Dear Burning Plasma Aficionados:

This newsletter provides a short update on U.S. Burning Plasma Organization activities. Comments on articles in the newsletter may be sent to the editor (Tom Rognlien trognlien at llnl.gov) or assistant editor (Rita Wilkinson ritaw at mail.utexas.edu).

Thank you for your interest in Burning Plasma research in the U.S.!

Director's Corner by J. W. Van Dam

Remote Seminar about ITPA Meetings

On Tuesday, February 22, beginning at 12:00 noon Eastern Standard Time, the USBPO will sponsor a remote seminar to report about the recent meeting of the ITPA Transport and Confinement topical group and the activities of this topical group. In addition, there will be a report about the December meeting of the ITPA Coordinating Committee and the status of the Joint Experiments and Joint Activities. The speakers will be John Rice and Erol Oktay. The seminar is planned to last for about one hour. Connection information is provided below. Please mark it on your calendars.

Connection information for the USBPO seminar on Tuesday February 22 at 12:00 p.m. Eastern Standard Time, 9:00 a.m. Pacific Standard Time: Two web-browser windows are needed to attend the seminar: one for video-plus-audio, and one for viewing the presentation slides.

1. Video and Audio (receive/listen only), using ESnet 'streaming':
   Use a web browser to connect to: http://mcu1.es.net or http://198.129.252.171
   Mac users: FireFox recommended. Safari may have problems.
   Sign-in name: <---- please type your name
   Conference ID: 411411
   Please do not enter a PIN number.
   Streaming rate: choose QuickTime or Real Player.
   Click "Stream this conference". If you see "invalid ID" then the conference has not yet started; try again in a few minutes.
2. Presentations, using ReadyTalk web conferencing:
   To view the seminar slides, open another browser window and connect it to: http://esnet.readytalk.com
   In the "Join a Meeting" window, type the Access Code of this seminar: 2705718 and click "Join".
   

NOTE: ReadyTalk Audio is *not* used for this seminar; please do not dial the telephone numbers displayed by ReadyTalk.

If you require alternate connection methods to attend this seminar then please send email to support@burningplasma.org.

Scholarships for 2011 ITER Summer School

The 5th ITER International Summer School will be held in Aix en Provence, France, during June 20-24, 2011. The theme of this year’s Summer School will be “MHD and Energetic Particles.” 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 and register. According to the web site, the registration deadline is February 25, but I was informed that this is to be extended to April 1.

The USBPO will make available eight scholarships for US participants to this year’s ITER Summer School. “US participants” must be US citizens working at US institutions. The scholarships will cover round-trip airfare, registration fee, and six nights of student housing, which are the three items that 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 Prof. Michael Mauel, chair of the USBPO Council. 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. Please submit applications for scholarships by March 23.

Retirement News

The ITPA Coordinating Committee congratulated Dr. Erol Oktay at its December 2010 meeting (photograph courtesy of ITER)

At the December meeting of the ITPA Coordinating Committee, the Committee members paid tribute to Dr. Erol Oktay of the U.S. Department of Energy (DOE), who announced that he is retiring after a career of 40 years in fusion research, of which 36 years were spent working in the DOE Fusion Energy Sciences program. For many of these years, Dr. Oktay was deeply involved in promoting international collaborations in fusion. For the last five years, he served as Acting Director of the ITER and International Division of the Office of Fusion Energy Sciences at DOE. He was a member of the US delegation to the meetings of the Science and Technology Advisory Committee of the ITER Council. He has been the program manager for the DIII-D National Fusion Facility. And, very importantly, at DOE, he has been the lead person for the USBPO, in which capacity he has attended biweekly Research Committee meetings, biweekly Executive Committee meetings, and quarterly USBPO Council meetings. It would be difficult to list all the ways in which he has nurtured the activities of the USBPO. I interacted with Erol frequently and was always amazed at the workload he carried. Most impressive and appreciated has been his genuine passion for fusion. On behalf of the US Burning Plasma Organization, I express sincere appreciation to him and best wishes for a pleasant retirement. Erol, we’ll miss you!

Upcoming ITPA Meetings

Proposals for five of the seven Spring 2011 ITPA Topical Group meetings are now posted on the USBPO web site. If you have any contributions to the topical areas and/or wish to attend any of these meetings, please contact the respective US Coordinator and/or Deputy Coordinator. For reference, here are their names:

Big Physics Symposium

Recently I was invited to give a talk with the title “An Overview of ITER and US Efforts” at a meeting, called Big Physics Symposium, held February 7 in Paris, France. The focus of the meeting was on control, diagnostic, and measurement for physics systems and experiments. There were five talks about plasma control systems and remote handling for tokamaks, including a keynote address by Dr. Dhiraj Bora, ITER Deputy Director-General for the CODAC Directorate. In addition, there were four talks about large particle accelerators, specifically at CERN, a talk about the European EISCAT facility for radar studies of the atmosphere and ionosphere, and specialized talks about control instrumentation and software. My talk is posted on the USBPO web site, Reference Files page.

