Study on Fluctuations during the RF Current Ramp-up Phase in the

CPD Spherical Tokamak

H. Zushi1), T. Ryoukai2), K. Kikukawa2), T. Morisaki3), R. Bhattacharyay2), T. Yoshinaga1,3), K. Hanada1), T.Sakimura2), H. Idei1), K. Dono 2), N. Nishino4), H. Honma2), S. Tashima2), T. Mutoh3), S.

Kubo3), K. Nagasaki5), M. Sakamoto1), Y. Nakashima6), Y. Higashizono1), K. N. Sato1), K. Nakamura1), M. Hasegawa1), S. Kawasaki1) H. Nakashima1), A. Higashijima1)

　1)RIAM, Kyushu University, Kasuga, Fukuoka, Japan, 816-8580, 2) IGSES, Kyushu University,

Kasuga, Fukuoka, Japan, 816-8580, 3) National Institute for Fusion Science, 4) Hiroshima University, 5) Kyoto University, 6) University of Tsukuba,

zushi@triam.kyushu-u.ac.jp

4th IAEA TCM on Spherical tori and 14th International workshop on Spherical TorusENEA Frascati, October 7-10, 2008

1

MotivationWhy steady Bz is required forThe initial condition ?

Forrest PRL 1992

1. Confine the trapped electrons => toroidal precession current

2. Pressure driven/ Uni-directional PS current => seed toroidal current

Motivation (Role of Bz)Slab-Annulus plasma is unstable because of bad curvature

QUEST CPD

Fast camera image shows vertically moving modes

Slab plasma in simple torus

Fluctuations & Conversion efficiency

1.00.80.60.40.20.0Tr

ansim

issio

n

0.200.150.100.050.00Ln(m)

pol=2cm

40% 20% 10% 1%

f=8.2GHz, fce=4.92GHz

TOX v.s. Ln at f=8.2 GHz. Fluctuation level= 1% (solid-circle), 10% (dot-dashed), and 20% (dotted), and 40 % (solid-square). lpol =20 mm, and parallel refractive index N||=0.7.

According to Laqua PRL 1997

Open to Closed flux surfaces during the current ramp

#507034 External Bv Field Closed Flux Surfaceon the Flattop

Ip increases slowly with the slow increase of Pinj (~60kW for Ip ~ 1.7kA).Less effective than current jump(Pinj ~ 30kW for Ip ~ 2kA).

Cen

ter Po

st

Vertical shift is suppressed

under Bv field with higherdecay index.

t = 0.146 sYoshinaga ICPP2008

OUTLINE

1. Diagnostics of fluctuations

2. Results during the current ramp-up 2-1) fluctuations in slab-annulus plasma role of Bz

2-2) fluctuations during the current jump

3. Summary

7

MM14MM13

赤道面ポート配置

(MH3 )180゜

270゜90゜

425 .81

(MM12 )

(MH2 )

346 .40

(MM15 )

(MM16 )

(MH4 )

402 .86

(MM11 )306 .41

(MM17 )

(MM18 )

(MH1 )

590.00

525.00

590.00

525.00

RF antennaHX(CdTe-PHA)

VUV spectrometer

SX array

Visible monitor

Li-CCD

Fast camera(10s)

RotatingPumplimiter

Pump(TMP,Cryo)

Probe

Ha filter

Li-BES(50 PMTs)

Rogowskii coil

Medium speed camera(1ms)

AM-reflectometer

Stray rf power

Stray rf power

IR-TV camera

IR spectroscopy

Visible spectroscopy

(Hyougo Univ)

CT injector

(NIFS)

(NIFS)(NIFS)

(NIFS)

(Hirosima Univ.)

(Tsukuba Univ.)

CS

I.

45 Flux loop coils

8

Ex.１ Ex.２

2fcefce fceOpen Closed+Open

Bt=0.29T, Bv=40G, Ip~3kARf 8.2GHz, 60kW

I. CPD Li-imaging (CCD & LBFS)4 TFcoils

CS

TF

CCD

R

Z

10x10 fiber+50PMTs

50x50mm

Li injector

-600mm

Typical discharge0.220.200.180.160.14

Rre

s(m

) (a)

5040302010

0

Bz(

G)

R=0.25 m

(b)

2.01.51.00.50.0

Ip (

kA

) (c)

60

40

20

0

Prf (

kW

)

0.400.300.20

(d)

-0.2-0.10.00.1

Vlo

op

(V

)

R=0.106Z=0.325

(e)

50004000300020001000

0Pst

ray(

a.u

.) Z=0.225(f)

43210

LiI

(a.u

.) R=0.182Z=0.04

(g)

0.60.40.20.0

H(a

.u.)

