News and Events

U.S. Burning Plasma Organization eNews
Dec 31, 2015 (Issue 103)

 

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
Delgado-Aparicio et al.
ITPA Update  
Schedule of Burning Plasma Events  
Contact and Contribution Information

Announcements

USBPO web seminar

We are pleased to announce our first web seminar of 2016:

Speaker: Dr. Francesca Poli, Princeton Plasma Physics Laboratory

Topic: Integrated modeling in support of ITER: the path from the commissioning phase to demonstration scenarios; issues and progress.

We will use Zoom for audio, video and slides (http://zoom.us); connection details will be sent in an email reminder in early January.

Please share this announcement with colleagues and students. We hope you can participate, and would like to take this opportunity to wish you a very happy holiday season.

Amanda Hubbard
USBPO Deputy Director

Director’s Corner

C.M. Greenfield

Holiday wishes

I would like to wish all of our readers a wonderful holiday and a happy and productive 2016.

ITER: Significant progress in 2015

There were a lot of changes at ITER in 2015. As construction progress continued and accelerated, new Director General Bernard Bigot took charge, who with his team has been working to put the project on a realistic schedule. The ITER Council reviewed this schedule at its November meeting and referred it to an independent review, due in time for its next meeting in June, 2016. Last year at this time I told you about progress on construction. At that time, the B2 slab supporting the tokamak complex had been completed, and walls starting to rise around the tokamak pit. Since then, the tokamak complex has continued to rise, with the B2 slab long since rendered invisible by walls and additional tokamak support structure. The Tokamak Assembly Hall frame was completed, with sheet metal siding now being applied.

Photo of the construction scene

The ITER site in November, 2015. The newly erected Tokamak Assembly Hall is at the left, and the tokamak pit is at the center (Photo © ITER Organization).

Progress isn’t limited only to development of the site. Recently, the first components of the tokamak arrived in the form of 12 segments of the cryostat, supplied by India. Also, during my recent visit for the annual ITPA Coordinating Committee meeting, we were given a site tour including the inside of the Poloidal Field Coil Winding building. Last year, this building was being used to store crates of electrical components, mostly from the US Domestic Agency. But now, the building is being outfitted to begin its assigned task of winding magnets.

The Poloidal Field Coil Winding Building: No longer just a warehouse!

ITER International School

The 2015 ITER International School was held earlier this month in Hefei, China. Seven post-docs and graduate students attended via scholarships provided by the USBPO (see photo).

The theme of this year’s school (http://www.iterschool2015.cn/iis/Sitehome.aspx) was “Transport and Pedestal Physics in Tokamaks.” These schools are primarily designed for graduate students, postdocs, and young researchers. The location rotates to different ITER partners, with the topic varying from year-to-year, so stay tuned for information on the next school.

USBPO scholarship winners at the 2015 ITER International School. From left: Tess Bernard (University of Texas), Mike Ross (University of Washington), Drew Elliott (West Virginia University), Matt Beidler (University of Wisconsin), Chris Everson (University of Washington), Jonathan Coburn (North Carolina State), Tim Younkin (University of Tennessee). Photo courtesy of Phil Snyder.]

There were also six US-based experts among the lecturers Phil Snyder (General Atomics), Gary Staebler (General Atomics), Rajesh Maingi (PPPL), Raffi Nazikian (PPPL), C.S. Chang (PPPL), and Joshua Burby (NYU).

Research Highlight

Diagnostics Topical Group, Leaders: Ted Biewer and Brent Stratton


Multi-energy x-ray cameras for magnetically confined fusion plasmas


L. Delgado-Aparicio 1, J. Maddox 1,2, N. Pablant 1, K. Hill 1, K. Tritz 3, D. Stutman 3, M. Bitter 1, J. E. Rice 4, A. Hubbard 4, M. Greenwald 4, E. Marmar 4, J. Irby 4, P. Efthimion 1, B. Stratton 1
1 PPPL, Princeton, NJ, 08540, USA 2 SULI fellow 3 The Johns Hopkins University, Baltimore, MD, 21218, USA 4 MIT - PSFC, Cambridge, MA, 02139, USA

