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U.S. Burning Plasma Organization e-News
December 15, 2008 (Issue 27)

CONTENTS


     -- Director's Corner by Jim Van Dam
     -- Reports
         -ITPA Pedestal and Edge Group Meeting by R. Maingi, T. Rognlien, P. Snyder
         -11th ITPA Meeting of SOL/Divertor Physics, by D. Whyte and B. Lipschultz
     -- 2009 Burning Plasma-Related Events

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 (R. Nazikian rnazikian at pppl.gov).

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


Director's Corner by J. Van Dam

The name for the month of January derives from Janus, who in Roman mythology was the god of doorways and of beginnings and endings. Janus is traditionally depicted with two faces, looking in opposite directions, symbolizing the gift to see both the past and also the future.

As we come to the end of calendar year 2008, we, too, look back and look forward. We look back on yet another extremely busy year of USBPO activities. To cite a few examples:

  • Strong US scientific involvement in addressing the urgent R&D issues for ITER (including the so-called 13 STAC Issues), with results presented at the IAEA Fusion Energy Conference
  • Rotation of ITPA topical group membership and leaders
  • Involvement in the development and analysis of the ITER Research Plan and the ITER Work Programme documents
  • ITER Town Meeting and ITER contributed oral session at the APS Division of Plasma Physics Meeting
  • National Research Council review of the Energy Policy Act Report

The USBPO web site continues to be an extremely useful tool for communicating scientific results and programmatic developments to the US fusion energy sciences community and beyond. Several recent presentations have been posted:

Incidentally, the current membership in the ten USBPO topical groups is 291, and the current list of subscribers for eNews stands at 429. I’d love to see these numbers become 300 and 450, respectively. Please encourage your colleagues to sign up—it’s easy and it makes you feel so good!

The USBPO is also hosting the web site for the Research Needs Workshop (abbreviated as ReNeW), to be held June 2009 in Bethesda, MD. Officially, this is not a USBPO activity; however, many in the USBPO membership are expected to be active participants in the ReNeW activities. If you click on http://burningplasma.org/renew.html, you will see that ReNeW is organized into five theme areas, one of which—Achieving and Understanding the Burning Plasma State in ITER—is directly relevant to the USBPO, and, in fact, the leaders for this theme area are Mickey Wade and myself. We strongly encourage the USBPO membership and the broader US fusion energy sciences community to be active in the ReNeW effort, since it will provide valuable planning information for the US program in future years and also (we hope) be useful for educating the new Administration, soon to take office in Washington, D.C., concerning the strategic importance of fusion.

As we look ahead to next year (the forward-looking face of Janus), we set our sights on the following items:

  • Restoration of ITER construction funding by the new Congress and continued support by the new Administration
  • A list of exciting scientific initiatives to emerge from the Research Needs Workshop
  • Continued US contributions to addressing vital research issues for ITER, related in the near term to its design and, on the longer term, to planning for operational scenarios and research programs
  • Further focusing of the US fusion effort on the grand-challenge frontier of burning plasma science

One note in closing: In January, Emily Hooks, our USBPO Administrator for the past two years, will be moving to a another position here at the University of Texas. She has served us most ably, with competence and grace. Most recently, the new USBPO logo and web page banner were due to her initiative, and she is now working on a redesign of the USBPO web site (in collaboration with Jim DeKock, our IT guru). Having done such a bang-up job for the USBPO, she has been recruited to work in contracts management for the petawatt laser project of the Center for High Intensity Laser Science. Thus, in a sense, she will remain within the larger plasma physics family here at the university. We sincerely thank her and wish her the very best in the future.

  

 

Emily HooksEmily Hooks

I wish all of you a Happy Burning Plasma New Year!

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Announcments

Submit BPO-related announcements for next month’s eNews to Raffi Nazikian at rnazikian at pppl.gov.

