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Measurement of ion acceleration characteristics of a laser-electrostatic hybrid microthruster for space propulsion applications
Vacuum 83 (2009) 213–216
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Measurement of ion acceleration characteristics of a laser-electrostatic hybrid microthruster for space propulsion applications Tomohisa Ono a, *, Yasufumi Uchida a, Hideyuki Horisawa 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 Laser propulsion Hybrid acceleration Ion velocity measurement Micro propulsion
For space propulsion applications, a fundamental study of a laser-electrostatic hybrid thruster was conducted. A new type of thruster, in which a laser-produced plasma was accelerated by an additional electrostatic field, was tested to optimize the ion acceleration process. A time-of-flight measurement by a Faraday cup showed that the average speed of ions was about 15 km/s when only 0.04 mJ of laser impulse was introduced to a copper target. When an accelerator electrode with a 6-mm-diameter hole was placed in front of the laser target, it was observed that the average speed of ions increased. The maximum velocity was 23 km/s, which corresponded to the case where the accelerator grid was biased to þ100 V for the target-to-electrode gap of 2 mm. It was found that the positively biased electrode was more effective than the negatively biased electrode for ion acceleration in the thruster. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Small-sized onboard laser plasma thrusters are under significant development with rapid evolutions of compact but high power laser systems [1–3]. One of the advantages of such laser thrusters is the use of solid-state materials for the propellant. Since any solid material can be used for the propellant, tanks, valves, or piping systems, which are necessary for thrusters with liquid or gaseous propellant, are not required for the laser propulsion system. Therefore, the laser thruster system can be very simple and compact. Also, significant controllability of thrust is possible by changing the input laser power [4–6]. In order to further improve the thrust performances and system simplicities of conventional laser propulsion systems, a preliminary study on a laser-electric hybrid propulsion system was conducted. Our laser-electric hybrid acceleration system is depicted in Figs. 1 and 2. The basic idea of these systems is that a laser ablation plasma, induced by laser irradiation on a solid target, is additionally accelerated by an accelerator grid. Because the laser-ablated plasma has an initial velocity of about 10 km/s, if the plasma is further accelerated by an electrostatic method, specific impulses can be significantly increased [6]. In this study, to reveal the acceleration mechanism of the laserelectrostatic hybrid thruster, the plasma plume near the ablation target was characterized by a Langmuir probe. Also, to elucidate ion
* Corresponding author. Fax: þ81 463 50 2060. E-mail address: [email protected] (T. Ono). 0042-207X/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2008.03.098
acceleration characteristics, a time-of-flight measurement with a Faraday cup was conducted. 2. Experimental apparatus A schematic illustration of a laser-electrostatic hybrid thruster is shown in Fig. 2. The hybrid thruster consists of a Cu target (propellant), an acceleration electrode, and a laser oscillator. The Cu target was mounted on an X–Y stage to refresh irradiated surfaces. For the laser oscillator, an LD-pumped Nd:YAG microchip laser (JDS UNIPHASE, PowerChip Nanolaser, wavelength: 1064 nm, pulse energy: 50 mJ/pulse, pulse width: 250 ps, repetition rate: 1 kHz) was used. Laser ablation plasmas or ions were accelerated by the acceleration electrode made of a 0.3 mm-thick Cu plate with a hole of 6 mm in diameter. To elucidate the effects of the electrostatic field, the acceleration voltage (Vac) was changed from 100 V to þ100 V. Temporal evolutions of the plasma plume temperatures and the plasma densities were measured by an electrostatic probe, as shown in Fig. 3 [5–7]. The electrostatic single-probe consisting of a tungsten rod of 0.1 mm diameter and 1.8 mm length was placed at 5–50 mm away from the thruster (target plate position) along the centerline. Temporal changes of the probe currents were acquired by an oscilloscope (Tektronix, spectral width: 300 MHz, maximum sampling rate: 2.5 GS/s). Since the transient probe data were highly reproducible, it was possible to obtain the current– voltage characteristic at each moment by taking a temporal slice of a current–voltage characteristic. From the current–voltage curve, temporal evolutions of the plasma temperature and the
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T. Ono et al. / Vacuum 83 (2009) 213–216
Propellant tape
z Vac
Power supply
Probe or Faraday cup
Laser
a Propellant tape
f = 150 mm Accel. electrode
(1) Amp
Vacuum cham ber
B
10-6 Torr
Pressure: 2.0 -
j
Al mirror
(2)
Lase r -
LD-pumped Nd:YAG Microchip laser (1) Differential amplifier DC to 10MHz JDS UNIPHASE, PowerChip Nanolaser, wave length: λ = 1064 nm, pulse energy: 50 (2) Oscilloscope Tektronix, TDS3034B, sampling rate: μJ/ pulse, pulse width: 250 psec, repetition 2.5GS/s, frequency bandwidth: 300 MHz rate: 1 kHz
Laser
b
c
Fig. 1. Schematics of laser ablation thrusters. (a) Pure laser thruster, (b) laserelectromagnetic hybrid thruster, (c) laser-electric hybrid thruster.