0.400.300.20

(h)

Fluctuations in Annulus Plasma w/o Bz3.0x10

17

2.0

1.0

0.0

ne(

m-3

)

240220200180R (mm)

507795 z=-80mm

0.1

2

4

1

2

4

10

<S>x

x (a

.u.)

6

103

2 4 6

104

2

f (Hz)

507795 t=0.18-0.2s

R=0.2m,z=-0.08m

1.0

0.8

0.6

0.4

0.2

0.0

2

6

103

2 4 6

104

2

f(Hz)

2R(z=-0.08m) 2

z(R=0.2m)

0.250.200.150.100.050.00

0.220.200.180.16

507795

1 kW

Rres=164 mm

LF mode with Long correlation length

1.0

0.8

0.6

0.4

0.2

0.0

2

0.240.220.200.180.16R(m)

f=1.1kHz

(a) 1.0

0.8

0.6

0.4

0.2

0.0

2

-0.10 -0.06Z(m)

507795_1_1.1kHz_175_200ms

-0.100

-0.090

-0.080

-0.070Z

(m)

0.220.200.18R (m)

f=1.1kHz (507795_1_175_25_1.1_CS)

1.00.80.60.40.20.0

(a)

R~ 5 cmz> 2.5 cm

Bz suppresses density fluctuations

600

400

200

LiI

(a.u

.)

300250200150R(mm)

'1fr_Bz=50G' '2fr_Bz=40G' '3fr_Bz=32G' '4fr_Bz=24G' '5fr_Bz=15G'

507918Z=-50mm

0.40

0.35

0.30

0.25

0.20

IL

i/<I L

i>

-50 -40 -30 -20 -10 0Bz(G)

Bz=0G 507803(11/ 14)others 507918(11/ 14)

Density profile is not significantly affected by Bz (< 50G)However, the fluctuation level is much reduced.

Bt=0.29T Bz< 50 G Liee

e nnIII

nn *,

~~

Power spectrum and coherency are much reduced

68

1

2

4

68

10

2

Sxx

ch1 (a

.u.)

6

103

2 3 4 5 6

104

frequency[Hz]

Bz=50G (t=100-110ms) Bz=8G (t=310-320ms) Bz= 0G (t=100-110ms)

507903-918(a)

-30x10-3

-20

-10

0

Z(m

)0.200.180.16

R (m)

cohrz_507918_1_90_30_2.05_CS

1.00.80.60.40.20.0

(b)Square coherency at 2kHz

Low frequency components are reduced as Bz increases.Correlation length is drastically reduced.

Better conversion

Bz is desirable to reduce fluctuationsSteady Bz is required for the initial condition !

Forrest PRL 1992

1. Confine the trapped electrons => toroidal precession current

2. Pressure driven/ Uni-directional PS current => seed toroidal current

Ryoukai, ICPP 20083. Reduced Fluctuations => more efficient conversion

1.00.80.60.40.20.0T

rans

imis

sion

0.200.150.100.050.00Ln(m)

pol=2cm

40% 20% 10% 1%

f=8.2GHz, fce=4.92GHz

2 Fluctuations during Current Jump0.220.200.180.160.14

Rre

s(m

) (a)

5040302010

0

Bz(

G)

R=0.25 m

(b)

2.01.51.00.50.0

Ip (

kA

) (c)

60

40

20

0

Prf (

kW

)

0.400.300.20

(d)

-0.2-0.10.00.1

Vlo

op

(V

)

R=0.106Z=0.325

(e)

50004000300020001000

0Pst

ray(

a.u

.) Z=0.225(f)

43210

LiI

(a.u

.) R=0.182Z=0.04

(g)

0.60.40.20.0

H(a

.u.)

0.400.300.20

(h)

Burst in LiI and during Ip Jump

2.01.51.00.50.0

Ip (

kA

) (a)

43210

LiI

(a.u

.) R=0.182Z=0.04

(b)

-0.15-0.10-0.050.000.05

Vlo

op (

V)

R=0.106Z=0.325

(c)

4 msBlue: t=218msGreen:t=220 msRed: t=221ms

4

3

2

1

0

LiI

(a.u

.)

0.2230.2220.2210.2200.2190.218

R=0.182Z=0.04

Contour of LiI(R,Z) during Jump

Li_508279_0.218_0.2181_CS(a)

Li_508279_0.219_0.2191_CS

0.50.40.30.20.1

(b)

Li_508279_0.2195_0.2196_CS(c)

Li_508279_0.221_0.2211_CS(e) Li_508279_0.2215_0.2216_CS(g)

Li_508279_0.2218_0.2219_CS(h)

Li_508279_0.222_0.2221_CS(i)

Li_508279_0.2209_0.221_CS(d) Li_508279_0.2212_0.2213_CS(f)

Vertically aligned contour => horizontally aligned contourOpen field => Closed field lines

159 <R<215 mm 27<Z< 52 mm

Rres=194mm

4

3

2

1

0

LiI

(a.u

.)

