Plasma Start-up, Sustainment, and Heating by RF Waves in TST-2

Y. Takase, A. Ejiri, Y. Nagashima, O. Watanabe, Y. Adachi, B. An, H. Hayashi,

S. Kainaga, H. Kobayashi, H. Kurashina, H. Matsuzawa, T. Oosako, J. Sugiyama,

H. Tojo, K. Yamada, T. Yamada, T. Yamaguchi, T. Masuda, Y. Ono, M. Sasaki

The University of Tokyo

Joint Meeting of the4th IAEA Technical Meeting on Spherical Tori

and the14th International Workshop on Spherical Torus

7-10 October 2008Frascati, Italy

TST-2 Spherical Tokamak

Nominal parameters: R = 0.38 m a = 0.25 m Bt = 0.2 T Ip = 0.1 MA

HHFW 21 MHz400 kW

ECH 2.45 GHz5 kW

Part I. Noninductive Ip Start-up and Sustainment

• Ip start-up by ECW (2.45 GHz)

– Three phases of Ip start-up

– Dynamics of closed flux surface formation

– MHD activity and Ip collapse

• Ip sustainment by RF (21 MHz) power alone

Three Phases of Ip Start-up by ECH

3Phase 1 2

Open Field Lines Current Jump

Closed Flux Surfaces

z[m]00.4

-0.4

-0.40

0.40

0.4-0.4 x[m]y[m]

0 0.5

0

0.6

-0.6

will use dIp/dt(17ms ～ 22ms)

0 0.5

0

0.6

-0.6

00.1-0.1

00.4

-0.4 0 0.4-0.4

z[m]

x[m]y[m]

RF (21MHz) power can induce a current jump.

Plasma current can be sustained by RF power alone.

Antenna excites waves with a broad spectrum of toroidal mode numbers, up to |n| ~ 20. But only |n| = 0, 1 can propagate to the core.

Ion heating is not expected due to high harmonic number (> 10). Ion (H/D, C, O) heating was not observed.

Electron heating is expected to be weak due to low T. Soft X-rays (~ 2 keV) were observed at high RF power (~ 30 kW).

RF onlyRF

RF sust.EC sust.

80 ms RF only

Sustainment by RF Power

Initial Current Ramp-up Rate and Ip-Wk Trajectory

Dependence on various parameters are summarized by a scaling lawJ. Sugiyama et al., Plasma Fusion Res., 3, 026 (2008).

0.0

0.2

0.4

0.6

0.8

0 0.02 0.04 0.06 0.08 0.1

Pla

sma

curr

ent

[kA

]

Stored kinetic energy [J]

Z

KP BR

WI

2

open: before jump

closed: after jump

KP WI is confirmed by equilibrium analysis

RFEC

Single-particle orbit theory predicts

A. Ejiri and Y. Takase, Nucl. Fusion, 47, 403 (2007).

mag. axis

jt

F term

p termTruncated boundary

LCFS

pol. flux tor. current force bal.

flux loops saddle loops pickup coils

pressure pol. field pol. flux

pol. angle pol. anglez

“Truncated equilibrium” was introduced to include finite pressure and current in the open field line region. A. Ejiri et. al., Nucl. Fusion 46, 709 (2006).

Truncated equilibrium can reproduce magnetic measurements (~80 channels), and can be used to analyze all three phases.

)()1( 00

000

0

FR

r

r

Rj

d

dff

Rd

dpRj

pp

Equilibrium Reconstruction

Dynamics of EC Induced Current Jump

EC induced current jump occurs when Ip exceeds a critical value Ip,crit where

Ip after current jump is given by

Equilibrium reconstruction reveals slow and soft formation of initial closed flux surfaces. While Ip increases rapidly, Wk and Rjmax increase slowly.

0.0

0.4

0.8

Pla

sma

curr

ent

[kA

]

(a)

IP I

PLCFS

0.2

0.3

0.4

0.5

0.6

0

0.1

20 25 30 35 40 45 50

Rjm

ax

Sto

red

En

ergy

[J]

Time [ms]

(b)

Just before and after closed flux surface formation

kA/mT 5.0, z

critp

B

I

kA/mT 2.1z

p

B

I

Conditions for RF Induced Current Jump and Sustainment

RF induced current jump occurs when the injected RF power exceeds a threshold, which is different for H and D.O. Watanabe, et al., Plasma Fusion Res. 3, 049 (2008) .

