Internal Transport Barriersin the DIII-D Tokamak
byE.J. Doyle
Dept. of Electrical Engineering and PSTI,
University of California, Los Angeles, California

For the DIII-D Research Team

Presented at the12th International Toki Conference and3rd General Scientific Assembly of theAsia Plasma Fusion Association,Toki, Japan
December 11–14, 2001

DIII–DNATIONAL FUSION FACILITYSAN DIEGOAUCLUCLAMOTIVATION —IMPORTANCE OF ITB RESEARCH
Obtaining ITBs with large radius and barrier width leads to:— Higher fusion— Improved MHD
performancestability limits●★ Assist in obtaining significant
fusion gain (Q~10) in Next
Step burning plasmas

Schematic ITB Ti profiles

8ITB radius, ρITB
1.5Optimal
ITB profile

1.02
Non-optimal
ITB profile
0.00.2

DIII–D0.4

ρ

0.60.8

MHD modeling of βN limit
βN (%-m-T/MA)★ Assist in achieving
steady-state tokamak
operation
ARIES-AT Modeling250200150
10050

Desired current
profile40.50.0★ More compact
and/or economic
powerplants

6ITB
halfwidth— Improved bootstrap
current alignment

1.00stable
unstable
ITB Radius, ΨITB

302520151050ITBTi (keV)00.2

0.4

0.6

0.8

1r/a

NATIONAL FUSION FACILITYSAN DIEGOEJD Toki 2001 12/14/2001 2

OVERVIEW● Significant progress on DIII-D in addressing critical issues for ITB research:— Improved understanding of physical mechanisms responsible for ITBformation★ Evidence for a range of turbulence/transport reduction mechanisms— New Quiescent Double Barrier (QDB) regime provides sustained, high quality
ITB operation with an ELM-free H-mode edge, allowing us to examine:★ Edge-core integration issues, e.g. effect of ELMs
★ ITB sustainment★ Impurity accumulation— MHD stability will determine ultimate performance limit of ITB plasmas★ Stabilization of resistive wall modes (RWM) and neoclassical tearingmodes (NTM) demonstrated on DIII-D . Invited talk by on RWM results by
M. Okabayashi, Wednesday

DIII–DNATIONAL FUSION FACILITYSAN DIEGOEJD Toki 2001 12/14/2001 3

UNDERSTANDING OF ITB FORMATION CONDITIONS FLOWSFROM UNDERSTANDING OF TRANSPORT DRIVE AND
SUPPRESSION MECHANISMS● Theory-based

Indicative
turbulence
scalesTurbulence/transportmechanisms
Affectedtransportchannels
Stabilizationmechanisms

0.1kθ ρs1.

101.

kθ (cm -1)

10100

ITG

modeling
predictions for turbulence andtransport drive and control
mechanisms are compared toexperiment● DIII-D results indicate following

TEM
ETG
Ion thermal
Momentum
Electron particle
Electron thermal
ExB shear
Reversed magnetic shear (NCS)
α-stabilization (Shafranov shift)
Impurity injection

turbulence control mechanisms
can be effective:— α-stabilization/Shafranovshift— q profile— Growth rate reduction via
impurity injection— Sheared ExB flows (rotation)● Direct evidence for ETG

DIII–Dmodesis lacking

NATIONAL FUSION FACILITYSAN DIEGOEJD Toki 2001 12/14/2001 4

α-STABILIZATION AND NEGATIVE MAGNETIC SHEAR
ARE PREDICTED TO REDUCE TURBULENCE GROWTH RATES● Theory calculations, e.g. Waltz et al, Phys Plasmas 1997, indicate that turbulence

growth rates can be reduced by negative magnetic shear and α-stabilization(Shafranov shift)— Where α is the normalized pressure gradient (ballooning parameter)

