14th ITPA CDBMTP Meeting

1
Energy Loss Associated with High Frequency, LFS
Pellet Injection Potential Impact on ELMsW.A.
Houlberg, A. R. PolevoiFusion Science
Technology (FST) Dept ITER OrganizationAcknowle
dgements L.R. Baylor, ORNL

• 14th ITPA CDBM/TP Meeting
• 22-25 April 2008
• Oak Ridge, Tennessee

2
Outline

• Ballooning nature of transport at the plasma
  edge
• H-mode, L-mode and ELMs
• Indications of ballooning nature of ELM
  triggering by pellets
• Potential changes in operating characteristics
  with high frequency small pellets
• Energy losses associated with pellet injection
• Summary

3
Ballooning Nature of Transport at the Plasma Edge

• Transport at the plasma edge generally displays a
  strong ballooning character
• In H-mode
• Radial fluxes, turbulence measurements, and
  pressure gradients indicate inter-ELM transport
  is concentrated on the outboard midplane
• Experiments indicate 102 larger radial fluxes on
  the outboard midplane than on the inboard
  midplane
• The asymmetry is not explained by geometric
  effects, ie compression of flux surfaces and
  steepening of gradients on outside relative to
  inside
• In L-mode limiter plasmas
• Similar observations to H-mode plasmas
• ? Asymmetry is not explained by the presence of a
  separatrix
• ELMs in H-mode plasmas appear to be driven by
  peeling-ballooning instabilities
• Can we take advantage of these features to
  develop effective and robust ELM amelioration
  methods?

4
Ballooning Nature of Transport in H-mode(Alcator
C-Mod)B. LaBombard, US Transport Task Force
Meeting, Boulder, 25-28 Mar 2008
5
Ballooning nature of Transport in L-mode(Tore
Supra)J.P. Gunn et al, J. Nucl. Mater. 363-365
(2007) 484
6
Indications of Ballooning Nature of ELM
Triggeringby Pellets

• Experiments on JET, DIII-D and AUG
• Seem to indicate that pellets injected from the
  LFS are more effective in triggering ELMs P.T.
  Lang et al, NF 44 (2004) 665
• ELMs triggered by LFS injection in DIII-D
• Stronger and longer-lasting than those from the
  inner wall injected pellets L.R. Baylor et al,
  NF 47 (2007) 1598
• Open issue in interpretation of AUG results
• Is the triggering delay after crossing the
  separatrix with HFS pellets related to the
  required penetration depth, or the time for the
  cloud to expand to the LFS?
• This could significantly change the present
  injection requirements
• Can ELMs be triggered by smaller pellets using
  LFS launch than using HFS launch?

7
Potential Changes in Operating Characteristics
with High Frequency Small Pellets

• Reduced Type I ELM size
• This is the anticipated result, with the hope
  that there is minimal degradation of global
  energy confinement
• Change in ELM character
• Smaller Type III or grassy ELMs, which are
  normally obtained by establishing a radiating
  zone close to the edge
• Much more frequent small Type II ELMs
• Regression to L-mode
• Not a desirable outcome
• Elimination of ELMs, but maintenance of H-mode
• Similar to an enhanced Da regime where ELMs seem
  to be replaced by a quasi-coherent edge mode
• For this possibility, we need to examine the
  energy losses associated with the ionization of
  the pellets mass and expulsion of the cloud
• More likely for LFS injection?

8
Energy Losses Associated with Pellet Injection - I

• Aside from the energy and particle losses from
  the triggered ELMs, there are other energy loss
  mechanisms associated with pellet injection
• Ionization of the pellet mass
• Expulsion of the partially heated ablatant cloud
  from LFS injection
• Enhanced turbulent transport in the pedestal by
  small LFS pellets
• Ionization of the pellet mass
• The evaporation, ionization and radiation from
  the cloud is estimated to be lt40eV
• For an upper limit of 100Pam3/s maximum input
  from pacemaking pellets, this would represent a
  negligible of lt350kW in ITER

9
Energy Losses Associated with Pellet Injection -
II

• Expulsion of the partially heated ablatant cloud
  from LFS injection
• Evaluations with the PRL code (P.B. Parks, L.R.
  Baylor, PRL 94,2005, 125002) for ITER cases
  (Baylor)
• 3mm pellets 4.0kJ/pellet ? 33eV/ion
• Complete mixing of the pellet ablatant with the
  density and temperature in the pedestal would
  yield a factor of 10 higher energy loss
• Analysis of LFS L-mode cases shows no detectable
  decrease in edge pressure (Baylor)
• Enhanced transport in the pedestal by small LFS
  pellets
• Although no enhanced losses from the pedestal
  have been observed with larger LFS pellets, can
  smaller high frequency enhance the losses?
• Possibly from the 3-D perturbations similar to
  RMP or vertical position that oscillations
  enhance neoclassical, non-ambipolar 3-D losses
• Frequency is much lower than turbulence
  frequencies, so turbulence will likely not be
  enhanced

10
ITER Inside and Outside Pellet Launch Locations

• Cross section of ITER showing the pellet
  injection and gas injection locations
• The dashed pellet trajectory is the proposed low
  field side location for ELM triggering
• The solid pellet trajectory is the proposed high
  field side location for fuelling
• Are these locations sufficiently optimal for
  separating ELM control and fuelling functions?
• Would a midplane launch capability be much more
  effective for ELM amelioration?

?
11
Summary

• The most likely outcome of injection of small
  pellets at high frequency will be to change the
  character of the ELMs
• Smaller Type I ELMS, Type II or grassy Type III
  ELMs
• A key issue is the possible degradation of the
  pedestal and consequently the global confinement
  as often already noted
• Nonetheless, energy loss associated with small
  high frequency pellets needs to be examined
• The ablation and ablatant drift of small
  low-velocity pellets in relatively high
  temperature pedestals could show strong
  deviations from existing models
• Deeper penetration of the electrons in the solid
  pellet leading to bulk heating surface instead of
  surface heating
• Shield size and cloud mass larger relative to
  pellet size
• These could lead to new conditions and effects
  not seen in present experiments