News and Events


U.S. Burning Plasma Organization eNews
January 31, 2014 (Issue 80)

 

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
S. Shiraiwa, et al.
Progress Towards Efficient Off-axis lhcd at Reactor-like Conditions
ITPA Update
Schedule of Burning Plasma Events
Contact and Contribution Information

Announcements

Alcator C-Mod Returns to Operation

The operation of Alcator C-Mod has resumed less than 2 weeks after the signing of the 2014 Omnibus Budget bill by President Obama which authorized funding for C-Mod operation and research. Plasmas were first produced on January 29th after two days of system tests needed to bring up power systems. The next two weeks of operation will be devoted to plasma conditioning of the plasma facing components and diagnostic commissioning. Some limited physics research may also be possible during this period. The investigation of pedestal and boundary physics will be a primary research goal of this run campaign including PMI and studies of I-mode in both single and double null discharges. Work on critical ITER research needs, including disruption physics and runaway electron suppression will also be a high priority. An Ideas Forum is being targeted for early March, during which input from the worldwide fusion community to help plan the campaign will be sought.

For more information, including ways to participate in research, see the Alcator C-Mod Homepage.

US Contributions to 25th IAEA Fusion Energy Conference

Abstract Submission Deadline: February 5, 2014

Details are available through the US Fusion Energy Sciences website at: Schedule and Submission Guidance (PDF, 28 KB)

USBPO Web Seminar

Wednesday, February 26th, 1PM EST, 10AM PST

Presenter: Todd Evans, General Atomics

Title: A Current Perspective on RMP ELM Mitigation

Connection information will be sent by email.


Director’s Corner

by C.M. Greenfield

Happy New Year!

Exciting times in the US fusion program

The recent passage of the FY2014 budget is presenting unexpected opportunities for us to advance the science of burning plasmas. Both DIII-D and C-Mod will be operating this year, while NSTX-U completes its upgrade and prepares to return to operation. With the concomitant increase in funding to $200M toward US contributions for ITER, 2014 should be a year of significant advance in the US Fusion Energy Science community to do its part both in science and technology readiness to make ITER and the ITER research program a success.

Some Changes in the US Burning Plasma Organization

Amadeo Gonzalez of the University of Texas has been the USBPO Administrator since June, 2012. Amadeo is now moving on to a position outside the university. I would like to thank Amadeo for his excellent service during his tenure, and wish him the best of luck in his new job. Teresa Garza, also of University of Texas, is taking over as USBPO Administrator effective immediately. Please join me in welcoming Teresa.

Matt Reinke, deputy leader of the Diagnostics Topical Group, has left his position at MIT to join the faculty of University of York. Matt’s departure is a loss to both the USBPO and the US fusion endeavor, but I am glad to see he will continue to work in fusion, and congratulate him on his new position. Ted Biewer (ORNL) has agreed to step in and complete Matt’s term. I look forward to working with Ted.

Finally, a change that I hope none of you will notice: During the last month or so, Mark London (MIT and USBPO Communications Coordinator) has quietly and transparently moved the USBPO infrastructure (web servers, etc.) from the University of Wisconsin to MIT. Of course, thanks are due to both the UW and MIT staffs who made the smooth move possible, and congratulations to all of them on a job well done. On the off chance that you should happen to notice a problem that has been missed, please let Mark know at webmaster@burningplasma.org. With this change, I believe Jim DeKock (previous USBPO Communications Coordinator) has finally and really retired I hope he’s still reading these, so I can thank him one last time!

Webinars

We will continue—and hopefully expand—our series of USBPO webinars this year. The first one, on February 26, will feature Todd Evans (GA) speaking about RMP ELM control. We are working now on scheduling a second webinar for community discussion of the output of the USBPO Task Group on modes of collaboration with ITER. Please watch for announcements of these and later webinars from Amanda Hubbard (USBPO Deputy Director). We are always looking for webinar topics of general interest to the Burning Plasma community. If you would like to suggest a topic or even volunteer please contact Amanda.

USBPO Task Groups

The USBPO currently has two task groups in place: The aforementioned Task Group on modes of collaboration with ITER (chaired by Rajesh Maingi) is nearing completion of its work, with plans to discuss its output with the community. We will be working in the coming weeks to revitalize the second Task Group, on disruptions and disruption mitigation. We hope to see this group helping to provide bridges between our now two operating tokamaks and with the theory community.

