outline experimental setup experimental results main features of lithium operations

Plasma behaviour in presence of a liquid lithium limiter

G. Mazzitelli1 on behalf of FTU Team1

P.Innocente2, S.Munaretto2

1Ass. Euratom-ENEA sulla Fusione, CR Frascati, C.P.65, 00044 Frascati, Roma, Italy2Consorzio RFX, -EURATOM/ENEA Ass. C.so Stati Uniti 4, Padova,Italy 35127

2nd International Symposium on Lithium Applications for Fusion DevicesApril 27 - 29, 2011

Princeton, New Jersey, USA 1

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OUTLINE

1. Experimental Setup

2. Experimental Results Main features of lithium operations Peaked electron density discharges Effect of Lithium on MHD Activity at FTU Heat load CPS Damages

3. Work in progress

4. Conclusions

2nd Int. Symp on Lithium Appl for Fusion Devices G. Mazzitelli

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1. Experimental Setup


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Liquid Lithium Limiter

Langmuir probes

Thermocouples

Heater electrical cables


5

The LLL system is composed by three similar units

Scheme of fully-equipped lithium limiter unit

Liquid lithium surface

Heater

Li source

S.S. box with a cylindrical support

Mo heater accumulator

Ceramic break

Thermocouples

100 mm 34 mm

CPS is made as a matt from wire meshes with porous radius 15 m and wire diameter 30 m Structural material of wires is S.S. and TUNGSTEN

Capillary Porous System (CPS)

Meshes filled with Li


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Liquid Lithium Limiter

Melting point 180.6 °CBoiling point 1342 °C

Total lithium area ~ 170 cm2 Plasma interacting area ~ 50- 85 cm2

Total amount of lithium 80 g LLL initial temperature > 200oC


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2. Experimental Results


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Main features of lithiums operations1. Better plasma performance with Lithium than boronization

2. Radiation losses are very low less than 30%

3. With lithium limiter much more gas has to be injected to get the same electron density with respect to boronized and fully metallic discharges > 10 times

4. Operations near or beyond the Greenwald limit are easily performed

5. For q>5 the Greenwald limit has been exceed by more than a factor 1.5 at Ip=.5 MA Bt=6T (ne=3.2 1020 m-3) and nG>1.3 at Ip=.7MA Bt=7.2T Bt=6T (ne=4 1020 m-3)


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Main features of lithiums operations

6. Te in the SOL is 50% higher while increase in ne is much smaller in lithium discharges

7. Operations are generally more easy to perform and the behavior of the machine is more reliable.

8. Discharge recovery after a disruption is prompt


Peaked electron density discharges

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The SOL densities do not follow the FTU scaling law

461.eeSOL nn

Central density increases while edge and SOL densities do not change


Spontaneously the density profile peaks for ne > 1.0 1020 m-3



Very similar peaked density profiles with Li and B at least up to <ne>vol ≈ 1.5*1020m-3 but:

with Li it is possible to operate at higher <ne>vol ne(0)/<ne>vol => 2.5 only with Li, in a regime not accessible with B



For lithizated discharges the linear ohmic confinement (LOC) extends at higher values, from 54 ms up to 76 ms, that corresponds to the new saturated ohmic confinement (SOC).

The ion transport is negligible with respect to the electron one.

From JETTO code:

χe ≈0.2 m2/s a factor 2

lower than in the unpeaked phase

χi ≈0.2-0.3 m2/s close to

its neoclassical value.



Gyrokinetic code GKW has been used for microinstability analisys

At 0.3 s Li is the only impurity (Zeff=1.9). Li ions change the turbulence spectrum of ITG modes moving the peak of ITG modes toward higher ki

-At 0.3 s, with Li, the amplitude of the turbolence of ETG modes is lower than without Li

At 0.8 s, with or without Li no difference (Zeff=1)

Effect of Lithium on MHD Activity at FTU

B (T) I (KA) Ne (1020 m-3) qe

2.5 480 0.5 2.8

Without lithium: •Instability starts after rump-up•Disruption at 0.580 s.

With lithium: •Instability starts after rump-up•No disruption.