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USBPO Topical Group Highlights

(Editors note: The BPO Pedestal and Scrape-Off Layer Topical Group works to facilitate U.S. efforts to understand the boundary region of existing and future magnetic fusion devices via experiments and simulations (leaders are Tom Rognlien and Tony Leonard). This month's Research Highlight by Xueqiao Xu et al. summarizes ongoing work to simulate nonlinear ELM dynamics including the radial ejection of pedestal plasma.)

Nonlinear simulations of peeling-ballooning modes with anomalous electron viscosity and their role in edge-localized-mode plasma ejection
X.Q. Xu, M.V. Umansky (Lawrence Livermore National Laboratory); B. Dudson, H.R. Wilson (University of York, UK); P.B. Snyder (General Atomics)

The edge-localized modes (ELMs) are quasi-periodic relaxations of the high-pressure edge pedestal during high confinement H-mode operation that are observed routinely in tokamaks [1]. The resulting hot plasma ejection could potentially damage the ITER divertor plates and first wall, as well as reduce energy confinement. Consequently, one of the ITER “urgent tasks” is to assess the effect of ELMs on the plasma confinement and the associated energy loss and fluxes to plasma-facing components (PFCs).

Through the development of the theory of peeling-ballooning (P-B) modes and their numerical implementation in codes such as ELITE [2,3], a robust prediction of edge MHD stability limits is available for existing and future tokamaks. It has been found that large ELMs are triggered, and pedestal height is constrained, by ideal peeling-ballooning stability. Peeling-ballooning modes are ideal MHD modes, which are driven by a combination of steep edge pressure gradients (ballooning) and bootstrap current in the pedestal. Onset of each ELM (type-I) is consistently found to correlate with crossing of the ideal peeling-ballooning stability boundary [4]; i.e., peeling-ballooning theory successfully describes the trigger of the ELM. However the nonlinear dynamics, and, in particular, the physics of the ELM energy loss and pedestal dynamics after the onset of each ELM (type-I) remain uncertain.

Nonlinear ELM simulations become computationally difficult because the ratio of the Alfvén wave transit time to the resistive diffusion time (the Lundquist number, S) is very large. In this parameter regime, very fine spatial resolution is needed to resolve the narrow current sheets and/or narrowing plasma fingers arising from the explosive ideal MHD instabilities predicted from nonlinear ballooning theory [5,6]. These very fine structures lead to dramatic reduction of the simulation time step at the early nonlinear stage of ELM evolution [6]. Similar problems appear in other fields, such as magnetospheric substorms, solar and stellar flares, and sawtooth crashes in tokamaks. A common practice is to use an anomalous resistivity and/or ion viscosity to complete nonlinear ELM simulations, which also leads to significantly different linear growth rates and instability thresholds. Furthermore, in nonlinear resistive MHD simulations, the pedestal pressure collapses deep into the plasma core, which yields much larger ELM sizes than observed experimentally.

In a recent paper published in Physical Review Letters [7], we describe nonlinear simulations of plasma edge pedestal collapse in the tokamak configuration using a three-field model (pressure, electrostatic potential, and magnetic vector potential), as illustrated in Fig. 1. The simulations are carried out in the BOUT++ two-fluid framework [8], which allows studies of nonlinear dynamics of ELMs with extensions beyond ideal MHD physics. The nonlinear simulations show that the peeling-ballooning modes trigger magnetic reconnection, which drives the collapse of the pedestal pressure. Anomalous electron viscosity, which appears as a hyperresistivity term for the parallel magnetic vector potential, is found to limit the radial spreading of ELMs by facilitating magnetic reconnection.

Fig. 1. Radial pressure profiles at several different Lunquist numbers S and time slices. Open magnetic field line plasma loss occurs for R > 4.58. From Ref. 7.

