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Micro-multi-plasmajet array thruster for space propulsion applications
Vacuum 85 (2010) 574e578
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Micro-multi-plasmajet array thruster for space propulsion applications Hideyuki Horisawa a, *, Fujimi Sawada a, Shuji Hagiwara a, Ikkoh Funaki b a b
Department of Aeronautics and Astronautics, School of Engineering, Tokai University, 1117 Kitakaname, Hiratsuka-shi, Kanagawa 259-1292, Japan Japan Aerospace Exploration Agency, Sagamihara, Kanagawa, 229-8510, Japan
a b s t r a c t Keywords: Electric Propulsion Micro-thruster Micro-multi-plasmajet array
Thrust performance tests were conducted for an arrayed multi-plasmajet structure consisting of rectangular nozzle elements each with an exit height of 0.5 mm and length of 0.5 mm. To evaluate the thrust characteristics of the arrayed micro-multi-plasmajet, the thrust was measured using a calibrated cantilever-type thrust stand in vacuum. Using 3 3 nozzle elements, the micro-plasmajet array showed stable DC operation. Compared to conditions with cold-gas flow, the application of a DC discharge showed a significant improvement in thrust performance of at least 20% for thrust and specific impulse. Typical values achieved for thrust, specific impulse and thrust efficiency at an input power of 6.3 W were 8.5 mN, 77 s and 0.21, respectively. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction The authors have been conducting investigations on small-sized very low-power (<10 W) arcjets, or plasmajets, studying discharge characteristics, thermal characteristics of discharging plasmas, and correlations of these characteristics with thrust performance [1e4]. Microfabrication of micro-arcjets with ultra-violet lasers, and the development of rectangular DC micro-arcjets of various sizes operated at less than 5 W have also been undertaken [5e9]. The geometry and dimensions in these experiments is of critical importance for efficient operation. In the tests described above the rectangular micro-nozzle was machined in a 1.2 mm thick quartz plate. Sizes of the nozzle exit were 0.44 mm in height and 0.1 mm in constrictor height. The anode was a thin Au film (e100 nm thick) coated by physical vapor deposition in vacuum on a divergent part of the nozzle. The cathode was an Au film also coated on an inner wall surface. In operational tests, a stable discharge was observed for mass flow of 0.4 mg/s, input power of 4 W. In addition, thrust performance tests were conducted for various nozzles with different exit heights (0.4e0.8 mm), or area ratios, for a fixed nozzle length and various lengths of the divergent part [7]. From the results, nozzles with larger exit height and a longer divergent part showed higher thrusts and specific impulses. It was also shown that variations of the background pressure in the vacuum chamber, in which the thruster were tested, had a significant effect on thrust performance as much as the nozzle sizes due to the enhanced under-expansion of the exhaust jet. To reduce the expansion of the
* Corresponding author. Fax: þ81 463 50 2060. E-mail address: [email protected] (H. Horisawa). 0042-207X/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2010.08.014
exhaust jet, the effect of multi-jet interaction exhausted from the multi-nozzle jet array was additionally investigated [8e10]. In the investigation, thrust characteristics of the 3 3 micro-multinozzle array were compared with those of a single-nozzle with identical elemental size. To compare the thrust performance between the arrayed thruster and single-nozzle thruster, thrusts and mass flows per nozzle, or average values of each nozzle element, of the array were estimated by dividing each of measured values of thrust and mass flow by number of nozzle elements of the array. From the results, it was shown that the thrust and specific impulse of the each nozzle element with the nozzle array were significantly higher than those of the single-nozzle [8e10]. In this study, thrust characteristics of the micro-nozzle arrays with different geometries were investigated. Moreover, after fabricating the electrodes in the micro-nozzle arrays, investigation of the discharge and propulsive characteristics were also conducted.
