Parametric family of the PlaS-type thrusters: development status and future activities

Parametric family of the PlaS-type thrusters: development status and future activities IEPC-2017-39 Presented at the 35th International Electric Propulsion Conference Georgia Institute of Technology Atlanta, Georgia USA M.Yu. Bernikova1, V.V. Gopanchuk2 FSUE EDB Fakel, Kaliningrad, 236001, Russia Abstract: This paper presents an overview of development of the PlaS-type parametric family in the power range from 100 W to 6 kw. Results of the research works aimed at creation of the perspective plasma thrusters ensuring high thrust performances by reducing of massand-dimension parameters are given. Distinctive features of the thruster operation and specific performances behavior of these new plasma thrusters at different operating points are estimated in this work. Advantages of PlaS-type thrusters in comparison to the well-known analogues, and also competitiveness of their application are demonstrated. Id Ud Ga Ucg ~Id = discharge current = discharge voltage = anode gas flow rate = «cathode-to-ground» voltage = discharge current oscillation (RMS) Nomenclature T I.Introduction he today s tendencies in spacecraft development are oriented to increase some part of payload by reducing massand dimension parameters of the on-board electric propulsion system (EPS). Besides, such an ESP should ensure a high operating efficiency. Nowadays, considering the extension of the application range of Electric propulsions (EP) used in on-board systems performing S/C orientation and station keeping, and also orbital transfers and other maneuvers in Space, EDB Fakel is carrying out development of new plasma Hall-effect thrusters with a hollow magnet anode named PlaS [1]. II. PlaS-type thrusters design scheme creation history Creation of high-pulse electric propulsion with the thrust specific impulse higher than 2500 s was made at EDB Fakel (Kaliningrad) in 1999-2000 [1]. Back at that time in frames of contractual works with «Atlantic Research Corporation» (ARC, USA) a high-voltage thruster experimental model was developed based on the separate modified elements and assembly units of the PPS 1350R and based on the anode new design scheme, and this model was conventionally named as SPT-1. The proposed design scheme of the SPT-1 thruster high-voltage experimental model, according to the authors, is a new type of Hall-effect thrusters. It is stipulated by the fact that this thruster discharge chamber (DCh) is combined: DCh exit part is formed by dielectric rings and its bottom part is made metallic by means of the walls of the adjoining hollow anode-gas distributor. 1 Design Engineer, Ph.D. 2 Leading Designer 1

SPT-1 research tests were performed in 1999 at EDB Fakel s test facilities, after that in 2000-2001 at the test facilities of the NASA Glenn Scientific-research center (GRC, USA) demonstration tests were successfully carried out. During these tests at the different operating modes at the discharge currents from 2.07 to 5.09 A and at the voltages from 300 to 1250 V the following maximum parameters were achieved: Efficiency η аnode=0.64, thrust F=144.5 mn, Isp аnode=3661 s. In 2002 at EDB Fakel after modification of the SPT-1 design (fig.1) research tests were also conducted with a so-called hollow magnet anode-gas distributor. Figure 1. Anode unit of SPT-1 thruster with magnet anode The next development stage consisted of a complex optimization of the magnet and discharge systems, what resulted in reduced losses in the thruster magnet contour and in increased propellant ionization in the channel and, as a result, the efficiency of the thruster operation with a hollow magnet anode was improved in a whole. The outcome of these works was a formation of the thruster new configuration with a hollow magnet anode named as PlaS-type thruster. The main difference between the known scheme of the stationary plasma thruster (SPT) and PlaS is in the anode-gas distributor configuration, which is made in PlaS thrusters from the magnet material (fig.3). Figure 2. Scheme of Stationary Plasma Thruster (SPT): 1 сoaxial discharge chamber; 2 metal anode gasdistributer;3 thruster axis Figure 3. Plasma thruster with a hollow magnet anode (PlaS): 1 outlet ceramic rings; 2 hollow magnet anode; 3 thruster axis The equidistant configuration of the anode relative to magnetic field lines in the accelerating channel ensures a magnetically insulated area near the anode, that decreases gas ionization intensity in the depth of anode cavity, i.e. in that part of DCh, where gas ionization is not reasonable from energy point of view, because this lower the probability of ion leaving thruster without collisional processes. Gas flow uniformity on ACh width near input boundary of ionization layer locating in the area of output ceramic rings allows to enhance intensity gas ionization near the ACh walls and thus "to expand" ionization center on the width of ACh what improves the thruster performances. III.Development of PlaS-type thrusters parametric family On the basis of the research tests results of the hollow magnet anode plasma thruster laboratory models at EDB Fakel, a parametric family of the PlaS-type of thrusters conceptual models with the power from 100 W to 6 kw was developed, namely: PlaS-34, PlaS-40, PlaS-55 and PlaS-120СМ. а. PlaS-34 thruster PlaS-34 is the smallest thruster in terms of dimension and power in the PlaS-type parametric family (fig.4). This thruster was tested in the power range from 80 to 360 W (table 1) and research works and tests are actively on-going. 2

