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, USA3 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

ELMs Triggered from Deuterium Pellets Injected ELMs Triggered from Deuterium Pellets Injected into DIII-D and Extrapolation to ITER into DIII-D and Extrapolation to ITER

2

OverviewOverview

• 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

Filaments evolving during ELM on MAST. ( Kirk, et al. PRL 2004. )

• 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.

Edge Localized Mode: periodic instability located at the H-mode edge transport barrier

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4

Size of ELM Loss Predicted for ITER as much as 20 MJ

Full performance ITER plasma W pedestal can be as much as 100 MJ leading to ~20 MJ ELM loss if *ped is the controlling parameter.

Loarte et al., Nuclear Fusion, ITER Physics Basis,Chapter 4

Minimum Acceptable Value (?)

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5

CFC erosion is negligible for QELM < 0.5 MJ/m2

[Zhitlukhin et al., JNM, 2007]

ITER SOL projection on the divertor plates is 3.0 m2

ITER ELM size needs to be < 1.5 MJ for low erosion

Erosion of ITER Divertor Tiles

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6

OverviewOverview

• 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

• Pellets injected from the 5 different injection locations on DIII-D are observed to trigger immediate ELMs.

• LFS injected pellets trigger larger longer lasting D 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.

DIII-D 99474 PoP 2000

Wtot ~ 1% Wtot ~ 4% Wtot ~ 8%

2.21 2.22 2.23 2.24 2.25

2210 2220

0.0

0.4

0.8

1.2

Div

erto

r D

Den

sity

2.4

2.8

3.2

3.6

n eL

(1014

m-2

)(a

.u.)

3.11 3.12 3.13 3.14

Time (s)

2210 2220

0.0

0.4

0.8

1.2

Div

erto

r D

Den

sity

2.4

2.8

3.2

3.6

n eL

(1014

m-2

)(a

.u.)

3.11 3.12 3.13 3.14

Time (s)

2.69 2.70 2.71 2.72 2.73

HFS Vertical LFS

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8

• 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 s time scales.

What Causes an ELM Trigger from a Pellet?

L

necs

Teqe

L

Pe

L

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9

0.00

0.02

0.04

0.06

0.08

0.10

1.5

1.6

1.7

1.8

1.9

-40

-20

0

20

40

60

80

2772.2 2772.4 2772.6 2772.8 2773.0-50

0

50

100

150

200

PelletEntersPlasma

ELMTriggered

ELMEnd

neL(1014m-2)

dBr/dt (a.u.)Inner wall

dBr/dt (a.u.)Outer shelf

Divertor D

HFS Pellet Induced ELM Details from DIII-D

TS profile time

Pellet D

• 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.

DIII-D 1291951.8mm pellet injectedfrom innerwall is ~10%density perturbation

No MHD precursorto pellet induced ELM

Pellet Drepresentsablation of pellet in the plasma with assumed constant radial speed.

Time (ms)

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Te ped (keV)

0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

% P

ed

0.0

0.2

0.4

0.6

0.8

1.0

Non-RMP HFSRMP HFS Non-RMP LFS

ITER Penetration Requirement

ASDEX-U Trigger Range

Non-RMP HFS RMP HFS Non-RMP LFS RMP LFS

10

• 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 47 1166 (2007).)

0

5

10

15

20

25

30

0.6 0.7 0.8 0.9 1 1.1

Pe

(kP

a)

Electron Pressure Pedestal

DIII-D 129195 2.772sPre-pellet and ELM

Pellet Locationwhen ELMTriggered

Depth of Pellet when ELM is Triggered in DIII-D Depth of Pellet when ELM is Triggered in DIII-D is Much Less Than Top of the Pedestalis Much Less Than Top of the Pedestal

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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

OverviewOverview

• 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

• 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

DIII-D Platform

PelletDropper

V+3at 0o

MicrowaveCavity

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14

Pellet Dropper Operation

Pictures of extruded solid deuterium (14K)and pellet chopped fromextrusion

Data from microwavecavity showingmass of each pellet.

15

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

0.7 0.8 0.9 1.0 1.05

n e (1

019 m

-3)

1.8mm 200 m/s

1.0mm 10 m/s

1.0mm 50 m/s

1.0

0.4

0.8

0.6

0.2

0.0

1.2

Te (k

ev)

DIII-D 120775

Pellet Dropper Now Operational on 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.

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16

Target Plasma for the Pellet Dropper is the ITER Scenario

ELMs of this size on ITERcould be 15 MJ per ELM

The dropper is designed to drop 50 Hz pellets to trigger50 Hz ELMs that have smaller energy loss

Can the dropper be used toObtain 5X increase in ELMFrequency?

Density

PNBI

Wtot

Divertor D

ORNL Filterscope Data

Time (s)2.0 3.0 4.01.00.0

E. Doyle 2008

DIII-D ITER Like Scenario

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17

Pellet Dropper Diagnostic Views

• 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) 032504. )

•Filterscope D 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.

Fast Camera Viewing Region

Dropper Pellet Trajectory

ITER Scenario Plasma Shape

Pellet Dropper Initial Results – April 2008

• The 1-mm 10/ms deuterium pellets dropped during L-mode Ohmic plasmas travel in a vertical path and are observed in the divertor D 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. Tangential view from fast framing camera –

images and movies by J. Yu

Ohmic L-mode

NBI H-mode

Z

18LRB Crete Jun2008

What Causes the Toroidal Defection?

• Plasma flow in the SOL is measured by CER to be ~ 10 km/s.

• Drag estimated from Fd = -1/2 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 1011 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.

NBI H-mode

Z

19LRB Crete Jun2008

ELM Frequency Increase Due to pellets ?

