An upgrade for MAST

IAEA ST workshop 2008, Roma 1

An upgrade for MAST

G. Cunningham for the MAST teamEURATOM/UKAEA Fusion Association, Culham

Science Centre,

Abingdon, Oxon, OX14 3DB, UK.


MAST stands on two legs

CTF DEMO physics


Strategic objective number 1: ST based component test facility (CTF)

Tritium consumption 0.6kg yr-1 ...

CANDU stock for 10 yr in parallel with ITER

major, minor radius 0.84, 0.54 m

Elongation 2.4

Ip, ITF 6.5, 10.5 MA

Paux 40 MW

βN, βT 3.5, 16%

fNI 100% (40% bootstrap)

IndicativeST-CTF parameters:


Strategic objective number 2: ITER and DEMO physics basis

EFDA mission MAST upgrade contribution

Burning plasmas High β, high fast particle content ( βFAST up to 20%), Alfvén mode physics

First wall materials, divertor design High, but adjustable, heat load, flexible magnetic design, accessible target area

Technology and physics of long pulse steady state ...

NB current drive, variable NBI geometry, q profile control, density control

Predicting fusion performance Confinement scaling, high rotation, high β (RWM), fast particle effects, ...


Features

• Vacuum vessel and main PF coils retained,• Closed, and pumped, divertor for long pulse density control,• Extensive set of divertor coils for plasma shaping and divertor control,• Fatter centre column for increased TF and flux,• Increased NBI power,• Off-axis NBI and on-axis counter-current NBI for q profile and rotation control,• 1MW EBW heating and current drive• Continuous pellet injector


NBI geometry (plan view)

2 double PINI boxes (1 on-axis, 1 off-axis PINI per box)

1 on-axis counter-current PINI


Design process (and talk outline)Non-inductive steady state (>> resistive timescale), NB drive

Bootstrap driven direct NB current drive

High β

q profile control

Strong shaping ()

Lower density

High T

Active pumping

Reduce J0

Substantial input power

Adequate divertor power handling

off-axis/counter NBI

Higher elongation

Higher density

Lower li


Divertor – magnetic design


Elongation is a function of li, and control system

•At suitable li (li(2)=0.65 here), and by optimising the divertor current, k>2.5 is achievable.•However such li is not readily sustained, •and vertical control is demanding

An additional 'X point' coilincreases triangularity,improves vertical control, and raises β limit.

The confinement region isnow satisfactory, but the divertor is problematic.


New array of divertor coils

• Array of divertor coils takes strike point out to larger radius• and gives good compensation for solenoid field.

Also, solenoid is enlarged to increase volt-seconds

‘Startup’ (merging- compression) coil is removed

Radial field coil is relocated to admit off-axis NBI


Wide range of divertor control

• The divertor strike point location can be moved considerably,• allows higher power density operation for divertor target testing,• and low power density for long pulse.

The divertor is remote from the main plasma, allowing closed divertor operation.


the same philosophy can be extended ...

• Possible DEMO divertor solution,

• Possible thanks to large MAST vessel,

• Known in the US as 'super-X',

• Ongoing development - collaborations welcome (simplification is no doubt possible!)


Central shaping coil gives further versatility

• Large centre column leaves space for a small central shaping coil

• gives access to increased triangularity


Divertor power handling

• 10 MW m-2 considered the limit for 5s operation• Experiment has found most of the power goes to

the outboard divertor ...

... outboard/inboard ratio, Roi up to 30


PNBI = 10 MW, PRAD = 3 MW,target = 90°, SOL = 1 cm, flux = 4

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

18.0

0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00

R (m)

HE

AT

FL

UX

(M

W m

-2)

CDN L-Mode (Roi = 30) CDN H-Mode (Roi = 10)

LSND L-Mode (Roi = 4)

FLUXSOLSP

targetlowerRADNBIHEAT R

fPP

sin

)(

INNER TARGET

Power deposition at the inner target (high )

EvenRstrike ~0.3m

is OK in DND


0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

40.0

0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00

R (m)

HE

AT

FL

UX

(M

W m

-2)

CDN L-Mode (Roi = 30) CDN H-Mode (Roi = 10)

LSND L-Mode (Roi = 4)

