News and Events

U.S. Burning Plasma Organization eNews

Jun 30, 2018 (Issue 131)


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
Research Highlight
R. D. Kolasinski
Schedule of Burning Plasma Events  
Contact and Contribution Information  

Announcements

ITER internships

An announcement for ITER internship opportunities for undergraduate and graduate students was recently posted. Please see the following link for a description: https://www.iter.org/jobs/internships

The proposed topics can be found here:

https://www.iter.org/doc/www/content/com/Lists/WebText_2014/Attachments/321/Call_for_topics-Internship_2018_V3.pdf

 

Director’s Corner By C.M. Greenfield

Burning Plasmas at the upcoming APS Division of Plasma Physics Conference

As I told you in the last two issues, the US Burning Plasma Organization invited submissions for the eleventh annual contributed oral session on Research in Support of ITER at the upcoming APS-DPP conference in Portland, Oregon. I’m pretty sure the 32 submissions we received are a record, and it made it very challenging to pick a representative set to populate our session with 15 talks. Apologies to those who we weren’t able to accommodate. I will share the agenda for that session in next month’s issue.

We are also in the process of scheduling a town meeting on the ITER Research Plan during the APS conference. The speaker will be Tim Luce, the new Director of ITER’s Science and Operations Division.

ITER news (now over 55% complete for first plasma)

I had a chance to attend my first ITER Council (IC-22) meeting this month, to report on the results of discussions at last month’s Science and Technology Advisory Committee (STAC) meeting. I found it rather fascinating to see a side of the project that most of us on the scientific and technical side take for granted. The IC is composed of government officials from the seven ITER Members, with a wide range of technical backgrounds, ranging from scientists and engineers to diplomats. I was impressed by the enthusiasm for the project shown by all attendees.

During this meeting, the Council heard about the status of the ITER project, and approved refinements to the construction strategy that are expected to keep ITER on schedule for first plasma in 2025. A press release describing some of the high points of the discussion can be found at:

https://www.iter.org/doc/www/content/com/Lists/list_items/Attachments/777/2018_06_IC-22.pdf.

The Council also heard about the status and plans of preparations for the Disruption Mitigation System (DMS), including the need for an extensive R&D program on both physics and technology in preparation for deployment of a Shattered Pellet Injection (SPI) based DMS. This will be a fairly major undertaking, with support sought from the ITPA and the world-wide suite of tokamak facilities. In the long term, consideration will be given not only to SPI, but to alternative strategies in case it is decided something better is needed. You will hear more about that in the coming months.

This was also my first time making two visits to ITER so close together (five weeks after the STAC meeting). Even in such a short time, there were visible differences on the construction site.

 

Research Highlight

Fusion Engineering Science (Leaders: Jean Paul Allain & Masashi Shimada)

One challenge to the study of plasma-material interactions expected in future plasma-burning fusion reactors is to establish an understanding of helium and hydrogen modification to candidate material surfaces that bound the tokamak plasma.  In this month’s research highlight Rob Kolasinski and his colleagues at the Sandia National Laboratory at Livermore (SNLL) summarize recent findings of temperature-dependent helium and hydrogen irradiation of candidate tungsten materials exposed to prototypical conditions.  The exciting new results highlight detailed surface morphology evolution and also unique in-situ and ex-situ PMI diagnostic approaches that is adding to our overall understanding of this complex synergy of He irradiation, microstructure and temperature effects critical to the extreme environments expected in future plasma-burning fusion reactors.

Materials science challenges in magnetic fusion: Effects of low-energy hydrogen and helium plasmas on tungsten surfaces

R. D. Kolasinski1, J. A. Whaley1, F. I. Allen2, and D. A. Buchenauer1

1Sandia National Laboratories, Energy Innovation Department, Livermore, CA 94550 USA

2University of California – Berkeley, Department of Materials Science and Engineering, Berkeley, CA 94720 USA

Author e-mail: rkolasi@sandia.gov

The plasma-facing surfaces of a fusion reactor will be exposed to a combination of high-flux deuterium-tritium (D-T) plasmas, high-energy fusion products, as well as impurities (including artificially introduced species to promote radiative cooling of the scrape-off-layer.) These energetic particles will continually reconfigure the exposed surfaces. While this is obviously one of the main mechanisms leading to material degradation, the implications are far broader than this. Surface structure affects many topics of importance to tokamak operation, including transport of tritium fuel through the plasma-facing material, sputtering (and therefore impurity transport into the core plasma), as well as recycling.

