EU Spherical Tokamak Approach to High Beta Steady State Operation

Brian LloydEuratom/UKAEA Fusion Association

This work was jointly funded by the UK Engineering & Physical Sciences Research Council and Euratom

OutlineIntroduction

Progress towards high beta steady-state operation

- stability- confinement- current drive- exhaust

Summary & Conclusions

- EU ST programme- ST Power Plant & CTF Requirements- Key issues for high beta steady state operation

MAST developments

IntroductionEU ST programme is centred at UKAEA Culham and focussed onthe Mega Ampere Spherical Tokamak (MAST)

MAST is the successor (first physics operation 2000) to the pioneering START experiment

Other EU Associations (e.g. IPP, ENEA, CRPP, IST, FOM) provide important contributions through collaborations and there are strong international links with the U.S., Russia, Japan and Brazil. ENEA is studying the SPHERA concept.

The goals of MAST are twofold:

- to advance key tokamak physics issues for optimal exploitation of ITER

- to explore the long-term potential of the spherical tokamak (ST).

MAST Parameters

Open divertor, up-down symmetric

Graphite protection on all plasma contacting surfaces

Adaptable fuelling systems - inboard & outboard gas puffing plus multi-pellet injector

Digital plasma control implemented June 2003(PCS supplied by GA)

Plasma cross-section and current comparableto ASDEX-U and DIII-D.

Parameter ValueAspect ratio, A 1.4Major/minor radius, R0/a (m) 3.416/2.44Elongation, 3.2Triangularity, 0.55Plasma current, Ip (MA) 31Centre rod current, Irod (MA) 30.2Vacuum field at R0, B0 (T) 1.77q0, qa 2.9, 15Line average density (1019 m-3) 10.8Greenwald density (1019 m-3) 16.6Vol average temperature (keV) 21.75, N 59, 8.2Zeff 1.6Fusion Power (GW) 3.1CD power (MW) 50Auxiliary CD (MA) 2.3Pressure driven current (MA) 28.7Confinement HIPB98(y,1), HIPB98(y,2) 1.4, 1.6

He* 4

1GW(e) ST Power Plant: physics design parameters

Howard Wilson, Garry Voss et alE

• Neutron wall loading (3.5MWm-2) drives the size: R=3.4m

• Cost of electricity limits toroidal field, Irod ~ Ip

• MHD limits N=8.2

• High elongation required for ~90% pressure-driven current;vertical instability =3.2 (fs = 3.0)

• Required fusion power (~3GW) Irod=30.2MA (Ip=31MA)

• Non-inductive current drive requires low density ~1.1x1020m-3

(~60% Greenwald)

• Confinement, E=1.6 IPB98(y,2) or 1.4 IPB98(y,1)

STPP parameter drivers

HighBootstrap

Low internalinductance

HighElongation

fBS ~ Nh()Irod/Ip

Howard Wilson et al

STPP Stability

Ballooning modes - a 2nd stable solution exists with 90% pressure driven current High central safety factor

~uniform magnetic shear across the plasma hollow current profileBUT in an ST q() can remain monotonic

Close fitting wall and high q(0) ensures n=1, 2 and 3 kink mode stable

0 0.5 1.0

q

12

10

8

6

4

2

0 0.5 1.0

J || M

A m

-2

0.8

0.6

0.4

0.2

NTMs? Considerable uncertainty but stabilising Glasser term very strong. High q(0)avoids low order modes.

Howard Wilson et al

0 0.5 1.0

J aux

MA

m-2

0.3

0.2

0.1

Required aux CD profile

0.14MA 2.2MA

0.5MeV-ve D beam

80keV+ve D beam

Current drive in the STPP

Rob Akers, Howard Wilson et al

Confinement

Pressure profiles chosen to be consistent with ideal MHD stabilityThe thermal diffusivities required have been calculated:

- A broad minimum in is required towards the edge- Control of may be necessary, eg through ITBs

ei

(m

2 s-1

) 6

4

2

0 0.2 0.4 0.6 0.8 1.0

Howard WilsonAlexei Dnestrovskij et al

Non-monotonic B(R) profile helps confine ’s

The full orbit code CUEBIT has been used to calculate

the lost -fraction

Despite large orbits in the core, the increasing B towards the edge “pinches” the orbits, improving confinement:

• Taking account of the ~1% TF ripple, losses are low, at the level <~1%

R(m)

CTF Parameters Free Boundary Equilibrium

Plasma current 8 MAMajor radius 0.71 mAspect ratio 1.6Elongation 2.7Triangularity 0.32Ip/Irod 0.9Normalised beta 3.6Fusion power (incl. beam fusion) 40 MWCurrent drive power ~45 MWTest module neutron wall load 2.0 MW/m2

Overall availablity ~40%Annual tritium consumption <1.0 kgH IPB98(y,1) , H IPB98(y,2) 1.45, 1.39Bootstrap current fraction 30% Shield plugs

TF coilsEquatorialtestmodule

Divertor coil

Centre rod

fieldVertical

coils

Polar testmodule

Component Test Facility: Physics design parameters

Component Test Facility (CTF)

Example:

Garry Voss, Howard Wilson et al

T = 11keV, ne = 1.8x1020m-3

Key issues for high beta steady state operation:

Confinement - the CTF is primarily ‘confinement limited’; confinement scaling with P, Ip important. Confinement scaling with beta important for the STPP.

