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
2nd Int. Symp on Lithium Appl for Fusion Devices G. Mazzitelli
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Liquid Lithium Limiter
Langmuir probes
Thermocouples
Heater electrical cables
2nd Int. Symp on Lithium Appl for Fusion Devices G. Mazzitelli
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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
2nd Int. Symp on Lithium Appl for Fusion Devices G. Mazzitelli
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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
2nd Int. Symp on Lithium Appl for Fusion Devices G. Mazzitelli
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2. Experimental Results
2nd Int. Symp on Lithium Appl for Fusion Devices G. Mazzitelli
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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)
2nd Int. Symp on Lithium Appl for Fusion Devices G. Mazzitelli
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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
2nd Int. Symp on Lithium Appl for Fusion Devices G. Mazzitelli
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
2nd Int. Symp on Lithium Appl for Fusion Devices G. Mazzitelli
Spontaneously the density profile peaks for ne > 1.0 1020 m-3
Peaked electron density discharges
112nd Int. Symp on Lithium Appl for Fusion Devices G. Mazzitelli
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
Peaked electron density discharges
122nd Int. Symp on Lithium Appl for Fusion Devices G. Mazzitelli
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.
Peaked electron density discharges
132nd Int. Symp on Lithium Appl for Fusion Devices G. Mazzitelli
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
162nd Int. Symp on Lithium Appl for Fusion Devices G. Mazzitelli
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
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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
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Standard discharge used for lithization
Ip = 0.5 MA
Bt = 6 T
LCMS=1.5 cm
#33206
Heat loads – 1° Case
192nd Int. Symp on Lithium Appl for Fusion Devices G. Mazzitelli
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
202nd Int. Symp on Lithium Appl for Fusion Devices G. Mazzitelli
Heat load on LLL is increased by shifting plasma
Ip [x105 A]
t (s)
z(m)
LiI [a.u.]
LiIII [a.u.]
212nd Int. Symp on Lithium Appl for Fusion Devices G. Mazzitelli
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
2nd Int. Symp on Lithium Appl for Fusion Devices G. Mazzitelli
Heat Load
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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
242nd Int. Symp on Lithium Appl for Fusion Devices G. Mazzitelli
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
2nd Int. Symp on Lithium Appl for Fusion Devices G. Mazzitelli
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Damage by fast electrons with LH
LLL-3
No surface damages
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Very good behaviour of tungsten structure
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3. Work In progress
2nd Int. Symp on Lithium Appl for Fusion Devices G. Mazzitelli
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
2nd Int. Symp on Lithium Appl for Fusion Devices G. Mazzitelli
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