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.

Director's Corner

by C.M. Greenfield

ITER advisory committees meet

The Science and Technology Advisory Committee (STAC) of the ITER Council held its fourteenth meeting May 14-16, and for the first time, the meeting took place in the Council Chamber in the new ITER headquarters building.

The Council at its November 2012 meeting (IC-11) had formulated a set of six charges to STAC-14. Here is a condensed version:

1. Assess the technical aspects of the Level-0 Reference Schedule
2. Assess the various options for deferred and delayed procurements in order to ensure appropriate investment protection and the earliest achievement of DT operation in the ITER Research Plan
3. Assess progress in the resolution of the outstanding issues related to neutronics shielding
4. Assess progress in the analysis of carbon and tungsten divertor options for the first ITER divertor (taking into account the accompanying report by the ITPA).
5. Assess progress in the outstanding issues on Magnets
6. As standing items of the STAC meeting, review brief IO status reports on the progress on key open technical issues

As in previous meetings, the STAC expressed concern about delays that continue to impact the schedule. However, there was encouraging news on some of the critical path schedule items. In particular, work has begun in earnest under the auspices of the European Domestic Agency on several of the buildings. The panorama below shows the sites of the future control, tokamak, and hot cell facilities. The second picture zooms in on the tokamak pit. Although you can’t see them in the pictures, a large and encouraging change from my last visit in December was the presence of a lot of construction activity – hard-hats and cranes now abound.

There was also discussion at the STAC meeting about two major systems that have many of us here in the US very interested. This fall, the ITER Council will consider two proposed changes to the baseline design. One is to formally approve the inclusion of in-vessel coils for ELM control. After hearing updates on the design, which incorporate both a great deal of redundancy (i.e. predicted ability to continue controlling ELMs even if several coils were to fail) and robustness (i.e. failure of a coil is unlikely), the STAC endorsed adding this system to the baseline design.

The ITER site in May, 2013; pictures taken through the rain-soaked windows of the new Headquarters Building. The preponderance of cranes and hard-hats (you may not be able to see them in these photos, but they’re there) is a sign of increased activity, with several “critical path” buildings underway (photos by C. Greenfield). Lower right: Attendees at the STAC-14 meeting standing outside the Headquarters Building (Photo © ITER Organization).

The second major change is to use tungsten divertor targets starting from the earliest tokamak operation. The present baseline design incorporates carbon targets, with the rest of the first wall made of tungsten and beryllium. The design change would remove the carbon and ITER would plan to operate through the first DT campaign with a single all-tungsten divertor (carbon is currently not considered acceptable for use during activated operation due to concerns about tritium retention). The ITPA has recently concluded a study of this change, with the main outstanding concern being the impact of events that might cause melting in the divertor. By the way, the reports from the ITPA Topical Groups on the carbon vs. tungsten divertor question were well prepared and very useful; our thanks to the ITPA for conducting this study. This will be discussed again at the STAC-15 meeting in October, where STAC will make a final recommendation to the ITER Council.

The US attendees at the STAC-14 meeting were Rob Goldston, Chuck Greenfield, Earl Marmar, Juergen Rapp, and Jim Van Dam.

The Management Advisory Committee (MAC) met the following week, on May 21-23, with their discussion focusing further on the schedule issues. Both the MAC-15 and STAC-14 reports will be presented to the ITER Council at its upcoming meeting in Tokyo, Japan, on June 19-20.

Working at ITER

I would like to call your attention to an opening that has been posted at ITER for a candidate with plasma control and operations expertise: Machine Operation Responsible Officer CIE-249. Please see http://www.iter.org/jobs for more information on this and several other recently posted positions, and to apply. Note that most of the currently posted positions, including the above, have an application deadline of June 9. New positions are frequently added, so if you’re interested in working at ITER, it’s worth checking on a regular basis.

USBPO update

As you probably know, the annual US Burning Plasma Council election concluded recently. Thanks to your nominations and an excellent job by our nomination committee (Tom Rognlien, David Brower, George McKee, Steve Knowlton, and Anne White), we had a slate of very strong candidates to choose from. It is now my pleasure to announce that the winners of the election were Chuck Kessel (PPPL) and Larry Baylor (ORNL).

In accordance with the Bylaws of the USBPO, two additional members were appointed to fill out the full Council membership of 12. The new appointed Council members are Dan Thomas (General Atomics) and David Maurer (Auburn University).