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Reports

Summary of the Meeting of the ITPA Pedestal and Edge Topical Group
Milan, Italy, October 20-22, 2008
Written by Rajesh Maingi, Tom Rognlien, and Phil Snyder

The reorganized ITPA Pedestal and Edge Topical Group held its first meeting at Milan, Italy on October 20-22, 2008.  Howard Wilson (EU) and Naoyuki Oyama (Japan) are serving as leader and co-leader, while Phil Snyder and Rajesh Maingi are U.S. coordinator and deputy, all respectively. The agenda was designed to focus on High Priority Research issues in the pedestal for ITER, In addition, a joint session was held with the ITPA Transport and Confinement Group to help intensified research for L-H transition questions. Research topics presented follow four working groups formed since the meeting on “short-term” issues, while additional information is being assembled for the “medium term” LH transition topic.  In more detail:

Working group 1 - ELM suppression by resonant magnetic perturbations (RMP).
For planned operating parameters, it will be necessary to strongly reduce Type I ELMs in ITER to fully meet its objectives. Coils to provide RMP are presently the only tool available known to suppress the ELMs in such high performance regimes, and the DIII-D tokamak is the only device to show full suppression for various discharges.  Other devices have shown some impact of RMP, and differences may be attributed to the very favourable RMP resonant spectrum to induce field-line stochasticity provided by the DIII-D coil set.  Clarifying the optimum coil design and understanding enhanced particle transport are among the key issues being investigated.

Working group 2 - ELM control by pellet pacing
Injection of pellets at rapid periodic intervals can induce many smaller ELMs to replace the more dangerous infrequent large ELMs.  Experiments have been preformed on AUG, DIII-D, and JET tokamaks. Issues include demonstration of adequate pellet frequency and compatibility with particle inventory, i.e., adequate pumping of pellet-injected particle inventory.

Working group 3 - TF ripple impact on pedestal
It is well known that toroidal field ripple can cause loss of energetic ions and increased heat load on plasma-facing components.  Here the specific impact on pedestal performance is investigated, which has been reported from experiments in JT-60U and JET.  Such ion loss can increase counter-current plasma rotation that may be one of the mechanisms impacting pedestal performance.

Working group 4 - Heating source impact on pedestal and ELMs
The scaling of the pedestal plasma pressure, which is directly connected to the height and width of the pedestal region, is believed key in determining the ITER fusion yield. Recent experimental data presented reinforced earlier results having the pedestal width scaling as the square root of poloidal bp, and weakly with normalized gyroradius r*.  A question to be answered is how the pedestal height depends on the type of auxiliary heating, as ITER will have multiple sources.

Improving understanding of LH transition power and maintainability
There is no well-accepted scaling for the threshold power for ITER to obtain H-mode, especially using multiple heating sources.  It is very important that ITER can obtain H-mode in the pre-DT hydrogen phase to test ELM control and pedestal scaling before activation. The joint ITPA session reviewed threshold data from various devices, and it was agreed that an assessment of the status of theory and a mechanism for collecting a multi-machine database would be performed by the next meeting.

Summary of the 11th ITPA Topical Group Meeting on SOL/Divertor Physics
Written by R. Dennis Whyte and Bruce Lipschultz

The meeting was held over the period September 15-18, 2008 at the Hotel New Urakami in Nagasaki, Japan. The local coordinators were M. Sakamoto (Advanced Fusion Research Center in the Research Institute for Applied Mechanics) and T. Tanabe (Interdisciplinary Graduate School of Engineering Sciences) of the University of Kyushu.

The new organization of ITPA was briefly presented showing, in particular, the strengthening of the links with ITER IO. This was illustrated in the meeting by sessions initiated by ITER IO, such as multi machine comparison of H levels and dust production.

The SOL/divertor group had previously reviewed (Avila meeting, January, 2008) the calculations of T retention in ITER made by the US and EU during the ITER review period last year. It was clear that the underlying assumptions and methods used by the 2 groups were not in agreement. Bruce Lipschultz and Jochen Roth then organized an effort to resolve these differences. 13+ scientists came to an ‘ITPA ITER Tritium Inventory Assessment workshop’, June 23-24, 2008 at MIT. Agreement was reached on a number of underlying parameters such as the flux of ions and atoms to all surfaces (and their energies), the co-deposition rate (T with Be and C) at various surfaces and the neutron damage to W which leads to D retention deep within the W bulk. Most projections to ITER have been redone. However the effect of neutron damage on retention deep within the W is still in the process of being modeled. As expected, the present selection of PFCs for ITER (Be/C/W) lead to large uncertainties in retention (4.5 - 260g at 105s of full operation or 250 shots). Only taking into account ion implantation into W (ignoring nuclear damage) leads to a lower projected range of T retention for an all-W ITER (3-30g for the same period of operation). Future work includes finishing the modeling of retention due to nuclear damage (W) and writing a summary of the work. We also intend to expand the group work in this area with the addition of experts outside of the US and EU (e.g. Japan).

Two sessions were devoted to fuel retention, on carbon and high Z PFCs respectively.

