EFDA ITER

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THE PHYSICS OF ITER-FEATpresented by D J Campbell

EFDA, Close Support Unit - Garching

Acknowledgements:

Members of the ITER Joint Central Teamand Home Teams

42nd APS-DPP/ ICPP-2000, Québec City, 23-27 October 2000

EFDA ITER

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Synopsis

• ITER-FEAT Goals

• Physics design rules for ITER

• New ITER design

• Performance predictions:

• operating space for inductive operation• requirements for steady-state operation

• Design basis and physics issues:

• Confinement and transport• MHD stability and control• Divertor performance• Alpha-particle physics

• Conclusions

EFDA ITER

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ITER-FEAT GoalsPlasma Performance

• achieve extended burn in inductively drivenplasmas with the ratio of fusion power to auxiliaryheating power of at least 10:

• for a range of operating scenarios

• with a duration sufficient to achieve stationaryconditions on the time scales characteristic ofplasma processes.

• aim at demonstrating steady-state operation usingnon-inductive current drive with the ratio of fusionto current drive power of at least 5

• the possibility of controlled ignition should not beprecluded

Technology

• demonstration of integrated operation oftechnologies essential for a fusion reactor

• testing of components for a fusion reactor

• testing of concepts for a tritium breeding module

EFDA ITER

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Physics Design Rules

Confinement

• IPB98(y,2) ITER Physics Basis energyconfinement scaling (variations of scaling havealso been investigated):

τE,thELMy = 0.144 × I0.93B0.15P−0.69n

e,200.41M0.19R1.97ε0.58κ

eff0.78

• H-mode threshold scaling with isotope correction:

Pthr = 2.84 ×M−1B0.82n

e,200.58R1.0a0.81

MHD stability

• safety factor: q95 = 3

• elongation: determined essentially bytriangularity: control requirements

• density: ne ≤ nGW

• beta limit: βN ≤ 2.5

EFDA ITER

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Scrape-off layer/ Divertor

• peak target power: ≤10MWm-2

• helium content: simplified core/edge transportmodel

or: τHe∗ / τE ~ 5

• impurity content: nBe / ne = 0.02plus contribution from sputteredcarbon and seeded noble gasto limit peak target power

EFDA

ITER _________________________________________________________________________________________________________________________

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H-Mode ScalingsPower threshold Energy Confinement

ASDEX

AUG

C-MOD

DIII-D

JET

JFT-2M

JT60-U

PBX-MPDX

10.001.000.100.01τth (s)IPB98(y,2)

0.01

10.00

1.00

0.10

τ th

(s)

DB2P8=1

ER

99.1

.164

ITER-FEAT

EFDA ITER

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Device ParametersParameter ITER

κ95, κx 1.70, 1.85

δ95, δx 0.33, 0.49

R, a (m) 6.20, 2.0

R/a 3.1

Vol (m3) 828

B (T) 5.3

Ip (MA) 15.0

tburn (s) ≥300

<n>/nGW 0.85

<n> (1020m-3) 1.01

<Te>, <Ti> (keV) 8.8, 8.0

Zeff,axis 1.69

nHe,axis/ne (%) 4.3βN 1.8

β (%) 2.5

Pfus (MW) 400

Lwall (MWm-2) 0.47

Q 10

EFDA ITER

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ITER Poloidal Elevation

OperatingTemperature

20000 1000

PF5

PF4

PF3

PF2

PF1

TF Coil

CS

Blanket

VacuumVessel

Cassette

PF6

EFDA

ITER _________________________________________________________________________________________________________________________

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ITER: Main Design Features

Central Solenoid

Outer Intercoil

Structure

Toroidal Field Coil

Poloidal Field Coil

Machine Gravity

Support

Blanket Module

Vacuum Vessel

Cryostat

Port Plug

(EC Heating)

Divertor

Torus

Cryopump

EFDA ITER

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Heating and Current Drive

• Heating and current drive functions:

