1/14 In/Out balance and time scales of ELM divertor heat load in JET and ASDEX Upgrade T.Eich 1, A.Kallenbach 1, W.Fundamenski 2, A.Herrmann 1, R.A. Pitts 3, J.C.Fuchs 1, S.Devaux 1, V.Naulin 4, ASDEX Upgrade Team and JET-EFDA contributors 1 Max-Planck-Institut für Plasmaphysik, 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 B tor Negative B tor

4/14 A free streaming particle approach (FSP) v/c s fvfv innerouter Working model: All particles during ELMs are released on a time scale, τ ELM, 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)) Note: v a = 0 -> E in /E out = N in /N out =1

5/14 Comparison of FSP with IR data ELM target energies E in,out and τ in,out enter as fitting parameters In/out ELM energy asymmetry changes with field, time scales stay similar Field normal (+B,+I): E i /E o = 1.4 Field reversed (-B,+I): E i /E o = 0.6 Inner Outer far SOL

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) Similar time scales in JET compared to AUG due to higher pedestal temperature and longer connection lengths

7/14 Comparison to scaling: τ IR For open divertor geometries we find a clear correlation For closed divertor geometries systematically larger τ IR are found The upper limit concerning material limits is given by the scaling since τ IR is shortest then Scaling suggests fast rise of instability ≤ 200us

8/14 p target time  IR JET - outer target 18% Comparison to scaling: E(τ IR ) Within FSP approximation the E(τ IR ) is 18% of ELM target energy The temperature peaks slightly later ~50-100us (IR resolution) The E(τ IR ) for peak temperature is around 23-27%

9/14 Consider a net particle velocity (v a ≠ 0) fvfv inner outer Conjecture: v shift arises from pedestal rotation and ExB drifts Changing the B tor direction inverses the field line pitch at outer midplane Introducing a v a = v shift = 0.1c s causes the in/out target energy & particle deposition to be asymmetric with values of E in /E out ~ 1.4 and N in /N out ~ 1.25 in the limit of fully free streaming particles

10/14 Change of ELM heat fluxes with v shift Instead of fitting E in and E out, the values for E in +E out and v shift = 0.1*c s is set The small delay between inner and outer target contains information about the instability process Typical E in /E out values as seen experimentally Same data as slide 5, #16725, normal field E in +E out E in & E out

11/14 IR-Picture when installed New Divertor IR-Camera For 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) before0us 380us 190us 570us 760us950us 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 v shift /c s = 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 v shift 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 B tor Positive B tor V shift > 0V shift < 0 For comparison of LP/IR see A.Kallenbach, submitted to Nuclear Fusion (2008) T.Eich, JNM , 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: τ ELM,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 Inner divertor Normal (+B,+I)Reversed (-B,+I) Toroidal angle poloidal angle LFS HFS LFS

21/14 Filament motion and velocities Normal (+B,+I) Toroidal angle poloidal angle 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 Charge (As) for ‘normal’ field Diagnostical artefacts (i.e. surface layer) are neglegible

25/14 Comparison JET and ASDEX Upgrade AUG JET JET & AUG + (ELM target energy < 100kJ) : 1 ≤ E inner / E outer ≤ 2 Only JET + (ELM target energy > 100kJ) : E inner / E outer ≈ 2 Focusing on ‘normal’ field:

26/14 Adjust Lo and Li Vshift=0.1*cs