- Email: [email protected]
Liquid-metal-ion source development for space propulsion at ARC
ARTICLE IN PRESS Ultramicroscopy 109 (2009) 442–446
Contents lists available at ScienceDirect
Ultramicroscopy journal homepage: www.elsevier.com/locate/ultramic
Liquid-metal-ion source development for space propulsion at ARC M. Tajmar , C. Scharlemann, A. Genovese, N. Buldrini, W. Steiger, I. Vasiljevich Space Propulsion & Advanced Concepts, Austrian Research Centers GmbH—ARC, Austria
a r t i c l e in f o
PACS: 68.37.Vj 79.70.+q Keywords: LMIS FEEP Field emission
a b s t r a c t The Austrian Research Centers have a long history of developing indium Liquid-Metal-Ion Source (LMIS) for space applications including spacecraft charging compensators, SIMS and propulsion. Specifically the application as a thruster requires long-term operation as well as high-current operation which is very challenging. Recently, we demonstrated the operation of a cluster of single LMIS at an average current of 100 mA each for more than 4800 h and developed models for tip erosion and droplet deposition suggesting that such a LMIS can operate up to 20,000 h or more. In order to drastically increase the current, a porous multi-tip source that allows operation up to several mA was developed. Our paper will highlight the problem areas and challenges from our LMIS development focusing on space propulsion applications. & 2008 Elsevier B.V. All rights reserved.
1. Introduction The Austrian Research Centers have been developing LiquidMetal-Ion Source (LMIS) for space applications since the late 1980s [1–6]. The first test of an LMIS under microgravity conditions in space was performed during the AUSTROMIR mission in 1991. A number of scientific instruments followed, including the application of the ion source for active spacecraft potential control of satellites (ASPOC) as well as part of a secondary ion mass spectrometer. Indium was chosen as propellant because its melting point of 156.6 1C is high enough such that it is solid during launch. This is important because the vibrations during launch may cause a liquid propellant to spill out and contaminate the ion emitter module. The melting point of indium is still low enough that the overall heater power consumption is low which is critical for space applications with a limited power budget. A standard ion emitter module for scientific instruments was developed with low mass and low power consumption and was used for a number of different missions with minor modifications [3]. Typical ion current requirements are between 10 and 20 mA with peaks up to 50 mA for a lifetime of several thousands of hours similar to standard LMIS used for terrestrial applications. Since the late 1990s, the ion source has been developed further towards the application as a thruster [4] to enable ultraprecise drag-free control of satellites required, e.g. by formation flying missions to detect gravitational waves (LISA) or Earth-like planets outside of our solar system (DARWIN). This required a number of
Corresponding author.
E-mail address: [email protected] (M. Tajmar). 0304-3991/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ultramic.2008.10.009
modifications that were pushing the limits of existing LMIS capabilities. For instance, the current requirements increased by 2–3 orders of magnitude along with even longer lifetime requirements. This paper will give an introduction to the LMIS development towards space applications performed at ARC and the challenges during its development.