2. Experimental procedure 2.1. Microfabrication of micro-nozzles A laser micromachining system developed for machining of micro-nozzles was used here as previously described [5e10]. In order to minimize and localize thermal influences and to achieve accurate geometries, a short-pulse ultra-violet laser system was utilized. For the UV laser oscillator, a fifth harmonic generation wave of an Nd:YAG laser beam (Fifth-HG, wavelength 213 nm, NEWWAVE RESEARCH, Tempeste10, repetition rate 10 Hz) was utilized. With this technique, 3 3 and 4 4 micro-nozzle arrays
H. Horisawa et al. / Vacuum 85 (2010) 574e578
575
Fig. 1. Scanning electron microscope images of micro-nozzles (g: separations between nozzle elements). (a) Micro single-nozzle (b) 3 3 micro-nozzle-array (c) 4 4 micronozzle-array.
Fig. 3. Comparison of 3 3 and 4 4 micro-nozzle arrays for nozzle length of l ¼ 0.5 mm and 1 mm (cold-gas flow experiment). (a) Thrust vs mass flow (b) Specific impulse vs mass flow.
were machined. The SEM images of the micro-nozzles are shown in Fig. 1. In this study, a single nozzle (a), 3 3 and 4 4 nozzle arrays, (b) and (c), consisting of identical nozzle elements with (a), were machined and tested. Sizes of the nozzle element, or single nozzle, are 0.5 mm in total length, 100 mm in throat height and length, 500 mm in exit height. 2.2. Thrust measurement
Fig. 2. Schematics of experimental setups. (a) Experimental setup (b) Cantilever-type thrust stand.
A sketch of an experimental setup is illustrated in Fig. 2(a). Thrust performance tests were conducted to elucidate effects of nozzle configuration on thrust characteristics for the micro single-nozzle and the novel nozzle array thrusters. For various mass flow rate conditions of nitrogen gas propellant thrusts were measured for both types of the thrusters, and thrust characteristics then compared. The thrust was monitored with a cantilever-type thrust stand consisting of a cantilever and structural members made of quartz glass to minimize influences of thermal expansion of the structure (Fig. 2(b)) [7]. To evaluate the substantial thrust induced from aerodynamic acceleration of the propellant flow through the nozzle, the propellant supplied to the plenum is not heated by any electrical means but maintained at room temperature. The thrust tests are thus performed for an effective cold-gas operational condition. In addition, after depositing the film electrode made of gold on both the plenum side (cathode) and divergent-nozzle side (anode), shown in Fig. 2(a), investigation of the discharge characteristics and preliminary thrust performance tests were conducted for the micro-multi-plasmajet array thruster.
576
H. Horisawa et al. / Vacuum 85 (2010) 574e578
3. Results and discussion 3.1. Comparison of thrust performance for different nozzle geometries To elucidate and enhance the multi-jet effect, augmenting the thrust through interaction of multi-jet boundaries [8e10], 3 3 and 4 4 micro-nozzle arrays were fabricated and compared. The values of thrust and specific impulse per nozzle element are plotted in Fig. 3(a) and (b), respectively. Note that two kinds of the nozzle length (l ¼ 1 mm and l ¼ 0.5 mm) were compared. It will be observed that the thrust performance is lowered when the nozzle length was decreased; this is because of the reduction of the expansion region for acceleration. When the nozzle length becomes the half (l ¼ 0.5 mm), thrust of the 3 3 nozzle array was reduced by 30%. However, the thrust of the 4 4 nozzle array with 0.5-mm nozzle length was higher than that of the 3 3 nozzle array with 1-mm nozzle length. For a larger mass flow rate of 1.0 mg/s, thrust and specific impulse of the 3 3 nozzle array were 0.56 mN and 56 s, respectively. While in the 4 4 nozzle array case, higher values of thrust and specific impulse could be obtained, which were 0.64 mN and 62 s, respectively. Hence it became clear that the 4 4 nozzle array was more effective than the 3 3 nozzle array. 3.2. Discharge characteristics in stable micro-multi-plasmajet array operation In this experiment, authors have confirmed the stable discharge operational conditions of a 3 3 micro-multi-plasmajet array thruster. This is the first successful operation of the micro-multiplasmajet array. Fig. 4(a)e(d) show photos of typical plasma