plasma density were estimated based on the electrostatic probe theory [7,8]. The temporal evolution of the ion current exhausted from the thruster was also measured using a Faraday cup. The Faraday cup was placed 120 mm away from the surface of the ablation target. The ion current was amplified by an amplifier (DC to 10 MHz) and was monitored by an oscilloscope (Tektronix, TDS3034B, sampling rate: 2.5 GS/s, frequency bandwidth: 300 MHz). The ion velocity was estimated from the time of signal arrival to the Faraday cup.
Fig. 3. Experimental setup.
Temporal evolutions of electron temperature and plasma densities are also obtained by following the same procedure to plot the current–voltage curve in Fig. 4b. They are plotted in Fig. 5 for a pure laser ablation plasma; it is monitored at a position 30 mm away from the Cu target. It is seen that the electron temperature was 1z2 eV, and that the plasma density had a peak value of about 2 1016/m3 at 1.6 ms; the plasma density then gradually decreased down to 0.1 1016/m3 at 8 ms. Diagnosing the temporal evolutions of the electron density and temperature at several positions from 5 mm to 50 mm away from the target, a spatial distribution of the electron density along the
500
a
3.1. Probe measurement of pure laser ablation plasma Temporal evolutions of probe currents for various probe bias voltages are shown in Fig. 4a. In Fig. 4b, the probe current curves in Fig. 4a are sliced at 1.2 ms, for example, and a current–voltage curve is replotted. From the current–voltage characteristic in Fig. 4b, the electron temperature and the density can be derived based on the probe theory [7,8].
Probe current, μA
3. Results and discussion 0 -500
-20 V +5 V
-1000
+10 V +20 V
-1500 -2000 0
2
4
6
8
Time, µs 10
b
z
Vac
Laser-induced d plasma plasma
φ 6 mm
Probe current Ip, -mA
Target 1
0.1
0.01
0.001
0
5
10
15
20
Probe bias Vp, V Laser pulse
0.3 mm Accel. electrode & 0.5mm insulator
Fig. 2. Laser-electrostatic hybrid thruster.
Fig. 4. Typical outputs from an electrostatic probe (probe position: 30 mm from the ablation target). (a) Temporal evolutions of probe currents for various probe biasing voltages. (b) Typical plots of probe current and voltage characteristic replotted from the data at 1.2 ms.
T. Ono et al. / Vacuum 83 (2009) 213–216
4
250
Electron density Ne, ×1015m-3
a
Te, eV
3 Low
high 2
1
0
215
a
High
200
Low
150 100 50 0
0
2
4
6
0
8
10
Time, µsec 2.0
400
b
Debye length, μm
Ne, ×1016m-3
30
b
high
1.5
1.0 Low 0.5
0.0
20
40
50
40
50
Probe position, mm
High Low
300
200
100
0 0
2
4
6
0
8
10
Fig. 5. Temporal evolution of pure laser ablation plasma (probe position: 30 mm away from the ablation target). (a) Temporal evolution of the plasma density. (b) Temporal evolution of the electron temperature.
centerline can be plotted, as shown in Fig. 6a. Furthermore a spatial distribution of the Debye length can also be obtained from these results, as shown in Fig. 6b. Since the plasma density drastically changes in time, only the peak values and the inflection points (marked in Fig. 5b) in each spatial position are selected and then plotted as ‘‘high’’ and ‘‘low’’, respectively, in Fig. 6a and b. It can be seen that the peak electron density decreased toward the downstream region of the target and reached about 11.5 1015/m3 at 50 mm away from the target. On the other hand, the peak Debye length increased and reached about 300 mm at 50 mm away from the target.