• heating plasmas through H-mode transition andto burn

• control of plasma burn point

• current drive for hybrid/ steady state operation

• localized current drive for mhd stability control

• plasma start-up assist, wall conditioning

• Proposed initial heating and current drivecapability: total power = 73MW

• 20MW of ECRF at 170GHz

• 20MW of ICRH in range 35-55MHz

• 33MW of 1MeV negative ion based NBI

• Additional capability for mhd control orsteady-state current drive foreseen, totalling>100MW

• this could include ~20MW of LHCD at 5GHz

EFDA

ITER _________________________________________________________________________________________________________________________

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ITER Plasma Equilibria

-5

-4

-3

-2

-1

0

1

2

3

4

5

3 4 5 6 7 8 9

Z, m

R, m

0.4MA

1.5MA

2.5MA

3.5MA4.5MA

5.5MA

SOH,SOB

XPF

11.5MA

6.5MA

-8

-6

-4

-2

0

2

4

6

8

0 1 2 3 4 5 6 7 8 9 10 11 12 13

Z, m

R, mC

S3U

CS

2UC

S1U

CS

1LC

S2L

CS

3L

PF1PF2

PF3

PF4

PF5PF6

g1g2

g4

g3

g5

g6

EFDA

ITER _________________________________________________________________________________________________________________________

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Performance in Pulsed Operation

Q=10 at 15MA (q95=3) Q=50 at 17MA (q95=2.6)

0.7 0.8 0.9 1.0 1.1 1.2 1.3

1000

800

600

400

200

0

HH(y,2)

Fusi

on P

ower

(M

W) β

N = 3.0

βN = 2.0

βN = 2.5ne / nGW = 0.85

Ploss/PLH = 1

No

Solu

tion

n e / n GW

= 1.0

Ploss/PLH = 1.3

IP=15 MA, Q=10 (AN=0, AT=2.15, Ar=0.12%)

0.7 0.8 0.9 1.0 1.1 1.2 1.3

1000

800

600

400

200

0

Fusi

on P

ower

(M

W)

βN = 2.0

Ploss/PLH=1

No

Solu

tion

n e/nGW

= 1.

0

Ploss/PLH=1.3

βN = 2.5

ne/nGW = 0.85

IP=17 MA, Q=50 (AN=0, AT=2.15, Ar=0.12%)

HH(y,2)

EFDA ITER

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Q=10: Plasma Profiles

• Plasma profiles I=15MA, Paux=40MW, H98(y,2)=1

EFDA ITER

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ITER Performance

• At Q=10, fusion power is 200-700MW atH98(y,2)=1

• Neutron wall loading at H98(y,2)=1 variesbetween 0.23MWm-2 and 0.80MWm-2

• so there is still scope for technology studies

• Q=10 operational space has a margin indensity against the Greenwald value:

• at βN=1.5, H98(y,2)=1, Q=10 can be achieved atn/nGW~0.7

• ‘Controlled ignition’ (Q=50) can be attained inITER:

• in an inductive advanced scenario (H98(y,2)~1.2)

• if operation at n>nGW is possible

• if high confinement can be sustained at q95<3

EFDA

ITER _________________________________________________________________________________________________________________________

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Hybrid Operation: Q=5

100

1000

0 10 20 30 40 50

R/a= 6.35m / 1.85m, βN≤2.5

R/a= 6.20m / 2.00m, βN≤2.0

R/a= 6.20m / 2.00m, βN≤2.0, 1.5D

Bur

n T

ime

(s)

Fusion Gain Q

ne/n

G = 0.85

3000

500

IP≤17MA, 400MW ≤ P

f ≤ 700MW

EFDA

ITER _________________________________________________________________________________________________________________________

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Steady-State Operation: Q=5open - without impuritiesclosed - with impurities

1.0

1.2

1.4

1.6

1.8

2.0

8 9 10 11 12 13

HH

-fac

tor

IP (MA)

