The Dialogue Between the Science of Turbulence and Transport and a Burning Plasma Experiment E.J. Synakowski Princeton Plasma Physics Laboratory December 11, 2000 UFA Workshop on Burning Plasma Science Austin, Texas

There is great value in discussing the scientific needs of and contributions from a burning plasma experiment Heartfelt, strong sentiments from every vantage point –At issue is the quality of the scientific exchange –What are the issues and concerns a BP must address to be viewed as an attractive scientific test bed? Give & take: What does a burning plasma experiment need from the science of turbulence and transport? –Assume that predicting the performance is of value What would a burning plasma experiment give to the science of turbulence and transport (T&T)? –What the integration goal means to T&T –Flexibility

The transport & turbulence community speaks consistently of a couple of major thrusts I. A strong desire, what inspires the transport community scientifically, and what it feels is required to advance the field, is to develop predictive models based on an understanding of turbulence and turbulence dynamics. From Snowmass: n “Goal 1: Comprehensive transport models – Goal 1a: Pursue the challenging, yet realistic goal of developing comprehensive predictive transport models, based on physically reasonable assumptions and well-tested against experiments. – Goal 1b: Detailed Experiment/Theory Comparisons” at the level of the turbulence

The second primary goal: develop tools for turbulence manipulation to control the plasma pressure Again, from Snowmass: n "Goal 2. Develop tools and understanding for control of transport and transport barriers...” – This is what the BP language of “integration” speaks to – Even a doubter of the value of transport barriers can benefit from control tools (managing a self-heated environment) – Intrinsically interesting – Many in the community ask: Is a BP the place to develop the understanding and the tools? Will a BP be able to find a self-consistent operating point?

What a BP needs from T&T: issues have been identified for predictive transport modelling during the next 10 years Highlight items from the Snowmass report are: n Need convergence of models for reliable extrapolation n Reliable models for plasma boundary needed n Need accurate models for transport barrier dynamics (edge and core)

There has been progress in developing transport models that capture experimental trends, but it is not decisive "Are you better off now than you were 8 years ago?" n For temperature profile predictions: several models are now reasonably successful, but no single model is generally preferred. All models have flaws and failures. n The underlying differences between reasonably successful models are large enough that we can't say that their characteristics point to the importance of particular processes n More complete physics is being included (electron dynamics, ExB shear, for example)

Turbulence dynamics understanding is key to developing predictive capability and is a premier challenge Example: Zonal flow dynamics n Several facets have been appreciated only in the last several years: – their existence (not verified) – interaction between zonal flows and microinstabilities - saturation levels – Some consistency in measured features (e.g. bursting), but there are other explanations

Knowledge of electron thermal transport is key to predicting BP performance n Some promising links between theory and experiment n Nevertheless, we are a long way from demonstrating that theory explains a lot of experimental data. n Critically important in prediction of  heating effectiveness and need for ash removal strategies n Issue is forcing progress in codes: high k modes, streamers n Need for a similar experimental assault

Predicting BP performance requires significant improvement in knowledge of pedestal height and width Many caveats, contradictory theories, contradictory experiments fi predictive capability not in hand n Over a range of theories, many have ∆ ~  2/3- 1. – JT-60U, JET support “standard” model of ∆ ~  and gradient near the ideal MHD limit n Others (DIII-D) support ∆ independent of  perhaps related to second stability n Progress: useful cross-machine database being developed – ITER H-mode Edge Pedestal Expert Group, March 2000). n Edge turbulence simulations becoming more realistic – Xu and Cohen (LLNL), Rogers and Drake (U. Md.), Scott, Jenko, Zeiler et al. (Garching))

Dynamical models capture many features of present experiments, but cannot yet predict Required to assess robustness of operating point of a BP and to develop a control strategy Self-consistent evolution of turbulence, fluxes, shear, and profiles beginning Character of some dynamics seen in codes, but robust predictions are not in hand

Present-day modelling cannot capture dynamics of new regimes For example, an opportunity: NSTX –Lots of exciting speculation about possible confinement characteristics - sheared flows, aspect-ratio-induced stabilization of drives: speaks very well of the science BUT No model has yet dared to try to capture the dynamics of the ST in advance in a way that guides our experimental choices H mode physics, core dynamics, particle transport,... Lack of predictive capability not intrinsically different from the situation on a BP –Strong self-heating a significant extrapolation. Perhaps no stable operating point without pressure profile control.