20
30
Probe position, mm
Time, µsec
Fig. 6. Spatial distributions of electron density and Debye length for pure ablation plasma measured at several positions along the centerline (5–50 mm away from the target). ‘‘High’’ and ‘‘low’’ correspond to the peak and inflection points of the density profile shown in Fig. 5b. (a) Spatial distribution of electron density. (b) Spatial distribution of the Debye length.
Beyond the bias voltage of þ40 V, it was found that the velocity gradually increased for an increased voltage, and finally, the velocity reached 23 km/s for þ100 V. However, in the case where the target–electrode gap was 2 mm, the maximum velocity was 17 km/ s at þ100 V. Since the Debye length is smaller than the gap size in the region between the target and the electrode, some positive ions are likely to experience repulsive forces from the positively biased electrode at the outside of the outlet. This effect seems to be exaggerated in larger target–electrode gap cases due to the larger Debye length where its electric field influences to longer distances.
3.2. Ion velocity and density measurement 25
Average velocity, km/sec
The averaged ion velocity and the number density of ions measured by an energy analyzer for various acceleration voltages are plotted in Figs. 7 and 8. When the voltages applied between the target and the electrode were negative, almost no effect was observed in the ion velocities from the pure laser ablation (14 km/s). This result means that the negative electrode voltage was not effective for the acceleration of the laser-induced plasma. This is attributed to the fact that the target-to-electrode distances of 2 mm and 18 mm were much larger than the local Debye length, which is at most 150 mm even in the case where the electrode position is 18 mm. Therefore, the charged particles never experienced the electric field between the electrodes due to the Debye shielding effect. On the other hand, when the acceleration voltages were positive, an increase of ion velocity was observed for an increased voltage. Especially for a target–electrode gap of 18 mm, the plasma was accelerated up to about 18 km/s in a range of þ20 V to þ40 V.
20
AVE(2mm) AVE(18mm)
15 10 5 0 -100
-50
0
50
100
Accelerating voltage, V Fig. 7. The average ion speed for various acceleration voltages (target-to-electrode gap: AVE ¼ 2 mm or 18 mm).
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T. Ono et al. / Vacuum 83 (2009) 213–216
2 2mm 18mm
Ion number, ×108
1.6 1.2 0.8 0.4 0 -100
-50
0
50
100
Accelerating voltage, V Fig. 8. The ion number density for various acceleration voltages (target-to-electrode gap ¼ 2 mm or 18 mm).
In Fig. 8, it was found that the number density of ions decreased when the electrode was positively biased, especially in the larger target–electrode gap case. For a target–electrode gap of 2 mm and the biasing voltage of þ100 V, the number density of ions decreased down to 50% from the case without the biasing voltage. For a larger gap of 18 mm and the biasing voltage of þ100 V, the number density of ions decreased to 16%. Extrapolating the Debye length curve shown in Fig. 6b, the value for the target–electrode gap of 2 mm, or at the acceleration electrode, is about 15 mm. On the other hand, the Debye length for the target–electrode gap of 18 mm is about 150 mm. Therefore, it can be presumed that with a larger gap, or larger Debye length condition, the ions are more likely to be trapped by the acceleration electrode. 4. Conclusions A temporal evolution of the plasma distribution of a laserelectrostatic hybrid thruster was experimentally evaluated to find the optimum electrode geometry for plasma acceleration. For the 6-mm-diameter electrode that was placed in front of the laserablated target, the following results were obtained.
