Report on ITER-relevant Scenario Experiments
on DIII-D
Presented byT.C. Luce
Presented at
ITPA Integrated Operation Scenarios
Topical Group Meeting
Kyoto, JapanOct. 18–21, 2011

What’s Here, What’s Not• Talk tries to cover DIII-D experiments of interest to ITER andpotential future joint experiments from the last run (May-Sept.)–––––

Steady-state scenario with off-axis NBI
Advanced inductive with low or no external torque input
QH mode with no external torque source
Divertor characterization
Closed-loop demonstration of current profile control with aphysics-based model• Plans for Oct. run will be shown– Plans for 2012 very uncertain at this point• Most, if not all, of the results directly in support of joint
experiments will be reported through the joint experiment
spokespersons

Steady State Scenario Work Focused onmin(q)>2 with Off-axis NBI• Previous attempts to probe the β limit
with min(q)>2 found central co-NBI
overdrove the central current at thepowers needed– Insufficient power at higher B• Theoretical and experimental studies
indicated higher β limits when thepressure and current profiles werebroader• Both considerations motivatedmodifying 1 beamline in DIII-D to tilt
vertically to give off-axis NBI

With On-Axis NBI, it is Difficult to Maintainmin(q)>2 and Achieve High βN
2009 ShotβN at ideal
wall n=1
limit
Targetqmin=2 notmaintained

BT=2 T, H89=2
require allavailable
co-NBIpower(Kinetic EFIT)

Modeling Indicates Only Modest Gain In β Limit byBroadening Only the Pressure Profile
P0/<P>=2.74
P0/<P>=3.1• Changed P at
fixed Ip
• DCON results
P0/<P>!N n=1 nw!N n=2 nw!N n=3 nw!N n=1 iw!N n=2 iw!N n=3 iw

3.1
2.59
3.32
3.66
3.34
3.66
3.8

2.74
2.84
3.41
3.61
4.25
3.93
3.86

Modest Improvement in the Calculated β Limit by
Increasing min(q) While Holding ρqmin Fixed• Scaled qmin at
fixed Ip
• DCON results
qmin

1.495

1.95

2.35

li

1

0.79

0.6

βN n=1
no-w	

3.56

2.59

2.48

βN n=2
no-w

3.59

3.32

3.15

3.66

3.64

βN n=3no-w
βN n=1
ideal

4.35

3.34

3.87

βN n=2ideal

3.62

3.66

3.97

βN n=3ideal

3.90

3.80

4.12

Broadening J Increases the Calculated
βN Limit Significantly
P0/<P>=2.9
P0/<P>=3.1
li=0.71
li=0.79
ρqmin=0.27
qmin=1.95

ρqmin=0.36, qmin=2.59• ΔJ|| derived fromdifference between
simulations of NB on-axis
and NB off-axis• Fixed Ip
βN n=1no-wall

2.59

2.69

βN n=2
no-wall

3.32

3.53

βN n=3no-wall

3.66

3.93

βN n=1
ideal-w

3.34

4.26

βN n=2
ideal-w

3.66

4.27

βN n=3ideal-w

3.80

4.31

Experimental Pressure and Current Profiles Are Broader
With Off-Axis NBI and More ECCD Power
BT=2 T, q95=6.8, Matched βN≈2.8 of 2009
Reference Shot
Time Averaged Profiles in βN flattop:
With central NBI, with off-axis NBI(Kinetic EFITs)

Density and Electron Temperature Profiles Are Broader
With Off-axis NBI
Time Averaged
Profiles inβN flattop
Not clear why
the density
profile is different

Stability Analysis Indicates the Ideal-Wall n=1 βN
Limit is Typically Over 4 With Off-axis NBI• Min(q)>2 easilysustained• Confinement limits
β with theavailable powerfor this B

Recent Experiments Are Pushing Advanced
Inductive Regime toward Zero External Torque Input• Previous results with a co-NBI startup followed by atransition toward more balanced injection showed:– Significant degradation of confinement as external torquewas reduced– Lower limit on external torque set by locked mode

• New approach taken this year to start with zero-torque or
low-torque from the beginningSignificant Reduction in Confinement Observed as
Torque Is Reduced• 2/1 NTM invariably
encountered at lowtorque• Power demand
increase ~70% forfixed bN (before 2/1onset)• H98 reduced from>1.5 to approx 1.0• Similar to previousresults[Politzer NF 2008]

H-mode Startup with ECCD (2 Gyrotrons) Resulted inStable AI Discharge with Zero Net torque Throughout• Modest ECH power allowed
2/1 to be reduced sufficiently
to allow stable operation atbN~2.5– Unknown whether additionalsources would havesuppressed mode completely– Unknown whether plasma
remains stable if remove ECCD
later in discharge• Sustained for max duration of210RT beam
• Note, no obvious 3/2 or 4/3
mode present– Is this a 2/1 AI plasma?
• In which case, what
happens if actuallystabilize 2/1?

