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U.S. Burning Plasma Organization eNews
March 31, 2015 (Issue 94)

 

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

Research Highlight
E.A. Belli, et al.
ITPA and ITER IMEG Updates  
Schedule of Burning Plasma Events  
Contact and Contribution Information

Research Highlight

Confinement and Transport Topical Group, Leaders: G. Staebler and S. Mordijck

The understanding of impurity transport is critical for ITER. At the high densities relevant for ITER, the accumulation of impurities in the core leads to undesirable radiative power loss or even radiative collapse. The theoretical work in this month’s Research Highlight by E. Belli, et al., demonstrates good agreement with experimental observations, which will improve our predictions for ITER scenarios in regards to impurity transport.

Modeling of the Neoclassical Boostrap Current and Transport of Heavy Impurities

E.A. Belli1, J. Candy1, O. Meneghini1, T.H. Osborne1, C. Angioni2 , and F.J. Casson3
1 General Atomics, P.O. Box 85608, San Diego, CA 92186–5608, USA
2 Max-Planck-Institute für Plasmaphysik, D–85748 Garching, Germany
3 CCFE, Culham Science Center, Abingdon, OX14 3DB, UK

The drift-kinetic code NEO [1] has recently been used to analyze the neoclassical flows and transport in experimental plasmas. NEO is a comprehensive neoclassical code which includes general 2D and 3D flux surface geometry, sonic toroidal rotation with full centrifugal physics, and full linearized Fokker–Planck collisions.

NEO has been used to assess the accuracy and limitations of the well-known Sauter model [2] for the bootstrap current. The bootstrap current, related to the neoclassical parallel flows, is important for the development of a steady-state reactor in which only a small fraction of the current can be driven externally. Because the contribution of the bootstrap current to the total plasma current is generally large in the pedestal, it also plays an important role in edge stability analysis. For representative DIII–D and NSTX H-mode plasmas, we find that NEO provides a 30% correction to the Sauter model in the core and a 10–15% correction in the pedestal. As shown in figure 1, an extensive study for a range of DIII–D cases found that the Sauter model generally underestimates the bootstrap current at low collisionality, with increasing error toward the banana regime, and overestimates the bootstrap current at high collisionality, with increasing error toward the Pfirsch–Schlüter regime. The Sauter model was found to not accurately capture the ion-impurity collisional interaction or the collisional effect of energetic species, which would be important for studies of beams or alphas in ITER [3]. Thus, using NEO directly is expected to improve the accuracy of edge stability calculations, which are highly sensitive to the bootstrap current profile.

Figure 1

Figure 1: Bootstrap current edge profiles comparing NEO simulation results with the Sauter model for DIII–D #149220 at low collisionality (ν∗e,ped = 0.068) and #145701 at high collisionality (ν∗e,ped = 4.202).

Integrated modeling of MHD equilibrium reconstruction coupling the NEO bootstrap current with the Grad–Shafranov magnetic equilibrium solution from kinetic EFIT [4] has recently been demonstrated for the first time. MHD equilibrium reconstruction is an important first step for accurate transport and stability analysis. The previous standard workflow used an analytic model for the bootstrap current, rather than a direct kinetic calculation, embedded into the EFIT total current constraint. A comparison of the EFIT magnetic reconstruction obtained using NEO and Sauter bootstrap currents for a high collisionality DIII–D H-mode case found a lower χ2 value for NEO. This indicates that the current constraint from NEO is more consistent with the pressure and magnetics measurements.

NEO has also been used to study the neoclassical transport of heavy impurities, such as tungsten which has been chosen as the material for the divertor in ITER. While tungsten is a good choice as a plasma facing material due to its high thermal-mechanical properties, its transport from the material surface into the high temperature core must be minimized since accumulation can lead to large radiation losses. NEO is an ideal tool for studying heavy impurity transport due to its inclusion of sonic toroidal rotation, which arises in tokamaks from torque due to neutral beam injection. This rotation produces a strong centrifugal force that pushes the ions toroidally outward causing them to redistribute non-uniformly around a flux surface. As a result of quasi-neutrality, a poloidally varying electrostatic potential is generated to balance the density asymmetry. For heavy impurities, like tungsten, the influence of the centrifugal force is amplified by their greater mass. In the banana regime, this force increases the effective trapped particle fraction. In the Pfirsch-Schlüter regime, it increases the effective toroidal curvature. Thus, it can lead to enhanced neoclassical transport.

