High Harmonic Fast Wave Experiments on TST-2

Y. Takase, A. Ejiri, S. Kainaga, H. Kasahara1), R. Kumazawa1), T. Masuda,

H. Nuga, T. Oosako, M. Sasaki, Y. Shimada, F. Shimpo1), J. Sugiyama,

N. Sumitomo, H. Tojo, Y. Torii, N. Tsujii, J. Tsujimura, T. Yamada2)

12th International Workshop on Spherical Torus 2006 Chengdu

11-13 October 2006

University of Tokyo, Kashiwa, 277-8561 Japan1)National Institute for Fusion Science, Toki, 509-5292 Japan2)Kyushu University, Kasuga, 816-8580 Japan

• TST-2 spherical tokamak and RF system

• HHFW experiment– Electron heating experiment– Wave diagnostics

• RF magnetic probes• Reflectometry

– Wave measurements• parametric decay• scattering

– TORIC full-wave analysis

• EC start-up experiment

• Plans – 200MHz experiments on TST-2– RF sustainment of high plasmas in UTST

Outline

TST-2 Spherical Tokamak

ECH: 2.45GHz (< 5 kW)HHFW: 21MHz (< 200 kW x 2)

ECH

HHFW

R / a = 0.38 / 0.25 m (A = 1.5)

Bt = 0.3 T / Ip = 0.1 MA

21 MHz Matching/Transmission System

• RF power

400 kW

• Frequency f = 13, 21, 30 MHz (/H ~ 7 at BT = 0.2 T, f = 21 MHz)

• Toroidal wavenumber k = n/R0 = 11, 16, 26 m-1

(n = 4.3, 6, 10)

varied by changing the strap spacing

• Faraday shield angle ~ 30°

currentstraps(0, )

Mo limiters

Faraday shield

Variable k Two-Strap Antenna

Single-pass Absorption Calculation

• Single-pass absorption is greater for double-strap excitation

• Single-pass absorption– increases with ne

– increases with Te

– decreases with Bt

(increases with e )

0.0

2.0

4.0

6.0

8.0

10

0.0 5.0 10 15 20 25 30

ne dependence for single and

double strap excitations

Single strap[%] Double strap[%] Single strap[%] Double strap[%] Single strap[%] Double strap[%]

Sin

gle

pas

s ab

sorp

tion

[%]

Toroidal mode number

Te = 300 eV

BT = 0.2 T

5.0x1019 m-3

1.0x1019 m-3

3.0x1019 m-3

double-strap

single-pass absorption

== 0.18

ELD + TTD

ELD + TTD + CROSS

ELD

ELD + CROSS

Imag ( k⊥)

dxke

)Im(21

　 Bt = 0.15 T　 ne = 1.0×1019 m-3

　 Te = 100 eV　 n = 10

Single-Pass Absorption Improves with e

• Analysis of HHFW heating scenarios used on TST-2 is being carried out using the TORIC full-wave code.

Bt = 0.2 T, f = 21 MHz, n = 10, ne0 = 2 1019 m-3, Te0 = 0.2 keV

TORIC Full-Wave Calculations

Electron absorption: 100%

• Soft X-ray increased, but density and radiated power did not change　 electron heating

• Strongest response near plasma center

t (ms)

Ip

nel

Prad

SX (> 200 eV)

360 kW RF

Electron Heating by HHFW

Low

field

sid

eH

igh fi

eld

sid

e

~ R0

180kW360kW

no HHFW

R=0.19m

R=0.26m

R=0.38m

R=0.43m

R=0.54m

Center

PS noise

• Increases in stored energy and visible-SX emission are greater for double-strap excitation

– Consistent with single-pass absorption calculation

Single-Strap vs. Double-Strap Excitation

double-strapsingle-strap

no RF

with RF

Edgeemission

visible-SXemission(A.U.)

