Ultra Low Work Function Coating for CombinedUltra Low Work Function Coating for Combined Passive Electron Emission and Collection for

Spacecraft Charging Neutralization and Electrodynamic Tether Applications

Lauren P. Rand, Colorado State UniversitySudhakar Mahalingam, TechX Corporation

Juan Sanmartin Universiad Politécnica de MadridzJuan Sanmartin, Universiad Politécnica de Madridzand

John D. Williams, Colorado State University

12CaO-7Al2O3 [Ca24Al28O64]4+ 2O2–

.4 n

m0

http://www.nanonet.go.jp/english/mailmag/2006/071a.html

[Ca24Al28O64]4+ 4e–

Toda, et al.  Advanced Materials.  19, 2007.

Cathode Flange

Cathode Tube

Bottom Crucible with one half insert locatedone half insert located over the C12A7 load

Two Insert

One Half of Insert

Crucibles

Cathode Tube

Two Insert Halves Enclosed

Keeper

Cathode Tube Mounted to Cathode Flange

Enclosed KeeperEnclosed Keeper Mounted to

Cathode Flange

Plasma Electron Emitters are Nice, but are there Other Applications for Electride Electron Emitters?

Plasma Contactor

DC-DC Converters& Electronics

Pressure Regulator

Solenoid Valve& Electronics

Hollow Cathode

Xenon Tank

Hollow Cathode with Keeper

Fill & Drain ValvePressure Transducer

• Why not use electride to coat a tether and passively EMIT and COLLECT electrons?

• One question to answer is where the floating point will end upfloating point will end up.– How long is the negatively biased section

relative to the positively biased section?relative to the positively biased section?

Electron Collection

Space Plasma

Collection

+V Plasma

0 VSheath

neoTeo

Positively Biased Tether

0.10OML

0.08

m2 )

OOPIC Pro

0.06

nsity

(A/

0.04

rent

Den

0.02

Cur

r

0.000 200 400 600 800 10000 200 400 600 800 1000

Tether potential (V)

ge (V

)Vo

ltag

Electron Emission

-VSpace Plasma

Emission

Plasma

0 VSheath

neoTeo

Negatively Biased Tether

Thermionic Emission

j AST 2 exp(

ekT

)kT

3V 1 S exp

e3V4d

1kT

4d o kT

Space Charge Limited CurrentSpace Charge Limited Current

2 e V32

j 2

9e

2mV 2

d29 2m d

j (2 25 106)V

32

j (2.25x10 6)d2

j j ? V32

jTE > jSCLC ? j (2.25x106)V 2

d2

j AST 2 exp(ekT

)jSCLC > jTE ? kT

(eV)Some typical

LaB6 2.66Some typical high current density

CeB6 2.43density emitters

Ba-W 2.14C12A7 0.6

TT (K)Same Geometry?

LaB6 1900Same Applied Voltage?

CeB6 1750Voltage?

Same Emission

Ba-W 1350Same Emission Current!

C12A7 440

0.10OML

0.08

m2 )

OOPIC Pro

0.06

nsity

(A/

0.04

rent

Den

0.02

Cur

r

0.000 200 400 600 800 10000 200 400 600 800 1000

Tether potential (V)

1.20 OML ElectronCollectionPreliminary

1.00m

2 )SCL ElectronEmissionTL Electron

Calculation

0 60

0.80

nsity

(A/m TL Electron

Emission

0 40

0.60

rent

Den 270 K, 0.6 eV

0.20

0.40

Cur 5X Higher

0.00

neo = 1011 m-3

0 200 400 600 800 1000

Tether potential (V)

ConclusionConclusion• A new material may be available soon that has a

l k f ti th t i t bl d hi hvery low work function that is stable under high emission currentP li i l l ti t th t l t• Preliminary calculations suggest that electron emission length of a bare tether might be ~1/5 of the positively biased sectionthe positively biased section

• Ohmic heating might reduce negative bias section length even moresection length even more

• Configuration of electride into gun geometry might be an alternative to coating tether g g