plumes exhausted from 3 3 micro-plasmajet array thruster stably operating under different background pressures for mass flow rate of 0.42 [mg/sec] per nozzle element. In (a) and (b), discharge currents were 17 mA in both cases with discharge voltages of 330 V (5.6 W) and 350 V (5.9 W), respectively. Magnified front views of the multi-plasmajet plumes observed from the nozzle exhaust are shown in Fig. 4 (c) and (d). From the figure, it can be seen that discharges, or micro-multi-plasmajets, are occurring at each nozzle element. Moreover, the plasma plume from the micro-multi-plasmajet array is exhausted uniformly among each element and is also interacting with each other at each jet boundary. However, after several hours of operation, depending on the conditions, because of the degradation of the electrode (anode, or Au film), unstable and non-uniform discharges were also observed. Fig. 5(a) and (b) show front close-up photos of a single plasma plume exhausted from a single nozzle-element of 3 3 micromulti-plasmajet array thruster after hours of operation. In (a), a single discharge is occurring only at the upper-right corner element and in (b) at the central element. These non-uniformities occurred probably because of the local remains of film-electrode elements on an anode or divergent side and are thought to be a primary cause of instability and non-uniform discharges. One way to overcome this problem is to use a divided film electrode of stronger thermal and erosion properties and hence the possibility of controllability of an elemental discharge, or “digital discharge” by “a pixel”, or one single nozzle element. Relations of discharge current and voltage characteristics are shown in Fig. 6 with variation of plenum pressure in the discharge chamber for different background pressures (PB). In each background pressure, the voltage decreased with the increase of the current, and a large drop of the voltage, about 100 V, was observed between 18 mA and 22 mA.
Fig. 4. Photos of stable plasma plumes exhausted from 3 3 micro-multi-plasmajet array thruster operated under different background pressures (PB) for mass flow rate of 0.42 [mg/sec] per nozzle element. (a) PB: 53 Pa (b) PB: 4 Pa (c) Front view without discharge (d) Front view with discharge.
H. Horisawa et al. / Vacuum 85 (2010) 574e578
a
577
1.2
Thrust , mN
1 0.8 0.6 0.4 cold
0.2
hot
0 0.5
0.7
0.9 1.1 1.3 Mass flow rate , mg/sec
1.5
b 100 Isp , sec
80
3.3. Thrust performance of micro-plasmajet array thruster Thrust and specific impulse per nozzle element of a 3 3 micromulti-plasmajet array thruster from the tests are shown in Fig. 7(a) and (b), respectively. Again, as was in Fig. 3, to compare the values with those of Fig. 3, values of thrust, specific impulse and mass flow are those averaged per nozzle element, or those divided by number of all nozzle elements. From the figure, it can be seen that values of
V 53Pa V 4Pa P 53Pa P 4Pa
D is ch arg e v o ltag e, V
450
0.095
400 0.090
350 300
0.085
P len u m p res s u re, M P a
0.100
500
250 200
0.080 0
10
20
30
Discharge current, mA Fig. 6. Discharge voltage and discharge plenum pressure vs discharge current.
40 20
Fig. 5. Photos of a single plasma plume exhausted from a single nozzle-element of 3 3 micro-multi-plasmajet array thruster after several hours of operation depending on conditions. (a) Front view of a single discharge at the upper-right corner element (b) Front view of a single discharge at the central element.
Although further investigation is needed, from the negative current-voltage characteristics the discharge partly has an aspect of typical arc discharge characteristics. Under lower background pressure conditions, higher voltages, about þ20 V, was observed, resulting in a tendency of more unstable discharges.
60
cold hot
0 0.5
0.7
0.9 1.1 1.3 Mass flow rate , mg/sec
1.5
Fig. 7. Thrust performance of 3 3 micro-multi-plasmajet array thruster, in which typical input powers in hot conditions were 6.3e6.8 W. (a) Thrust vs mass flow rate (b) Specific impulse vs mass flow rate.
thrust and specific impulse with very low-power DC discharges are significantly higher than those of cold-gas operation. In addition, thrust increases with mass flow under both cold-gas and discharge operations. On the other hand, values of specific impulse are gradually increasing with mass flow. The lower values of the specific impulse are probably due to the viscous loss of the low Reynolds number flow through very small nozzles. Typical thrust performance of the 3 3 micro-plasmajet array thruster for mass flow rate of 1.25 [mg/sec] per nozzle element is listed in Table 1. As described in our previous papers [8e10], the thrust decreases with decrease of the background pressure due to the under-expansion of the exhaust plume. However with the plasmajet array cases, effect of the background pressure was suppressed with the interaction of the multi-jets resulting in the increase of static pressure at the nozzle exit plane resulting in the augmentation of a pressure thrust component of the thrust.
