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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

Director’s Corner	C.M. Greenfield
Research Highlight	W.P. Wehner and E. Schuster
Schedule of Burning Plasma Events
Contact and Contribution Information

 

By C.M. Greenfield

USBPO beginning an outreach effort

I’m sure you’ve all had the conversation that starts with “when can I get my Mr. Fusion?” While it can be an amusing conversation starter, it also reveals that the information available about plasma science and fusion research outside our community tends to be rather limited. While the ongoing community workshops are (I hope) building support within the community for a common vision of where we need to go, we also need support from the broader scientific community and the general public. My belief is that the best way to do this is to create opportunities for those audiences to hear about our work, the progress we’ve made, and our vision for the future.

In that spirit, the US Burning Plasma Organization will be starting an outreach effort to speak to these audiences. Although there is already quite a bit of outreach activity in the plasma science community, it is mostly focused toward a K-12 audience. An example of this is the annual Plasma Science Expo held during the APS-DPP Conferences, most recently last week in Milwaukee. Over 1000 local students attended the Expo, being both entertained and educated about physics in general, and plasma science and fusion energy more specifically. There are still ways that we can participate and help to expand the reach of these activities, and I will address that more in future columns.

It is clear that there is much less community effort going into communicating to the scientific community and the general public. There is the APS Distinguished Lecturer in Plasma Physics (DLPP) program, which makes six speakers available at any given time to visit colleges and universities to give seminars. Many of you have made your own efforts to speak in many different venues. But I believe there is an opportunity space for the USBPO to make it easier for our members to be heard. Our starting point will be to form something like a “speakers bureau.” We are creating a collection of presentation material that you can use, either to take a canned presentation or create your own. This is at an early stage. David Pace began assembling a library of material about six years ago, but we will need to work on issues including getting permission from authors, how to do proper attribution, and the nuts and bolts of sharing the material (for various reasons we may not be able to just post the material on an open website).

You will be hearing more about this in the coming months. At some point, we will be looking for people who have prepared material to “donate” it to our library and for volunteer speakers. We have about 430 members, so I am hoping geography won’t be too much of a limitation for this effort.

We will also be looking for other ways to expand awareness of our research. There are potential partners for us in the international community, and we will certainly be speaking with them (or already have been, in some cases). Your suggestions will of course be welcome.

U.S. Magnetic Fusion Research Strategic Directions Workshop

The second in this series of community workshops will take place in Austin, Texas, on December 11-15. Full information is given at https://sites.google.com/site/usmfrstrategicdirections/. This is a very important discussion for the entire community. We’ve heard the message that “community consensus is worth money” from several different sources, and there are examples of where that has been successful in other scientific communities.

During last week’s APS-DPP Conference, Mickey Wade gave a presentation on the workshop. He indicated that the Austin workshop will be focused on developing key information and potential strategic approaches for delivering on the guideline given to the National Academy’s Burning Plasma committee, that “economical fusion energy within the next several decades is a U.S. strategic interest.” There are still a variety of opinions on how best to achieve that goal. These workshops are aimed at assembling a community-sourced strategic plan with the needed level of consensus.

In speaking with a number of early career scientists during the APS-DPP conference, I was struck by a widespread reluctance to participate. This was largely based on what I hope and believe is a faulty assumption – that they have nothing to add to the discussion due to their lack of experience. What I told them, and what I would say to any of you with the same concern, is that you not only have a right to strong opinions, but a responsibility to yourself and our entire field to voice them. After all, you have at least as much at stake as any of us with years of experience under our belts.

Operations and Control Topical Group (Leaders: Jim Irby and Eugenio Schuster)

Real-time optimization of the q profile evolution to assist steady-state scenario development at DIII-D

W.P. Wehner and E. Schuster

Plasma Control Laboratory, Lehigh University, Bethlehem, PA

Email: wehner@lehigh.edu

A primary goal for the DIII-D research program over the next five years is to develop the physics basis for a high q (q_min > 2), high b_N steady-state scenario[1] that can serve as the basis for a steady-state ITER scenario at fusion gain Q=5. Various approaches are being considered to maximize the bootstrap current contribution so that fully noninductive (f_NI=1) discharges can be obtained for several resistive current diffusion times. It is anticipated that the upgrades to DIII-D including an additional off-axis neutral beam injection (NBI) system in 2019 will provide sufficient auxiliary current drive to maintain fully noninductive plasmas at high b_N. However, much work is necessary to investigate MHD stability, adequate confinement, and early achievement and sustainment of the steady-state condition.