Stability - the STPP is confinement & stability limited. A stable route to N ~ 8 has to be demonstrated. Control/avoidance of NTMs. Bootstrap predictions need to be validated experimentally. Consistency of implied profiles to be determined.

Current drive- experimental demonstration of suitable schemes for off-axis current drive,e.g. NBCD, electron Bernstein wave current drive

Exhaust- SOL characteristics and extrapolation to the STPP and CTF; testing ofnovel divertor concepts e.g. ‘biased’ divertor, cascading pebble divertor etc.

0 2 4 6 8 100

10

20

30

40

50

MG 19/04/99 15:50:25

199619971998

DIII-D, #80108

conventionaltokamak

RECORD ON START(achieved through NB Heating)

T , %

normalised plasma current, Ip/aB

T

The inherent high beta capabilities of the ST were first confirmed in START

Alan Sykes, Mikhail Gryaznevich et al

N=5li

N

P

li

MAST Beta Operating Space 2002-03

T

I/aB

(%)

tmax = 16%

N > 5, approaching ideal n=1no-wall external kink limit

Avoidance of neo-classical tearing modes (NTMs)by operating in regimes where sawteeth aresmall or absent altogether.

Mikhail Gryaznevich, Richard Buttery et al

Typical high N discharges in MAST

Mikhail Gryaznevich et al

Ip

PNBI

N

W

D

t(s)

- #7020 - #8789

unstable

stable

Ideal no-wall beta limit approached

By avoidance of NTMs N > 5, (N > 5li) has been achieved approaching the ideal no-wall beta limit - no obvious MHD limit to performance. Main limit so far due to initial NBI system capabilities

Menard calculation:

- q*/q0=3

- q*/q0=1.5

a) b)High N confirmed by kineticmeasurements

KINX calculations

fBS ~ 40 - 50%

Wfast ~ 15 - 20%

PNBI = 2.8MW

Matthew Hole, Richard Buttery et al

Sawtooth triggered NTMs (m/n = 3/2, m/n = 2/1) have been observed in MAST

3/2 NTM is excited close to its saturated size and at p close to p

crit strong seeding process

3/2 NTM reduces confinement by typically ~ 10%; approximate agreementwith Chang & Callen belt model2/1 NTM can trigger HL transition followed by mode locking and disruption

0.40.5

0.6

Neoclassical tearing modes (NTMs)

Richard Buttery et al

Neo-classical island evolution

Island evolution confirms strong role of field curvature stabilisation term (Glasser term)(which cancels ~60% of the drive provided by the bootstrap current in MAST)

Small island stabilisation effects (finite island transport and/or polarisation current effects) also necessary to explain observed island evolution.

Typical island widths:~ 4cm for m/n = 3/2 ~ 10cm for m/n = 2/1Magnetic estimates confirmed by Thomson scattering

Richard Buttery, Olivier Sauter et al

Energetic particle driven modes

Sergei Sharapov, Mikhail Gryaznevich et al

= 4.3%

= 5.7%

= 6.1%

0 1

TAE eigenfunctions at different values of thermal beta theory predicts TAE modesstabilised in MAST at high betaas observed experimentally

0 5 10 150.00

0.05

0.10

mod

e am

p., a

.u.

t, %

Chirping modes- experimentalmeasurements showmode amplitude reducesas beta increases.

STs support a rich variety of EPD modes - but TAEs and chirping modesfound to be stabilised for > 5% and > 15% respectively.

MISHKA

Highest t discharge subject to n=1, n=2 tearing modes

Mikhail Gryaznevich,Richard Buttery et al

Quasi-stationary H-modes with E ~ EIPB98(y,2)

N ~3, HH ~ 1, ne/nGr ~ 0.5 sustainedfor ~200ms (~ 4E)

Normalised parameters achieved comparable with requirements of a Component Test Facility (CTF)

Ip = 0.73MA, BT = 0.46T

CTF

Martin Valovic et al

MAST H-mode transport close to ion neoclassical

Ti=Ti(CXRS)r/a>0.5, i~neoclassicalr/a<0.5, e~ion neoclassical

Sensitivity test

(strong e- D+ coupling)

R<1.2m, Ti=Ti(CXRS)R>1.2m, Ti=Te

r/a>0.5, i~ e ~ 6m2/sr/a<0.5, i ~ e ~ion neoclassical

i~ e ~ 2-4m2/s in the mid radius region of the plasma (as required by STPP).