Table 1 Typical thrust performance of 3 3 micro-plasmajet array thruster for mass flow rate of 1.25 [mg/sec] per nozzle element, where PB: background pressure, P: total input power for DC discharge, Pp: discharge plenum pressure, T: averaged thrust per nozzle element, Isp: averaged specific impulse per nozzle element, hth: thrust efficiency. PB [Pa]
53
P [W] Pp [MPa] T [mN] Isp [sec]
0 0.1 0.74 60
hth
4 6.3 0.12 0.94 77 0.21
0 0.1 0.61 50
6.5 0.13 0.77 62 0.15
578
H. Horisawa et al. / Vacuum 85 (2010) 574e578
Comparing the thrust with cold-gas thrust, effect of the discharge on improvement of the thrust was about 40% increase with 6.5 W input power at PB ¼ 4 Pa. Furthermore, this improvement corresponds to about 100% increase of the thrust comparing with the single nozzle result. In the table, values of thrust efficiency are also included, where kinetic energy of the cold-gas thrust was subtracted from the total energy of the thrust to examine a pure energy conversion rate from electric to kinetic energy [3]. Relatively high efficiency, over 20%, among the micro-thrusters could be obtained with the arrayed thruster. From the results, a possibility to overcome one of disadvantages of the micro-thrusters, i.e., low efficiency because of the viscous loss, was demonstrated with the arrayed plasmajet thruster.
4. Conclusions Thrust performance tests were conducted for the arrayed multiplasmajet structure consisting of rectangular nozzle elements each with exit height of 0.5 mm and length of 0.5 mm. To evaluate the thrust characteristics of the arrayed micro-multi-plasmajet, the thrust was measured by a calibrated cantilever-type thrust stand in vacuum. The following conclusions were obtained: 1) Stable DC operation of the micro-multi-plasmajet array with 3 3 nozzle elements was confirmed.
2) From the close-up observation of the exhausted plasma plume, a uniform plasma plume exhausted from all of the nozzle elements was observed. 3) From the cold-gas thrust performance test, it was shown that larger number of nozzle elements showed higher thrust performance. 4) From the thrust performance test, a significant improvement of the thrust performance was confirmed with application of the DC discharge, in which higher thrust and specific impulse of at least more than 20% compared to the cold-gas flow experiment were obtained. The typical values of the thrust, specific impulse and thrust efficiency at input power of 6.3 W were 8.5 mN, 77 s and 0.21, respectively. References [1] Horisawa H, Kimura I. AIAA Paper. AIAA-97-3202; 1997. [2] Horisawa H, Kimura I. AIAA Paper. AIAA-98-3633, 1998. [3] Horisawa H, Kimura I. Progress in Astronautics and Aeronautics 2000;187:185e97. [4] Horisawa H, Ashiya H, Kimura I. IEPC Paper; 2003. IEPC 03-0078. [5] Horisawa H, Onodera K, Noda T, Kimura I. Vacuum 2006;80(11e12):1244e51. [6] Horisawa H, Noda T, Onodera K, Kimura I. Thin Solid Films 2007;515 (9):4130e5. [7] Horisawa H, Onodera K, Noda T, Kimura I. AIAA Paper; 2006. AIAA-06-4496. [8] Koshiyama, A, Onodera, K, Sawada, F, Horisawa, H, Funaki, I. IEPC Paper. IEPC2007-201; 2007. [9] Sawada, F, Koshiyama, A, Hagiwara, S, Horisawa, H, Funaki, I. ISTS Paper. ISTS 2008-b-16; 2008. [10] Horisawa H, Sawada F, Onodera K, Funaki I. Vacuum 2008;83(1):52e6.