2. LMIS for scientific instruments Table 1 gives an overview of the missions using the ARC LMIS for spacecraft potential control and for secondary ion mass spectrometers. More than 16,000 h of operation could be accumulated on various spacecraft till date. The basic elements of our standard ion emission module is illustrated in Fig. 1 featuring the emitter itself, electrical and thermal insulation, the heater and electrical connections. The ion source usually uses an electrochemically sharpened tungsten tip with a tip radius of a few mm and a tank size varying between 0.22 and 1.2 g. The emitter is welded to a ceramic tube which houses the heater element. The ceramic insulation has the advantage that the heater can be operated at low voltage thus reducing the mass and complexity of the electronics since the emitter itself is usually operated with voltages between 3 and 8 kV. Several ion sources can be integrated into one module housing. They can be selected via relays that switch the heater between the ion sources. After final testing, the ion module is placed in nitrogen atmosphere and closed off by a pyro-spring cap. A dedicated storage test, however, showed that normal purging with a red-tag cover, which is removed prior to launch, is sufficient. For example, the voltage difference after a 6 month storage under nitrogen purging showed a change in the emitter voltage of less than 200 V at 10 mA which is
ARTICLE IN PRESS M. Tajmar et al. / Ultramicroscopy 109 (2009) 442–446
443
Table 1 Space experience of ARC indium LMIS (up to November 2008). Experiment
Function
Spacecraft
Nr1. of LMIS
Operation time
LOGION MIGMAS/A EFE-IE PCD ASPOC
Test of LMIS in m-Gravity Mass spectrometer S/C potential control S/C potential control S/C potential control
MIR MIR GEOTAIL EQUATOR-S CLUSTER
1 1 8 8 32
ASPOC-II COSIMA ASPOC/DSP
S/C potential control Mass spectrometer S/C potential control
CLUSTER-II ROSETTA DoubleStar
32 2 4
24 h (1991) 120 h (1991–94) 600 h (1992) 250 h (1998) Ariane 5 launch failure 1996 Still operational after crash 6516 h (2000–2008) Launched 2004 (tested in space) 8979 h (2004–)
such ion sources need to provide up to 1 mA of current for about 20,000 h of operation in a maintenance-free environment. Some missions even require several mN of thrust (several tens of mA) in order to move the satellite formation to a different orientation in addition to the usual drag-free compensation duty. This triggered research in several directions with special focus on the challenges with respect to the reservoir tank, lifetime challenges and the high current. Previous high-current LMIS were based either on slit emitters using caesium as propellant [8] or porous tungsten emitters which were mechanically sharpened [9]. Our goal was to avoid caesium since it is toxic, reacts violently with oxygen and is liquid during launch due to its low melting point at 28.4 1C which could cause spillage problems during vibrations. In contrast, our propellant indium is quite suitable for a FEEP thruster due to its relatively high-atomic weight and low ionization energy which results in high thrusts at moderate power-to-thrust ratios [4] as well as its relative inertness to oxidation which makes it possible, in contrast to Cs, to store the emitter for extended times up to years under normal laboratory conditions. Therefore, our first action was to greatly enlarge the propellant tank of our existing indium LMIS and use a cluster of ion emitters in order to achieve high currents. Our most recent design is a single multi-tip emitter using etched porous tungsten structures which enable much more compact designs while fulfilling the challenging requirements for FEEP thruster ion sources [6]. A summary of our thruster-related LMIS performances is shown in Table 3.
3.1. Cluster of large reservoir tank emitters
Fig. 1. Cut through ion emission module (top) and ASPOC instrument for CLUSTERII (bottom).
within the usual voltage deviations for an LMIS after re-starting. Table 2 lists the typical performance envelope for our standard LMIS ion emission module.
3. LMIS for propulsion purposes An LMIS is the core element of a Field-emission-electricpropulsion (FEEP) thruster [7]. In order to provide an extremely accurate attitude and orbit control of satellites during the entire duration of the missions, such thrusters need to provide a thrust level of several tens of mN up to 100 mN for several years of continuous operation. Translated into requirements for an LMIS,
The first challenge was to increase the propellant tank size in such a manner that it is compatible with operation in microgravity environment. Fig. 2 shows the progression of how our tank size increased up to 30 g. Inside the reservoir, fins are responsible for holding the propellant in place through capillary forces in order to ensure operation of the tank in every orientation. We performed a vertical expulsion test by emptying a completely filled 15 g reservoir against gravity by means of field emission. This test demonstrated that the design of the large tank is compatible with microgravity operation. The next challenge consisted of increasing the total current. A single LMIS can routinely provide currents in the 100 mA range. Higher currents may cause degradation, especially due to erosion as discussed in the next chapter. The first obvious approach was to cluster the emitters together. However, it is not straightforward to use a single high-voltage power supply for our assembly because there is quite a large spread in the current–voltage characteristics among the LMIS in the cluster. This is due to the fact that the electrical characteristics are greatly influenced by the size of the Taylor cone on the needle tip [10,11]. A change of only 1 mm can shift the voltage by a full kV at the same current. Obviously, oxide layers and different thicknesses of the indium film after wetting
ARTICLE IN PRESS 444
M. Tajmar et al. / Ultramicroscopy 109 (2009) 442–446
Table 2 Ion emission module. Number of emitters Propellant Reservoir size Diameter (approx.)a Height (approx.)a Weight (approx.)a Current/emitter Voltage Design lifetime (at 20 mA) a
1–4 Indium 0.22–1.2 g 5 cm 7 cm 100 g 1–100 mA 3–8 kV Up to 5300 h
Actual dimensions depend on number of emitters and reservoir size.
