Dear Burning Plasma Aficionados:

This newsletter provides a short update on U.S. Burning Plasma Organization activities. Comments on articles in the newsletter may be sent to the editor (David Pace) or assistant editor (Amadeo Gonzales).

Thank you for your interest in Burning Plasma research in the U.S.!

Director's Corner by C. M. Greenfield

ITER passes an important milestone

On November 10, French Prime Minister Jean-Marc Ayrault signed the official decree authorizing the ITER Organization to create the Installation nucléaire de base (basic nuclear installation) ITER. This is a very important landmark in the construction of the ITER facility, and marks the first time in the history of the world where the safety characteristics of a fusion device have been examined and approved by a nuclear regulator.

Progress has also continued in construction of the ITER facility. One recent highlight is the completion of the new Headquarters Building, with ITER Organization staff now moving into the new building.

[Left] The future tokamak pit with the now completed seismic bearings. The Poloidal Field Coil Winding Facility is in the background (Photo by C. Greenfield); [Right] The ITER Team has started moving into their offices in the newly completed Headquarters Building. Photo © ITER Organization.

Much of the information about ITER that appears here comes from publically available sources, and I can only touch on a few tidbits. The weekly ITER Newsline (http://www.iter.org/newsline) is an excellent source of up-to-date information.

ITER International School scholarships

The 6th ITER International School will be held in Ahmedabad, India, December 2-6, 2012. This year’s focus will be on RF heating and current drive in plasmas, and information can be found at http://www.iter-india.org/iis2012. Once again, the USBPO is providing 8 travel scholarships to students and post-docs to attend the school. This year’s scholarship winners, selected by a committee composed of Mark Koepke (West Virginia University), Bill Heidbrink (University of California – Irvine), and Nermin Uckan (Oak Ridge National Laboratory), are as follows:

Congratulations to all of the winners, and best wishes for an interesting, educational, and productive experience in India.

USBPO Research Committee membership

Over the summer, five USBPO Topical Group leaders completed their terms, and have now stepped down. I would like to thank John Rice (MIT; Confinement and Transport TG), Jim Terry (MIT; Diagnostics TG), John Ferron (GA; Integrated Scenarios TG), Ted Strait (GA; MHD and Macroscopic Plasma Physics TG), and Dylan Brennan (University of Tulsa; Modeling and Simulation TG) for their years of service to the USBPO and the community at large. I’d like to give special mention to Jim Terry, who was leader or deputy in the Diagnostics TG for six years, making him the last original Research Committee member.

The 2012-13 USBPO Research Committee membership is as follows:

In each case, the previous deputy leader (green) has been promoted to lead their respective topical groups. I am delighted to welcome the new deputy leaders (blue) to the Research Committee, and look forward to working with them during the coming years.

With the departure of Dylan Brennan from the Research Committee, his term as eNews editor has come to an end. I am grateful to Dylan, and happy to announce that David Pace has stepped in to become the new editor. To insure a smooth transition, Dylan and David have worked together to produce the issue that you’re reading.

Burning Plasma meeings

October was a busy month in our community, with the IAEA Fusion Energy Conference (San Diego, October 8-13), six ITPA Topical Group meetings (San Diego, October 15-18), and the APS Division of Plasma Physics annual meeting (Providence, October 29-November 2). The USBPO organized events at both the IAEA and APS meetings.

On Tuesday evening during IAEA, a Town Meeting on ITER was held, with two speakers from the ITER Organization. Each of these talks will be posted on the USBPO website in the coming days:

This year’s APS-DPP meeting was considerably smaller than usual due to competition from Hurricane Sandy. Nevertheless, we held a fifth annual contributed oral session on “Research in Support of ITER,” including a wide range of contributions spanning a range of topics.

Finally, over the summer, the USBPO held a pair of “Virtual Forum” web seminars to foster discussion and community input for the FESAC subcommittee on MFE priorities. These were very well attended, with 74 ReadyTalk connections recorded during the first (we can’t tell how many people there were). We plan to expand the use of web seminars for increased communication with the BPO community, continuing our series on ITPA topical groups and also other topics of interest. Suggestions for topics are welcome.

USBPO Task Groups

Finally, I’d like to call your attention to opportunities to contribute progress in burning plasma science: USBPO Task Groups that focus on specific burning plasma issues that may cut across Topical Group boundaries. Please contact the leaders directly, or myself, if you’d like to participate:

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USBPO Topical Group Highlights

(Editors note: The BPO Fusion Engineering Science Topical Group works to inform 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 Larry Baylor and Russ Doerner]. This month’s highlight by G.M. Wright, et al., summarizes the first experimental production of tungsten nano-tendrils (“fuzz”) in a tokamak. The experiment, conducted on the Alcator C-Mod tokamak, demonstrates that the tokamak environment is capable of producing the material surface conditions necessary to effect this nanoscale growth. Such material surface evolution is of concern for ITER due to the impact on dust production.)

