LISA Gravitational Reference Sensors

Transcription

1 Gravitational Reference Sensors Ke-Xun Sun Stanford University For the Research Community TeV Particle Astrophysics II University of Wisconsin, Madison, August 28-31, ppt

2 : A Spacecraft Constellation Each spacecraft houses two Gravitational Reference Sensors () has ppt

3 Gravitational Reference Sensor () in the Spacecraft is one of the core scientific instruments the heart of ppt

4 LTP, and BBO Configurations and Performance Space Fleet LTP/ST-7 BBO Configuration Observational Goal IMS/DRS test Discrete Sources Gravitational Wave Relics # of S/Cs Arm Length 20 cm 5x10 6 km 5x10 4 km Laser Power 20 mw 2 W 300 W Strain Sensitivity Acceleration Limit N.A ms -2 Hz -½ h~10-21 h~ ms -2 Hz -½ ms -2 Hz -½ ppt

5 Baseline Performance ppt

6 Mid and Low Frequency Range Sensitivity Determined by Performance } IMS ppt

7 LTP Gravitational Reference Sensor consists of: A freely-floating proof mass within a housing, Position measurement of the test mass w.r.t. housing Control of test mass orientation Charge control subsystem Disturbance reduction from: Solar magnetic field Solar radiation pressure Residual gas pressure Thermal radiation pressure Charging by cosmic rays Spacecraft self-gravity Spacecraft magnetic fields Spacecraft electric fields LTP Graphics thanks to Stefan Vitale ppt

8 LTP Engineering Model Testing ppt

9 LTP Testing LTP Graphics thanks to Stefan Vitale ppt

10 Stanford Gravity Probe-B and Previous Flight GP-B was launched April 2004 and started science measurement in August GP-B has experimentally measured frame-dragging effects GP-B experiences are an important assets to -BBO R&D based on Stanford experience with TRIAD (Stanford/APL, 1972, < 5x10-11 m/s2 RMS over 3 days) GP-B (Stanford, launch 2002, < 2x10-12 m.s-2/ Hz at 5x10-3 Hz ) Position sensing, charge control from GP-B GP-B: 17 DOF GP-B is a critical development for high precision space flight, including Drag-free technology Cryogenics Precision fabrication Caging Charge management ppt

11 ST-7 Development at Stanford Electronics and system integration Au/Pt PM Vacuum system Housing with compound material ppt

12 Noise Sources Additional leading term: Voltage Reference Instability ppt

13 Cross Talk is the Leading Noise Source Anyway to Reduce? Cross talk due to : 1) Proof mass shape 2) Two proof masses ppt

14 Each Spacecraft has Two Cubic Proof Masses Sensitive path Waveplate Transmissive optics To and from Remote Spacecraft Proof Mass Proof Mass Great elaborated structure, but Interlinked scheme for re-correlation Long sensitive path Coupling throughout the system dn/dt problem in transmissive optics Alignment coupling ppt

15 Modular has Only One Proof Mass Reduced Cross Talk Modular : The New Baseline Architecture 5 (2004) Amaldi 6 (2005) ppt

16 Single proof mass Modularized, stand-alone GW detection optics external to External laser beam not directly shining on test mass (Just learned that will follow! - - Heinzel talk before ours in this session) Internal optical sensing for higher precision Large gap for better disturbance reduction True 3-dim drag-free architecture Determine the geometric center and center of mass A Standalone Architecture Presented at 5 th Symposium Housing Optical Readout Beam Proof Mass 16 Incoming Laser Beam Outgoing Laser Beam ppt Large gap Telescope Details Shown in Figure 2 Sun, Allen, Buchman, DeBra, Byer, CQG (22) 2005 S287-S296

17 An All-Reflective Configuration for Modular (a) Laser for External Interferometer Optical Fiber Detector To and from Remote Spacecraft Thin, Doublesided, Dual Gratings Housing Wall Optical source for Gap Measurement Detector Reference Relay Region 1~2 cm gap (sensitive optical path) Optical Fiber Spherical PM. Diameter 2~10 cm (b) (c) (d) (e) ppt