0.2230.2220.2210.2200.2190.218

R=0.182Z=0.04

0.6

0.5

0.4

0.3

0.2L

i (R

)0.210.200.190.180.170.16

R (m)

'0.2206_0.2207' '0.2210_0.2211' '0.2212_0.2213' '0.2215_0.2216'

t=0.1ms

Profile flattening occurs 1ms before the burst

Coherency of 1kHz during Jump

cohrz_508279_1_211_2_1.17_CS (a) cohrz_508279_1_213_2_1.17_CS

1.00.80.60.40.20.0

(b) cohrz_508279_1_217_2_1.17_CS (c)

cohrz_508279_1_219_2_1.17_CS (d) cohrz_508279_1_221_2_1.17_CS (e) cohrz_508279_1_223_2_1.17_CS (f)

4

3

2

1

0

LiI

(a.u

.)

0.2240.2200.2160.212

R=0.182Z=0.04

Highly coherent mode at f~1 kHz dominates the viewing area

Summary

1. Initial Bz suppresses the density fluctuations, suggesting that the injected ECW can be converted efficiently to EBW

2. 2D structure of the density fluctuations shows that highly coherent mode at low freq. with very long correlation length grows just before the burst of LiI only during the current jump.

2 Current Jump in CPDNormal X-mode injectionCo-directional O-mode injection

ThresholdP ~ 20 kW

ThresholdP ~ 25 kW

Co-directional O-mode injection seems more efficient for achieving current jump than normal X-mode.

The critical parameter for current jump phenomenon should be the value of Ip, since Ip just before and just after current jump are almost identical.

The difference of the threshold power on the incident mode may be due to the heating efficiency in each mode. Ip itself may be determined from some equilibrium conditions.

Yoshinaga IAEA (2008)

for X-mode injection (X-B scenario) (Normal to B-field)

for O-mode injection (O-X-B scenario) (injection angle is adjustable)

Midplane

Reflector Shaft

Reflecting mirror

Microwave (8.2GHz) Launching System in CPD

Incident wave must be converted into Electron Bernstein Wave (EBW) mode to heat the core plasma region in overdense plasmas.

Microwave launchers from 8 klystrons are separated into normal X-mode injectors and co-directional O-mode injectors to study injection mode dependencies.

Current Jump discharge in CPD

Current Jump

3ms

EC

R

2nd

EC

R

EC

R

2nd

EC

R

External Bv FieldClosed Flux ConfigurationAfter Current Jump

Cen

ter Po

st

Cen

ter Po

stt = 0.153 s t = 0.157 s

#506967

Vertical Shift is observed both in magnetic flux and H image.

Non-Inductive Current Generation by ECH via Current Jump

Current Jump occurs under relatively high Bv (≥ 30 G).

In this range of Bv, Ip saturates at ~ 1.8 kA without current jump.This suggests that the current jump is necessary to obtain higher Ip under higher Bv.

Under Bv ≤ 30 G, Ip increases slowly and there is no clear current jump.

Finally achieved Ip with and without current jump increases roughly proportional to Bv.

Pinj < 60 kW

Summary of ECH current start-up experiments in CPDCurrent jump discharges have been observed in CPD.

This suggests that the current jump occurs commonly in ST devices.

Current jump is necessary to achieve higher Ip under higher Bv.

-2

0

2

ph

ase

(ra

d)

6

103

2 4 6

104

2

f(Hz)

R(z=-0.08m) z(R=0.2m)

(a)

10

8

6

4

2

0

S xx(a

.u.)

0.240.220.200.180.16R(m)

'f=1.2kHz' 'f=4.1kHz'

z=-84mm

外部垂直磁場Ip = 0 kA

電流ジャンプにおけるポロイダル磁場構造の時間発展Flat top1.5 kA

電流ジャンプ

60

400 2

0R(cm)

EC

R

2nd

EC

R

0.8 kA

0

20

40

60

-20

-40

-60

Z(cm)

1.5 kA1.4 kA1.2 kA1.0 kA

R=35.8cm

Ip = 0.8 kA 1.2 kA1.0 kA 1.4 kA

磁場構造の変化にあわせて tip B の浮遊電位が低下

tip A

tip B

-100V -1000V

-30V