RF Start-up, Heating and Current Drive Studies on

TST-2 and UTST

Y. Takase, TST-2 Team, UTST Team

The University of Tokyo

The 15th International Workshop on Spherical Tori 2009

22-24 October 2009Madison, Wisconsin, U.S.A.

TST-2

1UTST

R 0.38 m a 0.25 m B 0.3 TIp 0.2 MA

Outline

• Ip start-up experiments on TST-2

• High-harmonic fast wave (HHFW) experiments on TST-2 and UTST

• Plan for LHCD experiment on TST-2

2

Ip Start-up Experiments on TST-2

• In TST-2, Ip start-up, ST plasma formation and sustainment have been achieved by EC power (up to 5 kW at 2.45 GHz). – When Ip reaches a critical value, Ip increases abruptly (current jump) and

reaches a steady sustainment level Ipsus which is proportional to Bz.

– Before current jump the field configuration is open.

– After current jump an ST configuration with closed flux surfaces is sustained.

• Once initial plasma is formed, RF power (up to 30 kW at 21 MHz) injected using the HHFW loop antenna can induce a current jump and sustain the ST configuration with the same Ip

sus as the EC sustained case.

3

2-Strap HHFW Antenna(only 1 strap was used)

21MHz, up to 400 kW(up to 30 kW was used)

TST-2 Spherical Tokamak and Heating Systems

X-mode launch horn antenna for ECH2.45 GHz, up to 5 kW

R = 0.38 ma = 0.25 mA = 1.5

4

3 Phases of Ip Start-up by ECH

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

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]

Ip increases rapidly once Ip reaches a critical level determined by Bv.

5

particle orbit

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

Antenna excites a broad toroidal mode number spectrum, up to |n| ~ 20. But only |n| = 0, 1, 2 can propagate to the core.

Ion absorption is not expected due to high /ci (> 10). Ion (H/D, C, O) heating was not observed (< 10 eV).

Electron absorption is expected to be weak due to low e. Soft X-rays (up to 3 keV) were observed at high RF power (~ 30 kW).

Sustainment by RF Power Alone

0

2

4

6

8

10

0.5 1 1.5 2 2.5 3Energy [keV]

Log (N)

Exp(-E/42eV)

RF onlyRF

RF sust.EC sust.

Ip can be sustained by RF power alone.

6

Truncated Equilibrium

To treat finite p and j in the open field line region, “truncated equilibrium” is used. [A. Ejiri et. al., Nucl. Fusion 46, 709-713 (2006).]

. )4 ,2 ,1 ,5.0(over discretely variedare ,. channels) 80~( tsmeasuremen magnetic tofittingby

determined are ) , , and( , , Parameters

11 );(

11 );( where

);()1();(

as expressed isequation S-G

0

2

2

00

000

0

EddyPFpp

f

p

fppp

III

F

F

FR

rF

r

Rj

d

dff

Rd

dpRj

0.0

0.2

0.4

0.6

0.8

1.0

Fp

or F

()

(a)

=0.5 12

4

0.0

0.2

0.4

0.6

0.8

1.0

0 0.2 0.4 0.6 0.8 1

Pre

ssur

e

1/2

(b)

=0.5 1 2 4

Outboardlimiter

R

Top limiter

Bottom limiter

LCFS

Inboardlimiter

The following effects are not taken into account:• anisotropic pressure• parallel pressure gradient

Truncation boundaries

7

Equilibrium Reconstruction

Flux loops

Pickup coils

Saddle loops

Vacuum field and locations of magnetic measurements

x10

Red: measurementsBlack: fitDistribution of Ieddy is pre-calculated for given Ip(t).

Ieddy can become ~1/3 of Ip during current jump.

8

Evolution of Equilibrium

0.1 0.3 0.5 0.7-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.0

1.0

0.0

0.1

Pre

ssu

re [

Pa

]j

[kA

/m2]

Z [

m]

R [m]

#53783, t=25ms

jf

total

ff’

p’

(a)

0.1 0.3 0.5 0.7-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0

100.0

0.4

j f [k

A/m

2 ]Z

[m

]

R [m]

#53783, t=40ms

j

total

ff’

p’

LCFS

(b)

0.1 0.3 0.5 0.7-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0

80.0

0.4

j f [k

A/m

2 ]Z

[m

]

R [m]

#53783, t=50ms

j

total

ff’

p’

LCFS

-4

(c)

0.0

0.5

1.0 #53783

Plas

ma

curre

nt[k

A]

(a)Total

Inside LCFS

,

(b)

1

0.5

2

4

0.00

0.05

0.10

0

0.5

1

Stor

ed e

nerg

y[J

]

P max

[Pa]

(c)

0.4

0.6

Maj

or ra

dius

[m]

(d)

Rax

Rjmax

0.0

0.5

1.0

Plas

ma

volu

me

[m3 ] (e)

Total

Inside LCFS

0.0

1.0

2.0

20 30 40 50 60 70 80 90N

orm

aliz

ed

2

(f)

