JongGab Jo, H. Y. Lee, Y. H. An, K. J. Chung and Y. S. Hwang*
Effective pre-ionization using fundamental extraordinary mode with XB mode conversion in VEST
Department of Nuclear Engineering, Seoul Na-tional University, Seoul 151-742, Korea
2/15
1. Introduction• Motivation & Objectives
2. Experimental Setup• ECH system and diagnostics in VEST
3. Experimental Result• Heating effect with pure toroidal magnetic field• Comparison between O-mode and X-mode injection• Pre-ionization effect on trapped particle configuration start-up
4. Summary & Conclusion
Contents
3/15
IntroductionMotivation & Objectives
Device EC ModeASDEX-U X2
COMPASS-D X1, X2, O1
DIII-D X1, X2
FTU O1
JT-60U O1, X2
T-10 X2
TCV X2, X3
TEXTOR X2
TORE SUPRA O1, X2
KSATR X2
LHD O1, X2
W7-X X2
ITER O1
<ECH>
Device MC ScenarioMAST OXB
NSTX OXB. XB
CDX-U XB
LATE OXB
TST-2 XBW7-AS OXB
<EBW>
Conventional tokamak: O1 mode or harmonics of X mode
Spherical torus: EBW by XB or OXB mode conversion
4/15
X1 mode has large fraction of RH compo-nent at low density and cold plasma.
Electron cyclotron damping of O1 and X2 mode is FLR effect.
For effective pre-ionization in VEST, X1 mode with XB mode conversion must be utilized.
IntroductionMotivation & Objectives
Polarization, cold plasmaPrater, Phys. Plasmas 11, 2349 (2004)
5/15
LFS X-mode injection produces the largest electron density in preliminary experiment in linear device.
Production of overdense plasma by XB mode conversion. ECH launching system of VEST has been designed in a low field side injection
configuration by accounting the preliminary experimental results in linear device.
IntroductionMotivation & Objectives
Bt ~875G @ center2.45GHz microwave
300 400 500 600 700 800 9000.0
1.5
3.0
4.5
6.0
7.5
n e [1
017m
-3]
Microwave Power [W]
High Field Side O-mode High Field Side X-mode Low Field Side O-mode Low Field Side X-mode
L-cutoff density
1.45 x 1017m-3
H. Y. LEE
6/15
Experimental SetupECH System and diagnostics in VEST
2.45GHz, 6kW microwave generator and 3kW magnetron is installed in main chamber of VEST.
Low field side X-mode injection configuration. WR284 / WR340 rectangular waveguide for TE10 mode propagation. Directional coupler and rf power meter for microwave power monitoring. A triple probe is fabricated and installed to diagnose the time varying plasma den-
sity and temperature during discharges.
2.45GHz, 6kW, CW
2.45GHz, 3kW, pulse
Triple Probe
7/15
30 35 40 45 50 55 60 65 70 75 80 850.0
0.2
0.4
0.6
0.8
1.0
1.2
n e [1
017m
-3]
R [cm]
X-mode_2kW X-mode_3kW X-mode_4kW X-mode_6kW
Electron Cyclotron Resonance
Chamber Port
30 35 40 45 50 55 60 65 70 75 80 853
6
9
12
15
18
21
24
T e [e
V]
R [cm]
X-mode_2kW X-mode_3kW X-mode_4kW X-mode_6kW
Electron Cyclotron Resonance
Chamber Port
30 35 40 45 50 55 60 65 70 75 80 850.00
0.03
0.06
0.09
0.12
0.15
0.18
0.21
n ekT
e [J
/m3 ]
R [cm]
X-mode_2kW X-mode_3kW X-mode_4kW X-mode_6kW
Electron Cyclotron Resonance
Chamber Port
Power absorption in UHR(ne) and ECR(Te).
Initial breakdown occurs in ECR, and then UHR move outward with electron density build-up.
Doppler shift and relativistic effect in wave-particle resonance condition.