2.0

2.5

3.0

3.5

4.0

8 9 10 11 12 13β

N

IP (MA)

EFDA ITER

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Hybrid and Steady-StateOperation

• Hybrid operation allows long pulses (~2000s)to be produced for technology testing

• Q=5 requires H98(y,2)~1 and βN=2.5

• this mode of operation should allow true steady-state to be developed gradually

• 1.5-D analysis of steady-state operation showsthat Q=5 requires:

• H98(y,2)≥1.5, βN≥3.5 for 9≤Ip≤12 and n/nGW≤1

• Ibs/Ip~40-50%

• These requirements imply that scenarios withactive profile control would be required

• βN values required imply that stabilization forresistive wall modes necessary

EFDA ITER

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Design Basis and PhysicsIssues for ITER

• Confinement and transport

• MHD stability and control

• Divertor performance

• Alpha-particle physics

EFDA ITER

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J G Cordey et al, Plasma Phys Control Fusion 38 (1996) A67

H-Mode Confinement:Non-Dimensional Scaling

0.1��

0.1��

1��

10��

1��

10��B τITERH93–P�

�

Bτ t

h� �

(I) DIII–D��(I) JET��

(II) JET����

(II) DIII–D��

DIII–D βn = 2.0��

ρ* scans��

JET βn = 1.5��JET βn = 1.6��JET βn = 2.0��τ ITER��

JG97

.293

/5c�

�

• JET/ DIII-D comparisons (for example) showBτE scaling in an almost gyro-Bohm fashion

( B Eτ ρ~ *−3) - star shows ITER-1998

• independently derived global scalingexpressions have approximately gyro-Bohmdependence

• analysis of local transport coefficients confirmsgyro-Bohm form in ELMy H-modes

EFDA ITER

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Core-Edge Integration• At the reactor scale plasmas must

simultaneously:

• exhibit good core confinement• operate at high density (n~nGW)• possibly operate close to H-mode threshold• dissipate exhaust power (significant radiation)

• Core-edge integration issues

• core and pedestal confinement scale differentlyfrom existing experiments to ITER scale

• current experiments matching ITER coredimensionless parameters have ‘low density’edges, typically well above the H-modethreshold, and with low to moderate radiation

• only an ITER-scale device can maintain reactor-relevant core parameters with reactor-relevantedge

• operation at high density with low NBI fuellingwill necessitate application of reactor relevantfuelling techniques

EFDA ITER

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Triangularity Issues

• Wedged TF construction allows segmentedcentral solenoid, providing additional flexibilityin equilibrium control ⇒ higher triangularity

• limit in ITER is probably set by approach toDNX configuration - require ∆sep≥4cm fromdivertor modelling

• Although triangularity does not appearexplicitly in confinement scaling:

• increased triangularity increases currentcapability

• JET and ASDEX Upgrade have found highconfinement can be maintained at densitiescloser to nGW with increasing triangularity

• In contrast, with increasing triangularity, ELMfrequency decreases and heat pulses todivertor may cause increased erosion

• high density operation, pellet injection, oralternative access to alternative H-moderegimes may moderate ELM behaviour

EFDA

ITER _________________________________________________________________________________________________________________________

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(J Paméla et al, 18th IAEA Conference, Sorrento, 2000) (O Gruber et al, 18th IAEA Conference, Sorrento, 2000)

Influence of Triangularity on ConfinementJET ASDEX Upgrade

density / empirical Greenwald density

HH-98PITER-FEAT

EFDA ITER

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(Y Murakami et al, Journal of Plasma and Fusion Research (to be published))

Sawtooth Simulation in ITER

• Sawteeth have small effect on fusion power

EFDA ITER

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Disruptions

There are 3 main issues arising from disruptionsand vertical displacement events:

• Thermal quench, involving ~300-500MJ:

• vapour shield formation expected to mitigatethermal quench effects (energy totarget<<10%)

