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Announcements Director's Corner C.M. Greenfield Topical Group Research Highlight D.G. Whyte, H.S. Barnard, Z.S. Hartwig, et al. First Erosion and Redeposition Measurements of in situ Plasma-facing Surfaces using AIMS on Alcator C-Mod ITPA Update Schedule of Burning Plasma Events Contact and Contribution Information
26th Meeting of the ITPA Diagnostics Topical Group
May 19–22, 2014 at Pohang University of Science and Technology (POSTECH), Republic of Korea
Registration forms are available at the meeting website:
by C.M. Greenfield
Construction is moving along at a good pace now, with concrete pouring having begun in earnest in the Tokamak Complex. When completed, this will be 120 meters long, 80 meters wide, and 80 meters tall, and weigh 360,000 tons. It will house the ITER tokamak as well as the diagnostic and tritium systems. Many other buildings on the ITER site are also under construction. Most notable to visitors may be the extension to the ITER Headquarters Building, which will increase its capacity from 500 to 850 people.
Many other buildings are under construction at the ITER site, and Fusion for Energy, the EU domestic agency, recently signed contracts for construction to begin another seven.
(Left) Close to 1,000 cubic meters of concrete were employed in filling a 638 square-meter plot (P14) in the northeast corner of the Tokamak Pit. (Right) The Headquarters Building expansion is well underway. Photos © ITER Organization
The early-year ITPA meeting season is in full swing. The Scrape-off Layer and Divertor and MHD, Disruption and Control topical groups have already met with the remaining five groups meeting in the coming weeks:
• Energetic Particles meets March 31–April 3 in Madrid, Spain
• Integrated Operational Scenarios meets March 31–April 3 at MIT
• Transport and Confinement meets April 7–11 at MIT
• Pedestal and Edge Physics meets April 14–17 in Prague, Czech Republic
• Diagnostics meets May 19–22 in Korea
In the coming months you can expect to see and hear reports on these meetings via the eNews and our web seminar series.
Postings for ITER positions
ITER regularly posts their openings at http://www.iter.org/jobs. There are currently eight positions shown there, including one for a computational plasma physicist that may be of interest to our readers. The application deadline is April 23.
Research Highlights are selected by the leaders of the BPO Topical Groups on a rotating basis. The BPO Pedestal and Divertor/SOL Topical Group facilitates U.S. efforts to understand the boundary region of magnetic fusion devices through experiment and simulation (leaders are Rajesh Maingi and Peter Stangeby). For many years the fond but apparently hopeless dream of tokamak edge diagnosticians has been to find a way to take an MeV ion beam analysis accelerator inside a tokamak to measure the erosion and deposition of the wall and target tiles in situ. Dennis Whyte, Zach Hartwig and Harold Barnard, however, have now found a way to do this using a compact Radio-Frequency Quadrupole (RFQ) linear accelerator situated outside the vessel, with the tokamak coils being used to steer the beam to the desired location at the wall. This diagnostic development has the potential to bring about a sea change in tokamak plasma materials research, equivalent to the impact that Thomson scattering measurements of density and temperature have had on tokamak plasma physics research.
First Erosion and Redeposition Measurements of in situ Plasma-facing Surfaces using AIMS on Alcator C-Mod
D.G. Whyte, H.S. Barnard, Z.S. Hartwig, B.N. Sorbom, D. Terry, R.C. Lanza, P.W. Stahle, and the Alcator C-Mod Team
MIT Plasma Science and Fusion Center, Cambridge MA 02139
MIT Nuclear Science and Engineering Department, Cambridge MA 02139
Erosion and deposition of plasma-facing components (PFC) is a major concern for the lifetime and viability of long-pulse burning plasma experiments like ITER, FNSF or reactors. This concern stems from the fact that the PFC must remain thin, ∼several mm, in order to support heat removal through conduction, yet it is simultaneously subjected to enormous particle fluences. For example, a burning plasma strikepoint PFC will see approximately 5 × 1031 ions/m2/year, yet the total thickness of the mm PFC material is only 5 × 1025 atoms/m2; indicating a net removal probability of < 10−6 atoms per incident ion must be achieved in order to not expend the PFC. This effective yield is so low it may require the suppression of sputtering through plasma temperature reduction. A more difficult problem than erosion may be the viability of PFC surfaces where deposition occurs, because these plasma-deposited layers tend to have poor thermal properties and likely threaten the plasma with large-scale layer failure. Overall, erosion and deposition mitigation in burning plasmas will require both a comprehensive understanding of the controlling physics and a large suite of control tools.