18 Diffractive Optics Simplifies for Baseline with Cubic Proof Mass Grating fabrication studies - Funded by JPL DRDF - The progress is ahead of schedule - e-beam lithography at Stanford Nano-Science Facility - Grating pattern on dielectric demonstrated - Grating on noble metal in progress The only sensitive optical path Compact construction Displacement + Angular sensing 18 SEM Image of Grating ppt

19 Webb Space Telescope Multi-Mirror Steering Scheme From remote S/C CCD imager From interferometer laser to remote S/C 19 Multi-mirror design Telescope main heavy primary mirror and tube fixed Steering direction by moving (rotating) lighter tertiary and quaternary mirrors Assign coarse and fine adjustments to different mirrors Add CCD imager for seeking acquisition ppt

20 The Single Proof Mass Architecture It is now adopted by BBO Presented at BBO Working Group Meeting, Collocated with 5 th Symposium, ESTEC, Netherlands, July 2004 BBO new design as of December 2004, presented by JPL at Texas Symposium at Stanford ppt

21 Strap Down (Modular ) Has Become the New Interferometer Baseline ppt

22 Mission & DOF Counts Single SC Comparison of Control Complexity Displacement DOF Angular DOF Other DOF Stand-alone Simplifies Control GPB ST-7 BBO (New) ppt Stand-alone /BBO Total DOF Single S/C Decoupled DOF counts Time to setup experiment mo. Mission 3 mo. 1 1 Total Fleet DOF () 12 mo. (?) (New BBO) 3+7? Shorter than coupled scheme

23 An Alternative Also Considered by perceived a similar approach Single proof mass Fiduciary mirror based beam steering Simpler than half/half separation scheme System and Technology Report, July 2000, A.4.4, P270, Laser Metrology Harness ppt

24 Baseline Spacecraft Structure Before Modular Graphics thanks to Ulrich Johann ppt

25 New Proposed Structure after Strap-Down (Modular ) Graphics thanks to Ulrich Johann, EDS Astrium ppt

26 Structure Using Single Proof Mass Cleaner structure Smaller volume Smaller rocket Graphics thanks to Ulrich Johann ppt

27 Developing Modular External Interferometry Mass distribution Moment of Inertia measurement Grating design and fabrication Grating angular sensor Center of Mass Measurement Optical displacement sensing UV LED charge management system Electromagnetic modeling Thermal control 27 Surface studies ppt

28 Grating Cavity Experimental Setup Graham Allen, Ke-Xun Sun, and Robert L. Byer, A Grating Cavity as a Displacement Sensor in Drag-Free Satellites, 6th Symposium, June 2006, Goddard Space Flight Center, Greenbelt, MD ppt

29 Cavity Sweep Results 55 nm linear range Greater than 60% back-coupling into fiber Higher order mode clearly visible New grating may fix mv/nm slope mv/ Hz stability of DC amplifiers 33 db RF SNR 26 pm noise floor Limited by resistor noise 400 mw delivered via fiber 71 mw reflected signal Higher power result of weaker than expected side-band signal ppt

30 Recent Grating Angular Sensing Experiment with Two Detectors NPRO Quad Det. θ -1 ~ N Quad Det. Scope S.A. 6 cm Custom Grating 935 lines/mm Simple construction No extra optics No other uncertainty and noise Ke-Xun Sun, Patrick Lu, and Robert L. Byer, A robust, symmetric grating angular sensor, 6 th Symposium, June 2006, 30 Goddard Space Flight Center, Greenbelt, MD ppt

31 Enhancement of Angular Sensitivity 10 3 Grating angle amplification Enhancement of Angular Sensitivity Diffraction Angle of +1 Order [degree] Beam cross section compression cos( θ ) in Pm = K' P cos( θm) 2 in K. Sun, S. Buchman, and R. L. Byer, Grating Angle Magnification Enhanced Angular and Integrated Sensors for Applications, accepted for publication at J. Phys. C. Special issue of Almadi 6 Conference on Gravitational Waves ppt

