Implications of TFTR D-T Experiments for Burning Plasma Program

R. J. Hawryluk

IEA Large Tokamak Workshop (W60)Burning Plasma Physics and Simulation

Tarragona, Spain

July 4-5, 2005

Thanks for Useful Discussions

M. Bell, R. Budny, R. Goldston, N. Fisch, N. Gorelenkov., J. Hosea, C. Kessel, D. Meade, S. Medley, C. K. Phillips, R. Nazikian, N. Sauthoff, S. Scott, C. Skinner, J. Strachan, E. Synakowski, J. R. Wilson, K. L. Wong, M. Zarnstorff, and S. Zweben

Presentation reflects my personal perspectives.

ITER Must Demonstrate Fusion Power Production and Make Critical Scientific Contributions

• Transport and Turbulence: Extend the study of turbulent plasma transport to much larger plasmas

• Stability: Extend the understanding of pressure limits to much larger plasmas.

• Energetic particles: Study strong heating by fusion products, in new regimes where multiple instabilities can overlap.

• Plasma-boundary interface: Extend the study of plasma-materials interactions to much greater power and pulse length.

Today: ~10 MW for ~1 second, gain of < 1ITER: 500 MW for 400 seconds, gain > 10Power Plant: 2500 MW, continuous, gain > 25

What Should be the Goals for Research Prior to ITER?

Enable ITER to demonstrate fusion power production.

Enable ITER to make the scientific contributions needed for Demo.

and

• Continue to develop the scientific basis for an attractive power plant

Together with the ITER results, this will enable us to move forward with Demo

What are the Implications of the TFTR D-T Experiments for ITER and Research Prior to ITER?

• Transport and Turbulence– Isotope Effects

• Stability

• ICRF Heating

• Alpha-particle Physics

• Plasma-boundary Interface

Global Ethermal Increased, I Decreased

in Core of DT Supershots Compared to D

• ni(0)ETi(0) increased by ~55% from D to DT

– Some cases up to 80% increase• Enhanced confinement critical for fusion

power production.

Ethermal <A>0.89±0.1

itot <A>-1.8 ±0.2

S. Scott, M. Zarnstorff

ITG Model with Radial Electric Field Reproduced the Ion Temperature

• Maximum linear growth rate decreases with ion mass.

• Er shearing rate increases with Ti.

• Radial electric field shear reproduces strong isotope effect in supershots.

D. Ernst

Isotope Effect on Confinement Varied Widely Depending on Operating Regime

• Challenge to theory and to gyro-Bohm scaling: <A>-0.2

• Recent ITER scaling for ELMy H-mode: Ethermal <A>+0.19

EDT - τEDD

τEDD

(% )

-100102030<A>-0.25±.22Elm-free<A>0.16±.06ElmyJET H-ModeOHICRFL-mode

NBIL-mode

Reverse Shear

<A>0.85<A>0.3-0.5<A>0.3-0.5<A>0.5Supershot/High liTFTR

S. Scott, S. Sabbagh, C. K. Phillips

Understanding of Isotope Scaling Remains Incomplete

• Depends on operating regime– Not consistent with naive turbulence theory scaling– What is the role of radial electric field shear in the different regimes?– What are the implications for advanced operating modes?

• Power threshold for internal barrier formation increased with <A>.– Was this a transport effect or a consequence of the beam deposition

profile being different?

Operational Implications:• Though TRANSP was used extensively for experimental planning,

– Existing transport models were inadequate to predict both the isotope effect as well as variations in confinement.

• Occurrence of internal transport barriers or unforeseen improvements in core confinement occasionally resulted in disruptions.

Implications of Transport Studies

• Implications for ITER:– Burn control will require controlling the pressure and current

profile in the presence of dominant alpha heating.• Are the required profiles consistent with the underlying transport

rates?• Enhanced confinement at high density is key to optimizing

performance.

– Measure turbulence in ITER low * regime and comprehensive profile and edge diagnostics.

• Needed to apply the results to different configurations.

• Implications for research prior to ITER:– Develop predictive capability beyond empirical scaling that has

been experimentally established.– Develop fluctuation diagnostics for ITER.– Develop a deeper understanding of the relationship of “wall

conditioning” and “plasma performance.”– Aggressive goal: control the transport locally????

TFTR Stability was Limited by Kink-Ballooning Mode and Neoclassical Tearing Modes

Theory predicted observed amplitudes and growth rates for neoclassical

tearing modes (NTM). - NTM were observed to

occur without a seed. Why?

Nonlinear numerical simulations found n=1 kink excited local ballooning modes

Kink-ballooning mode limited fusion power performance.

E. Fredrickson, Y. Nagayama, W. Park Z. Chang. E. Fredricskon

Implications of Stability Studies• Implications for ITER:

– Control tools will be used to both establish and control a burning plasma.• Are they sufficient?

– Performance optimization entails increased likelihood of disruption.– Disruption mitigation and avoidance is very important.– Study of extended MHD effects at low * will be a major scientific

contribution.• For example NTM, RWM and sawteeth

• Implications for research prior to ITER:– Perform assessment of heating and current drive options to optimize

research and performance capabilities.– Develop techniques to modify the pressure and current profile.– Develop and test three dimensional nonlinear MHD codes with extended

physics.– Develop techniques to stabilize sawteeth, NTM and RWM that can be applied

to ITER.– Establish reliable disruption mitigation and avoidance techniques.

ICRF Successfully Heated D-T Supershot Plasmas in TFTR

• Power deposition calculations in good agreement with experiment.

