Accommodation of Prompt Alpha-Particle Loss in Compact Stellarators

1
Accommodation of Prompt Alpha-Particle Loss in
Compact Stellarators

• R. Raffray, T. K. Mau, F. Najmabadi,
• and the ARIES Team
• University of California, San Diego
• PSI-17
• Hefei, China
• May 21-26, 2006

2
Alpha Loss is an Important Issue in MFE, Governed
by the Magnetic Field Ripple
? loss can impact tokamaks, where size and
aspect ratio constraints can result in coil
placement inducing significant ripple, and more
importantly stellarators, where the inherent
non-axisymmetry of the magnetic geometry causes
an appreciable magnetic field ripple along the
flux surfaces inside the plasma. In this
case, a non-negligible fraction of the alpha
particles will either be born or kicked into
orbits that are trapped in these
ripples. Depending on the magnetic topology, a
fraction of these particles are promptly lost
from the plasma and hit the PFCs at energies
equal to or close to their born value of 3.5
MeV. The resultant footprints on the LCMS
show that the bulk of these particles are lost
in a region below the mid-plane on the outboard
edge of the plasma cross-section. On the other
hand, the more thermalized particles tend to
diffuse from the plasma uniformly on the LCMS.
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Several Classes of Stellarators Were Considered
As Part of the ARIES-CS Study, Including
NCSX-Based Configurations
Example In-Core ARIES-CS Configuration with
Port-Based Maintenance
Coil Configuration and Plasma Shape for Example
NCSX-Like 3-Field Period Concept
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Example Spectrum of ??Particle Loss from ARIES-CS
Compact Stellarator
? loss not only represents a loss of
heating power in the core, but impacts the
PFC design requirement and lifetime.
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Accommodating Alpha Particle Flux on PFC
High heat flux could be accommodated by
designing special divertor-like modules. e.g.
for typical ARIES-CS power plant
parameters - Pfusion 2350 MW - Max. neutron
wall load 5 MW/m2 - FW Surface Area 572
m2 - Assumed ? loss fraction 0.1 - Assumed
? footprint 0.05 - Ave. q on alpha modules
2.2 MW/m2 - Max. q lt10 MW/m2 for peaking
factor lt 4.5 - Divertor itself must be designed
to accommodate both the divertor heat load
and any incident ? loss power. Impact of
?-particle flux on armor lifetime (erosion) is an
additional concern.
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Divertor-Like Modules Can Accommodate Heat Flux
of Up to 10 MW/m2
T-tube divertor concept developed for ARIES-CS,
able to accommodate 10 MW/m2, utilizing a
He-cooled jet configuration with W-alloy as
structural material and a thin W armor. - Inlet
and outlet He temperatures 600C and
700C - Maximum W alloy temperature lt
1300C - Total stress intensity lt 370 MPa
Issues requiring further RD effort include
fabrication methods, W alloy development and
thermo-fluid performance verification.
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ARIES-CS Divertor T-Tube Concept
T-tubes can be assembled to form target plate
of required size.
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Alpha Particle Flux Impact on Armor Lifetime
For these high ? energies, sputtering is
less of a concern and armor lifetime would be
governed by some other mechanism, such
as exfolation, resulting from accumulation of
He atoms in the armor.
The implantation depth for 1 MeV He in
W is 1.5 ?m. - Pfusion 2350 MW - FW
Surface Area 572 m2 - Assumed ? loss fraction
0.1 - Assumed ? footprint 0.05 - ? flux
on PFC is 3 x 1018 ions/m2-s - Eff. Vol.
generat. rate 2 x 1024 ions/m3-s
Sputtering yield of W as a function of He energy
at different incidence angles (from W. Eckstein,
Sputtering, reflection and range values for
plasma edge codes, IPP-Report,
Max-Planck-Institut fûr Plasmaphysik, Garching
bei Munchen, Germany, IPP 9/117, March 1998.)
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He Implantation and Behavior in W Armor Quite
Complex, Consisting of a Number of Mechanisms
Due to high heat of solution, inert- gas
atoms essentially insoluble in most
solids. This can then lead to gas-atom
precipitation, bubble formation and
ultimately to destruction of the material.
He atoms in a metal may occupy either
substitutional or interstitial sites. As
interstitials, they are very mobile, but they
will be trapped at lattice vacancies,
impurities, and vacancy-impurity complexes.
The following activation energies were
estimated for different He processes in
tungsten 1,2 - Helium formation energy
5.47 eV - Helium migration energy 0.24
eV - He vacancy binding energy 4.15
eV - He vacancy dissociation energy 4.39
eV - From 3, D (m2/s) D0 exp (-EDif/kT)
D0 4.7 x 10-7 m2/s and EDif 0.28 eV
1. M. S. Abd El Keriem, D. P. van der Werf and F.
Pleiter, "Helium-vacancy interactions in
tungsten," Physical review B, Vol. 47, No. 22,
14771-14777, June 1993. 2. W. D. Wilson and R. A.
Johnson, in Interatomic Potentials and Simulation
of Lattice Defects, edited by P. C. Gehlen, J. R.
Beeler and R. I. Jaffee (Plenum New York, 1972),
p375. 3. A. Wagner and D. N. Seidman, Phys. Rev.
Letter 42, 515 (1979)
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Recent Experimental Data on Cyclic He
Implantation and Release in W (from S. B.
Gilliam, S. M. Gidcumb, N. R. Parikh, D. G.
Forsythe, B. K. Patnaik, J. D. Hunn, L. L. Snead
and G. P. Lamaze, J. Nucl. Mat. 347 (2005) 289.)
Results indicate that He retention decreases
drastically when a given He dose is spread
over an increasing number of pulses, each one
followed by W annealing to 2000C, to the
extent that there would be no He retention
below a certain He dose per pulse.
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Effective Diffusion Analysis of Experimental