Table 3 FEEP emitter cluster module.
Number of emitters Propellant Reservoir size (g) Current/emitter Voltage (kV) Thrust/emitter (mN) Thrust noise (mN/OHz) Thrust resolution (mN) Specific impulse (s)b Power/thrust ratio (W/mN)b Beam divergence (1) Design lifetime (at 100 mA) (h) a b
Clustered LMIS
Porous tungsten source
4–16 Indium 15 1–150 mA 3–8 0.1–12 o0.1 o0.1 4000–6000 40–55 o25 Up to 20,000
28 Indium 415a Up to 3 mA 7–18 0.1–500 o0.1 o0.1 6000–8000 60–85 o25 Up to 10,000
Fig. 3. Current–voltage characteristic of solid and porous tungsten needle LMIS.
Can be enlarged to customer specification. Varying over the current–voltage characteristic.
Fig. 4. 4 4 LMIS cluster.
They also contain a focusing electrode to reduce the half-angle beam spread to 251 even at maximum currents. A 9-emitter cluster is presently under development for the LISA Pathfinder satellite that can provide thrusts up to 100 mN. 3.2. Lifetime challenges
Fig. 2. Propellant tanks.
can influence the size of the Taylor cone by several mm which causes the spread in the electrical characteristics. The key to solve this problem was to have an ion source with a high electrical impedance in order to minimize the change in current for large voltage jumps. This was done by manufacturing very sharp tips (2–3 mm diameter) with tight manufacturing tolerances, which result in high impedance sources, and by implementing a pre-resistor before the emitter. The current– voltage characteristic of such a sharp-tip LMIS is shown in Fig. 3. The starting voltage is at around 2.5 kV and the impedance is close to 40 MO. Together with a pre-resistor of similar size, this results in typical current variations of only 710% across clusters of LMIS powered by a single HV power supply. Such clusters were built up to a size of 4 4 emitters in compact module housings (see Fig. 4).
Many endurance tests were carried out in order to test the lifetime of our ion sources. One pre-requisite was to make our vacuum facilities compatible for such tests. The obvious limitation here is the severe ion beam backsputtering. We implemented a collector with fins on the back side in order to avoid direct backsputtering. In addition, the collector fins were covered with indium to limit backsputtering of non-indium particles which could contaminate our ion sources. During testing, two lifetime limitations were found:
In addition to the usual ions, weakly charged microdroplets which can clog the extractor electrode and cause a shortcut are emitted. After modelling the droplet deposition, it was decided to greatly enlarge the extractor diameter to about 8 mm in order to create enough room to account for the deposition. Furthermore, we had to implement a negatively biased cover electrode in order to repel secondary electrodes from testing chambers and electrons from the plasma environment which
ARTICLE IN PRESS M. Tajmar et al. / Ultramicroscopy 109 (2009) 442–446
would otherwise react to the high positive potential from the ion source. The most severe lifetime limitation was found to be a slow erosion process of the tip. This erosion could be traced back to microsparks between emitter and extractor (a typical feature of LMIS) which heat up the emitter tip locally and cause dissolution of the tungsten tip into the liquid indium. The preresistor implemented for our cluster design helped to reduce the spark energies and therefore to slow down the erosion process. Details on the erosion process, models and verifications tests can be found in Ref. [12].