Time [ms]

(a) (b) (c)

appearance of closed flux surfaces

9

Comparison of Equilibria during Sustainment

Truncated boundary

LCFS

j

Inboard limiter LCFS Outboard

limiter

#53783 50ms

EC sustained, Ip = 0.6 kA

Truncated boundary

LCFS

j

Inboard limiter

LCFS Outboardlimiter

#53773 50ms

RF sustained, Ip = 0.6 kA

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

0

0

pLCFS

pLCFS

E

IIq

T

Truncated boundary

LCFS

j

Inboard limiter LCFS Outboard

limiter

#53197 90ms

EC sustained, Ip = 1.3 kA

%70/ ,50~ ,0.1

[ms] 05.0~[eV], 180~

0

0

pLCFS

pLCFS

E

IIq

T

%93/ ,50~ ,1.1

[ms] 02.0~[eV], 45~

0

0

pLCFS

pLCFS

E

IIq

T

10

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2v||/v0

v^ /v0

1.0

0.8

0.6

0.4

0.2

0.0

(I)

(II)

(IV)

(III)

A

D

B

C

EF

(a)

0.0 0.2 0.4 0.6 0.8

Orbits-t1

R [m]

D

E

F

(c)

-0.8

-0.4

0.0

0.4

0.8

0.0 0.2 0.4 0.6 0.8

Orbits

Z [

m]

R [m]

AB

C

(b)

Limiter

Velocity Space Structure in Vacuum Field

0v v:condition Stagnation

v: velocitysticCharacteri

||D

0

t

p

p

B

BR

Velocity space for orbits starting from R = 0.38 m for PF2+PF5 configuration classes of particle orbits

A. Ejiri et. al., Nucl. Fusion 47, 403-416 (2007).11

• Banana orbits under the influence of self-generated E are analyzed.

– Angular momentum is conserved from axisymmetry

– Banana particles are frozen to flux surfaces, and move with flux surfaces towards the low field side.

– This movement causes kinetic energy and plasma current to decrease (inverse of Ware pinch).

• Passing particles have short energy decay times. They are accelerated in the direction to reduce Ip. Movement of orbit is small.

Ip stops increasing when closed flux surfaces are formed.

Effect of E on Particle Orbits in Start-up Plasma

12

const.~ || qRAmRvqRAmRvp

– 14 mV/m

Toroidal Field(t=35 – 40 ms)

0.2 0.60.4R [m]

0.6

0.4

0.2

0.0

-0.2

-0.4

-0.6

–3 mV/m

Condition for Flux Conservation

-2 x 10-5

0

2 x 10-5

4 x 10-5

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

#53783

[

Vs/

2]

R [m]

25 ms 30 ms 35 ms40 ms50 ms

45 ms

55 ms

0

0 time [ms] 10

P, qRA

mRV1

2

3 1 2 3

surface.flux with themove Particlesconst.~

If ||

qRAp

qRAmRv

Angular momentum conservation Flux conservation

13

of Orbits in Velocity Space

14

Trapped regionInverse of Ware pinch; = 0

< 0, R, Z ~ 0Counter moving

Acceleration < 0, R, Z ~ 0Co movingDeceleration

E-field dominated region

Velocity normalized by V0=Re pol

Transition regionCo Trapped:

~< 0, R ~> 0

Mixed transition regionCo Counter : < 0, R < 0Co Trapped: ~< 0, R ~> 0,

Discussion of Current Drive Mechanism

iele

e

double

doubleS

VVVVS

VVpZd

mkTSen

ekT

I

VR

IRRV

LEV

III

/2/

/2~

||||

0.1 0.3 0.5 0.7-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0

80.0

0.4

Pre

ssu

re [

Pa

]j f

[kA

/m2 ]

Z [

m]

R [m]

#53783, t=50ms

j

total

ff’

p’

LCFS

-4

4V/1.5 A=3

650-150=400A

Because of the V.V. current, the poloidal current is always in the diamagnetic direction.

In addition to the precessional current of trapped particles, Pfirsch-Schülter current can give net toroidal current in the open field line region.

The vertical drift current (Id) returns partially through the plasma (IpZ) and partially throught the vacuum vessel (IVV).

jd

Id IpZIVV

RS/2

RS/2

RVV

Top limiter

Bottom limiter

Plasma

V.V.

RpZV

j||

Z

jvv

j

15

HHFW Experiments on TST-2 and UTST

• Up to 300 kW of RF power at 21 MHz has been injected into TST-2 plasmas. Two-strap antenna excites HHFW with a wavenumber spectrum peaked at k ~15 m-1 at the antenna.

– When parametric decay instability (PDI) is observed, the Te increment becomes smaller, the edge density shows a rapid increase, and the impurity Ti increment increases.

– Wave measurements by microwave reflectometry, electrostatic and electromagnetic probes are consistent with the HHFW pump wave decaying into the ion Bernstein wave (IBW) or the HHFW lower sideband, and the low frequency ion-cyclotron quasi-mode (ICQM).