22cepeUH
llllc vkn
Experimental ResultThe effect of ECH power on pre-ionization with pure TF
UHR
8/15
15 20 25 30 35 40 45 50 55 60 65 70 75 80 850.0
0.2
0.4
0.6
0.8
1.0
1.2n e [
1017m
-3]
R [cm]
TF current: 3.8kA
ECR UHR
15 20 25 30 35 40 45 50 55 60 65 70 75 80 850.0
0.2
0.4
0.6
0.8
1.0
1.2
n e [1
017m
-3]
R [cm]
TF Current: 5.4kAECR UHR
15 20 25 30 35 40 45 50 55 60 65 70 75 80 850.0
0.2
0.4
0.6
0.8
1.0
1.2
n e [1
017m
-3]
R [cm]
TF Current: 6.7kAECR UHR
15 20 25 30 35 40 45 50 55 60 65 70 75 80 850.0
0.2
0.4
0.6
0.8
1.0
1.2
n e [1
017m
-3]
R [cm]
TF Current: 8.2kAECR UHR
Experimental ResultThe effect of TF strength on pre-ionization with pure TF (ne)
9/15
Distance between the UHR and R-cutoff can be expressed by density scale length and magnetic field within the limit of .
Budden analysis (UHR, R-cutoff doublet) Steep density gradient and low magnetic field are favorable to XB mode conver-
sion.
When the TF current is 3.8kA, reflected wave from inner wall of the chamber makes situation similar to triplet case increasing mode conversion efficiency.
High density plasma is produced when the peak of density profile is near the inner wall or outer wall with the aid of high X-B mode conversion efficiency.
nB LL
TF Current T R C8.2kA 0.2754 0.5251 0.1995
6.7kA, 5.4kA 0.05 0.9 0.053.8kA 0.123 0.7691 0.1079
pe
ce
n
k
kkkLa
2
)1(2 2
(k, Ln: evaluated at the R-cutoff) )1(1
)1(22
22
2
eeRTC
eR
eT
20ak
Budden Parameter
Experimental ResultThe effect of TF strength on pre-ionization with pure TF (ne)
10/15
15 20 25 30 35 40 45 50 55 60 65 70 75 80 850
5
10
15
20
25
30
T e [eV
]
R [cm]
TF Current: 5.4kA1st ECR 2nd ECR
1st
2nd
15 20 25 30 35 40 45 50 55 60 65 70 75 80 850
5
10
15
20
25
30
T e [eV
]
R [cm]
TF Current: 6.7kA1st ECR 2nd ECR
1st
2nd
15 20 25 30 35 40 45 50 55 60 65 70 75 80 850
5
10
15
20
25
30 TF Current: 8.2kA
T e [eV
]
R [cm]
1st ECR
1st
15 20 25 30 35 40 45 50 55 60 65 70 75 80 850
5
10
15
20
25
30 2nd ECR TF Current: 3.8kA
T e [eV
]
R [cm]
1st ECR
1st
2nd
Experimental ResultThe effect of TF strength on pre-ionization with pure TF (Te)
11/15
250 300 350 4000
1
2
3
4
5
6
Forw
ard
Pow
er [k
W]
Time [ms]
ECH power ramp-up phase
Electron temperature peak is located in the 1st ECR at the beginning of break-down, and then another peak near the 2nd ECR layer appears at the ECH power ramp-up phase.
Second harmonic heating is observed when both 1st and 2nd ECR layer exist in chamber but X2 mode breakdown without 1st ECR layer is fail.
Pre-heated plasma will be needed for second harmonic heating (FLR effect)
Experimental ResultSecond harmonic heating
Te [eV]
TF Current: 3.8kA
1st ECR 2nd ECR
12/15
30 35 40 45 50 55 60 65 70 75 80 850.0
0.2
0.4
0.6
0.8
1.0
1.2
n e [10
17m
-3]
R [cm]
X-mode_6kW O-mode_6kW
Electron Cyclotron Resonance
Cham
ber Port
TF Current: 8.3kA
30 35 40 45 50 55 60 65 70 75 80 853
6
9
12
15
18
21 TF Current: 8.3kA
T e [eV
]
R [cm]
X-mode_6kW O-mode_6kW
Electron Cyclotron Resonance
Cham
ber Port
Experimental ResultComparison between O-mode and X-mode injection
300 320 340 360 380 400 420 440 460 480 5000
100
200
300
400
500
600O-mode Injection
Refle
cted
Pow
er [W
]
Time [ms]
Upper_X Main_X Upper_O
300 320 340 360 380 400 420 440 460 480 5000
100
200
300
400
500
600
Ref
lect
ed P
ower
[W]
Time [ms]
Upper_X Main_O Upper_O
X-mode Injection
X wave ~ X wave O wave X wave
X-mode injection is slightly better than O-mode. Power meter data shows that many of injected O-wave
is converted into X-mode in the chamber unlike X-mode injection.