The tungsten transport in a hybrid scenario in JET, which now has a tungsten divertor as will be the situation in ITER, has recently been extensively analyzed [5]. The analysis was done for two time slices: before (t=5.9s) and after (t=7.5s) the observed tungsten accumulation process. The sonic toroidal rotation was found to largely enhance both the diffusive and convective components of the tungsten radial particle flux by more than an order of magnitude. As a consequence, the neoclassical transport rather than the turbulent transport dominates the central part of the plasma inside r/a ≶ 0.3. This differs from the case of the main deuterium species. The results were validated by comparing SXR emission tomography measurements (with the bremsstrahlung radiation subtracted, assuming that tungsten dominates the remaining emission) and predicted SXR emission forward-modeled from the 2D tungsten density computed using the neoclassical transport from NEO and the turbulent transport from the gyrokinetic code GKW [6]. As shown in figure 2, good qualitative agreement was found between the simulation models and the measurements in both the radial and poloidal structure. A localization on the low-field side was seen at the earlier time due to centrifugal trapping. In contrast, an extreme central peaking was seen at the later time due to neoclassical accumulation, which ultimately degrades the performance. If the centrifugal effects were not included in the NEO simulations, then qualitative disagreements were seen with the tomography, indicating that they are an essential component to the tungsten modeling.

Figure 2 (upper)
Figure 2 (lower)

Figure 2: Contour lines of the reconstructed tungsten density for JET #82722 at t=5.9s (left) and t=7.5s (right), comparing the reconstructed SXR emission measurements (top) with the prediction from NEO+GKW (bottom).

An extension of the neoclassical model for studying poloidal asymmetries in the density induced by RF minorities with anisotropic temperature has also been developed and implemented in NEO. In contrast with sonic rotation effects (for which the density is localized at the outboard midplane), the poloidal distribution induced by ICRH produces a reverse in-out asymmetry. NEO simulations find that, unlike for sonic rotation, small temperature anisotropies reduce rather than enhance the impurity diffusivity. This may have implications for studies of heavy impurity transport in ICRH plasmas and has recently been validated in NEO+GKW analysis of tungsten transport in ASDEX Upgrade experiments [7].

This work was supported by the U.S. Department of Energy, Office of Science, Office of Fusion Energy Sciences, Theory program, under Awards DE–FG02–95ER54309 and DE–FC02–06ER54873.

References

[1] E.A. Belli and J. Candy, Plasma Phys. Control. Fusion 50, 095010 (2008); Plasma Phys. Control. Fusion 54, 015015 (2012)

[2] O. Sauter et al., Phys. Plasmas 6, 2834 (1999); Phys. Plasmas 9, 5140 (2002)

[3] E.A. Belli et al, Plasma Phys. Control. Fusion 56, 045006 (2014)

[4] L. Lao et al, Nucl. Fusion 25, 1611 (1985)

[5] C. Angioni et al, Nucl. Fusion 54, 083028 (2014)

[6] A.G. Peeters et al, Comput. Phys. Comm. 180, 2650 (2009)

[7] F.J. Casson et al, Plasma Phys. Control. Fusion 57, 014031 (2015)


ITPA and ITER IMEG Updates

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

Energetic Particles Topical Group

The 14th meeting of the ITPA Energetic Particles Topical Group took place at ITER headquarters over March 25–27, 2015. Two primary issues concerned energetic ion diagnostics in ITER, and the development of new joint experiments. For ITER, there is a great desire to have an energetic ion loss detector, but also concern over the safety issues surrounding the installation of a reciprocating probe. Possible future joint experiments involve plasma shaping effects on fusion alpha heating, and modeling of neutral beam current drive in the presence of energetic ion transport by MHD. The full agenda and a summary are available at the >BPO EP Forum. — D.C. Pace

People standing around looking at a construction scene

ITPA EP participants appreciating the incredible scale and rapid construction pace of ITER.

Transport and Confinement Topical Group

Due to a recent influenza outbreak in Ahmedabad, India, the Transport and Confinement ITPA spring meeting has been moved. The meeting will take place at ITER headquarters during May 5–7. Although the pedestal ITPA will not be held simultaneously, the joint session on the L to H transition will still be held and remote participation will be available. The agenda was only altered for remote participation purposes. Please contact one of the USBPO topical Transport leaders or your “local” ITPA T&C contact if you would like more information. — S. Mordijck

ITER IMEG Update

The 6th ITER Integrated Modeling Expert Group (IMEG) annual meeting was held in ITER Headquarters, December 15–17, 2014. IMEG consists of two representatives from each domestic partner. The two US representatives are Lang Lao of General Atomics and Steve Jardin of PPPL. Lang Lao chaired the IMEG annual meeting.

