News and Events


U.S. Burning Plasma Organization eNews
November 30, 2015 (Issue 102)

 

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.J. Goldston
Schedule of Burning Plasma Events  
Contact and Contribution Information
Image of the Month

Announcements

Career Opportunity Announcement

The Office of Fusion Energy Sciences is seeking qualified candidates for a Program Manager position in the Research Division. The position is to be posted at USAJobs (https://www.usajobs.gov/GetJob/ViewDetails/421353400) on Monday, November 16, and will remain open for applicationsfor fifteen (15) business days, closing on Monday, December 7.

The focus of this position will be to serve as a recognized scientific authority and expert in magnetic confinement of high-temperature plasmas and the operation of large toroidal magnetic fusion science experimental facilities and other technical areas, and as such involves the responsibility to plan, coordinate, implement, and evaluate research programs in plasma and fusion science on a national and international level.

The position will also involve planning, budgeting, justifying, and allocating funds among a variety of research programs; preparing analytical documents; giving oral presentations; selecting experts to review proposals; planning, directing, and evaluating complex research programs, projects, and policies; traveling to conferences; and identifying pioneering and strategic research needs.

Information about federal job benefits is available upon request. Moving expenses will be provided for this position.

UFA: Forum on the Future of Fusion Energy and Plasma Science Research in the U.S.

Since the beginning of the year, the UFA has been developing the details for a community-wide meeting: Forum on the Future of Fusion Energy and Plasma Science Research in the U.S. The meeting will be held December 14-15, 2015 on the University of Maryland campus. Additional and registration information can be found at https://sites.google.com/site/universityfusionassociation/forum. Please mark your calendars and plan to attend this important meeting. For more information please contact Uri Shumlak shumlak@uw.edu

Workshop on Integrated Simulations for Magnetic Fusion Energy Sciences Final Report

The Final Report of the Workshop on Integrated Simulations for Magnetic Fusion Energy Sciences is now available on the US BPO website at:

/resources/ref/Workshops2015/IS/ISFusionWorkshopReport.11.12.2015.pdf

The workshop and report assessed the role of integrated simulations in magnetic fusion energy sciences with a focus on identifying gaps, challenges, and new opportunities in fusion theory and computation in the science applications of disruption physics, the plasma boundary, and whole device modeling. The role of computational and enabling technologies was also considered in the crosscutting areas of multiphysics and multiscale coupling; numerical optimization and uncertainty quantification; data analysis, management, and assimilation; and software integration and performance. Strategies and a path forward in each of these areas were articulated in terms of a set of priority research directions. Numerous opportunities for collaborations between plasma physicists and applied mathematicians and computer scientists have been identified that will enable fusion energy scientists to take advantage of emerging extreme-scale architectures and ultimately develop the integrated simulation capabilities needed to predict key physical processes in burning plasmas and next-step devices.

A summary presentation on the findings from the workshop report was also made to the Office of Fusion Energy Sciences and the Office of Advanced Scientific Computing Research by Paul Bonoli (workshop chair) and Lois Curfman McInnes (workshop co-chair) on November 12, 2015 at a meeting in Germantown, MD. This presentation is also available on the US BPO website at

/resources/ref/Workshops2015/IS/doe_slides_11.12.2015b.pdf


Director's Corner

by C.M. Greenfield

ITER International School

Congratulations to the following winners of USBPO scholarships to attend the 8th ITER International School at the University of Science and Technology of China, in Hefei, China, December 14-18 2015:

  • Matthew Beidler (University of Wisconsin)
  • Tess Bernard (University of Texas)
  • Jonathan Coburn (North Carolina State)
  • Drew Elliott (West Virginia University)
  • Chris Everson (University of Washington)
  • Michael Ross (University of Washington)
  • Timothy Younkin (University of Tennessee)

The theme of this yearâs school (http://www.iterschool2015.cn/iis/Siteh\ ome.aspx) will be "Transport and Pedestal Physics in Tokamaks." These schools are primarily designed for graduate students, post-docs, and young researchers. Unfortunately, it is too late to register for this year's school, but these schools are held most years. The location rotates to different ITER partners, with the topic varying from year-to-year. There will also be six US-based experts among the lecturers Phil Snyder (General Atomics), Gary Staebler (General Atomics), Rajesh Maingi (PPPL), Raffi Nazikian (PPPL), C.S. Chang (PPPL), and Joshua Burby (NYU).

ITER Council meets

The ITER Council held its seventeenth meeting last week at ITER Headquarters in Saint Paul lez Durance, France. A highlight of the discussion was an updated long-term schedule. This was in turn referred to an independent review by outside experts. That review is expected to be completed in time for the Council to reach agreement on the updated, resource-loaded long-term schedule by its next meeting in June 2016.

This meeting also marked the end of the term of Bob Iotti of the US as chairman of the ITER Council. The Council elected Won Namkung, from Korea, as its new Chair.

