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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

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in out balance and time scales of elm divertor heat load in jet and asdex upgrade
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

slide2

Outline of the talk & data base

Data base:

  • 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:

  • 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
slide3

Motivation

Though good progress for understanding ELM SOL transport is reported, we still do not understand ELM in/out asymmetries

Negative Btor

  • 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

Positive Btor

slide4

Note: va= 0 -> Ein/Eout = Nin/Nout =1

A free streaming particle approach (FSP)

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))

outer

inner

fv

v/cs

slide5

Comparison of FSP with IR data

Field reversed (-B,+I): Ei/Eo = 0.6

Field normal (+B,+I): Ei/Eo = 1.4

Inner

Outer

far SOL

  • ELM target energies Ein,out and τin,out enter as fitting parameters
  • In/out ELM energy asymmetry changes with field, time scales stay similar
slide6

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

slide7

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

Scaling suggests fast rise of instability ≤ 200us

slide8

Comparison to scaling: E(τIR)

ptarget

18%

JET - outer target

time

tIR

  • 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%
slide9

Consider a net particle velocity (va≠ 0)

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

inner

outer

fv

  • Conjecture: vshift arises from pedestal rotation and ExB drifts
  • Changing the Btor direction inverses the field line pitch at outer midplane
slide10

Typical Ein/Eout values as seen experimentally

Change of ELM heat fluxes with vshift

Same data as slide 5, #16725, normal field

Ein+Eout

Ein & Eout

  • 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
slide11

New Divertor

IR-Camera

For JET

CFC

W

IR-Picture when installed

elm structure at jet

Small & fast window gives 26.3kHz or 38us

ELM structure at JET

Footprints of single filaments

Snapshot of IR camera

Spatial resolution is 1.7mm

elm structure evolution camera data
ELM structure evolution (camera data)

190us

76us

before

0us

38us

380us

570us

760us

950us

Note, this all happens in less than 1ms

slide14

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
slide15

Positive Btor

Negative Btor

Vshift > 0

Vshift < 0

Comparison of ELM target energy and charge

T.Eich, JNM 363-365, p. 989, (2007)

  • 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)

For comparison of LP/IR see A.Kallenbach, submitted to Nuclear Fusion (2008)

slide16

ELM time + parallel transport

  • Energy source function
  • FSP for Linner/cs
  • FSP for Louter/cs
  • Inner target power load
  • Outer target power load
slide17

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)

slide19

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
slide20

Filament motion and velocities differ

  • 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

Inner divertor

Inner divertor

HFS

HFS

poloidal angle

LFS

LFS

Normal (+B,+I)

Reversed (-B,+I)

Outer divertor

Outer divertor

Toroidal angle

slide21

Filament motion and velocities

Net velocities Blue:Particles Green : Filament

Note: Parallel expansion of filament usually unobservable

Toroidal motion of filament ONLY due to V_perp.

HFS

V_perp

V_par

V_pol

poloidal angle

V_tor

LFS

Normal (+B,+I)

Toroidal angle

V.Naulin

slide22

‘normal’ ion B x grad(B) direction

AUG upper divertor

  • 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
slide23

‘reversed’ ion B x grad(B) direction

AUG upper divertor

  • More energy (power) deposited on outer target than on inner
  • Charge (current) → net positive charge on outer target

Observation: target with net positive charge receives more energy

slide24

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

slide25

Comparison JET and ASDEX Upgrade

JET

AUG

Focusing on ‘normal’ field:

  • JET & AUG + (ELM target energy < 100kJ) : 1 ≤Einner / Eouter≤ 2
  • Only JET + (ELM target energy > 100kJ) : Einner / Eouter≈2
slide26

Adjust Lo and Li

Vshift=0.1*cs