The primary goal of the ITER Integrated Modeling (IM) Program is the development of an Integrated Modeling Analysis Suite (IMAS) that includes a framework to support ITER plasma operation and research. The main objectives of the IMEG annual meeting are to discuss advances in domestic integrated modeling programs that are of interest to ITER and to review IMAS progress and to advise ITER of its IM program. In particular, for the 2014 meeting the charges to IMEG are to review IMAS plans, trainings, and opportunities, review development and applications of IMAS infrastructure and physics components, and to discuss members’ plans to test, validate, and contribute to IMAS.

Progress in the domestic integrated modeling programs include a new 5-year European Union (EU) Integrated modeling Project WP-CD with the near-term goals of an extended core transport simulator, core-edge transport, and a discharge simulator with feedback controllers; development of a new 2D and 3D helical equilibrium high-Z impurity transport code TOTAL and a MHD Equilibrium Simulator MECS for JT–60SA in Japan; and development of integrated disruption simulations with the ASTRA/DINA transport packages in Russia. From the US, Steve Jardin gave a summary of the progress and plan for the TRANSP transport package and Lang Lao gave a summary of the new FES/ASCR SciDAC Advanced Tokamak Modeling (AToM) project, a summary of the progress in the Edge Simulation Laboratory (ESL) project, and a summary of recent DIII–D integrated modeling results.

The IMAS near-term plan includes implementation of a Plasma Simulator based on the European Transport Solver (ETS) as a modular physics workflow to demonstrate the capabilities and approach of IMAS that can be used as a template for the development of other workflows, and extension of data access capabilities to provide access to existing and future experimental data in the form of Interface Data Structures (IDSs). Key action items include invitation of ITER members to adapt their experimental analysis and modeling tools to the ITER data model, invitation of ITER members to help as testers of local IMAS installation and validation, and invitation of ITER members to participate in the SOLPS–ITER development and validation efforts. — L. Lao

Coil winding facility

ITER poloidal field coil winding facility standing at the ready to simultaneously construct the coils that are too large to be produced off-site. (March 2015)


Schedule of Burning Plasma Events

USBPO Public Calendar: View Online or Subscribe

 
2015
NSTX-U First Plasma —
— W7-X First Plasma —
 
March 23 – April 3, Winter School on Turbulence, Magnetic Fields and Self Organization in Laboratory and Astrophysical Plasmas, Les Houches, France
 
April 14, DEADLINE to apply for the DOE Office of Science Graduate Student Research (SCGSR) Program
 
April 14–17, 1st European Conference on Plasma Diagnostics, Frascati, Italy
 
April 14–17, ITPA: MHD Topical Group Meeting, ITER Headquarters, France
 
April 20–21, BaPSF Users Meeting, Los Angeles, CA, United States
 
April 20–23, ITPA: Integrated Operating Scenarios Topical Group Meeting, Barcelona, Spain
 
April 27–29, 21st Topical Conference on Radiofrequency Power in Plasmas, Lake Arrowhead, CA, United States
 
April 28 – May 1, US/EU Transport Task Force Workshop, Salem, MA, United States
 
May 4–6, FES Workshop: Plasma-Materials Interactions, Princeton, NJ, United States
 
May 5–7, ITPA: Transport and Confinement Topical Group Meeting, ITER Headquarters, France
 
May 18–22, 15th International Conference on Plasma-facing Materials and Components for Fusion Applications, Aix-en-Provence, France
 
May 19–23, ITPA Diagnostics Topical Group Meeting, NIFS, Japan
 
May 20, DEADLINE for submission of manuscripts to the Journal of Fusion Energy Special Issue on Strategic Opportunities
 
May 24–28, 42nd IEEE International Conference on Plasma Science (ICOPS), Belek, Antalya, Turkey
 
June 2–4, FES Workshop: Integrated Simulations for Magnetic Fusion Energy Sciences, Washington, D.C.
 
June 22–26, 42nd EPS Conference on Plasma Physics, Lisbon, Portugal
 
August 24 – September 4, 2th Carolus Magnus Summer School on Plasma and Fusion Energy Physics
 
September 1–4, IAEA Technical Meeting on Energetic Particles in Magnetic Confinement Systems, Vienna, Austria
 
September 7–9, ITPA: Energetic Particles Topical Group Meeting, Vienna, Austria
 
September 14–18, 12th International Symposium on Fusion Nuclear Technology, Jeju Island, South Korea
 
November 16–20, 57th APS Division of Plasma Physics Meeting, Savannah, GA, United States

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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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