Proper sizing of the pre-resistor as well as large extractor electrodes enabled us to design a FEEP thruster fulfilling the challenging lifetime requirements. We successfully demonstrated a test of more than 4800 h of continuous operation for two emitters and a test of 3000 h for a 4 4 cluster equipped with 8 emitters (the first 2000 h with 8 emitters and then the rest 1000 h with 6 emitters since 2 emitters were kept for analysis purposes)
445
[13,14]. Our models indicate that such a thruster could be operated in principle up to 20,000 h or more. 3.3. High-current porous tungsten source In order to further increase the number of emitters and to make the overall module more compact, a new type of highcurrent ion source was developed based on porous tungsten. There are two basic types of emitter geometries for LMIS: needleand capillary-based emitters. The needle emitters have usually much larger electrical impedances compared to the capillary emitter which makes them suitable for clustering (as discussed). However, capillary emitters are usually more stable and reliable. We therefore decided to design a multi-tip emitter using porous tungsten needles in order to combine the advantage of high electrical impedance with excellent stability. The emitter is manufactured using the Micro-Powder Injection Moulding technique from the ARC Powder Technology Center [6]. A special feedstock is prepared using tungsten micropowders and
Fig. 5. Porous tungsten source: porous needle tip (left), filled and wet with indium (middle), 28-needle crown emitter (right).
ARTICLE IN PRESS 446
M. Tajmar et al. / Ultramicroscopy 109 (2009) 442–446
the crown significantly differs from the one of a single needle. This also translates in higher power-to-thrust ratios that may be compensated by negatively biasing the extractor electrode if necessary.
4. Conclusion ARC developed a variety of LMIS for space applications emitting currents from 10 mA to several mA for lifetimes of 20,000 h. Our applications include active spacecraft compensators, secondary ion mass spectrometers as well as micropropulsion. Our latest development is a high-current porous multi-emitter that promises exceptional stability that can also trigger terrestrial applications such as high-current FIB manufacturing. Fig. 6. Current–voltage characteristic of porous crown emitter with 28 needle tips.
References the needle structure is formed using dedicated moulding tools. After sintering, the porous emitter structure can be etched similarly to the traditional solid tungsten needle emitters. Currently, 85% porosity and micron-sized pores can be achieved. We designed a 28-emitter crown emitter which can produce currents in the range of 3 mA (corresponding to about 350 mN). Our test data indicated an exceptional operational stability of the ion source and show no measurable change in performance after more than 20 thermal cycles and exposure to oxygen. This behaviour is attributed to the excellent capillary properties of the porous needle itself. Fig. 5 shows details of such a porous needle structure as well as the overall crown geometry. Fig. 3 compares the current–voltage characteristics in such a single porous needle with our standard solid tungsten needles and they appear to be quite similar. The slightly higher starting voltage and correspondingly lower impedance can be attributed to a blunter tip of 5 mm diameter since it is much harder to etch porous tungsten structures compared to solid ones which still leaves room for optimization. The current–voltage characteristic of a 28-emitter crown emitter is shown in Fig. 6. One immediately notices the higher operating voltages up to 15 kV compared to the usual 6 kV from single LMIS needles since the electric field configuration for
