1. ELMs Triggered from Deuterium Pellets Injected into DIII-D and Extrapolation to ITER
L.R. Baylor1, T.C. Jernigan1, N. Commaux1, S. K. Combs1,
P.B. Parks2, M. E. Fenstermacher3, T.E. Evans2,
T. H. Osborne2, R.A. Moyer4, J. Yu4
1Oak Ridge National Laboratory, Oak Ridge, TN, USA
2General Atomics, San Diego, CA, USA
3 Lawrence Livermore National Laboratory, Livermore, CA, USA
4University of California San Diego, San Diego, CA, USA
Fueling of Magnetic Confinement Machines
Hersonissos, Crete, Greece
16-17 June 2008

2. Overview
ITER has a potential significant ELM problem – erosion of divertor looks serious
Methods to mitigate the ELM problem have been proposed:
Pellet ELM pacing
Resonant magnetic perturbations
Pellets injected from all locations are known to trigger ELMs - Why???
DIII-D Pellet Dropper Installed to Investigate ELM pace making
ELM pacing requirements for ITER are challenging
RMP suppresses ELMs on DIII-D, but at high density some ELMs return
Further research into pellets for ELM mitigation is ongoing also at JET and AUG.

3. Introduction to ELM Pacing Physics
Edge Localized Mode: periodic instability located
at the H-mode edge transport barrier
ELMs result from edge pressure clamped at ballooning limit – transient breakdowns of edge barrier – filaments are ejected from the plasma
Significant fraction of the total plasma stored energy ~5% can be expelled by an ELM (>10 MJ on ITER)
Significant erosion of plasma facing materials will occur for ELM bursts greater than 1 MJ. (A. Loarte, Nucl. Fusion 47 No 6 (June 2007) S203-S263)
A method to eliminate or induce rapid small ELMs is badly needed for ITER.
Compatibility of method with pellet fueling needs to be demonstrated.
Filaments evolving during ELM on MAST. ( Kirk, et al. PRL )
LRB Crete Jun2008

4. Size of ELM Loss Predicted for ITER as much as 20 MJ
Loarte et al., Nuclear Fusion, ITER Physics Basis,Chapter 4
Minimum Acceptable Value (?)
Full performance ITER plasma W pedestal can be as much as 100 MJ leading to ~20 MJ ELM loss if n*ped is the controlling parameter.
LRB Crete Jun2008

5. Erosion of ITER Divertor Tiles
CFC erosion is negligible for QELM < 0.5 MJ/m2 [Zhitlukhin et al., JNM, 2007]
Erosion of ITER Divertor Tiles
ITER SOL projection on the divertor plates is 3.0 m2
ITER ELM size needs to be < 1.5 MJ for low erosion
LRB Crete Jun2008

6. Overview
ITER has a potential significant ELM problem – erosion of divertor looks serious
Methods to mitigate the ELM problem have been proposed:
Pellet ELM pacing
Resonant magnetic perturbations
Pellets injected from all locations are known to trigger ELMs - Why???
DIII-D Pellet Dropper Installed to Investigate ELM pace making
ELM pacing requirements for ITER are challenging
RMP suppresses ELMs on DIII-D, but at high density some ELMs return
Further research into pellets for ELM mitigation is ongoing also at JET and AUG.
LRB Crete Jun2008

7. Fueling Pellets Injected from all Locations on DIII-D Trigger ELMs
DIII-D PoP 2000
0.0
0.4
0.8
1.2
Divertor D a
Density
2.4
2.8
3.2
3.6 n e L
(10 14 m - 2 ) (
a.u .)
Divertor D a
Density
2.4
2.8
3.2
3.6 n e L
(10 14 m - 2 ) (
a.u .)
2210
2220
0.0
0.4
0.8
1.2
2.21
2.22
2.23
2.24
2.25
2210
2220
3.11
3.12
3.13
3.14
Time (s)
3.11
3.12
3.13
3.14
Time (s)
HFS
Vertical
LFS
DWtot ~ 4%
DWtot ~ 8%
DWtot ~ 1%
2.69
2.70
2.71
2.72
2.73
Pellets injected from the 5 different injection locations on DIII-D are observed to trigger immediate ELMs.
LFS injected pellets trigger larger longer lasting Da perturbations (larger ELMs). LFS is more sensitive for ELM triggering.
Natural ELM frequency on DIII-D is higher than available pellet fueling frequency so no chance to test ELM pacing with fueling pellets.
LRB Crete Jun2008

