News and Events


U.S. Burning Plasma Organization eNews
April 30, 2014 (Issue 83)

 

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
Topical Group Research Highlight
V.V. Mirnov, D.L. Brower, D.J. Den Hartog, et al.
Theoretical Support for Optical Diagnostics in Burning Plasmas
ITPA Update
Schedule of Burning Plasma Events
Contact and Contribution Information

Director’s Corner

by C.M. Greenfield

Fusion at the 1964 World’s Fair

Did you know that fusion was demonstrated 50 years ago? General Electric’s exhibit at the 1964 World’s Fair in New York included “actual demonstrations of controlled nuclear fusion”.

The discharges being demonstrated here were 6 microseconds long, produced in a theta pinch filled with deuterium gas and driven by capacitor banks. You can read more at http://www.nywf64.com/genele08.shtml.

Fusion at the Fair

The climax of a visit to “Progressland” at the New York World’s Fair was a public demonstration of “fusion — the energy source that may someday supply all the electricity we’ll ever need.”

ITER Progress

Fifty years later, we are engaged in construction of ITER, which we expect will be the first device to demonstrate a sustained, high gain, fusion reaction. Progress continues at the ITER site, with a recent announcement that 16 new building projects will begin during the remainder of 2014.

Preparations are also underway for next month’s STAC-16 (Science and Technology Advisory Committee) meeting at ITER Headquarters. The STAC meets twice a year to consider a number of technical charges from the ITER Council. At this meeting, the following four charges (condensed version) will be addressed with STAC’s findings being reported at the upcoming ITER Council meeting in June:

1. Review a report on the maturity of design of the ITER systems

2. Assess progress on the development of the ITER Research Plan

3. Assess progress on technology and physics issues related to the First ITER full-W divertor

4. Assess the ITER Organization’s plans and progress on the resolution of outstanding technical issues such as neutronics issues, in-vessel coils, and ICH design and analysis

For the US participants (myself, Rob Goldston, Earl Marmar, Juergen Rapp, and Jim Van Dam) this meeting will represent something of a return to normalcy, as four of us were prevented from attending STAC-15 (October, 2013) in person by the government shutdown. We joined that meeting through the magic of videoconferencing, and it worked surprisingly well. But there is no substitute for being there in person.

ITPA meetings

The early-year ITPA meeting season is wrapping up, with six of the seven topical groups having met already. The seventh, Diagnostics TG, will meet in Korea May 19–22. Some of these meetings have already been summarized in this and recent eNews issues. In the coming months you can expect to see and hear reports on the rest of these meetings via the eNews and our web seminar series.

Postings for ITER positions

ITER regularly posts their openings at http://www.iter.org/jobs. There are currently fourteen positions shown there, with application deadlines in May and June.

The 7th ITER International School

The 7th ITER International School will be held on August 25–29 at ITER Headquarters and Aix-en-Provence, France. This year’s focus will be highly parallel computing in modeling magnetically confined plasmas for nuclear fusion, and information can be found at http://iterschool.univ-amu.fr/. As in the past, the USBPO will provide scholarships to a select number of graduate students and post-docs. Details of the application and selection processes for this year’s travel scholarships will be sent shortly to all USBPO members by email.

Two new FESAC panels

Two new FESAC (Fusion Energy Sciences Advisory Committee) subcommittees have been formed, and once again, the USBPO will be assisting with communication to the community at large. You can find links to each at http://burningplasma.org/. Look for “Non-USBPO Activities.” The two committees are:

• The Subcommittee on Workforce Development, chaired by Hantao Ji. This group has already started its work, and they only have a short time to complete their work by June 30.

• The Strategic Planning Panel, chaired by Mark Koepke, is just getting started. Their work is due October 1, and is being done as input for the Fusion Energy Science strategic plan requested by Congress and due in January. The second panel’s website is in preparation now and will be available in the next few days.