[1] F.G. Ruedenauer, W. Steiger, H. Arends, M. Fehringer, R. Schmidt, J. Phys. C 6 (49) (1988) 161. [2] F.G. Ruedenauer, H.M. Fehringer, R. Schmidt, H. Arends, ESA J. 17 (1993) 147. [3] R. Schmidt, H.M. Fehringer, F.G. Ruedenauer, W. Steiger, Rev. Sci. Instrum. 64 (1993) 2293. [4] M. Tajmar, A. Genovese, W. Steiger, J. Propul. Power 20 (2004) 211. [5] C. Scharlemann, A. Genovese, N. Buldrini, R. Schnitzer, M. Tajmar, H. Fru¨holz, R. Killinger, Indium FEEP micropropulsion qualification test program for LISA pathfinder, in: Proceedings of the Fifth International Space Propulsion Conference, May 2008, Heraklion. [6] I. Vasiljevich, M. Tajmar, W. Grienauer, F. Plesescu, N. Buldrini, J. Gonzalez del Amo, B. Carnicero-Dominguez, M. Betto, Development of an indium mN-FEEP thruster, in: AIAA Joint Propulsion Conference, AIAA-2008-4534, July 2008. [7] M. Tajmar, Advanced Space Propulsion Systems, Springer, Wien,New York, 2003. [8] S. Marcuccio, A. Genovese, M. Andrenucci, J. Propul. Power 14 (1998) 774. [9] J. Ishikawa, T. Takagi, Vacuum 36 (1986) 825. [10] M. Tajmar, Appl. Phys. A 81 (2005) 1447. [11] M. Tajmar, J. Phys. D: Appl. Phys. 37 (2004) 3057. [12] M. Tajmar, Development of a lifetime prediction model for indium FEEP thrusters, in: AIAA Joint Propulsion Conference, AIAA-2005-4386, 2005. [13] A. Genovese, N. Buldrini, K. Andres, M. Tajmar, 5000 h endurance test of an indium FEEP 2 2 cluster, in: AIAA Joint Propulsion Conference, AIAA-20064827, 2006. [14] A. Genovese, N. Buldrini, R. Schnitzer, M. Tajmar, C. Scharlemann, 3000 hour lifetime test of an indium FEEP cluster for the LISA pathfinder mission, in: International Electric Propulsion Conference, IEPC-2007-140, 2007.
Contents lists available at ScienceDirect
Ultramicroscopy journal homepage: www.elsevier.com/locate/ultramic
Liquid-metal-ion source development for space propulsion at ARC M. Tajmar , C. Scharlemann, A. Genovese, N. Buldrini, W. Steiger, I. Vasiljevich Space Propulsion & Advanced Concepts, Austrian Research Centers GmbH—ARC, Austria
a r t i c l e in f o
PACS: 68.37.Vj 79.70.+q Keywords: LMIS FEEP Field emission
a b s t r a c t The Austrian Research Centers have a long history of developing indium Liquid-Metal-Ion Source (LMIS) for space applications including spacecraft charging compensators, SIMS and propulsion. Specifically the application as a thruster requires long-term operation as well as high-current operation which is very challenging. Recently, we demonstrated the operation of a cluster of single LMIS at an average current of 100 mA each for more than 4800 h and developed models for tip erosion and droplet deposition suggesting that such a LMIS can operate up to 20,000 h or more. In order to drastically increase the current, a porous multi-tip source that allows operation up to several mA was developed. Our paper will highlight the problem areas and challenges from our LMIS development focusing on space propulsion applications. & 2008 Elsevier B.V. All rights reserved.
1. Introduction The Austrian Research Centers have been developing LiquidMetal-Ion Source (LMIS) for space applications since the late 1980s [1–6]. The first test of an LMIS under microgravity conditions in space was performed during the AUSTROMIR mission in 1991. A number of scientific instruments followed, including the application of the ion source for active spacecraft potential control of satellites (ASPOC) as well as part of a secondary ion mass spectrometer. Indium was chosen as propellant because its melting point of 156.6 1C is high enough such that it is solid during launch. This is important because the vibrations during launch may cause a liquid propellant to spill out and contaminate the ion emitter module. The melting point of indium is still low enough that the overall heater power consumption is low which is critical for space applications with a limited power budget. A standard ion emitter module for scientific instruments was developed with low mass and low power consumption and was used for a number of different missions with minor modifications [3]. Typical ion current requirements are between 10 and 20 mA with peaks up to 50 mA for a lifetime of several thousands of hours similar to standard LMIS used for terrestrial applications. Since the late 1990s, the ion source has been developed further towards the application as a thruster [4] to enable ultraprecise drag-free control of satellites required, e.g. by formation flying missions to detect gravitational waves (LISA) or Earth-like planets outside of our solar system (DARWIN). This required a number of
Corresponding author.
E-mail address: [email protected] (M. Tajmar). 0304-3991/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ultramic.2008.10.009
modifications that were pushing the limits of existing LMIS capabilities. For instance, the current requirements increased by 2–3 orders of magnitude along with even longer lifetime requirements. This paper will give an introduction to the LMIS development towards space applications performed at ARC and the challenges during its development.