I want to thank the departing members of the Council (Richard Buttery, Troy Carter, Steve Knowlton, and Cynthia Phillips) for their valuable service to the community, and welcome the new members. I look forward to working with them during their tenure on the Council.

Now that the new Council is in place, our next step will be to appoint successors for five Topical Group leaders whose term expires this summer. The entire current leadership is shown in the table below, including the years when the terms of the leaders (not the deputy leaders) expire.

Members of each of these Topical Groups should be receiving a request for nominations from the leader of their respective groups. I encourage you to nominate yourself, your friends, your enemies... anybody who you think would be a good candidate. You may also send your suggestions directly to me and/or Deputy Director Amanda Hubbard, by June 18.

Upcoming webinar

The USBPO will resume its series of web seminars on Wednesday, June 12, at 10:00AM PDT/1:00PM EDT. The topic will be Recent Activities in the ITPA Energetic Particle Physics Topical Group. Eric Fredrickson, leader of the USBPO Energetic Particles topical group, will be the presenter. Watch for an announcement with more information and instructions for participation, arriving soon in your in-box.

Plans for APS-DPP conference

For the fifth time, last year's APS Division of Plasma Physics annual meeting included a contributed oral session on Research in Support of ITER, which included 15 talks from US and foreign participants. These sessions have become quite popular, and are always well attended.

Following up on last year's success, the US Burning Plasma Organization is organizing a similar session for the 55th Annual Meeting of the Division of Plasma Physics, which will take place in Denver, Colorado, on November 11-15. As was the case last year, we are looking for talks on research that has been done specifically to address ITER design, operation, or physics issues. These brief talks are “standard” contributed orals: 10 minutes in duration, followed by a 2 minute discussion period. We hope to have broad participation once again, so we can highlight the breadth of this work and the institutions performing it.

If you, or somebody from your institution, are interested in making a presentation in this session, please send the title and author's contact information as soon as possible (but no later than June 17) to Amanda Hubbard (hubbard@psfc.mit.edu) and Gary Taylor (gtaylor@pppl.gov). A sufficient description to understand the work and its importance to ITER will be helpful. An abstract is not required now, but will need to be submitted via the conference website no later than 5:00 PM Eastern Daylight Time on July 12. Please indicate “Research in Support of ITER” in the placement requests box.

Note that space in this session is limited to 15 talks, so we may not be able to include all submissions. We will inform authors by June 28, so any not selected for the ITER session may indicate a preference for other sessions, or allow the conference program committee to select an appropriate session.

It is our intention that these sessions will continue every year for the foreseeable future.

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USBPO Topical Group Highlights

[The BPO Pedestal and Divertor/SOL Topical Group facilitates U.S. efforts to understand the boundary region of magnetic fusion devices through experiment and simulation (leaders are Tony Leonard and Rajesh Maingi). This month's Research Highlight by L. Baylor, et al., describes the first experimental demonstration of increasing ELM frequency in a tokamak plasma through the rapid-fire injection of deuterium pellets. These higher frequency ELMs result in reduced power load along the vessel walls, encouraging further development of this technique for application to ITER. -Ed.]

Experimental Demonstration of High Frequency ELM Pacing by Pellet Injection on DIII-D and Extrapolation to ITER
L.R. Baylor¹, N. Commaux¹, T.C. Jernigan¹, S.K. Combs¹, S.J. Meitner¹, N.H. Brooks², T.E. Evans², M.E. Fenstermacher³, C.J. Lasiner³, A.W. Leonard², R.A. Moyer⁴, T.H. Osborne², P.B. Parks², E.J. Strait², E.A. Unterberg¹, and A. Loarte⁵

¹ Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
² General Atomics, San Diego, California 92186-5608, USA
³ Lawrence Livermore National Laboratory, Livermore, California 94550, USA
⁴ University of California San Diego, La Jolla, California 92093, USA
⁵ITER Organization, Route de Vinon sur Verdon, 13115 St Paul Lez Durance Cedex, France
Email: baylorlr@ornl.gov

Injection of small deuterium pellets at high repetition rate on the DIII-D tokamak has been used to successfully demonstrate for the first time the pacing of edge localized modes (ELMs) at a 12x higher frequency than the natural type I ELMs [1]. This demonstration of ELM pacing was made by injecting slow (< 200 m/s) 1.3 mm diameter D₂ pellets at 60 Hz from the low field side in an ITER shaped plasma at q₉₅=3.5 with a low natural ELM frequency of 5 Hz, β_Ν=1.8, normalized energy confinement factor H₉₈=1.1, and with the input power only slightly above the L-H power threshold. The non-pellet reference discharges had ELM energy losses up to 50 kJ (~8% of total stored energy), while the case with pellets demonstrated 60 Hz ELMs with an average ELM energy loss less than 5 kJ (< 1% of the total). Total divertor energy deposited by the ELMs was reduced by an average factor greater than 10 (as measured by an IR camera), and the peak particle flux to the divertor from ELMs was also greatly reduced. Central impurity accumulation of Ni was significantly reduced by the application of the 60 Hz pellets.