0.4 ν =0

● In comparisons to theory, extensive use● ExB shear predicted to suppress

turbulence when the shearing rate ωExB
exceeds the turbulence linear growthrate γ

q=2γ [cs/a]

is made of the GLF23 transport model— Drift-wave based model (ITG, TEM,ETG), providing quasilinear
estimates of transport— Includes ExB shear, α-stabilization,
magnetic shear and dilution effects

ei

α=1

α=00.2α=00-1α=10s^

12DIII–D

NATIONAL FUSION FACILITYSAN DIEGOEJD Toki 2001 12/14/2001 5

EVIDENCE FOR ROLE OF α-STABILIZATION PROVIDED BY
ELECTRON THERMAL ITBs OBTAINED WITH LOCALIZED ECH
65keV

ECH onset4ECH Te
0.5
Prior to ECHTe0.4
0.3

32m2/s

0.2

qECH

10.1

0010
● Electron transport reduced0.6

MW/m3● E-ITB develops rapidly following99696

χe1

χi

χineo

0.1

DIII–DNATIONAL FUSION FACILITYSAN DIEGO

0.0

0.2

0.4

ρ

0.6

0.8

1.0
EJD Toki 2001 12/14/2001 6

SIMULATIONS INDICATE α-STABILIZATIONIS CRITICAL IN FORMATION OF ELECTRON ITB● Results also provide indirect evidence● Dynamical simulations using GLF23

model maintain E-ITB only if α issufficiently large— GLF23 also reproduces dynamics ofbarrier evolutionfor ETG modes:— T e gradient at location of E-ITBs
consistently observed to be at
marginal stability to ETG mode
30

699696, 0.2 sExperiment

5TeSimulation

4keVa/LTe2032101a/LTe

Calculated
if α=0

Simulation with α=0

0
0.0

0.2

DIII–D

0.4

Calculated (GKS)ETG critical gradientExperiment

ρ

0.6

0.8

1.0099696, 0.20s

0.2e-ITB
0.4

ρ

0.6

NATIONAL FUSION FACILITYSAN DIEGOEJD Toki 2001 12/14/2001 7

SUBSTANTIAL EVIDENCE FROM MANY EXPERIMENTS FOR
ROLE OF q-PROFILE IN FACILITATING ITB FORMATIONElectron Density

1.2 s
5ne (x1019 m-3)

1.4 s

10

1.5 s

483

Steep
gradientzone

64

22

1050

0
7

Carbon Rotation6.

q-profile

4030

4.

5

2.

0.

20

Electron Temperature
6

Te (keV)

Toroidal rotation (kHz)7612● Without strong

negative shear,
ITBs on DIII- D
often limited toion thermal andangularmomentumchannels

Ion Temperature

14

Ti (keV)use ofstrong negativeshear, plus highheating power
results in ITBs inall four transportchannels

16● On DIII-D,

40.

0.2

0.4

0.6

ρ

0.8

1.0

1.2

3210195989
000.20.4

ρ 0.6

0.8

1000.20.4

ρ 0.6

0.8

1DIII–D

NATIONAL FUSION FACILITYSAN DIEGOEJD Toki 2001 12/14/2001 8

IMPURITY DILUTION CAN REDUCE TURBULENT TRANSPORTBY REDUCING GROWTH RATES AND INCREASING ExB SHEAR● Results from neon injection into co-NBI plasma with L-mode edge, no prior ITB● Results explain physics of RI-mode:

Central ion temperature

5— Linear growth rates reduced andExB shearing rate increased— Density fluctuations dramaticallyreduced— Core temperatures rise, energy
confinement and neutron rate
double, profiles broaden

Ti(0) (keV)

4Start ofinjection

3Neon

21No neon
600

BES Range

ωExB shearing rate
0.2 no neon0

DIII–D

1

1400

2

Growth rate,with neon
34-1

k (cm )

1600

ρ∼0.7, 1.1-1.2 s
No neon32

ωExB shearing ratewith neon

0.40.01200

40.6

105 s-1

5x10-8(<ñ>/n) /kHz

0.8

1000

Time (ms)~
BES spectra of n/n

Turbulence growth and shearing rates
Growth rate,
no neon

8002Neon105050100150200250300350

Frequency (kHz)NATIONAL FUSION FACILITYSAN DIEGOEJD Toki 2001 12/14/2001 9

ExB SHEAR FLOW IS MOST STUDIED TRANSPORTBARRIER FORMATION MECHANISM (EDGE AND CORE)● Self-consistent

Experiment92529
dynamical modeling(m)using GLF23 canTi (keV)
8 R1.771
1.834
explain details of step1.866
1.898
wise formation of ITBs
1.929
4 1.964
on DIII- D
2.038
2.121
— Steps are
generated by a0competition
3.0
Te (keV)between the ∇Pand vφB θcontributions to1.5Er and the ExBshearing rate inco-NBI discharges 0.0

