Possible Experimental Tests of Pedestal Width/ Structure, Motivated by Theory/Modeling R.J. Groebner PERSISTENT SURVEILLANCE FOR PIPELINE PROTECTION AND THREAT INTERDICTION Presented at DIII-D Pedestal Transport Workshop San Diego, CA Feb 17-18, 2010
General Comments Status of several theoretical models for pedestal transport or structure was discussed. A number of measurements were discussed that would be very useful for evaluating or guiding these models. Several of these measurements would be best motivated by calculations prior to performing experiments. There are codes available that can perform some of these calculations It will require some effort and collaboration between experimentalists and modelers to perform this work One major outcome: Pedestal width is likely to be a combination of more than one physics process. Therefore, we should not expect to get a simple width expression. No reason to expect different profiles to have same width. A major benefit of modelers comparing their models to data is that they can eliminate models that do not work (we might not hear about this but it does occur) Profile reflectometry and Doppler Back Scattering provide great new/ improved capabilities DIII-D Ped Workshop / rjg / 2/17/10 2
New and Upgraded Pedestal Diagnostics are Planned or Under Consideration for DIII-D Lithium beam edge current measurement Power supply and control improvements Expected operation late in FY2010 run campaign High resolution edge Thomson scattering Improved measurements of pedestal T e, n e and p e, midplane measurement? Plan to install during LTOA-II Laser-induced fluorescence edge neutral measurement Measure 1D neutral density profile in pedestal (above divertor) Plan to install machine interface during LTOA-II Visible cameras with intensifiers for density fluctuations Possibly image EHO, other activity in and near pedestal In progress Tangential SXR X-point camera Study structure of stochastic layer Under consideration Ion temperature fluctuations Under development by BES group DIII-D Ped Workshop / rjg / 2/17/10 3
Experimental Measurements that are Motivated by Theory/Modeling 1 Basic ExB bifurcation model for H-mode predicts S-curves with specific characteristics for T e vs Q (heat flux) and n e vs Γ (particle flux). Model also predicts growth rate for width Can we measure the S-curves, including hysteresis from back transitions And, does growth of width show predicted characteristics Several models need to know if particle transport has an important nondiffusive aspect that can give inward flux Measure transient behavior of density (after L-H transition or with oscillating gas puffing) to see if we can infer a pinch need some estimate of particle source to complement this measurement. Determine if beams are important particle source Paleoclassical theory has several predictions for pedestal scale lengths (and for ped height) key parameter is resistive diffusion rate Do these predictions agree with experiment? Make experimental measurements over a range of resistive diffusion rate DIII-D Ped Workshop / rjg / 2/17/10 4
Experimental Measurements that are Motivated by Theory/Modeling 2 Kinetic ballooning modes proposed to limit pedestal pressure gradient Need predictions of expected fluctuation signatures for KBM can we see them? Can we measure saturation of pressure gradient coincident with rise of KBM pedestal turbulence? Hypothesis: Pedestal exists where ExB shearing rate exceeds linear growth rate of most unstable long wavelength mode Does measured turbulence decorrelation rate compared to measured ExB shearing rate show this behavior? Test this at L-H transition. Spatially resolved measurements in pedestal and into nearby core. Hypothesis: Profile stiffness causes core turbulence to respond quickly to pedestal changes Can we prove this? Show that core turbulence changes due to local changes in local temperature profiles (ion?)? Or, does turbulence respond to changes in ExB shearing rate? Or neither? DIII-D Ped Workshop / rjg / 2/17/10 5
Experimental Measurements that are Motivated by Theory/Modeling 3 In many models, fluctuations drive dominant transport in the pedestal. Do we have any evidence that fluctuations cause transport? One approach, look for correlation between density turbulence and saturation of gradients TGLF shows H-mode turbulence is not completely quenched in pedestal; significant ITG or TEM remain and contribute to xport Do we see this in experiment? Need code calcs for fluctuation spectra for comparison to BES and DBS data Some models hypothesize that ETG is dominant electron thermal transport mechanism at top of pedestal Can we measure ETG at pedestal top and show its effect on electron thermal transport Some nonlinear models predict that fluctuation bursts from core contribute to pedestal turbulence Can we measure such a phenomena? DIII-D Ped Workshop / rjg / 2/17/10 6