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Topical Group Research Highlight

Research Highlights are selected by the leaders of the BPO Topical Groups on a rotating basis. The BPO Diagnostics Topical Group facilitates U.S. efforts in developing advanced measurement techniques for existing and future magnetic fusion devices (leaders are David Brower and Ted Biewer). This month’s Research Highlight by V.V. Mirnov, et al., discusses unique diagnostic considerations for several major optical diagnostics presently under development for ITER: Thomson Scattering (TS); toroidal interferometer–polarimeter (TIP) and poloidal polarimeter (PoPola). At anticipated ITER operating conditions, the effect of electron thermal motion and relativity on phase measurements by TIP and PoPola will be significant and must be accurately treated. In addition, the polarization of incoherent Thomson scattered light is modified and can be exploited to obtain temperature information.

Theoretical Support for Optical Diagnostics in Burning Plasmas


V.V. Mirnov1, D.L. Brower2, D.J. Den Hartog1, W.X. Ding2, J. Duff1, E. Parke1
1University of Wisconsin–Madison and the Center for Magnetic Self-Organization in Laboratory and Astrophysical Plasmas, Madison, Wisconsin, USA
2University of California at Los Angeles, Los Angeles, California, USA

Toroidal interferometry/polarimetry (TIP), poloidal polarimetry (PoPola), and Thomson scattering (LIDAR TS) are major optical diagnostics being designed and developed for ITER. Fundamentally, each of these diagnostics relies upon a sophisticated quantitative understanding of the electron response to laser light propagating through a burning plasma. Improvements in this understanding are being used to guide and constrain the design of these diagnostics, and, once they are operational, will be used to improve measurement accuracy. These improvements will enable proper application of these diagnostic measurements to direct real-time feedback control of ITER device operation. Interferometry and polarimetry (I/P) will be described first below, followed by Thomson scattering (TS).

Interferometry and Polarimetry

The ITER TIP system is designed for tangential plasma density measurement from both traditional interferometry and Faraday-effect polarimetry [1]. The Faraday-effect polarimetry is used to compensate fringe jumps appearing in the interferometer, not to measure magnetic field. The ITER PoPola diagnostic is based on Faraday and Cotton–Mouton effects on laser beams launched in the poloidal plane. It will provide a unique method of internal magnetic field and current profile measurement, in addition to electron density [2].

Interpretation of I/P measurements has typically been done using a cold plasma model. One source of error is finite electron temperature effects neglected in the cold plasma dispersion relation. These corrections are proportional to τ = Te/mec2 and are small at Te ∼ 1 keV, but become sizable at Te ≥ 10 keV. There are two physically different sources of thermal corrections that are comparable in magnitude but contribute with opposite signs: non-relativistic Doppler-like effects, and the relativistic electron mass dependence on velocity. The effects of finite electron temperature were addressed in the non-relativistic limit in Ref. 3. Our reevaluation of this problem demonstrated that weakly relativistic effects are equally important and cannot be ignored [4]. The relativistic effects turn out to be stronger than the non-relativistic contributions for interferometry and Faraday-effect polarimetry. They change the sign of the non-relativistic corrections for the interferometric phase and Faraday rotation angle, and reduce the magnitude of the non-relativistic thermal correction for the Cotton-Mouton effect. At Te = 25 keV, the resulting values of these quantities relative to their values in cold plasma are, respectively, -7.5%, -10% and +22.5%, while the non-relativistic model yields overestimated values, +5%, +15% and +60%, correspondingly.

Figure 1

Figure 1: Depolarization degree vs orientation and ellipticity angles ψ and χ at θ = 90°, Te = 10 keV. There is a local maximum of D at ψ ∼ 82° and χ = 0 (linear polarization), but the absolute maximum is reached at ψ = 90° and χ ∼ 9° (elliptical polarization). D is an even function of cos ψ illustrated in this figure by plotting ψ > 90°.

For formal analysis of the problem we developed an iterative technique for solving the relativistic Vlasov kinetic equation. The key element of the method is expansion in powers of Y = ωce ≪ 1 prior to integrating over azimuthal angle in the velocity space. This avoids the use of a complicated Bessel function series representation. Instead, expansion is performed by successive differentiations of simple standard trigonometric functions. The final result is in the analytic form of a double power series expansion of the dielectric tensor in Y ≪ 1 and τ ≪ 1 to any desirable order. The validity of the method has been proven computationally by comparison with the ray-tracing numerical code GENRAY. The theoretical predictions have also been confirmed by direct measurements on the JET tokamak [5]. Data collected from high-Te JET discharges demonstrated good agreement with the relativistic theory and disagreement with the cold plasma and non-relativistic models. These were the first experimental observations of relativistic effects in plasma polarimetry.

