USBPO Mission Statement:

Advance the scientific understanding of burning plasmas and ensure the greatest benefit from a burning plasma experiment by coordinating relevant U.S. fusion research with broad community participation.

Director's Corner

by C.M. Greenfield

ITER news

As we all know, ITER is being built in the south of France, inland and uphill from Marseille. Since pieces of ITER are being manufactured all over the world, transporting the larger components poses obvious challenges. As a matter of fact, in preparation for arrival of some of the largest components, many roads had to undergo significant modification to handle the large dimensions and heavy weights anticipated. The culmination of these preparations occurred last week, when a test convoy delivered a load as heavy as two fully loaded Boeing 747s on a special 88-axle vehicle that is 46 meters long, 9 meters wide, and 10 meters high. The 65-mile trip from Étang de Berre (a small inland sea) took four nights at 5-15 km/hour, arriving at the ITER side early last Friday morning (September 20). To learn more, please go to http://www.iter.org/newsline/282/1706.

I reported in a previous column that the ITER Headquarters building was completed and occupied at the end of 2012. The present building has room for 550 occupants. Construction is now starting on an expansion of that facility to house 900 workers. The expansion should be completed by May, 2014. More information on the expansion is available at http://www.iter.org/newsline/282/1698.

Truly a “convoy exceptionelle:” The test convoy arrives at the ITER site early in the morning on Friday, September 20 (Photo ©ITER Organization)

Preparations are also underway for next month’s STAC-15 (Science and Technology Advisory Committee) meeting at ITER Headquarters. This week, a committee of experts from the various ITER parties is meeting at ITER to review the ITER Research Plan in light of the various schedule changes and procurement deferrals that have taken place. This addresses one of the six charges to the STAC formulated by the ITER Council at its twelfth meeting in June. The full list (condensed version) of charges is as follows:

1. Assess the technical aspects of the Level-0 Reference Schedule
2. Assess the progress in updating the ITER Research Plan with expert community input
3. Assess progress in the analysis of carbon and tungsten divertor options and provide recommendations for the first ITER divertor choice
4. Assess progress in the design and prototype manufacturing of the in-vessel coils as well as in the plans for the associated Final Design Review
5. Assess progress in the characterization of disruptions and runaways and their mitigation
6. Assess status reports on the progress made on key open technical issues (includes ECH, ICH, neutral beams, and neutronics)

Plans for APS-DPP conference

For the sixth straight year, the US Burning Plasma Organization has organized a contributed oral session on Research in Support of ITER at the 55th Annual Meeting of the Division of Plasma Physics, which will take place in Denver, Colorado, on November 11-15. The agenda for this session can be found here: http://meetings.aps.org/Meeting/DPP13/SessionIndex2/?SessionEventID=200907

The USBPO is also organizing a Town Meeting on ITER, scheduled for Thursday evening (November 14) in the Sheraton Denver Downtown Hotel. We will have a compelling program, focusing on two ITER design decisions to be formalized late this year. The agenda will be as follows:

Richard Pitts (ITER Organization): Physics basis and design of the ITER full tungsten divertor

Edward Daly (Thomas Jefferson National Accelerator Facility): ITER In-Vessel Coils - Design and Status

Ned Sauthoff (US ITER Project Office): US ITER project status

Webinars

A USBPO webinar was held on September 25, with a presentation on “Challenges and R&D needs for combined thermal and magnetic energy mitigation in ITER” given by John Wesley of General Atomics. Approximately 30 sites participated (we have no way to tell how many people at each site) in the meeting. We are now working to arrange our next web seminar; scheduling around the busy meeting schedule in October and November will be a challenge.

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USBPO Topical Group Highlights

[The BPO Fusion Engineering Science Topical Group informs the members of the BPO of ongoing efforts in fusion technology and the relationship to research in other areas of the fusion program (leaders are Russ Doerner and David Rasmussen). This month's Research Highlight by M.J. Baldwin describes experiments conducted in the PISCES lab at UCSD to measure the effectiveness of baking in removing deuterium from beryllium layers coated onto a tungsten substrate. The results compare favorably with simulations from the TMAP code, contributing to the validation of that code as a predictive model for ITER. - Ed.]

Toward a Predictive Capability for Hydrogen Isotope Removal from ITER Be Co-deposits During Wall Baking
M.J. Baldwin

Center for Energy Research, University of California–San Diego, 9500 Gilman Dr., La Jolla, CA 92093-0417 USA
Email: m1baldwin@ucsd.edu

Regular periods of first wall and divertor ‘bake-out’ are planned for ITER in order to keep the accumulated ‘in-vessel’ tritium (T) inventory from exceeding levels mandated by its nuclear license [1]. A main driver of the in-vessel inventory, that accumulates between bakes, is identified with co-deposits [1], which are surface layers of deposited build-up in non-erosive areas, comprised of material from eroded parts of the inner vessel, impurities, and co-implanted/co-deposited hydrogen isotopes. In the ITER all-metal design, with its Be first-wall and full W divertor, Be rich co-deposits are considered to be most relevant to inventory accumulation [1].