Our group at Sandia/CA has focused on how low energy hydrogen and helium plasmas reconfigure the structure of solid plasma-facing surfaces. This research emphasizes tungsten plasma-facing materials, which will be used in the ITER divertor and potentially in a DEMO reactor. Once implanted into the surface, both hydrogen and helium diffuse through the material and interact with defects within the lattice, such as vacancies, dislocation loops, and grain boundaries. Since ITER will operate with tritium, trapping of hydrogen isotopes at such defects has received considerable attention from a safety standpoint. In addition, small bubbles or blisters containing high-pressure hydrogen isotopes or helium may nucleate, leading to degradation of the plasma-facing surface. A particularly dramatic consequence of low energy helium plasmas on tungsten surfaces is the evolution of near-surface He bubbles into a dense network of nanotendrils (approximately 100 nm in diameter.) This effect was unexpected prior to work by Baldwin and Doerner using the PISCES linear plasma devices in 2008 [1]; later studies at MIT showed that this structure forms under appropriate conditions in a tokamak environment (Alcator C-Mod [2].)

While plasma-induced damage in refractory metal surfaces have been studied widely, how these structures evolve during plasma exposure from atomic-scale defects in initially pristine surfaces is still not well understood. Much of the knowledge gap is simply due to the difficult nature of characterization: most conventional microscopy tools work perfectly well in tightly-controlled ultra-high vacuum (UHV) environments, but they are generally not compatible with the higher pressures and magnetic fields encountered in a plasma environment. The second issue arises with the difficulty in decoupling the complex combination of processes that occur in the near-surface. How the surface is reconfigured by plasma depends both on (a) the concentration of implanted species within the lattice and (b) how these implanted species interact each other, nucleate defects, and expand these structures over time. Recent improvements in microscopy techniques have led to considerable advances in our understanding of these phenomena, as evidenced by recent work by Parish and colleagues [3]. In addition, this progress has been complemented by commensurate advances in materials modeling approaches that are now being applied to plasma-materials interactions [4].

We have taken two approaches toward addressing the problems described above. First, using in-situ optical spectroscopy as well as ex-situ helium ion microscopy, we are examining the linkage between nanostructure evolution and near-surface He bubble growth. In addition, in parallel with testing practical material systems, our research effort aims to break down the complexity of plasma-material interactions into basic atomic-scale processes using low energy ion beam experiments performed in a well-controlled high-vacuum environment.

Text Box:  
Fig. 1: W target exposed to RF He plasma discharge generated using a Lisitano coil. 
Developing the experiments needed for in-situ experimental studies presents several technical challenges, the first of which is producing the relevant plasma exposure conditions. We developed an RF plasma source which can produce a linear plasma column with low energy (<70 eV) ion fluxes up to up to = 2.5×1021 m-2s-1 (c.f. Fig. 1.) Using this instrument, we exposed a series of W samples to the RF plasma over a range of exposure temperatures (Tsample = 550 - 930 °C) and total fluences (F = 7.9×1023 - 3.6×1025 m-2) to systematically explore in detail boundaries where the He-induced nanostructure is known to nucleate.

The cross sections in Fig. 2 clearly show how dramatically the He plasma alters the W surface, as well as how this effect accelerates as the surface temperature increases. To image nanostructured morphologies, we used a scanning helium ion microscope / focused ion beam instrument at the University of California, Berkeley. For each image in Fig. 2, the exposure conditions were identical except for the sample temperature (as indicated in each case). At low temperatures, small 20 nm diameter pits initially appear in the surface. As the temperature is increased above 650 °C, the topography then evolves as a network of protrusions and grooves approximately 50 nm in width. Variations in the initial structure appear to correlate with regions roughly the size of the tungsten grains. For the length of exposure considered here, any obvious dependence on the initial grain structure disappears at temperatures greater than 786 °C. While these experimental results provide interesting insight, it is important to keep in mind that they capture only part of the picture. We have been collaborating with researchers participating in the University of Tennessee-led SciDAC program on plasma-materials interactions, who are developing models of initial stage tungsten nanostructure growth. Our experimental results are complementary to their efforts and will help to provide a better understanding of the physics mechanisms underlying this phenomenon.

Fig. 2: Helium ion microscope images showing the evolution of tungsten nanostructure as a function of temperature. Note that both top view and focused ion beam (FIB) cross sections are shown. Where applicable, an estimate of the tungsten nanotendril layer thickness (t) is given below each image.

