Advancing Tokamak Physics with the ITER Hybrid … Tokamak Physics with the ITER Hybrid Scenario on...

Advancing Tokamak Physics with theITER Hybrid Scenario on DIII–D

byP.A. Politzer

Presented atForty-Ninth APS Meeting ofthe Division of Plasma PhysicsOrlando, Florida

November 12–16, 2007

with C.C. Petty*, R.J. Jayakumar† , T.C. Luce*, J.C. DeBoo*,E. J. Doyle#, J.R. Ferron*, P. Gohil* , R. Groebner*,W. Heidbrink§, C.T. Holcomb†, A.W. Hyatt *, J. Kinsey*,R.J. La Haye*, M.A. Makowski† , R. Nazikian‡ , T.W. Petrie*,Q. Ren¶, E. Ruskov§ , G. Staebler*, and M.R.Wade*

*General Atomics, San Diego, California.†Lawrence Livermore National Laboratory, Livermore, California.#University of California-Los Angeles, Los Angeles, California.§University of Irvine-Irvine, California.‡Princeton Plasma Physics Laboratory, Princeton, New Jersey¶academy of Sciences Institute of Plasma Physics, China

APS/DPP Orlando – 15Nov07 – Politzer – 2

The Hybrid Scenario is an High PerformanceInductive Operating Regime for ITER

Conventional H-mode:βN ≈ 1.8, q95 ~ 3, fBS << 1baseline

Advanced inductive:βN ≤ βno-wall, q95 ~ 3, fBS ~ 0.3high gain

Hybrid:βN ≈ βno-wall, q95 ≥ 4, fBS ~ 0.5high fluence

Advanced tokamak:βN ≤ βideal-wall, q95 ≥ 5, fBS ~ 0.8steady-state

inc

rea

sing

co

mp

lexi

ty &

be

nefit

“hyb

rids”

• The major tokamak programs have been developing the hybrid scenario for a number of years.

• It provides high gain and high neutron fluence options for ITER operation.

• This talk will cover two areas where we’ve made significant advances, leading to better capability to forecast performance:

– MHD & the current profile,

– rotation & confinement,

– and will give a brief sampling of other areas of tokamak physics being addressed in hybrids.


Stationary, High Performance Hybridsare Studied in DIII-D

• Stationary conditions are maintained for many τE and τR.➜ limited only by hardware.

0.00.51.01.5

024

012

05

10

time (s)

0.02.55.0

0 2 4 6 8 10 12

120675

Ip (MA)

PNB (MW/10)

βT (%)βN

H89PH98y2

ne (1019m–3)ZeffDα

Bn=1 (G; rms)~Bn=2 (G; rms)~

βT ≈ 4%βN ≈ 2.7

H89P ≈ 2.3H98y2 ≈ 1.3

Zeff ≈ 2

10 τE τR

ITER scenarios#2(base)

2.5%1.8 1.9

1.0 1.0

2.8%

#3(hybrid)

≈ 0.58 0.43 0.32βNH89q952

2.4 2.1


MHD and the current profile


MHD Activity is an Integral Part of Hybrid Operation– Usually a m/n = 3/2 NTM in DIII-D

• The effect of the ~stationary NTM

– is to broaden the current profile ➜ raises q(0) sawteeth are reduced (for q95 ≤ 4) or eliminated (for q95 > 4) › better confinement › removes one trigger for the 2/1 NTM

– increases stability of 2/1 mode

– with only a modest confinement reduction est ΔτE/τE ≈ – 6-15% depending on q95, rotation

– leading to high β operation;βN ~ 4li (~ no-wall limit)

0 1 2 3 4 5 6time (s)

H89P0

2

4

0.40.81.2

0.0

li

012302460

20

40125470125469

βN

Bn=2,rms (G)~

Bn=1,rms (G)~

➤ Without a 3/2 mode, the discharge evolves to an unstable 2/1 tearing mode

➜ controlled shut-down


The Current Near the Axis is Reduced, Increasing q(0)

10 ρ

0

1

2

–1

117740.5000

q=3/2

Johm+Jbs+JNB

Jtotal

J (M

A/m

2 )

ΔJ

DIII-D current profile simulation

• Measured current profile shows deficit at center compared to sum of calculated currents.

• Only ~5% of total current, but strongly affects q(0).

• TRANSP simulation switches to neoclassical resistivity and current transport at 3.5 s.

– current profile peaks and q(0) drops.

2.5 3.0 3.5 4.0 4.5

time (s)

1.1

1.0

0.9

0.8TRANSP

plasma

q(0

)

●


The NTM is Responsible for Modifying the q Profile:Changing the NTM Amplitude with ECCD Affects Sawteeth

• co-ECCD at q=1.5• suppresses 3/2 NTM• sawteeth appear

• counter-ECCD at q=1.5• enhances mode• sawteeth suppressed

n=23/2 NTM

n=1sawteeth

• Decreasing NTM amplitude increases sawtooth size, indicating peakingof the central current and reduction of q(0), and vice versa.

