News and Events


U.S. Burning Plasma Organization eNews
May 31, 2014 (Issue 84)

 

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
J.E. Rice
The Curious Rotation Reversal Phenomenon and Its Relation to Some Long-Standing Mysteries
ITPA Update
Schedule of Burning Plasma Events
Image of the Month
C. Collins
Spinning Unmagnetized Plasma
Contact and Contribution Information

Announcements

Application Deadline Extended for BPO Scholarships to 7th ITER International School

Application Deadline: Wednesday, June 4, 2014

Application Instructions: Submit the following materials to Dr. François Waelbroeck (flw@mail.utexas.edu)

1. Vita

2. List of Publications

3. Statement including the reason(s) your participation at this School would be beneficial

4. Reference letter from a senior scientist



New Question & Answer Section Begins in June Newsletter

Each month the BPO newsletter will present a fusion-related question answered by the community at large. Readers are encouraged to submit their own explanations/answers by emailing the editor. A single response will be chosen as the best answer and included in the newsletter. The only restriction is that the answer must be limited to no more than 618 characters (including spaces). Proposed answers may be as technical as the submitter desires.

June 2014 Question: What is an Edge Localized Mode (ELM)?


Director’s Corner

by C.M. Greenfield

ITER International School

The 7th ITER International School will be held in Aix en Provence, France, during August 25–29, 2014. The theme of this year’s Summer School will be “Highly parallel computing in modeling magnetically confined plasmas for nuclear fusion.” These Schools are primarily designed for graduate students, postdocs, and young researchers. If you are interested in attending, please go to the School’s web site (http://iterschool.univ-amu.fr/) and register. The registration deadline is being extended to June 15 (the website may still say June 1).

The US Burning Plasma Organization will once again make available several scholarships for US participants to this year’s ITER Summer School. The scholarships will cover round-trip airfare, registration fee, and six nights of student housing, which three items constitute the bulk of the expenses. Participants’ home institutions are encouraged to supplement the scholarships to cover other travel-related expenses.

Please send applications to Dr. François Waelbroeck (flw@mail.utexas.edu), chair of the selection committee. In each application, please include (1) a vita, (2) a list of publications, (3) a statement about the reasons why your participation at this School would be beneficial, and (4) a letter of reference from a senior scientist who is knowledgeable about you. Due to the rapidly approaching registration deadline, we ask that your applications be received no later than Wednesday, June 4 (note that the deadline for scholarship applications has been extended from the original May 30).

ITER Advisory Committees Meet

The Science and Technology Advisory Committee (STAC) of the ITER Council held its sixteenth meeting May 13–15. The Management Advisory Committee (MAC) is currently holding its meeting. The reports of each of these groups will be presented at the upcoming ITER Council meeting.

Major areas of discussion at STAC–16 included details of design and manufacture of several important ITER components, as well as the ITER Research Plan. Of interest to the burning plasma community are two broad areas where additional research is needed.

Attendees at STAC–16

Attendees at the STAC–16 meeting in May 2014 (Photo © ITER Organization)

ITER will begin its research operations with a non-activated phase during which hydrogen and helium will be the working gasses. It is anticipated that ITER will not obtain the needed high confinement H-mode in hydrogen, so the ITER Research Plan calls for the ELM control systems to be commissioned in helium. Experiments with helium plasmas in present-day devices should verify that the techniques used to control ELMs in deuterium plasmas behave in a similar manner in helium. Also, it is known that under some conditions helium operation on tungsten divertor targets can lead to changes in the surface morphology. It is hoped that additional research can clarify this issue.

Another area that still requires attention is the mitigation of disruptions, including avoidance and/or control of significant runaway electron populations that could damage plasma-facing components. The final design review for the ITER Disruption Mitigation System is still three years away, but further work is needed to validate the anticipated mitigation techniques (massive gas and/or shattered pellet injection) and to develop schemes for preventing damage from runaway electrons.

The US attendees at STAC-16 were Rob Goldston, Chuck Greenfield, Earl Marmar, Juergen Rapp, and Jim Van Dam. I am happy to say that we all attended the meeting in person. This contrasts with our experience at STAC–15, where four of us participated via video from the US as we were not allowed to travel to France during the government shutdown.

ITER site in May 2014; Aerial view of pit

Left: The ITER site in May, 2014 (Photo by C. Greenfield)
Right: Aerial view of the ITER seismic pit (Photo © ITER Organization)

The view of the ITER site from the headquarters building did not look extremely busy. However, there is a great deal of work going on in the seismic pit (where we didnt have a very good view), with about 100 workers laying rebar, securing embedded plates, and preparing for the next two segment pours for the Tritium Building. The entire basemat, which will support the tokamak and many of its ancillary facilities, is anticipated to be complete this summer.

