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Investigation of the Boundary Layer during the Transition from Volume to Surface Dominated H−
Production at the BATMAN Test Facility
Christian Wimmer, Loïc Schiesko, Ursel Fantz and the NNBI-Team
Max-Planck-Institut für Plasmaphysik
ICIS 2015 – 16th International Conference on Ion Sources(23 – 28 Aug 2015)
Investigation of the Boundary Layer during the Transition from Volume to Surface Dominated H−
Production at the BATMAN Test Facility
Max-Planck-Institut für Plasmaphysik
Outline
1. Prototype H− source for ITER N-NBI (⅛ size)
2. Source diagnostics at BATMAN
3. Influence of caesiation on
extr. currents, potentials and H− density
4. Conclusion
Prototype H− source for ITER N-NBI (⅛ size)
Driver:
plasma generation (H, Hx+, e−, …)
ICP: f = 1 MHz, P = 40-75 kW
Te = 10 eV, ne = 1018 m-3
Expansion volume with magnetic filter field:
reduction of Te and ne in front of
the plasma grid (≈ factor 10)
→ Reduce H− destruction:
H− + e− H + 2e−
Caesiated Plasma grid (PG):
production of H− and extraction:
H, Hx+ + surface e− H−
caesium layer
(low work function)
caesiumoven
driverØ = 24.5 cm ℓ = 14 cm
expansion chamber32x59x23 cm3
Faraday screen
RF coil
magn. filterfield
Cs
H-
Cs
3
Volume close to PG: boundary layer
Prototype H− source for ITER N-NBI (⅛ size)
4
Co-extracted electrons:
heat-load on second grid
(max. several 10 MW / m2) limits
tolerable amount:
je/jH− < 1
positivelybiased
Positive bias of PG:
reduction of je
grounded gridextraction grid
plasma grid
N S
S N
S N
1) 2) 3)SN
NS
SN
extractionsystem
1) plasma grid2) extraction grid3) grounded grid
Clean source without Cs:
volume production of H− dominant
H2(ν) + e− (Te ≈ 1 eV) → H− + H
Stripping losses in accelerator:
(H− + H2 → H + H2 + e−)
p ≤ 0.3 Pa
BATMAN test facility
BATMAN (BAvarian Test MAchine for Negative ions):
5
Pulsed operation:
Plasma phase: < 10 s (incl. 4 s beam extraction)
200 s vacuum phase between pulses
Standard configuration of magnetic filter field
(magnets in diagnostic flange)
Main aim:
Investigation of physical aspects!
BATMAN test facility
BATMAN (BAvarian Test MAchine for Negative ions):
6
Spring 2015 campaign:
• Starting with a cleaned source (Cs free → volume production of H−)
• Slowly evaporating Cs: transition to surface dominated H− production
@ constant operational parameters (p = 0.3 Pa, PRF = 60 kW, Uex = 5 kV)
• Following this process with diagnostics in boundary layer
7
Diagnostic setup
CRDS
probe bottom
probe top
Characterization of the plasma close to PG:
• Cavity Ring-Down Spectroscopy (CRDS)
→ nH− (2.2 cm to PG)
• Langmuir probes (0.7 cm to PG)
→ Φpl, Te, ni+
Standard measures:
• Source performance jH−, je
• Bias voltage Ubias
(constant bias current: Ibias = 5 A)
8
Cs conditioning: source performance
0
10
20
30
40
50
/17
1.64
je/jH-
x9.7
/1.8
24.3
je
Ext
r. c
urre
nt d
ensi
ty [m
A/c
m2 ]
jH-
14.8
BATMAN Cs conditioning 2015/04/02P = 60 kW, p = 0.3 Pa, Uex = 5 kV
start Cs
evaporation
102580 102600 102620 102640 102660 1026800
10
20
30
Rat
io j e/j H
-
Shot number
1st day of Cs conditioning:
• jH−: increase by a factor of 9.7
(1.7 mA/cm2 → 14.8 mA/cm2)
• je: decrease by a factor of 1.8
