum power drawn was 42 W. Mounted in bonded electrical contact with the spacecraft frame ground, the EBS was oriented so that the beam axis was perpendicular to the spacecraft spin axis. A protective aperture cover was removed by ground command when the spacecraft was in orbit. The PIBS consisted of a Penning discharge-chamber ion source and a control grid. The ion source consisted of an ionization chamber and beam formation electrodes. A cylinder of pressurized xenon constituted the gas source and was controlled by a leak valve with the flow rate commandable from the ground. The intensity and duration of the ion beam was also determined by ground command. The two beam bias voltages were 1000 and 2000 V dc, and the five selectable beam intensity levels were 0.3, 0.5, 1.0, 1.5, and 2.0 mA. During maximum beam ejection, the power drawn was 60 W. The PIBS nozzle was the element that controlled the nature of the ejected beam, and the thin wires mounted on top of the nozzle could neutralize all or a fraction (including zero) of the beam, depending on satellite experiment requirements. The expellant storage tank was connected to the ion source through a pressure regulator, a solenoid-operated latch, a porous plug, and an insulator. The ion source was maintained under vacuum and opened to the atmosphere in orbit on command.