We are also considering ideas for additional Task Groups. Of course these should address issues of interest and importance to the Burning Plasma community, but it is just as important that we can provide added value if a research area is proceeding just fine without a Task Group, there is no need. I think we have learned from previous Task Groups a lot about where a USBPO can be effective and where it can’t. If you have ideas for such an area, please feel free to contact me.

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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 Plasma-wave Interactions Topical Group facilitates U.S. efforts to understand radio frequency plasma heating and current drive in existing and future magnetic fusion devices via experiments and simulations (leaders are David Green and Robert Pinsker). This month’s Research Highlight by S. Shiraiwa, et al., describes experiment and modeling results from MIT’s Alcator C-Mod concerning the upper density limit for Lower Hybrid Current Drive. Gaining a detailed understanding of this phenomenon is the key to alleviating this limitation on this current drive method for possible application to a later phase of ITER or on future devices.

Progress Towards Efficient Off-axis LHCD at Reactor-like Conditions


S. Shiraiwa, S. G. Baek, P. Bonoli, R.R. Parker, G.M. Wallace, and the Alcator C-Mod Team
Massachusetts Institute of Technology, 77 Massachusetts Ave, Cambridge, MA 02139, USA
Corresponding Author’s Address: shiraiwa@psfc.mit.edu

Non-inductive current drive is a technique to drive the toroidal current necessary for confinement without using the Ohmic transformer. The cost of non-inductive current drive (CD) strongly affects the economic and scientific attractiveness of tokamak reactors, and thus the development of an efficient and reliable CD technique is critically important. Lower hybrid current drive (LHCD) [1] has achieved the highest efficiency among various non-inductive CD techniques. It is the only current drive method that has demonstrated sustained plasma current in a tokamak for hours. Advanced steady-state launcher technology providing reliable coupling has also been developed [2, 3]. Indeed, off-axis LHCD is considered a key for achieving the steady-state Q = 5 goal of ITER [4]. Nonetheless, efficient LHCD at reactor-relevant conditions in terms of density, magnetic field, wave frequency and driven current profile have not yet been fully demonstrated. Here, we report on one of the key remaining issues and how it is being addressed in Alcator C-Mod.

Mystery and a New Clue

What prevents us from driving current at the above-mentioned conditions is a large decrease of current drive efficiency at high density, which is often termed a “density limit” (or “lower hybrid density limit” to distinguish from the Greenwald density limit). For example, on Alcator C-Mod, the intensity of hard X-ray emission produced by non-thermal electrons (which carry the driven current) diminishes by nearly two orders of magnitude at ne ∼ 1 × 1020 m−3 compared to that at ne ∼ 0.5 × 1020 m−3 [5]. Observations and modeling have suggested that the injected wave power is not being absorbed in the core region but is lost in the vicinity of the last closed flux surface (LCFS). To help identify where and how the power is lost, a survey of LH wave frequency spectra has been performed using edge Langmuir probes (functioning as RF electric field probes) and high repetition-rate spectrum recorders.

A new clue to recover the current drive at high density was provided by two reciprocating Langmuir probes installed on the high field side (HFS) [6]. In Fig. 1, representative LH wave spectra measured at the HFS for three different line-averaged densities are shown. Note that in the highest density case, the LH wave is not accessible, and therefore is not expected to reach the probe location. Indeed in this case, the amplitude of the injected wave (at 4.6 GHz) is nearly two orders of magnitude smaller compared to the other cases.

Figure 1

Figure 1: LH wave spectra measured at ne = 0.6 × 1020 m−3 (red), 1.1 × 1020 m−3 (blue), 1.3 × 1020 m−3. Launched wave number is centered at N|| = 1.6, and in the case with ne = 1.3 × 1020 m−3, the LH wave does not have core accessibility. (f0 = 4.6 GHz)

At 1.1 × 1020 m−3, the spectrum amplitude at 4.6 GHz did not decrease compared to 0.6 × 1020 m−3 despite the fact that signatures of non-thermal electrons are almost disappeared. An important conclusion (leading to the approach discussed below) is that the LH wave power reaches this area before being lost. Furthermore, the spectrum at 1.1×1020 m−3 shows a secondary peak separated by about 60 MHz from the main peak. This frequency corresponds to the fundamental ion cyclotron (IC) frequency at the HFS of C-Mod. The integrated energy in the side band components is about the same as in the main peak. This observation suggests that the injected LH wave experiences strong IC parametric decay instability (PDI) near the HFS of Alcator C-Mod under these conditions. Due to this non-linear wave coupling, the injected LH wave power is transferred to daughter waves, which are predicted to be strongly damped near the LCFS on the HFS. Figure 2 shows the development of the sideband amplitudes at the fundamental and second harmonic of the HFS IC frequency as the density is increased, thus suggesting that the density limit is associated with the onset of PDIs.

