DIII-D Edge physics overview
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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