Cross-machine NTM physics studies and implications for ITER
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Transcript of Cross-machine NTM physics studies and implications for ITER
20th IAEA Fusion Energy - Vilamoura Cross-machine NTM physics - Buttery
Cross-machine NTM physics studies and implications for
ITERPresented by R. J. Buttery1,
On behalf of: P. Belo2, D. P. Brennan3, S. Coda4, L.-G. Eriksson5, J. Graves4, S. Günter6, C. Hegna7, T. C. Hender1, D. F. Howell1, H. R. Koslowski8, R. J. La Haye3, M. Maraschek6, M. L. Mayoral1,
A. Mück4, M. F. F. Nave2, O. Sauter4, E. Westerhof9, C. Windsor1, and the ASDEX Upgrade6 and DIII-D3 teams, and JET-EFDA contributors*.
1EURATOM/UKAEA Fusion Association2Association EURATOM/IST3General Atomics, San Diego4CRPP, Association EURATOM-Confédération Suisse5Association EURATOM-CEA, CEA-Cadarache
6MPI für Plasmaphysik, EURATOM Association7Dept. of Engineering Physics, University of Wisconsin8Association EURATOM-FZ Jülich IPP.9FOM-Rijnhuizen, Ass. EURATOM-FOM.*see annex 1, Pamela et al., Nuc Fus 43 (2003) 1540.
20th IAEA Fusion Energy - Vilamoura Cross-machine NTM physics - Buttery
2
1
03210
ITER?ITER?
AUG
JET
[M. Maraschek et al., EPS03]
3/2 NTMs
~i*0.9
Are NTMs a problem for ITER?•NTM physics is expected to scale with *
–often observed in local onset scalings
low threshold in ITER?
20th IAEA Fusion Energy - Vilamoura Cross-machine NTM physics - Buttery
0
1
2
3
4
4 6 8 10 12rho-i* x1000
be
tan
ASDEX-UDIII-DJ ET
N
i* x10-3
ITER
?IT
ER
?
?? ??
3/2 NTMs
[R. Buttery et al., NF04]
DIII-D AUGJET
Are NTMs a problem for ITER?•NTM physics is expected to scale with *
•But analyses in global suggests another possibility
–often observed in local onset scalings
low threshold in ITER?
•Key aspect in resolving the onset is the seeding process...
– which is it?
20th IAEA Fusion Energy - Vilamoura Cross-machine NTM physics - Buttery
Time (s)
40
0Neutron rate/a.u.
Magneticsspectrogram/kHz
4/3 NTM
5/4
7/5?
n=1 sawtooth precursor
-30%
-13%
Which modes are a concern? •2/1 NTMs terminate performance & unacceptable in ITER
•3/2 NTMs significant effect– typically 15-20% on confinement
~ -30% in fusion power
– trace Tritium experiments showconsistent with ~50% fall ininward pinch in vicinity of island
•Higher m/n NTMs also impact fusion performance at low q95
JET: 3.7MA, 2.9T, q95=2.7:– up to 13% effect on confinement– up to 30% effect on neutrons
20th IAEA Fusion Energy - Vilamoura Cross-machine NTM physics - Buttery
•2/1 NTMs terminate performance & unacceptable in ITER
•3/2 NTMs significant effect– typically 15-20% on confinement
~ -30% in fusion power
– trace Tritium experiments showconsistent with ~50% fall ininward pinch in vicinity of island
Which modes are a concern?
•Higher m/n NTMs also impact fusion performance at low q95
JET: 3.7MA, 2.9T, q95=2.7:– up to 13% effect on confinement– up to 30% effect on neutrons
AUG: 4/3 NTMs at q95=3.7:
– up to 20% effect on stored energy Time (s)
AUG
20th IAEA Fusion Energy - Vilamoura Cross-machine NTM physics - Buttery
Content •NTM * scalings:
–Onset criteria for NTMs
–How do the scalings do?
