cw rf cesium-free negative ion source development at snupsl.postech.ac.kr/kjw13/talks/jung.pdf ·...

CW RF cesium-free negative ion source development at SNU

Bong-ki Jung, Y. H. An,W. H. Cho, J. J. Dang, Y. S. Hwang

Department of Nuclear Engineering Seoul National University

JP-KO Workshop on Phys. and Tech. of

Y. S. Hwang

Dept. of Nuclear Engineering, Seoul National University,

San 56-1, Shillim-dong, Gwanak-gu, Seoul 151-742, Korea

[email protected]

JP-KO Workshop on Phys. and Tech. of Heating and Current Drive Jan. 29th 2013

Outline

• Introduction

• System setup• System setup

• Negative hydrogen ion source at SNUØ H- current depends on Pressure & Filter field strength

Ø H- current depends on Plasma Electrode

Ø H- current depends on Driving RF Frequency

2/29

• Summary & Conclusion

• Future Work

Introduction

High Energy Neutral Beam InjectorHigh Energy Neutral Beam Injector

• High energy(>1MeV) neutral beam injector is required for heating &current drive source of high density fusion plasma like ITER or DEMO

[1] [2] [3]

3/29

current drive source of high density fusion plasma like ITER or DEMOin considering penetration length and current drive efficiency

• To produce high-energy(>1MeV) neutral beam, development of highcurrent negative ion source is required due to higher neutralizationefficiency than positive ion source.

Reaction branches of negative hydrogen ion production process

H- ion production process

Volume production Surface productionVolume production Surface production

H2*(v”)formation

H2*(v”)destruction

H- ion creation

H- ion

H- ion creation

Atomic process

Ionic process

• Low energyelectronexcitation

• High energyelectronexcitation

• Dissociation Attachment

• Excited molecules relaxation• Excited molecule-ground state relaxation

4/29

H- ion destruction

excitation• Surfaceassistedexcitation

•Surfacerecombinationexcitation

relaxation• Excited molecule-Atom relaxation• Ionization reaction• Dissociation reaction•Wall relaxation

• Electronic detachment• Mutual neutralization• Associative detachment

Advantage of Cs free negative hydrogen ion source

Hydrogen negative ion

Hydrogen ion, atom

Cesiumion, atom

Magnetic Filter Field

SN

Extraction electrode

electron

• Production of negative ion increases with feeding Cs due to decrease of workfunction of surface. However, deposition of Cs on electrodes can lead to breakdown

SN

Plasma

Plasma electrode

Cs Layered surface

[2]

5/29

function of surface. However, deposition of Cs on electrodes can lead to breakdownbetween acceleration electrodes & complex chemistry interactions between Cs andplasma or wall is still a problem.

• Cs less operation of RF based negative ion source with large production of negativeions and lower operating pressure(<0.3 Pa[2.3mTorr]) to reduce heat load onelectrode is highly required for stable long-pulse operation of the NBI device.

Negative hydrogen ion production

e(>~20eV) + H2(v”=0) e + H2(v”) e(<1.0eV) + H2(v” 5) H- + H³

Step 1: Electron impact excitation Step 2: Dissociative electron attachment

Volume productionVolume production

fast electron

H2(v=0) H2(v>0) H2(v>0)slow electron

H-

H

6/29

[5] A.P. Hickman, Phys. Rev., A43, 3495, (1991)[4] J.R. Hiskes, J. Appl. Phys. 51, 4592, (1980)

Concept of RF based negative ion source

Multicusp Magnet Filter Magnet

H- Formation RegionPrimary Ionization Region

Electrodesfor Extraction

H2(v>0) H2(v>0)H-

e- (cold) e-

e- (hot)

H- ion

H2(v=0)

e- (hot)

Spiral RF A

ntenna

for Extraction

7/29

e- (hot)

e(>~20eV) + H2(v”=0) e + H2(v”) e(<1.0eV) + H2(v” 5) H- + H³

Spiral RF A

ntenna

Experimental Setup:TCP RF Volume Negative Ion Source at SNU

AccelerationElectrode

Extraction Electrode Quartz

Window RF Power

Plasma Electrode

Bias Electrode

PlanarSpiralCoil

V / I probe(MKS Corp.)

u Ion Source Characteristics

ü Volume production H- ion source

ü Longtime CW operation by RF

plasma source without filament

Discharge Chamber Size:

