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