Er =∇Pi− v Bϕ + vφ Bθ
eni Zi θDIII–D400900Time (ms)10Modeling 84736
Ti (kev)

864204Te (kev)3210
1400 0.00 0.12

0.24 0.36 0.48 0.60
Time (s)● Occurrence of steps sometimes correlates withpresence of rational q values

NATIONAL FUSION FACILITYSAN DIEGOEJD Toki 2001 12/14/2001 10

INTERPLAY OF TERMS IN ExB SHEARING RATEωE×B IS DIFFERENT FOR CO- AND COUNTER-NBI● Main ion shearing rate ω E×B can be5∇p

ωE×B = ωE×B + ωrotation
E×B
increasing the pressure
gradient component increases ω E×B , rather
than reducing it, as with co-injection— Counter-NBI favorable for ITBexpansion with L-mode edge
— Counter-NBI experiments led todiscovery of Quiescent Double Barrier(QDB) regime0DIII–DSAN DIEGOωE×Brotation
ωE×B-1087031 1.82 s

Counter-NBI

42

ω∇p
E×B

ωE×B0-2
NATIONAL FUSION FACILITY

ω-5

105 s-1● With counter-NBI,

105 s-1

separated into pressure and rotation terms

Co-NBI∇p
E×Brotation
ωE×B99849 1.17 s

0.00.20.4 ρ 0.60.8

1.0
EJD Toki 2001 12/14/2001 11

QUIESCENT DOUBLE-BARRIER (QDB) OPERATION● Will examine:— Performance obtained in QDB regime— Significance of QDB results— Transport and fluctuation analysis and modeling— Impurity issues● Some new acronyms:— QH-mode: Quiescent H-mode★ An ELM-free H-mode with density and radiated power control— QDB: Quiescent Double Barrier★ Operation with an internal transport barrier (ITB) inside a QH-mode edge

DIII–DNATIONAL FUSION FACILITYSAN DIEGOEJD Toki 2001 12/14/2001 12

ITB

L-modeedge

Ti

QDB

105L-mode

TeITB

54QDB

ITB +L-mode
3 edgekeV

15 ITB +QH-mode edge

pedestalelevates central
temperatures,
improving fusion
performancekeV● Edge2QH-mode edge

QDB REGIME OBTAINED USING COUNTER-NBI —COMBINES ITBs WITH ELM-FREE QUIESCENT H-MODE EDGE

L-mode06q

QDB
1069194.5 s
L–mode+ITB998511.4 s
L-mode998520.8 s

43

DIII–D43221100.0 0.20.4

ρ0.60.8 1.0

ne51019 m-3506

QH-mode edge

10.0 0.20.4

ρ

0.6

0.8 1.0

NATIONAL FUSION FACILITYSAN DIEGOEJD Toki 2001 12/14/2001 13

COMBINATION OF CORE ITB AND QH-MODE EDGERESULTS IN SUSTAINED HIGH PERFORMANCE PLASMAS–0.6

β N H 89=7 for 10 τE –0.8–1.0(1.6 s)● Duration limited by –1.2
–1.4
NBI sources–1.6● Have maintained–1.8
QH-mode for >3.5 8s, ~25τE6● Feature of4
QH-mode is ELMfree operation2with density and01.0radiated power
0.8 Dαcontrol
0.6— Density
0.4
controlledusing divertor 0.2
pumping
0.0

Plasma Current (MA)

●0.5

DIII–D106956

Central Density (1019 m–3)

141210864286

NBI Power (MW)Radiated Power (MW)βN*H89(7)4Line Average
Density (1019 m–3)

20654

Neutronrate (1015 s–1)