Table 1 - Main performances of PlaS-34 Performances Value Discharge voltage, V 120 300 Discharge current, А 0.50 1.5 Discharge power, W 80 360 Thrust, mn up to 22 Specific impulse, s up to 1300 Efficiency, % up to 35 Power-to-thrust ratio, W/mN 18 21 Mass, kg 0.97 Overall dimensions, mm 100х90 х85 Figure 4. Thruster PlaS-34 after manufacturing and during operation The thruster operating modes tested are shown in the table below. The cumulative diagram also demonstrates the operating range of PlaS-34 thrust performances (fig.5). PlaS-34 operating range U d, V 120 150 180 200 220 250 300 I d, A (G a, mg/s) 0.50 (0.63) 0.65 (0.74) 0.80 (0.90) 1.00 (1.00) 1.25 (1.26) 1.50 (1.50) 1.75 (1.76) Notes: - stable operating mode; - unstable operating mode; - flame out; - untested operating mode; - cathode with permanent ignition. Based on the test results, it is determined that the minimum discharge voltage at which the thruster has stable operation is equal to 120 V. The discharge voltage decrease lower than 120 V at low flow rates led to the thruster flame-out. At the minimum flow rate of 0.63 mg/s which is relevant to discharge current of 0.5 A the thruster operates in a stable manner, but with permanent ignition maintained on the cathode with ignition current of 0.5 A. It is determined that the operating mode with discharge voltage of 200 V is the most optimal and stable for the thruster. A test single-point Figure 5. PlaS-34 performance envelope firing at discharge voltage of 220 V has proven this fact. Current-voltage characteristics (fig. 6) were determined at the specified anode flow rates, which ensured discharge current of 0.50 to 1.75 А. Magnetic field was optimized by minimum discharge current at each operating point. As seen from the plot, the minimum value of discharge current with the growth of discharge voltage is achieved at discharge voltage of 200 V. 3

Figure 6. PlaS-34 Current-voltage characteristics Thrust, Specific impulse, Efficiency and Thrust-to- Power ratio of the PlaS-34 are shown in figures 7 and 8. Thrust increase is stable and is similar to linear dependence at the discharge voltage increase (fig. 7а). Maximum thrust was determined at the anode flow rate of 1.76 mg/s and at the discharge voltage of 200 V and was 22 mn. Minimum Thrust-to-Power ratio reached at the same operating mode is 16 W/mN (fig.8b). The total specific impulse calculated with an allowance for the discharge power and also with an allowance for cathode flow rate is 1320 s at discharge voltage of 300 V and power of 400 W (fig.7b). а) b) Figure7. а) Thrust and b) specific impulse dependence on discharge voltage Efficiency increases significantly at the discharge voltage increase up to 200 V followed by stabilization of the value up to 20% for anode gas flow less than 1.0 mg/s and 30% for anode gas flow 1.26 mg/s. As shown, maximum efficiency value and minimum power-to-thrust ratio value in the wide range of xenon flow rate were observed at the discharge voltage of 200 V. а) b) Figure 8. а) Efficiency and b) thrust-to power ratio dependence on discharge voltage During the test it was found that low level of discharge current oscillations is typical for this thruster (fig.9). Minimum of RMS discharge current is also observed at the discharge voltage of 200 V and equal to 0.05 А (less than 5 % of the discharge current value). During the test, the cathode КЭ-1Н had stable operation at the cathode flow rate of 0.12 mg/s. There is dynamics of the "Cathode-ground" voltage Ucg in figure 10. This parameter defines cathode operation efficiency at operating points with different discharge current. Ucg decrease at high discharge current is typical for a hollow cathode. 4

