Status and Plans Transport Model Validation in ITER-similar Current-Ramp Plasmas D. R. Mikkelsen, PPPL ITPA Transport & Confinement Workshop San Diego 4-5 April Mikkelsen, ITPA T&C, San Diego, 4-5 April 2011

Overview of Validation in I p -ramps Simulations do not yet provide robust predictions of ITER ramp-up/down, but they serve the useful purpose of identifying the larger sources of uncertainty. The periphery (r>a/2) dominates current evolution; focus model testing there. ITER-similarity discharges best for testing transport models used in I p ramp scenarios. Experimental data from ITER-similarity plasmas in Profile Database. C-Mod has four ramp-ups, DIII-D has three, JET has two. Full documentation is important for testing transport models – improvement needed. measure critical quantities (q,T i,T e,n e,Z eff,…) so they are not free parameters fluctuation measurements are desirable, to validate underlying transport theory. Benchmarking codes is the priority, but other (parallel) activities are useful: Further microinstability analysis of ITER-similar plasmas is desirable: are all tokamak's baseline ITER-similarity discharges in the same instability regime? ITER predictions based on prescribed profiles can play an important near-term role. Problems with data (next page) have stalled further work on this method. 2Mikkelsen, ITPA T&C, San Diego, 4-5 April 2011

Experimental data: ITER-similar current ramps Have data sets for ITER-similar current-ramp plasmas. C-Mod, DIII-D and JET contributions are now in the Profile Database. have both MDSplus access and ‘concatenated UFILEs’ have only one ramp-down so far (C-Mod) have both pure-OH and NBI/ICRH-assisted shots I p > half maximum in all data sets (not very different from non-ramp range of I p ) expect C-mod data from times with lower I p (analysis code problems). There are some problems/limitations with some diagnostic data: T e for r/a>0.7 is very problematic for two JET shots for most of the ramp-up ‘gray’ ECE, and low Thomson signal; is there any hope for improvement? no q profile measurement for C-Mod yet (perhaps in future campaigns) Can we get complete data sets from times with Ip below the flat-top value? Could more ramp-downs be made available for validation testing? Could get plasmas with H-mode transition during ramps (ramp-up and -down)? What further issues should be addressed? 3Mikkelsen, ITPA T&C, San Diego, 4-5 April 2011

Validating models: benchmarks first Need to compare predictive results drives code benchmarks. Currently: ASTRA, CRONOS, FASTRAN, JETTO. Others will join? We will cross-calibrate the simulation codes in current use for ITER simulation, by comparing implementations of transport models and predicted temperatures for several ITER-similar current ramps. Initial round will benchmark the ‘Mixed Bohm-gyroBohm’ model. Four modelers participating at this time (a fifth will join soon). Files shared on ITER T&C private web site (thanks to Houlberg & Hosokawa). Diffusivity calculations are all ‘on the same page’, but detailed study of the implementations is needed. All the modelers will be at the IOS meeting in Culham next week. Next: benchmarking the temperature predictions will test the use of the heating power and geometry data, as well as boundary conditions. 4Mikkelsen, ITPA T&C, San Diego, 4-5 April 2011

Benchmark of Bohm-gyroBohm Models Implementations differ a bit. the reasons must be identified. All modelers at IOS meeting next week at Culham. Which additional models should be benchmarked? 5Mikkelsen, ITPA T&C, San Diego, 4-5 April 2011

Backup slides follow this page Mikkelsen, ITPA T&C, San Diego, 4-5 April 20116

ITER predictions with prescribed profiles Near-term ITER ramp-up/down scenario development will determine if planned hardware provides sufficient flexibility in the face of transport uncertainties. Is the heating power sufficient to slow down current penetration? The magnitude of T e (r) may be controlled by heating, but does the less controllable shape lead to undesirable l i ? the largest uncertainty in T e (r) shape is caused by the pedestal height. Results may be found most quickly from simulations of current penetration with specified T e (r) taken from ITER-similarity experiments (L-mode and H-mode shapes). T e profile shapes in experiments are ~universal; go from expt. directly to ITER? This is the basis of 'profile consistency', 'canonical profiles',… ITER current ramp-up predictions could simplified by using a prescription for the T e profile shape that is accurate in the periphery - roughly, r > a/2. Need prescriptions for shape, normalization, and T e pedestal. Shape in the plasma periphery is needed for ITER current ramp-up predictions. A very simple empirical analysis is presented here, other prescriptions are invited (and see prescriptions in other talks).

Simplified ITER ramp-up simulations Normalized experimental T e profiles do have a common shape in the periphery. T e profiles in the periphery are a simple function of  n. The T e pedestal creates the largest shape variation. H factor is ~universal during current ramp-up, too. ITER current ramp simulations could use simple prescriptions for both T e (r) shape and normalization to quickly scope out a plausible operating range. The pedestal height should be scanned to characterize its impact.

Validation in the periphery A. Hubbard pointed out that no ITPA group is validating models in the "no man's land". There are fundamental reasons why the core validation effort avoided r/a>0.8 ELM influence is frequently a show-stopper (as with sawteeth and r/a<0.3) ELM avoidance/mitigation is a pre-requisite for validation. >>> work on L-mode, QH-mode, and I-mode plasmas. Many diagnostic issues become more serious as the edge is approached: Several parameters vary more and more rapidly: q, magnetic shear, L Te, L Ti, P rad, ??? so the mapping errors have greater consequences, greater accuracy is required. P rad becomes more important, it must be measured more accurately in magnitude and location. Q ie becomes more important, so T e and (T e -T i ) must be measured more precisely and the mapping errors must be reduced (measure both in same location to reduce mapping sensitivity). A serious effort in validation may require a new generation of diagnostics - start now!