1. Design window analysis of LHD-type Heliotron DEMO reactors
Fusion System Research Division,
Department of Helical Plasma Research,
National Institute for Fusion Science
Takuya GOTO, Junichi MIYAZAWA, Teruya TANAKA, Nagato YANAGI and Akio SAGARA
Special thanks to Dr. Y. Suzuki, Prof. F. Najmabadi
Japan-US Workshop on
Fusion Power Plants and Related Advanced Technologies
with participations of EU and Korea
February 22-24, 2011, NIFS, Toki, JAPAN
Thank you chairman. I’m Takuya Goto from NIFS. It’s my great pleasure to give a talk at this workshop. The title of my talk is “Design window analysis of LHD-type Heliotron DEMO reactors”. First of all, I thank all my co-researchers to give valuable advices. I also thank Dr. Yasuhiro Suzuki, who gives a great help in *** 1

2. LHD-type Heliotron DEMO is foreseeable
Helical system with net-current free plasma:
No disruptive event  steady-state easily achievable
No current drive power  self-ignition state (Q = ∞) is attainable
LHD has achieved:
High beta (~5% by volume average for ~100tE)
Extremely high central density (IDB-SDC, ne0 = 1.2×1021m-3 )
Steady state wo/ disruptive events
Other remarkable observations
Impurity hole
Divertor detach scenario
As you know, helical system with net-current-free plasmas has suitable properties considered as a DEMO and a commercial fusion reactor. Since it has no operational restriction due to the plasma current, a steady-state operation can be easily achieved. It needs no current drive power and self-ignition state can be attained, resulting in the low recirculation power and high plant efficiency.
Among helical system, heliotron system with two continuous helical coils has recorded several remarkable achievements in Large Helical Device, LHD. For example, LHD has achieved the reactor-level high beta state, around five percent by volume-averaged value, with the duration of much longer than energy confinement time. LHD also has achieved extremely high central electron density with internal diffusion barrier, greater than ten to the twenty-first per cubic meter. Long time discharge for about one hour without any disruptive event has also been demonstrated. There are other remarkable observations, such as impurity hole, and steady-state divertor detach scenario has also been proposed.
In this respect, the LHD-type heliotron DEMO reactor is now foreseeable and its design study is quite meaningful.
*Figure from A. Komori et al.,
Fusion Science and Technology 58 (2010) 1.
The LHD-type Heliotron DEMO reactor is foreseeable.
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3. Not optimum but ‘robust’ design
There are several uncertainties in the physics conditions of a burning plasma.
Density and temperature profiles
Impurity fraction (especially helium ash fraction fa)
Alpha heating efficiency ha
DEMO is the next step reactor
high reliability is required in its design
Direct extrapolation of experimentally-obtained plasma properties (presented by Dr. Miyazawa)
Finding of robust (insensititive to the change of uncertain parameters) design window
Generally, design point of a DEMO and commercial reactor is determined through a kind of optimization process. For example, the minimization of the cost of electricity or the maximization of the power density.
However, there are several uncertainties in the physics conditions of a burning plasma. For example, density and temperature profiles, helium ash fraction, and alpha heating efficiency.
On the other hand, DEMO is the next step reactor and high reliability and feasibility is demanded in its design.
Therefore it is required to reduce uncertainties. There are two ways to reduce uncertainties.
One is a direct exploration of experimentally-obtained plasma properties to the reactor condition. This method can minimize the ambiguity and will give a prediction with high certainty. About this method, Dr. Miyazawa will give a talk in the next presentation.
On the other hand, it is also important to explore wider possibilities.
In this respect, it is important to identify a robust design point, that is, the design point which is insensitive to the change in the uncertain physics parameters in place of finding the optimum point but with many assumptions.
Therefore we carried out the design window analysis using the system design code for heliotron reactors.
