JET anniversary 20th May 2004P.Mantica 1/20
Understanding physics issuesof relevance to ITER
• Brief summary of ITER relevant physics issues addressed at JET
• Detailed example: recent improvements in the understanding of transport and confinement
presented by P. ManticaIFP-CNR, Euratom/ENEA-CNR Association, Milano, Italy
on behalf of contributors to the EFDA-JET Work Programme
JET Anniversary P.Mantica 2/20
JET as a bridge to ITERJET is the tokamak nearest to ITER in size and achievable plasma parameters.
This allows
the best confidence in the extrapolation of results;
to carry out physics studies that are not possible in other machines;
to act as a focus for EU fusion research favouring exchange of results from various groups and the build-up of a EU team.
JET anniversary 20th May 2004P.Mantica 3/20
ITER relevant physics issues addressed at JET
Disruptions
Edge Localized Modes
Stabilization of MHD activity
Burning plasma physics
Tritium retention and migration
Understanding energy and particle transport
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Why is understanding transport important?
Progress towards ignition relies on improving energy and particle confinement.
Understanding transport physics allows to control transport.
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What do we need to understand?Transport is measured to be well above the levels foreseen by theory of neoclassical collision processes ==> “anomalous transport”
The anomaly comes from the existence of turbulence processes
Electrostatic fluid turbulence simulation
hat type of instabilities e the main players?
w do we control them?
D.McDonald
JET anniversary 20th May 2004P.Mantica 7/20
Current predictions of ITER performance assume a flat density profile
a 20% gain in central density would bring a 40% increase in fusion power
Particle and impurity transport
Density peaking
Pfus ~ nD nT
D and T transport
Impurity accumulation
He ash removal
JET has unique capability to study differences between T and D transport
Outward convection
0
1
4 5 6 7 8
n e (1
020 -
3 )
Major radius (m)
JG04
.169
-7c
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Is an anomalous pinch predicted by theory?
Does an anomalous pinch exist in experimental data?
Anomalous Curvature Thermo- Warediffusion driven pinch diffusion Pinch
Density peaking: anomalous pinch or not?
JET anniversary 20th May 2004P.Mantica 9/20
Theoretical predictions from turbulence simulations
X. Garbet
• electrostatic turbulence simulations indicate curvature pinch
• Thermo-diffusion changes sign when ∇Te/∇Ti is increased
0.6
0.4
0.2
0.00.80.60.40.2
Spe/Spi τe 0.5 2.0 1.0 2.7 2. 3.2 TEP
ρ
ne-n95
electron heating
No core particle source nor Ware pinch
==> no flattening due to αparticle electron heating expected in ITER plasmas dominated by Ion Temperature Gradient modes
ion heating
JET anniversary 20th May 2004P.Mantica 10/20H.Weisen / A. Zabolotsky
Curvature driven pinch in L-mode
No core particle source nor Ware pinch, still peaked ne
Peaked density in H-mode at low collisionality
Experimental results
Current peaking
Den
sity
pea
king
Strong dependence on collisionality
JET anniversary 20th May 2004P.Mantica 11/20
Tritium transport studiesT penetration after trace T gas puff in D plasma is resolved by powerful 14 MeV neutron diagnostics
T transport investigatedin all main JET plasma regimes, clarifying several physics issues for particle transport.
Dependence on ρ* can be assessedB τp
T ~ ρ*-2.9
Anomalous pinch present also for T transport
L.Bertalot
JET anniversary 20th May 2004P.Mantica 12/20
Impurity accumulationOccurs when impurity transport is neoclassical and the density gradient is peaked
Control of impurity accumu-lation as well as He ash removal is a result of control of density peaking.Method: application of central ion heating through RF in ITG dominated plasmas. Reduction of v observed.
No ICRH2 MW ICRH
Ar d
ensi
ty
M.E. Puiatti
JET anniversary 20th May 2004P.Mantica 13/20
Heat transport
Confinement scaling laws
Turbulence stabilization
Formation of Internal Transport Barriers
Temperature profile stiffness
How to stabilize turbulence and attain higher ∇T/T
Turbulence limits attainable ∇T/T
τE,th = 0.0562 M0.19κa0.78R1.39a0.58Ip
0.9 BT0.15ne
0.41P-0.69
ITPA
ITER 98
ITER
JET anniversary 20th May 2004P.Mantica 14/20
Dimensionless Scaling Laws
Dimensionless form of ITER 98 scaling law
BτE~ρ*-2.7 β-0.9 ν*
0.0
New results from JET
τE= τB ρ*a βb ν*
c τE= τB 1/ρ* βb ν*c
gyroBohm scaling
BτE~ρ*-2.7 β0 ν*
-0.35
needs to be confirmed by fut higher power experiments
gyroBohm scaling law, e collisions
0
5
10
15Pr
ojec
ted
Fusi
onPe
rform
ance
βN
Conventionalprediction
0 1 2 3
Ipu
= JET= DIII–D
20
G.Cordey, C.Petty
JET anniversary 20th May 2004P.Mantica 15/20
Turbulence and transport increase above a threshold value of ∇T/T preventing to reach high values of ∇T/T
Temperature Profile Stiffness
0.1
100
0 0.2 0.4 0.6 0.8 1
ρ
Assuming T profile
P.Mantica
A gain in threshold or an attenuation of the stiffness factor imply a gain in ITER central temperature/fusion power or alternatively allow a less demanding constraint on pedestal heigth.
JET anniversary 20th May 2004P.Mantica 16/20
A powerful tool to study
stiffness: power
modulation
Modulation data allow a stringent test of transport models and their validation for extrapolation to ITER predictions
P.Mantica
JET anniversary 20th May 2004P.Mantica 17/20
Results
• Coupled IonTemperature Gradient and Trapped Electron Modes are the main players. Stiffness models can be discriminated.
P.Mantica
Critical thresholdElec
tron
hea
t flu
xWeaker stiffness for dominant electron heatingStronger
stiffness with ion heating
JET anniversary 20th May 2004P.Mantica 18/20
One solution to overcome stiffness: Transport BarriersTransport barriers are regions where turbulence is stabilized and consequently transport is reduced.
ExB flow shear
Negative magnetic shear
X.Garbet
Turbulent vortices are broken and transport reduced
r
r
q q
r
zITB1
00.80.60.4
T
r/a
L RS q min =1.35 RS q min =1.5
JET anniversary 20th May 2004P.Mantica 19/20
JET has shown the importance of q profile for ITB triggering in large machines
Stabilizing effect of negative shear on electrons and ionsradius radius
0 0.2 0.4 0.6 0.8 1
Ion
tem
p er a
t ur e
(k e
V )
q
C.Challis
JET anniversary 20th May 2004P.Mantica 20/20
CONCLUSIONSJET is providing important information to clarify several ITER relevant transport issues due to
Continuing JET operation during ITER construction would allow significant progress in physics understanding, thereby making use of ITER more effective
• Proximity to ITER range of ρ*,ν*,β• T injection facility• Good diagnostic capabilities• Flexibility of heating systems