4th IAEA DEMO programme workshop KIT, Karlsruhe, Germany, 15-18 Nov 2016
Summary of session 2: DEMO physics gaps and impact on engineering design
W. Biela,b and H. Zohmc
aInstitute of Energy- and Climate Research, Forschungszentrum Jülich GmbH, Germany bDepartment of Applied Physics, Ghent University, Belgium cMax-Planck-Institut für Plasmaphysik, Garching, Germany
W. Biel and H. Zohm | Summary: DEMO physics gaps and impact on engineering | 4th DPWS, KIT 17th November 2016 No 2
General comments
Extrapolations of experimental findings from today‘s experiments towards
ITER and DEMO conditions often have limited validity, e.g.
some „global“ parameters can be chosen in agreement for both small and big
experiments (e.g. n/nGW, rad, N), while some more fundamental physics
parameters cannot be made to match simultaneously (e.g. r* and n*)
In order to clarify open issues towards DEMO, the following routes can be
pursued:
Development of new scaling laws which use a more physics oriented approach
(and which also better cover the DEMO relevant parameter range)
Improve theoretical understanding of underlying physics
Increased use of integrated modelling using detailed physics models
Benchmarking with the largest available experiments from today (JET, JT-60SA
will play a major role)
Final validation on ITER unfortunately only possible in ~ 20 years from now
W. Biel and H. Zohm | Summary: DEMO physics gaps and impact on engineering | 4th DPWS, KIT 17th November 2016 No 3
H Mode Core Plasma Confinement (R. Hawryluk)
IPB98(y,2) confinement scaling has
limitations:
not enough data points for high n/nGW
no data points for DEMO relevant
Prad,core
description of beta dependence is
questionable
applicability to low rotation cases not
clear
dependence on wall material (lower
confinement with JET ILW)
dependence on applied heating method
….
need for updated scalings, possibly
motivated by underlying improved
physics understanding
Kardaun scaling describes
density dependence better
than standard IPB98(y,2)
W. Biel and H. Zohm | Summary: DEMO physics gaps and impact on engineering | 4th DPWS, KIT 17th November 2016 No 4
H Mode Core Plasma Confinement (R. Hawryluk) (2)
H mode threshold power has large uncertainties
Very important both for required heating power and allowable power flux across
the separatrix
A large number of ‚hidden variables‘ exists
Scaling is with n, B and S, but PLH can be easily changed by > 50 % in an
individual device
plasma geometry / recycling
gradB drift direction
wall material
...
W. Biel and H. Zohm | Summary: DEMO physics gaps and impact on engineering | 4th DPWS, KIT 17th November 2016 No 5
H Mode Core Plasma Confinement (R. Hawryluk) (3)
Long term goal: validated physics based model should be further developed
pedestal from EPED-type stability/transport considerations
core profiles from gyrokinetic-based transport models (TGLF-type)
integrated modelling will play a crucial role in validating this approach
(example: ‚calculation of H-factor‘ by integrated modelling (V. Chan))
W. Biel and H. Zohm | Summary: DEMO physics gaps and impact on engineering | 4th DPWS, KIT 17th November 2016 No 6
H Mode Core Plasma Confinement (R. Hawryluk) (4)
New promising confinement regimes (QH Mode and I Mode)
what exactly is the recipe to achieve and maintain them?
what is the physics behind this?
compatibility with power exhaust requirements?
Achievable confinement quality under DEMO conditions?
W. Biel and H. Zohm | Summary: DEMO physics gaps and impact on engineering | 4th DPWS, KIT 17th November 2016 No 7
Power exhaust (H. Reimerdes) (1)
Fully detached plasma as a “baseline”
approach is proposed to spread heat loads
over larger area
demonstration/validation under fully DEMO
relevant conditions is not possible with today’s
devices (would need to reach simultaneously
high Prad,core and high Psep/R)
even for ITER, the simultaneous challenge is
lower
N.B.: not clear if any of the 0-D parameters
exhaustively describes the challenge
Predictive modelling of plasma detachment is
high priority
main trends are well described, but quantitative
predictive capability missing
need to combine first principles SOL transport
models and fluid/neutral codes
• Feedback control using
multiple species, e.g.
with N and Ar [A.