Φ10cm, length 10cm (750 cm3)

RF frequency: 11~27.12 MHz

Plasma→

←

14 pole cusp magnets

RF Matcher

2~3 kV Filter (Transverse) magnets

ü No contamination by external

antenna

ü Cs free H- ion source

8/29

(limited by matching capacity)

RF power: 500~1400W (CW)

Operating Pressure: >2 mTorr

RF antenna: planar spiral coil

u Filter magnetic fieldü Total: 150~210G

= 60~120G (Virtual Filter)

+ 90G (Dipole Magnet, fixed)

0~20 kV

magnets

H- current depends on Pressure

1.0

1.2

H- C

urre

nt (m

A)

13.56MHz_3mTorr 13.56MHz_5mTorr 13.56MHz_7mTorr

400 500 600 700 800 900 1000 1100 1200 13000.2

0.4

0.6

0.8

H- C

urre

nt (m

A)

9/29

RF power (W)• Stable minimum operating pressure of the TCP RF based negative ion

source is 3mTorr.• In contrast with previous work[2], negative ion beam current increase

with lower operating pressure.

H- current depends on Magnetic Filter field strength

3.5

4.0

Plas

ma

Elec

trode

Bia

s Cu

rren

t[A]

Low Filtering Middle Filtering High Filtering

1.2

1.4

1.6

H- io

n Be

am C

urre

nt[m

A]

Electron current flow to the plasma electrode

800 900 1000 1100 1200 1300 1400

1.5

2.0

2.5

3.0

Plas

ma

Elec

trode

Bia

s Cu

rren

t[A]

RF Power[W]

High Filtering

800 900 1000 1100 1200 1300 1400

0.4

0.6

0.8

1.0

H- io

n Be

am C

urre

nt[m

A]

RF Power[W]

Low Filtering Middle Filtering High Filtering

10/29

• Negative ion beam current increase with lower magnetic filter fieldstrength in low RF power regime but negative ion beam current increasewith higher magnetic filter field strength in high RF power regime.

• Electron transport near extraction region is related to filter field strength.

RF Power[W]RF Power[W]

Effect of bias electrode in negative ion source

Ceramic Electric

Electric schematics of bias electrodeElectric schematics of bias electrode

Plasma

Bipolar DC Power Supply

(±60V, 10A)

V

Bias Electrode

Ceramic Electric feed-through

20

24

28

32

plas

ma

pote

ntia

l [V]

nobias 0V 5V 10V 15V 20V 25V

11/29

0 2 4 6 80

4

8

12

16

plas

ma

pote

ntia

l [V]

probe position [cm]

25V 30V• In general ion sheath formation, negative

ion is hard to transport through the extraction hole due to sheath potential.

• Positively biased plasma electrode can enhance negative ion current.

Effect of bias electrode in negative ion source

0.80.91.01.1

3mTorr 5mTorr 7mTorr24

28

32

Plas

ma

pote

ntia

l [V]

nobias 0V 5V 10V 15V

Plasma potential profilePlasma potential profile H- current variationH- current variation

0 5 10 15 20 25 300.00.10.20.30.40.50.60.70.8

H- C

urre

nt [m

A]

Bias Voltage [V]0 2 4 6 8

0

4

8

12

16

20

Plas

ma

pote

ntia

l [V]

Probe position [cm]

15V 20V 25V 30V

12/29

• Plasma potential is measured for various bias voltage of plasma electrode.• Negative ion beam current increase with positively biased plasma

electrode but negative ion beam current decrease with too high positivelybiased condition due to decrease of electron density in extraction region.

Effect of plasma electrode materials

1.0

1.2

1.4

1.2sccm Mo 1.6sccm Mo 2.0sccm Mo 1.2sccm S/S

H- current variation with different electrode materials H- current variation with different electrode materials

H + H/surface → H2(v”)

Recombinative Desorption[4,5,6]

0 5 10 15 20 25 300.0

0.2

0.4

0.6

0.8

1.0

H- C

urre

nt [m

A]

PE Bias Voltage [V]

1.6sccm S/S 2.0sccm S/S

H2(v>0)


H3+ + surface → H2(v”) + H

H2+ + surface → H2(v”)

H, H2+, H3+

13/29

PE Bias Voltage [V]

• Increasing of negative ion beam current is observed with differentmaterial(stain less steel → Molybdenum) of plasma electrode.