Concerning carbon, a wide range of retention fraction is observed. The discussion showed that, because of varying pumping conditions, the absolute value of the retention rate, rather than the retention fraction, should be used for comparison between devices; Increased pump rates lead to increased injection rates, but the same amount of retention. Recent work on JET and Tore Supra show that the fuel retention estimated from post mortem analysis and particle balance can come to a reasonable agreement (factor 2 in TS), provided they are carried out on similar plasma conditions (dedicated campaign with repetitive pulses in TS) and that an extensive post mortem analysis is performed (for instance gaps in TS, louvers in JET).

Previous results have pointed to co-deposition as the dominant retention process for carbon PFCs, primarily occurring at the inner divertor for diverted tokamaks. New at this meeting was a report from JT-60U which showed that the erosion areas of the main wall had significant D in the near surface (perhaps implanted by fast particle losses) that, although low per unit area, may rival the divertor retention. A similarly surprising result was that co-deposition on tile sides in high erosion areas of the castellated Tore Supra limiter dominates the co-deposition in remote areas. Deep diffusion of D into CFC graphite (which might be due to co-deposition inside the porosity network of CFC) was evidenced in erosion zones, and shown to be ~ 10% of overall retention in Tore Supra.

It was proposed that the same method used for ITER T retention estimates be benchmarked against present day machines (estimate of wall/diverter fluxes → erosion sources → fuel inventory using D/C scalings) and the results be compared with particle balance (TS, JET, AUG, DIIIID, JT60U …), as well as gap modelling be benchmarked against experimental data (Textor, TS).

Concerning high Z materials, the number of hydrogenic retention processes are quite varied. While implantation of ions is certainly the initial entry point for retention of fuel it is not so clear what all the possible retention mechanisms are within lattice nor their relative importance relative to each other and to release from the surface. From the point of view of retention bubbles and blisters can be good as they often release their trapped gas and inhibit diffusion deep into the bulk. On the other hand such deformation of the surface obviously damages the surface and probably degrades the material properties in terms of heat load handling and shock resistance. We are just starting to address the effect of neutron damage, which creates traps for T retention throughout the tile. The talks at this meeting and at the MIT meeting indicate that such damage can lead to a maximum of ~ 1% [D+T]/W. Then, it is a matter of when the D+ T implanted at the surface can diffuse to those traps and thus be retained. Will it be slow or fast? Lastly it was pointed out that simultaneous implantation of He along with the D,T fuel can affect the diffusion of trapping of D,T. There are differing reports on whether the effect increases or decreases D,T retention and this will be a subject of review in upcoming meetings.

The present ITER dust safety strategy relies on measurements of gross erosion, and assumes as a conservative upper limit a conversion factor fd between gross erosion and mobilisable dust equal to 1. Even though the amount of C gross erosion as measured by spectroscopy is often much larger than the collected dust it is not clear whether they are related. In contrast, for high Z machines (C-Mod and AUG), preliminary results indicate that gross erosion estimated from spectroscopy is marginal to explain the dust produced. The meeting participants felt that it was likely that transients are playing a central role for high-Z dust formation. The collected metallic dust is often spherical, which again could be linked to droplets formation during transients. In all cases, dust is often evidenced in the plasma (cameras …) after disruptions. Further studies somehow directly linking dust types/quantities (flaking of C, droplets for high-Z) to erosion mechanisms (steady state chemical erosion and/or transients) are needed before projections to ITER can be made.  Dust can have serious effects on plasma operation. As an example Tore Supra compared operational space (and UFOs) with and without thorough extensive cleaning of dust and C flakes from all PFCs. Cleaning led to a dramatic reduction in UFOs and the operational space defined by heating power limits were greatly increased.

A session was devoted to the mitigation of transients (ELMs and disruptions), which is now included in ITER baseline, as it is recognized that non-mitigated events will lead to serious PFCs damage.

The characterization of mitigated ELMs is just starting. Pellet-induced ELMs in AUG are similar to intrinsic ELMs in terms of SOL plasma and heat loads at the same frequency. Results from pellet pacing in JET are expected to provide more data in this area, in particular to explore the scaling with pellet frequency. The measurements of the effect of ELMs mitigation with RMPs (Resonant Magnetic Perturbations) on divertor heat loads is primarily on DIII-D as other tokamaks either do not have the RMP capability operative at the moment or have no ELMs. The experiments are hampered by lack of IR measurements at the outer strike point and matching non-RMP and RMP equivalent discharges. However, the general assessment was that mitigated discharge heat fluxes were similar to averaging over the ELM, between-ELM periods of a non-mitigated discharge. Given the minimal amount of information available and the importance to ITER it is clear much more information from DIII-D and now JET is needed.