Figure 1: Comparison of 60 Hz pellet case (red) and natural ELM H-mode plasmas with 5 Hz ELMs (black). Divertor deposited energy and divertor particle flux are shown with nominal pellet times marked by blue tick marks. Central Ni emission, normalized energy confinement H98, and line integral density are shown.

No significant increase in density or decrease in energy confinement with the pellets was observed, as can been seen in Fig. 1. The individual pellets are undetectable in the interferometer density measurements, due to their small size and lack of fueling. The small ELMs that are triggered within 0.5 ms of the pellet entering the plasma eject the pellet mass. The plasma rotation is reduced at the edge from the pellets; however the central rotation is strongly increased without any increase in angular momentum input. Experimental details of the pellet ablation light and magnetic loop signals indicate that the ELMs were triggered before the pellet reached the top of the H-mode pressure pedestal, implying that very small, shallow penetrating pellets are sufficient to trigger ELMs. Fast camera images of the pellet entering the plasma from the low field side show a local triggering phenomenon. A single plasma filament became visible near the pellet cloud and struck the outer vessel wall within 200 μs. Additional ejected filaments were then observed to subsequently reach the wall. This is consistent with the hypothesis that the pellet cloud produces a local pressure perturbation that triggers a local high-n ballooning mode instability that manifests itself as the reduced size ELM. The plasma stored energy loss from the pellet triggered ELMs appears to be a function of the elapsed time after a previous ELM.

Figure 2: Pellet injection geometry used for ELM pacing studies on DIII-D. The lower trajectory mimics the planned ITER low field side injection line for pellet ELM pacing.

In these experiments, the pellets were injected from the low field side from both the midplane (Fig. 2, blue arrow) and newly installed lower port (Fig. 2, red arrow) that mimics the injection geometry that is proposed for ITER. Both trajectories lead to ELM triggering for this size and speed of pellet, agreeing with previous observations for vertical low field side injection [2,3]. No obvious difference is observed in the ELMs triggered from these three locations. The flux expansion in the plasma near the divertor enables a more localized pellet perturbation at the edge and precise measurement of the position of the pellet in the plasma at the ELM onset. The measurements there indicate that pellets of this size triggered ELMs when they reached the radial location that has approximately half of the peak pedestal pressure, which was ~2 cm inside the last closed flux surface along this trajectory.

The divertor energy deposited and peak heat fluxes from ELMs were determined for a series of discharges with different pellet injection frequencies, and a reference without pellets. The results of this frequency scan are shown in Fig. 3. Both the total energy and peak heat flux show a trend of reduced levels as a function of the pellet frequency. A curve showing the energy loss times ELM frequency equaling a constant is shown starting at the natural ELM frequency of 5 Hz. The data for the pellet triggered ELMs falls slightly under this curve. The peak heat flux on the outer strike point is found to be larger than on the inner strike point for all ELM frequencies including the 5 Hz natural ELMs.

The pellet ELM pacing reduces the average edge pedestal electron pressure compared to the natural ELMing case by approximately 20%. Despite the reduction in pedestal electron pressure, the normalized confinement H98y2 is not significantly affected. This is believed to be due to the profiles not remaining stiff when the pedestal electron temperature is reduced and to additional rotation shear that occurs when the pellets reduce the rotation at the plasma edge, while the central rotation is not reduced. The reduced impurity concentration also plays a role in maintaining the confinement when the pellets are applied. The edge Z_eff is reduced on the order of 50% based on the carbon reduction, which is the dominant impurity in these plasmas.

Figure 3: The average energy and peak heat flux deposited in the divertor as a function of pellet induced ELM frequency. The IR camera data is integrated assuming axisymmetric deposition.

