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.
Announcements Research Highlight Delgado-Aparicio et al. ITPA Update Schedule of Burning Plasma Events Contact and Contribution Information
US contribution on W-7X
The Wendelstein 7-X stellarator achieved its First Plasma milestone on 10 December 2015. The accuracy of the magnet system was verified by electron-beam field mapping (left) and expected island structures were observed. The first plasma (right) consisted of Helium and reached about 1 million degrees Celsius. (Photos: IPP)
The Wendelstein 7-X (W7-X) stellarator experiment officially opened on 10 December 2015 with the achievement of its First Plasma milestone. The facility, located at the Max Planck Institute for Plasma Physics (IPP) in Greifswald, Germany, uses a ring of 70 specially shaped superconducting coils (50 planar and 20 non-planar), to confine the hot plasma. It will provide a large-scale test of an innovative physics-optimized 3D plasma configuration design (Figure 1). It will test the compatibility of a high-performance core plasma with a unique 3D divertor, and be capable of extending plasma-material interaction research to pulse lengths up to 30 minutes. The W7-X is a key element in Europe’s fusion energy roadmap, with a mission to validate its optimized design and to qualify its so-called island divertor. Collaboration in W7-X research enables U.S. fusion scientists to deepen understanding of 3D plasma physics and to advance long-pulse PMI science using the world’s most advanced stellarator. A formal collaboration agreement between the Department of Energy and IPP, signed in 2014, provides opportunities for U.S. scientists to participate in W7-X as full team members, with access to all data and the ability to lead experiments and publish results.
The U.S. collaboration on W7-X began with an invitation from IPP in 2008 for the U.S. to join the project, then in construction, as a partner. A number of urgent engineering tasks, well matched to U.S. capabilities, were identified. After a proposal from Princeton Plasma Physics Laboratory, Oak Ridge National Laboratory, and Los Alamos National Laboratory was approved by DOE in 2010, work started on several projects, including the design and construction of a full system of low-order field perturbation ”trim” coils, which W7-X will use for balancing heat loads among its ten divertor chambers. U.S. Laboratory researchers participated in the commissioning of the trim coil system and in vacuum field mapping experiments to check that their effect on the magnetic configuration was as predicted.
In 2015, with W7-X research preparations intensifying, DOE approved proposals from four university and industry teams, bringing the Massachusetts Institute of Technology, the University of Wisconsin, Auburn University, and Xantho Technologies, LLC into the collaboration. By the time W7-X achieved first plasma, the U.S. had several tasks either completed or in progress. U.S. equipment items were installed and operating, and several U.S. scientists were on site, operating U.S. diagnostics, and contributing to the experiment from the first day of plasma operation. A list of U.S. institutions and their current roles in the W7-X collaboration is given in Table 1.
The goals of the brief first plasma campaign (known as OP1.1) are to establish reliable electron-cyclotron heated plasma operation in a limiter configuration, commission diagnostics, and begin to characterize plasma conditions. After a year-long outage to complete the installation of in-vessel components including an inertially-cooled test divertor system, the second campaign (OP1.2) will begin in mid-2017. The U.S. will be well prepared to make key physics contributions during that campaign, which will feature the first operation with divertors and neutral beam heating. This will explore the operating space of the device, and will address a diverse range of physics topics. Results from the OP1.2 campaign will help to determine the optimum in-vessel systems configuration and the most promising plasma scenarios for steady-state operation in the OP2 campaign to follow.
For further details on W7-X visit http://www.ipp.mpg.de/16900/w7x. For information on US participation, contact Hutch Neilson (email@example.com), U.S. Technical Coordinator for the W7 X collaboration, and visit http://advprojects.pppl.gov/home/w7-x.
Call for collaboration on WEST
As the construction of the WEST platform is now approaching completion, I would like to invite you to submit experimental and modelling proposals for the first phase of WEST exploitation (phase 1, 2016-2017).
The WEST program is targeted at supporting the ITER tungsten divertor operation and extending Hmode operation towards long pulse in a metallic environment. First plasma is scheduled in fall 2016. It is intended to run WEST as a user facility, open to ITER partners. The present call is the basis for the common scientific exploitation of WEST.