•Role of the seed: the Sawtooth
– Influence on thresholds
–Sawtooth control
–Advances in sawtooth prediction
•The seeding process
–Sawtooth coupling mechanisms
–Other trigger mechanisms and effects
•Implications for ITER
20th IAEA Fusion Energy - Vilamoura Cross-machine NTM physics - Buttery
metastable threshold
*
NTM onset criteria •NTMs driven by hole in bootstrap
–but onset criteria depend on small island stabilisation effects
– require a seed island to reachpositive growth
•Onset highly sensitive to seed size
– scaling of seeding processmay be the critical thing
–uncertainties both in seed needed and seed obtained
seed
positivegrowth
saturation
Island size
– these introduce a * dependencein the metastable threshold
20th IAEA Fusion Energy - Vilamoura Cross-machine NTM physics - Buttery
How do the * scalings do?Pretty good in terms of underlying NTM physics and metastable threshold...
0
1.5
0 0.3i *
p
e (r
s/L
p)
JETDIII-DASDEX UITER
ITER scenario 2operation point
Regression fitagainst i
* alone:
Pe=5.5i*1.08
–power ramp-down experiments measure at which 3/2 NTM self-stabilises
– ITER baseline operation point deeply into metastable region
small triggers can excite mode
mode removal requires driving island down to small sizes
20th IAEA Fusion Energy - Vilamoura Cross-machine NTM physics - Buttery
How do the * scalings do? •But they are not predictive of NTM onset and time on JET...
– stays close to NTM onset scaling prediction once H-mode reached
for both local and global parameter fits
JET Pulse No: 47282
0
0.2
0.4
0.6
0.8
1
1.2
-7 -6 -5 -4 -3 -2 -1 0
time relative to NT M onset (s)
exp
erim
enta
l
/
- f
it p
red
icti
on
Local fit
G lobal fit
rises 50%
while ratio close to 1
Time prior to NTM onset (s)
/
(NTM
fit
pre
dic
tion)
JET
20th IAEA Fusion Energy - Vilamoura Cross-machine NTM physics - Buttery
How do the * scalings do? •But they are not predictive of NTM onset and time
– stays close to NTM onset scaling prediction once H-mode reached in JET
for both local and global parameter fits
–similarly on AUG:
proximity to scaling is a necessary-but-not-sufficient condition for NTMs
–there must be an extra control parameter...
AUG
scaling
experiment
20th IAEA Fusion Energy - Vilamoura Cross-machine NTM physics - Buttery
What is the hidden control parameter?
•Employ neural network to look for pattern in data...–automatic optimisation from
choice of 27 input parameters
– train to predict onset time
•Network successful –predicts decreasing time to
NTM as onset approached unlike * scaling!
–best network uses just N, * and sawtooth period
period even more significant than * 0
0.5
1
1.5
2
0 0.5 1 1.5 2
Actual time to NTM (s)
Predicted time to NTM (s)
Data forms two clumps: Fast evolution during rise Slow evolution at high
JET
20th IAEA Fusion Energy - Vilamoura Cross-machine NTM physics - Buttery
Role of the sawtooth
•Long sawteeth can lead to many low modes (blue)
•Sawtooth control can offer substantial mitigation (red)– here achieved by:
ICRH phasing to avoid core pinch establishing sawtoothing heated
L-mode to avoid peaked profiles
– avoids all modes, even with much more heating power
•Sawtooth period plays key role in NTM onset
Time (s)
ICRH/MW
NBI/MW
termination
Magnetics: #58884 only
Expanded intime: 15-18s
2/1
3/2
4/3
0
15kHz
long sawtooth
SXR/a.u.
