News and Events


U.S. Burning Plasma Organization eNews
November 30, 2016 (Issue 110)

 

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
Research Highlight
D.R. Hatch, et al.
Schedule of Burning Plasma Events  
Contact and Contribution Information

Announcements

STELLCON Report Made Available for Community Input

A report has been prepared that outlines the research needs and lays out a research plan for a reinvigorated stellarator program in the US. The report is the product of a series of community meetings organized by the National Stellarator Coordinating Committee the first of which was held on February of 2016, and hosted by MIT. The meeting, called STELLCON, was attended by 40 scientists from 12 different institutions as well as by a representative from FES. The report describes a compelling vision for advanced stellarator concepts based on innovations developed in the last decade, which extend the optimization strategies used to design the Wendelstein 7-X stellarator. The case is made that the stellarator is an important research path that can help solve the issues that stand between current understanding and the achievement of practical fusion energy.

The authors invite community input on the report, which can be found here /activities/?article=STELLCON\%20Report. Comments can be sent via email to Dr. David A. Gates, Stellarator Physics Division Head at PPPL (dgates@pppl.gov) and to Professor David Anderson (dtanders@wisc.edu). Comments will be accepted until January 6th 2017.



Director's Corner

by C.M. Greenfield

Happy Holidays to all

I would like to wish all of our readers a wonderful holiday and a happy and productive 2017.

ITER Council endorses staged plan to DT operation

You may remember that back in June, the ITER Council accepted a resource-loaded integrated schedule that will bring the facility to readiness for first plasma by December, 2025. At their recent meeting on November 16 and 17, the ITER Council further considered and accepted a staged plan that takes ITER from first plasma in 2025 to high-power DT operation in 2035. There will be four research phases: First Plasma and Engineering Operation (2025-2026), Pre-Fusion Power Operation I (2028-2030) and II (2032-2034), and finally Fusion Power Operation (starting in 2035). The device capabilities will increase at each stage, and research will focus on preparing the basis for operating ITER with its full capabilities in the Fusion Power Operation phase. Although I know we would all like to see high fusion gain operation sooner, there is now a plan in place that takes into account credible estimates of each domestic agency's timetable for delivery of their contributions. And while this plan was being developed, ITER has demonstrated a track record that gives some reason for confidence. Progress on the ITER site has accelerated during the past 18 months (see photos) and all 19 project milestones to date in 2016 have been completed on time and on budget.

The ITER Facility in December 2015 (left) and November 2016 (right). Photos © ITER Organization.

Activities at the recent APS-DPP Conference

In case you missed the recent APS-DPP Conference in San Jose, we have posted David Campbell's talk entitled Progress in ITER Construction and Strategy Towards the Operations Phase on the USBPO website at http://burningplasma.org. If you'd like more detail on the staged plan approved by the ITER Council, this is a good place to start.

We also organized the ninth annual Research in Support of ITER contributed oral session earlier the same day. Those talks will be posted in the coming days as soon as they are approved for posting by the speakers' institutions.



Research Highlight

Confinement and Transport Topical Group, Leaders: Saskia Mordijck and Walter Guttenfelder

The extention of non-linear gyro-kinetic simulations to the pedestal area has been challenging for decades. D.R. Hatch et al. have succeed at performing these simulations and in the process have discovered important transport phenomena that might play an important role in parameter regimes that might occur in burning plasma scenarios.

Pedestal Transport in the Transition to Burning Plasma Regimes
D.R. Hatch, M. Kotschenreuther, S.M. Mahajan1
1 Institute for Fusion Studies, University of Texas at Austin

Introduction - The H-mode pedestal is central to the most pressing issues for fusion energy. The pedestal height is the determining parameter for confinement through its role as the boundary condition for the core, where temperature profiles are typically constrained to lie near critical gradients. Likewise, due to its propensity for ELMs, the pedestal is strongly tied to divertor considerations and material constraints.

