Disruption studies in ASDEX Upgrade in view of ITER

G.Pautasso, A. Herrmann, 1

Disruption studies in ASDEX Upgrade in view of ITER

Gabriella Pautasso

D. Coster, T. Eich, J.C. Fuchs, O. Gruber, A. Gude, A. Herrmann, V. Igochine, C. Konz, B. Kurzan,K. Lackner,

T. Lunt, M. Marascheck, A. Mlynek, B. Reiter, V. Rohde, Y. Zhang [1], X. Bonnin

[2], M. Beck, G. Prausner

and the ASDEX Upgrade Team

[1] Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, People's Republic of China

[2] CNRS-LIMHP, Universite` Paris 13, F-93430 Villetaneuse, France

Max-Planck-Institut für Plasmaphysik, Euratom Association, Garching, Germany

Based on an invited talk at the 36Based on an invited talk at the 36thth

EPS Conference on Plasma Physics and Controlled Fusion, EPS Conference on Plasma Physics and Controlled Fusion, Sofia, Bulgaria, 29.6 Sofia, Bulgaria, 29.6 --

3.7.093.7.09


Introduction

hazard for the tokamak:loss of energy confinement within ~ 1 ms -> thermal loadsdecay of plasma current and vertical instability ->

electromagnetic forces

self-induced toroidal electric field

->

runaway electrons

ASDEX Upgrade research program: characterization, avoidance, prediction and mitigation

mitigation with

massive gas injection (MGI)‏reduction of heat loads and forces on ASDEX UpgradeITER requirements for collisional

suppression of runaway electrons

dedicated experiments


Mitigation valves

electromagnetic MGI (piezo-released)(ex-vessel) (in-vessel) ‏

distance from plasma ~1.4 m ~10 cmopening time < 1ms

~ 1 ms

reservoir pressure < 15 bar < 50 barvolume 32, 64 ml 40-80 mlinjected n. atoms 4 x1021

(standard)

< 8 x1022

Sector 13

1 m

EM valves

in-vessel valve

plasmavolume ~ 13 m3


Discharges in H-mode were shut down by massive gas injection to study the dependence of the fuelling efficiency on:

distance plasma-valvegas pressuretype of gasgas quantityplasma energy

Parameter variationgas pressure = 1.7 – 40 barN. atoms injected: 0.3 – 8 1022

plasma parameters:Ip = 0.8 – 1 MAq95 = 4.3, 5.7 PNBI = 2.5 – 20 MWne = 0.6 – 1 1020 / m3

Eth = 0.3 – 0.7 MJEmag = 1 – 1.6 MJ

Experiments


Evolution of MGI-induced shut-down

valve open within 1 ms flight time ~ 0.1 ms

density rise and plasma cooling by radiation edge -> center

cooling of q=2 surface triggers thermal quench (AUG: CMOS fast camera, AXUV; DIII-D:

E.M. Hollmann

NF 45 ‏((2007)

m =1 structure of SXR profile at thermal quench

reduced spike or roll-over of plasma current starts current quench

(0.7 mbar.l of He)‏

cooltΔ

80%

20%cqtΔ


Cooling and current quench duration

Δtcool

and Δtcq

/S function of type and amount (Ninj) of gasindependent of thermal energytend to asymptotic values Δtcool -> 1 ms, Δtcq/S -> 3.5 ms/m2

Aims:short cooling phase to beat natural thermal quench but keep localized radiation below wall melting limitaccelerate current quench but keep