PNET = 120 kW PNET = 120 kW

• RF magnetic probes– Sensitive to electromagnetic component

– Plasma edge only

• Reflectometry– Sensitive to electrostatic component

– Can probe the plasma interior

• Both parametric decay instability (PDI) and frequency broadening due to scattering by density fluctuations were observed.– These processes can alter the wavenumber spectrum, and affect bot

h wave propagation and absorption.

Wave diagnostics on TST-2

φ＝ -60°

φ＝ -30°

φ＝ -55°φ＝ -65°

φ＝ -115°

φ＝ -120°

φ＝ 155°φ＝ 150°

φ＝ 145°

φ＝ 65°φ＝ 60°φ＝ 55°

φ＝ 30°

φ＝ 0°

Top viewS.S. enclosure

Slit

Core (insulator)

1-turn loop

Semi-rigid cable

2cm

Direction ofB field to bemeasured

RF Magnetic Probes at 14 Toroidal Locations

toroidal direction

Bz B

φ＝ 9°

φ＝ -9°

φ＝ -125°

B

RF21MHz

eit

One of three sources is used Frequency sweepable VCO for profile measurements Fixed Gunn Osc. (25.85 or 27.44 GHz) for RF measurements

Ep x Bt

Aeit

Aeit+i

I

cos(p+t+RF)

sin(p+t+RF)

VCO6-10GHz

X4

QLO RF

coaxial waveguidescalar horn

Digitizer or Oscilloscope

<250MHz sampling

F.G.

X5

X10

Gunn25.85 or 27.44 GHz

D.C.-3dB

5-20mW

DC-500MHz

24-40GHz100mW

TST-2 Reflectometer System

Window200

TFCoil

TST-2V.V.

Mirror

Mirror

HornAntennas

Microwave Reflectometer

Most probable decay process:

High Harmonic Fast Wave (HHFW)

Ion Bernstein Wave (IBW)+

Ion-Cyclotron Quasi-Mode (ICQM)

Magnetic field dependence

H at outboard edge

Threshold power ~ 20 kW

High power

Lowpower

Parametric Decay: FW IBW + ICQM

Parametric Decay Instability (PDI)

PDI

①

②

probe②

RF probe

1.8 MHz

HHFW250 kW

Time [s]

Reflectometer25.85 GHz (cosine)

Reflectometer25.85 GHz (sine)

RF Probe (dB/dt)Antenna Limiter, P12

Frequency [MHz]

Comparison of RF Probe and Reflectometer Spectra

QMIBW

Comparison of RF Probe and Reflectometer Spectra

Reflectometer (cos)

Reflectometer (sin)

RF Magnetic Probes

f (MHz)

FW ?

IBW

QMIBW

noise (Al reflector)

2510 20

0

-20

-40

-60

-800

P (dB)

0

-20

-40

-60

-80

P (dB)

0

-20

-40

-60

-80

P (dB)

155

PDI becomes stronger as the plasma outer boundary approaches the antenna

Rout

Dependence on Plasma Position

antennalimiter

①

②

③

④

① ②

③ ④

Outboard vs. Inboard Comparison

• Inboard spectrum similar to outboard, but weaker

RF probes

φ＝ 21°straps

φ＝ 39°

R=125 R=630mmR=700mm

Z = 0mm

Z = -150mm

port10

①

②③

④

① ②

③ ④

• Broadened spectrum is only weakly dependent on vertical position

midplane B

midplane Bz

Z = 150mm

Inboard Side Spectra

RF probes

Vacuum Plasma

① ①

② ②

Frequency broadening of the pump waveby the plasma is observed.