To date, MHD stability has been the prevailing paradigm for understanding and predicting pedestal dynamics, and has been quite successful in providing a framework for a broad class of pedestal scenarios over a range of experiments. In a typical ELMy H-mode, an ELM is triggered when the plasma evolves to a state that is unstable to peeling-ballooning (ideal MHD) modes. The inter-ELM trajectory - and ultimately the pre-ELM state - of the plasma is determined by the profile-constraining transport mechanisms at play in a given scenario. The EPED model [1], for example, appeals to the kinetic ballooning mode (KBM) - essentially a second MHD limit - as the salient transport mechanism.

Despite its success, this exclusively MHD-based picture has limitations. Here we highlight the role of non-MHD turbulent transport, which we will call drift-type transport (for example ion temperature gradient (ITG), electron temperature gradient (ETG) or microtearing turbulence). These transport mechanisms can produce robust heat flux with little particle flux, thus preferentially limiting the temperature profile. Drift-type mechanisms appear to be the dominant player in many JET-ILW discharges. Moreover, our work suggests that drift-type transport will likely be the prevailing consideration in pedestal dynamics in the transition to a burning plasma regime.

We describe here simulations using the gyrokinetic GENE code modeling JET-ILW and prospective ITER pedestal scenarios. We have demonstrated for the first time, quantitative agreement between simulations and experimental power balance [2]. Furthermore, an extensive exploration of parameter space qualitatively reproduces the major experimental JET-ILW observations. The insights provided by these studies may aid in the pursuit of Q=1 in prospective upcoming JET DT campaign, and, furthermore, chart out the optimal route to ensuring good confinement on ITER.

The JET ITER-like wall Pedestal - JET ITER-like wall (ILW) scenarios offer prominent examples of pedestal dynamics limited by drift-type transport mechanisms. Since installing the ILW (tungsten divertor and beryllium chamber) JET has been unable to recover the confinement levels that were typical of earlier carbon wall operation in certain important scenarios [3]. This confinement degradation is attributable almost entirely to a limitation on pedestal temperature. Recent experimental observations have determined that JET-ILW pedestals often exhibit temperature profiles that saturate midway through the ELM cycle, accompanied by density profiles that evolve unconstrained until the ELM crash [4]. Such behavior is a clear signature of drift-type transport and is difficult to reconcile with an MHD-like (i.e. KBM) mechanism.

Our results suggest that a framework oriented around drift-type transport is consistent with the major JET-ILW pedestal observations including: (1) the inaccessibility of high pedestal temperatures, (2) the degradation of confinement with gas puffing, (3) the partial recovery of confinement with impurity seeding, and (4) the stronger propensity for metal-wall confinement issues on JET in comparison with ASDEX Upgrade (AUG) (JET accessing lower values of ρ* than AUG).

figure_1.jpg

Figure 1. Contributions to the transport as modeled with the GENE code for an experimental JET discharge (Base-matching experimental power balance), a high temperature scenario (T2-with sharply increased transport), and a high temperature scenario including a carbon impurity (T2 Z2C-with reduced transport).

Basic Physics- Although quantitative conclusions rely on demanding nonlinear gyrokinetic simulations, the underlying physics is rather transparent and is often reflected in the basic properties of the linear instabilities. An important feature of drift-type pedestal instabilities is that they are more slab-like in nature in comparison with core modes, which are dominated by toroidal effects [5]. This is attributable to both the stronger flux surface shaping and stronger gradients characteristic of the pedestal. In such a regime, the following parameters are particularly important:

  1. η = Ln/LT, (where Ln and LT are, respectively, the density and temperature gradient scale lengths), which is the dominant parameter characterizing the drive of ITG and ETG modes and is particularly sensitive to the operational constraints imposed by the ILW;

  2. Z-effective, which is tied to ILW impurity constraints and strongly affects transport mechanisms (notably ion dilution for ITG modes);

  3. and ρ*, which is an important parameter for ITG turbulence through both intrinsic ρ* effects [6] and a decrease in E ×B shear rates with decreasing ρ*. Scaling arguments lead robustly to the conclusion that E ×B shear rates scale proportional to ρ* [5].

