a.c.c. sips 9 th eu-us ttf workshop, córdoba, 9-12 september 20021/25 physics of itb’s: recent...

A.C.C. Sips 9th EU-US TTF workshop, Córdoba, 9-12 September 2002 1/25

Physics of ITB’s:

Recent results from experiments

A.C.C. Sips

Max-Planck-Institut für Plasmaphysik, Euratom Assoziation, Boltzmannstrasse 2, Garching.

With contributions from:

Y. Baranov1, C. Challis1, G. Conway, B. Esposito2, T. Fujita3, T. Fukuda3, P. Gohil4, C. Greenfield4,

G.T. Hoang5, G. Huysmans5, R. Jaspers6, E. Joffrin1, N. Kirneva7, X. Litaudon5, D. Mazon5,

A. Peeters, E. Quigley, T. Tala8 and R. Wolf

1: EURATOM/UKAEA Association, Oxon, UK.

2: Associazione Euratom-ENEA sulla Fusione, Frascati, Italy.

3: JAERI, Naka Fusion Research Establishment, Naka, Japan.

4: General Atomics, San Diego, USA.

5: Association EURATOM-CEA Cadarache, France.

6:FOM Instituut voor Plasmafysica Rijnhuizen, The Netherlands.

7: RRC Kurchatov Institute, Moscow, Russia.

8: Association Euratom-Tekes, VTT, Espoo, Finland.

Max Planck-Institutfür Plasmaphysik


ITB formation.

Current Hole.

Electron Barriers.

International database.

Similarity experiments.

Sustainment and control.

ITB`s with quiesent edge (QDB).

Outline

However, we must keep in mind where we need to go !!!

Fukuda – EPS ´02

„Transport Barriers provide great opportunitiesto study the broad dynamics in fusion science“

Schema della proporzioni

Physics

Scenariodevelopment

Reactor application


ITB scenario – Barrier formation

LHCD power level is varied

to change initial q-profile

Challis, Tala – PPCF ´02

Reversed shear, more NBI torque

favours ITB formation. Challis - PPCF ´02

ITB

Strong ITB

Pea

k * T

e =

s/

L Te


ITB scenario – Barrier formation

Esposito – EPS ´02

Strong reversed shear

Weak shear

Turbulence is suppressed when

ExB>m, but well defined region

with s close to 0 is important.

ITB´s startnear s=0

O.II.12, dynamics of e-ITB´s and ion ITB´s

shea

r ITB

shea

r ITB

Time (s)

electrons

ions

Te (

keV

)T

e (

keV

)


ITB formation: synergy between s and ExB shearing rate

Before ITB

After ITB

ExB/ITG

Ma

gn

etic

she

ar

s

Tala – PPCC ´01

In modeling the experimental data:

• Bohm/GyroBohm empirical model, using (–0.14+s-1.47 ExB/m ) (O.II.11)• Simulation of JET data with the Weiland model show that the density

gradient term dominates over the ExB shearing rate (T. Tala, PPCF ´02).

FULL code calculation

0

10

1.00.0 r / a

T i [ k

eV ]

1.4

- 2.0

L [

10

5/ s

]

q

10

2

q

T i

8

6

4

2

1.2

1.0

0.8

0.6

0.4

0.2

0.0

4

5

7

9

3

8

6

Lmodel,

JT-60U 24715, t = 6.0

f (s) = 1

5

4

3

2

1

0

L F

UL

L /

L m

od

el

1.00.50.0-0.5-1.0-1.5-2.0

magnetic shear at L

max

f (s) = 0.42 s +1.37

JT-60U

Comparisons made for the FULL code results after the ITB formation

m f(s), f(s) = 0.42 s + 1.37 for JT-60U

JT-60UFukuda – EPS ´02


Barrier formation: ASDEX Upgade results

ASDEX Upgrade

• Formation of an ITB at low ne, applying the NBI power in one step.

• Good, transient performance: H89L=3.4, N =4.


Barrier position

ASDEX Upgrade (E. Quigley –EPS ´02)

• The foot of the barrier expands to the positive shear region.

• This is important for the alignment of jboot with jtot.

