R. A. Pitts: KFKI-RMKI, Budapest 12/04/2007 A summary of some recent edge physics research on TCV and JET R. A. Pitts Centre de Recherches en Physique.

R. A. Pitts: KFKI-RMKI, Budapest 12/04/2007

A summary of some recent edge physics research on TCV and JET

R. A. Pitts

Centre de Recherches en Physique des PlasmasEcole Polytechnique Fédérale de Lausanne, Switzerland

Association EURATOM-Swiss Confederationand

Leader, Task Force E, EFDA-JET


OutlineTCV

• Brief overview of the machine: first wall, heating systems, edge diagnostics

• Divertor detachment• Turbulent transport• Parallel flows• Not covered: ELMs, Infra-red investigations, SOLPS5 H-mode

modellingJET

• Retarding field energy analyser• Flows and far SOL ELM ion energies• A lot more not covered!


TCVTokamak à

Configuration Variable


The TCV tokamak R= 0.88m; a= 0.25m BT ≤ 1.5T; Ip ≤ 1.2MA 0.9< <2.8; -0.6< <0.9

•X2: 82.7GHz•6 0.5MW, 2s•Side launch ECH,

ECCD•ncut-off = 4.21019m-3

•X3: 118GHz•3 0.5MW, 2s•Top launch ECH•ncut-off = 11.51019m-3


TCV first wall All graphite machine

• Upgrade to ~90% coverage in 1998

• First wall tiled with polycrystalline graphite (~1700 individual elements)

• Cold walls (during operation)

• Regularly boronised (~220C, Glow with 10% B2D6/90% He)

• Pulse length typically ~1.2 s

• All tiles being B4C blasted this summer

R. A. Pitts, R. Chavan, J-M. Moret, Nucl. Fus. 39 (1999) 1433


TCV: configurational flexibility

16 independently powered poloidal field coils• Enormous scope for flexibility in plasma shape

• Nightmare for edge physics and PSI however!


TCV: Edge diagnostics• 80 tile embdedded Langmuir

probes • IR cameras• Fast reciprocating probe

(flows and turbulence)• In-vessel pressure gauges• Fast AXUV diode cameras


Divertor detachment Mandatory for successful ITER (and reactor) operation

• Without (partial) divertor detachment in the separatrix region, power fluxes will be beyond the design power handling capacity

• SOLPS5 (B2.5-Eirene) solutions show that this will be possible• But has the code been sufficiently benchmarked on today’s machines

for us to have confidence?

OS

P

ISP

ITER Divertor DDD 17 (SOLPS5 runs by A. Kukushkin)


Divertor detachment on TCV Studies always made in simple

ohmic discharges• Isolate physics, obtain best

possible data

• X2 ECR heating system precludes high density L-mode operation

• Studied effect of geometry on detachment – “plasma plugging”

R. A. Pitts et al., J. Nucl. Mater., 290-293 (2001) 940R. A. Pitts et al., IAEA-CN77/EXP4/23 (2000) Time (s)

en (1019m-3)

Zeff

P

PRAD,TOT

PRAD,DIV

D,divertor

Jsat(Acm-2)ISP

OSP

(kW)


“Anomalous” detachment TCV outer divertor does not

detach like in other tokamaks • Divertor densities too low• Neutral baffling insufficient• SOLPS4 simulations (with A. Loarte)

unable to reproduce observed detachment

M. Wischmeier, Phd Thesis (EPFL: TH3176 (2005))M. Wischmeier et al.,ECA 29C P-5.013 (2005)M. Wischmeier, R. A. Pitts, in preparation for Nucl. Fusion

3 year study with SOLPS5 tracked the problem down (probably)• Strong outward convective transport

main chamber recycling increased C release increased radiation “power detachment”

Jsat(Acm-2)

Te(eV)

ne(1019m-3)


Turbulent transport Expts. on TCV some of the first

to identify profile broadening with increased plasma density • Fast RCP under midplane

• ne, Te and fluctuation driven flux

• Large database in ohmic plasmas

Broad profiles at high density increased main chamber wall interactionWhy does this happen?


Intermittency In the far SOL, density

fluctuations more bursty and rare• Almost all the radial

transport in these regions carried by the blobs

• Convect plasma quickly to the wall regions

• Competes equally with parallel transport

• Consistency with known statistical distributions discovered on TCV

J. P. Graves, J. Horacek, R. A. Pitts, Plasma Phys. Control. Fusion 47 (2005) L1


Modelling the turbulence

O. E. Garcia, J. Horacek, R. A. Pitts, et al., Plasma Phys. Control. Fus. 48 (2006) L1

2-dimensional fluid turbulence simulations – ESEL code (Risø) • Centred on outer midplane

• ne, Te and vorticity evolution

• Collective motions driven by non-uniform B-field

• Linear SOL damping terms driven by SOL transport

Model parameters set by a high density TCV case • Sample turbulent fields over

long time series by an array of trial probes


Encouraging agreement with expt.