The WEST program is structured around 2 Task Forces. While the call remains open to innovative ideas for making the best use of WEST specific features, it is proposed to focus on addressing the high priority research areas identified below for the first phase of WEST exploitation:
|Testing ITER grade Plasma Facing Components (PFCs)||Towards long pulse H mode operation|
|(TF leader: E. Tsitrone)||(TF leader: C. Bourdelle)|
|Heat load pattern characterization, Wall protection system, Test of ITER grade PFCs 10 MW/m2, tile shaping, Operation with damaged/misaligned PFC, melting||Robust RF heating scenarios, Tungsten control: sources and transport, H-mode characterization, Preparatory work for long pulse operation|
Confinement and Transport Topical Group, Leaders: Gary Staebler and Saskia Mordijck
Recent multi-scale gyro-kinetic simulations are shedding new light on the transport short-fall in the heat flux channel. N.T. Howard’s work shows direct agreement between experiments and theoretical simulations, opening the door for predictive ITER simulations.
In tokamak plasmas, the measured heat and particle losses generally exceed predictions from classical transport theories by orders of magnitude . Small-scale turbulence (small compared to the size of the device), known as drift-wave turbulence, has been measured in detail in the core plasma of many experiments . This turbulence is believed to be the primary cause of the high transport levels. To better understand and predict the turbulent-transport in tokamaks, nonlinear gyrokinetic theory  and associated numerical codes  have been developed that capture much of the dynamics of plasma turbulence for realistic conditions. Despite the model’s success at matching experimental observations in many cases, several key disagreements between simulated and experimental heat fluxes remain unresolved. In particular, heat loss in the electron channel is notoriously difficult to model . Since burning fusion plasmas, such as ITER, will feature equilibrated ions and electrons, errors in the calculation of the electron heat flux will significantly impact the ability to correctly predict ion temperature profiles and fusion performance in ITER.
A handful of specific drift-wave instabilities, classified by their primary driving mechanism and by the spatial scales of their associated turbulent eddies, are generally thought responsible for cross-field heat transport in the core of tokamak plasmas . The turbulent driving mechanisms are often parameterized as the logarithmic gradients in density, electron temperature, or ion temperature normalized to the plasma minor radius, e.g., a/Lx = −a∇ln(x), where x = ne(r), Te(r) or Ti(r), r is the radial coordinate of the plasma, and a is the midplane minor radius. Eddy sizes of the turbulence are characterized by values of kθρs, where kθ is the poloidal wavenumber of the turbulence and ρs = cs/Ωi is the ion sound Larmor radius (with cs = (Te/mi)1/2 and Ωi = eB/mi). Turbulence classified as ion-scale (low-k), has eddy sizes in the range kθρs < 1.0, while electron-scale (high-k) turbulence is associated with spatial scales in the range 1.0 < kθρs < 60.0. Due to the large eddy size and low frequency, most experiment and simulation has focused on turbulence at the ion-scale, where the Ion Temperature Gradient (ITG) mode, driven by a/LTi, and Trapped Electron Mode (TEM), driven by a/Lne and a/LTe are thought to dominate. However, at electron scale, the Electron Temperature Gradient (ETG) mode, driven predominantly by a/LTe is also known to exist. Despite the small scale of this turbulence, which might imply negligible transport, it has been shown numerically that ETG is able to form radially elongated ETG streamers  capable of driving experimental levels of heat transport 
To date, almost all research has used single scale (ion or electron-scale simulation) to interpret and compare with experiment. Such approaches implicitly assume weak or no coupling between the turbulent scales. This approach is used in part due to the extreme computational requirements for multi-scale simulation, which captures ion and electron-scale turbulence simultaneously and therefore must resolve turbulence on both the ion and electron spatio-temporal scales. Until recently , all multi-scale simulation utilized artificially heavy electrons (to reduce the scale separation and computational requirements) and typically simulated only modeled parameters . As a result, the role of cross-scale coupling in experimental conditions was left largely unexplored.
Figure 1 Simulated potential fluctuations are plotted for conditions (left) representative of standard ion-scale turbulence and (right) demonstrating the multi-scale nature of turbulence where ion-scale eddies are seen to coexist with ETG streamers.
Figure 2 The ion and electron heat fluxes for a scan of the ITG turbulence drive (a/LTi) from standard ion-scale simulation (red diamonds) are compared with multi-scale simulation (blue squares). From the multi-scale simulation, the electron heat fluxes driven at low-k (yellow dots) and high-k (green triangles) are also plotted. Only multi-scale simulation can simultaneously reproduce Qi and Qe from experiment.
Recent results, published in Nuclear Fusion , have begun to shed light on the cross-scale coupling of ion and electron-scale turbulence in experimental plasmas. Motivated by a well-documented discrepancy between standard ion-scale gyrokinetic simulation and experimental electron heat flux in Alcator C-Mod L-mode discharges , a total of 6 realistic electron mass, multi-scale (capturing up to kθρs = 48.0) gyrokinetic simulations were performed using the GYRO code , which probe the 1 σ experimental uncertainties of the ion-scale and electron-scale drive terms (a/LTi and a/LTe). These simulations pushed the limits of current supercomputing, requiring approximately 100,000,000 CPU hours on NERSC supercomputers and for the first time shed light on the effects of cross-scale coupling in experimental plasma conditions.