2. LMIS for scientific instruments Table 1 gives an overview of the missions using the ARC LMIS for spacecraft potential control and for secondary ion mass spectrometers. More than 16,000 h of operation could be accumulated on various spacecraft till date. The basic elements of our standard ion emission module is illustrated in Fig. 1 featuring the emitter itself, electrical and thermal insulation, the heater and electrical connections. The ion source usually uses an electrochemically sharpened tungsten tip with a tip radius of a few mm and a tank size varying between 0.22 and 1.2 g. The emitter is welded to a ceramic tube which houses the heater element. The ceramic insulation has the advantage that the heater can be operated at low voltage thus reducing the mass and complexity of the electronics since the emitter itself is usually operated with voltages between 3 and 8 kV. Several ion sources can be integrated into one module housing. They can be selected via relays that switch the heater between the ion sources. After final testing, the ion module is placed in nitrogen atmosphere and closed off by a pyro-spring cap. A dedicated storage test, however, showed that normal purging with a red-tag cover, which is removed prior to launch, is sufficient. For example, the voltage difference after a 6 month storage under nitrogen purging showed a change in the emitter voltage of less than 200 V at 10 mA which is
ARTICLE IN PRESS M. Tajmar et al. / Ultramicroscopy 109 (2009) 442–446
443
Table 1 Space experience of ARC indium LMIS (up to November 2008). Experiment
Function
Spacecraft
Nr1. of LMIS
Operation time
LOGION MIGMAS/A EFE-IE PCD ASPOC
Test of LMIS in m-Gravity Mass spectrometer S/C potential control S/C potential control S/C potential control
MIR MIR GEOTAIL EQUATOR-S CLUSTER
1 1 8 8 32
ASPOC-II COSIMA ASPOC/DSP
S/C potential control Mass spectrometer S/C potential control
CLUSTER-II ROSETTA DoubleStar
32 2 4
24 h (1991) 120 h (1991–94) 600 h (1992) 250 h (1998) Ariane 5 launch failure 1996 Still operational after crash 6516 h (2000–2008) Launched 2004 (tested in space) 8979 h (2004–)
such ion sources need to provide up to 1 mA of current for about 20,000 h of operation in a maintenance-free environment. Some missions even require several mN of thrust (several tens of mA) in order to move the satellite formation to a different orientation in addition to the usual drag-free compensation duty. This triggered research in several directions with special focus on the challenges with respect to the reservoir tank, lifetime challenges and the high current. Previous high-current LMIS were based either on slit emitters using caesium as propellant [8] or porous tungsten emitters which were mechanically sharpened [9]. Our goal was to avoid caesium since it is toxic, reacts violently with oxygen and is liquid during launch due to its low melting point at 28.4 1C which could cause spillage problems during vibrations. In contrast, our propellant indium is quite suitable for a FEEP thruster due to its relatively high-atomic weight and low ionization energy which results in high thrusts at moderate power-to-thrust ratios [4] as well as its relative inertness to oxidation which makes it possible, in contrast to Cs, to store the emitter for extended times up to years under normal laboratory conditions. Therefore, our first action was to greatly enlarge the propellant tank of our existing indium LMIS and use a cluster of ion emitters in order to achieve high currents. Our most recent design is a single multi-tip emitter using etched porous tungsten structures which enable much more compact designs while fulfilling the challenging requirements for FEEP thruster ion sources [6]. A summary of our thruster-related LMIS performances is shown in Table 3.
3.1. Cluster of large reservoir tank emitters
Fig. 1. Cut through ion emission module (top) and ASPOC instrument for CLUSTERII (bottom).
within the usual voltage deviations for an LMIS after re-starting. Table 2 lists the typical performance envelope for our standard LMIS ion emission module.
