Development of VASIMR Helicon Source

Transcription

1 Development of VASIMR Helicon Source Verlin T. Jacobson, Franklin R. Chang Díaz, Jared P. Squire, Greg E., McCaskill, James E. McCoy, Andrew J. Petro, D. Scott Winter and Hugh M. Jamison Advanced Space Propulsion Lab. NASA/JSC. Houston, TX F. Wally Baity Oak Ridge National Laboratory, Oak Ridge, TN Roger D. Bengtson The University of Texas at Austin, Austin, TX Edgar A. Bering The University of Houston, Houston, TX Tim W. Glover Rice University, Houston, TX 43 nd Annual Meeting of the APS Division of Plasma Physics Mini-Conference on Helicon Sources Long Beach, California

2 Abstract The Variable Specific Impulse Magnetoplasma Rocket (VASIMR) is an RF driven plasma thruster concept with an open magnetic field configuration currently under development at JSC/NASA. Its design allows for a high energy density, high variable exhaust velocity plasma to achieve optimum performance with minimal impact to physical surfaces. To this end, a Helicon plasma source is used to generate high density plasma with modest magnetic fields (<.1T) and power input (3 kw, 25 MHz). The plasma is further heated with ICRH, and thrust is achieved via a magnetic nozzle. Unique to the VASIMR Helicon design is the need to generate a flowing plasma in light gasses (H 2, D 2, He). Current operation produces high density discharges (>1 12 cm -3 ) in all propellants of interest, as well as high exhaust velocity (>1 km/s) at high Mach numbers (M>1).

3 VASIMR System 1st stage: helicon plasma generator 2nd stage: ion cyclotron resonance power amplifier 3rd stage: magnetic nozzle

4 Advantages and future potential Electrodeless design implies high power density. Propellant is cheap and plentiful; chemical forms (Ammonia, Methane, etc.) may be easy to store and produce in-situ. Continuous acceleration (very low artificial g). Hydrogen provides the best radiation shield to GCR and SPEs. Operational flexibility with variable thrust and specific impulse. Very fast human and robotic missions throughout the solar system are possible. Solar electric applications for near Earth environment and high power nuclear electric for planetary exploration (far from the Sun.) Medium 12 MW electric power enables short (115 days) human Mars missions with large (61 MT) payloads. Enables power-rich mission architectures for human survival.

5 VASIMR Development Roadmap A variety of space flight experiments of increasing capability are envisioned

6 VASIMR Demonstration on ISS Pulsed (~ 1 min) 25 kw VASIMR test platform RF Amplifier Set (1 of 4) Radiator Benefits to ISS: ISS becomes adv.technology test bed Dramatic reduction of propellant requirements for reboost Improves µg environment Serves as plasma contactor Thruster Core Propellant Tanks Rechargeable Batteries Approximate Dimensions of Thruster Core: Diameter <.5 m Length < 1 m POTENTIAL ISS LOCATION Attachment at external payload site on P3 (or S3) truss segment shown

7 Fast Mission Architecture High thrust Earth spiral Heliocentric trajectory Mars orbit insertion IMLEO = 188 mt Payload = 6.8 mt Propellant = 7 mt hydrogen 12 MW nuclear electric a = 4 kg/kw 6% efficiency Isp profile for 115 day trip

8 Full-Up Aborts

9 Cutaway view of VX-1 VASIMR: Variable Specific Impulse Magnetoplasma Rocket electromagnets Vacuum Chamber Volume ~ 5 m 3 Pumping ~ 5 l/s ICRH antenna helicon antenna gas injection quartz tube

10 VX-1 Experiment at JSC Diffusion Pumps Vacuum Chamber Cryo magnets Gas Injector ICRH feeds Plasma Source

11 VX-1 Development &Testing

12 Achieved Steady-State Operation with Light Gases High density (~1e19 m -3 peak), stable plasma discharges in hydrogen, deuterium, helium and gas mixtures are now routine. Over 3% of the injected gas is accounted for in the plasma flow. Gas tube geometry is important for performance, longer tube shows improvement.