Our analytical expressions [4] were included in the initial error analysis and design projections of the ITER TIP and PoPola systems [1,2]. The precision of this lowest-order linear in τ model may be insufficient; using the same iterative technique we recently constructed a more sophisticated model [6] with τ2-order corrections to satisfy the accuracy requirements for the ITER TIP and PoPola systems. For example, for the ITER TIP system with a CO2 laser at λ = 10.6 μm, a central viewing channel optical path length of 21&nbs;m, a plasma density of 1020 m−3, and Te = 25 keV, the linear thermal correction to the interferometric phase is large (∼ 270°), and the quadratic correction is also significant (∼ 17°). All steps of expansion, angular, and velocity averaging over relativistic Maxwellian distributions were performed analytically. With the τ2-model and Te known from Thomson scattering, finite Te effects can be rapidly and accurately calculated. This capability is particularly important for fast real-time feedback corrections in ITER.

Figure 2

Figure 2: Contour lines of maximum value of the degree of depolarization Dmax(Te,θ) (maximized with respect to all possible polarization states of the incident light). Red curve is a boundary in (Te, θ) space that determines which of two maxima shown in Fig.1 provides the absolute maximum.

New effects come into play when the electron distribution function develops an anisotropy. This could be caused by a large mean electron drift velocity U||e (parallel equilibrium current), an enhanced perpendicular temperature in ECRH heated plasmas, or a large parallel temperature due to LH current drive. Motion of the electron component as a whole results in the Fizeau effect, that is, the phase velocity of electromagnetic waves depends on whether they propagate in a moving or stationary medium. This suggests a new interferometric scheme for measuring the equilibrium current density by comparing the phases of two counter-propagating laser beams [7]. In a cold non-magnetized plasma, only the perpendicular to plasma density isosurface mean electron velocity can be measured by this method because the Fizeau effect cancels for the parallel component [8,9]. In the presence of a magnetic field, the non-magnetized electron dispersion relation is modified. Applying our iterative technique to this case, we found new physical properties of the Fizeau effect. They appear in the form of birefringence of electromagnetic waves caused by the combining action of magnetic field and electron drift velocity. This may open new possibilities for diagnostics of [10].

Thomson Scattering

Incoherent Thomson scattering is routinely used for electron temperature measurement, with Te proportional to the width of the scattered spectrum [11]. The scattering process changes the polarization of the light, an effect that becomes large in high-temperature burning plasmas and is typically described by the relativistic depolarization factor q included in LIDAR analysis. This factor quantifies the reduction of scattered spectral intensity collected by a detector that has a specific polarization sensitivity. Employment of a relativistic scattering operator, Stokes vectors, and Mueller matrix formalism enables a more general approach. The superposition effect caused by a large number of randomly moving electrons renders the scattered radiation partially polarized even though the incident light is fully polarized. The loss of polarization is quantified by the degree of polarization P, or by the degree of depolarization D = 1 - P. The possibility of determining the plasma electron temperature by measuring the degree of depolarization was suggested in Ref. 12. If the dependence of the degree of depolarization on electron temperature is accurately known from theory, the accuracy of such a diagnostic could potentially exceed that of the conventional spectrum-based TS method. Thus motivated, we revisited this topic to analyze whether polarization effects may be suitable for application to advanced TS diagnostics on ITER. In our analysis, we follow Ref. 13, with some important corrections and improvements. In particular, the finite transit time effect [14] is properly incorporated into the scattering operator. Another important improvement is optimal choice of the reference frame for averaging over velocity space. This allows derivation, for the first time, of an exact relativistic analytical expression for the degree of depolarization without any approximations for the full range of incident polarizations, scattering angles, and electron thermal motion from non-relativistic to ultra-relativistic [15].

Figure 3

Figure 3: Contour lines of minimum value of the degree of depolarization Dmin(Te,θ) (minimized with respect to all possible polarization states of the incident laser light).

