Performance of Plume Characterization of the SPT100-B Thruster

Transcription

1 Performance of Plume Characterization of the SPT1-B Thruster IEPC-15-1 Presented at Joint Conference of 3th International Symposium on Space Technology and Science 34th International Electric Propulsion Conference and 6th Nano-satellite Symposium, Hyogo-Kobe, Japan July 4 1, 15 Damiano Pagano 1, Giovanni Coduti, Simone Scaranzin 3, Gianfranco Meniconi 4 and Fabrizio Scortecci 5, Aerospazio Tecnologie srl, Rapolano Terme (SI), Italy and Niccola Kutufa 6 ESA/ESTEC, The Netherlands Abstract: The present paper summarizes the most relevant results obtained during the test campaign carried out at AEROSPAZIO Tecnologie s.r.l. on a flight type SPT1-B Hall Effect Thruster provided by EDB-FAKEL through SNECMA acting as commercial agent. The SPT-1 was tested at the operating point corresponding to 75mN thrust at two different pressures. α = pitch angle of the probe A s = Langmuir probe surface β = yaw angle of the probe CRP = Cathode Reference Potential e = electron charge ε = energy EEDF = Electron Energy Distribution Function FP = Faraday Probe I = ion current IEDF = Ion Energy Distribution Function m e = electron mass n e = electron density RPA = Retarding Potential Analyzer T = thrust vector direction T e = electron temperature V = voltage Nomenclature 1 Researcher, damiano.pagano@aerospazio.com. Researcher, presently at Snecma - SAFRAN group, giovanni.coduti@snecma.fr. 3 Researcher, simone.scaranzin@aerospazio.com. 4 Researcher, gianfranco.meniconi@aerospazio.com. 5 Technical manager, fscortecci@aerospazio.com. 6 Propulsion Systems Engineer, TEC-MPE, Niccola.Kutufa@esa.int. 1 Joint Conference of 3th ISTS, 34th IEPC and 6th NSAT, Kobe-Hyogo, Japan July 4 1, 15

2 V p = plasma potential V s = voltage applied to Langmuir probe I. Introduction HE present paper describes the results of the test campaign performed in the framewor of the SGEO Tprogramme on an SPT1-B representative of the flight model. The test campaign was performed in the AEROSPAZIO LVTF-1 Test Facility. The aim of this campaign is to clarify some critical aspects concerning the plume parameters: in particular the thrust vector direction, the plume divergence, the ion energy at high angles of divergence These measurements was carried out at 1m distance form the thruster exit plane center. Moreover, electron parameters (electron temperature, electron density and plasma potential) were measured by means of Langmuir probes on a plane perpendicular to the thruster face in the near field region. A particular analysis has been devoted to understand the behavior of the thruster during the first instants after ignition in terms of thrust vector direction. Measurements were carried out different pressures in order to assess the effect on the plume parameters. II. The Test Equipment A. The Vacuum Facility The whole test campaign was carried out in the Large Vacuum Test Facility 1 (LVTF-1). It consists of a 11.5 m in length 3.8 m in diameter diamagnetic horizontal stainless steel cylinder with a total volume of 1 m 3. Fig.1 reports a schematic of the experimental set-up. The thruster is installed in the middle of the cryo-panel so that the center of the exit plane coincides with the center of the diagnostic system. Figure 1. Schematic of the experimental set-up in the LVTF-1. B. The Beam Diagnostic System The diagnostic system is constituted by an invar boom able to rotate, describing a semispherical surface 1 m in radius, in front of the thruster exit plane. The boom is equipped with 9 Faraday probes mounted each 5 deg, allowing measuring the ion current along the semicircular arcs described by each probe. Six additional probes are installed on the sides of the boom; these probes, together with the three corresponding probes located on the boom constitute a 3x3 array and are used to study the thrust vector behavior during the first instants after thruster ignition. Other two probes allow measuring the current at different distance from the center of the thruster exit plane in order to verify the inverse square law of the plume current density. Moreover a Retarding Potential Analyzer is also installed on the boom able to scan the plume in the equatorial plane. Figure shows an overall view of the diagnostic boom equipped with Faraday probes and RPAs. Langmuir probes are in front of the thruster face on a perpendicular plane according to the map reported in Fig. 4 showing the measurement positions moved by means of a dedicated system (Fig.3). Joint Conference of 3th ISTS, 34th IEPC and 6th NSAT, Kobe-Hyogo, Japan July 4 1, 15

