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High power light gas helicon plasma source for VASIMR
Thin Solid Films 506 – 507 (2006) 579 – 582 www.elsevier.com/locate/tsf
High power light gas helicon plasma source for VASIMR Jared P. Squire a,*, Franklin R. Chang-Dı´az a, Timothy W. Glover a, Verlin T. Jacobson a, Greg E. McCaskill a, D. Scott Winter a, F. Wally Baity b, Mark D. Carter b, Richard H. Goulding b a
Advanced Space Propulsion Laboratory (ASPL), Johnson Space Center, NASA, 13000 Space Center Blvd, Houston, TX 77059, USA b Oak Ridge National Laboratory (ORNL), Oak Ridge, TN 37831, USA Available online 20 December 2005
Abstract In the Advanced Space Propulsion Laboratory (ASPL) helicon experiment (VX-10) we have measured a plasma flux to input gas rate ratio near 100% for both helium and deuterium at power levels up to 10 kW. Recent results at Oak Ridge National Laboratory (ORNL) show enhanced efficiency operation with a high power density, over 5 kW in a 5 cm diameter tube. Our helicon is presently 9 cm in diameter and operates up to 10 kW of input power. The data here uses a Boswell double-saddle antenna design with a magnetic cusp just upstream of the antenna. Similar to ORNL, for deuterium at near 10 kW, we find an enhanced performance of operation at magnetic fields above the lower hybrid matching condition. D 2005 Elsevier B.V. All rights reserved. Keywords: Experimental methods; Helicon; Plasma propulsion
1. Introduction The Variable Specific Impulse Magnetoplasma Rocket (VASIMR) [1] space propulsion development effort relies on a high power (> 10 kW) helicon plasma source [2,3] to produce a dense (¨ 1019 m 3) flowing plasma (H, D and He) target for Ion Cyclotron Resonance Frequency (ICRF) acceleration of the ions [4]. Subsequent expansion in an expanding magnetic field (magnetic nozzle) converts ion kinetic energy to directed momentum. This plasma source must have critical features to enable an effective propulsion device. First, it must ionize most of the input neutral flow of gas, thus producing a plasma stream with a high degree of ionization for the application of ICRF power. This avoids propellant waste and potential power losses due to charge exchange. This has previously been demonstrated [5]. Next, the plasma stream must flow into a region of high magnetic field (¨ 0.5 T) for efficient ICRF acceleration. Third, the ratio of input power to plasma flux must be low, providing an energy per ion –electron pair approaching 100 eV. Lastly,
* Corresponding author. 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.08.061
the source must be robust, powerful (potentially >100 kW) and capable of very long lifetimes (years). Recent results at Oak Ridge National Laboratory (ORNL) [6] show an enhanced efficiency mode of operation with a high power density, over 5 kW in a 5 cm diameter tube. This result motivated the Advanced Space Propulsion Laboratory (ASPL) to explore this regime of operation. We are just beginning this high power density mode of operation in the VX-10 experiment. This paper presents recent results with 10 kW of operation in helium and deuterium discharges.
2. Experiment Fig. 1 shows a schematic of the VX-10 experimental system. The system consists of four cryogenic electromagnet coils integrated into the vacuum chamber. Fig. 2 shows typical magnetic field axial profiles for deuterium discharges and locates the positions of the antennas and probe. A quartz tube passes down the axis inside the vacuum and is sealed against the port cover on the upstream end. A gas choke (0.035 m inner diameter and 0.15 m long) located near the magnetic field maximum greatly reduces
580
J.P. Squire et al. / Thin Solid Films 506 – 507 (2006) 579 – 582
3. Results
Fig. 1. A schematic of the VX-10 device in the ASPL.
the neutral gas flow from the helicon to the main chamber. A 25 MHz, 10 kW source drives a water-cooled Boswelltype double-saddle antenna (10 cm long) located inside the vacuum around the quartz tube and roughly centered under the second magnet coil. Gas (He or D2) flows into the upstream end of the quartz tube (0.09 m inner diameter and 1.06 m long), which defines z = 0 in our coordinate system. The discharges are pulsed for approximately 2 sec to keep the pressure in the exhaust region below 10 4 Torr. Data presented here are measured downstream of the peak magnetic field in the exhaust. We use a reciprocating Langmuir triple probe to measure plasma density profiles. The probe has been calibrated against the line-integrated density from a microwave interferometer. We calculate the total plasma flux by integrating the probe ion saturation current radial profiles, which is very insensitive to errors in the electron temperature measurement. To carry out ICRF acceleration experiments, an antenna designed to couple to the ICRF slow wave is installed downstream of the helicon plasma source. The plasma loading on this antenna is approximately proportional to the plasma density, so the production from the helicon. For this paper we use the loading as a measure of helicon performance for deuterium discharges, since the Langmuir probe was not operating well for that data set. The loading is measured using a resonant circuit with a vacuum resistance of 0.24 V, and a network analyzer.