(1) The plasma density and the electron temperature profiles were obtained at positions of 5–50 mm from the ablated target. The Debye length was about 150 mm at a 5 mmposition from the ablation target, but at the downstream positions, the Debye length was much larger. Hence, to make the Debye length and the target-to-electrode gap comparable, the acceleration electrode should be located just in front of the ablation target. Otherwise, electrostatic acceleration is not possible. This explanation agrees with our experimental results that almost no velocity increase was observed with the negatively biased electrode, where positive ions are not likely to experience the attractive forces from the electrode in the gap. (2) It was shown that the positively biased acceleration electrode was more effective for ion acceleration than the negatively biased electrode. Further optimization is required to enhance the electrostatic acceleration in the laser-electrostatic hybrid thruster because the target-to-electrode gap can be tuned corresponding to the laserproduced plasma property on the target. References [1] Phipps C, Luke J. Diode laser-driven microthrusters: a new departure for micropropulsion. AIAA Journal 2000;40:310–8. [2] Pakhomov VA, Gregory DA. Ablative laser propulsion: an old concept revisited. AIAA Journal 2000;38:725–7. [3] Phipps C, Luke RJ, Lippert T, Hauer M, Wokaun A. Micropropulsion using a laser ablation jet. Journal of Propulsion and Power 2004;20:1000–11. [4] Horisawa H, Igari A, Kawakami M, Kimura I. Discharge characteristics of laserelectric hybrid thrusters. AIAA paper; 2004:3937. [5] Horisawa H, Kawakami M, Kimura I. Laser-assisted pulsed plasma thruster for space propulsion applications. Applied Physics A 2005;81:303–10. [6] Horisawa H, Sasaki K, Igari A, Kimura I. Laser-electric hybrid acceleration system for space propulsion applications. Review of Laser Engineering 2006; 34:435–41. [7] Teii S. Plasma basic engineering: augmented edition [in Japanese], vol. 1. Tokyo: Uchida Roukakuho; 1995. p. 112–203. [8] Uchida Y, Kino S, Horisawa H, Kimura I. Ion acceleration characteristics of laser-electric hybrid thrusters. In: Symposium on space transportation; 2006. p. 364–67 [in Japanese].
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Measurement of ion acceleration characteristics of a laser-electrostatic hybrid microthruster for space propulsion applications Tomohisa Ono a, *, Yasufumi Uchida a, Hideyuki Horisawa 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 Laser propulsion Hybrid acceleration Ion velocity measurement Micro propulsion
For space propulsion applications, a fundamental study of a laser-electrostatic hybrid thruster was conducted. A new type of thruster, in which a laser-produced plasma was accelerated by an additional electrostatic field, was tested to optimize the ion acceleration process. A time-of-flight measurement by a Faraday cup showed that the average speed of ions was about 15 km/s when only 0.04 mJ of laser impulse was introduced to a copper target. When an accelerator electrode with a 6-mm-diameter hole was placed in front of the laser target, it was observed that the average speed of ions increased. The maximum velocity was 23 km/s, which corresponded to the case where the accelerator grid was biased to þ100 V for the target-to-electrode gap of 2 mm. It was found that the positively biased electrode was more effective than the negatively biased electrode for ion acceleration in the thruster. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Small-sized onboard laser plasma thrusters are under significant development with rapid evolutions of compact but high power laser systems [1–3]. One of the advantages of such laser thrusters is the use of solid-state materials for the propellant. Since any solid material can be used for the propellant, tanks, valves, or piping systems, which are necessary for thrusters with liquid or gaseous propellant, are not required for the laser propulsion system. Therefore, the laser thruster system can be very simple and compact. Also, significant controllability of thrust is possible by changing the input laser power [4–6]. In order to further improve the thrust performances and system simplicities of conventional laser propulsion systems, a preliminary study on a laser-electric hybrid propulsion system was conducted. Our laser-electric hybrid acceleration system is depicted in Figs. 1 and 2. The basic idea of these systems is that a laser ablation plasma, induced by laser irradiation on a solid target, is additionally accelerated by an accelerator grid. Because the laser-ablated plasma has an initial velocity of about 10 km/s, if the plasma is further accelerated by an electrostatic method, specific impulses can be significantly increased [6]. In this study, to reveal the acceleration mechanism of the laserelectrostatic hybrid thruster, the plasma plume near the ablation target was characterized by a Langmuir probe. Also, to elucidate ion