• Different surface of material can affect to production of vibrational excitedmolecules which is important role of dissociative attachment(DA) process.

e- (hot)

Negatively biased or floating for recombinative desorptionPositively

biased for optimum

0,nH H+

Plasma Electrode (PE) Secondary Electrode (SE)

Study on effects of various material plasma electrode by using additional electrode

H2(v=0)H2(v”)

e- (hot)

H2(v”)

e-H-

optimum extraction

e(<1.0eV) + H2(v”≥5) → H- + He(~20eV) + H2 → e + H2(v”)

14/29

0,nH H+




0.70

Ta Ti

Study on effects of various material plasma electrode by using additional electrode

H- Current variation by using secondary electrodeH- Current variation by using secondary electrode

Total H- current decrease because secondary electrode may be an

0.50

0.55

0.60

0.65

H- C

urre

nt [m

A]

Ti Mo SS W

Titanium, Molybdenum and Tungsten H- current increase at low SE bias voltageSurface related effect

Positive ions are converted into vibrationally excited molecules with aids of metal surfaces.




secondary electrode may be an obstacle as an electron sink.

15/29

H- current profile as a function of secondary electrodebias voltage with various SE material(3mTorr, 0V PE bias voltage)

-40 -30 -20 -10 0 10 20SE Bias Voltage [V]

Mo, Ti, W > Stainless Steel

Titanium, Molybdenum and Tungsten also showed enhancement of H-current compared with S/S

H- current increase at low SE bias voltage

Effect of driving rf frequency on H- production

As rf frequency decreases ↓ As rf frequency increases ↑

Low driving rf frequencyLow driving rf frequency High driving rf frequencyHigh driving rf frequency

High energy electron Population increases

Power coupling Efficiency Increases

[7] Valery A. Godyak and Vladimir I. Kolobov, Phys. Rev. Lett. 81, 369–372 (1998) ] [8] V. A. Godyak, R. B. Piejak, and B. M. Alexandrovich, J. Appl. Phys. 85, 703 (1999)

Finding optimal driving rf frequency for TCP H- ion sourceFinding optimal driving rf frequency for TCP H- ion source

16/29

• Investigation of the effect of driving rf frequency on H- ion production in TCP H- ion source

• Plasma parameter diagnostics to find out the cause of rf frequency effect on H- production

• Searching for the optimal driving rf frequency of TCP H- ion source

Extracted negative ion beam current depends on the driving RF frequency

0.25

0.30

0.35

0.40

H- C

urre

nt (m

A)

3mTorr 10mTorr

10 12 14 16 18 20 22 24 26 280.00

0.05

0.10

0.15

0.20

0.25

H- C

urre

nt (m

A)

RF frequency (MHz)

Input RF power 500W

17/29

• Negative ion beam current is measured with various RF driving frequencies.

• Extracted negative ion beam current increase with higher RF driving frequencies at identical input power condition.

Acceleration Electrode14 pole cusp magnet Spiral RF Antenna

[ Schematic experimental setup ]

TCP RF Volume Negative Ion Source with various driving RF frequencies

→

←RF Matcher

RF Amplifier Function Generator

RF Choke

Plasma Electrode(PE)

18/29

Extraction Electrode Quartz Window

RF AmplifierGenerator

ü Operation condition : 500 rf Powerü RF Frequency of 11~27.12 MHz are used to generate plasma.ü Langmuir probe with compensation ring and rf choke circuit is used

for diagnostic of plasma parameters.

Plasma Electrode(PE)

Plasma Parameterswith various driving RF frequency

2.0

Electron Temperature Electron Density

Elec

tron

Tem

pera

ture

(eV) 1x1011

1x1011

7.37.47.5


Elec

tron

Tem

pera

ture

(eV) 1.8x1011

2.0x1011

Heating regionHeating region Extraction regionExtraction region

10 12 14 16 18 20 22 24 26 28 30

1.0

1.2

1.4

1.6

1.8

RF frequency (MHz)

Elec

tron

Tem

pera

ture

(eV)

500W 10mTorr 10keV3x1010

4x1010

5x1010

6x1010

7x1010

8x1010

9x1010

1x10 Electron Density (#/cm3)

10 12 14 16 18 20 22 24 26 28 306.56.66.76.86.97.07.17.27.3

RF frequency (MHz)

Elec

tron

Tem

pera

ture

(eV)

8.0x1010

1.0x1011

1.2x1011

1.4x1011

1.6x1011

1.8x1011 Electron Density (#/cm3)

500W, 10mTorr, 10 keV

19/29

• Electron density increase with higher driving RF frequency, whereas electron temperature decrease.