JET
20th IAEA Fusion Energy - Vilamoura Cross-machine NTM physics - Buttery [*Porcelli et al, NF44, 362]
Sawtooth control in ITER ITER has two possible strategies:
–early production to stabilise sawteeth - 50s may be possible* extend further with modified
start up and current drive but still limited and not steady state
Destabilisation of fast particle stabilised sawteeth now achieved:
– core ICRH stabilises sawteeth
Further progress with ECCD on AUG see Maraschek talk today
differ from resistive sawteeth in most present devices
– ICCD destabilises as inversion radius is approached
JET
– current drive destabilisation is this possible for fast
particle stabilised (ideal) sawteeth?...
20th IAEA Fusion Energy - Vilamoura Cross-machine NTM physics - Buttery
Sawtooth prediction is key •Good progress in the theory...
–kinetic effects stabilise sawteeth at high rotation
– important in reconciling data from present devices
Experiment: Theory:
[Gra
ves
et a
l, PP
CF42
, 104
9]
eg: Rotation dependence on JET...
JET
20th IAEA Fusion Energy - Vilamoura Cross-machine NTM physics - Buttery
Sawteeth with NNBI
[Kramer et al, NF40, 1383]
•JT60U also finds fast particles from energetic negative ion beams stabilising…
–350keV NNBI gives sawteeth of 300ms
– cf PNBI: 130ms
JT60U
20th IAEA Fusion Energy - Vilamoura Cross-machine NTM physics - Buttery
Pure coPure counter
Bal
ance
din
ject
ion
Co-deposit
ion
insid
e q=1
stabilis
es
sawte
eth
Injection direction
Sawteeth with NNBI
Deposition
outside q=1destabilising in co-direction
[Kramer et al, NF40, 1383][Graves et al, PRL92, 185003]
•JT60U also finds fast particles from energetic negative ion beams stabilising…
•Explained by Graves: –finite ion orbit effects
change free energy
–350keV NNBI gives sawteeth of 300ms
– cf PNBI: 130ms
–depends on deposition location...
•Possible mechanisms for sawtooth control in ITER?
20th IAEA Fusion Energy - Vilamoura Cross-machine NTM physics - Buttery
How is initial seed made?
–Magnetic coupling? NTM often too slow for
toroidal coupling to n=2
•Sawteeth often trigger 3/2 NTMs before the crash...
[*Hegna, Bull.Am.Phys.Soc.48, 280]
3 wave seeding possible: – bicoherence analysis shows phase lock
between driving (11+43) and 32 fields
but frequencies are not always consistent...
0 1 0 2 0 3 0 4 0 5 0- 5 0
- 4 0
- 3 0
- 2 0
- 1 0
0
1 0
2 0
1
( k H z )
2 (k
Hz
)
0 . 0 10 . 0 40 . 0 70 . 1 00 . 1 30 . 1 60 . 1 90 . 2 20 . 2 50 . 2 80 . 3 00 . 3 30 . 3 60 . 3 90 . 4 20 . 4 50 . 4 80 . 5 10 . 5 40 . 5 70 . 6 0
6 2 . 6 4 - 6 2 . 6 5 2 s
J E T # 5 1 9 9 52 / k H z
b i c o h e r e n c e o fn = 1 , 4 / 3 a n d 3 / 2
1 / k H z
SS pp ee cc tt rr oo gg rr aa mm :: ## 55 11 99 99 55
n = 1 p r e c u r s o r3 / 2
4 / 3
T i m e ( s )
n = 2
6 2 . 6 2 6 2 . 6 8
JET
– Ion polarisation effects? MHD can change island rotation*
potential to lower/reverse ionpolarisation effects enabling seeding
avoids need for frequency locking
20th IAEA Fusion Energy - Vilamoura Cross-machine NTM physics - Buttery
Forced reconnection at crash •At low , long sawteeth trigger NTMs directly at the crash–excite multiple NTMs & 2/1 much more likely concern for
ITER
– codes such as NFTC and NIMROD now able to 3D model such processes in detail...
n=1 perturbed pressure
n=2 perturbed current
Te
•Example: forced reconnection inducing a 3/2 in DIII-D–NIMROD simulation now includes rotation shear:
– island is still destabilisedby forced reconnection
–but as island grows itsstructure becomes distorted by rotation
•Shows viability of mechanism for NTM seeding
20th IAEA Fusion Energy - Vilamoura Cross-machine NTM physics - Buttery
Fishbone triggers - at higher N?