Postings for ITER positions

ITER regularly posts their openings at http://www.iter.org/jobs. There are currently sixteen positions shown there, with application deadlines through June 20. One posted job that may be of interest to our readers is for a Neutral Beam Section Leader. The application deadline for this post is June 20.

Two Current FESAC Subpanels Underway

The work of two FESAC (Fusion Energy Sciences Advisory Committee) subcommittees is underway, and once again, the USBPO is assisting with communication to the community at large. You can find links to each at http://burningplasma.org/. Look for “2014 FESAC Strategic Planning Panel” and “FESAC Subcommittee on Workforce Development.” The two committees are:

• The Subcommittee on Workforce Development, chaired by Hantao Ji. This group has only a short time to complete their work by June 30.

• The Strategic Planning Panel, chaired by Mark Koepke is holding two public meetings, in June and July. Their work is due October 1, and is being done as input for the Fusion Energy Science strategic plan requested by Congress and due in January.

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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 Confinement and Transport Topical Group works to facilitate U.S. efforts to understand plasma confinement via improved measurements and computational models for existing and future magnetic fusion devices (leaders are George McKee and Gary Staebler). Intrinsic toroidal rotation has been observed in numerous tokamaks and is of particular interest to burning plasmas since rotation impacts tearing mode stability, transport and energy confinement. Recent work on C-Mod is unraveling the parametric dependencies and testing theoretical mechanisms behind intrinsic rotation, rotation reversals, and the connection to turbulence behavior.

The Curious Rotation Reversal Phenomenon and Its Relation to Some Long-Standing Mysteries


J.E. Rice
Plasma Science and Fusion Center, Massachusetts Institute of Technology, Cambridge, MA 02139, USA

The curious rotation reversal phenomenon, where an abrupt change in the direction of self-generated toroidal flow occurs with very slight changes in macroscopic parameters, has been observed in several devices [1, 2, 3, 4, 5, 6, 7, 8, 9]. An example of dynamic reversals in a single C-Mod discharge, induced by changes in the electron density, is shown in Fig. 1. The central rotation velocity reversed from ∼−20 km/s in the counter-current direction to ∼+10 km/s in the co-current direction following a slight reduction in the electron density, beginning ∼0.67 s, and then reversed again as the density was raised by less than 10%. This reversal only transpired in the core plasma and was not seen in the outer regions.

Figure 1

Figure 1: Time histories of the average electron density (top) and the toroidal rotation velocity (bottom) in the center (solid red) and outside of r/a = 0.75 (dashed green), for a 1.05 MA, 5.4 T (q95=3.2) discharge which underwent two rotation reversals.

Rotation reversals in Ohmic plasmas have also been triggered through changes in the plasma shape, inverse rotational transform profile, total plasma current and toroidal magnetic field, with an example of the latter shown in Fig. 2. This is a comparison of two C-Mod discharges at constant density, one with ramps in BT, the other without. In the plasma with the reduction in BT there occured a rotation reversal, and then a return to the original state with an increase in magnetic field.

The critical density for rotation reversal is actually a function of q95. A large body of C-Mod data is summarized in Fig. 3, which shows the central steady state rotation velocity as a function of the product of ne and q95, for individual discharges. Above the value 3.7 in the product, the core rotation is directed counter-current; below this critical value the rotation is co-current. As can be seen, the transition is very sharp. The ranges for ne and q95 in this plot are from 0.3 to 2.0 × 1020/m3 and 2.6 to 7.2, respectively.

Figure 2

Figure 2: Time histories of 0.8 MA LSN discharges with (solid red) and without (dashed green) magnetic field ramps.

In order to address the question of what can cause the toroidal rotation to switch direction while other macroscopic parameters remain relatively unchanged, it is informative to examine the momentum flux. The momentum flux is proportional to the Reynolds stress, which can be written as the sum of three terms [10],

-χφ∂vφ/∂r + VPvφ + Πres

respectively proportional to the momentum diffusivity χφ, the momentum pinch VP and the residual stress Πres.

Figure 3

Figure 3: The central toroidal rotation velocity as a function of the neq95 product for Ohmic plasmas. The vertical line represents neq95 = 3.7.

