1/14 in/out balance and time scales of elm divertor heat load in jet and asdex upgrade t.eich 1,...
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1/14
In/Out balance and time scales of ELM divertor heat load in JET and ASDEX Upgrade
T.Eich1, A.Kallenbach1, W.Fundamenski2, A.Herrmann1 , R.A. Pitts3, J.C.Fuchs1, S.Devaux1, V.Naulin4,
ASDEX Upgrade Team and JET-EFDA contributors
1Max-Planck-Institut für Plasmaphysik, 85748 Garching, Germany
2 EURATOM-UKAEA Fusion Association, Abingdon, Oxon, United Kingdom
3 Association Euratom, CRPP-EPFL, 1015 Lausanne, Switzerland
4 Euratom-Association, Risoe-DTU, DK 4000 Roskilde, Denmark
28/05/2008, PSI-2008, Toledo, Spain
2/14
• Type-I ELMy H-Mode plasma discharges with deuterium
• ASDEX Upgrade upper single null discharges (+Ip/-Bt, +Ip/+Bt)
• JET lower single null discharges (+Ip/-Bt) optimised for IR studies
• All data are ELM averaged (~ 20) and thus filament averaged (~200)
Outline of the talk & data base
Data base:
Outline of the talk:
• A simplified picture for ELM energy transport
• Comparison to the empirical scalings for ELM power load time scales
• A possible contribution to the observed in/out ELM energy asymmetry
• Some preliminary results of the new JET divertor IR camera
3/14
Motivation
• Between ELMs most of the SOL power is deposited on the outer divertor target
• During ELMs the power load on the inner target is larger
Though good progress for understanding ELM SOL transport is reported, we still do not understand ELM in/out asymmetries
Positive Btor
Negative Btor
4/14
A free streaming particle approach (FSP)
v/cs
fv
innerouter
Working model: All particles during ELMs are released on a time scale, τELM, at the outer midplane and are free streaming along field lines to the inner and outer divertor target (W.Fundamenski et al., PPCF48, p.109 (2006))
)cL(. ELMsFSP 251
2
22
, /exp)(t
cvt
nt FSPsa
FSP
FSP
ELMoi
2
2
,, 1)()(t
tTtq FSPoioi
0av
0
,, )( dttN oioi
0
,, )( dttqE oioi Note: va= 0 -> Ein/Eout = Nin/Nout =1
5/14
Comparison of FSP with IR data
• ELM target energies Ein,out and τin,out enter as fitting parameters
• In/out ELM energy asymmetry changes with field, time scales stay similar
Field normal (+B,+I): Ei/Eo = 1.4 Field reversed (-B,+I): Ei/Eo = 0.6
InnerOuterfar SOL
6.0~*,topped
0av
6/14
Comparison to JET heat fluxes
Same exercise for JET ELM power load for inner and outer target
For outer target power load the agreement appears reasonable
For the inner divertor we use a best guess (due to reduced data quality)
1.0~*,topped
Similar time scales in JET compared to AUG due to higher pedestal temperature and longer connection lengths
7/14
Comparison to scaling: τIR
• For open divertor geometries we find a clear correlation
• For closed divertor geometries
systematically larger τIR are found
• The upper limit concerning material limits is given by the
scaling since τIR is shortest then
toppeds
scaling cRq ,95|| /2 scaling
IR ||27.1
Scaling suggests fast rise of instability ≤ 200us
8/14
ptarget
timeIR
JET - outer target
18%
Comparison to scaling: E(τIR)
• Within FSP approximation the E(τIR) is 18% of ELM target energy
• The temperature peaks slightly later ~50-100us (IR resolution)
• The E(τIR) for peak temperature is around 23-27%
)(E IR
9/14
Consider a net particle velocity (va ≠ 0)
fv
innerouter
sshift cv 1.0
• Conjecture: vshift arises from pedestal rotation and ExB drifts
• Changing the Btor direction inverses the field line pitch at outer midplane
Introducing a va = vshift = 0.1cs causes the in/out target energy & particle deposition to be asymmetric with values of Ein/Eout ~ 1.4 and Nin/Nout ~ 1.25 in the limit of fully free streaming particles
sshift cv /
10/14
Change of ELM heat fluxes with vshift
• Instead of fitting Ein and Eout, the values for Ein+Eout and vshift = 0.1*cs is set
• The small delay between inner and outer target contains information about the instability process
Typical Ein/Eout values as seen experimentally
sshift cv
Same data as slide 5, #16725, normal field
sshift cv 1.0
0shiftv
Ein+Eout
Ein & Eout
11/14IR-Picture when installed
New Divertor IR-CameraFor JET
CFC
W
12/14
ELM structure at JET
Snapshot of IR camera
Small & fast window gives 26.3kHz or 38us
Footprints of single filaments