Kallenbach, et al., NF (2012)]
AUG
W. Biel and H. Zohm | Summary: DEMO physics gaps and impact on engineering | 4th DPWS, KIT 17th November 2016 No 8
Power exhaust (H. Reimerdes) (2)
Need for impurity seeding in both core and SOL impacts plasma scenario
‚tailor‘ impurity species and concentration according to temperature profile
indications that at least two different impurity species are needed
encouraging start of integrated core/edge/SOL modelling (Chan; EU)
[M. Bernert, et al.,
PSI (2016)]
W. Biel and H. Zohm | Summary: DEMO physics gaps and impact on engineering | 4th DPWS, KIT 17th November 2016 No 9
Power exhaust (H. Reimerdes)
Comparison of different novel magnetic configurations (including the not-so-
new double null)
some appear feasible with ex vessel coils, others need in vessel coils
all novel magnetic configurations lead to lower use of the magnetic field volume
a comparative cost/benefit assessment would be helpful
but: physics base for advanced divertors not ready to characterise the benefits
need to focus the ongoing developments at clarifying the (dis)advantages for
DEMO (no advanced divertor for ITER)
W. Biel and H. Zohm | Summary: DEMO physics gaps and impact on engineering | 4th DPWS, KIT 17th November 2016 No 10
Steady State Scenarios for DEMO (V. Chan) (1)
Steady state tokamak scenario needs tailoring of profiles – sophisticated
1-D modelling indispensable
integrated model used to determine H&CD requirements, will also be used for
diagnostics & control assessment
physics uncertainties have large impact on the outcome (e.g. optimisation of
NBI energy as compromise between torque and CD)
W. Biel and H. Zohm | Summary: DEMO physics gaps and impact on engineering | 4th DPWS, KIT 17th November 2016 No 11
Steady State Scenarios for DEMO (V. Chan) (2)
This could be the right time to start an international joint effort on
integrated modelling of tokamak scenarios
several parties have sophisticated approaches that should be cross-validated
benchmarking against experiments on world-wide level would be a large step
forward and could sort out strenghts and weaknesses
will also be needed for ITER (ongoing activities already)
implement through ITPA?
W. Biel and H. Zohm | Summary: DEMO physics gaps and impact on engineering | 4th DPWS, KIT 17th November 2016 No 12
Operational Margins / Impact on Design (H. Lux) (1)
probabilistic analysis of margins
and uncertainties
best performance usually
observed for going to the
operational limits
what is the optimum distance
we should keep from operational
limits?
can we stay away from all
limits by about the same margin?
proposal to investigate cases
of „same disruptivity“ or „same
effect on fusion power“
W. Biel and H. Zohm | Summary: DEMO physics gaps and impact on engineering | 4th DPWS, KIT 17th November 2016 No 13
Operational Margins / Impact on Design (H. Lux) (2)
Powerful approach to test the effect of the uncertainties discussed before
different confinement scalings (e.g. Petty)
are different design points affected differently (e.g. CFETR versus EU DEMO)?
can also be used to direct R&D into the direction of highest impact
W. Biel and H. Zohm | Summary: DEMO physics gaps and impact on engineering | 4th DPWS, KIT 17th November 2016 No 14
Stellarator / Heliotron Physics (J. Miyazawa) (1)
Stellarator: advantage of steady state operation; no current drive needed
Stellarator DEMO has huge major radius but machine volume is not much
bigger than for a tokamak reactor (since centre is empty, no CS coil)
Predicted Fusion power for a stellarator reactor (Pfus = 3 GW) with plasma
volume of Vpl ~ 1500 m2 needs clarification
Will an intermediate step be needed and what is the optimum size (JET-size,
ITER-size)?
W. Biel and H. Zohm | Summary: DEMO physics gaps and impact on engineering | 4th DPWS, KIT 17th November 2016 No 15
Stellarator / Heliotron Physics (J. Miyazawa) (2)
Stellarator and Tokamak physics base have commonalities and
differences
turbulent transport generally expected to be in gyro-Bohm regime
perpendicular neoclassical transport of much greater importance in stellarators
beta limit of same order for both concepts, but so far quite benign in stellarator
density limit of similar mechanism (power balance problem in the edge), but
absolute value much higher in stellarator – what is the physics basis for
extrapolation to case without central source?
N.B.: Sorting out the differences will greatly benefit our understanding of fusion
plasma physics!