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DIII-D Edge physics overview

A.Leonard for the Plasma Boundary Interface Group

Presented at the PFC MeetingUCLA, August 4-6, 2010

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Plasma Boundary Interface 2010 Experimental Topics

• PBI was allocated 6.5 days of experimental time• Complete Joint Research Target on Boundary heat flux

– Complete parameter scan of divertor heat flux width– C-Mod comparison– Measure fluctuation driven radial heat transport in boundary

• Fuel Retention and Clean-up– Determine fraction of injected fuel tightly bound in vessel wall– Utilize bake in oxygen to remove fuel co-deposited with eroded carbon

• DiMES– Understanding divertor carbon chemical erosion

• ITER first wall design issues– Heat flux profile onto startup/rampdown limiters– ELM heat flux into secondary divertor

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Boundary Heat Flux Joint Research Task Completed DIII-D Scaling Studies

• Completed divertor heat flux width scaling– Power; No dependence– Collisionality; No dependence– Toroidal Field; Weak, or no dependence– Plasma Current; Inverse Dependence

• DIII-D Heat flux scaling q(cm, midplane) ~ 0.7/Ip (MA)

– implies for a diffusive model

• Size scaling to be tested across devices– A diffusive model of transport would imply

q∝R1/2

• Further Analysis– Power balance, radiated power– Statistical analysis

Heat Flux Width

Bp 1Bt

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DIII-D Divertor Heat Flux Compared with Alcator C-Mod

• A comparison between DIII-D and Alcator C-Mod has been completed– Comparison at same shape and q95

– Density scan matched collisionality– Power scan indicated no heat flux width

dependence on power; lends confidence to the noisier data low power for matched edge conditions

• Initial C-Mod data has been acquired– Initial estimate of C-Mod width indicates a

positive size scaling

• Additional analysis with C-Mod underway– Examine size scaling of divertor heat flux widths

and upstream profiles to determine consistency with diffusive transport, or a constant gradient

C-Mod Shape in DIII-D

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Characteristics of Near SOL Fluctuations to Offer Insight into Heat Flux Transport Mechanisms

• Initial set of measurements from midplane Langmuir probe of fluctuation driven heat transport– Initial data set from midplane and X-point

probe– Plasma current and density scan

• Analysis has only just begun– Examine correlation of turbulence region with

divertor heat flux footprint– Dominance of convective transport could

indicate interchange-like transport– Planning comparison with turbulence codes

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Heat Flux Joint Research Task Summary

• A complete parameter scan taken– Initial analysis indicates Ip dominant parameter affecting width

– Detailed statistical analysis tasks remain

• Upstream profiles acquired– Thomson data unclear if adequate to discern trends– Apply other analysis techniques to Thomson analysis– Upstream probe data yet to be fully compared with divertor heat flux

• Size scaling analysis underway– Comparison with C-Mod awaits further C-Mod analysis– Good dataset from JET/DIII-D pedestal comparison also to be

analyzed

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Goal of Fuel Retention Studies: Determine How Much Fuel Tightly Retained and Clean It Up

• Dynamic particle balance indicates most fuel is retained during the plasma ramp-up phase– Successful comparison of ‘static’ to ‘dynamic’ particle balance– Little retention during H-mode phase

• A bake after operations removed much of the retained fuel– A day of continuous particle balance preceded and followed by a

bake

• A successful Oxygen bake– Nothing destroyed(by oxygen)– Rapid restoration of high performance discharges– Removed expected fraction of carbon deposits and deuterium

co-deposits

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Typical Dynamic Balance has Phases of Retention

• Phase-I:Pre-fill & Breakdown

– No net retention• Phase-2: Ramp-up

– Large wall uptake– Dominated by puffing

• Phase-3: early H-mode– ELM-free; ne build-up– Net wall release

• Phase-4: H-mode steady-state

– No measurable retention

Phase-IPhase-2 Phase-3 Phase-4

Time (msec)

ΓIN [Torr-L/s]

Γwall [Torr-L/s]

∫Γwall [Torr-L]

Qpump [Torr-L/s]

Wall uptake

Wall inventory

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Pumping Speed Compares Well with Cryopump Regeneration