The multi-scale simulation results were rigorously compared with experiment, revealing that only multi-scale simulation is able to quantitatively reproduce experimental ion and electron heat flux as well as measured electron profile stiffness in the core (r/a = 0.6) of an Alcator C-Mod L-mode . The effects of cross-scale coupling are dramatic in conditions with marginally stable ion-scale turbulence or strongly driven electron-scale turbulence. ETG streamers are found to play a dominant role in the experimental conditions, able to coexist with ion-scale eddies (See Figure 1), and drive up to 70 % of the total electron heat flux. The presence of short wavelength turbulence (ETG) is found to not just to drive heat flux at electron scales, but also enhance ion and electron transport at ion scales; likely through cross-scale energy transfer and zonal flow shear modification. The interplay between ion and electron-scale turbulence fundamentally alters the response of heat fluxes to both turbulence drives (a/LTi and a/LTe) and can lead to electron heat fluxes that exceed standard ion-scale simulation by nearly a factor of 10 (See Figure 2.). Ultimately, these results provide a likely origin of missing electron heat flux and point to the importance of cross-scale turbulence coupling for interpretation of current day experiments and predictions for ITER.
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More information concerning the ITPA may be found at the Official ITPA Website.
Pedestal and Edge Physics Topical Group
A meeting of the Pedestal and Edge Physics ITPA Topical Group meeting was held October 22-23 in Garching, Germany, hosted by IPP. It immediately followed the H-mode and Transport Barrier Physics workshop at the same location, where many relevant results had been presented, hence the shorter duration. The meeting was co-located with a meeting of the Transport and Confinement group, and there were a few joint sessions. As is typical a main topic of the meeting was Resonant Magnetic Perturbations. G. Huijsmans reported on a recent workshop at ITER on the coils, which reaffirmed the need for the coil set and its main specifications. Interesting results were presented on ELM mitigation and suppression from DIII-D, EAST, KSTAR, MAST and AUG. Notably, AUG now achieves mitigation at low . It is becoming clear on several devices that edge response is localized at the top and bottom of the flux surfaces, not the midplane. Results on pellet mitigation were also presented. JET and AUG report that mitigation of transient ELM heat flux is less effective since they have switched to high Z PFCs. Modeling suggests high field side launch may be more effective for ITER. Vertical ’kicks’ are also being used on JET to mitigate ELMs, and predictions have been made for ITER. Research on I-mode, which naturally avoids ELMs, continues to be active. A paper on multi-device studies (C-Mod, AUG, DIII-D) is ready for submission and experiments are planned on several other tokamaks. Formation of a joint database to improve threshold and confinement scalings was proposed.
Pedestal structure was also a major topic. Phil Snyder (GA) reported on statistical analysis comparing the EPED model to a large multidevice dataset, with generally good results. The model has recently been combined with core transport simulations, which will be very useful for scenario modeling. Tim Luce (IOS Chair) attended a session and made a set of requests for further information to assist in scenario predictions, particularly for He or H plasmas which have been less studied to date. Understanding the generally lower pedestals observed in JET with ITER-like wall, and the effects of seeding and impurities generally, is a high priority topic.
The next meeting will be held in India, March 16-18, again co-located with the T&C group. Several joint sessions are planned. The deadline for registrations has now passed, but remote participation should be possible. Meeting web site is http://www.ipr.res.in/itpa-2016/index.html.
2016 — 10th Anniversary of USBPO Formation —
|March 16-18||ITPA T&C, Institute for Plasma Research||Gandhinagar, India|
|March 16-18||ITPA PEP, Institute for Plasma Research||Gandhinagar, India|
|March 29 - April 1|
|May 30-June 3||PSI Conference||Rome, Italy|
|June 19-23||International Conference on Plasma Sciences, ICOPS||Banff, Alberta, Canada|
|June 27-July 1||18th International Conference on Plasma Physics (ICPP2018)||Kaohsiung, Taiwan|
|July 4-8||European Physical Society Conference on Plasma Physics (EPS)||Leuven, Belgium|
|September 4-8||Joint EU-US Transport Task Force Meeting, Leysin, Switzerland]|
|October 13-15||ITER STAC Meeting||Kizu, Japan|
|October 17-22||26th IAEA Fusion Energy Conference||Kyoto, Japan|
|October 24-26||ITPA T&C, JAEA||Naka, Japan|
|October 31-November 4||58th APS Division of Plasma Physics||San Jose, California, USA|