Complexity and diagnosis are the major, and coupled, challenges to meet this goal. Complexity results from the orders of magnitude variation in plasma conditions at different PFC surfaces in a tokamak, the details of both near-surface and long-range turbulent plasma impurity transport, and the evolving nature of the surfaces themselves. Diagnosis is challenging because net erosion/deposition occurs due to relatively small imbalances between incoming impurity ion flux, resulting from impurities entrained in the plasma, and outgoing atomic flux, resulting from sputtering. Thus the net changes in surface depth and composition cannot be measured by plasma diagnostics and must be measured directly at surfaces; however to date we have very limited spatially and temporally resolved surface diagnosis. Dedicated surface probes are limited to a minute fraction of the wall, and surface instruments cannot be mounted on plasma-facing surfaces that receive large heat flux. Global surveys of PFC surfaces are possible with their physical removal from devices and ex situ ion beam analysis, but this is limited at best to campaign integrated values, where one cannot correlate plasma conditions to the erosion/deposition physics. In reality the expense and interruption to plasma operations of physically removing large fractions of the plasma-facing surfaces inhibits this measurement strategy in present experiments and in burning plasmas like ITER.
It is in the context of these measurement challenges that the Accelerator-based In-situ Material Surveillance (AIMS) diagnostic has been developed [1,2]. The essence of AIMS is adapting ion beam analysis, the preferred ex situ technique, to provide temporally and spatially resolved measurements of surfaces in a fusion environment. The AIMS technique has been prototyped on Alcator C-Mod as shown in Fig. 1. A compact accelerator injects a 900 keV deuteron ion beam into the tokamak between shots. The beam is steered, poloidally and toroidally, to PFC surfaces by low-amplitude B fields produced by the tokamak coils. The D+ induces nuclear reactions with isotopes in the surfaces, and remote, shielded detectors measure the penetrating gammas and neutrons that result. Proper interpretation of the gamma/neutron spectra can thus provide shot-to-shot, 2-D “maps” of the PFC surface properties, e.g. measurements of deuterium fuel retention in PFCs using AIMS . Here we describe the first in-situ measurement of boron erosion/deposition using AIMS. Boron is a natural choice for these first studies: boron films are intermittently applied to the high-Z molybdenum C-Mod PFCs, and it has nuclear reactions well suited for AIMS. However it should be noted that AIMS can be applied to measure the isotope concentrations of many other PFC materials such as lithium, carbon and beryllium.
Figure 1: Schematic of the AIMS surface diagnostic on Alcator C-Mod. The trajectories show the steering of the D ion beam by low magnitude B fields to four chosen locations at the inner wall and divertor.
Figure 2 is a demonstration of time and spatially resolved boron erosion and deposition in C-Mod at four locations along the inner wall and divertor as various wall conditioning techniques are applied. In this case the down- scattered neutrons from D-boron reactions are used; this neutron continuum is useful because it provides high count rates. As a result, the relative uncertainty in each measurement is down to ∼2-5% and the relative net change in boron thickness is readily followed. The plasma exposures clearly alter the boron film pattern. For example, the rate of boron deposition is much larger at the lower divertor, which is attributed to the fact that the electron cyclotron resonance during the boronization was located at a major radius just inside the R of the inner divertor plate. Thus ionized boron, undergoing ExB drift, predominately produces thick B films at larger R, in qualitative agreement with previous studies of boron deposition . The wall cleaning techniques of ECDC and GDC strongly affect the boron thickness in some locations, but not in others. Thus even simple wall conditioning techniques produce rather complex patterns of erosion and deposition, motivating the requirement for AIMS measurements.
Figure 2: Normalized boron thickness on C-Mod molybdenum tiles at four inner wall/divertor locations (right) through a sequence of wall conditioning events: application of B films, electron-cyclotron discharge cleaning (ECDC) and glow-discharge cleaning (GDC). The boron thickness is normalized to the data point taken before a vacuum break ad the removal of the tiles for external analysis. Note the varying vertical scales.
A section of C-Mod tiles measured by AIMS were removed and analyzed by ex situ ion beam analysis in the PSFC CLASS accelerator facility . The 2-D map of the inner wall boron thickness is shown in Fig. 3 with the AIMS measurement locations overlaid. One notes again the complex pattern of boron thicknesses present. The typical B film thickness is ∼500 nm. AIMS measurements of Fig. 2 indicate erosion/deposition sensitivity down to ∼10 nm.
Figure 3: Map of boron thickness from ex situ ion beam analysis of a C-Mod inner wall segment of Mo tiles after 2012 operations. Insert: photograph of tiles. Circles indicate location #1 and #2 (Fig. 2) measured in situ by AIMS.
Gammas produced by D-boron reactions were used to provide quantitative measurements of the boron thickness from AIMS, in addition to relative boron thickness from neutron measurements. The absolute thickness was calculated with no free parameters by using knowledge of the AIMS ion beam, the gamma-producing nuclear cross-sections, detector geometry, and detection efficiency. The gamma spectra thus provides absolute boron thickness but with larger relative uncertainty than the neutrons and with lower measurement frequency. The result of combining these two techniques is shown in Fig. 4, where the absolute B film thickness at the inner wall is tracked through both plasma operations and wall conditioning. The absolute B thickness from AIMS agrees very well with the ex situ analysis of the tiles at the same location, verifying AIMS as an absolute measurement technique . Also critical, one can see that ∼second-long tokamak discharges produce changes in the boron thickness similar to ∼hour-long wall conditioning. This stems from the extremely high flux densities and plasma temperatures involved in the C-Mod tokamak discharges. As an example of this intense plasma-material interaction, 18 lower single-null (LSN) discharges with RF-heated I-mode produced a ∼200 nm increase in the boron thickness (Fig. 4). The inferred redeposition rate is ∼5 nm/s at this location, which is equivalent to ∼15 cm/year! (a rate which would be unacceptable in a long-pulse device) The AIMS measurements also show that this location can switch from being in net deposition to being in net erosion just by changing the plasma topology to inner-wall limited (Fig. 4). A glance at Fig. 3 and Fig. 4 indicates the extreme difficulties, if not futility, of trying to understand PMI processes like erosion/deposition from campaign-integrated measurements.