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Micro-multi-plasmajet array thruster for space propulsion applications Hideyuki Horisawa a, *, Fujimi Sawada a, Shuji Hagiwara a, Ikkoh Funaki b a b
Department of Aeronautics and Astronautics, School of Engineering, Tokai University, 1117 Kitakaname, Hiratsuka-shi, Kanagawa 259-1292, Japan Japan Aerospace Exploration Agency, Sagamihara, Kanagawa, 229-8510, Japan
a b s t r a c t Keywords: Electric Propulsion Micro-thruster Micro-multi-plasmajet array
Thrust performance tests were conducted for an arrayed multi-plasmajet structure consisting of rectangular nozzle elements each with an exit height of 0.5 mm and length of 0.5 mm. To evaluate the thrust characteristics of the arrayed micro-multi-plasmajet, the thrust was measured using a calibrated cantilever-type thrust stand in vacuum. Using 3 3 nozzle elements, the micro-plasmajet array showed stable DC operation. Compared to conditions with cold-gas flow, the application of a DC discharge showed a significant improvement in thrust performance of at least 20% for thrust and specific impulse. Typical values achieved for thrust, specific impulse and thrust efficiency at an input power of 6.3 W were 8.5 mN, 77 s and 0.21, respectively. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction The authors have been conducting investigations on small-sized very low-power (<10 W) arcjets, or plasmajets, studying discharge characteristics, thermal characteristics of discharging plasmas, and correlations of these characteristics with thrust performance [1e4]. Microfabrication of micro-arcjets with ultra-violet lasers, and the development of rectangular DC micro-arcjets of various sizes operated at less than 5 W have also been undertaken [5e9]. The geometry and dimensions in these experiments is of critical importance for efficient operation. In the tests described above the rectangular micro-nozzle was machined in a 1.2 mm thick quartz plate. Sizes of the nozzle exit were 0.44 mm in height and 0.1 mm in constrictor height. The anode was a thin Au film (e100 nm thick) coated by physical vapor deposition in vacuum on a divergent part of the nozzle. The cathode was an Au film also coated on an inner wall surface. In operational tests, a stable discharge was observed for mass flow of 0.4 mg/s, input power of 4 W. In addition, thrust performance tests were conducted for various nozzles with different exit heights (0.4e0.8 mm), or area ratios, for a fixed nozzle length and various lengths of the divergent part [7]. From the results, nozzles with larger exit height and a longer divergent part showed higher thrusts and specific impulses. It was also shown that variations of the background pressure in the vacuum chamber, in which the thruster were tested, had a significant effect on thrust performance as much as the nozzle sizes due to the enhanced under-expansion of the exhaust jet. To reduce the expansion of the
* Corresponding author. Fax: þ81 463 50 2060. E-mail address: [email protected] (H. Horisawa). 0042-207X/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2010.08.014
exhaust jet, the effect of multi-jet interaction exhausted from the multi-nozzle jet array was additionally investigated [8e10]. In the investigation, thrust characteristics of the 3 3 micro-multinozzle array were compared with those of a single-nozzle with identical elemental size. To compare the thrust performance between the arrayed thruster and single-nozzle thruster, thrusts and mass flows per nozzle, or average values of each nozzle element, of the array were estimated by dividing each of measured values of thrust and mass flow by number of nozzle elements of the array. From the results, it was shown that the thrust and specific impulse of the each nozzle element with the nozzle array were significantly higher than those of the single-nozzle [8e10]. In this study, thrust characteristics of the micro-nozzle arrays with different geometries were investigated. Moreover, after fabricating the electrodes in the micro-nozzle arrays, investigation of the discharge and propulsive characteristics were also conducted.