A reversal in intrinsic rotation requires either a change in sign of VPvφ at the last closed flux surface or a change in sign of the residual stress, and the resulting intrinsic torque, -∂rΠres. (χφ is positive definite and cannot change sign.) The density gradient definitely does not change sign before, during or after the rotation reversal [5], so the flow pinch most likely does not play a role. The residual stress can change sign depending on the nature of the underlying turbulence, such as by a switch in the mode propagation direction; the sign of the residual stress is predicted to flip when the propagation direction of the turbulence in the plasma frame changes [11]. One example would be a change of wave propagation from the electron diamagnetic drift direction (electron drift waves or TEMs) to the ion diamagnetic drift direction (ion drift waves or ITG modes) as the density exceeds a critical threshold [11]. The observed reversal in rotation at a critical value of the product neq95, which is proportional to the collisionality ν*, is consistent with the ansatz that at low collisionality, TEMs are dominant while at high collisionality, ITG modes dominate. In fact, turbulence changes at a critical collisionality have been observed to accompany rotation reversals.

There are several other phenomena (and long-standing mysteries) that change in concert with rotation reversals, and most likely are related: the saturation of Ohmic energy confinement [12] (29 years), the cut-off of “non- local” heat transport [13] (19 years) and the appearance of edge up/down impurity density asymmetries [14] (37 years). This intimate relationship is demonstrated in Fig. 4 which shows the global energy confinement time, core electron temperature changes following edge cold pulses, the central toroidal rotation velocity and the edge up/down brightness ratio of argon emission as a function of electron density for 5.4 T C-Mod Ohmic discharges at 0.8 MA.

Figure 4

Figure 4: The global energy confinement time (top), percent change in electron temperature at R = 0.78 m following impurity injections (second frame), core toroidal rotation velocity (third frame) and up/down edge impurity brightness ratio (bottom) as a function of electron density for 0.8 MA, 5.4 T discharges.

The energy confinement time changes from the linear Ohmic confinement (LOC) regime to the saturated Ohmic confinement (SOC) regime at the same critical density as the rotation reversal, although this transition is not as sharp, and is somewhat subjective. The “non-local” effect cut-off occurs at the same critical density for this plasma current and is equally abrupt. The up/down impurity density asymmetry also appears, however at a slightly lower density. These changes are shown dynamically during a single C-Mod discharge with a density ramp across the LOC/SOC boundary, which is presented in Fig. 5.

Figure 5

Figure 5: Time histories of the electron density (top), core electron temperature (second frame), edge up/down forbidden line brightness (third frame) and core toroidal rotation velocity (bottom) for a 0.8 MA discharge with a rotation reversal. Impurity injection times for the cold pulses are shown by the vertical dotted lines. In the top frame, the critical density separating the LOC and SOC regimes is indicated by the horizontal dashed line.

This plasma had four impurity injections in order to provide edge cooling. During the low density (low collisionality) LOC phase (up to 1.2 s) the central electron temperature increased following the edge cooling (the “non-local” effect), the impurity density profiles were up/down symmetric and the rotation was directed co-current. At 1.4 s during the SOC phase, the core temperature dropped following the edge cooling demonstrating normal thermal diffusion. At this time the impurity density profiles were up/down asymmetric and the rotation switched to the counter-current direction. The connection between these phenomena for other plasma currents is demonstrated in Fig. 6 which shows the critical density for the various transitions as a function of 1/q95.

These effects all transpire at a fixed value of the neq95 product, suggesting that there is a critical collisionality which governs all of these changes. This is further supported from examination of similar changes from other devices, which suggests a 1/R dependence for the critical density at fixed q95. This indicates that the LOC/SOC transition should occur in ITER at an electron density of ∼1 × 1020/m3 with q95 = 3.5.

Figure 6

Figure 6: The density for rotation reversal (dots), electron temperature inversion (boxes), LOC/SOC transition (asterisks) and up/down impurity brightness asymmetry (triangles) as a function 1/q95.

Work supported at MIT by DoE Contract No. DE-FC02-99ER54512.

References

[1] A. Bortolon et al., Phys. Rev. Lett. 97 (2006) 235003.

[2] B.P. Duval et al., Plasma Phys. Contr. Fusion 49 (2007) B195.

[3] J.E. Rice et al., Plasma Phys. Contr. Fusion 50 (2008) 124042.

[4] B.P. Duval et al., Phys. Plasmas 15 (2008) 056113.

[5] J.E. Rice et al., Nucl. Fusion 51 (2011) 083005.

[6] J.E. Rice et al., Phys. Rev. Lett. 107 (2011) 265001.

[7] J.E. Rice et al., Phys. Plasmas 19 (2012) 056106.

[8] J.E. Rice et al., Nucl. Fusion 53 (2013) 033004.