3 +9 FP-G 3x3 FPs Array FP-H -9 β α Z Y +9-9 X Figure. Schematic of the reference system for probes position identification and view of the diagnostic boom. Thruster Firing Direction Langmuir Movement System Figure 3. Langmuir probes movement system. 3 Joint Conference of 3th ISTS, 34th IEPC and 6th NSAT, Kobe-Hyogo, Japan July 4 1, 15

4 Figure 4. Map of the measurement points for Langmuir probes. III. Beam Diagnostics Results Results reported in the present section was obtained for the 75 mn operating point (V d = 3 V, I d = 4.34 A) for two different pressures ( Best 8.7e-6 mbar; High 4.4e-5 mbar). A. Faraday Probes Diagnostics Fig. 5 shows a typical ion current density distribution as a function of (α, β) collected by Faraday Probes during a scan. In order to validate AEROSPAZIO diagnostic system, data acquired have been compared with data from literature 1. In particular Fig. 6 shows a comparison between the current density calculated from the current collected by the equatorial probe (collecting area =3.14 cm ) and data by NASA Lewis Research Center, and Fig.7 reports the effect of the operating pressure. Both NASA LeRC data and Aerospazio data show the same behavior as a function of pressure: an increase of the pressure determines an increase of the ion current in the tails. This comparison is only qualitative due to the fact that NASA data are obtained for 84.9 mn while Aerospazio data are for 75 mn operating point. Figure 5. 3D Ion current density for 75 mn. 4 Joint Conference of 3th ISTS, 34th IEPC and 6th NSAT, Kobe-Hyogo, Japan July 4 1, 15

5 1.8 AEROSPAZIO 75 mn LeRC 84.9 mn 1.6 Current Density (ma/cm ) α (deg) Figure 6. Comparison Ion current density collected on the equatorial plane at AEROSPAZIO for 75 mn at Best pressure and NASA LeRC 1 for 84.9 mn. 1 Comparison of current distribution at LeRC and AEROSPAZIO for different pressures LeRC, 84.9 mn, P=3.33e-5 mbar LeRC, 84.9 mn, P=7.45e-6 mbar Aerospazio, 75 mn, P=8.7e-6 mbar (TS-8: Best Pressure) Aerospazio, 75 mn, P=4.4e-5 mbar (TS-9: High Pressure) 1 Current Density (ma/cm^) (deg) Figure 7. Comparison of FP15 current at different operating pressures for 75 mn at 3 min and data from LeRC 1. 5 Joint Conference of 3th ISTS, 34th IEPC and 6th NSAT, Kobe-Hyogo, Japan July 4 1, 15

6 One of the most important parameter in Hall effect thruster plume characterization is represented by the deviation of the thrust vector in respect with the thruster geometrical axis. It is calculated as the center of mass of the current weighted on the measurement position. Fig.7 shows the thrust vector deviation in (α, β) coordinates, where the origin of the axis corresponds to the geometrical axis of the thruster. Measurements were carried out at different times from the thruster ignition instant and for different pressures, 5. Thrust vector position at different pressures for 75 mn 18 min*. 15 min* 1.5 Best Pressure (TS-8) High Pressure (TS-9) 1. 3 min ß (deg) 5 min 18 min*.5 15 min* 3 min 5 min (deg) NOTE: * High Accuracy FP acquisition Figure 8. Comparison of thrust vector position at different operating pressures. Another important parameter for understanding the effect of the plume on the satellite's structures is represented by the plume divergence. Results obtained according to different evaluation method of this parameter show strong differences. The right way should be represented by the calculation of the angle associated with 9% or 95% of the total ion current calculated on a hemispherical surface according to Eq. 1 (Divergence 3D),3. π / π A simplified version is obtained assuming the axial symmetry of the plume. I tot = i( φ, θ ) r sin( θ ) dφdθ (1) π / I tot = πr i( θ ) sin( θ ) dθ () Another approach (Full Width at Half Maximum) is based on 3D gaussian-lie behaviour of the ion current as a function of the angular position and maing a parallelism with a probability distribution, performed taing as independent variable the angular position (, ) of each probe. This approach is equivalent to calculate the total current performing the following integral: π / π I tot i( φ, θ ) r dφdθ (3) i.e. not considering that the current is collected on a hemispheric surface, neglecting the weight sin() and then considering the angular coordinates as linear. The same as the previous but assuming that the plume is axial-symmetric and therefore considering for the calculation only the current collected by the equatorial probe. This method neglects again the weight sin() and considers as a linear coordinate. 6 Joint Conference of 3th ISTS, 34th IEPC and 6th NSAT, Kobe-Hyogo, Japan July 4 1, 15