This is not the first time that PDIs have been observed in tokamak LH experiments. Instead, PDIs have been long considered to play a role in the LH density limit. However, past observations and theoretical considerations have focused mostly on PDIs occurring at the LFS, where the LH launcher is installed [7, 8]. What makes data from C-Mod a clear contrast with past observations is that PDIs are observed at the HFS, and that when they occur other probes located at the LFS do not detect them. In addition to the IC PDI, we also observed the LH pump wave broadening locally at the HFS, indicating that these non-linear phenomena may favor plasma and LH wave propagation conditions there. Similar observations were reported from the EAST tokamak recently, indicating that this could be common among diverted tokamaks, although a detailed cross-machine comparison needs to be done for better understanding. Such an effort is ongoing under ITPA joint experiment IOS-5.3[9].

Figure 2

Figure 2: Time trace of line-averaged density (top) and the dynamic spectra measured by a HFS Langmuir probe (bottom), showing an abrupt excitation of lower ion-cyclotron side bands (at t ∼ 1.0 s and 1.15 s).

Throughout the last C-mod run campaign, we consistently observed that a significant fraction of pump wave power is transferred to the HFS PDIs in lower single null magnetic configurations. In contrast, LFS PDIs were not observed to take over a significant amount of the injected spectrum power even when they are excited.

Improving First Pass Absorption is Key

As shown in Fig. 1, substantial wave power reaches the high field side up to the accessibility limit even when PDIs are excited. Although the HFS PDIs are apparently absorbing a significant fraction of power when excited, this would not be an issue if the wave power could be absorbed before reaching the HFS. The current drive efficiency observed at lower density should then be recovered. Our experimental approach to improving the current drive efficiency at high density is based on this simple consideration.

Figure 3

Figure 3: Schematics of LH3 launcher and C-Mod vacuum vessel. Front side of the launcher consists of four-way poloidal and two-way toroidal power splitter.

A key component of our approach is a newly designed additional off-midplane launcher (LH3), which launches the LH wave from a high poloidal angle ( 40 deg.) as shown in Fig. 3 [11, 12]. The purpose of LH3 is not only to double the net LH power but also to maximize the synergistic interaction in the velocity space with the LH waves launched from the existing mid-plane launcher. The detailed discussion on how LH waves interact in velocity space was presented in a previous BPO research highlight [13]. In short, the overall result of the interaction can be summarized in the two simulations computed by GENRAY/CQL3D (Raytracing + Fokker-Planck package) shown in Fig. 4. In Fig. 4, the ray color indicates the LH power normalized by the initial power of each ray (change of color from red to yellow and to blue shows power is absorbed.) On the left, when only the present launcher (LH2) is used, almost all rays reach the inboard side with red color indicating that the power is not absorbed during the first pass. However on the right, when LH3 is used together with the existing LH2 antenna, all rays show the change of color to yellow or blue before reaching the HFS wall. Some rays are completely absorbed during the first pass and do not even reach the HFS wall.

Planned Experiment Replicates LHCD in Reactor

Strong single pass absorption of LH waves is expected on ITER and future reactors due to high electron temperature. However, on present day tokamaks, particularly in experiments assessing LHCD at high density, the LH waves are in a weak damping multi-pass absorption regime, thus making it difficult to predict how LHCD would perform in a reactor. Our planned C-Mod LHCD experiments will focus on addressing this gap, providing crucial information for designing H/CD systems for future experiments. Furthermore, the increased LHCD power will enable C-Mod to operate fully noninductively over a wide range of plasma parameters allowing us to assess the effect of controlling current profile, e.g. q0 and shear, on core plasma performance, thus pointing the way toward efficient steady-state operation.

Figure 4

Figure 4: Comparison of ray-tracing/Fokker–Planck sumulations between when only the present launcher (LH2) is used (left) and when LH3 is used together with LH2 (right). The target plasma in these simulations has line-averaged density of 1.4 × 1020 m−3, and the central electron temperature of 5 keV, based on an existing I-mode discharge [10, 12].

Acknowledgments

This work supported by USDOE awards DE-FC02-99ER54512, DE-FG02-07ER84762 and DE-AC02-76CH03073.

