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

1. Energy Loss Associated with High Frequency, LFS Pellet Injection: Potential Impact on ELMsW.A. Houlberg, A. R. PolevoiFusion Science & Technology (FST) Dept ITER OrganizationAcknowledgements: L.R. Baylor, ORNL 14th ITPA CDBM/TP Meeting 22-25 April 2008 Oak Ridge, Tennessee 14th ITP CDBM/TP Meeting, Oak Ridge, TN 22-25 Apr 2008 - WAH, IRP

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 14th ITP CDBM/TP Meeting, Oak Ridge, TN 22-25 Apr 2008 - WAH, IRP

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? 14th ITP CDBM/TP Meeting, Oak Ridge, TN 22-25 Apr 2008 - WAH, IRP

4. Ballooning Nature of Transport in H-mode(Alcator C-Mod)B. LaBombard, US Transport Task Force Meeting, Boulder, 25-28 Mar 2008 14th ITP CDBM/TP Meeting, Oak Ridge, TN 22-25 Apr 2008 - WAH, IRP

5. Ballooning nature of Transport in L-mode(Tore Supra)J.P. Gunn et al, J. Nucl. Mater. 363-365 (2007) 484 14th ITP CDBM/TP Meeting, Oak Ridge, TN 22-25 Apr 2008 - WAH, IRP

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? 14th ITP CDBM/TP Meeting, Oak Ridge, TN 22-25 Apr 2008 - WAH, IRP

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 Dαregime 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? 14th ITP CDBM/TP Meeting, Oak Ridge, TN 22-25 Apr 2008 - WAH, IRP

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 <40eV • For an upper limit of 100Pa·m3/s maximum input from pacemaking pellets, this would represent a negligible of <350kW in ITER 14th ITP CDBM/TP Meeting, Oak Ridge, TN 22-25 Apr 2008 - WAH, IRP

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 14th ITP CDBM/TP Meeting, Oak Ridge, TN 22-25 Apr 2008 - WAH, IRP

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? ? 14th ITP CDBM/TP Meeting, Oak Ridge, TN 22-25 Apr 2008 - WAH, IRP

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 14th ITP CDBM/TP Meeting, Oak Ridge, TN 22-25 Apr 2008 - WAH, IRP