7 π / I tot i( θ ) dθ (4) This method was used in past Russian literature 4. Figure 9 summarizes the beam divergence calculated according to the described methods at different pressures, showing significant variations. Beam divergence at 75 mn, according to different calculation methods Best Pressure (TS-8) High Pressure (TS-9) 5. Beam divergence (deg) Divergence3D-9% FWHM3D-9% FWHM3D-95% EquatorD-9% EquatorD-95% Method Figure 9. Comparison of plume divergence at different operating pressures calculated according different methods. The thruster behavior from the point of view of the repeatability was investigated: the thruster was operated for 6 cycles and the plume characterization was performed just before the end of each cycle. In fig. 1 the ion current collected by the central probe is reported as a function of the angular position for each repetition. A good agreement among data for the 6 repetitions appears. Also the thrust vector position is in good agreement (Fig. 11): it is strongly localised in (,)-space Repeatability Analysis at 75mN (TS-11) FP 15 Current vs. alpha position 4.E-3 3.5E-3 3.E-3.5E-3 Rep 1 Rep Rep 3 Rep 4 Rep 5 Rep 6 Current (A).E-3 1.5E-3 1.E-3 5.E-4.E (deg) Figure 1. FP15 ion current as a function of the angular position at the end (9 min) of 6 repetitions. 7 Joint Conference of 3th ISTS, 34th IEPC and 6th NSAT, Kobe-Hyogo, Japan July 4 1, 15

8 Repeatability Analysis at 75mN (TS-11) Thrust vector position (ß vs. ) ß (deg) (deg) Figure 11. Thrust vector position at the end (9 min) of 6 repetitions. B. Transient Analysis The transient analysis has been carried out in order to show an eventual variation of the thrust vector position along the first instants of the thruster operation. Measurements have been performed eeping the diagnostic boom fixed along the thruster axis and acquiring the ion current from the 3-by-3-probes array continuously.the centre of mass of the ion current is calculated on this probes' array (Figure 1) according to Eq. 5. = α i i β i i T (5) It is important to point out that these results must be considered with their relative meaning, indeed they are useful to show just a relative variation of the thrust vector direction, but cannot be used to give an absolute estimation of the thrust vector direction. Figures 13 shows the time variation of the thrust vector position in respect with the thruster axis direction (, position) during the first instants of the thruster operation for the 75 mn nominal point. The "Thruster ON" point is associated with the increase in the current collected by the probes. The erratic behaviour of the thrust vector direction is considered to be related to the warming up of the magnetic system changing the shape of the magnetic lens in the discharge. The final position of the thrust vector direction is determined by the cathode position 8 Joint Conference of 3th ISTS, 34th IEPC and 6th NSAT, Kobe-Hyogo, Japan July 4 1, 15

9 3x3 Probes Array for Transient Analysis 1 8 FP A 6 FP 14 FP D 4 FP B FP 15 FP E ß (deg) FP C -4 FP 16 FP F (deg) Figure 1. 3x3 Faraday Probes array for transient analysis. Thrust vector position at 75 mn (TS-1: transient analysis) 1 min min 3 min 15 min 1 min 5 min 1. Thruster ON.6 ß (deg) (deg) Figure 13. Thrust vector position at different times at 75 mn. C. Retarding Potential Analyzer The plume behaviour has been fully characterised in terms of ion energy, in particular the mean energy has been determined as a function of the angular position in respect with the thruster s geometrical axis according to Eq. 6: 9 Joint Conference of 3th ISTS, 34th IEPC and 6th NSAT, Kobe-Hyogo, Japan July 4 1, 15

