This work is supported by U.S. DOE Grant No.DE-FG02-94ER54235 and Cooperative Agreement No. DE-FC02-99ER54512
Accelerator-based In situ Material Surveillance (AIMS) is providing first-of-its-kind ���in situ time-resolved 2-D maps of the plasma-facing surface properties in fusion devices ���
and has been successfully prototyped on the Alcator C-Mod tokamak
New In Situ Measurements for Plasma Material Interaction Studies in Alcator C-Mod D.G. Whyte1*, H.S. Barnard1, Z.S. Hartwig1, B.N. Sorbom1, P. Stahle1, D.R. Terry1, R.C. Lanza2 and the Alcator C-Mod Team 1 MIT Plasma Science and Fusion Center, Cambridge, MA, USA 2 MIT Nuclear Science and Engineering Dept. Cambridge MA USA *[email protected]
Motivation to develop in situ surface measurements • Plasma-material interactions (PMI) pose a severe science and technology challenge for ITER and
fusion reactors • PMI studies are severely hampered by a nearly complete lack of in situ, time-resolved quantitative
measurements of plasma-facing surface properties. • Critical PMI quantities such as erosion, material migration, and hydrogenic/tritium fuel retention
are the net effects of PMI à can only be obtained through surface diagnosis. • Ion beam analysis (IBA) using ~MeV ions is the standard ex situ surface diagnostic
Ø Provide non-destructive, depth resolved information of the element/isotope concentrations.
AIMS adapts ion beam analysis to fusion devices1,2,3
• ~MeV deuteron ion beam from compact RFQ accelerator injected into tokamak between shots • Beam is steered with intrinsic B fields to surfaces of interest • Deuterons induce high-Q nuclear reactions with isotopes in ~10 microns of surfaces • Penetrating neutrons and gammas measured with remote shielded detectors.
AIMS on Alcator C-Mod
Steady-state low magnitude toroidal and vertical B fields provide AIMS with poloidal and toroidal spatial resolution of axisymmetric plasma-facing surfaces1
!
Present TF power
supply limit
Beam focusing provides ~cm spatial resolution of the C-Mod surfaces appropriate for PMI studies1
• Dedicated ion beam model
developed to deal with space-charge effects and 3-D B field of the tokamak.
• Spatial resolution at divertor surfaces will be improved with future upgrades using active beam focusing
• PMI spatial patterns have ~cm scale-lengths from both plasma gradients & tile sizes.
Thickness of boron films measured with AIMS is validated with ex situ analysis of C-Mod tiles
25 mm
B=0T
B=0.05T
!
Particle Induced Gamma Emission (PIGE) Boron Film thickness measurement
Ex situ Map of Boron Thickness at C-Mod center-post shows complex
spatial patterns3
Sample gamma spectrum from��� AIMS on C-Mod3
• Excellent agreement to ex situ ion beam analysis performed at end of run campaign • Temporal evolution of boron at single location is complicated switching from erosion to deposition • I-mode shots: ~6-7 nm/s of deposition à ~ 20 cm per year! (Unacceptable in reactor) • Same location switches to net erosion with change to RF-heated inner-wall limited shot • Despite ~hr long exposure, wall conditioning has weaker effect on boron thickness (see below) • IT IS CLEARLY FUTILE TO UNDERSTAND THIS EVOLUTION BY A SINGLE
MEASUREMENT EVERY YEAR! IN SITU SURFACE DIAGNOSIS IS NEEDED TO UNDERSTAND PMI SCIENCE
• In general the deuterium retention follows application of boron films, i.e. the retained
deuterium in the C-Mod boron films is due to co-deposition during BZN itself • Boronization used electron cyclotron location just off of the center post. • Strongest boron deposition rate is at larger R “entrance” to the inner divertor which is
explained by ExB drift of boron ions away from EC resonance, consistent with previous boronization studies4
• EVEN “SIMPLE” WALL CONDITIONING PRODUCES A COMPLEX PATTERN OF EROSION, DEPOSITION AND FUEL RETENTION.
Boron film evolution measured on shot-to-shot timescale and during wall conditioning for first time 1,3
Wall conditioning effects the spatial pattern of Boron film thickness and Deuterium fuel retention
REFERENCES [1] HARTWIG, Z.S. et al. “An in situ accelerator-based diagnostic for plasma-material interactions science on magnetic fusion devices,” Review of Scientific Instruments 84 12503 (2013). [2] MARMAR E.S. et al 2007 Fusion Sci. Technol. 51 261(2007). [3] BARNARD, H.S. Ph.D. Thesis, MIT (2014). [4] OCHOUKOV, R. et al. Fusion Eng. Des. 87 1700 (2012).
Upgraded TF power supply for 2015 (only one B polarity shown, but both available)
AIMS in situ measurement locations
100
200
300
400
500
600
�Boron Evolution Measured with AIMS (B = 0 T)
Ex-situ Analysis
18 LSN I-m
ode shots
2 IWL shots + 1 disruption
2 IWL shots
Boronization
ECD
C
ECD
C
GD
C
1 Month dow
n time
AIMS NeutronsAIMS Gammas
Bor
on T
hick
ness
[nm
]
Ex-situ Analysis
Plasma shots Wall conditioningTime
C
D