Concerning disruptions, new, fast bolometry systems are just coming online at several machines to diagnose the wall and divertor heat loads with mitigated disruptions (AUG, C-Mod, TCV). Complex behaviours are in evidence, with pronounced toroidal /poloidal asymmetries (peaked near the injection location) during the initial gas penetration. While this certainly raises concerns for localized heating of the Be wall in ITER, the measurements show that when the radiation is highest (thermal quench) the asymmetries drop to ~ 1 (C-Mod). Different gas mixtures are under study for optimisation of the mitigation process, but understanding the physics of the impurity injected in these mitigated plasmas is critical to progress further (cross machine comparison essential). It is recommended that the ITER system remains flexible in terms of the nature/amount of gas injected. The results of the ITER workshop on runaway electrons during disruptions were presented to our group to keep them informed on the potential damage to in-vessel components.

Finally, a session on material migration was organised, covering all steps from erosion to transport and redeposition.

Chemical erosion of carbon is now being addressed in detached plasma parameters, but the contribution of atoms is essential in these conditions, and should be correctly diagnosed for valid erosion yield. The effect of seeding gases (Ne, Ar …) needs to be investigated. Erosion products (heavy hydrocarbons) and their sticking properties (strong impact on C migration modelling) should also be addressed.

While C13 injection has been used in the past to study the transport of C, a new experiment in JT-60U has allowed the study of W material migration: A localized set of W tiles (1/12 of toroidal circumference) served as the W source. W is found to be redeposited locally toroidally (probably due to prompt re-ionization and short range migration), while C is transported further toroidally (from 13C injection experiments). The amount of W in the core was strongly dependent on core rotation and conditions The W core concentration seems to depend even more sensitively on the particle transport properties (increasing with counter toroidal velocity) than the W source at the edge. In contrast, a complete ring of W divertor tiles installed in C-Mod has led to no observable level in the core. W migration seems to indicate a strong role of poloidal drifts in JT60U. Be migration was also studied in JET using Be evaporation.

In all cases, impurity transport is recognised as the most uncertain point to predict material migration in ITER (and associated fuel retention, dust production …) as well as impurity core concentration.

 

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Upcoming 2009 Burning Plasma-Related Events

Jan 13-14
FESAC
Gaithersburg, MD

Jan 18-23
8th IEA International Wksp on SiC/SiC
Daytona Beach, FL

Mar 2-4
4th Workshop on Stochastic Fusion Plasmas
Julich, Germany

Mar 31-Apr 2/3
Joint Transport & Confinement and Integrated Operational Scenarios ITPA TG Mtg
Naka, Japan

Apr 6-8
ITER Export Control Working Group Mtg
Washington, DC

Apr 28-May1
Transport Task Force Workshop
San Diego, CA

May 2-5
Sherwood Theory Fusion Conference/APS April Mtg
Boulder, CO

May 11-14
12th International Wksp on Plasma-Facing Materials & Components for Fusion Applications
Julich, Germany

May 25-30
STAC, MAC CPWG
TBD

May 31-Jun 5
ICOPS-SOFE 2009 Conference
San Diego, CA

Jun 14-18
ANS Annual Mtg
Atlanta, GA

Jun 24-26
18th Conf on RF Power in Plasmas
Gent, Belgium

Jun 29-Jul 3
14th International Conference on Emerging Nuclear Energy Systems (ICENES-2009)
Ericeira, Portugal

Jul 12-16
17th International Conference on Nuclear Engineering (ICONE 17)
Brussels, Belgium

Sept 7-12
14th International Conf on Fusion Reactor Materials (ICFRM-14)
Sapporo, Japan

Sept 21-24
14th International Symp on Laser-Aided Plasma Diagnostics (LAPD-14)
Castelbrando, Treviso, Italy

Oct 11-16
9th International Symp on Fusion Nuclear Technology (ISFNT-9)
Dalian, China

Oct 24-29
9th International Conference of Tritium Science and Technology
Nara, Japan

Nov 2-6
51st APS-DPP Mtg
Atlanta, GA

 

Fusion Research-related events can also be seen on the USBPO Events page at http://burningplasma.org/events.html. 

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