(43.1 mA/cm2 → 24.3 mA/cm2)
Thus:
je / jH− decreases by a factor of 17
Obviously:
Minority species reacts more sensitive on Cs conditions (here: H− in badly conditioned source)
9
Cs conditioning: transition to ion-ion plasma
Transition electron-ion towards ion-ion plasma:H− becomes dominant negatively charged particle
102580 102600 102620 102640 102660 1026800
2
4
6
8
10
12
14
16
0
10
20
30
je/jH-
j e/j H
-
Ext
r. H
- cur
rent
den
sity
[mA
/cm
2 ]
Shot number
jH-
start Cs
evaporation
-20 0 20 40 60-4
-3
-2
-1
0
1
2
3
4
pl = 39.1 V
Cur
rent
[mA
]
Voltage [V]
Langmuir probe TOPcharacteristics #102595
fl = 37.1 V
-20 0 20 40 60-4
-3
-2
-1
0
1
2
3
4
Cur
rent
[mA
]
Voltage [V]
Langmuir probe TOPcharacteristics #102670
fl
pl = 29.7 V
10
Cs conditioning: potentials
102580 102600 102620 102640 102660 1026800
25
30
35
40
45
pl,top
pl,bot
Pot
entia
l [V
]Shot number
UbiasIbias = 5 A
(current regulation)
start Cs
evaporation
Plasma potentials:
Φpl,top & Φpl,bot lowered by ≈ 10 V
(change of charge fluxes from & towards wall)
Bias voltage:
Lowered by the same value as plasma potential
102580 102600 102620 102640 102660 1026800
5
10
bottom probe
top probe
U
bias
- pl
[V]
Shot number
→ Potential difference in PG sheath (Ubias – Φpl) almost constant
11
0 2 4 6 8 10 12 14 160
5
10
15
20
25
30
35
H- d
ensi
ty [1
015 m
-3]
Extr. H- current density [mA/cm2]
transition
volume => surface
production
Cs conditioning: negative ions
Dependence nH− ↔ jH−:
• jH− > 6 mA/cm2:
linear correlation(regularly seen for const. operational parameters)
• jH− < 6 mA/cm2:
knee towards pure volume production
Possible reasons for knee:
• different measurement areas for nH− & jH−, possible gradient in Cs
coverage on PG
• change of transport of H− from surface towards volume during conditioning (nH− ↑, ne ↓)
Cs oven
extractionapertures
CRDS
CRDS LOS
PG
H− (volume)
H− (surface)
2.2 cm
12
Cs conditioning: negative ions
Timetrace of nH−:
Strong influence of beam extraction on nH−!
Removal of H− due to extraction (deep extraction meniscus) or change of plasma parameters?
0 2 4 6 8 100
10
20
30
40
H- d
ensi
ty [1
015
m-3]
Time [s]
BATMAN #102470 (no Cs)
extractionRF only
gaspuff
0 2 4 6 8 10 12 14 160
5
10
15
20
25
30
35
RF only
H- d
ensi
ty [1
015 m
-3]
Extr. H- current density [mA/cm2]
extraction
0 2 4 6 8 10 12 14 160
5
10
15
20
25
30
35
H- d
ensi
ty [1
015 m
-3]
Extr. H- current density [mA/cm2]
transition
volume => surface
production
During conditioning:
Reduction of nH− becomes lower!
→ Extraction meniscus does not penetrate into CRDS LOS!
13
Conclusions
BATMAN: Cs conditioning first time at 0.3 Pa, constant operational parameters followed by several diagnostics close to PG
• Langmuir probe characteristics:
transition from electron-ion plasma to ion-ion plasma
• Extracted currents:
jH− reacts relatively stronger (x 9.7) than je (/ 1.8)
• Potentials:
plasma potential & bias voltage (const. Ibias = 5 A) is lowered
Potential difference in PG sheath (Ubias – Φpl) almost constant
• Comparison nH− ↔ jH−:
linear dependence after reaching a certain conditioning stage deviation close to pure volume production reduction of nH− (extraction ↔ RF only) weaker with better conditioning