Possible processes:

• Parametric decay• Scattering by density fluctuations

Frequency Broadening

①

②

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

7.0 7.5 8.0 8.5 9.0

ch1ch4ch3ch2

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

7.0 7.5 8.0 8.5 9.0

ch1ch4

ch3ch2

f (HHFW)Pump wave power

0

0.0005

0.001

0.0015

0.002

0.0025

7.0 7.2 7.4 7.6 7.8 8.0 8.2 8.4 8.6 8.8 9.0

ch1ch4ch3ch2

Lower sideband power

①

②

③

④

0

0.5

1

1.5

2

2.5

7.0 7.5 8.0 8.5 9.0

R = 195

R = 260

R = 370

SX / SX

Density Dependence Varies with Probe Position

10dBf (HHFW)

• PDI is generally reduced at high density

• Only weak effect on heating

Preliminary

nelnel

• A k|| variable antenna was installed in TST-2, and the RF power capability was increased to 400kW. – Dependence on k|| spectrum (same spectral shape but different k||)

will be studied.– Single-pass absorption is expected to change from 10% to 35% wh

en ne0 = 3.01019 m−3 and Bt = 0.3T.

• In electron heating experiments, soft X-ray emission increased with RF power.– Stored energy increase was larger for double-strap excitation. – More direct measurement of Te is necessary (TS in preparation).

• Analysis of HHFW scenarios used on TST-2 is being carried out using TORIC.

Summary (HHFW Heatng)

• PDI and frequency broadening due to scattering were observed by RF magnetic probes.– The strength of PDI increased as the outer boundary of the plas

ma approached the antenna.

– Density dependence varies with RF probe location.

– Parametric decay became weaker at high densities where single-pass absorption is predicted to become stronger.

– The effect of parametric decay on plasma heating is not clear.

• Initial results of RF wave detection inside the plasma by microwave reflectometry were obtained.– PDI spectrum clearly observed

– Differentiation of ES and EM components may be made.

Summary (RF Measurements)

EBW Heating on TST-2@K (2003)

(dW/dt) indicates Pabs/Pin > 50% when ne in front of antenna is steep enough

Thursday: S. Shiraiwa, et al.,“Study of EBW Heating on TST-2”

< 200 kW@ 8.2 GHz

Typical EC Start-up Discharge

(a) Bt decreases gradually. (RECH decreases gradually.)

(b) IPF is kept constant.

(c) PEC is kept constant.

(d) Ip increases with time, but disappears when the = e layer moves out of the vacuum vessel.(up to 0.5 kA produced by 4 kW)

(e) ne is almost constant near the cutoff density.

0.00

0.25

RE

CH

[m

]

(a)

0

3

PF

2co

il c

urr

ent

[kA

] (b)

0

5

PE

CH

[k

W]

Forward

Reflection

(c)

0

0.2

0.4I p

[k

A]

(d)

0

6

12

10 20 30 40 50 60NL

[10

16m

-2]

Time [ms]

(e)

inboard limiter

Previously achieved: 1kA/1kW (2.45GHz) 4kA/100kW (8.2GHz)

Dependence on power and resonance position

0

0.1

0.2

0.3

0.4

0.1 0.15 0.2 0.25 0.3

#43371#43372

I p [

kA]

RECH

[m]

Diameter of the CS

0.0

0.1

0.2

0.3

#43371#43372

RE

CH

[m

] (a)

0.0

0.1

0.2

0.3

0.4

0 10 20 30 40 50 60 70

I p [k

A]

Time [ms]

(b)

0

0.1

0.2

0

5

10

0 1 2 3 4

I p [

kA

]

NL

[1016

m-3

]

PNET

[kw]

Ip depends on the = e resonance position (RECH).

Ip increases with PEC, whereas ne saturates around the cutoff density7 1016 m-3 [NL = (5-6) 1016 m-3].

time

inboardlimiter

Dependence on PF Strength and Decay Index

PF2+PF5 PF1PF2PF31m

-1m0.1m 0.7m

Mirror ratio

0

0.1

0.2

0.3

0.4

0 0.2 0.4 0.6 0.8 1 1.2

PF3PF2&PF5PF2PF1

I pm

ax [

kA

]

Bz [mT]

• Ip maximizes at a certain field strength.

• Highest Ip is achieved with PF2 (medium decay index).