2. Experimental procedure 2.1. Microfabrication of micro-nozzles A laser micromachining system developed for machining of micro-nozzles was used here as previously described [5e10]. In order to minimize and localize thermal influences and to achieve accurate geometries, a short-pulse ultra-violet laser system was utilized. For the UV laser oscillator, a fifth harmonic generation wave of an Nd:YAG laser beam (Fifth-HG, wavelength 213 nm, NEWWAVE RESEARCH, Tempeste10, repetition rate 10 Hz) was utilized. With this technique, 3 3 and 4 4 micro-nozzle arrays
H. Horisawa et al. / Vacuum 85 (2010) 574e578
575
Fig. 1. Scanning electron microscope images of micro-nozzles (g: separations between nozzle elements). (a) Micro single-nozzle (b) 3 3 micro-nozzle-array (c) 4 4 micronozzle-array.
Fig. 3. Comparison of 3 3 and 4 4 micro-nozzle arrays for nozzle length of l ¼ 0.5 mm and 1 mm (cold-gas flow experiment). (a) Thrust vs mass flow (b) Specific impulse vs mass flow.
were machined. The SEM images of the micro-nozzles are shown in Fig. 1. In this study, a single nozzle (a), 3 3 and 4 4 nozzle arrays, (b) and (c), consisting of identical nozzle elements with (a), were machined and tested. Sizes of the nozzle element, or single nozzle, are 0.5 mm in total length, 100 mm in throat height and length, 500 mm in exit height. 2.2. Thrust measurement
Fig. 2. Schematics of experimental setups. (a) Experimental setup (b) Cantilever-type thrust stand.
A sketch of an experimental setup is illustrated in Fig. 2(a). Thrust performance tests were conducted to elucidate effects of nozzle configuration on thrust characteristics for the micro single-nozzle and the novel nozzle array thrusters. For various mass flow rate conditions of nitrogen gas propellant thrusts were measured for both types of the thrusters, and thrust characteristics then compared. The thrust was monitored with a cantilever-type thrust stand consisting of a cantilever and structural members made of quartz glass to minimize influences of thermal expansion of the structure (Fig. 2(b)) [7]. To evaluate the substantial thrust induced from aerodynamic acceleration of the propellant flow through the nozzle, the propellant supplied to the plenum is not heated by any electrical means but maintained at room temperature. The thrust tests are thus performed for an effective cold-gas operational condition. In addition, after depositing the film electrode made of gold on both the plenum side (cathode) and divergent-nozzle side (anode), shown in Fig. 2(a), investigation of the discharge characteristics and preliminary thrust performance tests were conducted for the micro-multi-plasmajet array thruster.
576
H. Horisawa et al. / Vacuum 85 (2010) 574e578
3. Results and discussion 3.1. Comparison of thrust performance for different nozzle geometries To elucidate and enhance the multi-jet effect, augmenting the thrust through interaction of multi-jet boundaries [8e10], 3 3 and 4 4 micro-nozzle arrays were fabricated and compared. The values of thrust and specific impulse per nozzle element are plotted in Fig. 3(a) and (b), respectively. Note that two kinds of the nozzle length (l ¼ 1 mm and l ¼ 0.5 mm) were compared. It will be observed that the thrust performance is lowered when the nozzle length was decreased; this is because of the reduction of the expansion region for acceleration. When the nozzle length becomes the half (l ¼ 0.5 mm), thrust of the 3 3 nozzle array was reduced by 30%. However, the thrust of the 4 4 nozzle array with 0.5-mm nozzle length was higher than that of the 3 3 nozzle array with 1-mm nozzle length. For a larger mass flow rate of 1.0 mg/s, thrust and specific impulse of the 3 3 nozzle array were 0.56 mN and 56 s, respectively. While in the 4 4 nozzle array case, higher values of thrust and specific impulse could be obtained, which were 0.64 mN and 62 s, respectively. Hence it became clear that the 4 4 nozzle array was more effective than the 3 3 nozzle array. 3.2. Discharge characteristics in stable micro-multi-plasmajet array operation In this experiment, authors have confirmed the stable discharge operational conditions of a 3 3 micro-multi-plasmajet array thruster. This is the first successful operation of the micro-multiplasmajet array. Fig. 4(a)e(d) show photos of typical plasma