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4. System Design Code HELIOSCOPE
HELIOtron System design COde for Performance Evaluation
Parameter input
Coil design parameters
Rc, ac, g , a, jc, Ic, W/H, …
Plasma parameters
an, aT, fimp, ne0, Te0, …
Coil and magnetic surface structure
Bmax, Wmag, D, …
Plasma performance
`ne , Wp, Pfus, Prad, tE, HISS, <b>, nsudo, …
Magnetic surface configuration
<Bt>, <ap>, Rgeo, …
This flow chart shows the basic structure of the developed system design code, HELIOSCOPE.
In contrast to tokamak reactors, the shape of magnetic surfaces of heliotron reactors strongly coupled to the shape of external coils. So equilibrium calculations are needed to obtain the parameters related to the magnetic surface configuration, which are needed for the plasma performance evaluation. However, such equilibrium calculations are time-consuming. Therefore, these parameters are derived from the data established separately by vacuum equilibrium calculations for various shapes of the helical coils.
Database of magnetic
surface structure
(vacuum equilibrium calculation)
Plant power flow
Pn, Pth, Pe, Gn, Gdiv, …
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5. LHD-type Heliotron Reactor FFHR-2m2
FFHR-2m2* is the LHD-type Heliotron reactor
Based on LHD high-beta discharge
designed for the use in both a DEMO and commercial reactor
Design policy:
Pfus = 3GWth (Pe,net = 1GWe)
Long-term (~30 FPYs) operation with high plant availability
Average neutron wall load <Gnw > < 1.5 MW/m2 is assumed
Technology can be extrapolated from the present knowledge
Stored magnetic energy Wmag < 160 GJ is assumed**
First, I give a brief review of the LHD-type heliotron reactor design FFHR-2m2 as an example of a design window analysis. FFHR-2m2 is based on the LHD high beta discharges and designed for the utilization as both a DEMO and a commercial reactor. To obtain the enough electric output for commercial use, fusion power is set to be three Giga Watt. The averaged neutron wall load is suppressed below one point five Mega Watt per square meters to realize long-term operation with high plant availability. For superconducting magnet system, the use of technology can be extrapolated from the present knowledge is assumed and the stored magnetic energy is set to be less than one hundred and sixty Giga Joule.
* A. Sagara et al.,. Fus. Eng. Des. 83, 1690 (2008), A. Sagara et al.,. Nucl. Fusion 45, 258 (2005).
** S. Imagawa et al.,. Nucl. Fusion 49, (2009).
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6. Blanket space is a key design issue
Inboard side blanket space on vertically-elongated cross-section is limited for the LHD-type heliotron reactor
Inboard minimum blanket space Din > 1.0m is assumed in FFHR-2m2
assure the sufficient net TBR (>1.05) with ~40cm breeding layer
suppress fast neutron damage on superconducting coil < 1022n/cm2 after 30FPYs with ~60cm radiation shield
There is another important parameter, blanket space.
As shown in this figure, the space between helical coil and plasma of the LHD-type heliotron system has its minimum at the inboard side on the vertically elongated cross-section.
In the design of FFHR-2m2, blanket thickness at all position, including this minimum point, is kept greater than one meter to assure the sufficient tritium breeding ratio and to suppress fast neutron damage on superconducting coil after thirty full power years operation.
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7. Physic conditions of FFHR-2m2
Physics conditions:
Mainly based on LHD high-beta discharges
Inward-shifted magnetic axis position Rax/Rc = 3.6/3.9
Helical pitch parameter g =1.2 (plasma aspect ratio Ap ~ 6.3)
Density/temperature profiles as
with an=0.25, aT =0.75
Density limit:`ne < 1.5 nsudo
Assuming fa=0.03 and ha=0.9
Physics conditions of FFHR-2m2 are mainly based on LHD high-beta discharges.
It adopted inward-shifted magnetic axis position and helical pitch parameter of one point two. This figure shows the cross-sectional shape of magnetic surfaces and helical coils with this configuration.
