News and Events


U.S. Burning Plasma Organization eNews
February 28, 2014 (Issue 81)

 

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.

 

CONTENTS

Announcements  
Director's Corner
C.M. Greenfield
Topical Group Research Highlight
G.L. Delzanno and X.Z. Tang
Dust: Another Player in the Complex Plasma-material Interaction Problem for Long-pulse Tokamaks
ITPA Update
Schedule of Burning Plasma Events
Contact and Contribution Information

Announcements

Alcator C-Mod 2014 Ideas Forum

Participation is invited for the 2014 Alcator C-Mod Ideas Forum to be held on March 5–6, 2014. Ideas for which a presentation time slot is requested must be submitted by February 28, 2014. Submission information is available at
http://www.psfc.mit.edu/research/alcator/program/ideas2014/

The Forum aids in the planning of the FY14 C-Mod Experimental Campaign by providing an opportunity for presentation of specific ideas appropriate for experiments during the upcoming runs. As usual, the Forum will be open to all interested parties, including current and potential future collaborators. We actively solicit proposals from the entire fusion community, and we will make remote participation available.

Post Doctoral Fellowships at ITER

The application process is now open and the deadline to apply is March 17, 2014. Information concerning this opportunity is available online at http://www.iter.org/monaco2014


Director’s Corner

by C.M. Greenfield

Where to find information about ITER

As a reminder, I gather information for this newsletter from several sources. In some cases, it comes from personal contacts or observations during my own visits to the ITER site. But there are several sources of information that you can access yourself. The ITER Organization provides a weekly newsletter, ITER Newsline, which can be found at http://www.iter.org/whatsnew. ITER has also begun publication of an online magazine, which can be found at http://www.iter.org/mag. You can check them out on ITERs website, or subscribe to one or both of these. Domestic (US) information about ITER is available at the US ITER Project Office website, http://usiter.org. You can find various links to recent news stories as well as occasional News Updates (including a new one published just this week). And of course the US Burning Plasma Organization maintains its own website at http://burningplasma.org.

Many of you may be asked about ITER in the coming days by friends who have read Raffi Khatchadourian’s article in The New Yorker, entitled “A Star in a Bottle.” It is long, but is definitely recommended reading for those of us working in the field. Rather than attempt to review it for you, I leave it to you to form your own opinion. Mr. Khatchadourian has also posted a companion movie about ITER at http://www.newyorker.com/video?video-id=3250151664001.

ITER News

As 2014 began, the ITER site has been largely transformed from the empty area seen only a few years ago. The first buildings are complete, and the Headquarters building is already being expanded. The Tokamak Complex and associated buildings are taking shape in the middle of the site.

Figure 1

Figure 1: Aerial view of the ITER site. The long building at the right is the poloidal field coil winding facility. Excavation of the Tokamak Complex can be seen slightly above the center of the photo, with the ITER Headquarters building immediately to its left. Photo © IETR Organization.

Work was interrupted on the concrete slab that will support the ITER tokamak last June due to some concerns about the design. These concerns have now been addressed, with an updated design accepted by all parties. Laying of the rebar has resumed, with concrete pouring anticipated in the coming months.

US Burning Plasma Organization Task Groups

We welcome Valerie Izzo (UCSD) as the new leader of the USBPO Task Group on Disruptions. Bob Granetz (MIT) continues as the deputy leader of the Task Group. As the US FES large tokamak portfolio grows — from just DIII-D in 2013, with C-Mod starting up now and NSTX following later this year — we look forward to an exciting research program supporting a US response to one of ITER’s most pressing issues. I look forward not only to the Task Group working with the individual research programs, but in building stronger connections with the theory community and providing an interface to researchers at other labs and universities to participate in this important research line. If you are interested in joining in, I encourage you to contact either Val (izzo@fusion.gat.com) or Bob (granetz@psfc.mit.edu).

Our other current task group, on modes of ITER participation (led by Rajesh Maingi of PPPL) is nearing completion of their work. You can expect to hear the results of their study in the coming months.

Figure 2

Figure 2: Eight months after being interrupted, rebar operations resumed on this all-important area of the Tokamak Complex concrete slab. Photo © IETR Organization.

We are also considering ideas for additional Task Groups. Of course these should address issues of interest and importance to the Burning Plasma community, but it is just as important that we can provide added value… if a research area is proceeding just fine without a Task Group, there is no need. I think we have learned from previous Task Groups a lot about where a USBPO can be effective… and where it can’t. If you have ideas for such an area, please feel free to contact me.

