4th IAEA DEMO programme workshop - Pages - Home IAEA DEMO programme workshop KIT, Karlsruhe, Germany, 15-18 Nov 2016 Summary of session 2: DEMO physics gaps and impact on engineering

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

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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


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)


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

...



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))



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?


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


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)]


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)


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)


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?


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“


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


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)?


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!

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