•Fishbones also trigger NTMs–3/2 NTM thresholds on AUG
generally higher than for sawtooth
AUG
20th IAEA Fusion Energy - Vilamoura Cross-machine NTM physics - Buttery
Fishbone triggers - at higher N?
•Fishbones also trigger NTMs–3/2 NTM thresholds on AUG
generally higher than for sawtooth
•Fishbones recently observed to also trigger 2/1 NTMs:–at N=2.5 on JET
–although on JET these do not extend to low
unlike cases with fast particlestabilised sawteeth
i
*JET
6.6 Time (s)
Spectrogram:Magnetics /kHz0
7.4
254/3 NTM
fishbones
2/1 NTM
JET
20th IAEA Fusion Energy - Vilamoura Cross-machine NTM physics - Buttery
Ideal triggers at high N? •At high N often see weak/no seeding…
–modes often near ideal limit
–a particular issue for hybrid scenario and 2/1 NTMs
4li
[Bre
nnan
et a
l, PP
10, 1
643]
•Separate studies show 2/1 NTM threshold lowered by error fields
– possibly an ion polarisation effect...
DIII-D
•Modelling of DIII-D case showspoles in classical tearing stability:
0
2
0 12Bpen
N /
[6.4
n0
.86 /
B0
.92]
Rotating onset Locked onset
DIII-D
20th IAEA Fusion Energy - Vilamoura Cross-machine NTM physics - Buttery
Conclusions for ITER on NTMs •ITER deeply metastable to NTMs, but tractable?
–benign scalings for some NTM onset mechanisms– control of seeds possible for others
•Baseline scenario - key issues are fast particles & sawtooth– further triggers at higher N may remain at high N
•Hybrid scenario - main concern is 2/1 NTM (3/2 fairly benign) –does 2/1 onset threshold fall with *? - mitigate with high qmin?
•However, caution required for ITER...–adverse NTM physics scalings and high fast particle
populations
–need to confirm scalings of high N modes, especially 2/1 NTMs
–need to integrate control techniques into scenarios to develop ready to use tools (not lengthy research programmes) for ITER
Nevertheless, we now see the principal physics ingredients assembled, a new generation of codes
identifying the effects, and good progress in control and predictive
capability. Ongoing work is important to provide solutions for
ITER
20th IAEA Fusion Energy - Vilamoura Cross-machine NTM physics - Buttery
–higher e-dissipation raises w0 ~(De)0.5
Transient transport events can seed NTMs
• Does not require frequency matching between MHD modes and the island
• May explain error field effects
•Ion polarisation effects depend on island rotation - apol
~
Ion polarisation destabilising
w
e*
i*
w0
–naturally leads to small islands via ion polarisation effects
Rotation from balance of ion and electron dissipation:
MHD event: - ergodises the edge- increases e- transport, De
- rotation more in e- direction
Islands produce 3-D structure in |B|
Neoclassical ion viscosity
e- transport(De)
[Hegna, Bull.Am.Phys.Soc.48, 280]
20th IAEA Fusion Energy - Vilamoura Cross-machine NTM physics - Buttery
What is the hidden control parameter?
•Employ neural network to look for pattern in data...–automatic optimisation from
choice of 27 input parameters
– train to predict onset time
•Network successful –predicts decreasing time to
NTM as onset approached unlike * scaling!
