The goal of this protocol is to introduce the design of a 100 kilowatt class applied-field magnetoplasmadynamic thruster and the relative experimented method. Magnetoplasmadynamic thruster, that is MPD thruster, is a typical electric accelerator. It is well known for high specific impulse and a high thruster density, and is treated as primary candidates for our main propulsion in our future high power space missions.
In this article, we will introduce a design of a 100-kilowatt class applied-field MPD thruster, the necessary experiment systems to hold a relative thruster experiment and the operation steps to finish this experiment. Thruster designing. The thruster mainly consists of anode, cathode and insulator.
The anode is made of copper with a cylinder divergent nozzle, the minimum inner diameter of which is 60 millimeters. The cathode is built of tantalum tungsten with nine propellant channels, the outer diameter of which is 16 millimeters. There is a hollow cathode connector on the left side.
The propellant flows through the central of the connector and reach the hollow cathode. There is a large cavity inside the cathode base connecting with nine cylindrical channels. The cavity acts as a buffer to increase the uniformity of the propellant distribution in nine channels.
The cathode is connected to the electric cable with an annular copper block, which is installed around the cathode connector. Besides the main body of the thruster, external magnetic coil is also necessary for the working of MPD thruster. The coil consist of 288 turns circle copper pipes which act as the passage for both electric current and cooling water.
The inner diameter of the coil is 150 millimeters, while the outer diameter is 500 millimeters. The highest field strength in the central is 0.25 telsa. Experiment system.
Experiment system provides necessary conditions for the experiment, which mainly includes six subsystems. Firstly, vacuum system provides the necessary vacuum environment for the thruster. And the diameter of the chamber is three meters, while the length is two meters.
The environment pressure can maintain under 0.01 pascal. Secondly, power source system. Power source system consists of ignition power source, thruster power source, coil power source and cables.
The ignition power source can provide eight kilovolt or 15 kilovolt discharge voltage. The thruster power source provides a direct current up to 1, 000 ampere. The coil power source provides a direct current up to 240 ampere.
The third one is propellant supply system, which feeds gas propellant for thrusters. This system mainly includes gas source, mass flow rate controller and gas supply pipelines. The fourth one is water cooling system, which provides high pressure water to exchange the extra heat of the thruster, magnetic coil and power sources.
Then is acquisition and control system, which can record the signals of thruster operation conditions and control other systems. The last one is target thrust measurement system, which can be used to measure the thrust. The target thrust stand mainly consists of plate target, slender beam, displacement sensor, support frame, axial movable platform and radio movable platform.
The plasma can be intercepted by the target, and the target will be pushed by the plasma. The displacement of the target can be measured by the sensor placed behind the target. By this way, we can evaluate the thrust.
Preparation for experiment. Install the thruster. Wipe the components of the thruster with alcohol in a clean room.
Assemble the anode with the insulator. Bring together the cathode, cathode holder and cathode connector. Add the cathode part to the anode part.
Install the middle connector into assemblage and fix them with screws. Establish the coil seat on experiment platform with forklift. Place the experiment platform on guide rail of the vacuum chamber.
Install the thruster on the coil. Link the anode and cathode with corresponding electric cables. Link the magnetic coil with the coil power source.
Join up the water cooling pipes and propellant supply pipe with thruster. Join up the water cooling pipes with the coil. Install the movable platform inside the chamber and fix the main body of thrust stand on it.
Adjust the position of the movable platform to make the center line of the thruster and the target concede with each other. Then we collaborate the thrust stand. Firstly, load different weights on the collaboration device and record corresponding output of the thrust stand.
Repeat the process for three times at least. Then we can calculate the elastic coefficient of the thrust stand according to the calibration data. Vacuumize the vacuum chamber.
Close the door of the chamber. Start the mechanical pumps. Start the molecular pumps when the background pressure in the chamber is lower than five pascal.
Start the cryogenic pumps when the background pressure in the chamber is lower than 0.05 pascal. Wait for the pressure to reach 10 to the power of minus four pascal. Ignition and thrust measurement experiment.
We need to preheat the thruster if the thruster has been exposed to the air. Start recording the signal. Set the propellant mass flow rate at 40 milligrams and keep supplying for at least 20 minutes.
Switch on the cooling water supplying. Set the working frequency of anode and cathode water cooling pumps at 10 hertz. Move the thrust stand to the position far from the thruster.
Switch on the coil power source with the coil current of 90 ampere. Switch on the thruster power source with the discharge current of 240 ampere. Switch on the ignition power source.
Keep the thruster working for at least five minutes. Switch off the thruster power source and propellant supplying. Stop the recoding.
After the preheating, we can perform the thrust measurement. Move the thrust stand to the position 550 millimeters from the thruster. Start recording the signal.
Start the propellant supplying. Ignite the thruster with 90 ampere coil current and 240 ampere discharge current. Increase the coil current to 90 ampere.
Subsequently, increase the discharge current to 800 ampere. Then increase the coil current to 230 ampere. Switch off the thruster when the output of the thrust stand becomes stable.
Stop the propellant supplying. Stop the recoding. Representative results.
In the experiments, we control discharge current, propellant mass flow rate and applied magnetic field. Then we measure the value of discharge voltage and thrust based on which we can get other performance parameter like power, specific impulse and thrust efficiency. A typical signal of discharge voltage is shown in this figure.
After turning on the power source, a high voltage will be employed on the thruster to break down the neutral propellant. After the ignition, the voltage trends to a constant value and basically keeps constant. Then we can say the ignition is successful.
A typical thrust measurement result is shown in this figure. We start record the signal of the thruster stand before the propellant supplying, which is treated as a zero thrust point. There will be a slight thrust after supplying the propellant.
After the ignition, there will be a large oscillation. Then the thrust tends to a steady value. There will be a zero drift due to the thermal deformation of the target, the value of which is 50 millinewton.
The error caused by the drift is no more than 1%The figure shows the discharge characteristics during half-hour continue work. We confund that the thruster trends to steady state rapidly after ignition, and the voltage is very stable during this period. This figure shows the appearance changes of tantalum tungsten hollow cathode before and after experiments.
The total experiment time is more than 10 hours. We can find slight erosion distributing uniformly on the outer surface of the cathode, which means the thruster has the potential to work for a much longer time than 10 hours. After the continue work test, we explored the performance of the thruster in the power range of 50 to 100 kilowatt.
The best performance is obtained at 99.5 kilowatt while the thrust is 3, 052 millinewton. Specific impulse is 4, 359 seconds, and thrust efficiency is 67%It is noteworthy that when the thruster reaches the best performance, the background pressure is 0.2 pascal. Measured performance may be higher than the actual value due to the influence of high pressure.
The tester is made out of tantalum tungsten and it shows the operation resistance. The gas power is 100 kilowatt with a thrust of 3, 050 millinewton, the specific impulse of 4, 300 seconds, and the efficiency of 67%