3
ELM-free, QH-mode Edge

21

1.0

1.5

2.0 2.5 3.0
Time (s)3.5 4.0

0

0.5

1.0

1.5

2.0 2.5 3.0
Time (s)3.5 4.0

NATIONAL FUSION FACILITYSAN DIEGOEJD Toki 2001 12/14/2001 14

WHAT IS THE SIGNIFICANCE OF QDB OPERATION?
● H-mode is the operating regime of choice for next-step devices, but has non-optimal features due to the impact of Edge Localized Modes (ELMs)— Pulsed heat loads to the divertor can cause rapid erosion— Type I (Giant) ELMs can inhibit or destroy the ITBs desired for advanced
tokamak (AT) scenarios★ Double barriers have been achieved on JT-60U and JET● QDB plasmas address critical next-step and ITB issues:— Provides high quality ELM-free H-mode, eliminating pulsed divertor heat loads— The QH-mode edge is compatible with ITBs
— Sustained long pulse, high performance capability:★ >3.5 s or 25 τ E achieved, limited only by beam pulse duration★

β N H 89 =7 for 10 τ E— Long pulse capability provides opportunity to study impurity accumulation
issues in detail

DIII–DNATIONAL FUSION FACILITYSAN DIEGOEJD Toki 2001 12/14/2001 15

TRANSPORT ANALYSIS CONFIRMS PRESENCE OF DOUBLE(CORE AND EDGE) TRANSPORT BARRIERS

χi10χeITBITBITB +L-modeedgem2/s

L-mode

1.0

QDB

χineo
0.1

QDB
1069563.00s
L–mode+ITB998491.12s
L-mode998520.80s

0.0

0.20.4

ρ

0.6

0.8

1.0 0.0

0.20.4

ρ

0.6

0.8

1.0

● Core transport is similar to that in ITB plasmas with an L-mode edge

— ITB refers to region of reduced transport relative to L-mode● Edge transport is typical of H-mode● Core and edge barriers are kept separate by region of low ExB shear

DIII–DNATIONAL FUSION FACILITYSAN DIEGOEJD Toki 2001 12/14/2001 16

SIMULATIONS USING THE GLF23 MODELREPRODUCE THE QDB CORE ION BARRIER
25103740 3.305 s

DataT (keV)

20

ModelData

15ModelTi10Te5.00.000.20.4

ρ

0.60.81● Steady-state simulation reproduces core ion temperature barrier— Core Te profile not accurately reproduced● GLF23 also predicts core turbulence should not be completely
suppressed, as ExB shearing rate and turbulence growth rate in
DIII–D approximate balance.

NATIONAL FUSION FACILITYSAN DIEGOEJD Toki 2001 12/14/2001 17

CORE BARRIER EXISTS WITHOUT COMPLETE TURBULENCESUPPRESSION, IN AGREEMENT WITH GLF23 MODELINGFIR scattering
107001
k = 1 cm-1

completely suppressed as the QDBcore barrier evolves— Residual turbulence still significantly
above the FIR scattering system
detection limit— Contrasts with typical ITB in DIII–D,
where core turbulence issuppressed to the noise floor

Frequency (kHz)● Internal broadband turbulence is not1000
Corelocalize dquasi-coherent
modes500} Edge0modes

4.0-500-1000

1.0

2.0

3.0

4.0

1.5 s
2.5 s
3.5 sñ (a.u.)● High frequency coherent core modesare often detected.
— Reflectometer data indicate thesemodes are localized to ρ~0-0.4.

0.

Time (s)NBI initiation

System noise floor

0DIII–D2004006008001000 1200f (kHz)NATIONAL FUSION FACILITYSAN DIEGOEJD Toki 2001 12/14/2001 18

STEP SIZE FOR CORE TURBULENTTRANSPORT IS REDUCED IN QDB PLASMAS● In L–mode, correlation● In QDB plasmas, core

● Initial modeling using thelengths are observed toscale approximately as
5 –10 ρscorrelation lengths are
significantly lower than thescaling observed in L –modeUCAN global gyrokinetic
code tracks coreexperimental trends andmagnitude— ITG turbulence incircular geometry— Where ρs is the ion
gyroradius evaluated using Te
L–mode data set
Correlation
length, ∆r
5–10 ρs4