13 13.56 vs 25 MHz on He Discharge parameters are 2 kw, 325 sccm and.79 T mirror field. Both frequencies have similar ion rates MHz has higher Mach number MHz performs better at lower field as expected, but both are much higher than ω LH. Ion Rate (/s) <Mach Number> 14 x vs 25 MHz at 2 kw, 325 sccm, Bmirror =.79 T Helicon Field (T) MHz 25 MHz ω LH for 25 MHz Helicon Field (T)

14 13.56 vs 25 MHz on He 15 x MHz yields similar average density for a given field. <ne>[/m 3 ] Helicon Field (T) 8 25 MHz gives somewhat higher temperature. <Te>[eV] Helicon Field (T)

15 13.56 vs 25 MHz on D2 Ion rate significantly higher with MHz MHz operates significantly above ω LH, though data shows peaks that could be D+ and H+ resonance's. Mach probe data strongly indicates that flow is supersonic regardless of probe model. Ion Rate (/s) <Mach Number> x Helicon Field (T) ω LH for MHz ω LH for 25 MHz Helicon Field (T)

16 13.56 vs 25 MHz on D2 8 x MHz gives significantly higher density. <ne>[/m 3 ] MHz gives higher temperature, similar to He discharges. <Te>[eV] Helicon Field (T) Helicon Field (T)

17 Plasma Effects on Neutral Pressure We add a pitot tube to measure the pressure inside of the quartz tube just upstream of the antenna. Pressure measurements upstream and downstream show effects of neutral back pressure and pumping. Injector pressure varies with plasma performance and probe insertions are -5 clearly seen Pressure [mtorr] gas on triple probe enters rf on Baratron Pressure Mach probe enters time [ms] rf off no plasma T.32 T.52 T.73 T.94 T 1.15 T 1.3 T Pressure measurement.8m.73m 7 x T.32 T.73 T 1.15 T Chamber Pressure no plasma 1/4" OD S.S..1" wall tube 12" long Welded into a flange near the pressure measurement. 5 rf off 4.15m.5m P [torr] 3 Mach probe enters 2.16m quartz tube Antenna 1 rf on triple probe enters time [ms]

18 Axial Pressure Scan in 2 sccm He Two pitot tubes were placed on a sliding seal to obtain axial pressure measurements of static pressure and stagnation pressure to determine axial neutral pressure and neutral flow profiles. Pressure (mtorr) Downstream Upstream Distance (cm) Plasma On 1 8 Pressure (mtorr) 6 4 Plasma Off Mach Number Distance (cm) Distance (cm)

19 Effect of Antenna Position gas feed Pressure Sensor Quartz tube 5 cm dia H-alpha bolometer M1 M2 M3 Triple probe Ion gauge RPA Antenna position changed by 5 cm from previous configuration Antenna is now in a more uniform field Previous position RF source 3 kw 25 MHz Match box directional coupler Mach probe Downstream length of quartz tube is decreased

20 Effect of new antenna position on neutral flow rate Discharge parameters for new radial feed: 25 MHz, 3 kw RF at.58 T helicon field and.62 mirror field for D2. Parameters for previous axial feed: 25 MHz, 2kW RF at.76 T helicon field and.2 T mirror field D2. Optimal gas flow increased from 7 to 1 sccm for D2 Similar increases in flow rates observed in He and H2, ~ 3-5 sccm. Central density [m -3 ] 14 x New Position Old Position D2 gas flow [sccm]

21 Effects of Mirror Field 2 Old position: 2 kw, 25 MHz, 8 sccm,.3 T Helicon Field for H2 Ion Rate [1 19 /s] New position: 3 kw, 25 MHz, 13 sccm,.3 T Helicon field for H2 * Dramatic difference in required mirror field. Typical mirror field needed is greater than.5 T for all gases in present configuration Mirror Field [T]