Figure 9. RMS discharge current dependence on discharge voltage Figure 10. "Cathode-ground" voltage Ucg dependence on discharge voltage As shown, there is sharp increase of the "cathode-ground" voltage near the operating point with discharge voltage of 200V in the whole range of xenon flow rate. b. PlaS-40 thruster The first developed thruster of PlaS parametric family is PlaS-40 (fig.11). This thruster is the most investigated and tested and has status of engineering model. As shown by test, PlaS-40 thruster efficiently and stably operates in the discharge power range from 100 to 650 W (table 2). Table 2 - Main performances of PlaS-40 Performances Value Discharge volatge, V 100 500 Discharge current, А 1.00 2.25 Discharge power, W 100 650 Thrust, mn up to 44 Specific impulse, s up to 1880 Efficiency, % up to 50 Power-to-thrust ratio, W/mN 13 18 Mass, kg 1.2 Overall dimensions, mm 167х100 х87 Figure 11. Thruster PlaS-40 after testing and during operation Maximum thrust determined at the anode flow rate of 2.58 mg/s and at the discharge voltage of 300 V is 44.0 mn (fig.12). Minimum power-to-thrust is 12.3 W/mN and corresponds to discharge current of 2.0 A and discharge voltage of 150 V (fig.13a). The total specific impulse calculated with an allowance for the discharge power and power consumption for magnet field generation, and also with an allowance for cathode flow rate and for the level of vacuum chamber pressure is 1880 s at maximum discharge voltage of 500 V and power of 500 W. Efficiency increases significantly at the discharge voltage increase up to 250 V followed by stabilization of the value up to 35% for anode gas flow of 1.32 mg/s and 47% for anode gas flow 2.58 mg/s (fig. 13a). а) b) Figure 12. а) Thrust and b) specific impulse dependence on discharge voltage 5

c) d) Figure 13. а) Thrust, b) specific impulse, c) efficiency and d) power-to-thrust ratio dependence on discharge voltage Thruster has passed mechanical test according to level required for small S/c. besides, 400 hrs operation was performed at discharge power of 200 W on different gases: xenon and krypton [2]. The thruster PlaS-40 operating modes tested are shown in the table below Table 3 - PlaS-40 operating range I d, A (G a, mg/s) U d, В 100 150 180 200 250 300 350 400 450 500 1.00 (1.35) 1.25 (0.63) 1.50 (1.85) 1.75 (2.07) 2.00 (2.31) 2.25 (2.56) Notes: - stable operating mode; - unstable operating mode; - flame out; - cathode with permanent ignition. The cumulative diagram also demonstrates the operating range of PlaS-40 thrust performances (fig.14). Figure 14. PlaS-40 performances envelope 6

RMS discharge current amplitude dependencies on discharge voltage are shown in figure 15. As shown, discharge current oscillations decrease sharply by the discharge voltage increase, which minimum 20 ма was detected at the discharge voltage Ud=150 V. Minimum of the Power-to-thrust ratio was defined at this discharge voltage also. Figure 15. RMS discharge current amplitude dependencies on discharge voltage c. PlaS-55 thruster PlaS-55 is a new thruster of the PlaS-type parametric family [3]. By dimension type PlaS-55 is similar to SPT-70. The research test and reduced life test of PlaS-55 showed high efficiency of its operation in the discharge power range from 400 to 1350 W. Thruster performances are shown in table below [fig.16]. Table 4 - Main performances of PlaS-55 Performances Discharge volatge, V Discharge current, А Discharge power, W Thrust, mn Specific impulse, s Efficiency, % Power-to-thrust ratio, W/mN Mass, kg Value 200 450 2.00 4.50 400 1350 up to 80 up to 2050 up to 48 14 18 2.5 Overall dimensions, mm 167х100 х87 Figure 16. Thruster PlaS-55 after manufacturing and during operation Table 5 - PlaS-55 operating range Ud, V 200 225 250 Id, A (Ga, mg/s) 2.00 (2.27) 2.50 (2.76) 3.00 (3.18) 3.50 (3.69) 4.00 (4.12) 4.50 (4.51) Note: - stable operating mode; 275 300 325 350 375 400 425 - thruster flame-out; - untested operating mode. 7 450