8. What Causes an ELM Trigger from a Pellet?
Pellet cloud releases from pellet and expands along a flux tube.
Density from the cloud expands along flux tube at the sound speed cs.
Temperature ‘cold wave’ travels along the flux tube at the thermal speed. Heat is absorbed in the cloud resulting in a temperature deficit far from the cloud.
Pressure decays and expands along the flux tube with a lower pressure far from the cloud.
Strong local cross field pressure gradients result along the flux tube that form on ms time scales. L ne cs Te qe L Pe L f q
LRB Crete Jun2008 25

9. HFS Pellet Induced ELM Details from DIII-D
Triggered
ELM
End
Pellet Da
represents
ablation of pellet in the plasma with assumed constant radial speed.
DIII-D
1.8mm pellet
injected
from inner
wall is ~10%
density
perturbation
0.10
Pellet
Enters
Plasma
0.08
TS profile
time
Pellet Da
0.06
0.04
0.02
0.00
1.9
neL(1014m-2)
1.8
1.7
Divertor Da
1.6
1.5 80 60
dBr/dt (a.u.)
Inner wall 40 20
No MHD precursor
to pellet induced ELM
-20
-40
200
150
dBr/dt (a.u.)
Outer shelf
100 50
-50
2772.2
2772.4
2772.6
2772.8
2773.0
Time (ms)
The ELM is triggered 0.05 ms after the pellet enters the plasma or 147 m/s* 0.05 ms = 7.4 mm penetration depth.
LRB Crete Jun2008

10. Electron Pressure Pedestal
Depth of Pellet when ELM is Triggered in DIII-D is Much Less Than Top of the Pedestal
ITER Penetration Requirement
ASDEX-U Trigger Range
Non-RMP HFS
RMP HFS
Non-RMP LFS
RMP LFS 5 10 15 20 25 30
0.6
0.7
0.8
0.9 1
1.1
Pe (kPa) r
Electron Pressure Pedestal
DIII-D s
Pre-pellet and ELM
Pellet Location
when ELM
Triggered
Data from DIII-D indicates that the pellet triggers an ELM before the pellet reaches half way up the pedestal. (% Ped is % of Te pedestal height)
Note: Pellets must penetrate deeper to trigger an ELM when RMP is applied.
ASDEX-U trigger range is deeper (G. Kocsis, et al, Nucl. Fusion (2007).)
LRB Crete Jun2008

11. Pellet ELM Pacing Led to 2X Higher ELM Frequency on AUG
Initial pellet ELM pacing shown for 1 second on AUG. (P. Lang, et al Nuc. Fusion, 2004). Small fueling pellets from HFS doubled the natural ELM frequency.
Further optimization needed to reduce strong fueling induced confinement decay and greatly increase the natural ELM frequency.
LRB Crete Jun2008

12. Overview
ITER has a potential significant ELM problem – erosion of divertor looks serious
Methods to mitigate the ELM problem have been proposed:
Pellet ELM pacing
Resonant magnetic perturbations
Pellets injected from all locations are known to trigger ELMs - Why???
DIII-D Pellet Dropper Installed to Investigate ELM pace making
ELM pacing requirements for ITER are challenging
RMP suppresses ELMs on DIII-D, but at high density some ELMs return
Further research into pellets for ELM mitigation is ongoing also at JET and AUG.
LRB Crete Jun2008

13. Pellet Dropper for ELM Pacing on DIII-D
Platform
Pellet
Dropper
V+3
at 0o
Microwave
Cavity
The dropper was developed from existing equipment to make a simple ELM trigger tool.
It uses a batch extruder with pellet cutter to supply sub 1mm D2 pellets at up to 50 Hz for triggering ELMs on DIII-D
The extruder is cooled with a G-M cryocooler and LN2 for simplified installation and operation
Gravity acceleration limits pellet speeds to ~ 10 m/s
Low speed and small pellet size minimize fueling, but should make strong enough density perturbations to trigger ELMs
LRB Crete Jun2008

14. Pellet Dropper Operation
Pictures of
extruded solid
deuterium (14K)
and pellet
chopped from
extrusion
Data from
microwave
cavity showing
mass of each
pellet.