• Dropper pellets injected during ECRH pedestal control experiments :

• The ELM frequency is higher (~40 Hz) than without the pellets (~10 Hz)

Shot 132650

2 2.5 3 3.5 4 4.5 time (s)

Density (1019 m-3)

Divertor D(a.u.)

PNBI (MW)

Dropper pellets

ECRH0.0

2.0

1.0

0

0

8

4

5

1

2

34

20LRB Crete Jun2008

DIII-D Platform

PelletDropper

V+3at 0o

MicrowaveCavity

Pellet Dropper Future Plans for DIII-D

• 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.

21LRB Crete Jun2008

OverviewOverview

• 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.

22LRB Crete Jun2008

23

• ITER will have 2 pellet injectors that each provide D2, DT, T2 pellets (5mm @ ~10Hz, 3mm @ ~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 20-40 Hz. (A.Polevoi, ITER 2007 DCR)

Challenge of ITER Pellet Fueling and ELM Pacing

Pellet Path in ITER

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24

1/ E fpel

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Pel

let

Thr

ough

put/

Pla

sma

Vol

um

e

(mm

3/s

-m3)

0

2

4

6

8

10

DIII-DJETAUGJT-60U ITER

Pellet Normalized Throughput in Present Experiments is Heading Toward Projected ITER Pellet ELM Pacing Conditions

• 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.

ITER range covers:3mm@20Hz – 4mm@40Hz

(Time between pellets / E)

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25

Pellet Convective Loss of Energy Must be Low to Maintain Good Confinement During ELM Pacing

• 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 Bis 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) 125002.)

• 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 W as natural ELM in same plasma.

0.7 0.8 0.9 1.0

0

2

4

6

8

10

Te

(keV

),

n e(1

019

m-3

)

Te

5mm300 m/s

3mm300 m/s

2mm300 m/s

0.7 0.8 0.9 1.0

0

2

4

6

8

10

Te

(keV

),

n e(1

019

m-3

)

Te

5mm300 m/s

3mm300 m/s

2mm300 m/s

Ablation(No drift)

Cloudlet

ne

ResultingDeposition

Drift

1.00.9

Pellet Density Perturbation with no Drift

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26

OverviewOverview

• 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

• 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.

Evans, NF2008

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28

RMP Using Internal Coils is Successful at Eliminating ELMs

• RMP technique developed is successful in ITER like collisionality and shape at eliminating ELMs. (T. Evans, et al. Nucl. Fusion 48 (2008) 45009).

DIII-D RMP and non-RMP Comparison

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29

• 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.

Pellet Fueling During RMP Can Lead to Some ELM Activity

0

Density (1013 m-3) 131467

Density (1013 m-3) 129754

0

2

4

6

8

131467

0.0

0.2

0.4

0.6

0.8

129754

0

2.0 3.0 4.0 5.01.00.0 5.0

Time (s)

Icoil (kA)0.0

-6.0

Divertor D(a.u)

Divertor D(a.u)

6

0

3

Full ELMSuppression

Partial ELMSuppression

ELMs ~0.5 W from pre-RMP

10 Hz 1.8mm Pellets

5 Hz 1.8mm Pellets

30

Pellets in RMP ELM-free H-Mode Trigger ELM Like Events Pellets in RMP ELM-free H-Mode Trigger ELM Like Events

-2.5*10 11

1.9*1013

3.8*1013

5.7*1013

7.7*1013

density 129194

-5.0*106

0

5.0*106

1.0*107

1.5*107

pnbi 129194

-6000

-4000

-2000

0

2000

iu30 129194

-5.0*10 5

0

5.0*105

1.0*106

1.5*106

WMHD 129194

2000 2500 3000 3500 4000 4500

-0.78

0.66

2.10

3.55

4.99

phd02f 129194

Fri Jan 25 07:06:38 2008

• Pellet induces an immediate small ELM and some subsequent ELMs.

Time (s)

neL(1014m-2)

WMHD (MJ)

Divertor D

Icoil (kA)

PNBI (MW)

2.0 3.0 4.0

-4.4 kA

0.0

0.5

1.0

1.5

DIII-D 129194

ITER SimilarShape

Expanded view of this pellet

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31

Pellets in RMP ELM-free H-Mode Trigger Small Immediate Pellets in RMP ELM-free H-Mode Trigger Small Immediate ELM Like Events ELM Like Events

• 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.

Pellet D

neL(1014m-2)

Divertor D

WMHD (MJ)

dBr/dt (a.u.)Outer shelf

3.260 3.270 3.280 3.290

2.0

1.0

0.0

1.20

1.00

1.10

Time (s)

DIII-D 129194

4

8

12

16ITER SimilarShape

70

110

30

-10

2.0

7.0

12.0

17.0

ELM

~4% W

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32

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

Summary

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33

Plans for Future R&DPlans 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

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34

Pellet Frequency (Hz)

0 20 40 60 80 100

Pel

let S

ize

(mm

)

0

1

2

3

4

5

DIII-D JET AUG JT-60U ITER

Pellet Throughput in Present Experiments Well Below ThatProjected for ITER Pellet ELM Pacing

• 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)

HigherThroughput

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35

Pellet Throughput in Present Experiments Well Below ThatProjected for ITER Pellet ELM Pacing

• 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)

Machine Pellet Size (mm) Pellet Frequency (Hz)

Throughput

(Pa-m3/s)

AUG 1.2 83 12

1.2 150 23*

DIII-D 1.8 15 8

1 50 4*

JET 4.4 6 44

4 15 80*

1 50 4*

JT-60U 2.4 10 12

ITER 5.2 16 200*

4 40 220*

3 20 47*

* - Future

LRB Crete Jun2008