The degree of depolarization D depends on Te, scattering angle θ, and polarization characteristics of the incident light ψ and χ, where 0 ≤ ψ ≤ ≺π/2 is the azimuth (orientation angle) of the polarization ellipse and 0 ≤ χ ≤ ≺π/4 is the ellipticity angle (arctan χ = b/a). One particular example illustrating a maxima of D as a function of ψ and χ is shown in Fig.1 for Te = 10keV and θ = 90°. At any given θ and Te, extrema of D as a function of ψ and χ are reached not inside the area of definition of ψ and χ, but on its boundaries. This allows us to find absolute maximum Dmax(Te,θ), and minimum Dmin(Te,θ), with respect to all possible polarization states of the incident radiation, and to set upper and lower limits on D at given θ and Te. The structure of these two functions and a quantitative picture of their dependences on Te and θ are shown in Figs. 2 and 3. The planned ITER LIDAR TS system detects backscattered radiation at θ ≈ 180°. For such backscattered light, the degree of depolarization D is quadratic in τ ≪ 1 and, therefore, small (∼3–5%) at the temperatures expected in ITER. It is insensitive to ψ and reaches its maximum for circularly polarized incident light. This is illustrated in Fig. 4 by three dashed lines for linear, elliptical and circular incident polarizations. For a conventional Thomson scattering geometry with scattering angle θ ≈ 130°, the degree of depolarization is about five times larger (∼15–25%). It becomes sensitive to the orientation angle so that the absolute maximum of D is reached at ψ = 90° for elliptically polarized incident light. However, the difference between absolute maximum and depolarization of circularly polarized incident light is insignificant (see the two solid lines in Fig. 4). The feasibility of a polarization-based Thomson scattering diagnostic for ITER-like plasmas is further discussed in Ref. 16.

Figure 4

Figure 4: Depolarization degree vs Te for backscattered radiation measured by ITER LIDAR TS system (dashed) and for conventional TS system (solid lines: circular polarization (blue) and absolute maximum of D with elliptical polarization (red)).

This work was supported by the U.S. Department of Energy, the U.S. National Science Foundation, and the U.S. ITER Project Office. We would particularly like to acknowledge useful discussions with staff members of the MST experiment and members of the University of Wisconsin Center for Plasma Theory and Computation.

References

[1] M.A. Van Zeeland, R.L. Boivin, D.L. Brower, T.N. Carlstrom, J.A. Chavez, W.X. Ding, R. Feder, D. Johnson, L. Lin, R.C. ONeill and C. Watts, Rev. Sci. Instrum 84, 043501 (2013)

[2] R. Imazawa, Y. Kawano, and Y. Kusama, Nucl. Fusion 51, 113022 (2011)

[3] S.E. Segre and V. Zanza, Phys. Plasmas 9, 2919 (2002)

[4] V.V. Mirnov, W.X. Ding, D.L. Brower, M.A. Van Zeeland, and T.N. Carlstrom, Phys. Plasmas 14, 102105 (2007)

[5] O.P. Ford, J. Svensson, A. Boboc, D.C. McDonald, and JET EFDA contributors, Plasma Phys. Control. Fusion 51, 065004 (2009)

[6] V.V. Mirnov, D.L. Brower, D.J. Den Hartog, W.X. Ding, J. Du↵, and E. Parke, Nucl. Fusion 53, 113005 (2013)

[7] D.L. Brower, W.X. Ding, B.H. Deng, M.A. Mahdavi, V.V. Mirnov, and S.C. Prager, Rev. Sci. Instrum. 75, 3399 (2004)

[8] I. Lerche, Am. J. of Physics 43(10), 910 (1975)

[9] V.V. Mirnov, D.L. Brower, and W.X. Ding, Inter. Sherwood Fusion Energy Conf., Boulder, CO (2008) http://sherwood.colorado.edu/Uploads/MIRNOV onpossib3101 10.pdf

[10] V.V. Mirnov, D.L. Brower, D.J. Den Hartog, W.X. Ding, J. Duff, and E. Parke, submitted to AIP Proceeding of the Fusion Reactor Diagnostics Conference, Varenna, Italy (2013)

[11] J. Sheeld, D.H. Froula, S.H. Glenzer, and N.C. Luhmann, Plasma Scattering of Electromagnetic Radiation, 2nd ed. (Academic Press, 2011)