Fig. 2. Axial magnetic field profile for a typical deuterium discharge. This includes a scan of the reversed magnetic field strength.
Gas utilization is critical for rocket efficiency. We define this quantity as the ratio of plasma flux to gas input rate, g g K C p/C g. We observe similar behavior as we have in the past [5] with lower power (¨ 3 kW) and a smaller diameter (7.5 cm) source. Fig. 3a shows gas input scans for two fixed power levels, 6.5 and 10 kW. We see that the plasma flux saturates at a lower value for lower power and has a linear behavior below the saturation. For 10 kW, the data is near the g g = 1 line at a flux of about 6 1019/sec. In Fig. 3b we show a rf power scan for a fixed input gas flow rate of 7 1019/sec. This flow rate is above the saturation points in Fig. 3a, so the plasma flux is linear with power. We observe no mode jumps with power in this range, but the plasma flux is sensitive to the magnetic field strength, and we visually see a bright core, which indicates a helicon mode. For deuterium discharges at lower power, ¨ 3 kW, we normally find a narrow optimum in the performance with the lower hybrid resonance [7] (x LH/x = 1) at the downstream end of the antenna [8]. With the ICRF plasma loading as a performance indicator, Fig. 4 shows enhanced performance at approximately twice the magnetic field
Fig. 3. (a) Helium gas input flow rate scan for two fixed input powers. (b) Input power scan for a fixed gas input flow rate.
J.P. Squire et al. / Thin Solid Films 506 – 507 (2006) 579 – 582
Fig. 4. ICRF plasma loading measured during a scan of the magnetic field at the helicon antenna with deuterium gas. The magnetic field is held constant at the ICRF antenna, with a vacuum resistance of 0.24 V.
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Fig. 6. Langmuir probe ion saturation current profiles for optimal helium and deuterium discharges.
strength. Comparing a single plasma loading data point at lower power, the loading value is approximately proportional to input power. At about twice the magnetic field of the lower power optimum, the loading starts to drop, indicating a more efficient power balance limit. The plasma, however, is starting to scrape off on the gas choke, as measured by heating there. For deuterium, we also observe saturation of the plasma flux with input helicon power that indicates that we are near g g = 1 and the discharge is gas limit. This is achieved with nearly the same input atom rate, 6 1019/sec. Fig. 5 shows a power scan for this fixed input gas rate. The plasma flux indicates abrupt saturation at about 8.5 kW and even decreases as we over drive it somewhat. (The behavior of deuterium is typically more abrupt than for helium.) When we compare probe ion saturation current profiles, a measure of plasma flux, for helium and deuterium at high power, we find similar values. Fig. 6 shows such a comparison for optimal discharges.
Another interesting topic is the role of the magnetic cusp. We have explored this at lower power [8] by scanning the strength of the reversed field and maintaining the field strength at the center of the antenna constant, as shown in Fig. 2. Fig. 7 contains density profiles for a similar experiment, at high power, for deuterium discharges. For this case, the profiles become hollow as we reduce the reversed field strength. As previously stated, we have difficulty with plasma startup when the amplitude of the reversed field becomes small. As before, the upstream cusp substantially increases the plasma flux in the downstream direction.
Fig. 5. Input power scan for a fixed deuterium gas flow rate (3 1019 D2/sec).
Fig. 7. Plasma density profiles for a scan of the reversed field strength with a high power (8 kW) deuterium discharge.