PNBI = 10 MW, PRAD = 3 MW,target = 30°, SOL = 1 cm, flux = 4

FLUXSOLSP

targetlowerRADNBIHEAT R

fPP

sin

)(

OUTER TARGET

Rstrike >0.8m is OK

... outer target, declined plate


Divertor - active pump design


ATOM DENSITY MOL. DENSITYATOM DENSITY MOL. DENSITYPUMP OFF PUMP ON

ATOM DENSITY MOL. DENSITYATOM DENSITY MOL. DENSITYPUMP OFF PUMP ON

Closed divertor with active (cryo-) pumping

Closed divertor gives factor 5 reduction in vessel pressure, active pumping a further 50% decrease. (OSM/Eirene)

MAST, EXPERIMENTMASTMAST-U, PUMP OFFMAST-U, PUMP ON

R(m)

Dα

em

issi

on


Model is validated against MAST data

INVERTED D IMAGE

Observed Dα image Abel inverted image Model calculation


Plasma scenario design

• 8 scenarios developed (2 discussed here),• Transp (including Nubeam) is the main

design tool (run until profiles are relaxed),• Non-inductive current fraction 40% to 100%,

• βN from 3.7 to 6.7.


Most scenarios have CTF-like shape

• Three shapes in total

– CTF like– high (F)– MAST (E)

• Most scenarios have CTF like shape.

– A,B,C,D,G• Off-axis

beams require high .


• Objective: Test the confinement and stability of a monotonic q-profile with q>2• Avoid all low m/n MHD

– Major problem in current STs– No sawteeth crashes– No 2/1 and 3/2 NTMs

• Requires high TF to keep q > 2– Irod = 3.2 MA

• Scenario is ideally stable even without rotation.• Neoclassical transport increases with q

– safe to use ITER scaling?– but recent ST results suggest stronger

Bt and weaker Ip scaling, giving greater benefit from TFenhancement than has beenassumed.

Scenario A (CTF-like q profile)

A1 : □

A2 : +


Two ways to scenario AA1: “High” density: n/nG= 0.5• Conservative approach. N ~ 3.7 > N

ITER=2.2 (Nth = 3.1)

• Balance on-axis Ohmic current with off-axis NBCD

• classical fast-ion diffusion• Flat top: ft1.8s 3.7 R 37 E

• limited by TF to 2.2s

A2: Low density:n/nG = 0.2• Almost non-inductive fNI = 0.8 N = 4.8, with high fast particle

pressure N

fast=2.7 , Nth = 2.1

• Dominated by NBI current• anomalous fast-ion diffusion

Dfi= 0.3 m2/s.• Flat top: ft 1.8 s 2 R45E

• limited by TF to 1.8s


Flexible machine - range of scenarios

• A1,A2 : baseline, CTF-like q profile, 2 density variants

• B : high fast particle content - confinement, fNI=0.9, βN=6,

• C : long pulse, fNI>1, βN=6.7, reduced TF

• D : high βT, Ip=2MA, q0~1, test fast particle β limit

• E : 'touch-base', high li, low β

• F : high =0.6, β limit and confinement scaling

• G : high thermal βT (βN up to 7), Ip=2MA, ng=1, β limit testing

Common parameters:• Ip=1.2MA• κ=2.5• A=1.6• li(3)=0.5

(except where stated otherwise)


Main limiting instability: n=1 internal kink• Pressure driven internal n=1 kink in the low magnetic

shear region (infernal mode) is the limiting instability in most scenarios (except G).

– Mode is strongly stabilised by rotation.

– Mode is stabilised by fast particles.

• Wall has little influence on the stability.

CCB

D

Effect of rotation on reference scenarios

Toroidal rotation on axis (km/s)

Gro

wth

rat

e / A

lfven


MHD stability - MISHKA analysis

Once the internal mode is stabilised, the next least stable mode is an external kink, which is amenable to wall stabilisation. Stabilising plates are under consideration.


