News and Events


U.S. Burning Plasma Organization eNews
December 31, 2016 (Issue 115)

 

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
V.N. Duarte
Schedule of Burning Plasma Events  
Contact and Contribution Information

Director’s Corner

C.M. Greenfield

A farewell and a welcome

For about 2 1/2 years, ending last summer, Teresa Garza of the University of Texas ably served as the US Burning Plasma Organization Administrator. The involvement of the University of Texas in USBPO management goes back to my predecessor, Jim Van Dam, prior to his departure for DOE. The administrative tasks have now been moved to my home institution, General Atomics, where Michelle Metschel has now assumed the administrator position. I would like to express my gratitude to Teresa for her great work, and welcome Michelle to the US Burning Plasma Organization.

This change does not impact our website, mailing lists, and other communications media, which continue to be maintained by USBPO Communications Coordinator Mark London of MIT.

ITER in 3D

Those of you who visited the ITER booth at the recent IAEA Fusion Energy Conference in Kyoto may have had an opportunity to view a virtual reality tour of the ITER worksite, including views in and around the construction site, and even from drones flying above. Now the ITER Organization has published an updated version of that tour on the web, and it can be viewed either on your computer screen in 2D or in 3D using Virtual Reality (VR) goggles (Google Cardboard, Samsung Gear VR,...) and a smart phone. There is a link to the current version of the tour on the ITER home page at http://iter.org, or you can jump directly to http://static.iter.org/com/360/2016-10/. You can also scan the QR code embedded in the figure.

ITER1.png

Figure 1: A screen shot of the Main Map of the ITER VR tour. The QR code at the lower left hand corner is a shortcut to the tour.

This provides an excellent opportunity for all of us to follow construction progress almost as if we were there in person. And the progress is impressive. I visit the ITER site 2-3 times per year, and there are major, visible changes each time I visit. This wasn't always the case, but it is now. My latest visit was just a few weeks ago, for the ITPA Coordinating Committee meeting. Some of my photos are below, but of course there is much more to see in the VR tour.

Clockwise from top left: A prototype poloidal field coil is being prepared in the PF coil winding facility, and preparations are being made to begin winding the first “real’ PF coil in the next few months; the cryostat baseplate, with welding nearing completion, is prepared for an inspection; the ITER bioshield (the circular structure) is taking shape in front of the Assembly Building; The Assembly Building dominates a view of the worksite from the ITER Headquarters Building.



Research Highlight

Topical Group, Leaders: Nikolai Gorelenkov and Eric Bass


Criterion for chirping onset

V.N. Duarte 1,2, H.L. Berk 3, N.N. Gorelenkov 2
1 Institute of Physics, University of São Paulo, São Paulo, SP, 05508-090, Brazil
2 Princeton Plasma Physics Laboratory, Princeton, NJ, 08543, USA
3 Institute for Fusion Studies, University of Texas at Austin

Abstract

We use the generalized form of a criterion found by Lilley et.al. [1] that assesses whether marginally unstable Alfvénic modes destabilized by energetic particles are either likely to chirp or to remain as steady. This criterion is applied to NSTX and DIII-D data to predict whether chirping or steady Alfvénic oscillations are likely. Chirping arises in DIII-D when background turbulence markedly decreases.

Alfvénic instabilities in plasmas excited by energetic particles (EPs) can lead to a variety of nonlinear responses ranging from steady oscillations, excitation of side bands, chaotic behavior and finally to where the excited instability produces chirping signals [1.2]. Here we discuss how to assess the likelihood for the two extreme conditions to arise, steady or chirping oscillations, for specific experimental conditions. A more detailed discussion, can be found in [3]. A simplified analysis had been made in [1], but we found that the resulting predictions often contradicted the experimental results. In order to obtain predictions that correlated well with experimental data we needed to take into account the full phase space dependence of the parameters for the problem and to account for EP diffusion due to background turbulence. Indeed, we discovered in DIII-D data that chirping only arises when the background turbulence significantly decreases.