Experimental Measurements that are Motivated by Theory/Modeling 4 A generic need for models is basic time scale for profiles to be established, especially gradients after L-H transition and after ELM Use fast profile reflectometry to determine how quickly density gradient reaches equilibrium TGLF can do time dependent simulations from L-mode up to first ELM Need measured profiles to see if code can match them XGC0 modeling looking at how far neoclassical plus fuelling physics can explain pedestal can explain some things but not all. Turbulent models will be added. Clear trends from experiment are needed to test these models GEM predicts much stronger transport for EM than ES transport at ITG wavelengths Can we experimentally determine if EM effects are important in pedestal transport? DIII-D Ped Workshop / rjg / 2/17/10 7
Experimental Measurements that are Motivated by Theory/Modeling 5 Ion orbit loss predicts a loss cone? If so, can we measure edge ion distribution and see a loss cone? What is role of ExB shear at pedestal top? We can vary toroidal rotation to vary ExB shear Fundamental physics of paleoclassical theory: electrons diffuse with poloidal flux Can we see electrons diffusing with poloidal flux? BOUT shows very quick recovery of pedestal after ELM crash ~ 1 ms Need further BOUT simulations to see if this effect is related to strength of source Do we measure the recovery to be this fast? DIII-D Ped Workshop / rjg / 2/17/10 8
Outline-1 S-curve is clear signature of microturbulence that controls pedestal Questions about why it can take a long time for ELM to occur It is useful for modelers to look at data because they can rule out some models Hypothesis that ETG is electron transport mechanism at top of pedestal Are bursts of fluctuations from core contributing to pedestal turbulence? Pedestal width may be a hybrid of scales from more than one mechanism Spatial profile of ñ/n in pedestal and into core is there a difference? What would this test? Any evidence that fluctuations cause transport? Could look for correlation between density turbulence and saturation of gradients Look at fluctuation characteristics (from BES) even into core Statements about linkage of core turbulence to pedestal height if we change height and core turbulence changes, can we show it is due to stiffness? Can BES estimate transport from its measurements? DIII-D Ped Workshop / rjg / 2/17/10 9
Outline -2 Profile reflectometry and DBS capabilities are of great interest Can profile reflectometer look at ELM structure when an ELM starts to grow? Is there data for after L-H transition to look for evidence of spreading TGLF shows H-mode turbulence is not completely quenched in pedestal do code and observations agree on characteristics of remaining turbulence significant ITG or TEM contributing to xport do we see this? Time dependent simulation from L-mode up to first ELM to see if TGLF can fit data What is re-arrangement of momentum balance right after L-H transition And, different ways the pedestal profiles build after transition What controls shape of does density profile? GEM predicting much stronger transport for EM than ES transport at ITG wavelengths? Can neo predict poloidal flow for measured C+6 ions? Can we measure edge ion distribution and see a loss cone? Compare non-thermal Ti in SOL to code calcs DIII-D Ped Workshop / rjg / 2/17/10 10
Outline -3 What can we do to elucidate role of ExB shear at pedestal top? we can vary toroidal rotation to vary ExB shear Several equations presented for scale lengths from paleoclassical theory Can we see electrons diffusing with poloidal flux? Can we make S curves, including hysteresis, for T e vs Q and n e vs Γ? Would test basic ExB bifurcation model for H-mode Can we detect turbulence spreading from core to pedestal? Could be random walk? Perhaps has to be a code project first? Is there a non-diffusive aspect to particle transport that can give inward flux? Need perturbation experiment with SOLPS, UEDGE and FACETS time dependent analysis XGC0 modeling looking at how well neoclassical plus fuelling physics can explain pedestal can explain some things but not all clear trends from experiment are useful Need predictions of expected fluctuation signatures for KBM can we see them? BOUT shows very quick recovery of pedestal after ELM crash ~ 1 ms can we measure this? DIII-D Ped Workshop / rjg / 2/17/10 11
Outline -4 There is a need/request for experimentalists to assess a variety of errors, such as real uncertainties in q-profiles, due to fact that linear and non-linear models can be very sensitive to details of profiles DIII-D Ped Workshop / rjg / 2/17/10 12