– The lower sideband power varies approximately quadratically with the local pump wave power, which becomes smaller as absorption of the pump wave by the plasma increases.

• In UTST, direct wave measurements inside the plasma were made with a 2-D array of magnetic probes. – The measured wave field profile was roughly consistent with the result

of TORIC full-wave calculation.16

φ ＝ -60°

φ ＝ -30°φ ＝ -55°φ ＝ -65°

φ ＝ -115°φ ＝ -120°

φ ＝ 155°φ ＝ 150°

φ ＝ 65°φ ＝ 60°φ ＝ 55°

φ ＝ 30°

φ ＝ 0°

2cm

Direction of B field to be measured

Core (insulator)

1 turn loop

S. S enclosureSlit

Semi-rigid Cable

φ ＝ -125°

RF Diagnostics

Bφ Bz Reflectometer

φ ＝ 145°TOP VIEW

center stackprobes

strap

Parametric Decay Observed by Reflectometer

There is a threshold in pump wave power.

→ Parametric Decay Instability (PDI)

Ion Cyclotron Quasi-Mode

reflectometer reflectometer reflectometer

fci

RF probe

pumpQM

LS

pump

LS

Correlation Between PDI and Electron/Ion Heating

Stronger PDI Less electron heating More ion heating

inboard-shifted

outboard-limited

outboard-limited

inboard-shifted

Spectral Broadening of the Pump Wave

Spectral broadening can occur by• scattering by density fluctuations• parametric decay instability

Spectral broadening becomes larger farther away from the antenna.

Downshifted and broadened pump wave was observed at the inboard wall.

UTST Experiment (Univ. Tokyo and AIST)

• High- ST formation by double-null merging (DNM) • High- ST sustainment by additional heating: NBI and RF

Objectives:PF Pair Coils

PF Pair Coils

0.7m

HHFW antenna

2m

Magnetic probe arraylocated 45 away toroidallyfrom the antenna

Magnetic Flux Surfaces During HHFW Injection

4.9ms

RF B2 Profile Comparison

• HHFW field is stronger in the periphery for single-strap excitation.

• RF magnetic field strength is lower for double-strap excitation

Single-strap excitation Double-strap excitation

Wavenumber Measurement

Radial coherence

Vertical coherence

Reference

Reference

Radial and Vertical WavenumbersF

requ

enc

y [M

Hz]

Fre

que

ncy

[MH

z]

Plan for LHCD Experiment on TST-2

• Preparation is underway for lower hybrid (LH) current drive experiments on TST-2. – Up to 400 kW of power at 200 MHz will be used to

ramp-up Ip from a very low current (~ 1 kA), very low density (< 1018 m-3) ST plasma.

• Wave propagation and absorption were investigated using the TORIC-LH full wave code. – Core absorption is expected initially, but absorption is

predicted to move radially outward with the increase in Ip and density.

26

Preparation for LHCD Experiment on TST-2

200 MHz transmittersCombline antenna

(11 elements)

Initially, the combline antenna used on JFT-2M, adapted for use on TST-2, will be used to excite a unidirectional fast wave with n = 12 (corresponding to n|| = 5).

Direct excitation of the LH wave is planned in the future. The fast wave can mode convert to the LH wave and drive current.

(200 kW x 4, from JFT-2M)

TORICLH/TST2/101/One0 = 1 x 1017 m-3

Te0 = 1 keVIp = 10 kAn||0 = 7ant = 0

Collaboration with J. Wright, P. Bonoli (MIT)

TORICLH/TST2/101/Une0 = 1 x 1017 m-3

Te0 = 1 keVIp = 30 kA

TORICLH/TST2/101/Mne0 = 1 x 1017 m-3

Te0 = 1 keVIp = 100 kA

TORICLH/TST2/102/Ane0 = 1 x 1018 m-3

Te0 = 1 keVIp = 100 kAn||0 = +7ant = 0

TORICLH/TST2/102/Bne0 = 1 x 1018 m-3

Te0 = 1 keVn||0 = +3

TORICLH/TST2/102/Gne0 = 1 x 1018 m-3

Te0 = 1 keVn||0 = -3

TORICLH/TST2/102/Fne0 = 1 x 1018 m-3

Te0 = 1 keVn||0 = -7

TORICLH/TST2/101/One0 = 1 x 1017 m-3

Te0 = 1 keVIp = 10 kAn||0 = 7ant = 0

TORICLH/TST2/102/Pne0 = 1 x 1017 m-3

Te0 = 1 keVn||0 = 7ant = 90

TORICLH/TST2/102/Qne0 = 1 x 1017 m-3

Te0 = 1 keVn||0 = 7ant = -90

TORICLH/TST2/102/One0 = 1 x 1017 m-3

Te0 = 1 keVn||0 = 7ant = 180

TORICLH/TST2/101/Ane0 = 5 x 1018 m-3

Te0 = 1 keVLH res. at x = -5 cmFW cutoff at x = -14 cm