Thanks to important advances in the x-ray detector technology, especially, the manufacturing of twodimensional hybrid pixel array x-ray detectors of large areas and high count rate capabilities, it is now possible to record spatially resolved x-ray photons at multiple energy ranges from highly charged ions from tokamak plasmas [1]-[4]. Multi-energy x-ray imaging of magnetically confined fusion plasmas provides a unique opportunity for measuring, simultaneously, a variety of important plasma properties. The energy resolved measurements can be used to produce images of impurity concentrations (nZ & Zeff) - from the absolute image intensity at different energy bands - and the electron energy distribution function, both thermal (Te) and non-Maxwellian (ne,nM) from the variation of emissivity with x-ray energy. This novel capability offers also a unique opportunity to monitor high-Z impurities, calculate impurity transport coefficients and distinguish between contributions from medium-Z (e.g. Ar) to high-Z (e.g. Mo, W) impurities in reactor configurations with metal plasma facing components (PFCs).

In magnetically confined plasmas, acceleration of the runaway electrons can be limited by synchrotron losses that accompany pitch-angle scattering. The significance of this mechanism was first shown in Ref. [7] and then emphasized in Refs. [8,9].

Figure 1: a) Detector setup with the PILATUS2 detector shown in b). Inset (c) shows a vertical arrangement of energy thresholds which repeats every 13172m and is optimum for a 2D radial view. A horizontal arrangement of the thresholds useful for a 1D tangential con guration is shown in d).

A first proof-of-principle diagnostic system was deployed for testing at the Alcator C-Mod tokamak at MIT in 2012 with good results [1]. In a much recent installation a multi-energy camera was operated in a radial/poloidal configuration (see Fig. 1) and used in various regimes which include L→H transitions, impurity injections as well as radio frequency heating and current drive experiments. At the heart of the new proposed system is a Pilatus2 [5] x-ray detector depicted in Fig. 1-b) which has been the detector of choice for various designs of x-ray crystal imaging spectrometers. The configuration of pixilated detectors used is shown in Fig. 1-c). Since the x-ray emissivity is uniform along the toroidal magnetic field, the pixels in adjacent rows sample nearly the same USBPO Newsletter, December 31, 2015, Issue 103, BurningPlasma.org Page 5 of 10 plasma volume. Here, an entire row of pixels would be effectively used for each energy value; it is therefore possible to obtain coarse spectral resolution by setting the pixels in each column to varying energy thresholds, E1, E2, ... E13, etc. (from 4 to 16 keV), where a larger number of pixels can be set to the higher energy threshold to compensate for the exponential decrease of the photon intensity with energy. The diagnostic envisioned for C-Mod had a spatial and temporal resolution of 1 cm and 5-10 ms, respectively. Preselecting a detector response between 6 and 15 keV help eliminating the ‘contamination’ to the continuum introduced by the low- and high-energy line-emission from Argon and Molybdenum impurities (see spectra in Fig. 2) and facilitating the electron temperature measurements in Ohmic and ICRH heated plasmas. The x-ray signals with photon energies below 6 and above 15 keV can be used to calculate impurity concentrations.

Figure 2: SXR emissivity for Ar and Mo. The 13 detector response curves (dotted-lines) `bracket' the con- tinuum eliminating the plasma line-emission.

Previous attempts to develop this SXR capability have lack temporal-, spatial- or energy-resolution. Single-chord pulse-height-analyzers (PHA, see refs. [6]-[9]) are naturally restricted since they are line integrated measurements with limited spatial localization (e.g. one spectrum per instrument and with very poor profile definition). A better spatial coverage at the expense of lack of energy resolution can be gained using multiple 1D pin-hole x-ray detector arrays filtered using individual metallic foils [10]-[19]. This capability have shown remarkable flexibility and could be used for fast electron temperature measurements, impurity transport and macroscopic magnetohydrodynamic (RWM and NTM) studies. The novel diagnostic system installed in Alcator C-Mod at MIT combines the best features from both PHA and multi-foil methods, and represents a very large improvement in throughput and spatial resolution thanks to present state-of-the-art pixelated PILATUS2 detectors with a minimum of 100k pixels.

Figure 3: Multi-energy brightness pro les during L-modes and ICRH-heated H-modes.