Ti due to 2nd harmonic tritium heating

Te due to direct electron and 3He minority ion heating

G. Taylor, J. R. Wilson, J. Hosea, R. Majeski, C. K. Phillips

Implications of ICRF Heating and Current Drive Studies

• ITER Implications:– Fundamental heating and current drive physics for ICRF

has largely been established for D-He3 minority and second harmonic tritium heating experiments.

– Outstanding technology and coupling issues remain.

• Implications for research prior to ITER:– Need to develop predictive tools to design antennae and

obtain good power coupling.– Need to improve performance of couplers and matching

circuits.– Application of mode conversion or IBW to drive currents,

induce flows or alpha channeling requires further research.

Confined Alpha-particle Studies on TFTR were Confirmatory in Normal Shear (Supershot) Discharges

• Alpha birth rate and profile were adequately modeled.- Neutron flux in good

agreement with calculations based on plasma profile in normal shear discharges.

• Escaping alpha flux at 90o detector was consistent with classical first orbit losses

R. Budny, L. Johnson S. Zweben, D. Darrow

Confined Alpha-particle Studies Relied on New Diagnostics Developed for D-T Experiments

Confined alphas in the plasma core showed classical slowing down spectrum .

Alpha particles were well confined.

0 D 0.03 m2/s

Rapid ash transported from the core to the edge in supershots. (CHERS)

DHe/D ~ 1

R. Fisher, S. Medley, M. Petrov R. Fonck, G. McKee, B. Stratton

E. Synakowski

Initial Evidence of Alpha-particle Heating

• Alpha heating ~15% of power through electron channel

• Plasmas matched for dominant Te scaling in D only plasmas.

- Te(0) E0.5

- account for the isotope effect on confinement

G. Taylor, J. Strachan

Te(D

T)

- T

e(D

) (k

eV)

T

e(R

) (

keV

)

MHD Activity can Cause Enhanced Transport of Alpha Particles

• Sawteeth caused a large radial redistribution of alpha particles

• Strong toroidal anisotropic loss apparent as NTM mode was rotating.

• Enhanced loss also observed due to: - disruptions - kinetic ballooning modes, sawteeth

D. Darrow, S. Zweben S. Medley, M. Petrov, R. Fisher

TAEs Driven by Neutral Beam or ICRF Fast Ions Caused Substantial Fast Ion Losses

• In normal shear D-T discharges, TAE was stable.

• ICRF induced TAE with ripple trapping damaged the vessel during D-T operations.

E. Fredrickson, K. L. Wong, D. Darrow R. White

Alpha-particle Physics Studies in Reversed Shear Discharges Resulted in New Discoveries.

2.00PNBI(MW)TIME(s)NBI offβ(0)0 Neutral Beam Injection10-3310x-22.5 ( )TIME s3.0BB~=4n=5n=3n=2n2.82.93.00610x-9025103101

β(0)“ ”Damping“ ”Drive“ ”ResponsewallT

• Alpha-driven TAE (subsequently identified as Cascade Modes) were observed.

• TAEs redistributed deeply trapped alpha-particles–Further work required to benchmark models.

• Stochastic ripple diffusion affected confinement of deeply trapped particles

• Neutron emission in D-T enhanced reverse shear discharges disagreed with TRANSP analysis in some shots by factors of 2-3

–Source of discrepancy was not identified.

R. Nazikian, Z. Chang, G. Fu S. Medley, M. Petrov, R. Fisher, M. Redi

Implications of Alpha-particle Physics Studies

• Implications for ITER:– Nonlinear consequences of alpha heating will be studied for the

first time.– Comprehensive measurements of confined and lost alpha

particles are critical.– Scientific research is paced by diagnostic capabilities as well as

operating regimes.• ITER needs to be flexible to adjust to scientific developments.

• Implications for research prior to ITER:– Nonlinear consequences of alpha-particle driven modes need to

be put on a quantitative basis.– Interaction of ripple and alpha-particle driven modes needs to be

addressed.– Develop alpha-particle diagnostics for ITER.– Can alpha-particle channeling be experimentally established? (See

N. Fisch at this meeting and K. L. Wong’s DIII-D papers.)

Experience with Plasma-Boundary Interface

• “Wall conditioning” and wall coatings were critical for enhanced performance in TFTR.– Li coatings were crucial for supershot performance.– Other experiments exhibit strong dependence of

performance on wall conditions.

• Design of plasma facing components underwent several design iterations prior to D-T.– Carbon fiber composite tiles were reliable and effective.

• Tritium retention in graphite is a serious concern.– TFTR tiles 16% retention– JET 12% retention– One year after extensive removal efforts

C. Skinner

Implications of Experience with Plasma-Boundary Interface

• Implications for ITER:– If a reliable technique is not developed to remove the tritium from the

graphite tiles and coatings, it will have devastating operational consequences.

– Need to qualify plasma facing components prior to D-D/D-T operation.• Will H operation be sufficient? Are we prepared to resolve this in D-D?

What should the balance be between H and D operation?– What can ITER do to establish the basis for Demo with ~5 times the heat

exhaust?

• Implications for research prior to ITER:– Develop technique to remove the tritium from the plasma facing components.

• Increase removal rate by four orders of magnitude.– Ascertain whether high-Z materials (with and without coatings) can survive

the high heat fluxes and long pulses in ITER.– Are the E projections for ITER affected by the extensive use of wall coatings

in present experiments?– Develop backup strategy

• Li divertor??? What else??

Developing the Scientific Basis for a Power Plant is the Common Focus of both the

Ongoing Research Program and ITER

• Current research program will enhance the prospects for ITER:– Achieving a burning plasma and meeting its

programmatic goals.– Developing the science and technology that will also

impact a power plant.

• ITER’s success will be measured by:– Meetings its programmatic goals.– Developing the scientific understanding and making the

discoveries required to make an attractive power plant.