Litaudon – ITPA ´02

weak shearreversed shear

s/LTe 1.4 x 10-2

q=3]

shea

r

Normalised poloidal flux radius


JT-60U: EC preheat is used, to create a reversed shear target and try to expand the ITB radius, Fujita – PRL `01

JET: LHCD is used during the

current ramp phase at low

density, Hawkes – PRL `01

Current Hole, observation and explanation

4

0.0 1.0r / a

[ke

V]

0

25 Ti r6.30 s

6.40 s

6.55 s6.65 s7.18 s

0.0 1.0r / a

0

10

20

0

1

2

0

5

10

0

2

46

3 4 5 6 7 8

0

1

23

010

2030

[ke

V]

[MA

][M

J]

[MW

][1

0 1

6 / s

][ a

rb. ]

E040259

P NB

I P

P EC

Tio

Teo

Time [s]

Wdia

neutron ratem / n = 4 / 1 interchange mode

t = 7.2 s

q min < 2current

hole

q r

40

30

20

10

0 0

j [ M

A / m

2 ]j r

current hole

Te r at 7.2 s

Current Hole

4

0.0 1.0r / a

[ke

V]

0

25 Ti r6.30 s

6.40 s

6.55 s6.65 s7.18 s

0.0 1.0r / a

0

10

20

0

1

2

0

5

10

0

2

46

3 4 5 6 7 8

0

1

23

010

2030

[ke

V]

[MA

][M

J]

[MW

][1

0 1

6 / s

][ a

rb. ]

E040259

P NB

I P

P EC

Tio

Teo

Time [s]

Wdia

neutron ratem / n = 4 / 1 interchange mode

t = 7.2 s

q min < 2current

hole

q r

40

30

20

10

0 0

j [ M

A / m

2 ]j r

current hole

Te r at 7.2 s

Strongly reversed q-profile before NBI

heating starts, which persists during NBI.


Current Hole, explanation for j0=0

JET: Experimental observation: j(0) 0

Hawkes – `02, Huysmans – EPS ´02

J(0)

(A

m-2)

Global max DlnT

• m=1 mode grows exponentially as soon as a q= surface appears.• Re-connection flattens the current density to zero inside the q= surface.• JT-60U: current drive with ECCD inside the current hole: extremely difficult.

Simulation of the current density on axis

with and without the effect of the MHD.

0 2 4 6 8 10 12-0.04

-0.02

0

0.02

0.04

0.06

0.08

J (0)z

time [x10 ]5

A

Huysmans – EPS ´02

J z(0

)


Electron transport barriers

ASDEX UpgradeUsing ECCD:

Counter current drive in the

centre generates a reversed

shear AND a barrier.

However, short duration

MHD unstable

Wolf – IAEA ´00

NEW experiments at ASDEX

in 2001/2002.

Also ECRH: TCV and FTU...

T (

keV

)

T (

keV

)



ASDEX Upgrade new results,Peeters, ´02

More stable regime:

• Higher Ip (600 kA).

• Timing of ECCD.

• Qualitative agreement with

theoretical predictions of the

TEM stabilisation.



Textor

Textor

Jaspers - EPS ´02

Te(r) with 250 kW ECRH alone,

modeled with RTP q-comb

model for e.

ECRH heating a weakly reversed q(r):

• At different deposition radii.

• e-ITB at different rational q´s.e-ITB at q=1 e-ITB at q=2.5

* Results on e-ITB´s from FTU (O.II.10) and TCV (O.II.16) follow after this talk

* e-ITB,s Tore Surpa , Hoang PRL ´00

Te

(keV

) e

(m

2 s-1)

-1.0 0 1.0

2.5


Fluctuation measurements in electron ITB´s

• 2.5 MW LH only , Te > 8keV, ne ~1.5x1019 m-3

• e-ITB forms in negative shear region.

• No rotation shear.

Turbulence reduction coincides with reduced e

Low frequencies reduced not ETG, TEM ?

Conway – PPCF ´02

e (

m2 s

-1)


ITB scenario – International database

Kirneva – 8th TTF ´02

Data from many experiments, however:

• Most of the data are for Ti/Te > 1.

• Best confinement data for ne/nGW < 0.6.

• Confinement increases with ITB radius, favours large radius for qmin. Can these data be used to extrapolate to reactor conditions ?



Baranov – APS ´01 Sips & Fukuda – ´01

Combination of 1-D data from various Tokamaks show dependences

of access power to ITB: ne (or Ip), and size, weak dependence on Bt.



Hoang – EPS ´02

ion ITB

electron ITB

At same s , plasmas with stronger reversed shear, require lower input

power to form an ITB.