• Heating & extraction region has the similar characteristics of electron density and temperature.


Heating region

Heating region

Extractionregion

Extractionregion

Increased neIncrease




0.15

0.20

0.25

0.30

0.35

H- C

urre

nt (m

A)

10mTorr

Increase vibrational

excited molecules

Increase vibrational

excited molecules

decreased Tedecrease

vibrational excited molecules

decreased Tedecrease

vibrational excited molecules

Increase negative ion by

DA process

Increase negative ion by

DA process

decreased TeIncrease

negative ion by DA process

decreased TeIncrease

negative ion by DA process

10 12 14 16 18 20 22 24 26 280.00

0.05

0.10

0.15

H- C

urre

nt (m

A)

RF frequency (MHz)

Input RF power 500W

1.8

2.0


Elec

tron

Tem

pera

ture

(eV)

9x1010

1x1011

1x1011

Electron Density (#/cm

20/29

• Based on the plasma parameter, negative ion current variation can be explained with various driving RF frequency.10 12 14 16 18 20 22 24 26 28 30

1.0

1.2

1.4

1.6

RF frequency (MHz)

Elec

tron

Tem

pera

ture

(eV)

500W 10mTorr 10keV3x1010

4x1010

5x1010

6x1010

7x1010

8x1010

Electron Density (#/cm3)


0.20

0.25

0.30

0.35

H- C

urre

nt (m

A)

10mTorr

7.88.08.28.48.6

I_rf

I_rf

(A)

10 12 14 16 18 20 22 24 26 280.00

0.05

0.10

0.15

0.20

H- C

urre

nt (m

A)

RF frequency (MHz)

Input RF power 500W10 12 14 16 18 20 22 24 26 28

6.66.87.07.27.47.6

RF frequency (MHz)

I_rf

(A)

• RF antenna current is measured for various driving RF frequencies because Inductively coupled plasma mainly depends on the RF antenna

21/29

because Inductively coupled plasma mainly depends on the RF antenna current.

• The result show relation between negative ion current (plasma parameters) and RF antenna current for various driving RF frequency.

• Higher RF driving frequency can enhance negative ion production.

Extracted negative ion beam current with higher driving RF frequency & input power

0.8

0.9

1.0

1.1

1.2

H- C

urre

nt (m

A)

13.56MHz_3mTorr 13.56MHz_5mTorr 13.56MHz_7mTorr 27.12MHz_3mTorr 27.12MHz_5mTorr 27.12MHz_7mTorr

• Negative ion beam current is measured with two different driving frequencies RF power (13.56, 27.12 Mhz) as increasing input power.

400 500 600 700 800 900 1000 1100 1200 13000.2

0.3

0.4

0.5

0.6

0.7

H- C

urre

nt (m

A)

RF power (W)

22/29

frequencies RF power (13.56, 27.12 Mhz) as increasing input power.• Higher negative ion current is obtained with the higher driving RF

frequency in lower input RF power regime. However, lower negative ion current is obtained with the higher driving RF frequency in higher input RF power regime.

Plasma diagnostic by using bias electrode

Ceramic Electric feed-through

Electric circuit for measuring IV Curve by using Bias ElectrodeElectric circuit for measuring IV Curve by using Bias Electrode

Plasma

Bipolar DC Power Supply (±60V, 10A)

feed-through

OSC

23/29

V

10X Voltage Probe [Tektronix corp.] Measuring

Resistor(25ohm)

Bias Electrode

Add more bi-pass Capacitorto reduce RF perturbation

Low Pass Filter

(~1.9Mhz)

OSC

1.0

11Mhz0.8

3mTorr(0.4Pa)

I/V Curve w/ plasma electrode depends on operating conditions

I/V Curve for Various RF Frequency

I/V Curve for Various RF Frequency

I/V Curve for Various Pressure

I/V Curve for Various Pressure

-0.2

0.0

0.2

0.4

0.6

0.8

11Mhz 13Mhz 15Mhz 17Mhz 19Mhz 21Mhz

Curr

ent(A

)

-0.2

0.0

0.2

0.4

0.6

Curr

ent(A

)

3mTorr(0.4Pa) 5mTorr(0.65Pa) 7mTorr(0.9Pa)

500W, 3mTorr 500W,11Mhz

24/29

-10 -5 0 5 10 15 20 25 30Voltage(V)

-15 -10 -5 0 5 10 15 20 25Voltage(V)

• I/V Curve by using the bias electrode is successfully obtained forvarious driving frequency & Pressure condition.