Details of the line-integrated profiles across C-Mod cross section in thirteen different energy ranges for L- and H-modes is shown in Fig. 3. This detector has demonstrated an unprecedented flexibility in the configuration of an imaging x-ray detection system having a dynamic range spanning nearly 5 orders of magnitude. The strongest signals obtained with a detector setup with a 4 keV minimum energy threshold were of the order of 5 - 6 X 105 counts/sec/pixel, far from the its maximum count-rate of 2 X 106 counts/sec/pixel. The time-history of the inferred core electron temperatures using the multienergy line-integrated brightness as a proxy of the local emissivity is shown in Fig. 4 and is in very good agreement with the temperatures measured by the electron cyclotron emission (ECE) diagnostic [3, 4]. The use of tomographic codes for obtaining the 2D multi-energy SXR emissivities will facilitate the computation of Te and n2eZeff profiles from the.

Figure 4: ECE and SXR-inferred core electron temperature during Ohmic and ICRH-heated plasmas.

Further improvements in 2016 will include the use of new Pilatus3 systems which have maximum photon count rates of 2  107 counts/sec/pixel and the use lower minimum energy thresholds (e.g. 1.6 keV) sampling the line-emission contribution of molybdenum or tungsten. The potential for extracting valuable information from this compact x-ray system should be fully explored as a possible burning plasma diagnostic. Traditional techniques to infer basic plasma quantities based on the slope of the continuum radiation can still be employed since the tungsten continuum extends from 15 to 55 keV. The use of thicker (1 mm) silicon pixel array detectors will allow also the study of non-Maxwellian effects by probing higher x-ray energies up to ∼ 30−40 keV. An appropriate alternative for even higher energies is CdTe, which will allow photon detection up to  100 keV with nearly 70% efficiency. The latter is of interest of our community especially when radio frequency waves heat plasmas introducing non-Maxwellian “tails” in the distribution function, which are prone to generate harder x-rays. faster detectors have recently become commercially available and could provide the future framework needed for rapid estimates of impurity accumulation and radiated power for disruption avoidance.

In summary, due to its intrinsic energy-resolution, the multi-energy SXR technique can be used to provide simultaneous measurements of Te, nZ, Zeff and ne,nM in high-temperature plasmas; intense lineemission from high-Z impurities from metal PFCs can be ‘filtered’-out from the continuum using appropriate energy thresholds. This technique should be explored also as a burning plasma diagnostic in-view of its simplicity and robustness.

ITPA Update

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

Energetic Particles Topical Group

This meeting was held in Vienna September 7-9, 2015 immediately following the IAEA technical committee meeting on alphas particles in fusion research. The meeting was opened by Dr. Simon Pinches who reviewed recent developments at ITER and R&D modeling needs relevant to the energetic particle community. One particular problem he mentioned was the flexibility in the toroidal mode spectrum and rotation of the field by ELM/RWM coils is in need to reduce the potential heat fluxed to the level of < 10 MW=m2. Another specific need he described for energetic particle physics community is the transport due to the Alfvenic modes and the need to control it. The joint EP experiments and code benchmarking activities were discussed as well which include linear AE instability analysis for the ITER 15 MA beseline and half field/half current cases, nonlinear evolution studies, impact of both ECH and ICH on the stability of AEs. Important for future ITER operations were the discussions of the ICE (Ion Cyclotron Emission) physics to be used as a diagnostic tool for the burning plasmas. The location and the dates for the next ITPA-EP meeting were given as the spring of 2016 in the ITER site in Cadarach.

MHD, Disruptions, and Control Topical Group

The 26th Meeting of the ITPA Topical Group on MHD, Disruptions and Control was held October 19-22 in Naples, Italy. The agenda included sessions on ITER high priority needs (with a talk given by Yuri Gribov), contributed talks on a wide range of topics including experimental results on disruption mitigation (massive gas injection (MGI), shattered pellet injection (SPI), comparison of injection in normal plasma vs. ones with locked modes present), runaway electrons, MHD mode/error field control with spare coil configurations, and disruption simulations with non-linear MHD codes. Aspects of disruptions (their prediction, avoidance, and mitigation) remain among the most important and urgent unresolved issues for ITER, with special attention on the practical aspects of mitigation, as the final design review for this system on ITER will occur in 2017. Reports updating the status of MGI and SPI were given under joint experiment MDC-1 and radiation asymmetry during MGI in a summary of working group WG-08. A topical presentation on the comparison of MGI effectiveness in normal plasmas vs. plasmas with locked modes (by R. Granetz) made a favorable conclusion that based on global mitigation parameters and measured toroidal peaking of radiated power in C-Mod that MGI performance did not degrade in plasmas with locked modes. The ITER requested assessment of the levels of large scale plasma disturbances tolerated before plasma disruptions occur were assessed in NSTX plasmas disrupted by global mode destabilization and reported under joint experiment MDC-21 (S.A. Sabbagh). The magnitude of toroidal mode number n = 1 magnetic field perturbation,δB/B, reached prior to disruption was shown to increase strongly with plasma current. A further striking conclusion was that when expressed as scalings depending on combinations of Ip, li, a, and q95 as used for more localized modes (i.e. tearing modes), the maximum δB/B of global modes scaled with similar dependences to locked tearing modes with respect to these parameters. Also, in contrast, the maximum δB/B did not depend on parameters normally associated with the marginal stability point of the global modes (e.g. li, or pressure peaking). Joint experiment MDC-22 (G. Pautasso) on disruption predi2ction reported results of work on an extended disruption database in C-Mod including data at more time points as would be needed for work on disruption warning evaluation, a study of locking and disruptivity of initially rotating 2/1 tearing modes, and initial results from a new Disruption Event Characterization And Forecasting code (DECAF) utilizing NSTX data.