15. Pellet Dropper Now Operational on DIII-D
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
0.7
0.8
0.9
1.05 r
D ne (1019 m-3)
1.8mm 200 m/s
1.0mm 10 m/s
1.0mm 50 m/s
0.4
0.6
0.2
1.2
Te (kev)
DIII-D
NGS ablation model shows pellets should nearly reach Te pedestal in DIII-D H-mode. Good simulation of expected ITER 3mm pellet penetration depth.
Initial operation will be to determine penetration depth required to trigger ELMs.
LRB Crete Jun2008

16. Target Plasma for the Pellet Dropper is the ITER Scenario
DIII-D ITER Like Scenario
Density
ELMs of this size on ITER
could be 15 MJ per ELM
PNBI
Divertor Da
ORNL Filterscope Data
The dropper is designed to
drop 50 Hz pellets to trigger
50 Hz ELMs that have
smaller energy loss
Can the dropper be used to
Obtain 5X increase in ELM
Frequency?
Wtot
0.0
1.0
2.0
3.0
4.0
Time (s)
E. Doyle 2008
LRB Crete Jun2008

17. Pellet Dropper Diagnostic Views ITER Scenario Plasma Shape
Dropper Pellet Trajectory
Fast Camera Viewing Region
Fast camera at 90 degree midplane has an excellent view of the dropped pellets (and inner wall fueling pellets) (J. Yu, Phys Plasmas 15 (2008) )
Filterscope Da views of the divertor and midplane are used to observe the pellet perturbations to the SOL. Visible bremsstrahlung and interferometer can also see some density perturbation.
ITER Scenario Plasma Shape

18. Pellet Dropper Initial Results – April 2008
Ohmic L-mode
The 1-mm 10/ms deuterium pellets dropped during L-mode Ohmic plasmas travel in a vertical path and are observed in the divertor Da signals and edge interferometer.
In H-mode NBI plasmas the dropper pellets are observed to “skip” along the SOL and are swept toroidally. No interferometer signals for these pellets – no penetration inside the separatrix.
The largest pellets appear to coincide with ELMs and there may be a cause and effect, which is under investigation.
Drag and fast ion ablation in the SOL are candidates for the strong toroidal deflection and poor penetration.
Pellets with more momentum normal to the plasma are needed with this vertical trajectory.
NBI H-mode Z f
Tangential view from fast framing camera –
images and movies by J. Yu
LRB Crete Jun2008

19. What Causes the Toroidal Defection?
NBI H-mode Z f
Plasma flow in the SOL is measured by CER to be ~ 10 km/s.
Drag estimated from Fd = -1/2 r v2 Cd using Cd = 0.1 for a sphere yields a deflection of only ~ 1cm
NBI was in the co direction yielding fast ions that have trapped orbits into the SOL
Toroidal Force on the pellet is on the order of 1 m-Nt
On the order of fast ions (80 keV) giving up 10% of their energy to the pellet could explain this defection.
A counter NBI experiment is needed to verify this hypothesis.
LRB Crete Jun2008

20. ELM Frequency Increase Due to pellets ?
time (s)
Density (1019 m-3)
Divertor D (a.u.)
PNBI (MW)
Dropper pellets
ECRH
0.0
2.0
1.0 8 4 5 1 2 3
Shot
Dropper pellets injected during ECRH pedestal control experiments :
The ELM frequency is higher (~40 Hz) than without the pellets (~10 Hz)
LRB Crete Jun2008

21. Pellet Dropper Future Plans for DIII-D
Platform
Pellet
Dropper
V+3
at 0o
Microwave
Cavity
The low pellet speed at a glancing angle to the plasma edge limits the effectiveness in triggering ELMs. (No obvious proof of increase in the ELM frequency)
The plan is to modify the entry tube to deflect the pellets so they hit more normal to the plasma surface.
Less SOL to travel through
Higher momentum normal to the plasma. Speed normal to plasma surface to be 10 m/s instead of present 3.5 m/s.
Future modifications as necessary to obtain reliable ELM trigger in the ITER scenario plasma – larger pellets and/or faster. LFS trajectory as planned for ITER.
LRB Crete Jun2008

22. Overview
ITER has a potential significant ELM problem – erosion of divertor looks serious
Methods to mitigate the ELM problem have been proposed:
Pellet ELM pacing
Resonant magnetic perturbations
Pellets injected from all locations are known to trigger ELMs - Why???
DIII-D Pellet Dropper Installed to Investigate ELM pace making
ELM pacing requirements for ITER are challenging
RMP suppresses ELMs on DIII-D, but at high density some ELMs return
Further research into pellets for ELM mitigation is ongoing also at JET and AUG.
LRB Crete Jun2008

