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
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