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.