23. Challenge of ITER Pellet Fueling and ELM Pacing
ITER will have 2 pellet injectors that each provide D2, DT, T2 pellets ~10Hz, ~30Hz).
Inside wall pellet injection for efficient fueling beyond the pedestal utilizing a curved guide tube. Maximum pellet survival speed is 300 m/s.
Pellet injector must operate for up to 1 hour continuously and produce ~ 1.5 cm3/s (0.3 g/s) of ice.
A LFS tube is being added for pellet ELM pacing. Pellet speed maximum probably ~500 m/s.
Pellet pacing projected to require 3-4mm size pellets at Hz. (A.Polevoi, ITER 2007 DCR)
Pellet Path
in ITER
LRB Crete Jun2008

24. Pellet Normalized Throughput in Present Experiments is Heading
Toward Projected ITER Pellet ELM Pacing Conditions 10
DIII-D
JET 8
AUG
JT-60U
ITER 6
Pellet Throughput/Plasma Volume
(mm3/s-m3)
ITER range covers: – 4 2
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7 1/ t
E fpel
(Time between pellets / tE)
The normalized pellet throughput from current pacing and fueling experiments is far from the projected operating point.
The planned pellet ELM pacing experiments on JET, DIII-D, and ASDEX-U are all planning to be in a regime that is more ITER relevant in terms of normalized throughput and time between pellets scaled to energy confinement time.
LRB Crete Jun2008

25. Pellet Convective Loss of Energy Must be Low to
Maintain Good Confinement During ELM Pacing
Pellet Density Perturbation with no Drift
The planned low field side pellets for ELM pacing on ITER are expected to have shallow penetration.
Drift of pellet mass due to curvature and ÑB is expected to expel nearly all of the pellet particles.
Calculation of the pellet cloudlet reheating during the drift process with PRL+PELLET code shows the entire cloud drifts out after reheating to < 100 eV. (Parks and Baylor, PRL 94 (2005) )
The total convective loss for the 3mm pellet is 4 kJ and 8 kJ for the 4mm pellet. At 40 Hz this is less than 0.5 MW of convective pellet loss.
LFS pellet triggered ELM on D3D has same DW as natural ELM in same plasma.
Ablation
(No drift)
Cloudlet
Dne
Resulting
Deposition
Drift r
1.0
0.9
LRB Crete Jun2008

26. Overview
ITER has a potential significant ELM problem – erosion of divertor looks serious
Methods to mitigate the ELM problem have been proposed:
Pellet ELM pacing
Resonant magnetic perturbations
Pellets injected from all locations are known to trigger ELMs - Why???
DIII-D Pellet Dropper Installed to Investigate ELM pace making
ELM pacing requirements for ITER are challenging
RMP suppresses ELMs on DIII-D, but at high density some ELMs return
Further research into pellets for ELM mitigation is ongoing also at JET and ASDEX-U.
LRB Crete Jun2008

27. Resonant Magnetic Perturbation
Evans, NF2008
RMP created by currents in external coils (I-coil on DIII-D) and results in stochastic magnetic fields in the plasma edge that affect the plasma edge profiles and can keep the edge pressure below the ballooning limit.
LRB Crete Jun2008

28. RMP Using Internal Coils is Successful at Eliminating ELMs
DIII-D RMP and non-RMP Comparison
RMP technique developed is successful in ITER like collisionality and shape at eliminating ELMs. (T. Evans, et al. Nucl. Fusion 48 (2008) 45009).
LRB Crete Jun2008

29. Pellet Fueling During RMP Can Lead to Some ELM Activity
HFS Pellet Fueling (and gas fueling) into RMP ELM suppressed plasmas can lead to a return to ELMs when high density is reached.
Two examples with 5 Hz and 10 Hz 1.8mm HFS pellets are shown. The case with fully suppressed ELMs before the pellets actually has more ELM activity with the pellet fueling.
These results indicate that further optimization of RMP is needed for high density operation.
Density (1013 m-3)
131467
129754 2 4 6 8
0.0
0.2
0.4
0.6
0.8
2.0
3.0
4.0
5.0
1.0
Time (s)
Icoil (kA)
-6.0
Divertor Da (a.u) 3
Full ELM
Suppression
Partial ELM
ELMs ~0.5 DW from pre-RMP
10 Hz 1.8mm Pellets
5 Hz 1.8mm Pellets
This is an integrated effort to understand ELM mitigation using pellet pacing on JET and DIII-D. The studies involve edge plasma diagnostics, pellet diagnostics, modeling, and experimental analysis.
Mitigation of Type III ELMs on ITER would reduce peak heat flux to the divertor and other PFCs while also providing more favorable (more stable, longer density gradient) edge conditions for ICRF.
JET will use high frequency pellet injection. DIII-D will utilize an ORNL pellet dropper, scheduled for installation in August 2006.
Results from both experiments can be extrapolated to ITER.