Injection timing should be just before a current jump.

Current jump does not occur by RF power alone, and Ip stays at a low level < 0.3 kA.

Only low n|| waves can propagate to the plasma core, and formation of high energy electrons is expected. Soft X-ray energy spectrum indicates the presence of high energy (2-3 keV) electrons. However, Ip can be sustained even when soft X-rays are not observed. 0

2

4

6

8

10

0.5 1 1.5 2 2.5 3Energy [keV]

Log (N)

Exp(-E/42eV)

Comparison of Equilibria during Sustainment

Truncated boundary

LCFS

j

Inboard limiter

LCFS Outboardlimiter

#53773 50ms

Truncated boundary

LCFS

j

Inboard limiter LCFS Outboard

limiter

#53783 50ms

Truncated boundary

LCFS

j

Inboard limiter LCFS Outboard

limiter

#53197 90ms

RF sustained, Ip = 0.6 kA EC sustained, Ip = 0.6 kA EC sustained, Ip = 1.3 kA

%80/ ,70~ ,4.1[ms] 01.0~[eV], 20~

0

0

pLCFS

pLCFS

E

IIq

T

%93/ ,50~ ,1.1

[ms] 02.0~[eV], 45~

0

0

pLCFS

pLCFS

E

IIq

T

%70/ ,50~ ,0.1

[ms] 05.0~[eV], 180~

0

0

pLCFS

pLCFS

E

IIq

T

Ip Collapses are Often Observed during RF Sustainment

Power spectra of inboard BZ

RF sustainmentw/o collapse

ECH alone

Inboard Bz

Outboard Bz

Ip

4 discharges with almost the same operational conditions

Low and high frequency components are observed for collapsed discharges

Expansion of Open Field Line Region is Observed before MHD Activity

Phenomelogy of Ip collapse

Slow fluctuations

Rapid growth of high frequency

fluctuations

Ip collapse

EC

RF

Collapsed discharges are different from the beginning of RF pulse.

Inward shift and expansion of open field line

region

0.3

0.4

0.5

Rax

[m

]

0.0

0.4

0.8

42 44 46 48 50 52

Vol

ume

[m3 ]

Time [ms]

Inside LCFS

Whole

Summary

• Sustainment of ST plasma by low frequency RF power was demonstrated.

• Equilibrium analysis revealed detailed information during each phase of discharge.

• Initial current formation phase is characterized by a slow increase in Ip, proportional to the stored energy.

• During the current jump phase, initial closed flux surfaces are formed gradually, and changes in Wk and Rjmax are small. soft dynamics

• Sustained ST plasma has high p>1 and high q0>30

• MHD instability often terminates the RF sustained plasma, but no such phenomenon is observed for the EC sustained plasma.

Part II. HHFW Heating and Parametric Decay

• Electron heating

• Parametric Decay Instability (PDI)– Parameter dependences– Newly discovered sub-harmonic decay branch

Introduction

• A degradation of heating efficiency is observed during high-harmonic fast wave (HHFW) heating of spherical tokamak plasmas when parametric decay instability (PDI) is observed. Understanding and suppression of PDI is necessary to make HHFW a reliable heating and current drive tool in high plasmas.

• In TST-2, wave measurements were made using a radially movable electrostatic probe (ion saturation current and floating potential), RF magnetic probes distributed both toroidally and poloidally, microwave reflectometry, and fast optical diagnostic.

Typical Discharge Heated by HHFW(Inboard Shifted Plasma)

• Te = 140 170 eV over 0.4 ms after RF turn-on (PRF = 200 kW)

• PDI becomes stronger and Te decreases slowly after 0.4 ms (causality?)

ES probe = 165

inner wallprobes

Reflectometer = -75

RF Diagnostics

HHFWAntenna

front surface of S.S. enclosureat R = 635mm

RF magnetic probes

I

cos(p+t+RF)

sin(p+t+RF)

VCO6-10GHz

X4 5-20mW

DC-500MHzQLO RF

coaxial scalar horn

RF21MHzeit

Ep x Bt

Aeit Aeit+i

Digitizer (25MHz) or Oscilloscope (~250MHz)

F.G.