32 Signal Spectrum for 4 mw Incident Power Signal Noise floor lower than 1 nrad/hz 1/2 PZT displacement 10 nm Grating rotation 0.5 μrad 4 mw input power Noise floor level ~1 nrad/ /Hz 1 Potential applications: Telescope orientation Fiber collimator orientation Coarse Frequency stabilizatio ppt

33 Grating Fabrication for Grating Angular Sensor Transfer-imprinting of gold gratings This is performed by pressing a dielectric grating into a gold surface with force sufficient to exceed the yield stress of gold Centimeter-sized dielectric gratings were fabricated with e-beam lithography on quartz wafers. Gold gratings had 933 lines/mm and 300nm of depth. Various duty cycles have been demonstrated. 275nm depth 50% duty cycle gold gratings have been measured to have 26% diffraction efficiency in the +/-1 orders and 36% efficiency in the 0 th order. Dielectric grating, 50% duty cycle (AFM image) Imprinted gold grating, 50% duty cycle (SEM) Patrick Lu, Ke-Xun Sun, and Robert L. Byer, Methods of Fabricating Grating Patterns Dielectric grating 25% duty cycle on Dielectric and Metal Surfaces, 6 th Symposium, June 2006, Goddard Space Flight Center, Greenbelt, MD 33 (SEM) ppt

34 Grating Duty Cycle Variation Chrome etch mask 25% duty cycle. Chrome etch mask 75% duty cycle ppt

35 Pendulum Angular Measurement Grating Angular Optical Sensing*: Angular magnification -1 81, Independent of vertical motion Quad Photodetector: High sensitivity Accurate zero crossing Measure inertia to Position Sensing Detector: below 1 part in 10 4 Large dynamic range 1V Includes 0.25 pendulum mass rotation property uncertainties Ke-Xun Sun, Saps Buchman, and Robert Byer. Grating Angle Magnification Enhanced Angular and Integrated Sensors for Applications. Journal of Physics: Conference Series, 32: , ppt

36 Gravitational Self-Attraction Q = 1500 f = Hz FFT resolution < 1 mhz ppt

37 Displacement and Angular Measurements Meet Sensitivity Requirements Configuration Measurement Performance Fiber Michelson Interferometer Displacement 10 nm/hz 1/2 Free Space Michelson Grating Cavity Direct Detection Grating Cavity RF Detection Displacement Displacement (3 mw) Displacement (0.1 mw) 1 nm/hz 1/2 30 pm/hz 1/2 10 pm/hz 1/2 Direct Reflection Angular <10 μrad/hz 1/2 Grating Single Side Detection Angular (8 mw) 10 nrad/hz 1/2 Grating Two Side Detection Angular (4 mw) 1 nrad/hz 1/ ppt

38 Center of Mass Measurement R GC* r CM x ppt

39 Center of Measurement Initial measurement accuracy: 2-3 m Expect <1 m accuracy in future ppt

40 Deep UV LED Key Characteristics LED will be the UV Source Flown in UV LED Charge Management System Weight: 0.3 kg (including electronics) Electrical power ~1.5 W (0.1 W/LED. Est. power for control electronics ~1.2 W) Fast switching (<0.1 μs), wide selection of modulation frequency and duty ratio AC charge management at frequencies out-of signal band (e.g. at cap bridge freq.) Various AC charge management techniques for better performance Peak wavelength: nm, comparable to Hg line 254 nm FWHM: 12.5 nm, good photoemission for Au coatings Total UV power: mw, sufficient for charge management ppt

41 Positive and Negative AC Charge Transfer UV LED and bias voltage modulated at 1 khz ppt

42 UV LED Power and Spectral Stability Tests For AC Charge Management UV LED Operation > 2700 Hours UV LED Spectrum Stable ppt

43 Developing the Modular ppt

44 Summary technology made much progresses LTP ground testing performance met requirements LTP launch is still on schedule Novel architecture Modular ( Strap Down ) is adopted as the new IMS/ baseline Single proof mass configuration under serious considerations Much progress in modular development ppt