O. E. Garcia, R. A. Pitts, J. Horacek et al., PSI 2006, Heifei & Plasma Phys. Control. Fus. 48 (2006) L1

Code matches turbulent statistics • Relative fluctuation level

• Higher moments of PDF (Skewness, Flatness)

• Detailed “structure” of blobs – sharp front and trailing wake

Conditionally averaged density


Implications for main wall fluxes

J. Horacek, O. E. Garcia, R. A. Pitts et al., IAEA EXP4/21 (2006)

Good agreement between expt. & simulation provides extremely strong evidence for interchange motions as the origin of anomalous SOL transport• Flux-gradient paradigm: = Deffn/r not adequate to describe TCV data.

• Convection: = nVeff does better across region of broad SOL profile

• TCV results show that the scaling of wall flux with density seen elsewhere is due to turbulent interchange motions

PDF of turbulent flux at wall radius

2e

turb

walln

2ewall nn


B

BxB

ErxB, pxB

Ballooning

Pfirsch-SchlüterDivertor sink

ExB

Determine transport of impurities from source to destination in a tokamak – material migration – T-retention

FWD B

SOL Flows

B

BxBREV B

Poloidal

Parallel


#26092 #27585 #27582 #27588

Studying flows on TCV

BxBxBB BxBxBB

Mach probe

Use configurational flexibility of TCV to study flows in simplest possible diverted, ohmic plasmas• Emphasis on direction of B, configuration and density (|B| = 1.43 T)

• B and Ip always reversed together to preserve helicity


FWD-B/REV-B, Ip = 260 kA, density scan

wall

Strong field direction and density dependence near outer midplane• Flows always co-current

• Direction consistent with Pfirsch-Schlüter flow

• Slight, field independent negative offset

R. A. Pitts et al., PSI 2006


Ballooning drive?

+10 cm0 cm-10 cm en = 4.2 x 1019m-3

OU

TE

R d

iverto

r

Change of M|| with location above and below plasma midplane is consistent with a ballooning drive for the field independent flow offset• Not unambiguous owing to

presence of lower divertor sink – new results this week(!!) show that is a real “ballooning” drive

BxBxBB



260 kA density scan in FWD/REV-B:

OU

TE

R

diverto

r

INN

ER

div

erto

r

en (1019m-3) 1.7 2.5 4.2 7.36.3

REV B

OU

TE

R

diverto

r

INN

ER

div

erto

r

en (1019m-3) 1.7 2.5 4.2 7.36.3

REV B

FWD B

Choose radial band in the main SOL:8 < r-rsep < 12 mm

Take mean exptl. M|| and plot versus density

Compare with predicted Pfirsch-Schlüter flow

2B

Ben

pE

c er

s

2qcosMPS

||

Field dependent componentO

UT

ER

d

ivertor

INN

ER

div

erto

r

OU

TE

R

diverto

r

INN

ER

div

erto

r



Interchange driven flows Assume radial transport by

interchange motions in outer midplane vicinity – estimate parallel flow due to transients • Time averaged Mach No.

due to transport driven flow: M|| ~ 0.5fp>p

• fp>p = t(p > p)/t

• Duration of time series, t

• t(p > p) time over which p exceeds p by factor

R. A. Pitts et al., PSI 2006W. Fundamenski, R. A. Pitts et al., accepted for publication in Nuclear Fusion

Reasonable agreement with experiment• TCV results show that parallel flows can be explained by combination of

classical (drift driven) and transport (turbulence driven) components


JET


JET DOC-L Discharges

Retarding field energy analyserRFA

Designed and built at CRPP as part of enhancement project (JW0-ED-3.7) for reciprocating probe head upgrades • Previous attemps to make

such a device function had always failed on JET

Provides radial profile of SOL Ti

• Almost never measured, especially in large tokamaks

• Can also yield plasma potential and local Mach flow


-180 V-150 V 0 V

Vs

RFA Principle

Usual application is to sweep ion retarding grid to generate I-V characteristic and extract Ti, Vsheath agreement with experiment

• Negative slit bias allows simultaneous extraction of parallel ion flux Mach flows can be measured with a bi-directional device

• Long cable lengths (> 100 m on JET!) and small signals (A) prevent fast grid sweeping, but ELMs can be measured – see later


JET DOC-L Discharges

The JET RFA

Complex design – probes on JET subject to much greater constraints than elsewhere• Bi-directional – 2 RFA cavities

looking along B

• All boron-nitride design, like all JET probes

• 30 m wide entrance slits

• 2 mm grid separation

• Theoretical ion transmission ~0.2

March 2003

40 m

m

Slit plate

Grids

Collector

They don’t always last long either• Probe drive accident on last day

of operation in Campaign C14

R. A. Pitts et al., Rev. Sci. Instr. 74 (2003) 4644

After March 2004


Excellent flow data from top LFS

Probe samples at the near zero point for Pfirsch-Schlüter flow – and yet, large flows – not understood nor reproduced by edge codes (SOLPS5, EDGE2D)• BB strong parallel flow towards inner divertor at RCP

