UV LED charge control at 255 nm

Transcription

1 UV LED charge control at 255 nm Karthik Balakrishnan Department of Aeronautics and Astronautics Hansen Experimental Physics Labs Stanford University

2 Drag free concept and applications Cancel deviation from geodetic orbit Geodesy Aeronomy Autonomous orbit determination Fundamental Physics From presentation by Andreas Zoellner at the Spring 2012 CubeSat Developer's Workshop - Cal Poly

3 Drag-free history TRIAD I (1972) GRACE (2002)* Gravity Probe B (2004) GOCE (2009) Planned: LISA Pathfinder LISA * Accelerometer only From presentation by Andreas Zoellner at the Spring 2012 CubeSat Developer's Workshop - Cal Poly

4 Drag Free Control and MGRS MGRS: Modular Gravitational Reference Sensor: DOSS: Differential optical shadow sensor Nanometer level displacement sensitivity Grating angular sensor Nanoradian level angular sensitivity Grating displacement sensor Picometer level displacement sensitivity Launch lock mechanism UV-LED charge management

5 MGRS System Overview Differential Optical Shadow Sensor Grating Angular Sensor Grating Displacement Sensor Nanometer sensing for drag free signal Lower resolution, high dynamic range Andreas Zoellner Nanoradian level angular sensing Muflih Alrufaydah, Patrick Lu (alum) Picometer sensing for science signal High sensitivity, low dynamic range Graham Allen (alum) Proof Mass Caging UV LED Charge Management Full System 700 g clamping of proof mass during launch Minimal residual velocity on release No damage to proof mass surface Eric Hultgren, Chin-Yang Lui Solid state 255nm light source Charge control of proof mass and housing potential Karthik Balakrishnan 2.9 kg 70mm dia Au-Pt sphere Carbide coated sphere

6 Charging sources and effects Many disturbance forces: Solar, atmospheric, micrometeoroids Charging Charging mechanisms: Direct: charged particles accumulate on either proof mass or housing Secondary: charged particles interact with spacecraft, knocking off electrons which then accumulate on the proof mass or housing Potential imbalance leads to an electrostatic force

7 LED Current (ma) LED Optical Power (W) UV LED Properties PD Response Current (ma) Intensity (normalized) UV LEDs are: Now commercially available, space qualification w/ Stanford AlGaN based wide-bandgap (4.86eV) device with 255 nm line (12 nm FWHM) >10 uw output power possible High dynamic range (> 1 khz modulation is possible) Operate CM outside the science band 10 Voltage-Current Voltage-Power Current (L)-Current(P) Spectrum TFW1 - V-I during functional testing TFW1 - V-P during functional testing x TFW1 - LED Response I-I x TFW1 - Post Fact Spectra 8 6 Pre Test Post Thermal Post Shake Pre Test Post Thermal Post Shake Pre Test Post Thermal Post Shake Voltage (V) Voltage (V) LED Drive Current (ma) Wavelength (nm)

8 AC Charge Management Overview Positive Charge Transfer Negative Charge Transfer

9 Charge management experimental setup

10 Charge management results System capacitance to ground is 17 pf 10 W incident UV power (255 nm), modulated at 100hz, 50% duty cycle, 3.0 V pp bias Sphere potential was measured using floating probe relative to surrounding housing

11 Proof mass coatings Gold is soft and prone to sticking and scratching Alternatives: carbide coatings Very tough, wide bandgap (close to AlGaN) Test: carbide pellets coated on to Aluminum substrates via e- beam deposition Measured: reflectivity, quantum efficiency, surface resistivity Top row (from left): Au, Nb, Ir, SiC Bottom row (from left): TiC, Mo2C, ZrC, TaC

12 Proof mass coatings measurement Quantum efficiency (QE): Ratio between number of emitted electrons & Number of incident photons QE measurement setup nm UV light incident on a coated sample 2. Sample biased to -5V, sphere biased to +5V 3. Current required to hold sample at constant potential measured

13 Proof mass coatings - results Material QE R (255 nm) φ (ev)* Au 3.40E Nb 5.64E SiC 4.34E TiC 4.48E ZrC 3.85E MoC 6.82E TaC 6.35E Ir

14 Small satellite demonstration 16 total LEDs Four bias plates Gold coated sphere (e-beam dep n) Contact probe Gold coated Ultem tubes - shielding Lab model shown

15 References Balakrishnan, Karthik et. al., UV LED charge control of an electrically isolated proof mass in a Gravitational Reference Sensor configuration at 255 nm, submitted to Classical and Quantum Gravity (2012), preprint: Sun, Ke-Xun et. al., UV LED operation lifetime and radiation hardness qualification for space flights, Journal of Physics: Conference Series, Volume 154, Issue 1, pp (2009). Sun, Ke-Xun, et. al., LED deep UV source for charge management of gravitational reference sensors, Classical and Quantum Gravity, Volume 23, Issue 8, pp. S141-S150 (2006). Sun, Ke-Xun et. al., Spectral and Power Stability Tests of Deep UV LEDs for AC Charge Management, LASER INTERFEROMETER SPACE ANTENNA: 6th International LISA Symposium. AIP Conference Proceedings, Volume 873, pp (2006). Pollack, S. E. et. al., Charge management for gravitational-wave observatories using UV LEDs, Physical Review D, vol. 81, Issue 2, id (2010). Sumner, Tim et al., Description of charging/discharging processes of the LISA sensors, Classical and Quantum Gravity, Volume 21, Issue 5, pp. S597-S602 (2004). F. Antonucci et al., The interaction between stray electrostatic fields and a charged freefalling test mass, submitted to Physical Review Letters (2012), preprint:

16 Questions? arxiv: v1 Work was funded by grants from KACST and NASA Ames Research Center Karthik Balakrishnan is funded by the DoD Sponsored NDSEG (National Defense Science and Engineering Graduate) Fellowship