Webinars

Todd Evans’ (GA) excellent webinar on ELM control with Resonant Magnetic Perturbations earlier this week may have set a record for participation. We estimate about 85 of you were watching, but there’s always room for more! We are working to schedule additional webinars, including one for community discussion of the output of the USBPO Task Group on modes of ITER participation. Please watch for announcements of these and later webinars from Amanda Hubbard (USBPO Deputy Director).

We are always looking for webinar topics of general interest to the Burning Plasma community. If you would like to suggest a topic – or even volunteer – please contact Amanda.

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Topical Group Research Highlight

Research Highlights are selected by the leaders of the BPO Topical Groups on a rotating basis. The BPO Modeling and Simulation Topical Group facilitates US efforts to develop and apply numerical codes to the understanding and prediction of fusion device performance (leaders are David Mikkelsen and Xian-Zhu Tang). This month’s Research Highlight by Delzanno and Tang concerns the behavior of dust in reactor-grade tokamaks. The generation and accumulation of large amounts of dust in tokamak reactors could pose serious operational and regulatory concerns. Previous estimates of dust accumulation have relied on engineering extrapolation from present-day tokamaks, but a recent effort has begun to develop a physics basis for extrapolation that can be experimentally validated.

Dust: Another Player in the Complex Plasma-material Interaction Problem for Long-pulse Tokamaks


Gian Luca Delzanno and Xian-Zhu Tang
Los Alamos National Laboratory
delzanno@lanl.gov, xtang@lanl.gov

Burning plasmas in a deuterium-tritium fusion reactor generate intense fluxes - up to tens of MW/m2 - of thermal particles and 14 MeV neutrons that ultimately must interact with a material boundary. Plasma facing components (PFCs) must absorb and survive the enormous power and particle flux, while maintaining structural integrity and thermo-mechanical properties under neutron damage. While the liquid metal wall concept is currently under intensive study, a solid wall remains the front-runner in fusion reactor PFC design. The top consensus material for a solid PFC is tungsten due to its favorable thermo-mechanical properties: low sputtering yield, high thermal conductivity, and high melting temperature.

The conventional picture of the interaction between burning plasmas and material boundaries is the following. Wall materials are eroded and released to the plasma as atoms and molecules. These are ionized and brought back to the wall surface to redeposit by the plasma flow. Commercially viable magnetic fusion reactors must operate for many months between maintenance shutdowns, implying that a near perfect local balance between erosion and redeposition must be achieved everywhere on PFCs.

It is now well known, however, that plasma-material interaction can create mobile solid particulates or dusts (see the review [1]). While impurity neutrals can be ionized near the wall and promptly redeposited close to their initial release point, dust poses a challenge to maintaining the required erosion/redeposition balance: it can travel long distances in the chamber [2] and redeposit its material non-locally, or be lost and cause a net loss of wall material and hence areas of local net erosion. Based on data from current carbon-wall tokamaks, the fraction of wall materials “converted” into dust is estimated to be at least 10–15% of total gross erosion [3-4]. This number is unknown for tokamaks with metal walls because of the scarcity of available data.

Dust is normally not a concern in present day short-pulse tokamaks, which are characterized by relatively low energy fluxes to the wall (a reference number is Q ∼ 1 MW/m2 at the divertor plates). On the other hand, long-pulse tokamaks like ITER or steady-state reactors like DEMO are estimated to produce hundreds of kilograms of dust because of the larger energy fluxes to the wall (Q ∼ 10 MW/m2) and hence stronger plasma-material interaction [5]. Indeed, ITER is the first tokamak with dust safety issues and administrative limits on the total amount of dust that can be present at any given time in the vessel [5]. For this reason, research on dust in tokamaks has received a lot of attention in recent years, both experimentally [4,6,7] and on the modeling side [7-10].

In order to investigate the impact of dust on the local erosion/redeposition balance and the potential for accumulation of significant amount of dust in long-pulse tokamaks like ITER, in the February issue of Physics of Plasmas [10] we report on studies of dust transport near the divertor plates of a tokamak, contrasting the two scenarios corresponding to short-pulse and long-pulse tokamaks. We have focused on small injection velocities (∼ 0.1 m/s) normal to the divertor plate. In general large dust injection velocities normal to the plate lead to dust destruction, since the deep penetration of dust particles into the plasma exposes them to a plasma of much higher energy density. Large poloidal injection velocities increase the chance for survival, since the dust particles can move outside the divertor strike points and find a more benign plasma.

Figure 1

Figure 1: Transport simulations for tungsten dust particles injected at the divertor plate (lying horizontally at x = 0) with normal velocity 0.1 m/s. The plasma background is representative of that of a long-pulse tokamak. The top panels are for a spherical dust particle with initial radius rd = 1 micron, while the lower panels are for rd = 10 micron. The bouncing motion on the divertor plate induced by the plasma drag and the electrostatic force is showed on the left panels. The dust temperature and the dust radius showed are on the right panels. The micrometer dust particle evaporates within 1.5 ms, while the larger particle can move away from the divertor strike points and survive. From Ref. [10].