–best network uses just N, * and sawtooth period
period even more significant than *
•Sawtooth period plays key role in NTM onset
100% NBI
100% ICRH
JET
20th IAEA Fusion Energy - Vilamoura Cross-machine NTM physics - Buttery
Preemptive current drive on DIII-D •Use real time MSE tracking to put ECCD on NTM resonant surface, raising NTM thresholds
DIII-D
20th IAEA Fusion Energy - Vilamoura Cross-machine NTM physics - Buttery
Mode removal in ITER
– Island evolutions show scale length of small island term, wd, does not change much with *
–Mode removal in ITER will require driving islands down to similar size to those required in present devices
0
1
2
3
4
0.06 0.16i *
Fit
ted
wd
(cm
)
i*
20th IAEA Fusion Energy - Vilamoura Cross-machine NTM physics - Buttery
Use of correct local parameters
•Studies on AUG show that NTMs track correctly calculated bootstrap parameter, better than p
2/1 NTM
3/2 NTM
AUG
AUG
20th IAEA Fusion Energy - Vilamoura Cross-machine NTM physics - Buttery
How do the * scalings do? •But they are not predictive of NTM onset and time on JET...
– stays close to NTM onset scaling prediction once H-mode reached
for both local and global parameter fits
– JET NTM onsets align well with natural discharge evolution
(clue: ICRH phasings)0
1
2
3
4
0 0.009
BetaN
-90 RF
no RF
+90 RF
Onset fit
i*
N
discharge histories
NTM onset
JET
20th IAEA Fusion Energy - Vilamoura Cross-machine NTM physics - Buttery
* scalings sometimes work for NTM onset
•AUG discharges sometimes approach scalings from below and get NTM when the scaling is reached
AUG
20th IAEA Fusion Energy - Vilamoura Cross-machine NTM physics - Buttery
Formalism - origin of * scaling •Evolution of island size w governed by modified
Rutherford: wr
dt
dw
rr
44222222 2.028
35.065.0
b
pol
d
GGJ
bdbsP
ww
wa
ww
a
ww
w
ww
war
•Example: ion polarisation term, apol f( g(,) i
classical tearing finite
island transport
ion banana width
field curvature
ion polarisation currents
bootstrap
small island terms stabilising - lead to minimum size and for growth
•seed get > seed need
•uncertainties in both
gwa
awr
seedpol
polseedisonsetp .
])/(1[
/..'
2*
wseed
seed
positivegrowth
onset p rises rapidly as w
falls
saturation p
ons
et
wseed
20th IAEA Fusion Energy - Vilamoura Cross-machine NTM physics - Buttery
ITER possible figure? •Possibly how it looks...
q0
N
00.5
2
1
1
3
sawtooth: forced
reconnection 21/32/43
3wave/Hegna32
fishbone32/21
Poles in delta-prime 32/21
ELMs? 21?
20th IAEA Fusion Energy - Vilamoura Cross-machine NTM physics - Buttery
What is the hidden control parameter?
•Employ neural network to look for pattern in data...–automatic optimisation from
choice of 27 input parameters
– train to predict onset time
0
0.5
1
1.5
2
0 0.5 1 1.5 2
Actual time to NTM (s)
Predicted time to NTM (s)
Data forms two clumps: Fast evolution during rise Slow evolution at high
•Network successful –predicts decreasing time to
NTM as onset approached unlike * scaling!
–best network parameters:Parameters: Residual† Errors sawtooth i
* 34.3 17% sawtooth 34.4 20% i
* 35.7 26% 35.9 31%
i* 37.5 29%
†(predicted actual time to NTM)2
–Sawtooth period more useful than i* !
20th IAEA Fusion Energy - Vilamoura Cross-machine NTM physics - Buttery
Future work•Continue good progress on sawtooth models
•Demonstrate sawtooth control with strong FP populations at high beta
•Explore NTM triggering mechanisms and ways to control them
•Find beta limit in hybrid scenario and how it scales
•Resolve NTM small island physics and its scaling -particularly for 2/1 modes