Pellet ELM pacing has been proposed some time ago as a method to prevent large magnitude ELM events that can erode the ITER plasma facing components [4]. From these experiments it appears that the technique can be applied at pellet repetition rates approaching ITER requirements without deleterious effects while greatly reducing the spontaneous ELM generated divertor heat flux. ITER will require a reduced peak heat flux of more than a factor of 20 from the anticipated natural ELMs [5]. The technique is anticipated to be applicable in any plasma configuration without regard to edge q95 resonances; however this needs to be demonstrated. Issues that remain to be further investigated are the minimum size pellet perturbation that will reliably trigger ELMs and whether inner wall high field side pellet fueling, as planned for ITER plasma fueling, in concert with pellet ELM pacing can work synergistically to maintain high plasma performance. The heat flux in the divertor from pellet triggered ELMs has also been shown to be non-axisymmetric [6] and therefore better diagnostic coverage in the divertor will be needed to further study the impact on the divertor plasma facing components.

This work was supported by the US Department of Energy under DE-AC05-00OR22725, DE-FC02-04ER54698, DE-AC52-07NA27344 and DE-FG02-07ER54917.

References

1. This research highlight is for BPO information and is a summary of work that has been accepted for publication. L.R., Baylor et al., accepted for Phys Rev. Lett. (2013); accepted Phys. Plasmas (2013)
2. L.R., Baylor et al., Nucl. Fusion 49, 085 013 (2009)
3. L.R., Baylor et al., EPS (2010); Paper P2.117, http://ocs.ciemat.es/EPS2010PAP/html/author.html
4. P.T. Lang et al., Nucl. Fusion 44, 665 (2004)
5. A. Loarte et al., 25th IAEA Fusion Energy Conf., San Diego, USA 2012 (IAEA, Vienna) Paper ITR/1-2, submitted to Nucl. Fusion
6. R.P. Wenninger, et al., Plasma Phys. Contol. Fusion 53, 105002 (2011)

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ITPA Update

Summary information is prepared by the BPO Topical Group leaders with input from other meeting attendees. When possible, additional information is provided through BPO forum postings.

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Announcements

BPO Web Seminar: June 12 at 1:00 pm Eastern
“Recent activities in the ITPA Energetic Particle Physics Topical Group”
Eric Fredrickson, PPPL
Remote connection details will be sent to BPO members through email.

BPO Topical Group Leadership Rotation
New leaders are due for the following Topical Groups: Energetic Particles, Fusion Engineering Science, Operations and Control, Pedestal and Divertor/SOL, and Plasma-wave Interactions. Suggestions for new leaders may be sent (by June 7) to the present Leader of each group. The list of present leaders is available online at: burningplasma.org/groups.html

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Schedule of Burning Plasma Events

Click here to visit a list of previously concluded events.

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Contact and Contribution Information

This newsletter provides a monthly update on U.S. Burning Plasma Organization activities. Topical Group Highlight articles are selected by the Leader and Deputy Leader of those groups (burningplasma.org/groups.html). ITPA Reports are solicited by the Editor based on recently held meetings. Announcements, Upcoming Burning Plasma Events, and all comments may be sent to the Editor. Suggestions for the Image of the Month may be sent to the Editor. The images should be photos, as opposed to data plots, though combined graphics are welcome. The goal is to highlight U.S. fusion resources through interesting visualizations.

Become a member of the U.S. Burning Plasma Organization by signing up for a topical group:
burningplasma.org/jointopical

Editor: David Pace (pacedc@fusion.gat.com)
Assistant Editor: Amadeo Gonzales (agonzales@austin.utexas.edu)

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Image of the Month

Reaching Out with Plasma

The Science Education Department at the Princeton Plasma Physics Laboratory (PPPL) aims to expose the public to plasma physics and fusion technology through a variety of forums. Their programs target K-12 teachers, students, two- and four-year college undergraduates, and the general public. These programs include classroom visits, professional development workshops, research internships, mentoring, and public science expositions and talks. The bottom-left panel shows group leader Andrew Zwicker working on a plasma speaker in a Faraday cage with student Sean Lerch. A plasma demonstration led by Arturo Dominguez is given to a group of visiting students as shown in the bottom right panel. The present team includes Andrew Zwicker, Deedee Ortiz, Arturo Dominguez, and Aliya Merali (left to right in the top panel, posing in front of a plasma demonstration tube).

Contributed by Arturo Dominguez, Princeton Plasma Physics Laboratory, Princeton, NJ 08543 PPPL Science Education homepage,
http://science-education.pppl.gov/Science_Education_Main_Site/Welcome.html

PPPL Open House held on June 1, 2013,
http://www.pppl.gov/events/pppl-open-house

Click here to visit a Directory of Other Plasma Events

Please contact the administrator with additions and corrections.

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