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.

Director's Corner

by C.M. Greenfield

It has been a relatively quiet month for news. That will change over the next few months as we get into “meeting season.” There will be six ITPA Topical Group meetings in September and October, and of course, the APS Division of Plasma Physics meeting in November. At ITER, there will be a community review of the ITER Research Plan in late September, feeding into a STAC ITER Science and Technology Advisory Committee meeting in October and an ITER Council meeting in November.

Regarding ITER, there are two important design issues on the table with expectations that they will be settled this fall. The ITER Council will decide whether to include internal “ELM coils” and tungsten divertor targets in the ITER baseline design. Both of these changes have been undergoing extensive reviews in the community (via ITPA) and through design reviews at ITER. Reports on both of these will be presented during a Town Meeting to be held during the APS-DPP conference (see below).

As many of you have heard, there are also reviews of the ITER project going on within the United States. I hope to be able to report on these in a future column. Meanwhile, construction continues at ITER as concrete pouring for the B2 slab – the actual floor of the tokamak complex - is scheduled to begin right about now.

A smaller version of the next-phase Tokamak Complex basemat slab (B2) can already be found on the ITER site: the 150 m2 slab mockup even recreates the various-size anchor plates that will be embedded to support equipment. (Photo ©ITER Organization)

Fusion in the News

Earlier this week, CBS broadcast a report called “Powering the Future: What will fuel the next thousand years?” The focus of the report was on fusion and ITER, and featured a very nice interview with former USBPO Council Chair Mike Mauel. This serves as a reminder that if we want public support for our research, it helps to tell the public about what we’re doing. You can watch the report here: http://www.cbsnews.com/8301-35040_162-57599943/powering-the-future-what-will-fuel-the-next-thousand-years/

Plans for APS-DPP conference

For the sixth straight year, the US Burning Plasma Organization has organized a contributed oral session on Research in Support of ITER at the 55th Annual Meeting of the Division of Plasma Physics, which will take place in Denver, Colorado, on November 11-15. The scheduling of this excellent session can be found here: http://meetings.aps.org/Meeting/DPP13/SessionIndex2/?SessionEventID=200907

The USBPO is also organizing a Town Meeting on ITER, scheduled for Thursday evening (November 14) in the Sheraton Denver Downtown Hotel. We will have a compelling program, focusing on two ITER design decisions to be formalized late this year.

The agenda will be as follows:
Richard Pitts (ITER Organization): Physics basis and design of the ITER full tungsten divertor

Edward Daly (Thomas Jefferson National Accelerator Facility): ITER ELM Control Coils

Ned Sauthoff (US ITER Project Office): US ITER project status

Webinars

We are planning another web seminar in the coming weeks. Watch for an annoucement.

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USBPO Topical Group Highlights

[The BPO Energetic Particles Topical Group works to facilitate U.S. efforts to understand the behavior of energetic particles via improved measurements and computational models for existing and future magnetic fusion devices (leaders are David Pace and Nikolai Gorelenkov). This month's Research Highlight by B.N. Breizman, et al., describes recent advances in the theory and simulation of energetic ion instabilities driven through wave-particle resonances. As this Highlight demonstrates the steady progress in energetic particle-related theory applicable to fusion devices, the Image of the Month demonstrates a commensurate stride forward in experimental treatments of the underlying physics. -Ed.]

Long-range Frequency Sweeping Events and Convective Transport of Energetic Particles
B.N. Breizman¹, R.M. Nyqvist², and M.K. Lilley³

¹ Institute for Fusion Studies, The University of Texas at Austin, Austin, Texas 78712, USA
² Department of Earth and Space Science, Chalmers University of Technology, SE-412 96 Göteborg, Sweden
³ Physics Department, Imperial College, London, SW7 2AZ, UK