GENE Simulations- As a starting point [2], direct contact was made with a representative JET discharge (number 82585 described in detail in [7]). An extensive linear gyrokinetic analysis demonstrated microtearing modes to be the dominant ion-scale instabilities, whereas KBM is in a second-stability regime. Nonlinear GENE simulations were able to quantitatively match power balance across most of the pedestal (the first time this has been achieved with gyrokinetic simulations in the pedestal). This study, in effect, predicted the recent JET observations [4] of pedestal scenarios wherein the temperature profile is saturated while the density evolves unconstrained to an ELM crash.

In order to explore the ways in which drift-type transport is related to JET-ILW dynamics, several scenarios were constructed - variations around the base case (discharge 82585) that explore in a controlled manner the salient parameter transitions. The trend outlined in Fig. 1 reproduces perhaps the main JET-ILW observation - the inaccessibility of high pedestal temperatures that were accessible during earlier carbon operation and in nitrogen seeded ILW discharges. The simulations demonstrate that the prevailing mechanisms are related to shear suppression and ion dilution of ITG turbulence.

Figure 2: GyroBohm normalized transport over ρ* scans for JET-ILW (left) and ITER (right) parameter regimes. The ITG turbulence is negligible except at low ρ*.

We also seek to elucidate the reasons why JET is more-susceptible to metal-wall issues than other experiments (e.g. AUG and Cmod), and to explore implications for ITER. To this end, a dimensionless ρ* scan (varying ρ* while keeping all other dimensionless parameters fixed) was constructed. This exercise identifies two classes of transport mechanisms - those that scale close to gyroBohm scaling (which, in turn, is quite close to the standard H98 scaling law), and those that deviate strongly from gyroBohm scaling. The former category includes neoclassical transport, ETG transport, and microtearing transport (and presumably in other parameter regimes, KBM). The latter category includes ITG transport, which has an unfavorable ρ* scaling attributable to the decrease in E ×B shear rate with ρ*. Consistent with experimental observations, transport scales close to gyroBohm over the range of most present-day experiments. Deviation is only identified in the unique JET-ILW parameter regime (low ρ*, low Z-eff) corresponding to the regime of strong confinement degradation on JET [8]). These results are thus consistent with experimental observations (see, e.g., Ref. [9]) that pedestal structure is insensitive to variations in ρ*: the relevant effects appear only below a certain threshold in ρ*, and under conditions which are not commonly achieved on present day experiments. In such regimes, ρ*-sensitive ITG turbulence is no longer negligible and the transport begins to deviate strongly from gyroBohm scaling. Results are shown in Fig. 2 for both JET-ILW parameters and ITER parameters.

These results suggest that the low ρ* degradation of shear suppression is a major player in JET-ILW pedestal dynamics and may be a nascent manifestation of a fundamental challenge for ITER. Understanding the ρ*-dependence of pedestal transport should be a major point of emphasis - both experimental and computational. Optimized divertor operation (keeping SOL density as low as possible) and fine-tuned impurity seeding will be the major factors enabling acceptable pedestal transport levels on ITER [5]. JET, with its metal wall and access to low ρ*, will be an indispensable tool for examining such regimes. And in fact, by employing the optimizations on JET that will be needed on ITER, considerably higher energy gain should be obtained in JET-ILW DT operation.

Summary- The transition to burning plasma regimes may involve fundamental changes in pedestal transport dynamics. The impact of the relevant drift-type transport mechanisms is most strongly related to 1) divertor considerations, which tend to impact the drive of pedestal microinstabilities through the density profile (η effects), and 2) machine-size effects (i.e. ρ*) related to shear suppression, which exacerbate turbulence mechanisms that are negligible on smaller machines. JET-ILW is the best present-day laboratory for exploring such regimes. A deeper understanding via both experiment and computation holds the promise of enabling optimized pedestal scenarios that reconcile the conflicting demands of confinement and material constraints.

References
[1] P. B. Snyder et al., Phys. Plasmas, 16, 056118, (2009).