Although some uncertainties exist, current simulations agree that Be influxes into the divertor and also back to the wall will lead to thick Be co-deposited layers [2, 3]. For instance, [2] describes Be co-deposited layers as thick as ~0.4 μm developing in the locale of the inner divertor and dome baffles over a single 400 s ITER shot; and [3] predicts deposition in similar locations with projected T accumulation at a rate of several grams per shot. The layers are thus expected to be thick and highly saturated, as has been seen in previous PISCES divertor plasma simulator experiments where it was found that deuterium (D) readily traps with Be in co-deposited layers at compositional fractions up to, and even above, 0.1 D/Be, which exceeds the solubility for D in Be by many orders of magnitude [4].

The main T removal strategy in ITER will be to bake the inner vessel to a fixed temperature of 513 K on the first wall and 623 K in the divertor, but it is presently unclear how long is necessary, and if baking will be sufficient. Recent experiments at the UCSD PISCES laboratory have been undertaken with this question in mind. Experimental data on the thermal release of D from Be–D co-deposits have been collected for various layer thicknesses (up to 1 μm) and temperatures relevant to the proposed ITER bake out at 513 K and 623 K. These data have further been modeled with the Tritium Migration & Analysis Program (TMAP-7), for the purpose of developing a predictive capability that allows extrapolation from the laboratory scale to the ITER environment. The initial studies, to follow, show good agreement between experiment and modeling, and promise towards this goal.

A challenge with experiment, necessary for the ability to model thermal release data, is to create batches of samples with well-controlled co-deposited Be-D layers of suitable uniformity and reproducibility. To this end, we took advantage of a recently developed technique and apparatus, Fig. 1(a), for the manufacture of pure Be encapsulated National Ignition Facility (NIF) targets. [5]. Using similar practice, Be-D co-deposited layers were produced on 2 mm diameter W spheres. Batches (up to 30 layered spheres) were produced by rolling the spheres in a pan located beneath three sputter magnetron sources outfitted with Be targets, as shown in Fig. 1(b). Co-deposition with D is achieved through admixture of D to the sputter system Ar fill gas.

(a)	(b)
Figure 1: (a) The General Atomics NIF-target-research sputter magnetron system located in the PISCES-B Be safety enclosure at UCSD. (b) Production of a batch of Be-D co-deposited layers on 2 mm diameter W spheres.

The D₂ release from individual Be-D co-deposited spheres was measured using thermal desorption mass spectrometry (TDS). A desorption analysis involves heating the sample under vacuum and recording the time evolution of partial pressures corresponding to deuterium-containing species. For standard TDS analysis, the temperature of a sample is feedback controlled and driven linearly in time at a constant rate (β), of β K s^-1. The D₂ partial pressure-time profile, once obtained and calibrated, is a measure of the time and temperature dependent surface release flux of D₂ from the sample. The TMAP-7 hydrogen transport code is directly capable of the simulation of such profiles, as it computes the solution of the time dependent physics associated with gaseous and dissolved gaseous species concentrations and flows in connected systems of gas enclosures and solid structures [6].

Figure 2 shows an example of the agreement between data (open symbols) and TMAP-7 modeling (lines) for a simulated ITER wall bake at 513 K using a selection of Be-D coated sphere samples produced at 323 K. The open symbols are the experimentally observed D2 desorption flux from 3 thicknesses of Be-D co-deposited layer, denoted L_1-L₃. In this example, the samples are heated linearly at a rate of β₂=0.3 Ks^-1 (dashed line, RHS ordinate). Each sample displays an onset for D₂ desorption of ~420 K, but not all of the D₂ is out-gassed before the temperature is held fixed at the simulated ITER bake temperature of 513 K. At the onset of the bake segment (β=0 Ks^-1), D₂ desorption diminishes rapidly but continues at some low level as long as the fixed bake temperature is maintained. The amount of D₂ left in the co-deposited layer after 1 h and 10 h of such baking can be seen in panels (b) and (c) of Fig. 2, as revealed by elevated temperature excursion at a rate, again, of β₂=0.3 Ks^-1. Longer bake time results in less remaining inventory, as revealed visually by the integrals of corresponding profiles in panels (b) and (c) of Fig. 2. The computed TMAP-7 solutions, based on an initial retained fraction of D/Be=0.05 in the co-deposited layer, reproduces experiment reasonably well if traps of energy 0.8 and 1.0 eV are assumed.