Figure 4: Evolution of the AIMS measured boron thickness at the inner wall (location #1 of Fig. 2) through various RF-heated plasma shots and wall conditioning events. Time goes from left to right however the axis is not indicative of plasma exposure time. Error bars are the uncertainty in absolute boron thickness. The neutron continuum rates have been statistically normalized to the absolute B thickness from gamma photopeak measurements. The final AIMS measurement of B thickness ∼500 nm matches ex situ ion beam analysis within uncertainty.
It should be noted that these represent the very first results from AIMS. Nevertheless this initial glimpse at PMI processes in the C-Mod tokamak clearly compels further investigations. To that end the AIMS diagnostic is presently undergoing several upgrades to substantially improve its sensitivity, its PFC coverage and its time resolution. In addition, depth marker techniques are being developed in the CLASS laboratory  which would enable erosion/redeposition of bulk PFC surfaces using AIMS. An optimized AIMS diagnostic will provide exciting new opportunities for surface measurements and comparison to boundary/PMI models.
The authors are grateful to the outstanding technical and engineering staff at Alcator C-Mod and the PSFC. The work is supported by a diagnostic development grant U.S. DOE Grant No. DE-FG02-94ER54235, and Cooperative Agreement No. DE-FC02-99ER54512.
Z.S. Hartwig, et al.,
Rev. Sci. Instrum. 84, 123503 (2013)
 H.S. Barnard, “Development of accelerator based spatially resolved ion beam analysis techniques for the study of plasma-material interactions in magnetic fusion devices,” Ph.D. Thesis, Department of Nuclear Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA (2014)
 Z.S. Hartwig, et al., “First measurements of deuterium retention using Accelerator-based In-situ Materials Surveillance (AIMS) on the Alcator C-Mod tokamak,” USBPO eNews, Issue 73 (June 2013)
 R. Ochoukov, et al., Fusion Eng. Des. 87, 1700 (2012)
 H.S. Barnard, B. Lipschultz, and D.G. Whyte, J. Nucl. Mater. 415, S301 (2011)
 R. Sullivan, et al., Nucl. Instrum. Meth. B 319, 79 (2014)
More information concerning the ITPA may be found at the Official ITPA Website.
|Diagnostics Topical Group|
|26th Meeting, Pohang Univ. of Sci. and Tech., Republic of Korea, May 19–22, 2014|
|Energetic Particles Topical Group|
|12th Meeting, Madrid, Spain, March 31–April 3, 2014|
|Integrated Operation Scenarios Topical Group|
|12th Meeting, Massachusetts Institute of Technology, Cambridge, MA, United States, March 31–April 3, 2014|
|MHD, Disruptions, and Control Topical Group|
|23rd Meeting, Toki, Japan, March 10–14, 2014|
|Pedestal and Edge Physics Topical Group|
|26th Meeting, IPP, Prague, Czech Republic, April 15–17, 2014|
|Transport and Confinement Topical Group|
|12th Meeting, Massachusetts Institute of Technology, Cambridge, MA, United States, April 9–11, 2014|
|2014 — NSTX-U First Plasma —|
|March 31–April 3, ITPA: 12th Meeting of the Energetic Particles Topical Group, Madrid, Spain|
|March 31–April 3, ITPA: 12th Meeting of the Integrated Operation Scenarios Topical Group,|
|Massachusetts Institute of Technology, Cambridge, MA, United States|
|April 9–11, ITPA: 12th Meeting of the Transport and Confinement Topical Group,|
|Massachusetts Institute of Technology, Cambridge, MA, United States|
|April 15–17, ITPA: 26th Meeting of the Pedestal and Edge Physics Topical Group,|
|IPP, Prague, Czech Republic|
|April 22–25, Transport Task Force (TTF) Meeting, San Antonio, TX, United States|
|April 22–25, 18th Joint Workshop on Electron Cyclotron Emission and Electron Cyclotron Resonance Heating (EC–18), Nara, Japan|
|May 19–22, ITPA: 26th Meeting of the Diagnostics Topical Group|
|Pohang University of Science and Technology, Republic of Korea|
|June 1–5, 20th Topical Conference on High-Temperature Plasma Diagnostics (HTPD), Atlanta, GA, United States|
W7-X First Plasma
ITER First Plasma
JT60-SA First Plasma
ITER Full DT Operations
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