CW RF cesium-free negative ion source development at SNUpsl.postech.ac.kr/kjw13/talks/Jung.pdf ·...
Transcript of 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
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 ¤t drive source of high density fusion plasma like ITER or DEMO
[1] [2] [3]
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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
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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
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[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
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(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
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• 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)
H + H/surface → H2(v”)
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+
H + H/surface → H2(v”)
H3+ + surface → H2(v”) + H
H2+ + surface → H2(v”)
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.
H + H/surface → H2(v”)
H3+ + surface → H2(v”) + H
H2+ + surface → H2(v”)
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
Electron Temperature Electron Density
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.
Extracted negative ion beam current depends on the driving RF frequency
Heating region
Heating region
Extractionregion
Extractionregion
Increased neIncrease
Increased neIncrease
Increased neIncrease
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
Electron Temperature Electron Density
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)
Extracted negative ion beam current depends on the driving RF frequency
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
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-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 )
13.56MHz_3mTorr 27.12MHz_3mTorr
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)
13.56MHz_3mTorr 27.12MHz_3mTorr
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.