References

[1] N. Pablant, et al., Rev. Scientific Instrum., 83, 10E526, (2012).

[2] L. Delgado-Aparicio, et al., PPPL-report, 4977, (2014).

[3] J. Maddox, et al., proc. of the 57th APS-DPP, November, Savannah, GA, (2015).

[4] J. Maddox, et al., to be submitted to Nucl. Fusion, (2016).

[5] See https://www.dectris.com

[6] E. H. Silver, et al., Rev. Sci. Instrum., 53, 1198, (1982).

[7] K. W. Hill, et al., Rev. Sci. Instrum., 56, 840, (1985).

[8] K. W. Hill, et al., Nucl. Fusion, 26, 1131, (1986).

[9] J. E. Rice, et al., Phys. Rev. A, 25, 1645, (1982).

[10] J. Kiraly, et al., Rev. Sci. Instrum., 56, 827, (1985).

[11] L. Delgado-Aparicio, et al., Plasma Phys. Control. Fusion, 49, 1245, (2007).

[12] L. Delgado-Aparicio, et al., Journal of Applied Physics, 102, 073304, (2007).

[13] L. Delgado-Aparicio, et al., Rev. Sci. Instrum., 81, 10E303, (2010).

[14] K. Tritz, et al., Rev. Sci. Instrum., 83, 10E109, (2012).

[15] D. J. Clayton, et al, Plasma Phys. Control. Fusion, 55, 095015, (2013).

[16] L. Delgado-Aparicio, et al., Nucl. Fusion, 49, 085028, (2009).

[17] D. J. Clayton, et al., Plasma Phys. Control. Fusion, 54, 105022, (2012).

[18] L. Delgado-Aparicio, et al., Nucl. Fusion, 51, 083047, (2011).

[19] L. Delgado-Aparicio, et al., Plasma Phys. Control. Fusion, 53, 035005, (2011).

This work was supported by ITER under Contract No. ITER-CT-12-4300000273, and by the U.S. Department of Energy Contract No. DEFG02-04ER54742. The views and opinions expressed herein do not necessarily reflect those of the ITER Organization.

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Schedule of Burning Plasma Events

USBPO Public Calendar: View Online or Subscribe

2015         NSTX-U First Plasma — December 10 W7-X First Plasma —

2016         — 10th Anniversary of USBPO Formation —

January 13-14FESAC MeetingWashington DC area, USA
January 25-2822nd ITPA DivSol, ENEA Research CenterFrascati, Italy
March 16-18ITPA T&C, Institute for Plasma ResearchGandhinagar, India
March 16-18ITPA PEP, Institute for Plasma ResearchGandhinagar, India
May 30-June 3PSI ConferenceRome, Italy
June 19-23International Conference on Plasma Sciences, ICOPSBanff, Alberta, Canada
June 27-July 118th International Conference on Plasma Physics (ICPP2018)Kaohsiung, Taiwan
July 4-8European Physical Society Conference on Plasma Physics (EPS)Leuven, Belgium
October 13-15ITER STAC MeetingKizu, Japan
October 17-2226th IAEA Fusion Energy ConferenceKyoto, Japan
October 24-26ITPA T&C, JAEANaka, Japan
October 31-November 458th APS Division of Plasma PhysicsSan Jose, California, 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: David Pace (pacedc@fusion.gat.com)

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