Foto Credit: Sebastian Ringleb.
The Justus-Liebig University of Giessen operates several vacuum test facilities. The full scale 30 m3 JUMBO vacuum facility is suitable for testing large thrusters. The smaller test facilities, such as the BigMac EVO, can accommodate for testing low power thrusters.
The JUMBO test facility
The JUMBO test facility is a space simulation vacuum chamber with 2.6 m diameter and a length of 6 m, corresponding to a volume of ~ 30 m3. The chamber is equipped with various vacuum pump systems for the different pressure regimes.
The Jumbo facility was characterized by simulations (performed by the theory group at JLU) of the neutral particle distribution inside the vacuum chamber using state-of-the-art Particle-in-Cell (PIC) computer codes with massive parallelization together with Direct Simulation Monte Carlo Methods (DSMC).
First, a combination of a roots pump and a rotary pump, isolated from the chamber by an angle valve, with 2000 m3/h total pumping speed provides low and medium vacuum. Three turbo-molecular pumps with 5.100,00 l/s facilitate to reach high vacuum conditions. A special set of nine cryogenic pumps with total pumping speed of ~ 160.000,00 l/s for xenon is used during the operation of thrusters to hold the high vacuum level. After 12 hours of pumping the facility can be used to run a thruster.
The vacuum facility offers a great number of mechanical ports, e.g., for electric feedthroughs or cooling water. The chamber is equipped with a water-cooled beam target in Chevron configuration dissipating ion beams up to 50 kW (unfocussed, 25 kW for focused beams) at room temperature.
Ultimate pressure without mass flow is below 4e- 7 mbar.
1. Turbo molecular pump (or turbo pump)
A special vacuum pump to achieve a high vacuum (below 1e-7 mbar). The turbo pumps are needed to achieve this base pressure without the engine running. During operation, these pumps are still switched on, but they can no longer efficiently pump out the large quantities of gas that are admitted into the chamber via the thruster. This is where the cryogenic pumps take over.
2. Window
Observation of thruster during operation.
3. Pump control system
Controls the turbo molecular pumps and the associated rotary vane pumps. It is integrated into the control process of the facility.
4. Graphite beam target
The ion beam hits this graphite surface at the end of its path in the vacuum chamber. The ions of the beam travel so fast that they can remove material from the graphite upon impact (sputtering). Graphite withstands this bombardment best compared to other materials, so this is the usual material for it. The angle of inclination of the impact surfaces is intended to ensure that the sputtered particles are given a preferential direction towards the side walls, i.e. they are not directed directly towards the engine. The illustration still shows the old beam target. In the meantime, a new target has been installed whose graphite is supposed to produce even fewer sputter particles.
5. Various relevant feedthroughs for operating the thruster (feedthroughs for electricity, gas, cooling water etc.)
In the test mode, the high-voltage power supply units supplying the engine are located outside the vacuum chamber. The electrical supply lines are fed through special vacuum feedthroughs, as are the supply lines for propellant and for any cooling water that may be required. Typically, the radio frequency generator of the thruster is always in a vacuum, i.e. it is a component that must be permanently cooled, since even low resistive electric losses in a vacuum can lead to strong heating. In the case of testing a complete engine unit in a vacuum, important telemetry data can be sent outside via these feedthroughs.
6. Control desks
Flexible workspaces.
7. Rotary vane pumps
They take over the main work at the beginning of pumping process to remove most of the air from the chamber.
8. 1 of 8 of cold head cryogenic pumps
The BigMac-EVO test facility
The BigMac EVO test facility is a space simulation vacuum chamber with cylindrical shape, length 3.2, diameter 1.6 m and volume 6.4 m3. It is equipped with turbo and cryogenic pumps with total pumping speed of ~ 35 000 liter/s for xenon is used during the operation of thrusters to hold the high vacuum level. A rotating C-scanner is currently under construction and will be commissioned in autumn 2021. It will make it possible to record ion beam profiles on a spherical surface that is scanned by the scanner. Other measuring systems such as energy analyzers or ExB-probes can be integrated into the system if required.
1. Wheel to open dished bottom
2. Window flange
3. Cold head of the cryogenic pump
4. Hinge (for opening the tank)
5. Test power supply (TPS); provides power supplies for the operation of thrusters and computer controlled data acquisition and thruster control system.
6. Fluidic ground support equipment; mass flow controllers and valves.
7. Fluidic ground support equipment; xenon gas bottle
8. Manual valve for venting the chamber
9. 19’’ rack; power supplies and control units for the vacuum system; data acquisition of vacuum relevant data (pressures, temperatures of cryogenic pumps)
Within the framework of the project, special near-field diagnostics are being developed for the test facilities, which make it possible to measure the ion current density with very high accuracy directly in front of the grid of the thruster and also provide an indication of the background pressure that is present directly in front of the grid. These findings are to be used in the development of theoretical models, which will be used to calculate the erosion of the grid for very long operating times. The erosion of the grid significantly determines the lifetime of such an engine and is an important parameter for operating times of several years in space (permanent operation). The erosion is caused by the interaction of the ion beam with neutral propellant atoms. This results in charged slow ions, which are then attracted to the electrically biased grid of the engine. The impact of these ions results in atomisation processes that slowly erode the grid.