δB (

G; r

ms)

0246 119729

0123

ECCD on

3 4 5 6t (s)

δB (

G; r

ms)

119736642

0

012

3

ECCD on

3 4 5 6t (s)

• Raises the question: how does the NTM act on the current profile?


Several Mechanisms Suggested to Explain the Effectof the 3/2 Mode on q & J

• Making progress, but no definitive conclusion yet.

– Direct current drive by the 3/2 mode – as seen from the magnetic axis, the NTM island looks like an Alfvén wave antenna

• interesting physics; calculated magnitude too small

– Broadening of the fast ion spatial profile by the 3/2 mode• change in fast ion profile is observed;

modeling indicates small effect on current profile

– Modulation of the NTM amplitude by ELMs – asymmetry in time ⇒ flux pumping (analogous to the effect of sawteeth on q & J)

• evidence for effect is seen

– Dynamo – conversion of kinetic to magnetic energy via 〈v×B〉

• no data; need nonlinear resistive MHD modeling

~~

➜

➜


The Central Fast Ion Density is Reduced by the 3/2 NTM

0.4

0.3

0.2

0.1

0.0

be

am

cur

rent

de

nsity

(M

A/m

2 )

0.0 0.2 0.4 0.6 0.8 1.0ρ

Dfi = 0.01 m2/s

0.1

1.0

4.0

• Preliminary TRANSP analysis using uniform fast ion diffusion shows a large drop in central NBCD.

• TRANSP indicates that most of this is replaced by increased ohmic current, yielding a very small change in q(0).

• The FIDA (fast ion Dα) diagnostic indicates that the density of fast ions drops in the inner region of the plasma when the 3/2 NTM appears.

129257

n=2

no MHD

1.7 1.8 1.9 2.0 2.1R (m)

400

300

200

100

0

FID

A fa

st io

n d

ens

ity (

arb

)

ρ = 0 ρ = 0.5


Modulation of the 3/2 NTM by ELMs Leads to Current Profile Broadening

1042761

0

–1

–2

–3

–4

ΔBz (mT)

ρ0.0 1.0 0.0 1.0ρ

0.04

0.02

0.00

–0.02

–0.04

Δq

Profile change at an ELM(from direct MSE analysis)

• Averaging over many ELMs, analysis of MSE data shows that,at an ELM, q increases inside the q=3/2 surface.

100

0

–100

dB/

dt (

T/s)

118444

0

4

8

Dα

(a

rb)

4.0 4.1t (s)

• This modulation can lead to a poloidal magnetic flux pumping effect(similar to the process whereby sawteeth maintain q0 ~ 1).

[Petty, JP8.00084]


Rotation and confinement


Hybrid Performance Depends on Toroidal Rotation

• Most of the tokamak experience base has been limited to plasmas with strong toroidal rotation (thanks to NB heating).

• There is concern that ITER (& DEMO & reactors) will have low rotation.

➜ To study this issue, we’ve used the recently modified NB configuration in DIII-D to study the effect of rotation on the performance of hybrid plasmas. (5 sources co-NBI; 2 sources counter-NBI.)

– We did systematic scans of rotation for both hybrid (q95 ~ 4.2 & 4.6) and advanced inductive (q95 ~ 3.2) plasmas.

– The central Mach number has been reduced by up to a factor of 5, to M(0) ≈ 0.1, maintaining stationary conditions.

➜ Both the confinement and the MHD properties are affected.


Reducing Rotation Strongly Affects Plasma Characteristics

Pnb(MW)

0.0

0.2

0.4

0.6

0.8

M(0)

125499

0 1 2 3 4 5 60

1

2

3

4

t (s)

Bn=2(G; rms)

~

12

4

8

0

co

ctr

total

• Density and βN held constant, under feedback control

• High vs. low rotation: – add counter-NBI – reduce torque; rotation decreases – confinement decreases; total power increases – 3/2 NTM amplitude increases

• Experiments and modeling are sorting out what’s happening.