4. Discussion The ASPL has recently begun helicon operations with an excess of 10 kW rf of power. The helicon operates stable with both helium and deuterium and produces good
582
J.P. Squire et al. / Thin Solid Films 506 – 507 (2006) 579 – 582
target discharges for ICRF experimentation. We are able to produce similar plasma fluxes for both gases with a high degree of ionization. This is important for rocket efficiency and to minimize charge exchange losses in the ICRF stage. For deuterium at high power, we find that we can operate the helicon at substantially higher magnetic field than the lower hybrid matching condition. This is similar to the results found at ORNL [6]. Finally, we find that a magnet cusp upstream of our Boswell type antenna is still necessary to produce a peaked high density discharge downstream. Presently, the Boswell antenna is being replaced by a half-turn helical type, of the same size and at the same location. Since this antenna is directional, a computational model [9] predicts the cusp will not play a strong role in this configuration. We will soon have a direct comparison of the two antenna structures with a power level of 10 kW.
References [1] [2] [3] [4] [5]
[6] [7] [8]
[9]
F.R. Chang Diaz, Sci. Am. 283 (2000) 72. R.W. Boswell, F.F. Chen, IEEE Trans. Plasma Sci. 25 (1997) 1229. F.F. Chen, R.W. Boswell, IEEE Trans. Plasma Sci. 25 (1997) 1245. A.V. Arefiev, B.N. Breizman, Phys. Plasmas 11 (2004) 2942. J.P. Squire, F.R. Chang-Diaz, V.T. Jacobson, T.W. Glover, F.W. Baity, R.H. Goulding, R. Bengtson, E.A. Bering III, K.A. Stokke, 28th International. Electric Propulsion Conference, Toulouse, France, 2003. Y. Mori, H. Nakashima, F.W. Baity, R.H. Goulding, M.D. Carter, D.O. Sparks, Plasma Sources Sci. Technol. 13 (2004) 424. T. Stix, Waves in Plasmas, American Institute of Physics, New York, NY, 1992. J.P. Squire, F.R. Chang-Dı´az, V.T. Jacobson, T.W. Glover, F.W. Baity, M.D. Carter, R.H. Goulding, R.D. Bengtson, E.A. Bering III, in: C.B. Forest (Ed.), 15th Topical Conference in Radio Frequency Power in Plasmas, Moran, Wyoming, USA, May 19 – 21, AIP Conference Proceedings, vol. 694, 2003, p. 423. M.D. Carter, F.W. Baity Jr., G.C. Barber, R.H. Goulding, Y. Mori, D.O. Sparks, K.F. White, E.F. Jaeger, F.R. Chang-Dı´az, J.P. Squire, Phys. Plasmas 9 (2002) 5097.
High power light gas helicon plasma source for VASIMR Jared P. Squire a,*, Franklin R. Chang-Dı´az a, Timothy W. Glover a, Verlin T. Jacobson a, Greg E. McCaskill a, D. Scott Winter a, F. Wally Baity b, Mark D. Carter b, Richard H. Goulding b a
Advanced Space Propulsion Laboratory (ASPL), Johnson Space Center, NASA, 13000 Space Center Blvd, Houston, TX 77059, USA b Oak Ridge National Laboratory (ORNL), Oak Ridge, TN 37831, USA Available online 20 December 2005
Abstract In the Advanced Space Propulsion Laboratory (ASPL) helicon experiment (VX-10) we have measured a plasma flux to input gas rate ratio near 100% for both helium and deuterium at power levels up to 10 kW. Recent results at Oak Ridge National Laboratory (ORNL) show enhanced efficiency operation with a high power density, over 5 kW in a 5 cm diameter tube. Our helicon is presently 9 cm in diameter and operates up to 10 kW of input power. The data here uses a Boswell double-saddle antenna design with a magnetic cusp just upstream of the antenna. Similar to ORNL, for deuterium at near 10 kW, we find an enhanced performance of operation at magnetic fields above the lower hybrid matching condition. D 2005 Elsevier B.V. All rights reserved. Keywords: Experimental methods; Helicon; Plasma propulsion
1. Introduction The Variable Specific Impulse Magnetoplasma Rocket (VASIMR) [1] space propulsion development effort relies on a high power (> 10 kW) helicon plasma source [2,3] to produce a dense (¨ 1019 m 3) flowing plasma (H, D and He) target for Ion Cyclotron Resonance Frequency (ICRF) acceleration of the ions [4]. Subsequent expansion in an expanding magnetic field (magnetic nozzle) converts ion kinetic energy to directed momentum. This plasma source must have critical features to enable an effective propulsion device. First, it must ionize most of the input neutral flow of gas, thus producing a plasma stream with a high degree of ionization for the application of ICRF power. This avoids propellant waste and potential power losses due to charge exchange. This has previously been demonstrated [5]. Next, the plasma stream must flow into a region of high magnetic field (¨ 0.5 T) for efficient ICRF acceleration. Third, the ratio of input power to plasma flux must be low, providing an energy per ion –electron pair approaching 100 eV. Lastly,
* Corresponding author. 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.08.061
the source must be robust, powerful (potentially >100 kW) and capable of very long lifetimes (years). Recent results at Oak Ridge National Laboratory (ORNL) [6] show an enhanced efficiency mode of operation with a high power density, over 5 kW in a 5 cm diameter tube. This result motivated the Advanced Space Propulsion Laboratory (ASPL) to explore this regime of operation. We are just beginning this high power density mode of operation in the VX-10 experiment. This paper presents recent results with 10 kW of operation in helium and deuterium discharges.