Effect of Lithium on MHD Activity at FTU

B (T) I (KA) Ne (1020 m-3) qe

2.5 250 0.3 5.3

Without lithium: •Instability starts after rump-up•Disruption at 1.2 s.

With lithium: •Low intensity instability during discharge.

Heat load


The heat loads on the three units are evaluated starting from the measure of the surface temperature.The temperature rise in a planar surface under a power flux density q (t) can be written :

where CP is specific heat of the material, its density and k the thermal conductivity.

''

)'(1)(

0

dtt

ttq

kCtT

t

p

Heat Loads


T is the difference between the maximum temperature and the initial value for each shot. The difference among the three LLL units is a cloud without any systematic behavior

Heat loads - 1° Case


Standard discharge used for lithization

Ip = 0.5 MA

Bt = 6 T

LCMS=1.5 cm

#33206

Heat loads – 1° Case


The temperature rise up to 450 °C at the end of the pulse and 1.5 MW/m2 are withstood for about 1 sec

#33206

#33206q(MW/m2)

HEAT LOADS – 2° Case


Heat load on LLL is increased by shifting plasma

Ip [x105 A]

t (s)

z(m)

LiI [a.u.]

LiIII [a.u.]


HEAT LOADS#33209

Although the heat load on the LLL is increasing or it should be constant during the time in which the plasma is pushed on the LLL, the temperature doesn’t increase in time but saturates at a maximum value.

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Rate of lithium evaporation in vacuum versus temperature


Heat Load


HEAT LOADS#28568 - Ip=0.5MA,ne=1.1020m-3, Bt=6T

CCD camera view: the bottom brigth green annular ring develops just in between LLL and TZM

wall

core

TZM i-side

TZM e-side

LLL

Prad

3D sketch (TECXY) of PradMost (60%) Li radiation (not in coronal equilibrium) in between TZM and LLL Strong interaction plasma - LLL => also density peaks in front of LLL => shorter n


HEAT LOADS

For the central unit heat load in excess of 5 MW/m2 are withstood with a strong peak up to 14 MW/m2 during the plasma disruption. Of course the lithium radiating cloud around the units strongly reduces the heat load and avoids damages to CPS structure.

q(MW/m2)

q(MW/m2)

q(MW/m2)

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No Surface Damage on CPS



Damage by fast electrons with LH

LLL-3

No surface damages


Very good behaviour of tungsten structure

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3. Work In progress


B2-Eirene Code - FTU edge simulation Collaboration with RFX - Padua

Transport code for the SOL

Multifluid

Toroidally symmetric configurations (toroidal limiter or poloidal divertor)

It solves a reduced set of fluid equations (Braginskii) on a 2D grid in the poloidal cross-section of a Tokamak

Neutral gas transport code

It is a multi-species code

It solves simultaneously a system of time dependent or stationary linear kinetic transport equations

It is coupled to external databases for atomic and molecular data and for surface reflection data

B2-Eirene Code - FTU edge simulation

Lithization studies in 2 steps:1. Toroidal limiter covered by Li

2. Influx of Li particles from the LLL

PROBLEM: No Li database available for ionization and recombination in B2 yet

In a first phase Be is used instead of Lithium

The code is going to be improved to read ADAS database

Preliminary simulation B2-Eirene

• Particle flux from the core: 1021 m-2 s-1

• Power input from the core 0.5 MW

• Recycling at the limiter: 0.75

• D = 1

Compared with electron density and temperature from Langmuir probes @ -70°in the poloidal plane

Agreement with the density profile

A sink of energy is needed (Molybdenum?)

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1

2

5

1 3

The Cooled Lithium Limiter (CLL)

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CONCLUSIONSCONCLUSIONS

•Lithiumization is a very good and Lithiumization is a very good and effective tool for plasma operations effective tool for plasma operations and performancesand performances•Exposition of a liquid surface on Exposition of a liquid surface on tokamak is possible but the tokamak is possible but the temperature of the liquid lithium temperature of the liquid lithium must be kept below 500 °Cmust be kept below 500 °C


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