Energetic particle driven instabilities in burning plasmas are frequently viewed as a risk factor for fusion reactor performance because the excited waves have the potential to degrade alpha particle confinement. However, the free energy source for these instabilities is notably weak. The energetic particle pressure in a tokamak is typically smaller than the magnetic pressure of the poloidal field, suggesting that the excited waves should saturate at a relatively low level. A single saturated wave will therefore only affect a tiny fraction of the energetic particle population, which indicates that many perturbations are essential for undesirable global transport. Such perturbations can create a wide-ranging tangle of wave-particle resonances in phase space, giving rise to quasilinear diffusion. A plausible alternative to such diffusive transport is a convective transport mechanism that allows particles to travel long distances in phase space by being locked into a single resonance that itself evolves in time. The convection involves non-perturbative waves, whose existence relies on the nonlinear response of the resonant particles themselves rather than just the linear response of the background plasma. The non-perturbative modes of interest are nonlinear BGK-type waves that represent hole/clump phase-space structures. Such modes can form spontaneously in dissipative plasmas, and the dissipation forces them to have time-dependent phase velocities. As the number of resonant particles is typically small in any given wave, a sequence of many convective events is required to produce a global change in the energetic particle population. Moreover, it was recently demonstrated [1] that these modes are in fact generated continuously at the location of the linear wave-particle resonance and can chirp through a broad frequency range (Fig. 1).

Figure 1: Modes with time-dependent frequencies in nonlinear simulations of the bump-on-tail instability. Fourier analysis of the wave field shows that waves are continuously produced at the resonance.	Figure 2: Snapshots of the particle distribution function during repetitive hole/clump production show plateau formation above the original resonant velocity. Dashed line indicates the marginally stable slope.

The non-perturbative convective transport scenario stands in stark contrast with the quasilinear case that precludes global relaxation in the near-threshold regime. The notion that a marginally unstable resonance can produce a dramatic change in the fast particle population reflects a fundamental difference between the perturbative (soft) and non-perturbative (hard) regimes of the near-threshold instability. The quasilinear model, which involves only perturbative modes, forces the particle distribution to become marginally stable, but it cannot push the distribution far below the linear instability threshold because such distribution would suppress all eigenmodes. In this way particle relaxation is restricted. In contrast, the continuously produced non-perturbative holes and clumps (Figs. 1 and 2), which represent the hard nonlinear regime, provide a new channel for particle relaxation when linear modes are suppressed [2]. It is therefore pertinent to assess the role of convective transport in experiments. This consideration is an interesting topic for future work. To be predictive, such work should involve particle sources and sinks as well as multiple wave-particle resonances.

Figure 3: Phase space hole corresponding to the uppermost spectral line in Fig. 1. The snapshot is taken at t × γL = 8000. Note that particles are well phase mixed in the hole.	Figure 4: Seed structure used for simulations of holes in the reduced numerical model. The separatrix is highlighted in red.

The convective transport scenario calls for accurate nonlinear description of isolated non-perturbative waves. For that reason, a reduced theoretical model [3] and an economical numerical procedure [4] were developed that describe frequency-sweeping events in the 1D electrostatic bump-on-tail set-up with fast particle sources and collisions. By ignoring the rapid and complex hole/clump formation stage, the model enables an efficient description of initially prescribed phase space structures on time scales larger than the bounce period of the particles trapped in the wave field, and it allows for significant frequency shifts (on the order of the mode frequency itself) during which the mode structure may evolve considerably. The developed framework constitutes a logical step towards predictive modeling of nonlinear mode evolution in tokamak geometry.

Recent numerical simulations of evolving phase space holes mimic three regimes of experimentally observed frequency shifts: monotonic upward sweeping; solitary hooks; and formation of steady holes. Figures 5 and 6 display examples of each pattern and the corresponding ranges of slowing down and diffusive collision factors. Related analytic theory suggests that the hook pattern is a signature of unstable steady holes. For phase space clumps, the spectral pattern is essentially indifferent to the type of fast particle collision operator. The modes shift down in frequency monotonically and decay at a rate governed by collisions.

Figure 5: Phase space hole corresponding to the uppermost spectral line in Fig. 1. The snapshot is taken at t × γL = 8000. Note that particles are well phase mixed in the hole.	Figure 6: Seed structure used for simulations of holes in the reduced numerical model. The separatrix is highlighted in red.