Here we give a brief outline of the methodology of our analysis. For the cases where steady oscillations are expected one can then apply quasi-linear theory to predict the evolution of the EP distribution function in the presence of Alfvénic excitations. Indeed, we are in the process of implementing a line broadened quasi-linear theory [4] that is applicable whether or not the excited modes have overlapped. For the case where frequency chirping is expected, particle loss by convection is expected rather than diffusion. It is a major challenge to the plasma physics community to build a transport code that would reflect the losses due to such convective transport. Perhaps an approach similar to ref. [5] is appropriate.

The equation that needs to be analyzed is an extension of a time delayed cubic equation in the mode amplitude which describes the dynamics of a system close to marginal stability. If the stochastic processes are sufficiently strong, steady solutions are established. However, if the stochastic processes are too weak or if the drag processes are strong enough, the steady solutions are unstable and the cubic equation predicts an explosive solution that is a precursor to a chirping solution [6].

Specifically, the inclusion of both drag and stochastic terms lead to the following time delayed cubic equation [1] for an amplitude A(t),

05.jpg

Solutions of this equation were analyzed in Ref [1], when νstoch and νdrag have constant values. The results, presented in Fig.(2a), show that as a function of νstochdrag that there are regions where steady solutions are: (A) stable, (to the left and above the solid curve); (B) unstable (below the solid curve and above dotted line) and (C) non-existent (the hatched region of the figure). Wherever the non-existence of a steady solution applies, the cubic equation explodes and the emergence of a chirping solution is expected. For this simplified case the specific quantitative condition for the non-existence of a steady solution, which is a sufficient condition for chirping to arise, is:

07.jpg

In Fig.(1b), which is a plot Int(νstochdrag), we see when νstochdrag < 1.04, that there is no steady solutions so that a chirping response is expected in this case. Hence, experiments that lie in this region should produce chirping, while experiments that lie in the stable region of Fig.(1a) are expected to produce steady fixed frequency oscillations. The red diamonds and black dots in Fig.(1a) is where the experimental points lie for NSTX and DIII-D respectively when characteristic values for the transport parameters and resonance condition are taken. We find that many of these points are incompatible with experiment.

In order to obtain better results with experiment, we need to weight the response from all the regions of phase space where resonances arise. Then the modified criterion for not being able to obtain a steady solution, when we then expect a chirping response, is found to be of the following form,

06.jpg

where the integration is over the resonant surfaces Pϕ(E,μ) defined by:

n,j(E,μ) ≡ nωϕ(E,Pϕ(E,μ),μ)+jωθ(E,Pϕ(E,μ),μ)−ω = 0

with ωϕ(E,Pϕ(E,μ),μ) and ωθ(E,Pϕ(E,μ),μ) the mean angular toroidal and poloidal frequencies respectively. In addition |Vn,j| is the magnitude of the wave-particle resonant Hamiltonian which depends on the eigenfunction structure and the phase space positions of the resonances and Int(νstoch/νdrag), given in fig.(1b) depend on the phase space positions of νstoch and νdrag.

The dominant classical collisional process for νstoch is determined by pitch angle scattering. However, the background plasma turbulence, which typically has little effect on the overall EP distribution, can still play an important role in determining whether or not chirping arises. Hence, in our modelling we add the effect of background turbulence on the EPs based on an electrostatic turbulence model (ETM) developed in reference [7]. To calculate the expression for Crt we use the NOVA/NOVAK codes [8] to find respectively the structure of the eigenfunction and the wave-particle Hamiltonian, Vn,j and we use the TRANSP code to calculate the diffusion of the background plasma from both neo-classical effects and from turbulent diffusion. The turbulent diffusion is inferred in the TRANSP code by choosing the needed diffusion coefficients that achieves power balance. Hence, with the background diffusion determined, we apply the ETM theoretical model that determines the diffusion coefficient of EPs due to the background turbulence. For passing particles the EP diffusion coefficient was found to be DEP ≈ 5DiTi/EEP. However, there is considerable uncertainty about the accuracy of the ETM model (e.g. the result is sensitive to the background wavelength spectrum and there can be significant magnetic turbulence in addition to electrostatic turbulence). Hence, until the turbulence spectrum is more accurately determined, we take the uncertainty of EP diffusion to range from half to twice the diffusion coefficient predicted by the ETM model.

