A 100 KW Class Applied-field Magnetoplasmadynamic Thruster

Summary

The goal of this protocol is to introduce the design of a 100 kW class applied-field magnetoplasmadynamic thruster and relevant experimental methods.

Abstract

Applied-field magnetoplasmadynamic thrusters (AF-MPD thrusters) are hybrid accelerators in which electromagnetic and gas dynamic processes accelerate plasma to high speed; they have considerable potential for future space applications with the significant advantages of high specific impulse and thrust density. In this paper, we present a series of protocols for designing and manufacturing a 100 kW class of AF-MPD thruster with water-cooling structures, a 130 V maximum discharge voltage, a 800 A maximum discharge current, and a 0.25 T maximum strength of magnetic field. A hollow tantalum tungsten cathode acts as the only propellant inlet to inhibit the radial discharge, and it is positioned axially at the rear of the anode in order to relieve anode starvation. A cylindrical divergent copper anode is employed to decrease anode power deposition, where the length has been reduced to decrease the wall-plasma connecting area. Experiments utilized a vacuum system that can achieve a working vacuum of 0.01 Pa for a total propellant mass flow rate lower than 40 mg/s and a target thrust stand. The thruster tests were carried out to measure the effects of the working parameters such as propellant flow rates, the discharge current, and the strength of applied magnetic field on the performance and to allow appropriate analysis. The thruster could be operated continuously for significant periods of time with little erosion on the hollow cathode surface. The maximum power of the thruster is 100 kW, and the performance of this water-cooled configuration is comparable with that of thrusters reported in the literature.

Introduction

MPD thrusters are well known for a relatively high thrust density and a high specific impulse^1,2,3. However, the typical thrust efficiency¹ of MPD thrusters is relatively low, especially with propellants of noble gases^4,5,6. For most MPD thrusters, a part of the propellant flow is injected into the discharge chamber from a slit between anode and cathode^7,8 , with the result that a radial component is a significant proportion of the total discharge. However, in order to generate thrust, radial kinetic effects need to be converted into axial kinetic motion with a physical nozzle or a magnetic nozzle. Accordingly, a key feature of the new design MPD thruster is that all propellant is supplied through the cathode, which can act to inhibit radial discharge; in this way, the proportion of axial energy can be increased. There is an added effect in that the Hall parameter in the plasma around the anode can be increased by the decrease of the number density around the anode, which can strengthen the Hall acceleration component⁹. Since the propellant is close to the inner surface of the cathode where large quantities of initial electrons are emitted in this mode of injection, the propellant ionization rate can be increased greatly. Furthermore, the anode length has been minimized to decrease the wall-plasma connecting area and reduce anode power deposition^10,11. As a divergent anode is applied, this will decrease the angle between the anode and magnetic field lines and decrease anode power deposition further^12,13.

Despite the advantages noted above to improve performance, complete propellant supply by cathode injection can increase the risk of anode starvation which can result in "onset" phenomena¹⁴. To inhibit this behavior, we have retracted the cathode back to the base of anode. The electrons can then diffuse sufficiently in the radial direction before leaving the anode exit, which will act to relieve anode starvation. Further, a multichannel hollow cathode is adopted; compared to the single channel hollow cathode, a multichannel hollow cathode can increase the electron emission area and make the distribution of the propellant more uniform. With this modification, both the lifetime and stability of the thruster can be increased^15,16,17.

The designed power of the thruster is 100 kW and a cooling structure is necessary with steady state operation. In the present laboratory experiments, an efficient water-cooling structure is employed. However, to evaluate the performance of the MPD thruster design, it is critical to obtain the thrust. With the application of a high-pressure water system to transfer heat, there will be strong vibration during the operation of such cooling, which can create significant interference if we used traditional thrust measurements. Accordingly, a target thrust stand is employed to measure the thrust.