3. LMIS for propulsion purposes An LMIS is the core element of a Field-emission-electricpropulsion (FEEP) thruster [7]. In order to provide an extremely accurate attitude and orbit control of satellites during the entire duration of the missions, such thrusters need to provide a thrust level of several tens of mN up to 100 mN for several years of continuous operation. Translated into requirements for an LMIS,
The first challenge was to increase the propellant tank size in such a manner that it is compatible with operation in microgravity environment. Fig. 2 shows the progression of how our tank size increased up to 30 g. Inside the reservoir, fins are responsible for holding the propellant in place through capillary forces in order to ensure operation of the tank in every orientation. We performed a vertical expulsion test by emptying a completely filled 15 g reservoir against gravity by means of field emission. This test demonstrated that the design of the large tank is compatible with microgravity operation. The next challenge consisted of increasing the total current. A single LMIS can routinely provide currents in the 100 mA range. Higher currents may cause degradation, especially due to erosion as discussed in the next chapter. The first obvious approach was to cluster the emitters together. However, it is not straightforward to use a single high-voltage power supply for our assembly because there is quite a large spread in the current–voltage characteristics among the LMIS in the cluster. This is due to the fact that the electrical characteristics are greatly influenced by the size of the Taylor cone on the needle tip [10,11]. A change of only 1 mm can shift the voltage by a full kV at the same current. Obviously, oxide layers and different thicknesses of the indium film after wetting
ARTICLE IN PRESS 444
M. Tajmar et al. / Ultramicroscopy 109 (2009) 442–446
Table 2 Ion emission module. Number of emitters Propellant Reservoir size Diameter (approx.)a Height (approx.)a Weight (approx.)a Current/emitter Voltage Design lifetime (at 20 mA) a
1–4 Indium 0.22–1.2 g 5 cm 7 cm 100 g 1–100 mA 3–8 kV Up to 5300 h
Actual dimensions depend on number of emitters and reservoir size.
Table 3 FEEP emitter cluster module.
Number of emitters Propellant Reservoir size (g) Current/emitter Voltage (kV) Thrust/emitter (mN) Thrust noise (mN/OHz) Thrust resolution (mN) Specific impulse (s)b Power/thrust ratio (W/mN)b Beam divergence (1) Design lifetime (at 100 mA) (h) a b
Clustered LMIS
Porous tungsten source
4–16 Indium 15 1–150 mA 3–8 0.1–12 o0.1 o0.1 4000–6000 40–55 o25 Up to 20,000
28 Indium 415a Up to 3 mA 7–18 0.1–500 o0.1 o0.1 6000–8000 60–85 o25 Up to 10,000
Fig. 3. Current–voltage characteristic of solid and porous tungsten needle LMIS.
Can be enlarged to customer specification. Varying over the current–voltage characteristic.
Fig. 4. 4 4 LMIS cluster.
They also contain a focusing electrode to reduce the half-angle beam spread to 251 even at maximum currents. A 9-emitter cluster is presently under development for the LISA Pathfinder satellite that can provide thrusts up to 100 mN. 3.2. Lifetime challenges
Fig. 2. Propellant tanks.
can influence the size of the Taylor cone by several mm which causes the spread in the electrical characteristics. The key to solve this problem was to have an ion source with a high electrical impedance in order to minimize the change in current for large voltage jumps. This was done by manufacturing very sharp tips (2–3 mm diameter) with tight manufacturing tolerances, which result in high impedance sources, and by implementing a pre-resistor before the emitter. The current– voltage characteristic of such a sharp-tip LMIS is shown in Fig. 3. The starting voltage is at around 2.5 kV and the impedance is close to 40 MO. Together with a pre-resistor of similar size, this results in typical current variations of only 710% across clusters of LMIS powered by a single HV power supply. Such clusters were built up to a size of 4 4 emitters in compact module housings (see Fig. 4).
