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Solid Fuel Vacuum Arc Thrusters - New Concepts for Space Propulsion Solid Fuel Vacuum Arc Thrusters - New Concepts For Space Propulsion IEPC-2017-35 Presented at the 35th International Electric Propulsion Conference Georgia Institute of Technology { Atlanta, Georgia { USA October 8{12, 2017 Marvin Kuehn∗, Marina Kuehn-Kauffeldty, Lars Seippz and Jochen Scheinx University of The Federal Armed Forces Germany, Munich, Germany Abstract In this work the performance of a magnetic nozzle coupled with a pulsed vacuum arc thruster was investigated using high speed imaging and thrust measurement. Here titanium was used as a solid propellant. A magnetic nozzle was simulated using an arc current of 150 A. For the power supply a 10-stage pulse forming network (PFN) with a discharge length of 1 ms was used. The expanding plasma was investigated using high speed imaging and a µN-thrust balance. More over electrical characteristics of the system (current and voltage) were recorded. The results are in good agreement with literature, predicting a significant higher thrust level for magnetically enhanced vacuum arc thrusters. I. Introduction odern spacecrafts and satellites use various types of thrusters to fulfill their mission tasks. For longterm Mmissions and precise controlling operations electric propulsion systems offer very good characteristics. Those systems have a very long lifetime, high exit velocities resulting in a high ISP and a high energy efficiency. They consist of a power supply unit, a propellant storage system, a control unit and the thruster itself. Most of electric propulsion systems generate plasma in order to produce thrust. They use gas (e.g. Xenon) or a solid (e.g. Teflon) as fuel. The plasma can be created by means of an electric arc (Pulsed Plasma Thruster), radio frequency fields (Radio Ion Thruster) or the hall effect (Hall Effect Thruster). Since in gas feed thrusters only ions are extracted for thrust generation a neutralizer is needed to prevent the thruster from becoming negatively charged. Compared to gas feed systems, solid propellant thrusters advantages are the lower weight of the fuel supply system and smaller size. They do not have a complex propellant feed mechanisms (valves, tubes, etc...) which might fail during longterm operations. However solid propellant thrusters are less efficient and require high voltage for the ignition of the plasma. Moreover deposition of the solid propellant on the spacecraft can be a problem1. One example of a solid propellant system is the vacuum arc thruster. It uses the solid cathode material to generate a plasma, which is formed from metal evaporated by an electric arc. The arc can be operated in DC or pulsed DC mode. The latter offers a much lower power consumption resulting in lower heat dissipation, which makes it more attractive for this application An electric arc operated in vacuum produces ions with a relatively high kinetic energy from the cathode material. Thus the plasma expands from the cathode surface. Here electrons and ions are accelerated, so that the plasma remains quasi-neutral. Hence a neutralizer is not required here. Such a system can be ∗LPT, University Of The Federal Armed Forces Of Germany, [email protected]. yLPT, University Of The Federal Armed Forces Of Germany ,marina.kuehn-kauff[email protected] zUniversity Of The Federal Armed Forces Of Germany, [email protected] xLPT, University Of The Federal Armed Forces Of Germany, [email protected] 1 The 35th International Electric Propulsion Conference, Georgia Institute of Technology, USA October 8{12, 2017 supplied by an inductive or capacitive energy storage2;3;4;5. Moreover it is known, that an external magnetic field can increase the velocity of the produced ions and thus the thrust8;11. The aim of this work is to examine the effect of an external magnetic field on the efficiency of a vacuum arc thruster. II. Theory A. Vacuum Arcs The vacuum or cathodic arc has been intensively studied in the past and it has applications in coating technology, ion sources and others. Many works deal with the modification of vacuum arc systems for solid propellant thrusters 2;3;4;8;9;12. Vacuum arcs are generally characterized by high currents at comparably low burning voltages from 10 V to 40 V depending on the cathode material8;9. The vacuum arc plasma consists of the vaporized cathode material which is highly ionized (30 % to 100 %)8;9;15;17. The plasma is generated from so called cathode spots which have a diameter in the micrometer scale. The ions move with a velocity up to 2 × 104 m=s8;9 away from the cathode. For Titanium the ion current corresponds to around 7 % to 10 % of the total arc current7;8. Besides the plasma also macro particles which consist of molten cathode material are created. Their