• Current quench/ VDE involving ~0.5GJ ofenergy:

• eddy currents and halo currents give rise toelectromagnetic forces (up to ~104 tonnes)

• Runaway electrons might be produced byavalanche effect in cold, impure post-disruption plasma:

• calculations for the new ITER design indicatethat the total energy involved could be limited to~20MJ

EFDA ITER

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β-Limit - Neoclassical Modes

• Evidence from many tokamaks shows thatmost severe constraint on β is the growth ofneoclassical tearing modes:

• such modes are often observed in the regionβN~1.5-3

• extensive experimental evidence that critical βNdepends on (ρ*)µ, with 0.7≤µ≤1

• Experimentally (3,2) and (2,1) modes are mostcommon:

• (3,2) modes lead to degradation of confinement• (2,1) modes often cause disruption

• Theory of such modes is well-developed:

• however, predictive capability limited by needfor a ‘seed-island’ to trigger mode growth

• Expected mode growth time in ITER in range10-100s, allowing time for counter-measures:

• ECCD stabilization experiments now underway

EFDA ITER

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R J LaHaye et al, Phys Plasmas 7 3349 (2000)

β-Limit - Neoclassical Modes

• Analysis of the critical βN for the onset of (3,2)NTMs has been carried out across severaldevices:• βN∝ρ*f(ν) is consistent with theory based on

(stabilizing) ‘polarization current’ theory

• Indicates neoclassical modes could beexpected in ITER operating region

EFDA ITER

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G Gantenbein et al, Phys Rev Lett 85 1242 (2000)

Stabilization of NTMs

#12257

NBI Power (MW)

4.0

8.0

ECRH Power x 5 (MW)

0.0

4.0

8.0

Stored Energy (kJ)

400500600700

n=2 Amplitude (a.u.)

0.0

Scaled Energy (kJ)

2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.82.6time (s)

Shift of EC Resonance (cm)

02468

• Experiments with modulated ECCD in ASDEXUpgrade have successfully suppressed NTMs

• success achieved on several tokamaks• recovery of initial β remains a key issue• calculations predict that ~20-30MW of ECRF

power required for stabilization in ITER

EFDA ITER

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MHD Stability

• Main influence of sawteeth is likely to be viageneration of seed islands for neoclassicaltearing modes (NTMs)

• however, test of m=1 theory is required atreactor scale to address role of α-particles insawtooth stabilization and fishbones

• Disruption thermal loads, forces, and halocurrents will allow investigation of reactor-relevant phenomena

• ITER will operate in range βN~1.5-2.5, whereNTMs might occur

• stabilization of NTMs by ECCD/ LHCD has beensuccessfully demonstrated on several devices - such a system is foreseen for ITER

• In steady-state scenarios, resistive wall modesare likely to determine β-limit - if theoreticallimit can be reached

• a system of external stabilization coils forlow-m, n=1 RWMs is in under design

• coil set also used for error field correction

EFDA ITER

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Divertor Issues

• Long pulse capability of ITER makes divertorperformance critical - main issues:

• peak power load• helium fraction• control of density and fuel mixture• impurity content• transient power loads - ELMs, disruptions

• Divertor design developed from experience incurrent tokamaks

EFDA ITER

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A S Kukushkin et al, 14th PSI Conference, Rosenheim, 2000

Divertor Modelling

0

4

8

12

16

qpk

[MW/m 2]

ns [10 20m-3]

0.24 0.26 0.28 0.3 0.32 0.34 0.36FEAT:

geometry variation

86MWstraight86MWold V86MWnew V100MWstraight100MWold V100MWnew V130MWnew V

• Modelling using B2-EIRENE for ITER showsthat under partially detached conditions, peakpower load on outer divertor remains below10MWm-2 over a range of separatrix densities

• V-shaped geometry used in target regionfavours development of partial detachment

• influence of impurity seeding investigated

• core Zeff lies below 1.6

EFDA ITER

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A S Kukushkin et al, 14th PSI Conference, Rosenheim, 2000

Helium Exhaust - Modelling

0.001

0.01

0.1

100 150 200 250Γ

DT [Pa-m 3s-1]

c He

limit

limit

OK

75 MW

86 MW

100 MW

FEAT: Power Variation (Straight, Sp=75, C)

• Predictions of core helium concentration as afunction of fuel throughput, ΓDT, for ITER

• an installed fuelling capacity of 200Pam3s-1

should ensure that the core heliumconcentration can be held below 6%.