The data and TMAP modeling, in this way, are relevant to achieving a predictive capability for determining the efficacy of the ITER bake procedure. The eventual aim is to supply ITER team with a wide range of information similar to that shown in Fig. 3(a) and (b). In this plot, the normalized remaining D inventory in a co-deposited layer is shown as a function of the layer thickness, for fixed temperatures of 513 K and 623 K. The full lines are TMAP-7 modeling for bake periods of 1 h and 10 h,

Figure 2: Experiment (symbols) and TMAP modeling (full lines) showing the D2 release flux versus time for co-deposited Be-D samples. The dashed line is the desorption temperature read from the RHS ordinate. L1, L2, L3 denote Be-D layer thicknesses of 0.2, 0.4, and 0.7 μm respectively. (a) T excursion at β2=0.3 Ks-1 to 513 K fixed T bake. (b) and (c) show remaining inventory after 1 h and 10 h respectively.

validated by experiment (symbols), while the dashed lines are TMAP projections from the lab experiments to more ITER relevant thicknesses and bake durations. The blue regions approximately define the ITER relevant portion/objective of the plot. Figure 3 demonstrates that the proposed ITER bake, on the time scale of days to weeks, is effective for removing a large fraction of the trapped D inventory in laboratory Be-D co-deposited samples, but, thicker layers desorb less efficiently and require quasi-exponentially more time to outgas to a similar level of inventory removal as thinner layers.

Further work is currently underway to examine other effects and test further the integrity of the TMAP model. Such effects include, but are not limited to, the roles of deposition temperature, compositional effects, and impurity fractions, all of which are currently little explored and will nevertheless be active influences on co-deposits formed in the ITER environment.

Figure 3: Plot of the normalized remaining D inventory versus layer thickness for Be-D co-deposited layers formed at 323 K after fixed temperature bake at (a) 513 K and (b) 623 K. Symbols are experiment. Lines are TMAP-7 modeling, extrapolated to more ITER relevant layer thicknesses and bake times. Blue regions approximately define the ITER relevant portion/objective of the plot.

Acknowledgements

This work is supported by USDoE Grant Award: #DE-FG02-07ER54912. The author also gratefully acknowledges Prof. Glen Longhurst of Southern Utah University for help with the TMAP-7 code and Dr. Hong Wei. Xu of General Atomics–San Diego.

References

1. J. Roth, et al., J. Nucl. Mat. 390-391, 1 (2009)
2. K. Schmid, Nucl. Fusion 48, 105004 (2008)
3. J.N. Brooks, et al., Nucl. Fusion 49, 035007 (2009)
4. R.A. Causey, J. Nucl. Mat. 300, 91 (2002)
5. H.W. Xu, et al., Fus. Sci. Tech. 51, 547 (2007)
6. G. Longhurst, TMAP7 User Manual, INEEL/EXT-04–02352, Rev. 2, Idaho National Engineering & Environment Laboratory, Idaho Falls, Idaho (2008)

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ITPA Update

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Schedule of Burning Plasma Events

Click here to visit a list of previously concluded events.

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Contact and Contribution Information

This newsletter provides a monthly update on U.S. Burning Plasma Organization activities. Topical Group Highlight articles are selected by the Leader and Deputy Leader of those groups (burningplasma.org/groups.html). ITPA Reports are solicited by the Editor based on recently held meetings. Announcements, Upcoming Burning Plasma Events, and all comments may be sent to the Editor. Suggestions for the Image of the Month may be sent to the Editor. The images should be photos, as opposed to data plots, though combined graphics are welcome. The goal is to highlight U.S. fusion resources through interesting visualizations.

Become a member of the U.S. Burning Plasma Organization by signing up for a topical group:
burningplasma.org/jointopical

Editor: David Pace (pacedc@fusion.gat.com)
Assistant Editor: Amadeo Gonzales (agonzales@austin.utexas.edu)

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Announcements

DIII-D Team Wins Fifth APS Excellence Prize

DIII-D team members have won the John Dawson Award for Excellence in Plasma Physics Research. The award goes to Dr. Phil Snyder, Dr. John Ferron, and Dr. Tom Osborne, (all of General Atomics) and Prof. Howard Wilson (University of York, UK), “for experiments and theory that explained the limiting edge instabilities of high performance tokamak plasmas, including the key role of peeling-ballooning modes, thus enabling quantitative predictions of the edge pressure in fusion plasmas.” Congratulations to them!

For more information, contact R. Buttery, buttery@fusion.gat.com General Atomics, PO Box 85608, San Diego, California 92186-5608, USA

Special Events at the APS-DPP Meeting in Denver, November 11 - 15, 2013

The Edge Coordinating Committee (ECC) will hold a scientific session during the APS Division of Plasma Physics Meeting in Denver, CO. It will feature a review of recent collaborative efforts contributing to the 2013 US Joint Research Target in the physics of enhanced confinement regimes without ELMs. A period of Q/A and discussion will follow.

DATE: Wednesday, November 13, 2013
TIME: 12:30 to 2:00PM
LOCATION: Governor’s Square 12, Plaza Building, Sheraton Denver Downtown Hotel
SPEAKERS: Stefan Gerhardt, Rajesh Maingi, PPPL

This meeting is open to the fusion and plasma physics community. As in past ECC meetings, the focus will be on fostering communication between experimentalists and theorists, and between experts in different aspects of boundary and pedestal physics.

For more information, contact:
Jerry Hughes (jwhughes@psfc.mit.edu), ECC Chair
Ilon Joseph (joseph5@llnl.gov), ECC Vice-Chair

Click here to visit a Directory of Other Plasma Events

Please contact the administrator with additions and corrections.

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