* Corresponding author. Fax: þ81 463 50 2060. E-mail address: [email protected] (T. Ono). 0042-207X/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2008.03.098
acceleration characteristics, a time-of-flight measurement with a Faraday cup was conducted. 2. Experimental apparatus A schematic illustration of a laser-electrostatic hybrid thruster is shown in Fig. 2. The hybrid thruster consists of a Cu target (propellant), an acceleration electrode, and a laser oscillator. The Cu target was mounted on an X–Y stage to refresh irradiated surfaces. For the laser oscillator, an LD-pumped Nd:YAG microchip laser (JDS UNIPHASE, PowerChip Nanolaser, wavelength: 1064 nm, pulse energy: 50 mJ/pulse, pulse width: 250 ps, repetition rate: 1 kHz) was used. Laser ablation plasmas or ions were accelerated by the acceleration electrode made of a 0.3 mm-thick Cu plate with a hole of 6 mm in diameter. To elucidate the effects of the electrostatic field, the acceleration voltage (Vac) was changed from 100 V to þ100 V. Temporal evolutions of the plasma plume temperatures and the plasma densities were measured by an electrostatic probe, as shown in Fig. 3 [5–7]. The electrostatic single-probe consisting of a tungsten rod of 0.1 mm diameter and 1.8 mm length was placed at 5–50 mm away from the thruster (target plate position) along the centerline. Temporal changes of the probe currents were acquired by an oscilloscope (Tektronix, spectral width: 300 MHz, maximum sampling rate: 2.5 GS/s). Since the transient probe data were highly reproducible, it was possible to obtain the current– voltage characteristic at each moment by taking a temporal slice of a current–voltage characteristic. From the current–voltage curve, temporal evolutions of the plasma temperature and the
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T. Ono et al. / Vacuum 83 (2009) 213–216
Propellant tape
z Vac
Power supply
Probe or Faraday cup
Laser
a Propellant tape
f = 150 mm Accel. electrode
(1) Amp
Vacuum cham ber
B
10-6 Torr
Pressure: 2.0 -
j
Al mirror
(2)
Lase r -
LD-pumped Nd:YAG Microchip laser (1) Differential amplifier DC to 10MHz JDS UNIPHASE, PowerChip Nanolaser, wave length: λ = 1064 nm, pulse energy: 50 (2) Oscilloscope Tektronix, TDS3034B, sampling rate: μJ/ pulse, pulse width: 250 psec, repetition 2.5GS/s, frequency bandwidth: 300 MHz rate: 1 kHz
Laser
b
c
Fig. 1. Schematics of laser ablation thrusters. (a) Pure laser thruster, (b) laserelectromagnetic hybrid thruster, (c) laser-electric hybrid thruster.
plasma density were estimated based on the electrostatic probe theory [7,8]. The temporal evolution of the ion current exhausted from the thruster was also measured using a Faraday cup. The Faraday cup was placed 120 mm away from the surface of the ablation target. The ion current was amplified by an amplifier (DC to 10 MHz) and was monitored by an oscilloscope (Tektronix, TDS3034B, sampling rate: 2.5 GS/s, frequency bandwidth: 300 MHz). The ion velocity was estimated from the time of signal arrival to the Faraday cup.
Fig. 3. Experimental setup.
Temporal evolutions of electron temperature and plasma densities are also obtained by following the same procedure to plot the current–voltage curve in Fig. 4b. They are plotted in Fig. 5 for a pure laser ablation plasma; it is monitored at a position 30 mm away from the Cu target. It is seen that the electron temperature was 1z2 eV, and that the plasma density had a peak value of about 2 1016/m3 at 1.6 ms; the plasma density then gradually decreased down to 0.1 1016/m3 at 8 ms. Diagnosing the temporal evolutions of the electron density and temperature at several positions from 5 mm to 50 mm away from the target, a spatial distribution of the electron density along the
500
a
3.1. Probe measurement of pure laser ablation plasma Temporal evolutions of probe currents for various probe bias voltages are shown in Fig. 4a. In Fig. 4b, the probe current curves in Fig. 4a are sliced at 1.2 ms, for example, and a current–voltage curve is replotted. From the current–voltage characteristic in Fig. 4b, the electron temperature and the density can be derived based on the probe theory [7,8].