X-mode has a high rate of single pass absorption while O-mode experiences multiple reflection and then con-verted X-mode is absorbed in the fundamental ECR and UHR layer.
RF power meter with directional coupler to collect the chosen wave polarization
13/15
390 392 394 396 398 400 402 404 406 408 410-1
0
1
2
0
1
2
-2
-1
0
shot #7129: PF 1 shot #7131: PF 1 + PF 3&4
PF 1 + PF 3&4 PF 1 only
Plas
ma
Curr
ent [
kA]
Time [ms]
PF 1 + PF 3&4 PF 1 only
Loop
Vol
tage
[V] R=0.089m, Z=0m
PF 1 PF 3&4
PF C
urre
nt [k
A]Experimental ResultPre-ionization effect on plasma current kick up
Trapped Particle Config-uration by PF 3&4
PF 3&4 make trapped par-ticle field structure and PF 1 provide loop voltage.
Check the plasma current kick up without vertical field for force balance.
More plasma current is generated when loop volt-age is applied in trapped particle configuration.
14/15
Experimental ResultPre-ionization effect on plasma current kick up
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.80.0
0.3
0.6
0.9
1.2
1.5 TF + PF 3&4 TF only
n e [10
17m
-3]
R [m]
Inner Wall Outer WallTF current: 8.2kA
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.80
5
10
15
20
25
30
35TF current: 8.2kA
TF + PF 3&4 TF only
T e [eV
]
R [m]
Inner Wall Outer Wall
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.80.0
0.3
0.6
0.9
1.2
1.5TF current: 5.6kA
TF + PF 3&4 TF only
n e [10
17m
-3]
R [m]
Inner Wall Outer Wall
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.80
5
10
15
20
25
30
35 TF current: 5.6kA TF + PF 3&4 TF only
T e [eV
]
R [m]
Inner Wall Outer Wall
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.80.0
0.3
0.6
0.9
1.2
1.5TF current: 3.9kA
TF + PF 3&4 TF only
n e [10
17m
-3]
R [m]
Inner Wall Outer Wall
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.80
5
10
15
20
25
30
35 TF current: 3.9kA TF + PF 3&4 TF only
T e [eV
]
R [m]
Inner Wall Outer Wall
Enhancement of pre-ionization by trapped particle configuration in overall chamber makes plasma current kick up with low loop voltage of ~1V.
15/15
390 392 394 396 398 400 402 404 406 408 410
0
2
4
6
8
10
Plas
ma
Curr
ent [
kA]
Time [ms]
shot #7124_TF current 3.9kA shot #7125_TF current 5.6kA shot #7127_TF current 8.2kA
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.80
5
10
15
20
25
30
35
T e [eV
]
R [m]
TF current 3.9kA TF current 5.6kA TF current 8.2kA
Inner Wall Outer Wall
0.0
0.3
0.6
0.9
1.2
1.5
1.8
2.1
Loop Voltage
Loop Voltage [V]
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.80.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4Inner Wall
n e [10
17m
-3]
R [m]
TF current 3.9kA TF current 5.6kA TF current 8.2kA
Outer Wall
0.0
0.3
0.6
0.9
1.2
1.5
1.8
2.1
Loop Voltage
Loop Voltage [V]
Plasma current of ~8kA is sustained using additional vertical field for force balance.
Enhanced pre-ionization plasma by trapped particle configuration.
Current ramp-up rate, maximum current and pulse length are increased as TF strength decrease.
Effect of pre-ionization and EBW heating.
~400ms
~400ms
Experimental ResultPre-ionization and EBW heating effect on plasma current
16/15
Summary & Conclusion
Fundamental X-wave injected from low field side is absorbed in UHR (ne) and fundamental ECR (Te) layer.
High density plasma is produced when the peak of density profile is near the inner wall or outer wall with the aid of high X-B mode conversion efficiency.
O-wave injected from low field side is converted into X-mode in the chamber and then absorbed with lower absorption efficiency.
Plasma current ramp-up rate and pulse length are increased by ef-fective pre-ionization and consequent higher heating efficiency.