Δtcq

/S > 1.7ms/m2


Reduction of localized thermal load

Reduction of power deposited on divertor (thermography) by injection of 1 x1022

atoms of neon

85 % +/-

25 % of total plasma energy is measured in mitigated shut-downs foil-bolometer 180o

toroidally away from valvelarge scatter ~ toroidal asymmetry


Toroidally asymmetric radiated power

plasma thermal energy radiated “close” to valve during cooling phasethermal quench starts toroidally symmetric radiation phase (Ar) or reverses the toroidal asymmetry (Ne)‏radiated energy toroidally asymmetric (0-30 %)‏AXUV diodes measure photon emission with reduced (down to 30 %) responsivity

for Ephoton

< 100 eV

AXUV vertical cameras Δφ = 180o

in-vessel valve thermal quench

B. Reiter et al, this conference, P1-161


Reduction of mechanical forces

Prompt current decay and slower vertical displacement -

G. Pautasso

et al., NF 47 (2007) -

reduction of halo current and its toroidal asymmetry

total vertical force on the vessel (Fvv) reduced to force during controlled ramp-up and -down


primary or Dreicer

generation e E > me

ve

νee

= e ED

ED

~ ne

secondary generation or avalanche (JET and ITER)‏

ED -> Ec

~ ne

/c2

•

Compton scattering of gamma rays (tritium phase of ITER)‏

Mechanisms of generation of REs

100 102 104 106 108

E [eV]

»»»»»»»»»»»»»»»»»»» >

RE

γ

primary

secondary

Compton

suppressed

in AUG with ne

~ 4 x1019

m-3

in ITER with ne

~ 5 x1020

m-3

(Tritium phase: ne

~ 4 x1022

m-3)


Fuelling efficiency and effective density

Feff

= Δ

free electron number / atoms injectedtime averaged in ΔtF_eff

[(∫ne

dl )V-1

/ lV-1

+ (∫ne

dl )V-2

/ lV-2

]*Vol/2time averaged in ΔtF_eff

ne,eff

= ne

(t0

) + Δne,free

[ 1 + 0.5 (Z-1) ]time averaged in ΔtF_eff

assumption: impurity gas singly ionizedneutrals neglected


Dependence of Feff

on other plasma and gas parameters

on type of gasFeff

(He) = 0.5 –> 0.3 Feff

(Ne) = 0.4 –

0.1Feff

(Ar) < 0.1

degradation of Feff

with increasing Ninj

no dependence on gas pressure (not

shown) ‏

dependence on thermal energy at large Ninj

expected q95

dependence -

E.M. Hollmann

48 (2008) -


Feff

and thermal energy at large Ninj

At large Ninj

:

large poloidal density asymmetrydegradation of impurity assimilation at large thermal energyoutward drift of impurities on low-

field sidelate gas penetration (AXUV)‏

Ne


Towards nc

toroidal E field tends to asymptotic value

neff

/nc

~ 24 % with neon and Eth < 0.45 kJ (but inhomogeneous ne

‏(


Summary and conclusion

Localized heat load and large mechanical forces reduced by moderate neon MGI

-

should work in ITER too -fast cooling and current quench time above ITER design

24 % of density needed to suppress REs

in ITER has been reached in AUG

At large Ninj

: Feff

~ 20 % at moderate thermal energyasymmetric density distributionFeff

decreases with thermal energy

several valves at different poloidal and toroidal positions required in AUG (and ITER) to achieve critical density


Feff

dependence on plasma-valve distance

With the in-vessel valve:time delay trigger - current quench is eliminated dIp/dt doubles, reduction of vertical force radiated power doubles (max of 0.5 GW)‏Feff is higher (by 2-4) and ne rise starts earlier

5.80 5.81 5.82time (s)‏

0

5 102021758

V-1

V-2

òn_e

dl (m

-2‏(

-

6.800 6.810 6.8200

5*1020

V-1

21861

V-2

P_rad

(GW) ‏

#21861 EM valves#21758 IV valve

5.80 5.81 5.82

I_p

(MA)‏ 0.8

0.5

- 0.5z_curr

(m)‏

E_th

(MJ)‏

trigger

time (s)‏5.80 5.81 5.82

0.4

0.2

0.0

EM valves

in-vessel valve

Disruption studies in ASDEX Upgrade in view of ITER

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