2. Experiment Fig. 1 shows a schematic of the VX-10 experimental system. The system consists of four cryogenic electromagnet coils integrated into the vacuum chamber. Fig. 2 shows typical magnetic field axial profiles for deuterium discharges and locates the positions of the antennas and probe. A quartz tube passes down the axis inside the vacuum and is sealed against the port cover on the upstream end. A gas choke (0.035 m inner diameter and 0.15 m long) located near the magnetic field maximum greatly reduces
580
J.P. Squire et al. / Thin Solid Films 506 – 507 (2006) 579 – 582
3. Results
Fig. 1. A schematic of the VX-10 device in the ASPL.
the neutral gas flow from the helicon to the main chamber. A 25 MHz, 10 kW source drives a water-cooled Boswelltype double-saddle antenna (10 cm long) located inside the vacuum around the quartz tube and roughly centered under the second magnet coil. Gas (He or D2) flows into the upstream end of the quartz tube (0.09 m inner diameter and 1.06 m long), which defines z = 0 in our coordinate system. The discharges are pulsed for approximately 2 sec to keep the pressure in the exhaust region below 10 4 Torr. Data presented here are measured downstream of the peak magnetic field in the exhaust. We use a reciprocating Langmuir triple probe to measure plasma density profiles. The probe has been calibrated against the line-integrated density from a microwave interferometer. We calculate the total plasma flux by integrating the probe ion saturation current radial profiles, which is very insensitive to errors in the electron temperature measurement. To carry out ICRF acceleration experiments, an antenna designed to couple to the ICRF slow wave is installed downstream of the helicon plasma source. The plasma loading on this antenna is approximately proportional to the plasma density, so the production from the helicon. For this paper we use the loading as a measure of helicon performance for deuterium discharges, since the Langmuir probe was not operating well for that data set. The loading is measured using a resonant circuit with a vacuum resistance of 0.24 V, and a network analyzer.
Fig. 2. Axial magnetic field profile for a typical deuterium discharge. This includes a scan of the reversed magnetic field strength.
Gas utilization is critical for rocket efficiency. We define this quantity as the ratio of plasma flux to gas input rate, g g K C p/C g. We observe similar behavior as we have in the past [5] with lower power (¨ 3 kW) and a smaller diameter (7.5 cm) source. Fig. 3a shows gas input scans for two fixed power levels, 6.5 and 10 kW. We see that the plasma flux saturates at a lower value for lower power and has a linear behavior below the saturation. For 10 kW, the data is near the g g = 1 line at a flux of about 6 1019/sec. In Fig. 3b we show a rf power scan for a fixed input gas flow rate of 7 1019/sec. This flow rate is above the saturation points in Fig. 3a, so the plasma flux is linear with power. We observe no mode jumps with power in this range, but the plasma flux is sensitive to the magnetic field strength, and we visually see a bright core, which indicates a helicon mode. For deuterium discharges at lower power, ¨ 3 kW, we normally find a narrow optimum in the performance with the lower hybrid resonance [7] (x LH/x = 1) at the downstream end of the antenna [8]. With the ICRF plasma loading as a performance indicator, Fig. 4 shows enhanced performance at approximately twice the magnetic field
Fig. 3. (a) Helium gas input flow rate scan for two fixed input powers. (b) Input power scan for a fixed gas input flow rate.
J.P. Squire et al. / Thin Solid Films 506 – 507 (2006) 579 – 582
Fig. 4. ICRF plasma loading measured during a scan of the magnetic field at the helicon antenna with deuterium gas. The magnetic field is held constant at the ICRF antenna, with a vacuum resistance of 0.24 V.