The cumulative diagram also demonstrates the operating range of PlaS-55 thrust performances (fig.17). How you can see, thruster is able to operate with high efficiency in the wide range of discharge power. Figure 17. PlaS-55 performances envelope Maximum thrust determined at the anode flow rate of 4.51 mg/s is 80.0 mn (fig.18). Minimum power-to-thrust reached in the mentioned discharge voltage range is about 14 W/mN and corresponds to and discharge voltage of 200 V. The total specific impulse is 2050 s at maximum discharge voltage of 450 V and power of 1125 W. Significant growth of the efficiency is observed at discharge voltage increase up to 250 V and stabilized after at value about 45 % for flow rate of 2.27 mg/s and about 48 % for flow rate more than 2.76 mg/s (fig.18c). а) b) c) d) Figure 18. а) Thrust, b) specific impulse, c) efficiency and d) power-to-thrust ratio dependence on discharge voltage 8

Discharge current oscillation (RMS) dependence on discharge voltage for PlaS-55 thruster is presented in figure 19. For this range of discharge voltage the current oscillations behaves stable for each operating mode and do not exceed 30 % of discharge current value. Dependence between «Cathode-to-ground» voltage and discharge voltage for the PlaS-55 thruster has a typical character and the value of this parameter decreases with the flow rate increase. An average value for various thruster operating modes is in the range 15.0-16.5 V (fig. 19b). а) b) Figure 19. a) RMS discharge current amplitude and b)"cathode-to-ground" voltage U cg dependence on discharge voltage c. PlaS-120CM thruster Except for low and middle-power thrusters, works on creation of a new PlaS-120CM high-impulse hollow magnet anode plasma thruster with the operating power up to 6 kw are in progress at EDB Fakel (fig.20). Table 6 - Main performances of PlaS-120CM Performances Value Discharge voltage, V 200 1000 Discharge current, А 3.50 18.0 Discharge power, W 1.2 6.0 Thrust, mn up to 310 Specific impulse, s up to 3000 Efficiency, % up to 60 Power-to-thrust ratio, W/mN 13 18 Mass, kg 6.6 Overall dimensions, mm 248 245.5 96 Figure 20. Thruster PlaS-120 after testing and during operation The design of PlaS-120CM is similar to scheme of a low-power PlaS thruster as, for example, PlaS-40 and PlaS-55.The PlaS-120 main distinctive design feature is the cathode-compensator, which is placed in the center along the thruster axis. The cathode ignitor is placed on the external surface of the internal magnet pole [4]. The thruster was tested in the wide range of discharge voltage from 200 to 1000 V. The thruster research works were carried out both in high thrust specific impulse modes and in the high thrust modes. Representative performances of the PlaS-120 CM at different operating mode are given in the cumulative table 7. Test results of the PlaS-120CM demonstrated that this constructive scheme is more effective at the operating modes with high discharge power. Similar to the thruster with a hollow magnet anode of smaller dimension type PlaS-40, the PlaS-120CM thruster is able to effective operate at the modes, which are typical for the thrusters of bigger dimension type and higher discharge power without mass-size and energy characteristics expansion. 9