In the case of short-pulse tokamaks, it is shown that micrometer dust particles can survive near the divertor plate with relative ease because thermal radiation provides enough cooling. Indeed, dust particles are plentifully collected on all existing tokamaks [11]. For long-pulse tokamaks, however, the situation is dramatically different. Micrometer (tungsten) dust particles cannot survive near the divertor plates because radiative cooling does not compensate for the energy fluxes collected from the background plasma. The dust particles always move in the direction of the local plasma flow and redeposit their material nonlocally. This leads to a poloidal mass migration across the divertor. Smaller dust particles (characteristic size ∼ 0.1 micron) are also destroyed by the plasma-material interaction but this happens on a faster time scale so that they redeposit their material locally. Larger dust particles (∼ 10 micron) can instead survive partially, leading to some nonlocal redeposition and dust accumulation in the vessel. See Fig. 1.

The scoping study just described needs to be complemented by the material science experiments that will provide the characteristic dust size and injection speed relevant to long-pulse tokamaks, but it can be used to draw some general conclusions. For instance, if the characteristic dust size generated in ITER is ∼ 1 micron, our study indicates that large quantities of dust should not be accumulated in the machine and the dust safety limits might not be a problem. On the other hand, the nonlocal redeposition and the related net poloidal mass migration suggest that matching local erosion and redeposition profiles might be a challenge. A characteristic dust size of ∼ 10 micron is possibly worse, since large quantities of dust could potentially be accumulated in addition to the challenge of matching local erosion and redeposition profiles.

This research was supported by the U.S. Department of Energy Office of Science, Office of Fusion Energy Sciences, under the auspices of the National Nuclear Security Administration of the U.S. Department of Energy by Los Alamos National Laboratory, operated by Los Alamos National Security LLC under contract DE-AC52-06NA25396.

References

[1] S.I. Krasheninnikov, R.D. Smirnov, and D.L. Rudakov, Plasma Phys. Control. Fusion 53, 083001 (2011)
[2] S. Krasheninnikov, et al., Phys. Plasmas 11, 3141 (2004)
[3] C. Grisolia, et al., J. Nucl. Mat. 386-388, 871 (2009)
[4] D. Rudakov, et al., Nucl. Fusion 49, 085022 (2009)
[5] J. Roth, et al., J. Nucl. Mat. 390-391, 1 (2009)
[6] A. M. Litnovsky, et al., J. Nucl. Mat. 438, S126 (2013)
[7] S. Ratynskaia, et al., Nucl. Fusion 53, 123002 (2013)
[8] R. D. Smirnov, et al., Plasma Phys. Control. Fusion 49, 347 (2007)
[9] M. Bacharis, M. Coppins, and J. E. Allen, Phys. Plasmas 17, 042505 (2010)
[10] G.L. Delzanno and X.Z. Tang, Phys. Plasmas 21, 022502 (2014)
[11] J.P. Sharpe, D.A. Petti, and H.-W. Bartels, Fusion Eng. Design 63-64, 153 (2002)

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ITPA Update

More information concerning the ITPA may be found at the Official ITPA Website.

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
A summary of the 25th Meeting (Kyushu University, Japan, October 7–9, 2013), written by R. Maingi is available at the BPO Forum (PEP).
 
Transport and Confinement Topical Group
  12th Meeting, Massachusetts Institute of Technology, Cambridge, MA, United States, April 9–11, 2014

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Schedule of Burning Plasma Events

2014 — NSTX-U First Plasma —
 
March 5–6, Alcator C-Mod Ideas Forum
  Massachusetts Institute of Technology, Cambridge, MA, United States
 
March 10–14, ITPA: 23rd MHD Disruptions, and Control Topical Group Meeting, Toki, Japan
  Held jointly with the US-Japan MHD Workshop on “3-D field effect on MHD control: the roles of magnetic island and stochasticity in optimum MHD control”
 
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
 
2015
W7-X First Plasma
2021
ITER First Plasma
2019
JT60-SA First Plasma
2028
ITER Full DT Operations

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Contact and Contribution Information

This newsletter provides a monthly update on U.S. Burning Plasma Organization activities. The USBPO operates under the auspices of the U.S. Department of Energy, Fusion Energy Sciences (FES) division. All comments, including suggestions for content, may be sent to the Editor. Correspondence may also be submitted through the USBPO Website Feedback Form.

Become a member of the U.S. Burning Plasma Organization by signing up for a topical group.

Editor: David Pace (pacedc@fusion.gat.com)

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