[9] R.M. McDermott et al., Nucl. Fusion 54 (2014) 043009.

[10] P.H. Diamond et al., Nucl. Fusion 49 (2009) 045002.

[11] P.H. Diamond et al., Phys. Plasmas 15 (2008) 012303.

[12] R.R. Parker et al., Nucl. Fusion 25 (1985) 1127.

[13] K. Gentle et al., Phys. Rev. Lett. 74 (1995) 3620.

[14] J.L. Terry et al., Phys. Rev. Lett. 39 (1977) 1615.

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ITPA Update

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

Diagnostics Topical Group
  26th Meeting, Pohang Univ. of Sci. and Tech., Republic of Korea, May 19–22, 2014
 
Energetic Particles Topical Group
  12th Meeting, Madrid, Spain, March 31–April 3, 2014
  A summary written by D. Pace with contributions from P. Bonoli, D. Brower, B. Harvey, and D. Spong is available on the BPO Forum (EP).
 
Integrated Operation Scenarios Topical Group
  12th Meeting, Massachusetts Institute of Technology, Cambridge, MA, United States, March 31–April 3, 2014
  A summary written by S. Gerhardt with help from T. Luce, C. Kessel, and C. Holcomb is available on the BPO Forum (IS).
 
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
 
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 —
 
June 1–5, 20th Topical Conference on High-Temperature Plasma Diagnostics (HTPD), Atlanta, GA, United States
 
June 16, Students participating in the National Undergraduate Fellowship Program (NUF) arrive at their host institutions.
 
June 23–27, 41st EPS Conference on Plasma Physics, Berlin, Germany
 
August 25–29, 7th ITER International School, Aix-en-Provence, France
 
September 8–11, 19th Joint EU-US Transport Task Force Meeting (TTF), Culham, United Kingdom
 
October 13–18, 25th IAEA Fusion Energy Conference (FEC 2014), St. Petersburg, Russia
 
October 27–31, 56th APS Division of Plasma Physics Conference, New Orleans, LA
 
2015
W7-X First Plasma —
 
January, Due date for report concerning the ten-year strategic plan of the Fusion Energy Sciences division of the US Department of Energy.
 
2016
— 10th Anniversary of USBPO Formation —
 
2017
JET DT-campaign —
 
2019
JT60-SA First Plasma —

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Image of the Month

Image of the Month

Spinning Unmagnetized Plasma

Beginning as a sketch on the whiteboard in 2007, the Plasma Couette Experiment (PCX) in the Department of Physics at the University of Wisconsin-Madison has since successfully demonstrated a new concept for producing a sufficiently hot, fast flowing, magnetic field-free plasma suitable for laboratory studies of flow-driven MHD phenomenon such as the magnetorotational instability (MRI). MRI is of interest for its role in angular momentum transport in one of the most powerful processes in the universe: accretion. While poorly understood, the accretion process converts 25–40% of rest mass to energy in some neutron star and black hole systems (hydrogen-isotope fusion is 0.7% efficient). In this image, graduate student C. Collins† prepares the water-cooled assembly of over 1900 permanent magnets for installation inside the 1 m tall, 1 m diameter vacuum chamber. Rings of alternating polarity magnets form an axisymmetric cusp field that quickly vanishes away from the boundaries, leaving a large, unmagnetized bulk plasma. A view through the magnet rings (bottom-left image) shows an argon plasma (Te < 10 eV, ne < 1011 cm−3) produced by 6 kW of 2.45 GHz microwave heating. Flows (up to 12 km/s, M = V/Cs ∼ 0.7 in helium) are induced through applied J × B torque, where current is driven by electrostatically biased hot cathodes (upper-right image) at both the inner and outer magnetic boundaries of the device. Probe measurements show that the azimuthal flow viscously couples momentum from the magnetized edge into the unmagnetized bulk, creating controlled, differential rotation profiles modified only by the presence of neutral particle drag. The stirring technique developed on PCX is now being implemented in the larger, recently constructed Madison Plasma Dynamo Experiment, a 3 m diameter sphere designed to study the spontaneous generation of magnetic fields through dynamos while serving as a training ground for future plasma physicists.

Contributed by C. Collins, Department of Physics, University of Wisconsin-Madison, Madison, WI 53706

† Presently a postdoctoral researcher with the Department of Physics and Astronomy, University of California–Irvine conducting magnetic fusion research at the DIII-D National Fusion Facility in San Diego, CA.

C. Collins, et al., “Taylor–Couette Flow of Unmagnetized Plasma”, Phys. Plasmas 21, 042117 (2014)


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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