For typical pedestal plasma parameters, S ≈10⁸−10¹⁰, the growth rate of the peeling-ballooning mode is γPB ≈ 0.1ω_A (with ω_A the Alfvén frequency), and the width of the resistive current sheet is Δ_J ≈ R(ω_A/γ_PB/S)^1/2 ~ 10-100 microns (with R the major radius), which is comparable to the electron Larmor radius. In the absence of the hyper-resistivity, the simulation time-step collapses as the radial scale-length of the current sheet approaches the radial grid spacing Δ_x for typical resistive MHD simulations L_p >> Δ_q >> Δ_x >> ρ_i ~ δ_i ~ δ_e >> Δ_J > ρ_e. Here L_p is the pedestal width, Δ_q is the rational surface spacing, δ_i and δ_e are the ion and electron skin depths, and ρ_i and ρ_e are the ion and electron Larmor radii. With the hyper-resistivity η_H (or anomalous electron viscosity), the width of the hyper-resistive current sheet is Δ_H ≈ R(ω_A/γ_PB/S_H)^1/4. Here S_H = μ₀R₀³v_A/η_H is defined as the hyper-Lundquist number. The hyperresistivity could arise, for example, from small-scale electron turbulence in the H-mode pedestal [9]. Under the assumption that the electron viscosity is comparable to the anomalous electron thermal diffusivity, we have S_H = 10¹²; hence L_p >> Δ_q >> Δ_H > Δ_x.

The radial pressure profiles at the outer mid-plane at several different time slices and different Lundquist numbers are shown in Fig. 1. It is clearly shown at late times that the ELM causes the pedestal pressure to collapse deeply inside the core plasma at low Lundquist number (S = 10⁵). It is also shown that for high Lundquist number there are two distinct processes in the evolution of pressure profiles: a fast collapse greatly flattening the pressure profile near the peak pressure gradient on the order of tens of Alfvén times after the onset of nonlinear peeling-ballooning mode, t = 74ω_A, and a subsequent slow buildup of pressure gradient. We can characterize the fast collapse as a magnetic reconnection (triggered by peeling-ballooning modes) -> magnetic island formation -> bursting process, and a slow buildup as a turbulence transport process. The radial-poloidal pressure profiles clearly show the characteristics of the ballooning mode. As is well known from linear instability analysis, φ and A|| have ballooning parity for the peeling-ballooning mode. In the nonlinear stage, however, a tearing parity component appears due to the nonlinear mode coupling, which facilitates magnetic reconnection and island formation.

Defining the ELM size as the ratio of the ELM energy loss to the pedestal stored energy, the ELM size is calculated from each nonlinear simulation. The ELM size scaling vs. classical Lundquist number S is given in Fig. 2, which plots the ELM loss fraction as a function of S for a fixed hyper-Lundquist number of S_H = 10¹². The size of the ELM ejection varies dramatically between the high and low resistivity cases. As the Lundquist number S is increased, the loss of thermal energy during the ELM drops from 50% to 10% over a range of S = 10⁴ − 10⁷ and then stays independent of S. This plateau region is due to hyper-resistivity now dominating over classical resistivity in Ohm’s law, ηJ << η_H∇?² J, and therefore leads to a significant reduction of the inward spreading of the perturbed magnetic field from the region of the peeling-ballooning drive (peak gradient region). With a fixed hyper-Lundquist number S_H = 10¹², when S > S_c = (S_Hω_A/γ_PB) > 10⁶, the ELM size is insensitive to the resistivity, which is relevant to today’s modestly sized tokamaks and ITER.

Fig. 2. Simulation ELM size vs Lundquist number S with SH =1012 showing reduction to a  minimum size; the experimental range for S is 108-1010.

This work was performed for U.S. DOE by LLNL under Contract DE-AC52-07NA27344, grants DE-FG03-95ER54309 at general Atomics, and by the UK Engineering and Physical Sciences Research Council under grant EP/H012605/1 and the Euro. Commun. under the contract of Association between EURATOM and CCFE.

References

1. M. Keilhacker, G. Becker, and K. Bernhardi et al., Plasma Phys. Controlled Fusion 26, 49 (1984).
2. H.R. Wilson et al., Phys. Plasmas 9, 1277 (2002).
3. P.B. Snyder et al., Phys. Plasmas 9, 2037 (2002).
4. P.B. Snyder et al., Nucl. Fusion 49, 085035 (2009).
5. H.R. Wilson and S.C. Cowley, Phys. Rev. Lett. 92, 175006 (2004).
6. P.B. Snyder et al., Phys. Plasmas 12, 056115 (2005).
7. X.Q. Xu, B. Dudson, P.B. Snyder, M.V. Umansky, and H.R. Wilson, Phys. Rev. Lett. 105, 175005 (2010).
8. B.D. Dudson, H.R. Wilson, M.V. Umansky, X.Q. Xu, and P.B. Snyder, Comput. Phys. Commun. 180, 1467 (2009).
9. H. Biglari and P.H. Diamond, Phys. Fluids B 5, 3838 (1993).

