ITPA-Transport TG Particle & impurity workgroup Discussion, future plans Milano,

Issue1: Density peaking with electron heating Peaking at low eff is robustly documented in ITG and consistent with theory. Truly relevant to ITER in electron heated regime? Smaller machines experience ‘pumpout’ with strong electron heating How to address question? - Empirical: study trends with relevant parameters Ti/Te,  Ti/  Te,  ei /  E - Study ITG/TEM transitional region (scan electron/ion heating ratio) - Resolve correlation between Ti/Te in expts and NBI source, compare to theory

Particles:talks summary H.Weisen et al presented GS2 linear simulations, reproduced JET density peaking dependencies on nueff, source,li, gradTi/Ti C.Angioni presented collisional, non-linear GYRO calculations of particle flux. High k, outward flux from trapped electron, low k inward. Explains agreement in linear case, if representative max gamma mode chosen. K. Tanaka showed detailed comparison between LHD&JT-60U. LHD at large major radius shows clear, non-axissymmetrical NC transport. Comparison useful as tokamak community struggles and/or makes use of non- axisymmetrical effects: TF ripple, MHD modes, Resonant magnetic perturbations. Watch out! Y. Camenen et al presented GK equations with inclusion of Coriolis and centrifugal forces. Important for heavy impurities in strong rotation. N. Tamura showed CORE BORN impurity accumulation of in LHD occurs at highest densities and with strongest ne peaking (pellets). Ergodic edge layer can screen impurities born at PFC, thereby preventing accumulation. D. Mikkelsen reported GYRO GK simulations for CMOD, consistent with observations. Density peaking if R/LTi not to far above threshold, otherwise flattening.

Particles: discussion summary Electron heating: Effect of electron heating by alphas in ITER. Will ITER remain in ITG (in which case we expect density peaking) or will TEM’s flatten density peaking? If ITER remains in ITG, will alpha power drive out core impurities? L versus H: How can we understand differences in peaking behaviour in L and H-mode? Fuelling: What evidence do we really have that gas fuelling will be ineffective in ITER? Z-scaling of impurity transport. Research on several devices and operating regimes. Very wide subject, too large for a focussed task. He-transport: Received little attention in recent years, especially in advanced scenarios. Felt we had some catching-up to do. Chosen as a priority 1 subject for joint research, because answers can be obtained by experiments and by mining existing data in resonable time frame.

CDB-?He profiles and transport coefficients (draft) Confinement Database & Modeling TG Spokesperson: H. Weisen Background - Previous results With the exception of DIII-D and JET in the ninenties, little is known of He transport in most devices and especially in advanced regimes, as envisioned for ITER. The old DIII-D results indicate that He transport coefficients are similar to those of the background particles and also comparable those of trace tritium ions investigated in JET. We know that He data were acquired on JET in the past few years, however raw data interpretation appears to be difficult and is therefore still in progress. He ash transport enters into reactor performance via the ratio of helium to energy confinement time. Similarly the possible existence of convection would impact reactor performance positively if V/D is larger for He than for D and T (if He profiles are less peaked than hydrogen profiles in the absence of a core source) or negatively in the opposite case.

Outline of Experiment Coordinated experiments Highest priority target conditions for the proposed studies include hybrid and ITB scenarios for ITER. Experiments most likely would involve He gas puffs of NBI injection of He in D plasmas and measurements of the resulting He density evolution using CXRS. The resulting analysis should provide empirical transport coefficients (if time resolved measurements are available), steady-state profiles and V/D otherwise as well as He residence times. These quantities would have to be related to the corresponding quantities characterising the other transport channels. In addition to advanced ITER scenarios, He transport experiments in the following operating conditions are expected to help consolidating our physics knowledge base and provide data for code (de-) validation: standard H-modes, H-modes in electron heated conditions, L-modes. Some such experiments are already planned, such as He transport in advanced regimes in JET and He transport in H-modes in TCV. Coordinated data mining He transport data have been obtained in some devices (like JET) although not yet fully analysed and validated. We call ITPA participants to make such data, even historic data, available for inclusion in a common profile database in addition to those from the proposed experiments. Ideally the contents of the database should be the same as for the international database, plus profiles for V He & D He if available and n He in steady state, if available. CRPP volunteers for assembling and maintaining the database, which would be similar to the one assembled from AUG,JET and C-MOD for density peaking. The matter of the format would be discussed as collaborating institutions signal their readiness to contribute data. The database would ideally be constructed such as to allow a future extension towards including other impurities, however other impurities are not included at this stage in order to avoid creating an open-ended investigation.

Issue 2: Density peaking in H versus L-modes 2) Can we understand the apparent conflicting behaviour between most of the H-mode database (strong nueff scaling, weak or no li or magnetic shear scaling) and most of the L-Modes (strong magnetic shear scaling, weak or no nueff scaling)? Ideas: -Clues to be obtained from exceptions: C-MOD and other devices? -Repeat GS2 comparison with good L-mode profile data (JET) -

Issue 3: Fuelling Plasma fuelling -Do we understand gas fuelling? -Is gas fuelling aided by an inward pinch in the pedestal or not? -Are we sure we can write off gas fuelling in ITER? -Are large devices truly harder to gas fuel than small ones? -Prediction of pellet fuelling in ITER? -Relation with ELM pacing? How to address issues?

Issue 4: De-ashing -He transport poorly documented -He profiles harder to obtain than C profiles in JET He transport coefficients directly impact reactor performance.( For instance inward pinch/outward convection would worsen/relieve divertor compression requirements) -Do He transport coefficients vary as you go from ion heated to electron heated? -Believe this should be high priority How to address issue? -Analysis effort at JET (Carine Giroud) -Proposal for measuring He profile in ECH –H-mode in TCV -

Issue 5: Impurity transport, Z scaling -W adressed in AUG, will be at JET -Medium Z in several devices -Impact of rotation, may introduce bias that’s wrongly attributed to Ti/Te or other parameters correlated with heating method -So far no multi-machine effort as for density profiles How to address issue? -Need HFS & LFS profiles in machines with strong rotation (DIII-D, AUG, JET?) -Proposal for measuring He profile in ECH –H-mode in TCV -

Issue 6: Particle, He, impurity transport in ITB’s - So far little systematic studies? How to address issue? Priority?

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