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

14th ITP CDBM/TP Meeting, Oak Ridge, TN 22-25 Apr 2008 - WAH, IRP1

14th ITPA CDBM/TP Meeting

22-25 April 2008

Oak Ridge, Tennessee

Energy Loss Associated with High Frequency, LFS Energy Loss Associated with High Frequency, LFS Pellet Injection: Potential Impact on ELMsPellet Injection: Potential Impact on ELMs

W.A. Houlberg, A. R. PolevoiW.A. Houlberg, A. R. PolevoiFusion Science & Technology (FST) Dept Fusion Science & Technology (FST) Dept

ITER OrganizationITER Organization

Acknowledgements: Acknowledgements: L.R. BaylorL.R. Baylor, ORNL, ORNL


OutlineOutline

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


Ballooning Nature of Transport at the Plasma Ballooning Nature of Transport at the Plasma Edge 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?


Ballooning Nature of Transport in H-modeBallooning Nature of Transport in H-mode(Alcator C-Mod)(Alcator C-Mod)B. LaBombard, US Transport Task Force Meeting, Boulder, 25-28 Mar 2008


Ballooning nature of Transport in L-mode(Tore Supra)J.P. Gunn et al, J. Nucl. Mater. 363-365 (2007) 484


Indications of Ballooning Nature of ELM TriggeringIndications of Ballooning Nature of ELM Triggeringby Pelletsby 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?


Potential Changes in Operating Characteristics Potential Changes in Operating Characteristics with High Frequency Small Pelletswith 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 Dα 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?


Energy Losses Associated with Pellet Injection - IEnergy 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


Energy Losses Associated with Pellet Injection - IIEnergy 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


ITER Inside and Outside Pellet Launch ITER Inside and Outside Pellet Launch LocationsLocations

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?

??


SummarySummary

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

Documents

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