Predictive transport codes, in particular TRANSP, are being used to develop a scenario that maximizes plasma performance (high b and high confinement) and reaches a steady-state condition, defined by a zero loop-voltage profile, as early as possible in the discharge. The scenario development work provides a feedforward control policy (set of actuator waveforms) that under ideal conditions guides the plasma evolution to the desired state. However, variability in the plasma startup and tokamak wall conditions amongst other things can lead to poor performance with feedforward control alone. Therefore, the feedforward control policy is combined with feedback control, meaning the control is computed in real-time as a function of the measured plasma state, to improve shot-to-shot reproducibility.

The feedback control signal is computed by optimizing in real-time the plasma response to the available actuator set over a finite horizon (number of future control time-steps). A control-oriented model is used to predict the evolution of the poloidal magnetic flux (y) profile, which is directly related to the plasma safety factor q, and the plasma pressure (as measured by b_N) in response to the actuator set over a short time horizon [1]. An optimization routine selects the control actions to minimize the difference between the model-predicted plasma evolution and the desired evolution.

At time k during the discharge, the feedback control is computed as the minimizer to the optimization problem (P1). The feedback control (u^FB) represents an update to the feedforward control (u^FF) with the aim of minimizing deviations from the target q profile evolution, q^des. Additionally, if violations to a set of constraints including bounds on q and b_N, or deviations from the desired loop voltage profile are predicted, then the feedback controller makes updates to the feedforward control so as to minimize the constraint violations.

The optimization problem considers a short horizon (number of future time steps), k, k+1, k+2, …, k+N, and a linearized form of the system dynamics (poloidal flux evolution model) is used in problem (P1) to make the problem real-time solvable. Because of limits on the actuators and other constraints, the optimal solution to problem (P1) cannot be obtained in closed-form, and the problem must therefore be solved numerically.

If the model completely and accurately described all of the underlying dynamics of the system, we could simply apply the entire control sequence for time-steps k, through k+N. However, since the model is only approximate, we expect to see some discrepancy between the predicted and measured state at the subsequent time-step. Therefore, only the first update of the optimized control sequence is applied, the optimization problem is then updated with a new plasma state measurement on the next time-step, and the optimization procedure repeated. Introducing feedback in this manner, i.e. solving the optimization problem, sampling the state, and solving the updated optimization problem again in a repetitive fashion leads to the feedback scheme known as “model predictive control” (MPC). Because the MPC approach involves solving a series of similar optimization problems, the information from a previous solution can be used to initiate (warm start) the next solve allowing for fast average computation times.

The feedback control approach defined by problem (P1) is expected to be fully tested during the 2018 DIII-D experimental campaign. Initial tests were carried out during the 2016 experimental campaign, in which the feedback control optimization problem took a form similar to problem (P1), but without the constraints associated with MHD stability and steady-state achievement [2]. The problem was solved in real-time by an active set method, which combined with warm-starting of the optimization problem allowed for sufficiently fast control computation times on average of 1 ms (see [2] for details).

Experimental testing showed the MPC feedback approach could successfully reach monotonic q profile targets with relatively high q_min > 1.5. Figure 1 shows control results for shots 163743 (target: q_min = 1.6 and q₉₅ = 5) and 163832 (target: q_min = 1.9 and q₉₅ = 5). For both shots, the obtained q profile is plotted at the top of the respective figures in comparison to the q profile obtained with feedforward control alone. The failure of the feedforward control action to reach the target is primarily due to slight modeling errors in the resistive diffusion rate. This emphasizes the importance of feedback control, which is able to account for the modeling errors and bring the q profile back on target. In both cases, the desired q profile (q^des of problem (P1)) and the feedforward control signal were taken as design parameters and obtained via scenario planning by model-based optimization (see [3,4]).