581
Fig. 6. Langmuir probe ion saturation current profiles for optimal helium and deuterium discharges.
strength. Comparing a single plasma loading data point at lower power, the loading value is approximately proportional to input power. At about twice the magnetic field of the lower power optimum, the loading starts to drop, indicating a more efficient power balance limit. The plasma, however, is starting to scrape off on the gas choke, as measured by heating there. For deuterium, we also observe saturation of the plasma flux with input helicon power that indicates that we are near g g = 1 and the discharge is gas limit. This is achieved with nearly the same input atom rate, 6 1019/sec. Fig. 5 shows a power scan for this fixed input gas rate. The plasma flux indicates abrupt saturation at about 8.5 kW and even decreases as we over drive it somewhat. (The behavior of deuterium is typically more abrupt than for helium.) When we compare probe ion saturation current profiles, a measure of plasma flux, for helium and deuterium at high power, we find similar values. Fig. 6 shows such a comparison for optimal discharges.
Another interesting topic is the role of the magnetic cusp. We have explored this at lower power [8] by scanning the strength of the reversed field and maintaining the field strength at the center of the antenna constant, as shown in Fig. 2. Fig. 7 contains density profiles for a similar experiment, at high power, for deuterium discharges. For this case, the profiles become hollow as we reduce the reversed field strength. As previously stated, we have difficulty with plasma startup when the amplitude of the reversed field becomes small. As before, the upstream cusp substantially increases the plasma flux in the downstream direction.
Fig. 5. Input power scan for a fixed deuterium gas flow rate (3 1019 D2/sec).
Fig. 7. Plasma density profiles for a scan of the reversed field strength with a high power (8 kW) deuterium discharge.
4. Discussion The ASPL has recently begun helicon operations with an excess of 10 kW rf of power. The helicon operates stable with both helium and deuterium and produces good
582
J.P. Squire et al. / Thin Solid Films 506 – 507 (2006) 579 – 582
target discharges for ICRF experimentation. We are able to produce similar plasma fluxes for both gases with a high degree of ionization. This is important for rocket efficiency and to minimize charge exchange losses in the ICRF stage. For deuterium at high power, we find that we can operate the helicon at substantially higher magnetic field than the lower hybrid matching condition. This is similar to the results found at ORNL [6]. Finally, we find that a magnet cusp upstream of our Boswell type antenna is still necessary to produce a peaked high density discharge downstream. Presently, the Boswell antenna is being replaced by a half-turn helical type, of the same size and at the same location. Since this antenna is directional, a computational model [9] predicts the cusp will not play a strong role in this configuration. We will soon have a direct comparison of the two antenna structures with a power level of 10 kW.
References [1] [2] [3] [4] [5]
[6] [7] [8]
[9]
F.R. Chang Diaz, Sci. Am. 283 (2000) 72. R.W. Boswell, F.F. Chen, IEEE Trans. Plasma Sci. 25 (1997) 1229. F.F. Chen, R.W. Boswell, IEEE Trans. Plasma Sci. 25 (1997) 1245. A.V. Arefiev, B.N. Breizman, Phys. Plasmas 11 (2004) 2942. J.P. Squire, F.R. Chang-Diaz, V.T. Jacobson, T.W. Glover, F.W. Baity, R.H. Goulding, R. Bengtson, E.A. Bering III, K.A. Stokke, 28th International. Electric Propulsion Conference, Toulouse, France, 2003. Y. Mori, H. Nakashima, F.W. Baity, R.H. Goulding, M.D. Carter, D.O. Sparks, Plasma Sources Sci. Technol. 13 (2004) 424. T. Stix, Waves in Plasmas, American Institute of Physics, New York, NY, 1992. J.P. Squire, F.R. Chang-Dı´az, V.T. Jacobson, T.W. Glover, F.W. Baity, M.D. Carter, R.H. Goulding, R.D. Bengtson, E.A. Bering III, in: C.B. Forest (Ed.), 15th Topical Conference in Radio Frequency Power in Plasmas, Moran, Wyoming, USA, May 19 – 21, AIP Conference Proceedings, vol. 694, 2003, p. 423. M.D. Carter, F.W. Baity Jr., G.C. Barber, R.H. Goulding, Y. Mori, D.O. Sparks, K.F. White, E.F. Jaeger, F.R. Chang-Dı´az, J.P. Squire, Phys. Plasmas 9 (2002) 5097.