Our simulations also demonstrate that particle trapping in the wave field can enhance the mode and expand the trapping area in a feedback like manner. This effect has an order of unity impact on the spectral patterns. Another important aspect of long-range sweeping concerns the global distribution of fast particles. As the holes and clumps voyage through fast particle phase space they encounter varying local slope of the fast particle distribution, which affects the nonlinear amplitude and frequency of the modes. In particular, hooked frequency sweeping appears due to distribution function with a decaying slope in the presence of slowing down collisions alone. In contrast, previous simulations with a globally linear unperturbed distribution of fast particles would not exhibit Hooks without diffusive or Krook collisions (in addition to the slowing down collisions).

This work was supported by the U.S. Department of Energy Contract No. DE-FG02-04ER54742, the Swedish Research Council, and EURATOM.

References

1. M.K. Lilley, B.N. Breizman, and S.E. Sharapov, Phys. Plasmas 17, 092306 (2010)
2. M.K. Lilley and B.N. Breizman, Nucl. Fusion 52, 094002 (2012)
3. R.M. Nyqvist, M.K. Lilley, and B. N. Breizman, Nucl. Fusion 52, 094020 (2012)
4. R.M. Nyqvist and B. N. Breizman, Phys. Plasmas 20, 042106 (2013)

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ITPA Update

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Schedule of Burning Plasma Events

Click here to visit a list of previously concluded events.

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Contact and Contribution Information

This newsletter provides a monthly update on U.S. Burning Plasma Organization activities. Topical Group Highlight articles are selected by the Leader and Deputy Leader of those groups (burningplasma.org/groups.html). ITPA Reports are solicited by the Editor based on recently held meetings. Announcements, Upcoming Burning Plasma Events, and all comments may be sent to the Editor. Suggestions for the Image of the Month may be sent to the Editor. The images should be photos, as opposed to data plots, though combined graphics are welcome. The goal is to highlight U.S. fusion resources through interesting visualizations.

Become a member of the U.S. Burning Plasma Organization by signing up for a topical group:
burningplasma.org/jointopical

Editor: David Pace (pacedc@fusion.gat.com)
Assistant Editor: Amadeo Gonzales (agonzales@austin.utexas.edu)

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Image of the Month

Fast Ions Making an Impact

Confinement properties of fusion-grade plasmas are greatly influenced by fusion-produced alpha particles and fast-ions from auxiliary heating. These energetic-particles excite numerous wave-modes with which they non-linearly interact, thereby affecting stability and energy transport. The fusion-campaign at the Large Plasma Device (LAPD) at UCLA addresses this important research area. A photo of the LAPD is displayed in the top panel of the graphic. LAPD is a highly-flexible linear magnetoplasma device operating at 1 Hz repetition rate with typical plasma parameters: n ≈ 10¹² cm^-3, T_e ≈ 4.0 eV, B =1.0 - 1.8 kG, 19 m length, and 0.6 m diameter. The magnetic-field profiles and plasma parameters of the LAPD are adjustable, allowing for the study of a variety of fundamental processes relevant to fusion and space plasmas. The principal instrument for carrying out fast-ion studies on the LAPD is an ion beam source (25 kV, 10 A, bottom-right panel). The ion source uses a hot-cathode LaB₆ plasma source and a multi-aperture three-grid extractor to inject super-Alfvenic ions (v_beam ≤ 1.2 v_Alfven) from one end of the LAPD. The pitch-angle of the beam is varied from 0°-75° by changing the beam-injection angle. Interaction of the spiraling ion-beam with the LAPD magnetoplasma is diagnosed in great detail using fast-ion collectors and a variety of probes. Numerically calculated ion trajectories are compared with measurements from the fast-ion collectors. The trajectory of an 18 keV helium ion beam (pitch angle ≈ 25°, B = 1kG) and its profile (j_max ≈ 82 mA/cm², recorded in the plasma at a 12 m distance from the ion-source exit-grid) are depicted in the middle-left and bottom-left panels. Initial results include observations of beam-driven Alfven waves and resonant-interaction of waves with fast-ions. This project is jointly supported by US DOE and NSF and performed at the Basic Plasma Science Facility, UCLA.

Contributed by Shreekrishna Tripathi, Walter Gekelman, Patrick Pribyl, and the LAPD team
University of California at Los Angeles, Los Angeles, CA 90095, USA
S.K.P. Tripathi, P. Pribyl, and W. Gekelman, Rev. Sci. Instrum. 82, 093501 (2011)

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