[12] F. Orsitto and N. Tartonni, Rev. Sci. Instrum. 70, 798 (1999)

[13] S.E. Segre and V. Zanza, Phys. Plasmas 7, 2677 (2000)

[14] R.E. Pechacek and A.W. Trivelpiece, Phys. Fluids 10, 1688 (1967)

[15] V.V. Mirnov, D.L. Brower, D.J. Den Hartog, W.X. Ding, J. Duff, and E. Parke, Proc. 24th Int. Conf. on Fusion Energy 2012, ITR/P5-32 (San Diego, CA, 2012) (Vienna: IAEA) http://www-naweb.iaea.org/napc/physics/FEC/FEC2012/papers/300 ITRP532.pdf

[16] E. Parke, V.V. Mirnov, and D.J. Den Hartog, Proc. of the 16th International Symposium on Laser-Aided Plasma Diagnostics (Madison, WI, USA, 2013), JINST 9 C02030 (2014) http://stacks.iop.org/1748-0221/9/i=02/a=C02030

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ITPA Update

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

Diagnostics Topical Group
  26th Meeting, Pohang Univ. of Sci. and Tech., Republic of Korea, May 19–22, 2014
 
Energetic Particles Topical Group
  12th Meeting, Madrid, Spain, March 31–April 3, 2014
  A preview of the summary written by D. Pace with contributions from P. Bonoli, D. Brower, B. Harvey, and D. Spong is provided here, while the agenda and the full summary are available on the BPO Forum (EP).
The meeting was hosted at CIEMAT in Madrid, Spain over March 31 through April 3, 2014. ITPA discussions were separated into the following categories: ITER physics priorities and diagnostics, updates on the four open Joint Experiments, ion cyclotron physics, and theory and modeling validation exercises. Interspersed within this agenda were assorted presentations of ITER needs, new experimental/simulation results from existing facilities, other diagnostic issues, and work from the TJ-II stellarator program housed at CIEMAT. This summary does not detail every single presentation from the meeting, rather, it discusses highlights and provides personal commentary concerning the US role in the area of energetic particles physics.
 
Integrated Operation Scenarios Topical Group
  12th Meeting, Massachusetts Institute of Technology, Cambridge, MA, United States, March 31–April 3, 2014
  A preview of the summary written by S. Gerhardt with help from T. Luce, C. Kessel, and C. Holcomb is provided here, while the full summary is available on the BPO Forum (IS).
The Spring ITPA Integrated Operating Scenarios (IOS) meeting was held in Boston Massachusetts from March 31st to April 3rd. This meeting focused on the development of integrated discharge scenarios for ITER operations, including the non-activated phase in H or He, the Q=10 baseline scenarios, and potential steady-state scenarios at reduced fusion gain. In addition, simulations of discharges on JET, DIII-D, and JT-60U were presented.
 
MHD, Disruptions, and Control Topical Group
  23rd Meeting, Toki, Japan, March 10–14, 2014
 
Pedestal and Edge Physics Topical Group
  26th Meeting, IPP, Prague, Czech Republic, April 15–17, 2014
 
Transport and Confinement Topical Group
  12th Meeting, Massachusetts Institute of Technology, Cambridge, MA, United States, April 9–11, 2014

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Schedule of Burning Plasma Events

2014
NSTX-U First Plasma —
 
May 19–22, ITPA: 26th Meeting of the Diagnostics Topical Group
  Pohang University of Science and Technology, Republic of Korea
 
June 1–5, 20th Topical Conference on High-Temperature Plasma Diagnostics (HTPD), Atlanta, GA, United States
 
June 16, Students participating in the National Undergraduate Fellowship Program (NUF) arrive at their host institutions.
 
June 23–27, 41st EPS Conference on Plasma Physics, Berlin, Germany
 
October 13–18, 25th IAEA Fusion Energy Conference (FEC 2014), St. Petersburg, Russia
 
October 27–31, 56th APS Division of Plasma Physics Conference, New Orleans, LA
 
2015
W7-X First Plasma —
 
January, Due date for report concerning the ten-year strategic plan of the Fusion Energy Sciences division of the US Department of Energy.
 
2016
— 10th Anniversary of USBPO Formation —
 
2017
JET DT-campaign —
 
2019
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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