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-Kauﬀeldt†, Lars Seipp‡ and Jochen Schein§ 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]. †LPT, University Of The Federal Armed Forces Of Germany ,marina.kuehn-kauﬀ[email protected] ‡University Of The Federal Armed Forces Of Germany, [email protected] §LPT, 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. Hence, there are a lot of particles with an approximately perpendicular velocity vector near to the cathode surface.Consequently the particles are deflected according to their mass and velocity, which results in a gyration of the charged particles (ions and electrons) with the cyclotron frequency ωp. For a single particle the following equations can describe this movement: |qp|B ωp = (3) mp with qp the charge and mp as the mass of each particle. The resulting rotation radius rp is described by

v⊥ rp = (4) ωp with v⊥ is the velocity perpendicular to the magnet ﬁeld lines. These eﬀect can be used to compress and expand the plasma and also decrease the amount of generated molten cathode material (macro particles). The compression can contribute to protect the surrounding geometries against coating. Additionally it leads to a higher collision rate coupled with an increase of the ion temperature. Moreover, during the expansion mode the plasma can be accelerated. 10,11,14.

III. Experimental Setup

In ﬁgure 4 the experimental setup is shown. Besides the thruster with a power supply unit it consists of a high speed camera for plasma plume visualization, an oscilloscope with current and voltage probes measuring the electric characteristics of the discharge and a thrust balance. The thruster and the thrust balance are positioned inside a vacuum chamber with an initial pressure of 5 × 10−6 mbar

thrust balance vacuum chamber

thruster camera

PFN V A

trigger signal osciloscope Figure 4: Experimental setup.

The power supply consists of a 10 staged PFN with a maximum load voltage of 800 V and a fixed pulse length of 1000 µs for a load resistance of 1 Ω. The pulse shape and length change when the additional inductance for magnetic field generation is used. For experiments the load voltages where set to 300 V with a resulting maximum energy of 18 J per pulse. It is possible to connect the thruster with the inductance in series or directly to the cathode. This allows to compare and evaluate the influence of the magnetic field. For high speed imaging a PCO type pco.dimax camera was used. The camera was triggered using the current signal from the vacuum arc. The signal was recorded by a Rohde & Schwarz oscilloscope and a Rhode & Schwarz current probe (RT ZC02). With the camera it is desired to take images of the evolving plasma with and without a magnetic field and to see in how far the plasma expansion changes. Side on images of the operating thruster were taken with a frequency of 40 kHz and an exposure time of 1 µs.

4 The 35th International Electric Propulsion Conference, Georgia Institute of Technology, USA October 8–12, 2017 Figure 5: Pulse current shape for the thruster operated with and without an external magnetic ﬁeld.

Figure 6: Pulse voltage shape for the thruster operated with and without an external magnetic ﬁeld.

For thrust measurement a µN thrust balance (ARC, Austria) was used. This measurement system is designed for continues driven systems. However it can be used for pulsed systems if only qualitative measurements are desired. For the thrust measurements three consecutive pulses with a frequency of 0.1 Hz were used in order to minimize oscillations phenomena resulting from the mass inertia of the thrust balance system. The measurements where evaluated only qualitatively due to the low temporal resolution of the thrust balance system.

IV. Results and Discussion

Current and voltage characteristics of the PFN with the different thruster setups are shown in figure 5 and figure 6. In comparison to the standard setup (without magnetic field) the pulse shape is significantly changed when the external magnetic field is added. The arc current of the standard vacuum arc thruster is around 50 % lower than it is the case when the magnetic field is applied. Therefore the arc burning voltage is more than 100 % higher in the latter case. This behaviour is expected as the magnetic field leads to an increased burning voltage of the vacuum arc8,10.

5 The 35th International Electric Propulsion Conference, Georgia Institute of Technology, USA October 8–12, 2017 Figure 7: Thrust measurement with and without an applied magnetic ﬁeld.

(a) Without magnetic ﬁeld (b) With additional magnetic ﬁeld

Figure 8: High speed images taken from a vacuum arc thruster without an additional magnetic ﬁeld (a) and with a magnetic ﬁeld (b). Both pictures were taken at t ≈400 µs after ignition.

Also the pulse length is extended by the additional inductance used for magnetic field generation by about 20 %. This behavior occurs because no additional matching network is used in this case. Hence it leads to a higher output resistance and to the extension of the pulse length. The achievable thrust is around 100 % higher with the additional magnetic field compared to the standard setup which can be seen in figure 7. This can be explained with more directed ion flight trajectory eliminating vectorial thrust losses. The magnetic field also leads to a higher current per spot which results in higher energy input in nearly the same volume and also creates higher ion charge state levels 14,18. Higher charged states and energies can cause higher ion velocities and therefore significantly increase the ISP3. High speed imaging results show that the plasma within a magnetic field(figure 8 (b)) evolves more directed. The plasma forms spike-like features which can be assumed to have a higher ion density than the unguided plasma( figure 8 (a)).The spike features can be explained by the to constriction of the plasma in the external magnetic field directly after it emerges from the cathode spots. More over the spike features evolve similar to the shape of the magnetic field lines as shown in figure 3, which confirms the initial assumption. The more directed trajectories create higher thrust levels compared to the thruster without a magnetic field as was already demonstrated in the thrust measurement. The performed measurements are in good agreement with other experiments on vacuum arcs8. The magnetic field can act as a magnetic nozzle and also influences the plasma formation and the expansion process. Besides the effect on the plasma, a magnetic field also changes the erosion rate of the cathode material 8,13,18. This effect can be used to optimize the thruster geometry for later work and build much smaller and more fuel efficient thrusters. The PFN in this work is not exactly matched. A better matching PFN will result in better thrust

6 The 35th International Electric Propulsion Conference, Georgia Institute of Technology, USA October 8–12, 2017 performance. Size and mass of the PFN can also be better fitted when using high optimized inductances and a capacitors. A PFN for this type of thruster could have a mass around 1 kg. The simple arrangement of the PFN allows a good fitting for the thruster design. In this way a shorter pulse duration of the PFN could increase the thruster performance as the dissipated heat and load time decrease. For a thruster and power supply comparison with other system the overall effenciy and the thrust to power ratio have to be calculated.