At low * ITB´s form at lower power when confinement is good, easier to

create rotation shear, and peaked profiles.


ITB scenario – similarity experiments

Similarity experiments on ITB formation: ASDEX Upgrade – JET

• First results from ASDEX Upgrade (to be analysed in detail).

• JET experiments in 2003 to match dimensionless parameters (q,,*,*).

t=0.942t=0.968t=0.994t=1.072

However, this is the collapse of the ITB, due to the ELM´s !!!

ASDEX Upgrade


ITB scenario - duration

Litaudon – PPCF ´02

LHCD to create and sustain q (r).

• 2MA/3.4T, q95 = 5.5.

• H89P = 2.0, p = 1.1,N = 1.7.

• Duration = 36 E (e-ITB).

• Duration = 27 E (i,ne,vtor-ITB).

• Type III ELM`s at the edge due to

high jedge (M. Bécoulet, Y. Sarazin - ´01).

Iboot 1.0 MA

ILHCD 0.5 MA

INBI 0.3 MA

6

5

4

3

2

q-p

rofi

le

1.00.80.60.40.20

normalised radius

#53521 target q-profile from

MSE t=4.3s Polarimetry t=4.5s

6

4

2

0

15

10

5

0

0.8

0.4

0

1412108642Time [s]

3.0

2.0

1.0

015

10

5

0

#53521

PLHCD[MW] Ip[MA]

MW

keV

PNBI PICRH

Tio

neo [1019 m-3]

D [a.u.]

Vs [V]li

Teo


NBI

ITB scenario – duration, but impurity accumulation

MHD collapse, Hender/Hennequin- PPCF

High Z impurities:

- Accumulate (neo-classical

behaviour) (R. Dux – PSI ´02).

- Due to continued density

peaking accumulation of high Z

impurities only becomes worse.

- Cause (radiative) collapse.

s/LTe 1.4 x 10-2

Nickel concentration on axis

ITB reforms, what if PICRH neutron rate, simulating conditions in a reactor ?


ITB scenario – duration and control

(Mazon – PPCF ´02)

Control at “slightly“ lower performance

Pulse 53521 collapses 2x.


QDB combines an ITB with ELM

free steady state H-mode edge,

modulated by MHD activity.

• Counter NBI only.

This also maintains qmin > 1

and reversed q (r).

• Low edge density, due to MHD

and divertor cryo pumping.

Using counter NBI, this ELM free edge

has now been reproduced at ASDEX.

(O.I.01)

ITB scenario - QDB


ITB scenario - QDB

Edge pedestal pressure in QDB Type I ELMy H-mode.

More stable compared to L-mode, No ELMs ITB stays


ITB scenario - QDB

Also in QDB, high Z impurity accumulation is a problem.

Experiments with ECRH in the core in progress (Casper – EPS ´02) 10

15 n

eutr

ons/

s


Some concluding remarks

1.Document differences or similarities on ITB formation:

q-profile is crucial to ITB formation. Reversed shear is favourable for ITB

formation: extreme is a current hole. For ion-ITB ExB shear is required.

Need to improve our models to predict ITB formation in reactor (at least

come to a consensus).

2.Compatibility with ELM´s

We should document ITB collapse with ELM´s.

Type III ELM´s okay, but H factor lower (low edge pedestal pressure).

QDB demonstrates that it is possible to combine an H-mode edge with ITB,

but only with counter NBI, at low density and peaked ne(r).

3.Impurity accumulation: problem in long pulses

In order to avoid this we need a flatter ne(r ), and broader T(r) profiles.

Is this compatible with sustaining an ITB ?


Finishing with ........Open issues

4. High (edge) density:

Pellet injection (PEP) or operation at high triangulrity.

Why is sustaining an ITB at high density difficult (is it impossible) ?

5. Control in long pulses – good progress has been made:

Still need: duration of ITB >> current diffusion time scale.

Still need: demonstration of control schemes in reactor relevant conditions.

6. Reactor with -heating, Te = Ti and D-T fuel:

Electron heating: ECRH, N-NBI, LHCD and ICRH.

Warning: Even the best results in D-D may be difficult to extrapolate to a

D-T phase (even without -power this was difficult in JET & TFTR !).

After > 7 years of intensive research – still a long way to go

a.c.c. sips 9 th eu-us ttf workshop, córdoba, 9-12 september 20021/25 physics of itb’s: recent...

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