QDB data set

8

Correlation
length, ∆r
5–10 ρs

6cm

6cm

86cm

8

4

Simulations
Correlation
length, ∆r
5–10 ρs4

22

2

00.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

00.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

00.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8ρρρ

DIII–DNATIONAL FUSION FACILITYSAN DIEGOEJD Toki 2001 12/14/2001 19

QDB DISCHARGES ALLOW US TO STUDY IMPURITY
ACCUMULATION IN DIII-D ITB PLASMAS
10.0
7.5
5.0
2.5
0.0
1.00
0.75
0.50
0.25
0.00
7.00
5.25
3.50
1.75
0.00
3.00
2.25
1.50
0.75
0.00
43210

NBI Power (MW)Radiated Power (MW)DαELM-free, QH-mode Edge

Central Density (1019 m–3)
Line Average Density (1019 m–3)
Ni XXVI (1015 ph/cm2/s)

Central Carbon Fraction (%)

1.02.03.0

4.0106940
5.0Time (s)● Nickel content increases with time, but contribution to radiated power

DIII–Dis low, < 0.3 MW. Large impact on Zeff
● Low-Z impurities, e.g. carbon, stay approximately constant

NATIONAL FUSION FACILITYSAN DIEGOEJD Toki 2001 12/14/2001 20

NEOCLASSICAL MODELING PREDICTS CENTRALPEAKING OF HIGH-Z IMPURITIES, DUE TO PEAKED ne PROFILE● Measured impurity convection and diffusivity is larger than neoclassicalfrom 0.1 < ρ < 0.5

Pinch velocity
VNEO - helium-2

VNEO - neon

-4Diffusivity

3m2/sm/s0V - helium

2D - neonD - helium

1V - neon

DNEO - helium

0.00.10.2

ρ

0.3

0.4 0.5● Measured neon profileis less peaked thanprofile calculated using
neoclassical transport

DIII–DNATIONAL FUSION FACILITYSAN DIEGO

Density Profile (au)

-6

1.2
1.0
0.8
0.6
0.4
0.2
0.0
0.0

00.1

0.00.2

ρ

DNEO - neon

0.3

0.40.5

Profile reconstructed using
calculated neon D and V

Measured Profile
3200 ms

Profile using DNEO and VNEO

0.20.4

ρ

0.6

0.8 1.0
EJD Toki 2001 12/14/2001 21

CONTROL TOOLS EXIST TO MODIFYDENSITY PROFILE AND REDUCE DENSITY PEAKING● Example of use ofcentral ECH tomodify density
profile● ne (o)/n AVE

129Te (0) (keV)

8PNBI (MW)

664

3

20
20

0
3.0

Ti at several radii (keV)

Prad (MW)

Neutron rate (1015 s-1)

15

decreases from 2.6
10
to 1.7

1.55● MISTmodeling
indicates Ni
concentration is
reduced

0400

Upper divertor photodiode (a.u.)

0C VI rotation at several radii(103 m/s)

3002008ne (0) (1019 m-3)

4ne100● Reduceddensity0
peaking would also2
improve bootstrap
current alignment 10

ECH power (MW)42

Ni XXVI (1014 ph/cm2/s)

00
3600

40004400

Time (ms)

4800

3600

4000

4400

4800

Time (ms)

DIII–DNATIONAL FUSION FACILITYSAN DIEGOEJD Toki 2001 12/14/2001 22

CONCLUSIONS● DIII-D results have improved our understanding of ITB formation conditions— Evidence for the effect of α-stabilization/Shafranov shift, magnetic shear,
impurity injection, and sheared ExB flows● QDB results demonstrate that it is possible to have long pulse, highperformance ITB operation with an ELM-free H-mode edge, with density andradiated power control— >3.5 s or 25 τ E achieved, limited only by beam pulse duration— β N H 89 =7 for 10 τE— Pulsed divertor heat loads eliminated— Core and edge transport barriers are compatible— Turbulence and transport behavior of QDB discharges is reproduced byinitial simulations and modeling— Issues are increasing the operating density, impurity accumulation andobtaining QDB with balanced or co-NBI (JT-60U has unique capability!)

DIII–DNATIONAL FUSION FACILITYSAN DIEGOEJD Toki 2001 12/14/2001 23