plumes exhausted from 3 3 micro-plasmajet array thruster stably operating under different background pressures for mass flow rate of 0.42 [mg/sec] per nozzle element. In (a) and (b), discharge currents were 17 mA in both cases with discharge voltages of 330 V (5.6 W) and 350 V (5.9 W), respectively. Magnified front views of the multi-plasmajet plumes observed from the nozzle exhaust are shown in Fig. 4 (c) and (d). From the figure, it can be seen that discharges, or micro-multi-plasmajets, are occurring at each nozzle element. Moreover, the plasma plume from the micro-multi-plasmajet array is exhausted uniformly among each element and is also interacting with each other at each jet boundary. However, after several hours of operation, depending on the conditions, because of the degradation of the electrode (anode, or Au film), unstable and non-uniform discharges were also observed. Fig. 5(a) and (b) show front close-up photos of a single plasma plume exhausted from a single nozzle-element of 3 3 micromulti-plasmajet array thruster after hours of operation. In (a), a single discharge is occurring only at the upper-right corner element and in (b) at the central element. These non-uniformities occurred probably because of the local remains of film-electrode elements on an anode or divergent side and are thought to be a primary cause of instability and non-uniform discharges. One way to overcome this problem is to use a divided film electrode of stronger thermal and erosion properties and hence the possibility of controllability of an elemental discharge, or “digital discharge” by “a pixel”, or one single nozzle element. Relations of discharge current and voltage characteristics are shown in Fig. 6 with variation of plenum pressure in the discharge chamber for different background pressures (PB). In each background pressure, the voltage decreased with the increase of the current, and a large drop of the voltage, about 100 V, was observed between 18 mA and 22 mA.
Fig. 4. Photos of stable plasma plumes exhausted from 3 3 micro-multi-plasmajet array thruster operated under different background pressures (PB) for mass flow rate of 0.42 [mg/sec] per nozzle element. (a) PB: 53 Pa (b) PB: 4 Pa (c) Front view without discharge (d) Front view with discharge.
H. Horisawa et al. / Vacuum 85 (2010) 574e578
a
577
1.2
Thrust , mN
1 0.8 0.6 0.4 cold
0.2
hot
0 0.5
0.7
0.9 1.1 1.3 Mass flow rate , mg/sec
1.5
b 100 Isp , sec
80
3.3. Thrust performance of micro-plasmajet array thruster Thrust and specific impulse per nozzle element of a 3 3 micromulti-plasmajet array thruster from the tests are shown in Fig. 7(a) and (b), respectively. Again, as was in Fig. 3, to compare the values with those of Fig. 3, values of thrust, specific impulse and mass flow are those averaged per nozzle element, or those divided by number of all nozzle elements. From the figure, it can be seen that values of
V 53Pa V 4Pa P 53Pa P 4Pa
D is ch arg e v o ltag e, V
450
0.095
400 0.090
350 300
0.085
P len u m p res s u re, M P a
0.100
500
250 200
0.080 0
10
20
30
Discharge current, mA Fig. 6. Discharge voltage and discharge plenum pressure vs discharge current.
40 20
Fig. 5. Photos of a single plasma plume exhausted from a single nozzle-element of 3 3 micro-multi-plasmajet array thruster after several hours of operation depending on conditions. (a) Front view of a single discharge at the upper-right corner element (b) Front view of a single discharge at the central element.
Although further investigation is needed, from the negative current-voltage characteristics the discharge partly has an aspect of typical arc discharge characteristics. Under lower background pressure conditions, higher voltages, about þ20 V, was observed, resulting in a tendency of more unstable discharges.
60
cold hot
0 0.5
0.7
0.9 1.1 1.3 Mass flow rate , mg/sec
1.5
Fig. 7. Thrust performance of 3 3 micro-multi-plasmajet array thruster, in which typical input powers in hot conditions were 6.3e6.8 W. (a) Thrust vs mass flow rate (b) Specific impulse vs mass flow rate.
thrust and specific impulse with very low-power DC discharges are significantly higher than those of cold-gas operation. In addition, thrust increases with mass flow under both cold-gas and discharge operations. On the other hand, values of specific impulse are gradually increasing with mass flow. The lower values of the specific impulse are probably due to the viscous loss of the low Reynolds number flow through very small nozzles. Typical thrust performance of the 3 3 micro-plasmajet array thruster for mass flow rate of 1.25 [mg/sec] per nozzle element is listed in Table 1. As described in our previous papers [8e10], the thrust decreases with decrease of the background pressure due to the under-expansion of the exhaust plume. However with the plasmajet array cases, effect of the background pressure was suppressed with the interaction of the multi-jets resulting in the increase of static pressure at the nozzle exit plane resulting in the augmentation of a pressure thrust component of the thrust.