Density and temperature profile is approximated by the power of parabolic function of normalized minor radius. And the exponent is selected as zero point two five for density and zero point seven five for temperature according to the experimentally obtained profile.
There are other three assumptions. Density is limited by one point five times of Sudo density scaling. Helium ash fraction is three percent, and alpha heating efficiency is ninety percent.
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8. Design point of FFHR-2m2
Rc=17m, Bt,c=4.7T has been selected by engineering constraints
HLHD~1.3 can realize this design point with the assumed physics conditions
HLHD is quite sensitive to physics conditions
Design point with lower HLHD is favorable
Here I’ll show how these engineering constraints determine the design window using this figure. Here the horizontal axis corresponds to the helical coil major radius and the vertical axis corresponds to the averaged toroidal field at the winding center. In case of the similar extension of LHD, the blanket space is proportional to the major radius. In case that the current density of helical coil is fixed, the contour lines of the blanket space becomes upward-sloping lines like this.
The stored magnetic energy is proportional to the cube of major radius and the square of magnetic field strength.
Then contour plot becomes like this. Note that these boundaries are fixed regardless of plasma conditions.
In case of fusion power of three Giga Watts, contour line of neutron wall load becomes vertical lines like this.
Thus the design window which satisfies all engineering constraints is this white region. Then the design point with major radius of seventeen meter and magnetic field strength of four point seven Tesla, here, has been selected as a candidate for FFHR-2m2. As I mentioned before, these physics conditions are assumed in the design of FFHR-2m2. In this case, one point three times the confinement improvement to the present experiment can realize this design point.
However, the required confinement improvement is quite sensitive to the change in the physics condition.
For example, if the line-averaged electron density is limited by Sudo density scaling, the contours for the required confinement improvement shift upward like this. In addition, the dependence does not so much change with both the reactor size and the magnetic field.
Then design point with lower required confinement improvement is favorable for DEMO reactor to assure the its design robustness.
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9. Flexible DEMO design concept can reduce the minimum blanket space
DEMO should demonstrate;
steady-state, self-ignition plasma operation and electric power generation
integrity of engineering components
30FPYs operation is not needed and higher neutron flux can be accepted
Thinner inboard blanket can be realized by
Using special material (ZrH1.65, WC, etc)
accepting higher neutron flux on superconducting coil
Plasma
Breeding
layer
(Flibe+Be)
32 cm
Magnet
Radiation
shield
68 cm
Thus we changed our way of thinking on the DEMO design concept.
We returned to the definition of DEMO.
The first priority of DEMO is a demonstration of steady-state, self-ignition plasma operation with plant-scale electric power generation, and the confirmation of the integrity of engineering components.
From this viewpoint, the fusion power of three Giga Watts is not a necessarily condition. Thirty full power years operation is also not necessary. and the constraint of the neutron wall load can be changed.
The reduction in the inboard minimum blanket space is also achievable.
This figure shows neutronics calculation result of FFHR-2m2 blanket system. In the case of thirty full power years operation, fast neutron flux on the superconducting coil must be suppressed to this level, ten to the tenth neutrons per square centimeter per second. It shows that the shield thickness can be reduced by about twenty centimeter by using a specific materials, such as tungsten carbide or zirconium hydride.
The blanket thickness can also be reduced if the the operation period is shorter than thirty full power years and higher neutron flux can be accepted.
Fluence limit for 30FPYs operation
~20cm
thinner
Courtesy of Dr. Teruya Tanaka
(original figure is found in
A. Sagara et al., FED 83, 1690 (2008))
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10. Reduction in inboard blanket thickness can expand design window
FFHR-2m2
With thinner inboard blanket
With thinner inboard blanket & higher neutron wall load
This reduction in the minimum blanket thickness can greatly expand the design window.
This figure shows the dependence of the required confinement improvement on the fusion power with various blanket space and neutron wall loading.