Reproduced Low Torque Reference, and with All 6
Gyrotrons, NTM Completely Suppressed• Slight increase inperformance without
2/1 NTM, but largely
offset by increased useof EC power– Similar bN for similar
combined NBI+ECH

WM Solomon/FSM/Sep2011

bN~3.2, Approaching ITER Q=10 Equivalent Achieved
with Torque~1 Nm, H98~1.1, G~0.38• 1 Nm in DIII-D drives
approximately thesame rotation as ITER
beams (30+ Nm)– Scaling for momentof inertia andconfinement• H98>~1• Normalized fusion
performance, G~0.38– G~0.4 is ITER Q=10
equivalent
WM Solomon/FSM/Sep2011

Also Achieved Higher Fusion Performance with Net
Torque~0 Nm at Higher Current, q95~4• Torque stepped downfrom 1 Nm in startup
phase back to 0•

bN~2.7, H98~1, G~0.32

WM Solomon/FSM/Sep2011

High bN~3, Torque~1 Nm, H98~1 Achieved Using
Central ECH (no q=2 ECCD)• Previous results
needed ECCD at
q=2 to operate athigh beta and lowtorque• Most recent results
still required EC
power, butinsensitive todeposition locationor heating vs current
drive– Here, ECH ~ 0.3 andconfigured forheating
WM Solomon/FSM/Sep2011

QH Mode May Be an ITER-relevant
ELM Avoidance Scheme• QH mode has a saturated (or stationary) edge mode
instead of ELMs
– Observed on all 4 large divertor tokamaks– Traditionally observed with ctr-NBI• Recent results have used the plasma response to nonaxisymmetric (n=3) magnetic fields to supply the edge
rotation in the counter direction believed necessary for
QH mode in the absence of external torque input

Neoclassical Torque from 3D Fields Can Replace
NBI Torque for QH-mode Access• Previously, QH-mode required significant NBI torque– Minimum pedestal velocity shear necessary for QH-mode, consistent withtheory predictions [Burrell et al., PRL (2009)]• Counter-Ip torque from nonresonant magnetic fields (NRMFs) enables QHmode in plasmas with zero-net NBI torque[Garofalo et al., NF (2011)]➠ Path toward QH-mode in self-heated burning regime

Recent Experiments Aimed at Testing ITER-like
NBI Torque and External Coils for 3D fields• C-coil only maintains QHmode with zero-net NBI
torque• Confinement improves atlow NBI torque• Large counter-rotation
obtained with zero-net NBI
torqueIn=3=6.2 kA (C-coil)Divertor Experiments Seek to Validate Basic Scalingsand Look for Impact on Pedestal and Core Performance• For fixed midplane temperature (fixed power flow?), one
expects:– Divertor target temperature to scale with 1/[Rtarget]2 from basic
geometry– Divertor target temperature to scale with 1/[L||]0.57, where L||
is the parallel path length from the midplane to the divertor• BUT, one can turn the argument around and assume fixed
divertor temperature and look for the impact on thepedestal and core parametersThree Basic Shapes used in the Divertor
Shaping Experiment

The Base Case for the RTAR Comparison
Have Similar Plasma Properties
146010 145878

1.0

Ip (MA)0.0
8.0
1010152025
30
3515202530
35

40

40

4545

Pinj (MW)

0.0
3.0

HL89

2.0
55

55

50

50

1.0
0.0
0.6

RTAR =1.67 m
fEXP ~–1.8

RTAR =1.20 m
fEXP –~3.25511146010

DIII–DNATIONAL FUSION FACILITY

145878

nped (1020)

0.0
1000

0

Tped (eV)

01000

2000

3000
4000
Time (ms)

5000

6000

107-11/TP/jy

The Time-averaged Emissivity Along the Outer Divertor
Leg is Markedly Different in These Attached Plasmas

DIII–DNATIONAL FUSION FACILITY

ne/nG ≈ 0.35

ne/nG ≈ 0.38

PRAD/PIN ~ 0.38

PRAD/PIN ~ 0.43

107-11/TP/jy

Electron Density and Temprature at the Outer Target
During H-mode Do Not Show the Expected Behaviors
2TTAR ∝ 1/RTAR
2nTAR ∝ RTAR