Table 7 - PlaS-120СМ representative performances Operating mode G a, mg/s U d, V I d, А N, W F, mn I sp, s Eff, % Minimum power level 7.1 200 6.2 1240 100 1400 52 Maximum F 16.5 322 19.8 6376 315 1860 46 Maximum Isp 7.1 800 6.9 5520 220 3020 58 Power level N=2.5 kw Maximum F 9.4 253 9.7 2455 168 1700 59 Maximum Isp 7.1 400 6.1 2440 145 2030 58 Power level N=3.8 kw Maximum F 11.6 299 12.6 3764 226 1877 61 Maximum Isp 7.1 600 6.4 3840 187 2670 61 Power level N=4.5 kw Maximum F 16.5 220 21.0 4613 274 1615 51 Maximum Isp 7.1 700 6.5 4550 200 2780 58 Power level N=6.0 kw Maximum F 16.5 229 20.2 5999 307 1810 48 Maximum Isp 7.1 855 7.2 6156 216 2990 52 Current-voltage characteristics (fig. 21) were measured at the specified anode flow rate, which corresponds to the ensured discharge current of 4.5 to 20.0 А. Figure 21. Current-voltage characteristics of PlaS-120CM Thrust, power-to-thrust ratio, total specific impulse and total thrust efficiency of the PlaS-120CM dependences in the discharge voltage range from 200 to 400 V are shown in figures below. Maximum value of the thrust at the anode flow rate of 16.5 mg/s and at the discharge voltage of 300 V is amounts to 307 mn. Average value of the minimum power-to-thrust ratio is about 14 W/mN at the discharge voltage of 200 V. Total specific impulse calculated with an allowance for vacuum and for the cathode flow rate is equal to 68 % at the operating mode with Id=6 А and Ud=400 V. а) b) Figure 22. а) Thrust and b) specific impulse dependence on discharge voltage 10

a) b) Figure 23. а) efficiency and b) power-to-thrust ratio dependence on discharge voltage Possibility of the PlaS-120CM ser.01 thruster operation in the high voltage range was researched during test. Thrust performances at this operating point are given in figure 24. Maximum value of the total specific impulse reached at the operating mode with Id= 6 А and Ud= 800 V is amount to 3050 s, at the same time total thrust efficiency at this high voltage mode is 58 %. а) b) c) d) Figure 24. а) Thrust, b) specific impulse, c) efficiency and d) power-to-thrust ratio dependence on discharge voltage The "cathode-ground" potential was also measured during test (fig.25). This potential increases sufficiently at the discharge voltage and discharge current increase, what partially explains the thrust efficiency decrease at the Ud growths at the discharge current of Id=20 А. The "cathode-ground" voltage behaves itself similar at the flow rate corresponding to the discharge current of 15 A, however thrust efficiency has another dependence. This fact indicates that the thrust efficiency decrease at the discharge current of 20 A is caused by more serious reason. Most 11

probably efficiency decrease is determined by plasma plume defocusing owing to insufficient power of the external magnet coils. Figure 25b) demonstrates discharge current oscillations dependence on discharge voltage. As shown, current oscillations level decrease more than 3 times at the anode xenon flow rate. Figure 25. a) RMS discharge current amplitude and b)"cathode-to-ground" voltage U cg dependence on discharge voltage IV. PlaS-type thrusters functional peculiarities Experimental research works of the presented models of the PlaS parametric family allowed us to reveal a row of thrusters main operating peculiarities. One of the peculiarities compared to the known analogues at the equivalent or close PlaS thrusters operating modes are lower power-to-thrust ratio values. Besides, for all the operating modes a low level of current oscillations is typical. The thruster is also resistant to the magnet field level variation at various currents in the magnet coils its parameters are practically not changed. One of the most distinctive peculiarity is achievement of high values for thrust efficiency [fig.26]. This effect is supposed to be stipulated by a lower intensity of ions formation inside the anode cavity, namely, in the discharge chamber part, where ions formation is not efficient energetically, because their exit from the thruster rapidly decreases passing by parasitic collisions. Uniformity of the gas flow along the channel width near the entry border of ionization zone, which is located in the area of the exit ceramic rings, allows one to increase ions formation intensity near the walls of the acceleration channel and to additionally stretch the ionization core along all the channel width, what finally has a positive effect on the thruster output characteristics. In the future plasma parameters investigations are planned in this direction. Figure 26. Thrust efficiency dependence on discharge power for SPT and PlaS-type thrusters V.Future works The magnet system optimization, made for PlaS thrusters, allowed us to improve the magnet contour efficiency in some way and decrease magnet flow losses in the contour. However, the introduced design changes do not allow us to achieve a significant increase in the system functional efficiency. As it is known, the thruster magnet field lines have an oval shape. Therefore, this shape determines the shape of the magnet contour itself, namely, exclusion of the rectangular-shape joints of the thruster magnet system elements. It resulted in an idea to make a magnet system with a shape that is close to the natural shape of the magnet field lines. 12