The question of what a BP gives back to turbulence and transport, and the value of the investment, causes much debate If the question is confined narrowly, "I have a passion for turbulence and transport dynamics: what is the best way to learn about it?", so far, it is not accepted that the BP experiment is the experiment of choice.

The strongest case arises for a BP when two issues are discussed Transport control in a self-heating environment –But many feel that the flexibility of a D-D experiment is better suited for developing control tools (e.g. shear flow generation tools)  * scaling of core transport and edge pedestal characteristics –This is of value in predicting the performance of a DEMO or ITER, but the range of  * in present experiments exceeds the change that a next-step BP will provide –Main value is to prediction of yet another BP; intrinsic value is debated

The BP feature of integration strongly suggests the need for turbulence manipulation tools to modify P(r) To the Snowmass transport and TTF audiences, the BP goal of integration speaks to transport barrier control tool development. Discussions focus on the value of confronting the BP, self- heated core and transport control self-consistently vs. $ for $, getting more bang-for-the-buck in one's investment by developing a flexible control strategy in existing machines No doubt the self-consistent alpha heating profile will be hard to simulate to everyone's satisfaction –But the science of turbulence manipulation, bifurcations, flow shear, Ti/Te effects, would benefit greatly from an investment in flexibility “Of lasting value to a BP is learning to control what we have”

The T&T community is motivated by the possibility of manipulating turbulence dynamics to modify the pressure profile From AT Workshop, GA, March 1999: “The single issue that was virtually unanimously agreed to be the most pressing in terms of the ultimate viability of the Advanced Tokamak is the following: In both the experimental and modeling efforts... significant progress... would result if local pressure profile control, through manipulation of the local transport, can be realized...” The transport community equates the challenge of BP integration as the challenge of pressure profile control tool development based on manipulating turbulence. –What does a BP contribute to help this effort? –What development is required in advance of a BP?

Flexibility, not rules for enhanced confinement access, will help ensure the success of a BP experiment The question, "What is the power threshold of a core barrier?" is too narrow. For example, –Supershot is transitionless (no bifurcation) –ERS regime access related to the conditions required for V  (E r ) shear layer development what is the physics of this event? –Slow transitions to enhanced confinement observed. DIII-D NCS, TFTR RS with co-rotation Low or no power threshold –Some transitions with simple q values Wide range of dynamics fi BP needs flexible tools. There is no scaleable rule.

Flexibility allows theories of dynamics to be challenged External E r variations are a powerful teaching tool Independent variations of T e, T i push theory to different extremes The BP community would strengthen their case with quantitative discussion regarding flexibility and tools, and what will be learned about –Transport and turbulence dynamics and control, e.g. bifurcations, spontaneous shear flow generation –barrier dynamics, expansion, front propagation –ion vs. electron transport that cannot be learned better elsewhere

Challenge to the BP community: make a scientific argument for what BP physics can teach about transport and turbulence dynamics n The T&T community is passionate about understanding of turbulence and turbulence dynamics – The scientific basis for predictive capability – Intrinsically front-line physics, and a great export to other fields – Flexibility, accessibility essential for making progress in a BP or elsewhere n Advances being made, but ability to predict performance of a BP has not changed in a decisive sense n Flexibility is required for the integration goal of a BP to be compelling to T&T – Tools to vary E r, T i /T e, diagnostics need high priority – Control tool development required to ensure a stable operating point – Many feel that the place to do this is in an invigorated base program n Integration P(r) control via turbulence manipulation