Goldston wins Nuclear Fusion Award

The editorial board of the journal Nuclear Fusion has selected Rob Goldston of PPPL as winner of the 2015 Nuclear Fusion Award. The award recognizes Goldston's paper describing a new model for estimating the width of the scrape-off layer (see also the research highlight later in this issue) as the most outstanding paper published by the journal in 2012.


Research Highlight

Pedestal and Divertor/SOL Topical Group, Leaders: John Canik and Michael Jaworski

In recent years, few parameters in fusion devices have been upended so strongly as the power exhaust channel in the scrape-off layer (SOL) of tokamaks. The ITER design basis made provision for an SOL width of 5mm [1] which, in the face of experimental evidence [2] and theoretical treatments [3] has been scrutinized. In fact, the theoretical treatment by Goldston predicts a heat flux width approximately 1/5 the ITER reference leading to a 5-fold increase in parallel heat flux. The importance of the result was recently reflected in the recent announcement that the paper on the heat-flux scaling had been awarded the 2015 Nuclear Fusion Journal Award. An overview is given below.

[1] A.S. Kukushkin, et al., J. Nucl. Mater. 438 (2013) S203-S207.
[2] T. Eich, et al., J. Nucl. Mater. 438 (2013) S72-S77.
[3] R. Goldston, Nucl. Fusion 52 (2012) 013009.

The Heuristic Drift Model
R.J. Goldston 1
1Princeton Plasma Physics Laboratory, Princeton, New Jersey, USA
fig1.png

Figure 1. Experimental measurements of the inter-ELM exponential feature, λq, in a range of low-gas-puff H-Mode tokamaks, plotted against the HD value [4].

One of the main challenges to make fusion energy a reality is controlling the interface of the plasma with the divertor surface. In order for the divertor to survive, the heat flux to its surface needs to be limited to values that can be tolerated by practical materials. In tokamaks we observe that the heat flux peaks close to strike point location and then exhibits an exponential fall off, characterized by the parameter, λq. The Heuristic Drift (HD) model [1-5] for the scrape-off-layer in low-gas-puff H-Mode tokamaks exhibits remarkable agreement with the exponential power width, λq, measured in a wide range of tokamak experiments [6-8], figures 1 and 2. This model allows us to make predictions with respect to the heat flux width and thus also the peak heat flux in future devices, assuming other physics does not intervene. The key result can be framed most basically, that tokamak plasmas appear to carry a narrow layer of very high heat flux, of width approximately equal to the poloidal ion gyro-radius. Importantly, this width does not scale explicitly with system size, suggesting it is caused by a non-diffusion mechanism.

fig2.png

Figure 2. Regression coefficients and their 1−σ error bars for data shown in the figure on the left, compared with HD model scalings. R/a  ∼ 3 includes ASDEX-Upgrage, C-Mod, DIII-D and JET.

This HD model balances cs/2 ion parallel flows in the Scrape-Off Layer (SOL) against the ion grad B and curv B drifts to find λq = 2(a/R) ρp. Here a/R is the ratio of the minor radius with to major radius of the tokamak and ρp is the poloidal gyroradius. The edge temperature needed to evaluate ρp is derived from Spitzer parallel electron thermal conduction in the conduction-limited regime (TSpitzer), on the assumption that a density channel is established in the SOL by ion dynamics, and the electron parallel heat flux is confined to this channel. The success of the HD model both with respect to the absolute value of the power scrape off width and its scalings with BT, qcyl, P, R and a/R, is illustrated in figures 1 and 2.

More recently, measurements in inner-wall limited L-mode plasmas show that the limiter heat flux contains both a near and a far scrape-off zone, which can be characterized by a "summed exponential" form:

q|| = q||,0,nearexp(−r/λnear) + q||,0,farexp(−r/λfar)
(1)

The HD model was not developed for L-Mode, limiter plasmas. However it is reasonable that similar drifts and parallel flows would take place within the SOL, albeit in a noisier environment and fueled more strongly from the main plasma. In this picture λnear would be determined by HD physics, while λfar would arise from turbulence. In effect λfar would play a similar role to Eich's S parameter [6] for divertor plasmas. Figure 3 shows the result of plotting data for λnear from "summed exponential" fits, kindly provided by the C-MOD [9], COMPASS [10, 11], DIII-D [12], JET [13] and TCV [14] teams, against the HD model. Also included are single points for T-10 [15] and TEXTOR [16]. The similarity in trend and in absolute magnitude to the divertor data in figure 1 is striking, supporting the hypothesis that the same mechanisms are at play.

fig3.png

Figure 3. Experimental measurements of "near" exponential feature in a range of tokamaks [9-16].