Many endurance tests were carried out in order to test the lifetime of our ion sources. One pre-requisite was to make our vacuum facilities compatible for such tests. The obvious limitation here is the severe ion beam backsputtering. We implemented a collector with fins on the back side in order to avoid direct backsputtering. In addition, the collector fins were covered with indium to limit backsputtering of non-indium particles which could contaminate our ion sources. During testing, two lifetime limitations were found:
In addition to the usual ions, weakly charged microdroplets which can clog the extractor electrode and cause a shortcut are emitted. After modelling the droplet deposition, it was decided to greatly enlarge the extractor diameter to about 8 mm in order to create enough room to account for the deposition. Furthermore, we had to implement a negatively biased cover electrode in order to repel secondary electrodes from testing chambers and electrons from the plasma environment which
ARTICLE IN PRESS M. Tajmar et al. / Ultramicroscopy 109 (2009) 442–446
would otherwise react to the high positive potential from the ion source. The most severe lifetime limitation was found to be a slow erosion process of the tip. This erosion could be traced back to microsparks between emitter and extractor (a typical feature of LMIS) which heat up the emitter tip locally and cause dissolution of the tungsten tip into the liquid indium. The preresistor implemented for our cluster design helped to reduce the spark energies and therefore to slow down the erosion process. Details on the erosion process, models and verifications tests can be found in Ref. [12].
Proper sizing of the pre-resistor as well as large extractor electrodes enabled us to design a FEEP thruster fulfilling the challenging lifetime requirements. We successfully demonstrated a test of more than 4800 h of continuous operation for two emitters and a test of 3000 h for a 4 4 cluster equipped with 8 emitters (the first 2000 h with 8 emitters and then the rest 1000 h with 6 emitters since 2 emitters were kept for analysis purposes)
445
[13,14]. Our models indicate that such a thruster could be operated in principle up to 20,000 h or more. 3.3. High-current porous tungsten source In order to further increase the number of emitters and to make the overall module more compact, a new type of highcurrent ion source was developed based on porous tungsten. There are two basic types of emitter geometries for LMIS: needleand capillary-based emitters. The needle emitters have usually much larger electrical impedances compared to the capillary emitter which makes them suitable for clustering (as discussed). However, capillary emitters are usually more stable and reliable. We therefore decided to design a multi-tip emitter using porous tungsten needles in order to combine the advantage of high electrical impedance with excellent stability. The emitter is manufactured using the Micro-Powder Injection Moulding technique from the ARC Powder Technology Center [6]. A special feedstock is prepared using tungsten micropowders and
Fig. 5. Porous tungsten source: porous needle tip (left), filled and wet with indium (middle), 28-needle crown emitter (right).
ARTICLE IN PRESS 446
M. Tajmar et al. / Ultramicroscopy 109 (2009) 442–446
the crown significantly differs from the one of a single needle. This also translates in higher power-to-thrust ratios that may be compensated by negatively biasing the extractor electrode if necessary.
4. Conclusion ARC developed a variety of LMIS for space applications emitting currents from 10 mA to several mA for lifetimes of 20,000 h. Our applications include active spacecraft compensators, secondary ion mass spectrometers as well as micropropulsion. Our latest development is a high-current porous multi-emitter that promises exceptional stability that can also trigger terrestrial applications such as high-current FIB manufacturing. Fig. 6. Current–voltage characteristic of porous crown emitter with 28 needle tips.
References the needle structure is formed using dedicated moulding tools. After sintering, the porous emitter structure can be etched similarly to the traditional solid tungsten needle emitters. Currently, 85% porosity and micron-sized pores can be achieved. We designed a 28-emitter crown emitter which can produce currents in the range of 3 mA (corresponding to about 350 mN). Our test data indicated an exceptional operational stability of the ion source and show no measurable change in performance after more than 20 thermal cycles and exposure to oxygen. This behaviour is attributed to the excellent capillary properties of the porous needle itself. Fig. 5 shows details of such a porous needle structure as well as the overall crown geometry. Fig. 3 compares the current–voltage characteristics in such a single porous needle with our standard solid tungsten needles and they appear to be quite similar. The slightly higher starting voltage and correspondingly lower impedance can be attributed to a blunter tip of 5 mm diameter since it is much harder to etch porous tungsten structures compared to solid ones which still leaves room for optimization. The current–voltage characteristic of a 28-emitter crown emitter is shown in Fig. 6. One immediately notices the higher operating voltages up to 15 kV compared to the usual 6 kV from single LMIS needles since the electric field configuration for
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