Set #1 Set #2 Set #3 Set #4 Set #5

5% 4% 4% 2% 0.5%

Exh

aust

ed P

arti

cles

[T

orr

-L]

Comparison of Time-resolved vs Shot-integrated Exhausted Particles

• Careful calibration of dynamic balance diagnostics completed

• Multiple shots used for better accuracy in static balance

• Allows confidence in dynamic balance results

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Complete Run-day Analysis Gives ~ 20% Gas Retention

• Static balance used capacitance manometer– Multiple shots to increase accuracy; Error ~ 1.5%

• Dynamic balance for whole shot (ramp-up & steady-state H-mode)

4 shots4 shots

1 shot

Cumulative Exhaust vs Injected Particles for a Full Run-day

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Vacuum Bake Before and After Single Run-day Used to Deplete and Recover Wall Inventory

• Goal is to determine fraction of injected particles than can be easily removed

• Pre-operation vacuum bake to deplete recoverable wall inventory• Post-operation vacuum bake after run day to recover loosely

bound particles – Limited bake time (i.e., pressure did not turn over)

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Vacuum Bake Before and After Single Run-day Returned Large Fraction of Retained Particles• Particle balance summary

– Total injected : 2400 [torr-L]

– Exhausted: 1010-1140 [torr-L]

– Bake released: 1090 [torr-L]

• Post-bake retention/total injected

– 170-300 [torr-L]/2400 ~7-12%

7-12%

Would more be released from higher/longer bake?

–But co-dep removal starts ~700°C

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Co-deposited D Could Account for Fuel Left After Vacuum Bake

• D left in machine– 1.2-2.1 x1022 D atoms (170-300 Torr-L)

• Co-deposition retention estimate:

• Net C erosion yields: – Yphys~1%, Ychem~2%

• From experiment: ~ 4x1023 D/s

• Assumed D/C ratios:– D/CHard ~ 0.1-0.3; D/CSoft ~ 0.7-1.4

• RetHARD: 1.4-4.3 x 1021 D atoms• RetSOFT: 1.5-3 x1022 D atoms

= 165sec

Deposition Pattern in Lower Divertor Region

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Oxygen Bake Goals

• Demonstrate an oxygen bake on the DIII-D tokamak and recover high performance plasma operation (with only clean vents).– Assess “collateral damage” to tokamak systems– Operate tokamak systems – Pumps, ECH, ICH– Demonstrate 13C removal on a few inserted tiles– Measure reaction products – RGA and FTIR

• Demonstrate removal of 13C from several tiles with a second oxygen bake– DiMES will be only indication that 13C deposited during 10-15 repeat

plasma shots– Oxygen Bake– Tiles removed for analysis at start of LTOA

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Oxygen Bake Timeline

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The Heat Flux to the Secondary Divertor is a Significant Concern for ITER

• ITER will create secondary divertor when operating at high triangularity

• Primary concern is the ELM heat flux which can broader profile than the steady heat flux between ELMs

• Local recycling and SOL interaction also a concern due to re-deposition of eroded beryllium and co-deposition of tritium

138229 3745.00

BXB

IRTV

Secondary divertor

Primary divertor

Langmuir probes

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Secondary divertor heat flux profile between ELMs is relatively broad but peaks at strike point

• The steady state heat flux in the secondary divertor (between ELMs) is:– peaked at the

strike point with typical initial spatial decay

– broader and more uneven than typical primary strike point profiles

IRTV secondary divertor steady state heat flux

EFIT OSP2

EFIT ISP2

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Target plate heat flux profiles during an Elm show large peaks in the far SOL

• Large ELM peaks far outside strike-point– Not tied to physical

features; tile gaps, or thin layers

– Smaller ELMs have narrower profile

• Future analysis– ELM power balance;

thermocouples, probes

– Radial decay of ELM heat flux versus ELM size

Δt = 0.2 msec Δt = 0.1 msec Δt = 0.0 msec

Δt = 0.3 msec Δt = 1.0 msec

Tile gaps

EFIT OSP2

EFIT OSP2

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Scaling of Limiter Heat Flux Consistent with ITER Assumptions

• Limiter heat flux examined to test ITER first wall design assumptions• Measured widths are within range of assumed scaling, though parameter

dependence not observed