It is interesting to derive the MHD α parameter in the SOL based on the assumption that the pressure gradient scale length at the separatrix is approximately equal to the experimentally measured λq. If we use Spitzer conductivity to determine Te and assume nsep = n̄/3 we find that α so defined is given by:


ɑ̃
 
= 1.92 ·10−26 R−9/7 a−2/7 qcyl18/7


 
λq−9/7BT−2 PSOL2/7 (1 + κ2)−1/7
Zeff + 4

5

2/7

 
(2)

Figure 4 shows that this parameter, evaluated for the data shown in figure 1, rises rapidly with fGW, largely independently of other parameters, as can also be seen for the theory by substituting the HD formula for λq into the above equation.

fig4.png

Figure 4. Estimated MHD α parameter vs. Greenwald parameter, for data in Figure 1.


ɑ̃
 

HD 
=
0.123 fGW nsep

ñ


qcylRB

a
PSOL
1/8

 
(1 + κ2)3/2
·

2

Ā
 

(1+


 
)


−9/16


 

Zeff + 4

5

1/8

 
(3)

For typical plasma parameters in this data set, this gives ā ∼ (2−3) ·fGW. These observations suggest that the high-density limit of the H-Mode, and of the Greenwald limit more generally, could be caused by the onset of MHD-driven turbulence in the SOL, rather than in the core or the pedestal.

References
[1] R.J. Goldston, EPS Conference on Plasma Physics (2011) P1.074
[2] R.J. Goldston, Nucl. Fusion 52 (2012) 013009
[3] R.J. Goldston, J. Nucl. Mat. 438 (2013) S372-374
[4] A.V Chankin and D.P. Coster, J. Nucl. Mat. 438 (2013) S463-466
[5] R.J. Goldston, J. Nucl. Mat. 463 (2015) 397-400
[6] T. Eich et al., Phys. Rev. Lett. 107 (2011) 215001
[7] T. Eich et al., Nucl. Fusion 53 (2013) 093013
[8] R.J. Goldston, T. Eich, IAEA Fusion Energy Conference 2012, TH/P4-19
[9] B. Labombard, private communication, 2014; SOL probe data
[10] J. Horaek et al., J. Nucl. Mat. 463 (2015) 385-388; Inner wall IR camera data from recessed limiter.
[11] R. Dejarnac et al., J. Nucl. Mat. 463 (2015) 381-384; Inner wall probe data.
[12] P. Stangeby et al., J. Nucl. Mat. 463 (2015) 389-392; Inner wall IR camera data.
[13] G. Arnoux et al., Nuclear Fusion 53 (2013) 073016 83
[14] F. Nespoli et al., J. Nucl. Mat. 463 (2015) 393-396; Inner wall IR camera data. Data selected to reduce error associated with finite IR coverage: λfar < 3 Rmax − 45 mm based on simulated error analysis.
[15] A.V. Chankin et al., Journal of Nucl. Mat. 145-147 (1987) 789
[16] T. Denner, K.H. Finken, G. Mank, N. Noda, Nucl. Fusion 39 (1999)


Schedule of Burning Plasma Events

USBPO Public Calendar: View Online or Subscribe

2015 -- NSTX-U First Plasma - August 10 W7-X First Plasma --

December 14-15, UFA forum on the Future of Fusion Energy and Plasma Science Research in the U.S., University of Maryland, Maryland, United States

December 16-17, Fusion Power Associates 36th Annual Meeting and Symposium: Strategies to Fusion Power, Washington, DC, United States

2016 -- 10th Anniversary of USBPO Formation --

January 25-28, 22nd ITPA DivSol, ENEA Research Center, Frascati, Italy

March 16-18, ITPA T&C, Institute for Plasma Research, Gandhinagar, India

March 16-18, ITPA PEP, Institute for Plasma Research, Gandhinagar, India

May 30 - June 3, PSI Conference, Rome, Italy

June 19-23, International Conference on Plasma Sciences, ICOPS, Banff, Alberta, Canada

July 4-8, European Physical Society Conference on Plasma Physics (EPS), Leuven Belgium

October 17-22, 26th IAEA Fusion Energy Conference, Kyoto, Japan

October 24-26, ITPA T&C, JAEA, Naka, Japan

October 31 - November 4, 58th APS Division of Plasma Physics, San Jose, California, United States

2019 -- JET DT-campaign - JT60-SA First Plasma --

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Image of the Month

nstxu_mtm_bpo_figure.jpg

NSTX-U (Walter Guttenfelder and Filippo Scotti): Electron thermal transport due to microtearing mode turbulence in NSTX-U. Contours of magnetic field fluctuations (left) and density fluctuations (right) from microtearing mode turbulence simulated using GYRO [J. Candy, R.E. Waltz, Phys. Rev. Lett. 91, 045001 (2003)]. The magnetic perturbations cause field lines to become stochastic, enhancing the loss of electron energy. Upcoming experiments in NSTX-U will attempt to exploit the distinct structure of these microtearing mode fluctuations (compared to traditional ballooning mode turbulence) in an effort to determine their existence and effect on confinement in high-beta plasmas across a range of collisionality.


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: Saskia Mordijck (mordijck@fusion.gat.com)

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