MPD Thruster

As shown in Figure 1, the MPD thruster consists of anode, cathode and insulator. The anode is made of copper with a cylindrical divergent nozzle, the minimum inner diameter of which is 60 mm. There is an S-shaped cooling channel around the inner wall of the anode. The inlet and outlet of the channel are on the top of the anode, which are separated by a baffle. A slender copper block is employed to connect the anode and electric cable. The junction is on the outer surface of the anode.

The cathode material is tantalum tungsten, with nine propellant channels. The outer diameter of the cathode is 16 mm. The cooling of the cathode is achieved with a water-cooling holder around the cathode base. There is a ring-shaped channel inside the holder. The cold water is injected into the holder from the bottom and flows out from the top. There is a hollow cathode connector on the left side of the cathode. The propellant flows through the center of the connector and into the hollow cathode chamber; there is a large cavity inside the cathode base connecting with nine narrow 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.

In addition to the main body of the thruster, an external magnetic coil is also necessary to generate fields for the mechanisms in the AF-MPD thruster; magnetic fields provide a convergent-divergent magnetic field to accelerate the plasma together with the electric field. The field coil consists of 288 turns of circular copper pipes, which act as the passage for both electric current and cooling water. The inner diameter of the coil is 150 mm, while the outer diameter is 500 mm. The highest field strength in the center is 0.25 T with current of 230 A.

Experiment System

The experiment system includes six subsystems. The schematic diagram of the overall layout of the experimental system is shown in Figure 2; the layout of the thruster inside the vacuum chamber is shown in Figure 3.

First, the vacuum system, which provides the necessary vacuum environment for the thruster operation, consists of one vacuum chamber, two mechanical pumps, one molecular pump and four cryogenic pumps. The diameter of the chamber is 3 m, and the length is 5 m. The environment pressure can be maintained under 0.01 Pa when the flow rate of (argon) propellant is no more than 40 mg/s.

Second, this source system provides a high voltage pulse to ignite the thruster, provides power for the thruster to accelerate the plasma, and provides power for the magnetic field coil to sustain the external magnetic field. The power source system consists of an ignition power source, a thruster power source, a coil power source and cables. The ignition power source can provide 8 kV or 15 kV discharge voltage. The thruster power source provides a direct current up to 1000 A. The coil power source provides a direct current up to 240 A.

Third, the propellant supply system feeds gas propellant for thrusters. The system mainly includes the gas source, the mass flow rate controller and gas supply pipelines.

The fourth sub-system is the water-cooling system, which provides cool high-pressure water to exchange the heat of the thruster, magnetic coil and power sources. As shown in Figure 4, the system consists of pumps group, water tank, refrigerator, water supply pipelines and pumps controllers. The non-conducting pipes inside the vacuum chamber provide a cooling water terminal for the thruster and magnetic coil, and ensures that electrical insulation among the anode, the cathode, and the ground.

The acquisition and control system can record the signals measuring thruster operation conditions and control operation of other systems. It is composed of three computers and corresponding software, data acquisition card and cables.

As shown in Figure 5, the target thrust stand consists of plate target, slender beam, displacement sensor, support frame, axial moveable platform and radial moveable platform. The target can intercept the plasma which pushes the target. The displacement of the target can be measured by a sensor placed behind the target, in this way enabling evaluation of the thrust¹⁸.