Figure 1: Shot 163743 (left) with target: q_min = 1.6, q₉₅=5, and shot 163832 (right) with target: q_min = 1.9, q₉₅=5. The achieved q profile at the target time for both feedforward control (FF) and feedforward + feedback control (FF+FB) cases is displayed in the top plots followed by the q_min value in the middle plot and q₉₅ at the bottom.

During the initial tests, sufficient performance was obtained with a short horizon time (<10 control time-steps). However, for the steady-state type control strategy defined by problem (P1) which includes the additional constraints associated with MHD stability and the steady-state condition, it is anticipated a much larger horizon time will be necessary (»50 control time-steps) requiring more sophisticated numerical optimization techniques, which are currently under development.

Acknowledgements

This material is based upon work supported by the U.S. Department of Energy using the DIII-D National Fusion Facility, a DOE Office of Science user facility under awards DE-SC0010661, DE-AC05-00OR23100, DE-FC02-04ER54698.

References

[1] J. Barton et al., “Physics-based control-oriented modeling of the safety factor profile dynamics in high performance tokamak plasmas”, in 52^nd IEEE Conference on Decision and Control (2013).

[2] W. Wehner et al., “Predictive control of the tokamak q profile to facilitate reproducibility of high-q_min steady-state scenarios at DIII-D”, Proceedings of the 2016 IEEE Multiconference on Systems and Control (2016).

[3] J. Barton et al., “Physics-model-based nonlinear actuator trajectory optimization and safety factor profile feedback control for advanced scenario development in DIII-D”, Nuclear Fusion 55, 093005 (2015).

[4] W. Wehner et al., “Optimal current profile control for enhanced repeatability of L-mode and H-mode discharges in DIII-D”, Fusion Engineering and Design (2017).

USBPO Public Calendar:

2017

October 30-November 1	22nd Workshop on MHD Stability Control	Madison, Wisconsin
November 7-9	ITPA Coordinating Committee	ITER Headquarters, St. Paul-lez-Durance, France
December 6-7	Fusion Power Associates Annual Meeting	Washington, DC, USA
December 11-15	U.S. Magnetic Fusion Research Strategic Directions Community Workshop	Austin, Texas, USA

2018                                               

January 30-February 2	ITPA DSOL Topical Group meeting	Chengdu, China
April 9-11	ITPA T&C Topical Group meeting	Daejeon, South Korea
April 16-19	High Temperature Plasma Diagnostics (HTPD) conference	San Diego, CA
April 23-25	Sherwood Theory Conference	Auburn, AL
May 8-11	US Transport Task Force (US-TTF) meeting	San Diego, CA
June 17-22	International Conference on Plasma Surface Interactions (PSI)	Princeton, NJ
June 24-28	2018 IEEE International Conference on Plasma Science (ICOPS)	Denver, CO
July 2-6	EPS Conference on Plasma Physics	Prague, Czech Rep.
Sept 11-14	EU Transport Task Force (EU-TTF) meeting	Seville, Spain
October 22-27	IAEA Fusion Energy Conference	Gandhinagar, Gujarat, India
November 5-9	60th Annual Meeting of the APS Division of Plasma Physics	Portland, OR, USA

2018

January 30-February 2	ITPA DSOL Topical Group meeting	Chengdu, China
June 24-28	2018 IEEE International Conference on Plasma Science (ICOPS)	Denver, Colorado, USA
October 22-27	IAEA Fusion Energy Conference	Gandhinagar, Gujarat, India
November 5-9	60th Annual Meeting of the APS Division of Plasma Physics	Portland, Oregon, USA

2019

JET DT-campaign and JT60-SA First Plasma
October 21-25	61st Annual Meeting of the APS Division of Plasma Physics	Fort Lauderdale, Florida, USA

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: Walter Guttenfelder (wgutten@pppl.gov)