UV LED charge control at 255 nm - Stanford University...LISA 9 Symposium, May 2012 UV LED charge...

LISA 9 Symposium, May 2012

UV LED charge control at 255 nm

John Conklin for the UV-LED Team Hansen Experimental Physics Labs

Stanford University

[email protected]

Stanford

University

King Abdulaziz City for

Science and Technology

NASA Ames

Research Center

LISA 9 Symposium, May 2012 Information contained herein is not subject to Export Control or ITAR

Science

– Special/General Relativity

– Gravitational waves

– Earth Geodesy/Aeronomy

Science & Technology Implementation on Small Satellites

Education

Grad, Undergrad

3-5 year projects

Student led tasks

Technology

Gravitational Reference Sensors

Ultra-stable optics

Precision Navigation

formation flying Science & Technology on Small Satellites

Education driven

International collaborations


MGRS System Overview

Grating Displacement Sensor

• Picometer sensing for science signal • High sensitivity, low dynamic range • In work

Grating Angular Sensor

• Nanoradian level angular sensing • Completed lab demo

Differential Optical Shadow Sensor

• Nanometer sensing for drag free signal • Lower resolution, high dynamic range • 2014 Launch

UV LED Charge Management

• Solid state 255nm light source • Charge control of proof mass and housing • 2013 Launch

Proof Mass Caging

Full Drag-Free System

• 70-30 Au-Pt sphere • Silicone Carbide coating • 2015 Launch

•Model of the test mass caging system


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


GRACE follow-on

Geodesy, Aeronomy

STAR

Gravitational Science

Small Sats Technology Demo Program

LAGRANGE

Gravitational Waves

UV LED Sat - 2013

(funded)

Shadow Sat - 2014

(partially-funded)

Optical Sat – 2016

(in lab)

Drag-free CubeSat – 2015

(in lab)


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


UV LED Properties

• 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

4 4.5 5 5.5 6 6.50

2

4

6

8

10TFW1 - V-I during functional testing

Voltage (V)

LE

D C

urr

en

t (m

A)

Pre Test

Post Thermal

Post Shake

4 4.5 5 5.5 6 6.50

0.5

1

1.5

2

2.5

3x 10

-7 TFW1 - V-P during functional testing

Voltage (V)

LE

D O

ptica

l P

ow

er

(W)

Pre Test

Post Thermal

Post Shake

-2 0 2 4 6 8 100

0.2

0.4

0.6

0.8

1x 10

-9 TFW1 - LED Response I-I

PD

Re

sp

on

se

Cu

rre

nt

(mA

)

LED Drive Current (mA)

Pre Test

Post Thermal

Post Shake

200 220 240 260 280 300 3200

0.2

0.4

0.6

0.8

1TFW1 - Post Fact Spectra

Wavelength (nm)

Inte

nsity (

norm

aliz

ed)

Voltage-Current Voltage-Power Current (L)-Current(P) Spectrum


UV LED Charge Control

• Charge control by UV photoemission using 254 nm line of an rf mercury source successfully demonstrated on GP-B

• LAGRANGE: Newer commercial UV LEDs (240-255 nm) – Fast-switchable (> 100 MHz) allowing ac charge management through

synchronization with bias electrode

– 10 mW 10 μW at 252 nm

• Passive charge management possible for LAGRANGE – Virtual wire between TM and S/C (~5 pF capacitance)

• Space qualification complete – Lifetime: >5 years

– Radiation

– Shake, bake, shock


AC Charge Management Overview

“Positive Charge Transfer” “Negative Charge Transfer” “Positive Charge Transfer” “Negative Charge Transfer”


Charge Management Experimental Setup


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 Vpp bias

Sphere potential was measured using floating probe relative to surrounding housing


TRL 4

TRL 5

Collaboration: Stanford – ARC – KACST

Flight hardware, management

TAA with KACST

I&T, operations

Ames

Research

Center

Basic research supports

Basic research at KACST

Flight program at ARC

Stanford

University

S/C, Launch, basic res.

TAA with ARC

I&T, operations

Basic research with SU

KACST Flight data

Flight payload

“Non Conventional Collaboration” on UV-LED SAT


Small Satellite Demonstration

• 16 total LEDs • Four bias plates • Gold coated sphere (e-beam dep’n) • Contact probe • Gold coated Ultem tubes - shielding

Scheduled for launch in 2013


UV LED Small Sat Components

Payload completion: May 2012

Spacecraft CDR: Aug 2012

Payload Integration: Dec 2012

Russian launch: Sep 2013


Kelvin Probe for Measurements of Patch Effects


Kelvin Probe Results for Au and TiC Au – Nb – Alumina 11-03-04 TiC – Alumina 11-03-04

Kelvin probe measurements: investigations of the patch effect with applications to ST-7 and LISA N. A. Robertson et al Class. Quantum Grav. 23(7) pp. 2665-2680 (2006) http://iopscience.iop.org/0264-9381/23/7/026


Coatings Tested at Stanford

Nb MoC Au SiC

TaC ZrC TiC Nb


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

– Carbides are an attractive alternative for the traditional gold proof mass coatings

Material QE R (255 nm) φ (eV)*

Au 3.40E-07 0.17 4.57

Nb 5.64E-07 0.17 4.30 SiC 4.34E-07 0.12 4.80

TiC 4.48E-07 0.15 3.80

ZrC 3.85E-07 0.11 3.70 MoC 6.82E-07 0.15 4.74

TaC 6.35E-07 0.13 5.0

Ir -- 0.6 --

Top row (from left): Au, Nb, Ir, SiC Bottom row (from left): TiC, Mo2C, ZrC, TaC

QE measurement setup


Material

Current Source

Drive Voltage

(mV)

LED

Current

(mA)