Results for He Implantation and Release in W
(from S. B. Gilliam, S. M. Gidcumb, N. R. Parikh,
D. G. Forsythe, B. K. Patnaik, J. D. Hunn, L. L.
Snead and G. P. Lamaze, J. Nucl. Mat. 347 (2005)
289.)
Simple effective diffusion model
Set effective activation energy to reproduce
final He retention level in experiments for
given number of cycles, He dose implantation
and temperature anneals Activation energy
derived from analysis would not be that of bulk
diffusion but of the rate- controlling
mechanism, much probably some form of
trapping/detrapping mechanisms. - D0
4.7 x 10-7 m2/s - ? 1.5 ?m (for 1 MeV He in
W)
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Effective Diffusion Activation Energy (Eeff,diff)
as a Function of Dose per He Implantation from
Modeling of Experimental Results
(The curve fit has been drawn to suggest a
possible variation of the activation energy with
the He dose or concentration)
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Possible Explanation of Effective Activation
Energy Results
In general, trapping would increase with He
irradiation dose, which creates defects and
vacancies (followed by an anneal of the
unoccupied trapped sites during the ensuing
temp. transient). At very low dose, He
migration in W should be governed by bulk
diffusion (Eeff,dif 0.24-0.28eV) As the
dose per cycle increases, trapping sites are
formed or activated and Eeff,dif increases.
It seems that there is a near- threshold of
He dose at which Eeff,dif increases rapidly to
3.3-3.6 eV and stays there over a dose range
of 2 orders of magnitude. Above this range,
Eeff,dif increases rapidly to 4.2-4.8 eV,
indicating an increase in trapping perhaps due
to He build up in vacancies (the vacancy
dissociation energy is 4.4 eV) Overall, the
dependence of activation energy with dose is
about the same for SC and polycrystalline W
except that it is shifted to lower doses for
the latter case.
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Application of Modeling Results to Estimate He
Retention in ARIES-CS W Armor Due to ? Loss
For ARIES-CS, both He irradiation and
temperature are steady-state - Choice of
Eeff,diff is tricky as the experimental results
were obtained for cyclic conditions with a
finite He dose per implantation. As a rough
guide, the experimental temperature anneals were
integrated and manipulated to estimate the
Tuniform which would result in the same diffusive
time scale as the 60 s shown for each
cycle. - Effective Tuniform 1590C, -
Corresponding maximum dose rate 1.6x1017
ions/m2-s. - Increasing Tuniform would result
in shorter effective time and larger max.
dose rates (e.g. 3 s and 3 x 1018 ions/m2-s
for a 1800C temperature), and reducing it
would have the opposite effect. For ARIES-CS,
the steady state ? dose rate is 3 x 1018 He
ions/m2-s and the experimental results would be
best applied for high temp. cases with Eeff,diff
corresponding to the max. dose, 4.8 eV.
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Estimate of He Retention in W Armor Due to ? Loss
as a Function of Operating Temperature and
Characteristic Diffusion Length
Simple effective diffusion analysis for
Eeff,dif 4.8 eV Not clear what is the max.
He conc. limit in W to avoid exfolation
- perhaps 0.15-0.2 at.4,5 High W temp.
and shorter diffusion dimensions help,
perhaps a nano- structured porous W - e.g.
50-100 nm at gt1800C
4. G. Lucas, personal communication. 5. S. B.
Gilliam, S. M. Gidcumb, N. R. Parikh, D. G.
Forsythe, B. K. Patnaik, J. D. Hunn, L. L. Snead
and G. P. Lamaze, J. Nucl. Mat. 347 (2005) 289.
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Effect of Changing the Activation Energy of
Governing He Migration Process on Estimated He
Retention
A reduction in Eeff,diff would have a major
effect, e.g. for a diffusion distance of
100 nm and a temperature of 1800C, the steady
state He to W inventory ratio, IHe/W - IHe/W
0.1 for Eeff,diff 4.8 eV - IHe/W 4x10-5
for Eeff,diff 3.4 eV - IHe/W 8x10-13 for
Eeff,diff 0.24 eV. An interesting question
is whether at a such high W operating
temperature some annealing of the defects
might further help helium release. Further
RD requiired to understand and better
characterize integrated effect of governing
He migration processes under prototypical
conditions
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Plasma Processes Incorporated is Working on
Manufacturing Porous W with Nano Microstructure
Which Could Enhance He Release

• After W precursors are injected into the plasma
  flame, the vapor phase is quenched rapidly to
  solid phase yielding nano-sized W powder
• Nano tungsten powders have been successfully
  produced by plasma technique and the product is
  ultra pure with an ave. particle size of 20-30
  nm. Production rates of gt 10 kg/hr are feasible.
• Process applicable to molybdenum, rhenium,
  tungsten carbide, molybdenum carbide and other
  materials.
• The next step is to utilize such a powder in the
  Vacuum Plasma Spray process to manufacture porous
  W (10-20 porosity) with characteristic
  microstructure dimension of 50 nm .

TEM images of tungsten nanopowder, p/n
S05-15. (from S. ODell, PPI, personal
communication)
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Conclusions
Alpha loss is a major issue in stellarators,
impacting the survival of the first wall.
The armor lifetime under the alpha flux would
depend on a number of parameters
- a-particle energy spectrum - armor material
choice, configuration and temperature - activati
on energies of processes governing He behavior in
armor Use of a nano-sized porous W armor and
high operating temperature would help to
enhance the release of implanted He.
Further RD required to make sure that a
credible solution exists for a CS.