A schematic of the system is shown to the right. Arrows indicate
the direction of liquid lithium flow through the system. The entire
system is constructed of stainless steel and is designed to fit on the
back of the inverted pendulum thruster stand upon which the thruster is
mounted. Solid lithium is loaded into the reservoir (to the right
in the schematic) in the glove box facility and the sealed reservoir
is transferred to the testing facility. A steady flow of argon protects
the lithium from contamination as it is attached to the feed system.
The facility is then brought to vacuum (~5 x 10-5 Torr).
The system is then heated (>200 C) allowing for the gravity feed of lithium
into the reservoir line, the lithium chamber, and the thruster line.
Once melting is complete, the line to the reservoir is closed by means
of a freezing valve which takes advantage of the thermal properties of
the lithium to seal the line. A mechanical stepper motor-actuator
system then
drives the piston pushing the lithium through the thruster line to the vaporizer multi-channel hollow cathode at speeds which can be pre-programmed or controlled real-time. The lithium flow rate to the cathode is directly proportional to the piston velocity, depending only on this velocity, the density of the liquid lithium within the chamber (499 +/- 6 kg/m3) and the effective inner diameter of the lithium chamber. A calibration curve relating piston velocity to lithium mass flow determines the value of this effective diameter and provides real-time knowledge of lithium flow rate.