If the blanket space of zero point eight meter can be achieved, the design point with required confinement improvement factor of one point two can be selected by reducing fusion power to two point five Giga Watt. If the neutron wall load of two Mega Watts per square meters is accepted simultaneously, the required confinement improvement becomes further low with increasing fusion power.
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11. Reduction in the blanket thickness can expand design window
If Din=0.8m and <Gnw > = 2MW/m2 is accepted, design points with
Rc ~ 15 m
Pfus = 3.5 GW
HLHD ~1.15
can be selected
Design robustness of DEMO reactor to the change in the physics conditions
This figure shows the latter case. If the blanket thickness of zero point eight meter and the averaged neutron wall load of two Mega Watts per square meters are accepted, the design point with major radius of about fifteen meters and magnetic filed strength of about six Tesla can be selected with the fusion power of three point five Giga Watt and the required confinement improvement of around one point one five.
This design point has the robustness to the change in the physics conditions.
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12. Summary
Reduction in the minimum blanket thickness can greatly expand the design window
Major radius can be reduced by ~2m
give the robustness to the change in physics conditions
can adopt flexible magnetic configurations
Progress in engineering research that enables the reduction of the minimum blanket space and acceptance of higher neutron wall load is expected.
Larger confinement improvement is also expected to make DEMO and commercial reactors more economically attractive
Finally, I summarize my talk.
The design window analysis of LHD-type heliotron DEMO reactor has been carried out. It was found that the design window strongly depends on the engineering constraints, and the reduction in the minimum inboard blanket thickness can greatly expand the design window. The major radius can be reduced around two meter and the required confinement improvement ca also be reduced with keeping the other constraints. It lead to the design robustness of heliotron DEMO reactor to the change in uncertain physics conditions. I didn’t mention about it, but this reduction in the minimum blanket thickness also enables a flexible selection of magnetic configurations, which also leads to design robustness of the heliotron DEMO reactor.
This reduction of the blanket space and the acceptance of higher neutron wall load can expand the design window of not only a DEMO reactor but also a commercial plant. So the progress in the engineering research that enables such improvement is strongly expected. If we can reduce the magnetic field strength, the design feasibility of the superconducting magnet system can be enhanced and the cost of reactor system can be reduced. So larger confinement improvement is also expected to make a DEMO and a commercial reactors to be more economically attractive ones.
That’s all. Thank you for your attention.
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13. Adjustment of poloidal coil position
w/ adjustment
w/ adjustment
wo/ adjustment
wo/ adjustment
Adjustment of poloidal coil position enables the increase of blanket space as well as increase of plasma volume（~22%）.
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14. High-beta equilibrium analysis
Calculation by HINT2 code.
<b>~6.5% equilibrium with the almost same volume as that in the vacuum condition can be obtained by applying an appropriate vertical field.
Of course both the pressure profile and the magnetic surface structure change by the increase in the beta value.
Generally, nested surface volume shrinks due to the outward shift of magnetic axis and ergodization of periphery region with the increase in the beta value. However, the volume with finite pressure can be restored by applying an appropriate vertical field. This figure shows the comparison of magnetic field structure in vacuum equilibrium and in high-beta state with volume averaged beta value of about five point five percent. The high-beta equilibrium was calculated by HINT2 code by courtesy of Dr. Suzuki. This HINT2 code can calculate three dimensional equilibrium without assuming the existence of nested magnetic surfaces. As you can see, almost the same volume with finite pressure as in vacuum equilibrium can be obtained. The shape of pressure boundary, shown in green line, is also almost the same as the last closed flux surface in vacuum condition. Therefore, the analysis based on vacuum equilibrium can be applied to the finite beta case.
Courtesy of Dr. Yasuhiro Suzuki
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15. Heliotron Reactor Design FFHR-2m2
LHD-type Heliotron reactor which can be utilized as both a DEMO and commercial reactor
Design policy:
Pfus = 3GWth (Pe,net = 1GWe)
Long-term (~30 years) operation with high plant availability
Technology can be extrapolated from the present knowledge
First I give a brief review of the heliotron reactor design FFHR-2m2 as an example of system analyses.