22 Axial Mach probe Measurement Mach probe positioned to measure plasma upstream and downstream of mirror field gas feed Pressure Sensor Quartz tube 5 cm dia H-alpha bolometer M1 M2 M3 Triple probe Ion gauge RPA Field Ratio B down /B up ~7% RF source 3 kw 25 MHz Match box directional coupler Mach probe Mach probe

23 Mach Probe Consists of two pins 3mm in length, 1mm in diameter and separated by a stainless steel divider 5mm long Pins are aligned along the same field line and biased into ion saturation Plasma is free to stream to the upstream pin, but must diffuse across the magnetic field and be accelerated back to the downstream pin by the probe sheath The ratio of upstream and downstream saturation current gives an indication of plasma velocity

24 Results of axial measurement 1.5 He data with.2 T helicon field and 25 sccm. Upper and lower bounds indicate difference of plasma viscosity in probe models Downstream flow velocity is a factor of 2 higher than upstream velocity, probably due to acceleration by ambipolar field. Profile averaged downstream flow velocity is close to Mach 1. Mach number 1.5 Upstream Magnetic Mirror field [T] Downstream

25 Results of Axial Measurement Ion rate calculated from Mach probe data using Γ α M*I sat Upstream density calculated using I sat from Mach probe and downstream T e. Upstream density is greater than downstream density by factor of 3. Too large for field line mapping alone. Effective probe area may be changing causing an underestimation of output.

26 Effects of Non-uniform Field Optimal field for He.3.2 r (m) B LH = T z (m) Magnetic Field Along Z B (r=) (Tesla) z (m)

27 Effects of Non-uniform field Independent operation of Helicon field magnets yielded optimal Helium performance with nonuniform axial field. Lower-hybrid resonance was present under antenna in new configuration, unlike previous results. Similar results were also obtained with D2 Ion Rate (1 18 s -1 ) Helicon Field (Tesla)

28 Gas mixture experiments show super fast ions Super fast ions at > 15km/sec (15, sec I sp ) are being measured with two independent sensors from U.H. and Rice U. Same high energy tail observed in D2, H2 mixture Even without a fast ion tail, bulk flow is >3 km/sec.

29 Preliminary Light Data Power Scan, Helium Density [ions/cm^3] 2.1E E E E+12 9.E+11 6.E+11 3.E+11.E+ Density Radiated Power Radiated Power [kw] Helicon Power [kw]

30 Power scans show no sign of saturation The ion output rate increases linearly with input power with no sign of saturation. Discharges will not run below 5 W. The plasma Mach number also increases with power.

31 1 kw ICRF System 1 kw, 3MHz Transmitter 3-1/8 Heliax 9-degree Power Splitter 7/8 Heliax 7/8 Heliax Impedance Matchbox Impedance Matchbox Upstream antenna Downstream antenna

32 Solid state RF system design Design draws from ORNL expertise in RF heating of fusion plasmas. System architecture is robust and failure tolerant. Prototype hardware has been built and is undergoing testing. Plasma flow

33 Superconducting Magnet Technology BSCCO 2223 Superconductor Operates at 4 o K.3 Tesla field Flight like magnet designed by NASA/ORNL and DuPont. Delivered April, 21. Undergoing acceptance tests. 5 kg superconducting magnet will replace 15 kg conventional LN 2 -cooled magnet

34 VASIMR Team Government NASA: JSC, MSFC, GRC, GSFC, LaRC DOE: ORNL, LANL, PPPL VASIMR Workshop March 21 Industry Lockheed Martin Muñiz Eng. Boeing Barrios Eng. SAIC MSE Inc DuPont Strong participation of students at both graduate and undergraduate levels Academia UT-Austin Rice U U of Maryland U of Houston MIT U Michigan Princeton U

35 Summary VASIMR approach provides power-rich and fast transportation capability for human and robotic missions to Mars and beyond. Experiments in progress are exploring plasma performance and operational regimes, as well as helicon source refinement. Brisk technology development emphasizes ground and early space testing. Multi center team approach produces rapid results. Current activities include ICRH experiments and transition to high temperature superconductors.