Spatial resolution is 1.7mm
13/14
ELM structure evolution (camera data)
before 0us
380us
190us
570us
760us 950us
38us 76us
Note, this all happens in less than 1ms
14/14
Summary
• Parallel time scales of type-I ELMs are described reasonably well for the limit of low collisionality with assumption of free streaming particles
• The FSP approach gives a conservative limit for critical power loads
• Introducing a shift in the Maxwellian distribution for the particle velocities can reproduce ELM target energy in/out asymmetry
• Values to explain ELM energy asymmetries of ~1-2.5 are vshift/cs= 0 - 0.25
• Small observed delay of peak power load between and inner and outer target contains information about ELM instability which we need to understand
15/14
Comparison of ELM target energy and charge
• The effect of vshift must be working differently for ions and electrons
• Comparison of LP and IR (●) reveals energy asymmetry is due to ions• More detailed studies should be adressed with PIC modelling (next talk)
Negative BtorPositive Btor
Vshift > 0 Vshift < 0
For comparison of LP/IR see A.Kallenbach, submitted to Nuclear Fusion (2008)
T.Eich, JNM 363-365, p. 989, (2007)
16/14
ELM time + parallel transport
• Energy source function• FSP for Linner/cs• FSP for Louter/cs• Inner target power load• Outer target power load
17/14
Variation of energy source function Additionally to the FSP approach, we can numerically assume finite numbers for the ELM energy efflux duration and poloidal extension
Result: Only very little change in the resulting target heat fluxes which are beyond the diagnostic resolution
Which implies : From target fluxes no detailed conclusion on the poloidal extend nor ELM energy release time can be drawn
Shown: τELM,release = +/-75us, pol. FWHM = 13m (outer midplane)
18/14
First results of new JET divertor IR cam
19/14
JET target power load in ELMy H-Mode
• Between ELM most of the SOL power is deposited on the outer divertor target
• During ELMs the power load on the inner target is larger (1.5:1)
• Example here from the JET MKII-Gas Box divertor and IR optimised Type-I ELMy H-Mode discharges
20/14
ELM filaments are observed to decelerate toroidally (e.g. talk by A.Kirk)
Note that velocity components of particles and filament structures are different
Parallel particle velocity in filament does not result in filament rotation
Filament toroidal rotation solely due to perpendicular drifts, dominated by radial electric field
In/Out asymmetry due to field line pitch and pedestal top toroidal rotation direction
Filament motion and velocities differ
Inner divertor
Outer divertor Outer divertor
Inner divertor
Normal (+B,+I) Reversed (-B,+I)
0shiftv 0shiftv
Toroidal angle
polo
idal
ang
le
LFS
HFS HFS
LFS
21/14
Filament motion and velocities
Normal (+B,+I)
0_ shiftv
Toroidal angle
polo
idal
ang
le
LFS
HFS
Net velocities Blue:Particles Green : Filament
V_tor
V_pol
V_par
V_perp
Note: Parallel expansion of filament usually unobservable
Toroidal motion of filament ONLY due to V_perp.
V.Naulin
22/14
‘normal’ ion B x grad(B) direction
• More energy (power) deposited on inner target than on outer target
• Charge (current) → net positive charge on inner target
• Charge (current) for inner and outer target are equal in absolute size and opposite in sign
AUG upper divertor
23/14
Observation: target with net positive charge receives more energy
‘reversed’ ion B x grad(B) direction
• More energy (power) deposited on outer target than on inner
• Charge (current) → net positive charge on outer target
AUG upper divertor
24/14
ELM energy difference vs. charge difference
• The difference of ELM energy on inner and outer target is well correlated with charge difference
• Both quantities switch sign with field direction
• Situation is not symmetric but line passes through zero
line goes through zero
Cha
rge
(As)
for ‘normal’ field
As
kJ
C
EE
ELM
innerouter
5
5
1outer
inner
E
E
Diagnostical artefacts (i.e. surface layer) are neglegible
25/14
Comparison JET and ASDEX Upgrade
AUG JET
• JET & AUG + (ELM target energy < 100kJ) : 1 ≤ Einner / Eouter ≤ 2
• Only JET + (ELM target energy > 100kJ) : Einner / Eouter ≈ 2
Focusing on ‘normal’ field:
26/14
Adjust Lo and Li
Vshift=0.1*cs
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