0.20

0.00

0.10τE (s)

M(0.5)


Density, Temperature, and Current Profilesare Unaffected by Changing Torque

1.5

1.0

0.5

0.0

-0.5

torque density(N-m/m3)

0.0 0.2 0.4 0.6 0.8 1.0ρ

Changing torque and power:

Results in a large change inthe rotation profile:

But the density and temperatureprofiles vary little(βN and density are controlled):

1254990.50.40.30.20.10.0

–0.10.0 0.2 0.4 0.6 0.8 1.0

ρ

M (=vφ/cs)

4

3

2

1

0

Te (keV)

Ti (keV)8

6

4

2

0

8

6

4

2

0

ne (1019 m–3)

0.0 0.2 0.4 0.6 0.8 1.0ρ

0.0 0.2 0.4 0.6 0.8 1.0ρ

0.0 0.2 0.4 0.6 0.8 1.0ρ

Also, JNBCD is halved(0.12→0.06 MA), butthe effect on q and Jtotalprofiles is negligible.


Angular Momentum is Roughly Proportional To Torque

• Vary the applied torque (T) over the range 0.4-4.6 N-m.

• Characterize rotation by either L = total angular momentum M(0.5) = Mach number at ρ = 0.5

› these are strongly correlated

• L ∝ T⇒ τΩ = L/T ~ independent of L,

except for a possible indication of ‘inherent’ rotation at low torque.

• Very low (and zero) rotation was inaccessible because of error field penetration and locking of the 3/2 NTM.

0 1 2 3 4 5torque (N-m)

0.8

0.6

0.4

0.2

0.0ang

ula

r mo

me

ntum

(kg

-m2 /

s)

?

uuu

u

uu

u

▲▲▲▲▲▲

▲

▲▲

▲

▲▲nnn

n

n n

n

nnnnnnnnu

u

q95

4.5-4.9

3.1-3.44.1-4.2


Global Confinement Improves As Toroidal Rotation Increases

0.20

0.15

0.10

0.05

0.00

M(0.5)0.50.40.30.20.10.0

nnnnnnnuuunun

nnnn

nn

u

uuuu

▲▲

▲▲▲

▲▲

▲▲▲

▲

▲

τΩ(s)

τE(s)

• Weak dependence of τE and τΩ on M(0.5) at q95 > 4; stronger for low q95.

• Medium & high q95 indistinguishable.

• τE and τΩ close to numerically equal, except at low NB torque.

0.200.150.100.050.00

0.20

0.15

0.10

0.05

0.00

nnnnnunnuu▲uu

uuunnn

n

▲

n

▲▲

▲▲▲

▲▲▲▲▲

q95

4.5-4.9

3.1-3.44.1-4.2

low q95

low q95

med q95high q95

med q95high q95

τE (s)

τΩ(s)

0.20

0.15

0.10

0.05

0.00

M(0.5)0.50.40.30.20.10.0

nnnnnuuuuu

uuu u

nnnnn

n

n

n

▲▲▲

▲▲▲▲

▲▲▲ ▲

▲


ExB Flow Shearing Rate Increases With Rotation

M(0.5) incr.M(0.5)incr.

high q95

0.0 0.2 0.4 0.6 0.8 1.0ρ

200

150

100

50

0

x103 s–1 q=1.5100

80

60

40

20

00.0 0.2 0.4 0.6 0.8 1.0

x103 s–1

ρ

low q95M(0.5)0.120.140.200.25

M(0.5)0.130.160.240.36

q=1.5

• ExB flow shear at high q95 is is ~2x value at low q95.


Turbulent Transport Coefficients Decrease As Rotationand Rotation Shear Increase (Tglf Modeling)

• At high q95: χe & χi are comparable, and well above χi,neo.

• At low q95: χi is comparable to χi,neo, but χe is much larger.

➜ Using χi+χe as a measure of overall turbulent transport, going from high to low rotation the turbulent transport increases by ~25% at low q95 and by ~40% at high q95.

4

3

2

1

0

m2/s

0.1 0.2 0.3 0.4M(0.5)

q95 = 4.5-4.9

χi,turb

χe,turb

χi,neo

●

● ●● ●●

◆

◆

◆

◆ ◆◆

▲ ▲ ▲ ▲ ▲

dashed blue lines give χe due to kθρi > 1 modes

avg ρ=0.5-0.75

0.1 0.2 0.3 0.4

4

3

2

1

0

m2/s

M(0.5)

q95 = 3.1-3.4

●

◆

▲

χi,turb

χe,turb

χi,neo

◆ ◆ ◆ ◆

●●

● ●

▲▲ ▲ ▲

avg ρ=0.5-0.75


Turbulent Transport Coefficients From TGLF ReproduceTrends Seen In Transp Analysis of Profiles

◆●

●

●●●

●

●

●

● ●●

◆

◆

◆

◆

◆◆◆◆ ◆ ◆

(χi–χi,neo)TRANSP

χe,TRANSP

(χi–χi,neo)TRANSP

χe,TRANSP

4

3

2

1

0

m2/s

0.1 0.2 0.3 0.4M(0.5)

q95 = 4.5-4.9

χi,turb

χe,turb

●

● ●● ●●

◆

◆

◆ ◆◆

avg ρ=0.5-0.75

◆

0.1 0.2 0.3 0.4

4

3

2

1

0

m2/s

M(0.5)

q95 = 3.1-3.4

●

◆

χi,turb

χe,turb

◆ ◆ ◆ ◆

●●

● ●

avg ρ=0.5-0.75


Change In 3/2 Island Width Is Calculated To HaveA Moderate Effect On Confinement

– Island width is larger at low q95 at all rotations.