This idea is currently evaluated based on the low-power PlaS-34 thruster design. In the FG-34 thruster (fig.27) the magnet contour is designed oval. In this design the joints between the elements are made with smooth transitions along the radii, namely, steep rectangular-shaped transitions are excluded in the places where the magnet inductance losses are maximum. The length of such a contour is minimum and, consequently, the magnet resistance is the lowest, what leads to the decrease of the magnet flow losses. The oval shape of the magnet system has a row of advantages: Minimization of the dissipation fields and losses in the magnet contour; Possibility of thruster operation at lower magnetizing currents; Magnet-isolated near-anode zone along all the width of the acceleration channel; Decrease of destroying influence of the charged particle due to of tangential orientation of a surface of magnetic pole; The improved mechanical strength; Minimum quantity of constructive elements. Thruster FG-34 has passed test in the range of discharge power from 130 to 390 W with discharge voltage from 160 to 300 V and discharge current from 0.8 to 1.3 A. Thruster performances are presented in table below. Table 8 - Main performances of FG-34 Performances Value Diascharge volatge, V 160 300 Discharge current, А 0.8 1.3 Discharge power, W 130 390 Thrust, mn up to 18 Specific impulse, s up to1360 Efficiency, % up to 35 Power-to-thrust ratio, W/mN 18 21 Mass, kg 0.97 Overall dimensions, mm 100 92 85 Figure 27. Thruster FG-34 after manufacturing Investigation test in the wide range of discharge voltage and discharge current is planned to be performed on FG-34 thruster. Based on test results, optimization of the thruster magnetic system and discharge system including improvement of its mechanical and thermal interfaces will be carried out. VI.Conclusion Enhancement of plasma thruster efficiency in combination with their reliability is reached by improvement of design of well-known SPTs or by developing of plasma thruster of a new alternative schemes. Realized in the PlaS-type thrusters constructive scheme showed possibility of thruster efficiency improvement at invariable power costs for thrust generation. Results of investigation test of thruster with a hollow magnet anode of different power demonstrated that PlaS-type thrusters operated stable in the wide range of discharge power. The parametric family of a PlaS-type thruster with a hollow magnet anode in the power range from 100 to 6 kw has been developed at Fakel. These thrusters have a line of advantages and feature, namely: lover values of thrust-to-power ratio; lover level of discharge current oscillations; stability of operation over a wide power range; increased thrust efficiency. Research works aimed on improvement of the design of the tested thrusters in order to enhance their thrust and specific performances, operating efficiency and their reliability are being continued. References 1 M.Yu. Potapenko, V.V. Gopanchuk. Characteristic Relationship between Dimensions and Parameters of a hybrid Plasma Thruster // IEPC-2011-042, 32 nd International Electric Propulsion Conference, Wiesbaden, Germany, September 11 15, 2011. 13

2 M.Yu. Potapenko, V.V. Gopanchuk. Development and Research of the Plasma Thruster with a hollow magnet Anode PlaS-40 // IEPC-2013, 33rd International Electric Propulsion Conference, The George Washington University, Washington, D.C., USA, October 6 10, 2013. 3 M.Yu. Potapenko, V.V. Gopanchuk. Plasma thruster of a middle power PlaS-55: development and experimental research first results // SP-2016, 5th Space Propulsion Conference, Rome, Italy, May 1 6, 2016. 4 M.Yu. Potapenko, V.V. Gopanchuk, D.V. Merkuriev, P.G. Smirnov. Experimental study of a high specific impulse plasma thruster PlaS-120СМ // IEPC-2015-154/ISTS-2015-b-154, 34th International Electric Propulsion Conference, Cobe, Japan, July 4 10, 2015. 14