Tungsten nano-tendril growth in the Acator C-Mod tokamak
G.M. Wright¹, D. Brunner¹, M.J. Baldwin², R.P. Doerner², B. Labombard¹, B. Lipschultz¹, J.L. Terry¹, and D.G. Whyte¹

¹ MIT Plasma Science and Fusion Center, 77 Massachusetts Ave, Cambridge, MA, USA, 02139
² Center for Energy Research, University of California in San Diego, 9500 Gilman Dr, La Jolla, CA, 92093-0417, USA

The growth of tungsten nano-tendrils (or “fuzz”) has been well documented in a wide range of linear plasma devices [1-3]. The growth conditions used to create tungsten fuzz in these devices are elevated surface temperatures (T_s = 1000-2000 Κ) implanted with helium ions with sufficient incident energies (E_He+ > 20 eV). In future devices, there will be sections of the first wall that reach surface temperatures >1000 K and helium ions will always be present as a product of deuterium-tritium fusion reactions, thus the growth of tungsten fuzz in future fusion devices is a possibility. The impact of tungsten fuzz on how the plasma interacts with the tungsten tokamak wall and what this means for tokamak operations is still largely unknown, but has the potential to be significant. For example, these fine structures are very fragile and a major concern is the mechanical failure of the individual nano-tendrils and, in turn, significant production of tungsten dust and erosion; perhaps compromising the viability of a tungsten first wall. Until now, tungsten fuzz growth has not been observed in tokamak experiments. It is unclear if the significant differences between the plasmas in a tokamak and a linear plasma device, such as different ion incident angles, parallel heat fluxes, and magnetic fields, would inhibit or even prevent tungsten fuzz growth entirely.

Despite these differences we will show, for the first time, that tungsten fuzz growth can occur in a tokamak under plasma–material exposure conditions similar to those conducive to fuzz formation in linear plasma devices. An experiment performed on the compact, high magnetic field Alcator C-Mod tokamak has successfully grown tungsten fuzz on the surface of a tungsten probe. Helium plasmas were used for 14 sequential and identical 1.5 s discharges and the plasma heat flux received on the probe surface was ~ 30 MW/m². These high heat fluxes heat the probe surface up to ~2300 K in some cases. This resulted in an integrated time of ~11 s at the surface temperatures conducive to growing the tungsten nano-tendrils (T_surf = 1000-2300 K).

Figure 1: a) The ramped Mo tiles and W probe upon removal from Alcator C-Mod, b) SEM image of the W probe surface showing nano-tendrils on the surface, and c) high magnification SEM image showing the average thickness of the individual nano-tendrils to be ~100 nm. [4]

Upon the removal from Alcator C-Mod, visual inspection revealed that the tungsten probe was optically black (figure 1a). Inspection of the same tungsten probe tip with a scanning electron microscope shows fully developed nano-tendrils with almost complete coverage of the surface (figure 1b). A higher magnification image shows the individual tungsten nano-tendrils are typically ~100 nm in diameter (figure 1c), which is ~600 times thinner than a human hair. A cross-section of the surface shows that the depth of the fuzz layer is 600 ± 150 nm (figure 2). This is in-line with calculations from an empirical formula obtained with data from the PISCES linear plasma device [1]. One can also see that the tungsten in the fuzz layer is pocked with voids/bubbles. These are small bubbles formed by the precipitation of helium in the tungsten lattice. These bubbles are also seen in tungsten fuzz grown in linear plasma devices and are thought to play a significant role in defining the temperature regime for the fuzz growth and the diameter of the nano-tendrils.

Figure 2: Cross-section of the tungsten nano-tendril layer grown in the Alacator C-Mod Tokamak. [5]

In conclusion, it has been shown for the first time that tungsten fuzz can be grown in a tokamak environment. The surface morphology and growth rates seen in the Alcator C-Mod tokamak are also in good agreement with tungsten fuzz grown in linear plasma devices. This helps further support the notion that the surfaces of tungsten plasma-facing components in future fusion devices will transform into this nano-tendril morphology. While this work helps validate the use of data from linear plasma devices to determine the growth mechanisms of these nano-tendrils, the impact of these types of surfaces on fusion reactor design and tokamak operations remains largely unknown and needs to be investigated with high priority.

References

1. M.J. Baldwin, R.P. Doerner, Nucl. Fusion 48 (2008) 035001.
2. S. Takamura et al., Plasma Fusion Res. 1 (2006).
3. G. de Temmerman et al., J. Vac. Sci. Technol. A 30 (2012) 041306.
4. G.M. Wright et al. Nucl. Fusion 52 (2012) 042003.
5. G.M. Wright et al. 20th International Conference on Plasma Surface Interactions, Aachen, Germany, 2012, submitted to J. Nucl. Mater.

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