Protocol

1. Preparation for experiment

1. Install the thruster.
   1. Wipe the components of the thruster withnon-dust cloth,soaked with anhydrous alcohol, in a clean room.
   2. Assemble the anode with the insulator.
   3. Bring together the cathode, cathode holder and cathode connector.
   4. Add the cathode part to the anode part.
   5. Install the middle connector into the assemblage and fix them with screws (hexagon socket head screw, M5×16).
   6. Establish the coil seat on experiment platform with forklift.
   7. Place the experiment platform on guide rail of the vacuum chamber.
   8. Install the thruster on the coil.
   9. Link the anode and cathode with corresponding electric cables.
   10. Link the magnetic coil with the coil power source.
   11. Join up the water-cooling pipes and propellant supply pipe with the thruster.
   12. Join up the water-cooling pipes with the coil.
   13. Install the moveable platform inside the chamber and fix the main body of thrust stand on it.
   14. Adjust the position of the radial moveable platform to make the control lines of the thruster and the target coincide with each other.
2. Calibrate the thrust stand.
   1. Load different weights (10 g, 50 g, 100 g, 200 g), one by one, on the calibration device and record the corresponding output of the thrust stand.
   2. Unload the weights one by one.
   3. Repeat the process for three times at least.
   4. Calculate the elastic coefficient of the thrust stand according to the calibration data.
3. Evacuate the vacuum chamber.
   1. Close the door of the chamber.
   2. Start the mechanical pumps.
   3. Start the molecular pumps when the background pressure in the chamber is lower than 5 Pa.
   4. Start the cryogenic pumps when the background pressure in the chamber is lower than 0.05 Pa.
   5. Wait for the pressure to reach 1 x 10^-4 Pa.

2. Ignition and Thrust Measurement Experiment

1. Preheat the thruster if it has been exposed to the air.
   1. Start recording the signal.
   2. Set the propellant mass flow rate at 40 mg/s and keep supplying for at least 20 minutes
   3. Turn on the cooling water supply.
   4. Set the working frequency of cooling water pumps at 10 Hz.
   5. Move the thrust stand to the position far from the thruster.
   6. Switch on the coil power source with the coil current of 90 A.
   7. Switch on the thruster power source with the discharge current of 240 A.
   8. Switch on the ignition power source.
   9. Keep the thruster working for at least 5 minutes.
   10. Switch off the thruster power source and propellant supply.
   11. Stop the recording.
2. Thrust measurement
   1. Move the thrust stand to the position 550 mm from the thruster.
   2. Start recording the signal.
   3. Start the propellant supply.
   4. Ignite the thruster with 90 A coil current and 240 A discharge current.
   5. Increase the coil current to 150 A.
   6. Increase the discharge current to 800 A.
   7. Increase the coil current to 230 A.
   8. Switch off the thruster when the output of thrust stand becomes stable.
   9. Stop the propellant supply.
   10. Stop the recording.

Results

In the experiment, we control discharge current (Id), propellant mass flow rate(m) and applied magnetic field (Ba). In operation, we measure the value of discharge voltage (Vd) and thrust (T), from which base we can get other performance parameters like power (P), specific impulse (Isp) and thrust efficiency (η)¹.

A typical signal of discharge voltage is shown in Figure 6

Discussion

This protocol describes the processes of ignition, operation, and thrust measurement of a 100 kW class applied field MPD thruster. The key point in designing an MPD thruster for optimum performance is choosing the proper configuration according to the specific objective. MPD thrusters with convergent-divergent anode can function steady-state in a large operation range. However, the performance may be lower than the thruster with divergent anode. The hollow cathode, especially the multichannel hollow cathode, is superior ...

Materials

Name	Company	Catalog Number	Comments
Cryogenic Pumps	Brooks Automation		Pumping speed: 10000L/s
Displacement Sensor	Panasonic	HG-C1030	Repetition precision: 10μm Linearity: ±0.1% F.S.
Mass Flow Rate Controller	Brooks Automation		Range: 0-120mg/s
Molecular Pump	Oerlikon		Pumping speed: 2100L/s
Moveable Plantform	Beijing Weina Guangke Automation equipment Co., Ltd.		Range:0-2000mm
Plsatic Water Pipes	Xingye Xingye fluoroplastics (Jiaxing) Co., Ltd.		Ultimate pressure: 4.5MPa
Propellant Argon	Beijing Huanyu Hinghui Gas Technology Co., Ltd.		Purity:99.999%
Refrigerator	Beijing Jiuzhou Tongcheng Technology Co., Ltd.		Refrigeration power:45kW
Water Pumps	Liaocheng vanguard Motor Co., Ltd.; Shanghai people pump industry group Manufacturing Co., Ltd.; Nanfang Pump Limited company		Maximum Output pressure: Centrifugal pump:1MPa Plunger pump:10MPa