ITPA-Transport TG Particle

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ITPA-Transport TGParticle impurity
workgroupDiscussion, future plans

• Milano, 20.10.2008

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Issue1 Density peaking with electron heating

• Peaking at low neff 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,tei/tE- Study ITG/TEM
  transitional region (scan electron/ion heating
  ratio)
• - Resolve correlation between Ti/Te in expts and
  NBI source, compare to theory

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Particlestalks summary

• H.Weisen et al presented 1000 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
  LHDJT-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.

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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 TEMs
  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.

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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.
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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 VHe DHe if available and nHe 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.
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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) -
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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?

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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

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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 thats
  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

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Issue 6 Particle, He, impurity transport in ITBs
- So far little systematic studies? How to
address issue? Priority?
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Issue N
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