Sphere

Voltage

Sample

Voltage

Sample

Current

(LED off)

Sample

Current

(LED on)

Vacuum

Pressure

(torr)

Sample

current

(net)

#

electrons # photons QE mean

QE,

relative

to Au

Au 53 10 5 -5 -4.5E-13 3.15E-12 2.1E-03 3.60E-12 2.25E+07 6.41E+13

53 10 5 -5 -1.5E-13 3.45E-12 6.5E-04 3.60E-12 2.25E+07 6.41E+13 3.40E-07 1.00

Nb 53 10 5 -5 -4.3E-13 4.71E-12 2.1E-03 5.14E-12 3.21E+07 6.41E+13

53 10 5 -5 -3.4E-13 5.80E-12 5.7E-04 6.14E-12 3.84E+07 6.41E+13 5.64E-07 1.66

SiC 53 10 5 -5 -4.4E-13 3.56E-12 1.8E-03 4.00E-12 2.50E+07 6.41E+13

53 10 5 -5 -3.5E-13 4.25E-12 5.8E-04 4.60E-12 2.88E+07 6.41E+13 4.26E-07 1.25

TiC 53 10 5 -5 -1.1E-12 3.60E-12 2.0E-03 4.72E-12 2.95E+07 6.41E+13

53 10 5 -5 -2.0E-13 4.70E-12 4.7E-04 4.90E-12 3.06E+07 6.41E+13 4.51E-07 1.32

ZrC 53 10 5 -5 -6.7E-13 3.65E-12 2.0E-03 4.32E-12 2.70E+07 6.41E+13

53 10 5 -5 -3.5E-13 3.52E-12 5.1E-04 3.87E-12 2.42E+07 6.41E+13 3.92E-07 1.15

Mo2C 53 10 5 -5 5.0E-13 5.75E-12 2.0E-03 5.25E-12 3.28E+07 6.41E+13

53 10 5 -5 -2.5E-13 6.83E-12 5.5E-04 7.08E-12 4.43E+07 6.41E+13 6.48E-07 1.90

TaC 53 10 5 -5 -7.0E-13 5.42E-12 1.7E-03 6.12E-12 3.83E+07 6.41E+13

53 10 5 -5 1.0E-13 6.75E-12 5.2E-04 6.65E-12 4.16E+07 6.41E+13 6.27E-07 1.84

Photoemission Efficiency of Carbide Films


UV LED Small Satellite

Technology Objectives

Raise TRL levels (4/5 8/9) for

Deep UV LEDs

ac charge control

Beneficiaries:

LISA

GRACE follow-on

Drag-free CubeSat

Payload

Isolated “test mass”

16 UV LEDs & photodiodes

Charge amp

Voltage bias plates

ac charge control

electronics

Mission Design

Spacecraft: Saudi Sat

Russian launch in 2013

2 month mission

Fully funded ($1.5M)

Management

NASA ARC: Flight payload,

PM, SE, SMA

Stanford: Payload design, SOC

KACST: Spacecraft, Launch, MOC

Demonstrates unconventional

international collaboration

55 kg

50 W

Saudi Sat 3

222277180 mm

6.5 kg


Differential Optical Shadow Sensor (DOSS)


Raise TRL level for miniature high-

sensitivity displacement sensor

nm/Hz1/2 sensitivity

No forcing

Non-contact

Payload

Light source:

SLED, 1545 nm

InGaAs quad-photodiode

Ultra-low current Difet amp

Mission Design

2U CubeSat

Any orbit

Launch ~ 2014

1 month ops

Payload funded

Management

Stanford & KACST:

Payload, CubeSat structure

I&T & Launch: pending

2 kg

4 W

2U Cube


The Drag-free CubeSat

Science

Earth Aeronomy, space weather

Demo < 10–12 m/sec2 for future

Geodesy

Earth observation

Gravity science

Gravity-waves

Mission Design

3U CubeSat

Secondary launch via P-POD

Launch ~ 2015

1-2 month drag-free ops in LEO

Target program: Franklin/Edison

Payload

Drag-free sensor + micro-thrusters

Management

NASA ARC: PM, SE, SMA,

mission operations

Stanford: Payload design,

drag-free control, data analysis

4 kg

6 W

3U Cube


Fiber-coupled Optical Cavity Small Sat


Raise TRL levels for:

Frequency stabilized laser

Fiber coupled optical train

Beneficiaries:

STAR, GRACE follow-on, LISA

Mission Design

Saudi-sat compatible secondary

Any orbit, slow tumble OK

Launch ~ 2015

1 month mission lifetime

Payload

High-finesse Optical cavity

Optical fiber coupling

Low-cost laser

Modulator

photodiodes

Management

NASA ARC: PM, SE, SMA, ops

Stanford: Payload design, science

KACST: Spacecraft

55 kg

50 W

Saudi Sat 3


Space-Time Asymmetry Research (STAR)

Science

STAR will measure:

Isotropy of speed of light to 10–17

The direction of anisotropy

Dependence on boost velocity

Mission Design

ESPA compatible on EELV

Circular sun-sync 650 km orbit

Launch ~ 2016

2-year lifetime

Class D mission

Payload

Molecular clocks

Orthogonal

optical cavities

Low-noise

comparator

Multi-layer thermal enclosure

“Deviations from Einstein’s predictions would

cause us to rethink one of the foundational

pillars of all of physical science” – Astro2010

decadal

Management

NASA ARC: PM, SE, SMA, ops

Stanford: Payload design, science

KACST: Spacecraft & Launch

DLR: molecular clocks

180 kg

150 W

ESPA


UV LED Small Sat Integration and Test

Construction of engineering model

ongoing at Ames, March 21, 2011

Thermovac chamber

testing


Questions?

arXiv:1202.0585v1

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