Probe current, μA
3. Results and discussion 0 -500
-20 V +5 V
-1000
+10 V +20 V
-1500 -2000 0
2
4
6
8
Time, µs 10
b
z
Vac
Laser-induced d plasma plasma
φ 6 mm
Probe current Ip, -mA
Target 1
0.1
0.01
0.001
0
5
10
15
20
Probe bias Vp, V Laser pulse
0.3 mm Accel. electrode & 0.5mm insulator
Fig. 2. Laser-electrostatic hybrid thruster.
Fig. 4. Typical outputs from an electrostatic probe (probe position: 30 mm from the ablation target). (a) Temporal evolutions of probe currents for various probe biasing voltages. (b) Typical plots of probe current and voltage characteristic replotted from the data at 1.2 ms.
T. Ono et al. / Vacuum 83 (2009) 213–216
4
250
Electron density Ne, ×1015m-3
a
Te, eV
3 Low
high 2
1
0
215
a
High
200
Low
150 100 50 0
0
2
4
6
0
8
10
Time, µsec 2.0
400
b
Debye length, μm
Ne, ×1016m-3
30
b
high
1.5
1.0 Low 0.5
0.0
20
40
50
40
50
Probe position, mm
High Low
300
200
100
0 0
2
4
6
0
8
10
Fig. 5. Temporal evolution of pure laser ablation plasma (probe position: 30 mm away from the ablation target). (a) Temporal evolution of the plasma density. (b) Temporal evolution of the electron temperature.
centerline can be plotted, as shown in Fig. 6a. Furthermore a spatial distribution of the Debye length can also be obtained from these results, as shown in Fig. 6b. Since the plasma density drastically changes in time, only the peak values and the inflection points (marked in Fig. 5b) in each spatial position are selected and then plotted as ‘‘high’’ and ‘‘low’’, respectively, in Fig. 6a and b. It can be seen that the peak electron density decreased toward the downstream region of the target and reached about 11.5 1015/m3 at 50 mm away from the target. On the other hand, the peak Debye length increased and reached about 300 mm at 50 mm away from the target.
20
30
Probe position, mm
Time, µsec
Fig. 6. Spatial distributions of electron density and Debye length for pure ablation plasma measured at several positions along the centerline (5–50 mm away from the target). ‘‘High’’ and ‘‘low’’ correspond to the peak and inflection points of the density profile shown in Fig. 5b. (a) Spatial distribution of electron density. (b) Spatial distribution of the Debye length.
Beyond the bias voltage of þ40 V, it was found that the velocity gradually increased for an increased voltage, and finally, the velocity reached 23 km/s for þ100 V. However, in the case where the target–electrode gap was 2 mm, the maximum velocity was 17 km/ s at þ100 V. Since the Debye length is smaller than the gap size in the region between the target and the electrode, some positive ions are likely to experience repulsive forces from the positively biased electrode at the outside of the outlet. This effect seems to be exaggerated in larger target–electrode gap cases due to the larger Debye length where its electric field influences to longer distances.
3.2. Ion velocity and density measurement 25
Average velocity, km/sec
The averaged ion velocity and the number density of ions measured by an energy analyzer for various acceleration voltages are plotted in Figs. 7 and 8. When the voltages applied between the target and the electrode were negative, almost no effect was observed in the ion velocities from the pure laser ablation (14 km/s). This result means that the negative electrode voltage was not effective for the acceleration of the laser-induced plasma. This is attributed to the fact that the target-to-electrode distances of 2 mm and 18 mm were much larger than the local Debye length, which is at most 150 mm even in the case where the electrode position is 18 mm. Therefore, the charged particles never experienced the electric field between the electrodes due to the Debye shielding effect. On the other hand, when the acceleration voltages were positive, an increase of ion velocity was observed for an increased voltage. Especially for a target–electrode gap of 18 mm, the plasma was accelerated up to about 18 km/s in a range of þ20 V to þ40 V.