10 ε ( α ) V ( α ) = V max V I V V max I V ( α ) dv ( α ) dv (6) where di/dv is proportional to the ion energy distribution function (IEDF). As for the ion current, the effect of the bacground pressure on the energetic parameters of the ions was evaluated. The difference in the FP current already observed in Fig. 7 due to the bacground pressure appears also when the characteristics of the ion current as a function of ramp voltage are compared (Fig. 14). Obviously this difference reflects on the derivative of the characteristic. Comparing the normalised distribution it is clear that the position of the maximum, giving the most probable energy, is the same while the shape of the pea presents a slight difference that reflects on the mean energy. Indeed an increase of the bacground pressure produces a loss in the ion energy due to a higher collision probability. Departing from the thruster axis this effect is more evident as shown in Figure 15. In particular, while for the central positions the shape of the distribution is essentially unchanged increasing pressure, far from the central position a strong effect is observed. A low energy tail appears in the distribution, resulting in a lower mean ion energy; moreover a larger population of beam s ions is present at low pressure, as it is clear looing at the area of the associated pea at low and high pressure. This behaviour is due to the larger number of collisions occurring when pressure is increased. Measurements have been performed according to two different grounding schemes. In the first case the retarding voltage and the corresponding collected ion current are measured referring them to the chamber ground. In this case final results must be corrected with the cathode reference potential (CRP) in order to obtain the actual energy values. In the second case these measurements are referenced directly to the CRP in order to enhance the sensitivity of the measurements in the low-energy tails of the plume; they need no correction in order to obtain the actual energy values. Fig. 16 shows the comparison of the IEDF obtained according to the two grounding schemes as obtained by the instruments without applying any correction. The amount of peas shift in the derivative between the two approaches corresponds to the Cathode Reference Potential value ( -18 V). Fig. 17 summarises this behaviour reporting the mean ion energy as a function of the angular position for different operating pressures..5 x 1-4 Current vs. ramp voltage at α= deg for 75 mn Best Pressure (TS-8) High Pressure (TS-9).1 Derivative of the ion current vs. ramp voltage at α= deg for 75 mn Best Pressure (TS-8) High Pressure (TS-9) Current (A) di/dv normalized to the integral (V -1 ) Voltage (V) Voltage (V) Figure 14. pressures. Ion current as a function of the ramp voltage and IEDF at = deg for different operating 1 Joint Conference of 3th ISTS, 34th IEPC and 6th NSAT, Kobe-Hyogo, Japan July 4 1, 15

11 .9 1 x 1-5 Current vs. ramp voltage at α = -4 deg for 75 mn Best Pressure (TS-8) High Pressure (TS-9) 9 x Derivative of the ion current vs. ramp voltage at α = -4 deg for 75 mn Best Pressure (TS-8) High Pressure (TS-9).8 7 Current (A) di/dv normalized to integral (V -1 ) Figure 15. pressures Voltage (V) Voltage (V) Ion current as a function of the ramp voltage and IEDF at = -4 deg for different operating -di/dv vs retarding voltage at α= deg for Id=4.33A 3.E-6.5E-6 ~ V CRP TS-8_B (Chamber Grounding) not corrected with CRP TS-8_C (CRP Grounding).E-6 -di/dv (A/V) 1.5E-6 1.E-6 5.E-7.E Voltage (V) Figure 16. IEDF at = deg obtained according to the Chamber grounding and CRP grounding. 11 Joint Conference of 3th ISTS, 34th IEPC and 6th NSAT, Kobe-Hyogo, Japan July 4 1, 15

12 3 5 Mean Energy vs. position at different pressure for 75 mn Best Pressure (TS-8) High Pressure (TS-9) ME (ev) (deg) Figure 16. Mean Ion Energy (corrected for V CRP ) as a function of the angular position for different operating pressures. D. Langmuir Probes A single cylindrical Langmuir probe is used to characterise the plasma in terms of electron density, electron temperature and plasma potential. The probe is constituted by a tungsten wire routed through a 99.8% pure alumina tube. The collector is 1.5 mm in length and.15 mm in diameter. In the operation of a Langmuir probe, the probe bias voltage is swept across the voltage range of (-8 to +8V) and oscillates in a triangle wave pattern at 33Hz. This procedure generates the characteristic I-V (current-voltage) trace that is analysed using Langmuir probe theory. The plasma oscillations determine a fluctuation of the current collected by the probe, thus a smoothing is applied to the I-V characteristic in order to perform data elaboration. The first plasma parameter to be calculated is the plasma potential determined as the maximum in the I-V characteristic first derivative. Once, the plasma potential is nown, the EEDF is calculated according to the Druyvensteyn method 6 : f m e A ev m e S ( ) = S e d I dv ε (7) where ε = V V is the electron energy. Then the electron density is determined as the integral of the EEDF on the energy according to: p s while the electron temperature is calculated as: n e = 1 T = ε = 3 3 n f ( ε ) dε ε f ( ε ) S dε e (9) e (8) 1 Joint Conference of 3th ISTS, 34th IEPC and 6th NSAT, Kobe-Hyogo, Japan July 4 1, 15