Table 1 Typical thrust performance of 3 3 micro-plasmajet array thruster for mass flow rate of 1.25 [mg/sec] per nozzle element, where PB: background pressure, P: total input power for DC discharge, Pp: discharge plenum pressure, T: averaged thrust per nozzle element, Isp: averaged specific impulse per nozzle element, hth: thrust efficiency. PB [Pa]
53
P [W] Pp [MPa] T [mN] Isp [sec]
0 0.1 0.74 60
hth
4 6.3 0.12 0.94 77 0.21
0 0.1 0.61 50
6.5 0.13 0.77 62 0.15
578
H. Horisawa et al. / Vacuum 85 (2010) 574e578
Comparing the thrust with cold-gas thrust, effect of the discharge on improvement of the thrust was about 40% increase with 6.5 W input power at PB ¼ 4 Pa. Furthermore, this improvement corresponds to about 100% increase of the thrust comparing with the single nozzle result. In the table, values of thrust efficiency are also included, where kinetic energy of the cold-gas thrust was subtracted from the total energy of the thrust to examine a pure energy conversion rate from electric to kinetic energy [3]. Relatively high efficiency, over 20%, among the micro-thrusters could be obtained with the arrayed thruster. From the results, a possibility to overcome one of disadvantages of the micro-thrusters, i.e., low efficiency because of the viscous loss, was demonstrated with the arrayed plasmajet thruster.
4. Conclusions Thrust performance tests were conducted for the arrayed multiplasmajet structure consisting of rectangular nozzle elements each with exit height of 0.5 mm and length of 0.5 mm. To evaluate the thrust characteristics of the arrayed micro-multi-plasmajet, the thrust was measured by a calibrated cantilever-type thrust stand in vacuum. The following conclusions were obtained: 1) Stable DC operation of the micro-multi-plasmajet array with 3 3 nozzle elements was confirmed.
2) From the close-up observation of the exhausted plasma plume, a uniform plasma plume exhausted from all of the nozzle elements was observed. 3) From the cold-gas thrust performance test, it was shown that larger number of nozzle elements showed higher thrust performance. 4) From the thrust performance test, a significant improvement of the thrust performance was confirmed with application of the DC discharge, in which higher thrust and specific impulse of at least more than 20% compared to the cold-gas flow experiment were obtained. The typical values of the thrust, specific impulse and thrust efficiency at input power of 6.3 W were 8.5 mN, 77 s and 0.21, respectively. References [1] Horisawa H, Kimura I. AIAA Paper. AIAA-97-3202; 1997. [2] Horisawa H, Kimura I. AIAA Paper. AIAA-98-3633, 1998. [3] Horisawa H, Kimura I. Progress in Astronautics and Aeronautics 2000;187:185e97. [4] Horisawa H, Ashiya H, Kimura I. IEPC Paper; 2003. IEPC 03-0078. [5] Horisawa H, Onodera K, Noda T, Kimura I. Vacuum 2006;80(11e12):1244e51. [6] Horisawa H, Noda T, Onodera K, Kimura I. Thin Solid Films 2007;515 (9):4130e5. [7] Horisawa H, Onodera K, Noda T, Kimura I. AIAA Paper; 2006. AIAA-06-4496. [8] Koshiyama, A, Onodera, K, Sawada, F, Horisawa, H, Funaki, I. IEPC Paper. IEPC2007-201; 2007. [9] Sawada, F, Koshiyama, A, Hagiwara, S, Horisawa, H, Funaki, I. ISTS Paper. ISTS 2008-b-16; 2008. [10] Horisawa H, Sawada F, Onodera K, Funaki I. Vacuum 2008;83(1):52e6.