We have applied this enhanced theory and obtained predictions for chirping with data from the NSTX and DIII-D experiments. The results are shown in Figs.(2a, 2b). They show values of snCrt1/4 ≡ |Crt|1/4 multiplied by the sign of Crt, as a function of the ratio of phase-space averaged stochasticity and drag for Alfvénic modes. The values of snCrt1/4 for the NSTX data is displayed in Fig.(2a) when radial diffusion is neglected. Using the TRANSP code, the background plasma transport was found to be neo-classical for the experiments analyzed. If the diffusivity from plasma turbulence and the the pitch angle scattering contributions to νstoch3 are added together, the change in the value of snCrt1/4 was found to be imperceptible in the display of Fig.(2a), even though νstoch3 increases by a factor ≈ 1.3. Thus these results indicate that chirping modes are likely to arise in these NSTX experiments as is the case in the experiments that were examined.

08.jpg
The determination of snCrt1/4 for DIII-D leads to quite different conclusions. The analysis using the TRANSP code indicated that substantial turbulent diffusion is present. Fig.(2b) shows for several DIII-D experiments, the values of snCrt1/4 for several values of the spatial diffusion. The points on the tail of the curvy arrows is the result when the ion turbulent diffusion is neglected and the points on the head of the arrow are cases where the turbulent radial diffusion is included. The limits of the error bars indicate the values of snCrt1/4 respectively at half and twice the base value of the turbulent diffusivity that produces the lower and upper limits of the estimated diffusivity. The experiment shows steady modes in these DIII-D shots. By in large the theoretical curves agree with these observations, especially when the upper limit for snCrt1/4 is taken.

Figure 3: Correlation of the onset of chirping with the decrease of turbulent heat conductivity. The top graphs are the heat conduction coefficients and the bottom graphs are the excited Alfvén spectrum for the three different experimental shots.

In the DIII-D experiment chirping of Alfvén wave instability has only rarely been observed. When these rare shots were investigated and correlated to the background diffusivity, dramatic correlation was found as is seen in Fig.3 where the onset of chirping phenomena only arises with the reduction of the overall particle diffusivity, which in this case is due to a reduction in the background turbulent heat conductivity.

We summarize as follows:
1. A theoretical criterion for chirping onset of Alfvénic modes in experiment was compared with experimental data in NSTX and DIII-D.
2. Very good correlation was obtained for all the data examined only when the theory incorporated accurate phase space dependence of physical quantities, including: profiles for the mode structure; inclusion of diffusion from both pitch angle scattering and from background turbulence; proper phase space averaging over the Alfvénic mode structure.
3. NSTX data displayed a strong tendency for chirping in agreement with theoretical predictions, as background ion transport, which is low (due to neo-classical diffusion) so that classical pitch-angle scattering is the main contributor to the diffusive processes.
4. Most DIII-D shots produced steady oscillations during Alfvénic instability driven by EPs. In these shots the background turbulence appeared large enough to prevent chirping from arising.
5. For the rare DIII-D cases, where a chirping response was observed, it was found that chirping occurs when there is a pronounced reduction in the background ion-turbulence level.
6. This investigation suggests an answer to a previous puzzle for why, when Alfvénic oscillations characteristically appear in experimental data in NSTX and DIII-D, chirping Alfvénic modes usually arise in NSTX but only rarely arises in DIII-D.
7. This method of analysis needs to be applied to other experiments including ITER.

This work was supported by the São Paulo Research Foundation (FAPESP, Brazil) under grants 2012/22830-2 and 2014/03289-4, and by US Department of Energy (DOE) under contracts DE-AC02-09CH11466 and DE-FC02-04ER54698.

References
[1] M.K. Lilley, et al., Phys.Rev.Lett. 102 (2009) 195003.
[2] H.L.Berk, et al.,Phys.Rev.Lett. 76 (1996) 1256.
[3] V. N. Duarte, et. al., http://iopscience.iop.org/article/10.1088/1741-4326/aa6232, (submitted to Nuclear Fusion)
[4] K. Ghantous, et al., Phys.Plasma 19 (2012) 092511.
[5] R. M. Nyqvist and B. N. Breizman Phys. Plasmas 20 (2013) 042106.
[6] H.L.Berk, et al., Physics of Plasmas 6 (1999) 3102.
[7] W. Zhang, et al., Phys. Rev.Lett., 101 (2008) 095001.
[8] N.N. Gorelenkov, et al., Phys.Plasmas 6 (1999) 2802.

