Lab. Results

Differential Photoelectric Charging and Super-charging near the Lunar Terminator

UV Radiation

• Differential photoelectric charging near the boundary between lit and shadowed region.

• It has been suggested that time dependent charging at the terminator region may lead to ‘super-charging’, and the lift-off of lunar fines [Criswelland De, 1977].

+

+

+

+

+

+

+

+

-

-

-

-

-

-

-

-photons e

e

Surface Potentials Near Static Lit-Dark Boundaries

-1 0 1 2 3 4 5-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

L

VSL

= 2V V

SL= 0.8V (floating)

VSL

= 0V V

SL= -1V

DarkLit

Pot

entia

l (V

)Surface #

When all surfaces float, surface L charges positively and charge on dark surfaces remain small and E// at lit/dark boundary can be as large as 800 V/m.

e

Surface Potentials Near Moving Lit-Dark Boundaries

Surface L 0 1 2 3 4 5

Shadow

UV light

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.350.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Surface 4

Surface 3

Surface 2

Surface 1

Surface 0

Surface L

Pot

ent

ial (

V)

Time (s)

0 1 2 3 4 5 6 7 8Position of the shadow (cm)

Surface L is ‘supercharged’ when the shadow approaches it (i.e. the progression of sunset).

Dust Transport and Levitation above the Lunar Surface

The image of lunar horizon glow taken shortly after sunset [Criswell, 1973]

Dust Transport on A Surface in Plasma

Dust spreading process after plasma is turned on

Dust Ring

Bull’s Eye Pattern

Initial Pile

Uniform Spreading

Observations

Insulator

Initial dust pile

6 mm

Dust hopping

Potential contours above an insulating disc sitting on the graphite surface biased at -80V

E

EE

Potential Dip

Dust Transport on Surface in Plasma with An Electron Beam

Emissive probe

Filament

Dust pile

Graphite plate

Vacuum pump

Filament & Mesh

CCD camera

Observations with beam energy at 75eV

0 1 2 3 4 5 6

-40

-36

-32

-28

-24

-20

-16

-12

Po

ten

tial (

V)

Distance from the surface (cm)

Plasma Only 30eV 40eV 60eV 80eV 100eV

Sheath profiles with different beam energy (Jb Ji)

• E in the sheath increases significantly when Eb is sufficiently large and Jb Ji.

• Secondary electron emission is believed to plays a role.

Plasma Probes for Lunar Surface

Cylindrical probe

Zr Surface

Insulator standoffs

UV Light

Reference surface

V2

V1

V1 and V2 are adjusted to make net current through two probes to be zero

The probe data becomes useful when the photoemission from the reference surface is sufficiently large.

2

22/1

2

22)(

dV

Id

m

eV

Se

mf

)(

)(f

F

0

)( dfne

0

)(3

2 dfn

Te

eff

Both spherical and cylindrical probes show nearly identical Maxwellian electron distribution.

Electron energy probability function (EEPF)

and

Druyvesteyn’s second derivative method

Electron Energy Distribution