size is in the µm range while their velocity is far less than the ion velocity. Generated macro particles are not influenced by external magnetic or electric field. The amount and size of the particles can be controlled by magnetic fields, filter ducts or design parameters of the vacuum arc source8. For the arc ignition various methods can be applied. However for the electric propulsion application the so called trigger-less ignition mode is commonly used due to its simple and robust realization16;17. The thruster geometry used in this work is shown in figure 1. Here the cathode consists of a pure titanium rod with a diameter of 10 mm surrounded by ceramic insulator and copper anode. The copper anode itself has a conic shaped end and is fitted inside an aluminum coil holder. The exit nozzle diameter is 18 mm. On the other side of the thruster a nylon cap is installed to insulate and protect the back end of the thruster. Electric connection terminals are realized via screw connectors in the anode and the cathode (back end). The coil, which is used as a magnetic nozzle, consists of 0:5 mm diameter insulated copper wire and is double layered. The winding number is 40 with a resulting inductive of 7:24 µH which was determined with a LC-Meter type AE20204 ASCEL Electronic. For insulating and wear resistance reasons an additional PET-foil is applied to the outer surface of the coil holder. The overall mass of the thruster is 82:4 g including all screws, connector terminals and the coil. 18 mm aluminium coil 10 mm copper anode copper coil holder insulator titanium cathode Nylon cover 36.5 mm Figure 1: Setup of the magnetically enhanced vacuum arc thruster, left: cut though view right: front view. B. Power Supply Unit The current pulse for the vacuum arc thruster is generated using a \Pulse Forming Network"(PFN). A PFN is an arrangements of several capacitors and inductances which leads to a special shaped pulses depending 2 The 35th International Electric Propulsion Conference, Georgia Institute of Technology, USA October 8{12, 2017 on their electrical element values. The total energy stored in a PFN is given by n E = · C · U 2 (1) PFN 2 i load with n the number of section of the PFN, Ci the capacitance of each section and Uload the specified load voltage. In figure 2 a so called Raleigh PFN is illustrated. Here the same size of each capacitor and inductance is used which makes it convenient for realizationPFN (Li = Ci). Thus a rectangular shaped current pulse can be formed. The higher is the section number the more rectangular the pulse shape will be. L1 L2 Ln Switch I [A] VCC C1 C2 Cn t [s] Figure 2: Multi-staged Raleigh PFN with Li = Ci. Essential for the best pulse shapes is a matching of the impedance of the applied load. If this matching fails the pulse shape and the output characteristic of the PFN is significantly changed. If for example an additional inductance is added in series to the arc (as it will be the case in this work), the total impedance will be higher than the ideal matching impedance. However if the additional inductance is sufficiently small with respect to the inductance of the PFN, this effect can be neglected.17. C. Magnetic Field In order to make the vacuum arc thruster more efficient an external magnetic field can be applied. A radial magnetic field with field lines perpendicular to the cathode surface causes a retrograde movement of the cathode spots 18. This leads to a more efficient erosion of the cathode surface and reduction of macro particles due to a faster cathode spots movement8. This is shown in figure 3. The magnetic field configuration used in this work was calculated using Ansys 18.1 with included Maxwell 2d Electronics. The magnetic field was simulated using a constant current of 150 A and a double layered solenoid with 40 turns and the used thruster geometry. As visualized in figure 3 the magnetic nozzle has a slightly focusing and an expanding regions. Figure 3: Magnetic field calculated for I=150 A. The magnetic field strength has its maximum at the cathode surface and then expands when leaving the nozzle. The magnetic field influences the velocity vectors of charged particles due to the Lorenz force law: F~L = q · E~ + q · ~v × B~ (2) 3 The 35th International Electric Propulsion Conference, Georgia Institute of Technology, USA October 8{12, 2017 Here q is the charge state of a particle, E~ the electric field vector, ~v the particle velocity and B~ the magnetic field vector. If the absence of the electric field is assumed it becomes clear that only particles with a perpendicular velocity to the magnetic field lines are influenced. It is known that the plasma plume expanding distribution in absence of the magnetic field can be described as cosinusoidal or Gaussian10.
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