RTAR =1.20 m

DIII–DNATIONAL FUSION FACILITY

Expect ~2× differences

RTAR =1.67 m

107-11/TP/jy

Current Profile Control Has Demonstrated
Impressive Closed-Loop Results• Primary motivations are:– Feedback control to explore suitability of target current
profiles for advanced scenario operation– Provide reproducible target conditions to clearly distinguisheffects of variations in the high performance phase• Two approaches were successfully demonstrated on DIII-D
this year– System identification approach will be discussed by Moreau inIOS-6.1 talk
– Model-based control results discussed here

First-principles Model-based Current Profile Control• Why Current Profile Control?
– Effect on steady-state operation, MHD stability and confinement.
• Why Model-based Control?
– The high dimensionality and nonlinearity of the system dynamics,
and the strong coupling between magnetic and kinetic variables,
motivate the design of a model-based, multi-variable controller thattakes into account the dynamic responses of both magnetic andkinetic profiles to the different actuators.
• Why First-principles Control?
– The model can be expanded/modified as our system understanding
improves. New physics can be easily incorporated into the model.
– No need for a system identification experiment for each scenario ofinterest. Model can be more easily adapted to different scenarios.
– The high dimensionality and nonlinearity of the system can bepreserved in the model, overcoming limitations of linearized models.

LEHIGHU N I V E R S I T Y	  	  !

First-principles Model-based Current Profile Control• Our Approach to Control-oriented Modeling:– A first-principle model of current profile evolution in response to auxiliary
heating and current drive systems (NBI, ECH, ECCD), line-averaged densityand electric field due to induction was developed for an L-mode plasma.

$(Te ) 1 " & ˆ ˆ ˆ "# )j NI , B"#
ˆ=%ˆ FGH(+ + Ro H$(Te )2 ˆ2 ˆˆ
ˆ"t µo %b F % "% '"% *B- ,oDiffusivity Control

Magnetic diffusion equation

!

"#=0
ˆ
"$ $ˆ =0FuelingInterior Control

Radio Frequency H&CD

Boundary Control"#µoRo=%ˆ"$ $ˆ =12& Gˆ Hˆ$ˆ =1

I(t)Induction
Neutral Beam Injection$ˆ =1Y. Ou, T.C. Luce, E. Schuster, J.R. Ferron, M.L. Walker, C. Xu, and D.A. Humphreys,“Towards Model-based Current Profile Control at DIII-D,” Fusion Engineering and Design 82(2007) 1153–1160.

LEHIGHU N I V E R S I T Y	  	  First-principles Model-based Current Profile Control

1.2
1.1

FBK ON: 1<t<1.2
FBK ON: 1.6<t<1.8 	

FBK ON: 2.2<t<2.7
FBK ON: 3.2<t

1
0.9
0.8
0.7

123

Time(seconds)
Time:2.658
sec

ï0.05

4

ï0.1

32.8
2.6
2.4
2.2

1.8

2Feedforward

123Time (seconds)
Time:3.158 secï0.0545

ï0.1

1.9
1.85
1.8
1.75
1.7
1.65
1.6
1.55

ï0.2

ï0.25

ï0.25

ï0.25e

ï0.2e

ï0.2

ï0.3

ï0.3

ï0.3

ï0.35

ï0.35

ï0.35

ï0.45
00.2

0.4

0.6

Normalized l

0.8

1

ï0.45
0

ï0.4

FBK OFF	

0.2

0.4

0.6

Normalized l

0.8

3Time (seconds)
Time:3.958 sec

1Disturbance is artificially introduced at t=2.2 sec.	

ï0.45
04

5

Target Profile
Actual Profile

ï0.1
ï0.15

ï0.4

2

Time: 3.958 sec

ï0.15

FBK ON	

1

ï0.05

Target ProfileActual Profile

Time: 3.158 sec

1.95

ï0.15

ï0.4

Feedforward
Feedforward + Feedback

ï33.2

Feedforward + Feedback
52.05

193.42Target Profile
Actual Profile

Time: 2.658 sece

Total Noninductive Power (MW)Plasma Current (MA)1.3m )

3.6

Feedforward
Feedforward + FeedbackLineïaveraged Plasma Density (101.4

FBK ON	

0.2

0.4

0.6

Normalized l

0.8

1LEHIGHU N I V E R S I T Y	  	  Plans for “October” run period• DIII-D management identified 3 areas of focus for this 3 weekrun– ITER baseline scenario (especially n=1 tearing stability)– QH mode (focus on fusion performance metric and co-NBIstartup)– Performance extension in steady state scenario with off-axis NBI• Additional experiments will be done in the following areas– Correction of TBM fields with coils– Pellet pacing of ELMs
– Runaway channel control