Vertical position control

• High elongation is a major objective

• Little passive stabilisation close to plasma on LFS (NBI and diagnostic access)


Vertical position control Rigid plasma analysis (RZIP)

stable operating zone

>1kHz mode

<150 Hz mode

Growth rate (s-1)

Resonant frequency (Hz)

derivative gain

prop

ortio

nal g

ain


Pulse length limitations

Scenario pulse length τR limiting factor

A1 1.8s 0.5s TF joints I2t

A2 1.8 0.9 TF joints I2t

B 2.4 0.9 TF joints I2t

C 4.7 0.9 TF joints I2t fully NI

D 2.6 1.2 Solenoid I2t less NI current

E 3.4 Solenoid I2t "

F 2.4 0.8 TF joints I2t

G 1.9 0.5 Solenoid I2t "


ELM control coils

Poloidal magnetic spectrum for the n=3 configuration of the dipole (left) and monopole (right) coil sets. Superimposed as the blue crosses are the q=m/3 rational surfaces of the CTF-like equilibrium. The dipole set is closer to resonance.

Alternative ELM control coil geometries - both ex-vessel.

(left, preferred) n=3 dipole(right) n=3,4,6 monopole


1MW 18GHz EBW heating/current drive

Harmonic accessibility via O-X-B mode conversion. Midplane resonance topology with Doppler broadening. (shot #8694, Bφ = 0.55T at R=0.8m.)

0.9 1 1.1 1.2 1.3 1.4R [m ]

5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

f [G

Hz]

0

0 .2

0.4

0.6

0.8

1

Te

[keV

]

UHRPlasma resonanceTe

4fce

3fce

5fce

2fce

fce

This diagram is calculated for present day MAST, will need to be scaled for higher TF. Shows optimum access at ~18GHz


In conclusion, addressing CTF goals

0

0.5

1

1.5Inon-ind/I_p

Beta_N

rho-starRgrad(beta_fast) %

Pdiv/m^2

MAST08

MAST upgrade

ST CTF

Indicative values(not all simultaneous)

MAST today

Divertor

Steady state

Stability

Transport step size

Fast particles

No * extrapolation

• Upgrade exceeds most physics parameters of CTF:


Upgraded MAST is a key facility to progress fusion

The MAST team welcomes international partners to help explore the exciting new physics we will reach with the upgrade

and remembering the synergy with ITER/DEMO physics priorities,


This work was funded jointly by the United Kingdom Engineering and Physical Sciences Research Council and by the European Communities under the contract of Association between EURATOM and UKAEA. The views and opinions expressed herein do not necessarily reflect those of the European Commission.


blank


High NHigh N High fBS

High fBS

High q0High q0

Low liLow li

High High

)(~ hI

If

P

rodNBS

Hollow j(R)

Improves MHD stability

Improves vertical stability

Physics basis of a long pulse ST

Irod: limited by engineering and economics

Ip : maximise for confinement

... but competes with fBS - required for off-axis current drive, and optimisation


A Ip Irod qmin q0 li(3) n/nG N fNI Te,i0 neo

MA MA keV 1019m-3

A.1 2.5 0.5 1.6 1.2 3.2 2.1 2.3 0.4 0.6 3.7 0.5 1.4 8.8

A.2 2.5 0.5 1.6 1.2 3.2 2.1 2.2 0.5 0.2 4.7 0.8 2.4 3.5

B 2.5 0.5 1.6 1.2 2.9 2.0 4.5 0.5 0.2 5.9 0.9 2.4 3.6

C 2.5 0.5 1.6 1.2 2.2 1.5 3.2 0.5 0.2 7.7 1.1 2.4 3.6

D 2.5 0.5 1.6 2.0 2.4 0.6 0.6 0.5 0.2 5.5 0.5 3.2 6.1

E 2.0 0.4 1.4 0.7 2.0 1.5 1.5 0.7

F 2.4 0.6 1.6 1.2 2.9 1.7 2.2 0.5 0.2 5.2 0.7 2.4 3.6

G 2.5 0.5 1.6 2.0 2.4 0.8 0.8 0.4 1.0 7.0 0.4 1.4 24.6

Main parameters• All the scenarios have 4 beams at 2.5 MW each (1 on-axis, 2

off-axis, 1 cntr.)• Scenarios A.2, B, and C differ mainly in Irod and the assumed fast

ion diffusion.– “low” density almost fully non-inductive.

• Scenario D and G are high plasma current Ip= 2 MA.

An upgrade for MAST

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