V. Conclusions and Outlook

In this work it was demonstrated that the introduction of an magnetic field in the vicinity of the vacuum arc thruster could significantly increase the evolving thrust. In the chosen configuration it could be shown that the thrust could be doubled with respect to operation without magnetic field, while the same energy was provided by the power supply in both cases. In the future work quantitative thrust measurements need to be performed in order to be able to calculate the corresponding specific impulse, which is needed for comparison with other electric propulsion systems. In order to make the vacuum arc thruster system operational on space missions, the PFN needs to be optimized concerning its efficiency, weight and size. This includes an optimization of the matching and single electric components for minimal size and weight of the power supply unit. More over an optimal thruster operation pulse length has to be evaluated in order to decrease the losses by heat dissipation and the load time of the PFN for a faster operation mode.

VI. References

1 M. Martinez-Sanchez.,J.E. Pollard, “Spacecraft Electric PropulsionAn Overview,” Journal of propulsion and power, Vol. 14, No. 5, 1998, pp. 688-699. 2 Polk, J.E. and Sekerak, M.J. and Ziemer, J.K. and Schein, J. and Niansheng Qi and Anders, A., “A The- oretical Analysis of Vacuum Arc Thruster and Vacuum Arc Ion Thruster Performance,” IEEE Transactions on Plasma Science, Vol. 36, No. 5, 2008, pp. 2167-2179. 3 Keidar, Michael and Schein, Jochen and Wilson, Kristi and Gerhan, Andrew and Au, Michael and Tang, Benjamin and Idzkowski, Luke and Krishnan, Mahadevan and Beilis, Isak I, “Magnetically enhanced vacuum arc thruster,” Plasma Sources Science and Technology, Vol. 14, No. 4, 2005, pp. 661-669. 4 Mathias Pietzka, Stefan Kirner, Marina Kauffeldt, Claudia Arens Jos-Luis Marques-Lopez, Gnter Forster and Jochen Schein, “Development of Vacuum Arc Thrusters and Diagnostic Tools,” IEPC-2011-080, 5 Schein, J. and Qi, N. and Binder, R. and Krishnan, M. and Ziemer, J. K. and Polk, J. E. and Anders, A., “Inductive energy storage driven vacuum arc thruster“, Review of Scientific Instruments, Vol. 73, No. 2, 2002, pp. 925. 6 Hohenbild, Anders and Schein, “Geschwindigkeitsmessungen an Vakuumlichtbogenplasmen in Magnet- feldern“, Master Thesis, 28.04.2008 7 B.F. Coll and David M. Sanders,“Design of vacuum arc-based sources“, Surfaceand Coatings Technol- ogy,Vol. 81, 1996, pp. 42-51. 8 A. Anders,“Cathodic Arcs - From Fractal Spots to Energetic Condensation“, Springer, 2008 9 C. W. Kimblin,“Erosion and ionization in the cathode spot regions of vacuum arcs“, Journal of Applied Physics, Vol. 44, 3074 (1973); doi: 10.1063/1.1662710 10 M. Pietzka,“Development and Characterization of a Propulsion System for CubeSats based on Vacuum Arc Thrusters“, Dissertation (2016); urn: nbn:de:bvb:706-4744 11 M.Keidar, I. Beilis, R. L. Boxman and S.Goldsmith“2D expansion of the low-density interelectrode vacuum arc plasma jet in an axial magnetic field“, Journal of Applied Physics, Vol. 29,3727 (1996); doi: 10.1088/0022-3727/29/7/034 12 J. Kutzner and H.C. Miller,“Integrated ion flux emitted from the cathode spot region of a diffuse vacuum arc“, Journal of Applied Physics, Vol. 25,686-693 (1992); doi: 10.1088/0022-3727/25/4/015 13 I. Brown,“The Physics and Technology of Ion Sources“, Wiley-VCH, 2nd Edition (2004); ISBN: 978-3-527-40410-0 14 A. Anders,“Ion charge state distributions of vacuum arc plasmas: The origin of species“, Phys. Rev.,Vol E 55, 696 (1997); DOI: 10.1103/PhysRevE.55.969 15 R. L. Boxman and S. Goldsmith,“Principles and Applications of Vacuum Arc Coatings“, IEEE Transactions on Plasma Science, Vol. 17 (1986); DOI: 10.1109/27.41186

7 The 35th International Electric Propulsion Conference, Georgia Institute of Technology, USA October 8–12, 2017 16 A. Anders,“Triggerless’ triggering of vacuum arcs“, Journal of Applied Physics,Vol 31,584 (1998); 17 G. Teel, J. Lukas, A. Shashurin, M. Keidar,“Analysis of Ignition of the Micro Cathode ArcThruster“, IEPC,53 (2015); 17 C. Burkhart and M. Kemp,“Pulsed Power Engineering Basic Topologies“, Power Conversion Depart- ment SLAC National Accelerator Laboratory, (2011) 18 Isak I. Beilis,‘Vacuum Arc Cathode Spot Grouping and Motion in Magnetic Fields“, IEEE , (2002); DOI: 10.1109/TPS.2002.807330

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