– Change in width with rotation is larger at low q95.

– Largest island is 9.5 cm wide, ~16% of minor radius.

• Assess effect on τE using Chang-Callen ‘belt model’: – At high rotation, island reduces τE to 90-96% of est’d unperturbed value. – For higher q95, increasing width has a small effect. – For low q95, τE is reduced to ~85% of unperturbed value.

M(0.5)0.0 0.1 0.2 0.3 0.4 0.5

τE (with island)/τE (without island)

1.00

0.95

0.90

0.85

0.80

nnnnnnn

n n

nnn

n

n

uuuuu

uu

u

n▲▲

▲▲▲▲

▲▲

▲▲▲▲

NTM

isla

nd w

idth

(m

)

M(0.5)

0.10

0.08

0.06

0.04

0.02

0.000.0 0.1 0.2 0.3 0.4 0.5

n

n

nn▲

nnnnnnn

nnnn uuuu

uuu

u

▲

▲▲▲▲

▲

▲

▲▲▲

q95

4.5-4.9

3.1-3.44.1-4.2

low q95

high q95

low q95

high q95


Other tokamak physics with hybrids


Many Other Physics Studies And Results Using Hybrids

– The role of Te/Ti; changes in turbulence levels & confinementof energy and momentum. [Doyle, GO3.00006]

– The effect of shaping on confinement (triangularity, squareness,upper/lower null, double null). [Groebner, GO3.00012, Leonard BI1.00003]

– The effect of triangularity and squareness on the pedestal.[Leonard, BI1.00003, Groebner GO3.00012]

– The effect of rotation on the pedestal pressure. [Leonard, BI1.00003]

– Demonstration of radiative divertor operationand divertor heat flux reduction. [Petrie, UP8.00035].

– ELM suppression with RMP. [Fenstermacher, BI1.00002]

– The effect of wall conditioning on hybrid performance. [West, GO3.00005]


Increasing Te/Ti Leads to Increased Fluctuationsand Reduced Confinement

• Increase Te/Ti by adding 2.3 MW of ECH.

• Te/Ti at center increases by 22% (0.67-0.82).

• Rotation is reduced.

• Low-k & medium-k density fluctuations increase.

• To separate Te/Ti and rotation effects, compare with NBI-only plasma, matched for the same rotation and β; – by adding ~0.6 MW of counter-NBI instead of ECH.

• H89 is ~15% lower with ECH.

1.41.21.00.80.60.40.20.0Fl

uctu

atio

n le

vel (

a.u.

)

5004003002001000Frequency (kHz)

ECH (129282)

No ECH (128284)

ALT-CP, Ch1:4,17,18(acc),NP:1024/950, 3000-4000ms

r/a ~ 0.7

BES (0-3.5 cm–1)

FIR scattering (7 cm–1)


Confinement, Pedestal, And ELMs All Depend On Shape

• With increased triangularity:– confinement improves

at fixed βN– pedestal β increases

●

●●

●

■■

■■

1.6

1.4

1.2

1.0

0.81.0 1.5 2.0 2.5 3.0 3.5

βN

H98

y2

high δ

low δ

3.3 3.4 3.5 3.6 3.70

2

4

time (s)Dα

(a

rb)

• With increased squareness:– pedestal β decreases– smaller, faster ELMs

(consistent with theory)– reduce triggering of

2/1 NTM119748119750


Conclusions

• The presence of a stationary NTM is an inherent feature of ITER hybrid scenario plasmas in DIII-D – the NTM contributes to the beneficial modification of the current profile – the confinement penalty due to the NTM is modest

(5-15% lower than no-NTM estimate)

• Rotation and rotation shear have strong effects on confinement:

– Scanned the central Mach number 0.1-0.5

– At low rotation, the fusion performance parameter G (= βNH89/q952) is reduced by 10-30%, but remains above the ITER level for Q fus = 10 operation at low q95.

– 3/2 NTM island width increases as rotation is reduced, with a moderate effect on confinement.

– Primary confinement effect of changing rotation is via changes in ExB flow shear and turbulent transport.

• In addition to being a demonstrated scenario for improved ITER operation, the hybrid scenario is a good place for studying tokamak physics.

Advancing Tokamak Physics with the ITER Hybrid … Tokamak Physics with the ITER Hybrid Scenario on...

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