ITPA Update

More information concerning the ITPA may be found at the Official ITPA Website. Energetic Particles Topical Group

The 17th ITPA topical group meeting was held at Kyoto University, Japan from 24th to 26th October 2016 following the IAEA FEC conference. Several research topics were discussed including joint experiments: impact of ECH on AE activity; transport problem of thermal plasma and energetic particles (EP) in steady-state scenarios; assessment of Ion Cyclotron Emission (ICE) for diagnosing lost and barely confined fast ions. Several presentations on these topics were given. Among them is the report on the successful conclusion of ECH excitation of AEs in DIII-D tokamak which unambiguously showed that AEs are stabilized by applying significant power of ECH > 2.5 MW. An explanation of this was given in terms of the GAM frequency dependence on thermal electron temperature. At higher electron temperature GAM frequency goes up to TAE frequency suppressing RSAE modes and significantly altering the Alfven continuum in a way that global TAEs experience strong continuum damping.

Another important topic discussed in depth was on the potential of ICE application as a diagnostic of fusion products in burning plasma conditions. ICE is expected in experiments at low field side of the plasma if fast ions are present. Even though this technique needs more study several conclusions with regards to ICE diagnostics can be already be made. The ICE intensity is expected to correlate with the fast ion population at the edge. The spectrum of ICE should be sensitive to the details of fast ion distribution function which is currently the subject of the research. More studies of ICE theory especially in the nonlinear regimes has to be done perhaps under the guidance of the ITPA community.

MHD, Disruptions, and Control Topical Group

The ITPA topical group meeting on MHD was held from October 24-26th in Kyoto, Japan and was attended by approximately 35 participants. Many of the joint experiments and working groups recapped their progress. Several joint experiments were deemed to have met their goals and were slated for closing, specifically the MDC-1 "Disruption Mitigation by Massive Gas Jets", MDC-16 "Runaway electron generation, confinement and loss", and WG-11 "Control of Locked Modes". Urgent ITER requests were discussed by Y. Gribov and M. Lehnen. A proposal to remove the top and bottom ex-vessel error field correction coils from ITER was discussed and the need to perform predictive modeling to assess the impact of this change identified. Further required improvements to our understanding of impurity penetration into mature runaway electron beams was also highlighted. Finally, the urgent need to understand the effect of shattered pellet injection angle (grazing, or direct) was identified. Further experiments are needed to deal with the second and third item and are planned for 2017. The next meeting of the ITPA-MHD group is scheduled to be in Chengdu, China, in March 2017.


Schedule of Burning Plasma Events

USBPO Public Calendar: View Online or Subscribe

2017

March 13, ITPA MHD Topical group meeting, Southwestern Institute of Physics (SWIP), Chengdu, China
April 3-6, ITPA IOS Topical group meeting, ITER Headquarters, Cadarache, France
April 18-20 31st meeting of ITPA PEP Topical group, York, UK
April 18-21, 2nd European Conference on Plasma Diagnostics (ECPD), Bordeaux, France
April 25-28, EU-US Transport Taskforce Meeting (TTF), Williamsburg, Virginia
April 26-28, ITPA EP topical group meeting, Sevilla, Spain
May 1-3, Sherwood Fusion Theory meeting, Annapolis, MD, USA
May 1-3, ITPA TC Topical group meeting, Princeton, USA
May 8-12, ITPA DIAG topical group, Chengdu, China
May 21-25, 44th International Conference on Plasma Science (ICOPS), Atlantic City, New Jersey
May 30 - June 2, ITPA Divertor Scrape-Off (DSOL) Meeting, York, UK
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 11-13, ITPA EP topical group meeting, Princeton, USA
September 18-20, ITPA PEP topical group meeting, Helsinki, Finland
September 18-20, ITPA TC topical group meeting, Helsinki, Finland
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 9-12, ITPA IOS Topical group meeting, Lisbon, Portugal
October 23-27, 59 th Annual Meeting of the APS Division on Plasma Physics, Milwaukee, Wisconsin
December 5-7, ITPA Coordinating Committee, ITER Headquarters, Cadarache, France

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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