R. Akers et al

e ~3m2/s in the barrier region

i ~2m2/s (comparable to Z-corrected Chang Hinton

neoclassical value) in the barrier region

~0.7m2/s in the barrier region (3x lower than i)

*T(e-,D+)~0.14 (cf. *

ITB=0.014) - ITB criterion easily achieved in MAST

ITBs with co-NBI - early heating, fast Ip ramp

Rob Akers, Anthony Field et al

Electron ITB with counter-NBI

Te,

High performance counter injection plasmas on MAST:

Neutron rate down by ~2/3 but plasma energy comparable to co-injection.

Slightly higher Zeff (~2-2.5, but no peaking of profile).

Te profiles are broader, ne profiles more peaked than for co-injection.

Maximum plasma energy ~120kJ in ELMing counter injection H-mode (no ears).

Rob Akers, Anthony Field et al

Te,

e ~1m2/s in the barrier region (~Chang Hinton i)

i ~10m2/s (no evidence of ion ITB)

~2.0m2/s in the barrier region (5x lower than i)

Electron ITB with counter-NBI

Current Drive

Two main options being pursued:NBCDElectron Bernstein Wave (EBW) current drive

Initial NBCD studies promising -

Co-NBI

Cntr-NBI

Preliminary studies at modest beam power, indicate a neutral beam drivencurrent (~ 90kA) in approximate agreement with theoretical predictions (LOCUST).

Rob Akers et al

dZ=-0.1m

dZ=-0.2m

dZ=-0.3m

dZ=-0.3m no tilt

5 degree tilt

RT=0.7m, E0=70keV, 2.65MWPeaked Te(r) Te0 = 4keVBroad ne(r) ne0 = 4.45 x 1019 m-3

Off-axis NBCD

Alternative beam geometries are under consideration for off-axis current drive.

e.g. vertically offset beams, viz.

INBI ~ 250 - 300kA for PNBI = 2.65MW(dZ = 0.3m)

Rob Akers et al

EBW current drive

MAST 60GHz EBW antenna (21 mirrors)for proof-of-principle O-X-B EBW tests

An optimised low frequency system ( 20GHz) is under consideration for off-axis EBW current drive

EBW current drive demonstrated inCOMPASS-D(Shevchenko et al PRL 2002)

20 ~ 0.035 A/W/m2

Vladimir Shevchenko et al

Favourable divertor target power distributionin MAST

Pup ~ Pdown in DND Pout >> Pin

Glenn Counsell, Andrew Kirk et al

Outboard strike points shift radially outwards by 2 - 3cmduring low frequency ELMs but no significant broadeningof target power deposition

ELM power efflux is to outboard targets

Andrew Kirk, Glenn Counsell et al

Power loading in the STPP

The divertor target loading depends on the SOL width, which is uncertain

More work is needed to reduce theseuncertainties and develop novel divertor schemes (eg ‘biased’ divertor or cascading pebbles)

MAST data is compatible with a number of models (e.g. resistive interchange model)and incompatible with others.

Andrew Kirk, Joon-Wook Ahn et al

Compatible models predict powerloads in the STPP to be in the range- outboard 5 - 10 MW/m2

- inboard 15 - 25 MW/m2

but there are a number of uncertainties.

Divertor Biasing

ExB drift shifts peak power severalcm in opposite directions on live and grounded ribs - explicit prediction of theory

G Counsell et al

Ppeak reduced &slight broadening of power deposition

x3 broadening of power deposition but Ppeak rises (NB. Pbias/P ~ 0.3)

First experimental tests of toroidally asymmetric divertor biasing

Plant Developments 2003 - 04

present for M4New centre column New divertor

PF system improvements - e.g. P2 reversing switch

New error field correction system

NBI Upgrade 2003-04

Actively cooled calorimeter: Gate in closed position

Residual Ion Dumps

NBI system upgrade for reliable long pulse operation at high power (5MW, 5s)- replacement of sources with JET-style PINIs- new calorimeters & residual ion dumps employing hypervapotrons

MAST Schedule

2003/04

2004/05(provisional)

Apr Mar

M3b Improved divertor, new centre column, NBI upgrade (beam line 1)

Apr Mar

restart M4 M5NBI upgrade (2)

Operations

Engineering Break

Oct

Summary

The inherent high beta capability of the ST ultimately offers the possibility of a compact power plant and/or component test facility - viable designs forthese are being developed.

The EU ST programme is centred on MAST at UKAEA Culham but there is widespread involvement of other organisations through collaboration.

However, there are many key issues which have to be addressed to bring such ideas to fruition, e.g. exhaust techniques, current drive, stability & confinement. Since construction of MAST (& NSTX), considerable progresshas been made in tackling these key issues.

Many of the problems are similar to those faced by the conventionaltokamak community - there is a productive two-way relationship betweenSTs and the conventional tokamak STs are providing valuable input toITER but at the same time can directly benefit from the extensive knowledgebase of the conventional tokamak leading to rapid progress.