20
AVE(2mm) AVE(18mm)
15 10 5 0 -100
-50
0
50
100
Accelerating voltage, V Fig. 7. The average ion speed for various acceleration voltages (target-to-electrode gap: AVE ¼ 2 mm or 18 mm).
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T. Ono et al. / Vacuum 83 (2009) 213–216
2 2mm 18mm
Ion number, ×108
1.6 1.2 0.8 0.4 0 -100
-50
0
50
100
Accelerating voltage, V Fig. 8. The ion number density for various acceleration voltages (target-to-electrode gap ¼ 2 mm or 18 mm).
In Fig. 8, it was found that the number density of ions decreased when the electrode was positively biased, especially in the larger target–electrode gap case. For a target–electrode gap of 2 mm and the biasing voltage of þ100 V, the number density of ions decreased down to 50% from the case without the biasing voltage. For a larger gap of 18 mm and the biasing voltage of þ100 V, the number density of ions decreased to 16%. Extrapolating the Debye length curve shown in Fig. 6b, the value for the target–electrode gap of 2 mm, or at the acceleration electrode, is about 15 mm. On the other hand, the Debye length for the target–electrode gap of 18 mm is about 150 mm. Therefore, it can be presumed that with a larger gap, or larger Debye length condition, the ions are more likely to be trapped by the acceleration electrode. 4. Conclusions A temporal evolution of the plasma distribution of a laserelectrostatic hybrid thruster was experimentally evaluated to find the optimum electrode geometry for plasma acceleration. For the 6-mm-diameter electrode that was placed in front of the laserablated target, the following results were obtained.
(1) The plasma density and the electron temperature profiles were obtained at positions of 5–50 mm from the ablated target. The Debye length was about 150 mm at a 5 mmposition from the ablation target, but at the downstream positions, the Debye length was much larger. Hence, to make the Debye length and the target-to-electrode gap comparable, the acceleration electrode should be located just in front of the ablation target. Otherwise, electrostatic acceleration is not possible. This explanation agrees with our experimental results that almost no velocity increase was observed with the negatively biased electrode, where positive ions are not likely to experience the attractive forces from the electrode in the gap. (2) It was shown that the positively biased acceleration electrode was more effective for ion acceleration than the negatively biased electrode. Further optimization is required to enhance the electrostatic acceleration in the laser-electrostatic hybrid thruster because the target-to-electrode gap can be tuned corresponding to the laserproduced plasma property on the target. References [1] Phipps C, Luke J. Diode laser-driven microthrusters: a new departure for micropropulsion. AIAA Journal 2000;40:310–8. [2] Pakhomov VA, Gregory DA. Ablative laser propulsion: an old concept revisited. AIAA Journal 2000;38:725–7. [3] Phipps C, Luke RJ, Lippert T, Hauer M, Wokaun A. Micropropulsion using a laser ablation jet. Journal of Propulsion and Power 2004;20:1000–11. [4] Horisawa H, Igari A, Kawakami M, Kimura I. Discharge characteristics of laserelectric hybrid thrusters. AIAA paper; 2004:3937. [5] Horisawa H, Kawakami M, Kimura I. Laser-assisted pulsed plasma thruster for space propulsion applications. Applied Physics A 2005;81:303–10. [6] Horisawa H, Sasaki K, Igari A, Kimura I. Laser-electric hybrid acceleration system for space propulsion applications. Review of Laser Engineering 2006; 34:435–41. [7] Teii S. Plasma basic engineering: augmented edition [in Japanese], vol. 1. Tokyo: Uchida Roukakuho; 1995. p. 112–203. [8] Uchida Y, Kino S, Horisawa H, Kimura I. Ion acceleration characteristics of laser-electric hybrid thrusters. In: Symposium on space transportation; 2006. p. 364–67 [in Japanese].