30. Pellets in RMP ELM-free H-Mode Trigger ELM Like Events
-2.5*10 11
1.9*10 13
3.8*10
5.7*10
7.7*10
density
-5.0*10 6
5.0*10
1.0*10 7
1.5*10
pnbi
-6000
-4000
-2000
2000 iu 5
WMHD
2500
3000
3500
4000
4500
-0.78
0.66
2.10
3.55
4.99
phd02f
Fri Jan 25 07:06:
Pellets in RMP ELM-free H-Mode Trigger ELM Like Events
DIII-D
neL(1014m-2)
PNBI (MW)
Icoil (kA)
-4.4 kA
1.5
1.0
0.5
0.0
WMHD (MJ)
ITER Similar
Shape
Divertor Da
2.0
3.0
4.0
Time (s)
Pellet induces an immediate small ELM and some subsequent ELMs.
Expanded view
of this pellet
LRB Crete Jun2008

31. Pellets in RMP ELM-free H-Mode Trigger Small Immediate ELM Like Events
2.0
DIII-D
1.0
neL(1014m-2)
0.0
17.0
ELM
12.0
Pellet Da
7.0
2.0
110 70
dBr/dt (a.u.)
Outer shelf 30
-10
1.20
~4% DW
WMHD (MJ)
1.10
1.00
ITER Similar
Shape 16 12
Divertor Da 8 4
3.260
3.270
3.280
3.290
Time (s)
Pellet induces an immediate ELM, but much smaller stored energy decrease than occurs with a larger ELM 20ms later.
Further optimization needed to eliminate ELMs altogether.
LRB Crete Jun2008

32. Summary
ELMs are triggered by fueling pellets on DIII-D by the time the pellet reaches half way up the pressure pedestal.
Pellet dropper pellets velocity normal to the plasma is too low to penetrate through the SOL and trigger ELMs reliably.
A change in trajectory is planned and will be used for future pacing studies.
RMP for ELM suppression has been tested with pellet fueling and is found to have some ELM activity with the increased density
Needs further optimization to be compatible with high density operation and pellet fueling.
Fueling pellets also lead to ELMs in QH-mode (Baylor, et al., JNM 2005)
The pellet injection system for ITER ELM pacing presents challenges for the technology in throughput and reliability, gas gun concept looks promising
Tests of convective losses and high repetition rate pellets needed to project to ITER
Extruder and accelerator prototypes will be produced which can be available to test on existing tokamak devices
LRB Crete Jun2008

33. Plans for Future R&D
ELM triggering with vertical pellets on DIII-D – test pellet size and speed normal to the plasma necessary to trigger ELMs.
Investigate ELM size and confinement properties with pellet pacing > 5x natural ELM frequency
Compare DIII-D and JET pellet ELM pacing experiments
New JET pellet injection system utilizes ORNL pellet mass and cloud diagnostics
LFS pellets enter through the RF antenna, interesting coupling effects
Scaling of results to ITER and development of a 3D model for pellet ELM triggering
Future possible upgrade of D3DPI to use high frequency pellets with similar guide tube to ITER – test of newly developed technology for ITER
LRB Crete Jun2008

34. Pellet Throughput in Present Experiments Well Below That
Projected for ITER Pellet ELM Pacing 5
DIII-D 4
JET
AUG
JT-60U
ITER
Pellet Size (mm) 3
Higher
Throughput 2 1 20 40 60 80
100
Pellet Frequency (Hz)
The anticipated pellet throughput for ITER pellet ELM pacing is well above that in present day experiments. (Technically challenging)
A multi-barrel injection system is likely needed to meet this high ITER throughput requirement (Pellet sizes are cylindrical equivalent)
LRB Crete Jun2008

35. Pellet Throughput in Present Experiments Well Below That
Projected for ITER Pellet ELM Pacing
Machine
Pellet Size (mm)
Pellet Frequency (Hz)
Throughput
(Pa-m3/s)
AUG
1.2 83 12
150
23*
DIII-D
1.8 15 8 1 50 4*
JET
4.4 6 44 4
80*
JT-60U
2.4 10
ITER
5.2 16
200* 40
220* 3 20
47*
* - Future
The anticipated pellet throughput for ITER pellet ELM pacing is well above that in present day experiments. (Technically challenging)
A multi-barrel injection system is likely needed to meet this high ITER throughput requirement (Pellet sizes are cylindrical equivalent)
LRB Crete Jun2008