X5

X10

Gunn25.85 or 27.44 GHz

DC-100MHz

waveguide

D.C.-3dB

cutoff surface

Mirror

Second Mirror

~500mm

Launching horn

Receiving horn

24-40GHz100mW

Microwave Reflectometer

0

100

200

300

HH

FW

[kW

]

0

20

40

60

4521845219452244522745189

I p

[kA

]

0

0.1

Sof

t X

-ray

[a.u

.]

0

5

10

Rad

iati

on[a

.u.]

0

20

40

PF

3[k

A]

45218

45219 45224

45227

45189

0

5

16 17 18 19 20 21 22

NL

[1018

m2 ]

Time [ms]

PDI Spectra Measured by Reflectometer

Reflectometer

H plasma

f=1MHz

Pow

er [

10dB

]

1.7 MHz

19.3 MHz

21 MHz

22.7 MHz

#45244

20.0 20.5 21.0

Pow

er [

10dB

]

Time [ms]

1.7 MHz

19.3 MHz

21 MHz

22.7 MHz

#45227

HHFW

HHFW

10-7

10-6

10-5

10-7 10-6 10-5 10-4

P(-f ci

)

P(f0)

P(f0)2

Time Evolution and Power Correlation

Reflectometer

Sideband power varies quadratically with the pump wave power

Electrostatic Probe

Digitizer channelsCh. 1: mag. probe at = 155Ch. 2-4: ES probe at = 165 2: f1

3: Iis

4:f2

)()(2tY )()(

2tZ

)()()(

)()(22

2*

tZY

YZ

)(

)()(Re

)()(Imtan

*

*

1 tYZ

YZ

titi eZtzeYty )( ,)(

sampling rate: dt (2 ns)data window for FFT: N (10000)overlapping of data window: N/2 (5000)points for smoothing along time : m (49)

time resolution for : dtNm/2 = 0.49 ms tfZ2

)(

Spectral Analysis of RF Data

tfZ pump

2)(

tz ty

0t

pumpf 0

2)( tfZ

Time Evolution and Power CorrelationB and Iis

B~

isI~

correlation becomes higher and phase shift becomes definite during second half of RF pulse

H plasma

1

~f2

~f

Time Evolution and Power Correlationf1 and f2

correlation is nearly one and phase shift is almost zero for the pump wave

isI~

1

~f

Time Evolution and Power Correlationis and f1

correlation is intermediate and phase shift is non-zero

Newly Discovered Sub-Harmonic Decay Modes

f3

f2

f1

f0

f

deuterium

hydrogen

f increases with B

Two additional peaks were discovered between f0 and f0 – fcH in H plasmas(note that there is a dip at f0 – fcD)

– These modes may involve molecular ions or partially ionized impurity ions.

R (mm)R (mm)

isI~

2

~f

Radial Fall-off is Steeper for is than f2

outboard limiter

Phase Difference Between Neighboring RF Probes

2/

2/

t (ms)

t (ms)

2/

t (ms)

t (ms)

RF

B~

= - 65 = - 55

= - 0.5 corresponds to |n| = 18 Phase shift is not constant throughout the RF pulse.

B~

@

sampling rate: dt (2 ns)data window for FFT: N (500)

no window overlapping no smoothing time resolution = 1 s

B Dependence of Frequency Spectrumat Different Locations

D plasma

• RF probes on the outboard side have similar signal levels.

• RF probe on the inboard side has much smaller signal levels compared to the outboard side in low B discharges, but comparable in high B discharges.

• The vertical (poloidal) polarization is much weaker than the horizontal (toroidal) polarization.

• The frequency difference between the pump wave and the lower sideband wave increases with the magnetic field.

• The lower sideband becomes weaker, and the lower sideband peak becomes unresolved at low magnetic field.

Summary of RF Magnetic Probe B Scan

Summary of PDI Observations

• The frequency spectrum exhibits peaks at ion-cyclotron harmonic sidebands f0 ± nfci and low-frequency ion-cyclotron harmonics nfci, consistent with the HHFW pump

wave decaying into the HHFW or ion Bernstein wave (IBW) sideband and the ion-cyclotron quasi-mode (ICQM).

– PDI becomes stronger at lower densities, and much weaker when the plasma is far away from the antenna.

– The lower sideband power was found to increase quadratically with the local pump wave power.

– The lower sideband power relative to the local pump wave power was larger for reflectometer compared to either electrostatic or magnetic probes.

– The radial decay of the pump wave amplitude in the SOL was much faster for Iis than for f.

Conclusions