Effect of HHFW Injection

3 cases are compared:

● ECH only

● ECH + HHFW

● ECH turned off during HHFW

0

5

PE

CH

[k

W] Forward

Reflection

(b)

0.0

0.1

0.2

RE

CH

[m

]

RCS

(a)

0.0

0.3R

adia

tion

[a.u

.](e)

0

0.1

0.2

0.3

0.4

#42904#42905#42915

I p [k

A]

(d)

0

10#42904#42905

PH

HF

W [

kW

]

(c)

Net Power

0

4

8

10 20 30 40 50

NL

[10

16m

-2]

Time [ms]

(f)

• Ip responds quickly to HHFW

• ne and Prad increase during HHFW

• After ECH turn-off, Ip decays and HHFW reflection increases.

Single Particle Orbit Analysis

• Phase space for confined orbits is large for low A devices.

• Orbit-averaged toroidal precession is co-directed for all confined orbits.

(c) ctr trapped(b) co trapped(a) circulating

0.1 0.8 m-1

+1

Confined region in phase space(inside outermost blue boundary)

(b)

(a)(c)

V||/V0

V /V0

0

1

-0.4 0 1.2

Electron orbits starting from R = 0.38m / Z = 0m

Velocity is normalized by V0=Rsp

(p is the cyclotron frequency corresponding to Bp)

A. Ejiri, et al., to be published.

Driven Current Based on Single Particle Analysis

LMINS

LMINScurv

e

e

pse

RRR

ZZRR

F

FmkT

Renj

,2

Max ,12

32)( where

)(2

2

1~

2max

2max

2/32

2

3

3

Under a low Te (or high BZ) approximation (VTe << V0), j is given by

)( 34F

RR

B

Bp

Sp

p

)(~ 3 FBRp

jpS

This current has the same parameter dependence and the same order of magnitude as pressure driven currents

The generated field is

where R is the thickness of the EC deposition layer

Comparison with Experiment

0

0.1

0.2

0.3

0.4

0 0.2 0.4 0.6 0.8 1 1.2

PF3PF2&PF5PF2PF1

I pm

ax [

kA

]

Bz [mT]2

/[mT] 5.0

ee

pSz

mT

RB

• Bz dependence agrees qualitatively with experiment.

• Dependence on PF curvature is different.

• Predicted current for PF3 is negligibly small.

Assuming correspondence of Ipmax for PF2

[eV] 200~eT

0

0.1

0.2

0.3

0.4

0 1 2 3 4 5 6

<d/

dt>

RS/(

Te/m

e)1/2

RS

p/(T

e/m

e)1/2

PF1

PF2

PF2+PF5PF1 (Analytic)

,1~)( ],[m 10

[m], 01.0 [m], 4.0

3317

max

Fn

RZ

e

Further assuming

][kA 4.0~pI

estimate for driven Ip is

Conclusions (EC Start-up)

• Ip of up to 0.5 kA was obtained by ECH start-up (2.45 GHz / 4 kW) .

• Ip increases with the decrease in Bt.

• Highest Ip was achieved with PF configuration with medium index and appropriate field strength.

• Ip and ne increase with PEC, but ne saturates around the cutoff density.

• Ip increased by up to 0.4 kA by addition of HHFW power.

• Analytic expressions for generated current were derived based on single particle orbits:

– Generated current has a similar form to pressure driven currents. – Self-field generation becomes siginificant at high p.

• The proposed model and experimental results are not inconsistent, but further theoretical and experimental studies are needed.

Preparation in Progress for 200 MHz Experiments

E

Bt = 0.3 T, f = 200 MHz, n = 10, ne = 2 1018 m-3, Te = 0.3 keV

Full-wave calculation by TASK/WM

TST-2200 MHz transmitters (from JFT-2M)

Combline antenna (GA)

UTST

• HHFW (~ 20 MHz)• LHFW (~ 200 MHz)

TS-3 / TS-4

TST-2

UTST

A New Ultra-High ST Based on Plasma Merging

Merging scenario

Tokyo

Formation of ultra-high ST plasma using plasma merging

Sustainment using innovative RF methods

Friday: Y. Ono, et al., “Initial operation of UTST High-Beta Spherical Tokamak Merging Device”