1.8

2.0

2.2

2.4

Elec

tron

Tem

pera

ture

(eV)

13.56MHz_3mTorr 27.12MHz_3mTorr

3x1011

4x1011

5x1011

El

ectro

n De

nsity

(#/c

m3 )


Plasma Parameterswith higher driving RF frequency & input power

400 500 600 700 800 900 1000 1100 1200 1300

1.2

1.4

1.6

Elec

tron

Tem

pera

ture

(eV)

RF power (W)

Extraction Region

400 500 600 700 800 900 1000 1100 1200 13000

1x1011

2x1011

Extraction RegionElec

tron

Dens

ity (#

/cm

RF power (W)• Electron Temperature and Density increase as increasing input RF power.• Electron Temperature at higher RF frequency is lower than that at lower RF

25/29

frequency.• Electron Density at higher RF frequency is higher than that at lower RF

frequency.• But, above 1000W, Electron density at higher RF frequency is lower than

that at lower RF frequency

Extracted negative ion beam current with higher driving RF frequency & input power

10

11

12

13

I_rf (

A)

13.56MHz 27.12MHz

0.80.91.01.11.2

H-

Cur

rent

(mA)


400 500 600 700 800 900 1000 1100 1200 1300

7

8

9I_rf (

A)

RF power (W)400 500 600 700 800 900 1000 1100 1200 1300

0.40.50.60.7

H- C

urre

nt (m

A)

RF power (W)• RF antenna current is measured for driving RF frequencies and input power.• The result also show relation between negative ion current (plasma

26/29

parameters) and RF antenna current. • In contrast to lower input power regime, unfortunately negative ion current

decrease with higher driving frequency in high input power regime.• Capacitive loss of ICP plasma with higher driving frequency and input power

condition can cause decrease of electron density and temperature[9].

Summary & Conclusion

• To reduce heat load to acceleration electrodes, RF based negative ion source operation at low pressure(3mTorr) is achieved and Higher magnetic filter field strength shows less electron flow to plasma electrode.

• Effects of various plasma electrode materials are compared with negative • Effects of various plasma electrode materials are compared with negative ion current in consideration of recombinative desorption and negative ion current is enhanced with plasma electrode materials of Ti,Mo,W.

• Negative ion current changes with various driving RF frequencies and the results is explained by variation of plasma parameters(ne,Te) due to effect of different power coupling with various frequencies.

• Higher negative ion current is obtained with the higher driving RF

27/29

• Higher negative ion current is obtained with the higher driving RF frequency in lower input RF power regime. However, lower negative ion current is obtained with the higher driving RF frequency in higher input RF power regime due to capacitive loss in ICP.

Comparison of negative ion source

Total Current Current density

NNBI for ITER(RF source, 40A(CW):100kW

for ICP module*8 24 mA/cm2(RF source,Cs seeded) for ICP module*8 24 mA/cm

DESY (RF source)

40mA(Pulse[15ms]):30kW

120mA/cm2

J-Parc(Filament source) 18mA(CW) 28.3 mA/cm2

SNU 1.67 mA(CW):1.4kW, 0.4 Pa 3.3 mA/cm2

28/29

:1.4kW, 0.4 Pa

• In comparing extraction current with other negative ion source, TCP based RF negative ion source has competitiveness in considering RF power density but still needs enhancement of total current and current density.

Future Work

• Characterization of negative ion production in various conditions more

precisely by using diagnostics(measurement of negative ion density,

vibrationally excited molecules) to optimize negative ion production and

extraction respectively.

• Geometry effect of the TCP negative ion source to optimize in considering

transport of plasma.

• Effect of more various plasma electrode materials in others approach.

(LaB6, Cs-doped Metals,…)

29/29

• Research on the method for higher power coupling with various RF

frequency to increase negative ion current efficiently.

cw rf cesium-free negative ion source development at snupsl.postech.ac.kr/kjw13/talks/jung.pdf ·...

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