 

Text Box:  Fig. 3: Index of refraction and extinction coeffi-cients measured by spectroscopic ellipsometry for tungsten samples exposed to varying He plasma fluences. For clarity, only the optical properties at 650 nm are shown here.Spectroscopic ellipsometry provides a promising avenue for characterizing the development of these surface morphologies in-situ. In this approach, one uses elliptically polarized light to probe the sample. The light undergoes a polarization state change upon reflecting from the surface. The principal task is to then determine the ellipsometric angles that characterize this state change. One can also extract meaningful data on the wavelength dependence of the refractive index and extinction coefficients. The main challenge associated with ellipsometry is that it characterizes optical, rather than physical properties of the surface. Thus, extracting meaningful insight from the measurement requires developing an appropriate model that correlates these features. Using ellipsometry, we performed ex-situ benchmarking measurements how the index of refraction (n) and extinction coefficient (k) of surfaces exposed to He plasmas of different fluences. The various curves correspond to different exposure times ranging from a pristine surface (0 hr) to a surface with a fully developed nano-tendril network (12 hr.) As depicted in Fig. 3, one can use the change in optical properties at 650 nm to assess nanostructure changes in the near-surface. Over the past month, we have performed our first set of in-situ measurements using the ellipsometer directly attached to the RF plasma system.

The desired goal of the research described above is to advance the physical picture of hydrogen and helium behavior in the near-surface to enable more accurate predictions of surface composition and structure evolution during plasma exposure. While this of course represents a considerable technical challenge, we have been fortunate to collaborate with researchers in the U.S. and internationally who have been dedicated to these longstanding problems. Going forward, this scientific basis for understanding these near-surface effects will have an essential role in guiding the development of new plasma-facing materials for future magnetic fusion experiments.

Acknowledgements

This work was supported by the DOE Office of Fusion Energy Sciences, through the Materials and Nuclear Science program. RDK also acknowledges support from the DOE Early Career Research Program. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia LLC, a wholly owned subsidiary of Honeywell International Inc. for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

References

[1] M. J. Baldwin and R. P. Doerner, Nucl. Fusion 48 (2008) 035001.

[2] G. M. Wright, D. Brunner, M. J. Baldwin, R. P. Doerner, B. Labombard, B. Lipschultz, J. L. Terry and D. G. Whyte, Nucl. Fusion 52 (2012) 042003.

[3] K. Wang, R. P. Doerner, M. J. Baldwin, F. W. Meyer, M. E. Bannister, A. Darbal, R. Stroud, and C. M. Parish, Scientific Reports 7 (2017) 42315.

[4] K. D. Hammond, S. Blondel, L. Hu, D. Maroudas, and B. D. Wirth, Acta Mater. 144 (2018) 561.

 

Calendar of Burning Plasma Events

2018

July 2-6

EPS Conference on Plasma Physics

Prague, Czech Rep.

August 27-31

Theory of Fusion Plasmas , Joint Varenna-Lausanne International Workshop

Varenna, Italy

Sept 3-5

ITPA Energetic Particles Topical Group meeting

Lisbon, Portugal

Sept 11-14

EU Transport Task Force (EU-TTF) meeting

Seville, Spain

Sept 17-20

ITPA Transport & Confinement Topical Group meeting

ITER HQ, Franc

October 1-3

ITPA MHD Disruptions & Control Topical Group meeting

Napoli, Italy

October 22-27

IAEA Fusion Energy Conference

Gandhinagar, Gujarat, India

October 29-31

ITPA Integrated Operation Scenarios Topical Group meeting

ITER HQ, France

October 29-31

ITPA Pedestal & Edge Physics Topical Group meeting

ITER HQ, France

November 5-9

60th Annual Meeting of the APS Division of Plasma Physics

Portland, OR

November 11-15

ANS 23rd Topical Meeting on the Technology of Fusion Energy (TOFE)

Orlando, FL

November 12-14

21st MHD Stability Control Workshop

UCLA, CA

November 12-18

2nd Asia-Pacific Conference on Plasma Physics

Kanazawa, Japan

December 4-5

Fusion Power Associates 39th Annual Meeting & Symposium, Fusion Energy: Strategies & Expectations through the 2020s

Washington, DC

Dec. or Jan.

ITPA Coordinating Committee & CTP ExComm

ITER HQ, France

2019

JET DT-campaign (https://www.euro-fusion.org/newsletter/jet-full-throttle/)

April 15-17

Sherwood Theory conference

Princeton, NJ

October 21-25

61st Annual Meeting of the APS Division of Plasma Physics

Fort Lauderdale, Florida

2020

JT60-SA First Plasma (http://jt60sa.org/)



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: Walter Guttenfelder (wgutten@pppl.gov)

 

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