FFHR-2m2 is the LHD-type heliotron fusion reactor that can be utilized as both a DEMO and commercial reactor
Thus it assumes fusion power of three Giga Watts and achievement of reasonable COE is considered.
On the other hand, it is based on the technology can be extrapolated from the present knowledge.
Thus the following engineering constraints is imposed.
First, the magnet system is based on the ITER-relevant technology. Thus helical coil current density is fixed to be twenty five Ampere per square millimeter and magnetic stored energy is up to one hundred and sixty Giga Joule.
Second constraint is the minimum blanket space. To assure the sufficient tritium breeding ratio and neutron shielding effect, the minimum blanket space is set to be greater than one meter.
Third one is the averaged neutron wall load and it is limited to be less than one point five Mega Watts per square meter. This condition is required to realize the long-life blanket concept, which can suppress neutron damage on the structural material under one hundred dpa for thirty years operation.
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16. Design constraints of FFHR-2m2
Engineering constraints :
Magnet system based on the ITER-relevant technology
Helical coil current density: jc=25 A/mm2
Magnetic stored energy: Wmag < 160 GJ
Minimum blanket space: Dmin > 1m
Assure the sufficient net TBR (>1.05)
Suppressing fast neutron fluence on SC coil <1022n/cm2 after 30FPY operation)
Averaged neutron wall load: <Gnw > < 1.5 MW/m2
Suppressing neutron damage on the structural material <100dpa after 30 FPY operation with long-life blanket concepts
Physics condition :
Based on LHD high-beta discharges
Assuming ne < 1.5 nsudo, fa=0.03 and ha=0.9.
First I give a brief review of the heliotron reactor design FFHR-2m2 as an example of system analyses.
FFHR-2m2 is the LHD-type heliotron fusion reactor that can be utilized as both a DEMO and commercial reactor
Thus it assumes fusion power of three Giga Watts and achievement of reasonable COE is considered.
On the other hand, it is based on the technology can be extrapolated from the present knowledge.
Thus the following engineering constraints is imposed.
First, the magnet system is based on the ITER-relevant technology. Thus helical coil current density is fixed to be twenty five Ampere per square millimeter and magnetic stored energy is up to one hundred and sixty Giga Joule.
Second constraint is the minimum blanket space. To assure the sufficient tritium breeding ratio and neutron shielding effect, the minimum blanket space is set to be greater than one meter.
Third one is the averaged neutron wall load and it is limited to be less than one point five Mega Watts per square meter. This condition is required to realize the long-life blanket concept, which can suppress neutron damage on the structural material under one hundred dpa for thirty years operation.
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17. 3 Key Elements in the LHD-type Heliotron Reactor Designs
Increase in magnetic field strength
vs.
Reduction in stored
magnetic energy
Maximization of plasma volume,
Increase in output
vs.
Blanket space,
Reduction of Wall and divertor loading
Plasma
・Beta value
・Confinement
improvement
SC Coil
Blanket/
・Shield
・Max. field on coil
・Coil current density
・Stored magnetic
energy
・Neutron fluence
・Nuclear heating
・Neutron flux
・Thickness
・Materials
Increase in the coil cross-sectional area
vs.
Blanket space
In the design of magnetically confined fusion reactor, the relation between many engineering and physics parameter needs to be clarified with a comprehensive standpoint.
Especially, these three elements, core plasma confinement, superconducting coil, and blanket and shield are quite important because these parameters strongly affect the design and coupled with each other.
In helical system, the shape of plasma is simply defined once the shape of external coils is given. In the LHD-type heliotron system, the space between plasma and helical coil has its minimum at the inboard side on vertically-elongated cross-section. As I show later, this minimum blanket space is an important parameter of its design.
System analysis is necessary to identify design window.
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