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Announcements

Submit BPO-related announcements for next month’s eNews to Tom Rognlien at trognlien at llnl.gov.

Upcoming Burning Plasma Events

Click here to visit a list of previously concluded events.

2011 Events

March 1-4, 2011 send title of talk to senabhijit@gmail.com ITPA MHD Topical Group Meeting Ahmedabad, INDIA

March 7-8, 2011 Fusion Energy Science Advisory Committee (FESAC) Bethesda, Maryland, USA

March 30 - April 1, 2011 20th ITPA Pedestal and Edge Topical Group Meeting Cambridge, Massachusetts, USA

April 4-5, 2011 ITPA Transport & Confinement Topical Group Meeting San Diego, California USA

April 5, 2011 abstract deadline: March 4, 2011  send to: snyder@fusion.gat.com Edge Coordinating Committee Workshop on RMP San Diego, California USA

April 6-9, 2011 Joint US-EU Transport Task Force (TTF) Workshop San Diego, California USA

April 11-13, 2011 ITPA Energetic Particles Topical Group Meeting Frascti, ITALY

April 11-14, 2011 ITPA Integrated Operation Scenarios Topical Group Meeting JET, Culham, UNITED KINGDOM

April 11-14, 2011 abstract deadline closed Workshop on Stochasticity in Fusion Plasmas (SFP 20110) Jülich, GERMANY

April 20-22, 2011  abstracts due: Feb 28, 2011 send to: marian1@llnl.gov 2011 IEA Fusion Modeling Workshop on Materials Lawrence Livermore National Laboratory, Livermore, California USA

May 2-4, 2011 2011 International Sherwood Fusion Theory Conference Austin, Texas USA

May 9-13, 2011 13th International Workshop on Plasma-Facing Materials and Components for Fusion Applications (PFMC-13) and 1st International Conference on Fusion Energy Materials Science (FEMaS-1) Rosenheim, GERMANY

May 15-19, 2011 15th International Conference on Emerging Nuclear Energy Systems (ICENES) San Francisco, California USA

May 16-19, 2011 ITPA SOL and Divertor Topical Group Meeting Helsinki, FINLAND

May 23-26, 2011 ITPA Diagnostics Topical Group Meeting FOM, NETHERLANDS

June 1-3, 2011 19th Topical Conference on Radio Frequency Power in Plasmas (C.K. Phillips and J.R. Wilson, PPPL) Newport, Rhode Island USA

June 20-24, 2011 ITER International Summer School on MHD and Energetic Particles Aix en Provence, FRANCE

June 26-30, 2011 38th IEEE International Conference on Plasma Science (ICOPS) and the 24th Symposium on Fusion Engineering (SOFE) Chicago, Illinois USA

Jun 27-Jul 1, 2011 38th European Physical Society Conference on Plasma Physics Strasbourg, FR EPS

Fall 2011 ITPA Diagnostics Topical Group Meeting CHINA

Sept 5-7, 2011 IAEA Technical Meeting on Theory of Plasma Instabilities Austin, Texas  USA

Sept 8-10, 2011 IAEA Technical Meeting on Energetic Particles in Magnetic Confinement Systems Ausitn, Texas USA

TBA ITPA Energetic Particles Topical Group Meeting Austin, Texas  USA

TBA---tentatively scheduled ITPA MHD Topical Group Meeting Padova, ITALY

Sep 11-16, 2011 10th International Symposium on Fusion Nuclear Technology Portland, Oregon USA

Sep 19-21, 2011 Plasma Edge Theory Meeting South Lake Tahoe, California USA

October 5-7, 2011 ITPA Transport & Confinement Topical Group Meeting Cadarache, FRANCE

October 5-7, 2011 ITPA Pedestal and Edge Topical Group Meeting York, UNITED KINGDOM

Oct 16-21, 2011 15th International Conference on Fusion Reactor Materials (ICFRM-15) Charleston, South Carolina USA

October 18-21, 2011 ITPA Integrated Operational Scenarios Topical Group Kyoto University, JAPAN

Nov 14-18, 2011 53rd APS Division of Plasma Physics Annual Meeting Salt Lake City, Utah USA

December 12-15, 2011 ITPA CC & CTP-ITPA Joint Experiments Meeting Cadarache, FRANCE

December 2011 or January 2012 ITPA Divertor and SOL (PSI Selection Committee) Topical Group Meeting Jüelich, GERMANY

Click here to visit a Directory of Other Plasma Events

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