EFDA ITER

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A Loarte et al, 18th IAEA Conference, Sorrento 2000

ELM Power Loading

0

0.05

0.1

0.15

0.2

0.25

0.01 0.1 1 10

DIII-DJETASDEX-U

ν*

∆W

EL

M/W

ped

ν*ITER

• Recent analysis of ELM energy loss indicatesthat pedestal collisionality and paralleltransport time in the SOL are important

• extrapolation to ITER would imply type I ELMamplitude of ~10MJ

• this would pose problems for the divertorlifetime

• alternative H-mode operational regimes wouldbe desirable (eg type II ELMs, EDA)

EFDA ITER

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Divertor Performance• Detailed modelling underway:

• steady-state peak power load on outer divertorcan be kept below 10MWm-2 design limit

• core helium concentration can be kept below6%, as required

• ∆sep≥4cm required to limit power load in vicinityof upper null to that of first wall generally

• Transient power loads due to ELMs anddisruptions might prove the most severe limiton target lifetime

• Use of inside pellet launch and hightriangularity plasmas can provide tools forachieving high confinement at high density

• Co-deposition and retention of tritium must beaddressed by development of appropriateconditioning techniques

EFDA ITER

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Alpha Particle Physics• Key issue is that α-particles should slow down

classically and provide efficient heating

• extensive experience in experiments withenergetic particle populations produced byauxiliary power systems

• TFTR and JET DT experiments confirmα-heating as expected (within uncertainties)

• TF ripple losses must be within first wallpower loading constraints:

• theory well validated by experiments in severaltokamaks

• acceptable TF ripple losses in steady-stateconditions will require ferromagnetic inserts

• ITER will permit models of interaction withmhd instabilities to be tested:

• formalism exists for analyzing interaction withsawteeth, fishbones, kinetic ballooning modes,localized interchange modes

• interaction with NTMs and ELMs conjectural

EFDA ITER

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• Alfvén eigenmodes:

• extensive validation of numerical codes againstexperimental observations

• ITER-1998 expected to differ from presentexperiments in that many modes with n>10could be excited

• many of critical parameters in ITER (βα(0),vα/vA, R∇βα) differ little from ITER-1998(~20%)

• certain parameters (ρα/a) differ by up to afactor of 1.5

• Analysis of α-particle behaviour for ITERplasma conditions is now being initiated

• it is expected that unless unstable modesoverlap and extend to wall, non-linearredistribution of α-particles may simply resultsin profile broadening

• complications arising from 1MeV beam ions willhave to be addressed in parallel

EFDA ITER

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Conclusions• The new ITER design has been derived from:

• the ITER Physics Basis, which has beenvalidated in the experimental tokamakprogramme

• engineering methodologies and guidelineswhich have been established during the ITEREDA

• The design can fulfil the requirements of theITER programme:

• a significant margin for Q=10 inductiveoperation

• long pulse inductive operation appropriate forstudy of mhd stability and divertor operation(including helium exhaust)

• capability for studying steady-state scenarios atQ=5

• possibility of achieving ‘controlled ignition’ underfavourable conditions

• physics processes, including α-particle physics,will be characteristic of reactor scale plasmas

EFDA ITER

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• Major physics issues:

• maintenance of high confinement at highdensity

• control of NTMs and their impact on the β-limit

• impact of ELMs on divertor target lifetime

• tritium inventory control

• development of steady-state scenarios