References

[1] P. Bonoli, “Review of Recent Experimental and Modeling Progress in the Lower Hybrid Range of Frequencies at ITER Relevant Parameters”, Phys. Plasmas (submitted)
[2] P. Bibet, X. Litaudon and D. Moreau, Nucl. Fusion 35 1213 (1995)
[3] A. Ekedahl, et. al., Nucl. Fusion 50 112002 (2010)
[4] F. M. Poli and C. E. Kessel, Phys. Plasmas 20 , 056105 (2013)
[5] G. Wallace, et. al., Phys. Plasmas 17, 082508 (2010)
[6] S. G. Baek, et. al., Plasma Phys. Contrl. Fusion 55, 052001(2013)
[7] Y. Takase and M. Porkolab, Physics of Fluids 26, 2992 (1983)
[8] R. Cesario, Nature Communications 1 (2010)
[9] A. A. Tuccillo, et al 2012 “On the use of lower hybrid waves at ITER relevant density” Proc. 24th Int. Conf. on Fusion Energy (San Diego, 2012) (Vienna: IAEA)
[10] D.G. Whyte, et al., Nucl. Fusion 50 105005 (2010)
[11] G. Wallace, et. al., Nucl. Fusion 53 073012 (2013)
[12] S. Shiraiwa, et. al, Nucl. Fusion 53 113028 (2013)
[13] G. Wallace, et. al, USBPO Newsletter, Dec. 2012 issue

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ITPA Update

Coordinating Committee
  4th Meeting, ITER Site, France, December 9–11, 2013
 
Diagnostics Topical Group
  25th Meeting, ITER Site, France, October 15–18, 2013
  The meeting summary written by J.L. Terry is available at the BPO Forum (D).
 
Energetic Particles Topical Group
  12th Meeting, Madrid, Spain, March 31–April 3, 2014
 
Integrated Operation Scenarios Topical Group
  12th Meeting, Massachusetts Institute of Technology, Cambridge, MA, United States, March 31–April 3, 2014
 
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
  25th Meeting, Kyushu University, Japan, October 7–9, 2013
The following is an abbreviated version extracted from the complete summary written by R. Maingi and posted to the BPO Forum (PEP).
  Individual sessions were conducted, along with a set of parallel sessions with the Transport and Confinement and Integrated Operating Scenarios topical groups. The scientific program of the meeting covered many key issues for ITER including:
1) An update on RMP physics from a number of devices, and a review of the status of the ITER in-vessel control coils
2) Pedestal structure and turbulence, and ELM physics
3) Summaries from outstanding PEP joint experiments
4) Summaries of working group activities for pedestal structure, RMP, and pellet ELM pacing, and discussions on topical prioritizations of the working groups
Scrape-off Layer and Divertor Topical Group
  19th Meeting, Kanazawa, Japan, January 20–23, 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 —
 
January 28, Resumption of Operations at Alcator C-Mod
 
February 26, USBPO Web Seminar
  T. Evans, “A Current Perspective on RMP ELM Mitigation”
 
March 10–14, ITPA: 23rd MHD Disruptions, and Control Topical Group Meeting, Toki, Japan
  Held jointly with the US-Japan MHD Workshop on “3-D field effect on MHD control: the roles of magnetic island and stochasticity in optimum MHD control”
 
March 31–April 3, ITPA: 12th Meeting of the Energetic Particles Topical Group, Madrid, Spain
 
March 31–April 3, ITPA: 12th Meeting of the Integrated Operation Scenarios Topical Group, Massachusetts Institute of Technology, Cambridge, MA, United States
 
April 9–11, ITPA: 12th Meeting of the Transport and Confinement Topical Group, Massachusetts Institute of Technology, Cambridge, MA, United States
 
April 15–17, ITPA: 26th Meeting of the Pedestal and Edge Physics Topical Group, IPP, Prague, Czech Republic
 
April 22–25, Transport Task Force (TTF) Meeting, San Antonio, TX, United States
 
April 22–25, 18th Joint Workshop on Electron Cyclotron Emission and Electron Cyclotron Resonance Heating (EC–18), Nara, Japan
 
June 1–5, 20th Topical Conference on High-Temperature Plasma Diagnostics (HTPD), Atlanta, GA, United States
 
2015
W7-X First Plasma
2021
ITER First Plasma
2019
JT60-SA First Plasma
2028
ITER Full DT Operations

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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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