• BB flow stagnates at RCP

• Mean flow offset towards inner divertor – consistent with transport driven flow as in TCV – verified also with ESEL on JET

S. K. Erents, R. A. Pitts et al., PPCF. 46 (2004) 1757


SOL flow confirmed by Ti data

Large change in ion-side/electron side Ti ratio with field reversal• BB: Ti,i-side/Ti,e-side > 1

• BB : Ti,i-side/Ti,e-side ~ 1

• Due to the strong perturbing effect of the probe itself

• Ions depleted on the “downstream” side strong electric fields develop ion f(v||) modified

R. A. Pitts et al., ECA Vol. 27A, P-2.84R. A. Pitts et al., J. Nucl. Mater. 337-339 (2005) 146

• JET RFA provided first ever demonstration of this theoretically expected effect – quantitative agreement with theory for FWD B


D

JET #62218

t = 19.05 s, ELM-free t = 19.06 s, Type I ELM

Time (s)

H-mode Edge MHD instabilities periodic bursts of particles and energy into the SOL. Type I ELMing H-mode is currently the baseline ITER scenario

Edge Localised modes


Far SOL ELMs on the RFA Delicate edge probes can

never be used close to separatrix in high power discharges• But measurements in the far

SOL just as important (determine wall interaction)

• Use RFA to detect the ELM transient near the limiter radius

• Use constant grid bias and catch ELM convected ions able to surmount the potental barrier (~400 V)

• Only a few measurements in H-mode Hydrogen plasmasTime (s)

dsep at probe (mm)

jslit (Acm-2)

Icoll (A)

H (outer) /1015

Vslit (V)

Vgrid1 (V)

Vgrid2 (V)

Wdia (kJ)

#63214


Individual ELMsdsep ~134 mm

H

Wdia (kJ)

jslit (Acm-2)

Icoll (A)

dsep ~86 mm dsep ~74 mm dsep ~73 mm


Far SOL ELM ion energies

Clear filaments in each ELM• Net apparent flow to

inboard side ELM enters SOL mainly on outboard side

• Multiple filaments and clear trend for lower energies in successive filaments suggest picture of ELM as a train of toroidally rotating, feld aligned structures

r - rsep ~ 80 mm at the probe

Current of ions with energy > 400 eV

R. A. Pitts et al., Nucl. Fusion 46 (2006) 82


• Solves dynamical particle and energy two-fluid equations in the ELM filament frame subject to parallel losses determined by sheath boundary conditions

• Normalisation parameter: n,0 = L||/cs

• Characteristic parallel loss time evaluated at the initial conditions of the transient

• Filament origin location

• Te, Ti, ne at ELM origin

• ELM radial speed, vELM

• Parallel connect. length, L||

Input

Output

• Te, Ti, ne in the ELM filament at

any radial distance

Compute ion collector current with simple model of RFA function compare with expt.

Modelling the ELM transient

W. Fundamenski & R. A. Pitts PPCF 48 (2006) 109

New transient model of ELM parallel losses


Model consistent with RFA data Good agreement with i-side

fluxes• Assume ELM starts anywhere

from pedestal top to separatrix but with “mid-pedestal” Ti, Te, n

• Semi-adiabatic broadening

• vrELM = 600 ms-1 (from previous

JET scaling)

• Predicts Ti,RFA/Ti,ped = 0.30.5

• Te,RFA/Te,ped = 0.130.25

• ne,RFA/ne,ped = 0.30.4

Filament cools faster than it dilutes, electrons cool more rapidly than ions

W. Fundamenski & R. A. Pitts PPCF 48 (2006) 109


ELMs can interact strongly with main chamber surfaces

New KL8 visible (Photron) camera at JET picks up ELM induced main wall interactions during recent dedicated ELM ablation session (1 MJ ELMs!)

Images courtesy of CIEMAT team and C. Silva


~260 eV

~600 eV

~1100 eV

Model prediction for ITER

Model implies significant ELM wall erosion in ITER and beyond, even for high Z-wall

ELM starts mid-pedestal

D+ W: 0.5% yield(1% for T+)

D+ W threshold(209 eV D+, 136 eV T+)

Confirmed experimentally on JET (RFA)

Ion impact energy = 3Te + 2Ti R. A. Pitts et al., PPCF 47 (2005) B303W. Fundamenski & R. A. Pitts PSI 2006

R. A. Pitts: KFKI-RMKI, Budapest 12/04/2007 A summary of some recent edge physics research on TCV and JET R. A. Pitts Centre de Recherches en Physique.

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