13 Figure 17. Electron Density as a function of the radial position (averaged over probe 1 and ) for different axial positions. Figure 18. Electron Temperature as a function of the radial position (averaged over probe 1 and ) for different axial positions. 13 Joint Conference of 3th ISTS, 34th IEPC and 6th NSAT, Kobe-Hyogo, Japan July 4 1, 15

14 Figure 19. Electron Density as a function of the axial position (averaged over probe 1 and ) for different radial positions. Figure. Electron Temperature as a function of the axial position (averaged over probe 1 and ) for different radial positions. E. Plume oscillations Langmuir probe (without bias voltage) and RPA (configured as Faraday probe) have been also to measure the oscillation frequencies of the thruster 7,8. Measurements have been performed using the probes as antennas, positioning them along the thruster axis, respectively at 19mm and 1m far from the thruster s exit plane. The spectra obtained have been compared with the typical oscillations measured on the thruster discharge voltage line. 14 Joint Conference of 3th ISTS, 34th IEPC and 6th NSAT, Kobe-Hyogo, Japan July 4 1, 15

15 The analysed spectral range allows obtaining information just in a narrow interval around the thruster breathing mode frequency ( 16 KHz). At higher frequencies a low signal to noise ratio prevents an unambiguous frequencies interpretation. Table 1 summarizes results for the main frequencies, obtained from Langmuir probe, RPA and discharge voltage spectrum for different operating pressure. Discharge voltage spectrum shows a greater number of high frequency components than Langmuir and RPA spectrum. Table 1. Comparison of pea frequencies measured from different sources. P = mbar P = mbar Langmuir probes KHz 1 KHz RPA without bias KHz 17 KHz RPA with bias 18 KHz 19 KHz Measurements performed on the thruster power line KHz 11 KHz 1. MHz 8.54 MHz 17.6 MHz KHz, 118 KHz 1.3 MHz 4.5 MHz 8.88 MHz 1.5 MHz 18. MHz IV. Conclusion In this wor we investigated some aspects of Hall Effect thrusters plume that are not described in literature. In particular, interesting results have been obtained concerning the repeatability of the behavior of the thruster in terms of plume shape and thrust vector direction. The strong differences concerning the plume divergence values obtained by different authors have been clarified. A representation of the behavior of the thruster at the start-up has been provided illustrating, although with a relative meaning, the migration of the thrust vector during the first time instants after ignition. For what concerns the energetic parameters, the effect of the grounding scheme used for the RPA has been described. Acnowledgments The present wor has been performed with the financial support of ESA under the contract No. 376/9/NL/CLP Extended Test of SPT-1 for Small Geo Propulsion System. References 1 Manzella, D. H., Sanovic, J. M., Hall Thruster Ion Beam Characterization, 31 st Joint Propulsion Conference and Exhibit, AIAA-95-97, San Diego, California, Waler L. R., et al., The effects of nude Faraday probe design and vacuum facility bacpressure on the measured ion current density profile of Hall thruster plumes, 38 th AIAA/ASME/SAE/ASEE Joint Propulsion Conference, AIAA--453, Indianapolis, IN, July 7-1,. 3 Hofer R. R, et al., Ion voltage diagnostic in the far field plume of a high specific impulse Hall thruster, 39 th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit, AIAA , Huntsville, AL, July -3, 3 4 Absalamov, S. K., et al., Measurements of plasma parameters in the stationary plasma thruster (SPT-1) plume and its effect on spacecraft components, 8 th AIAA/SAE/ASME/ASEE Joint Propulsion Conference and Exhibit, AIAA , Nashville, TN, Hofer R. R., et al. Characterising Vacuum Facility Bacpressure Effects on the Performance of a Hall Thruster, 7 th International Electric Propulsion Conference, IEPC-1-45, Pasadena, CA, 1. 6 Godya, V., Pieja, R., Alexandrovich, B., Probe diagnostics of non-maxwellian plasmas, Rev. Sci. Instrum.,, Vol. 73, 1993, pp Choueiri, E. Y., Plasma Oscillations in Hall Thrusters, Phys. Plasma Vol. 8, 1, p Boeuf, J. P., and Garrigues, L., Low-frequency oscillations in a stationary plasma thruster, J. Appl. Phys, Vol. 84, No. 7, 1998, pp Joint Conference of 3th ISTS, 34th IEPC and 6th NSAT, Kobe-Hyogo, Japan July 4 1, 15