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