[2] D. R. Hatch, et al., Nucl. Fusion, 56, 104003 (2016).
[3] C. Giroud et al., Nucl. Fusion 53, 113025 (2013).

[4] C. F. Maggi et al., Proceedings of the 26th IAEA Fusion Energy Conference, Kyoto, Japan (2016).
[5] M. Kotschenreuther, Bulletin of the American Physical Society 60, (2015); M. Kotschenreuther, et al., Burning Plasmas in the High Confinement Mode and ITER, submitted to Nuclear Fusion 2016.

[6] B. F. McMillan et al., Phys. Rev. Lett. 105, 155001 (2010).

[7] M. J. Leyland et al., Nucl. Fusion, 55, 013019 (2015).

[8] I. Nunes and JET Contributors, Plasma Phys. Control. Fusion, 58, 014034 (2016).

[9] M. N. A. Beurskens et al., Phys. Plasmas, 18, 56120 (2011).


ITPA Update

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

Transport and Confinement Topical Group

The 17th Transport and Confinement Topical Group meeting was held at the Naka Fusion Institue in Japan from October 24-26 2016. Aside from an updates on ongoing joint experiments, the meeting focused on several topics and there were joint sessions with the pedestal group and the scenario group. The T&C group is leading the effort to update the ITER database. Aside from updating the database using new experimental data, the database is also looking at including more parameters related to confinement, that were not included last time. An update was also given on the database related to intrinsic rotation. The two topics that were discussed in more detail are related to edge shortfall and stiffness of the temperature profiles. The focus of these areas is mostly on linear, quasi-linear and non-linear code validation. How to capture the multi-scale interactions with reduced models is an active area of research along with the comparison of the different non-linear codes.

The joint session with the pedestal group focussed on the I-mode regime and the joint session with the scenario group was on 4 different topics: Particle transport, neural nets for core transport, transport in the current rise and discussion on isotope effect on core transport. In this joint session with scenarios, the speakers were from both the transport community as well as the scenario community in order to approach these topics from different angles and perspectives. The next ITPA T&C meeting will be in Princeton, New Jersey, USA from May 1-3. Potential topics for this meeting are

  • Revision of ITER confinement database
  • Dependence of momentum/ particle pinch on collisionality
  • L-H transition, Impurity transport
  • Particle transport during transient H-mode phases by pellets
  • Understanding of the LOC-SOC transition and connected phenomenology

Schedule of Burning Plasma Events

USBPO Public Calendar: View Online or Subscribe

2016       ---10th Anniversary of USBPO Formation ---

December 6-8, 7th Coordinating Committee meeting, ITER Organization, St. Paul Lez Durance, France
December 8, 7th Executive Coordinating Committee ITPA meeting, ITER Organization, St. Paul Lez Durance, France
December 13-14, 37th Fusion Power Associates Annual Meeting and Symposium, Fusion Power Development: An International Venture

2017

April 18-21, 2nd European Conference on Plasma Diagnostics (ECPD), Bordeaux, France
April 25-28, EU-US Transport Taskforce Meeting (TTF), Williamsburg, Virginia
May 1-3, Sherwood Fusion Theory meeting, Annapolis, MD, USA
May 21-25, 44th International Conference on Plasma Science (ICOPS), Atlantic City, New Jersey
June 4-8, 27th IEEE Symposium on Fusion Engineering (SOFE2017), Shanghai, China
JUne 26-30, 44th EPS Conference on Plasma Physics, Belfast, Northern Ireland
September 18-22, 1st Asia-Paficic Conference on PLasma Physics (AAPPS-DPP), Chengdu, China
September 27-29, Plasma Edge Theory in Fusion Devices (PET16), Marseille, France
October 23-27, 59 th Annual Meeting of the APS Division on Plasma Physics, Milwaukee, Wisconsin

2018

June 24-28, 2018 IEEE International Conference on Plasma Science (ICOPS), Denver, Colorado, USA

2019        --- JET DT-campaign - JT60-SA First Plasma ---

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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: Saskia Mordijck (smordijck@wm.edu)

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