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 C.M. Greenfield Research Highlight S. Wukitch Schedule of Burning Plasma Events Contact and Contribution Information
As you know, the annual US Burning Plasma Council election concluded recently. Thanks to your nominations and an excellent job by our nomination committee (Dan Thomas [chair], David Maurer, Susana Reyes, Francesca Poli, and Jim Terry), we had a slate of very strong candidates to choose from. It is now my pleasure to announce that the winners of the election were David Newman (University of Alaska) and Ted Biewer (ORNL). In accordance with the Bylaws of the USBPO, two additional members will be appointed to fill out the full Council membership. These appointments have not yet been finalized and will be announced soon.
I want to thank the outgoing members of the Council (Larry Baylor, David Maurer, and Dan Thomas) and especially Mark Koepke, who is completing his term both as a Council member and its Chair, for their valuable service to the community, and hope they will continue to be active as we move forward.
I also thank Mark London, our communications coordinator, and all who voted. We had an astounding 203 voters this year... the previous record was 168, set last year.
Chuck Kessel has agreed to take over as the new Council Chair. The new vice chair will be named when the membership is finalized.
|USBPO Council 2015-16||USBPO Council 2016-17|
|Larry Baylor||Ted Biewer (ORNL)|
|Stan Kaye (PPPL)||David Brower (UCLA)|
|Chuck Kessel (Vice Chair, PPPL)||Chuck Kessel (Chair, PPPL)|
|Mark Koekpe(Chair, West Virginia)||Earl Marmar (MIT)|
|Earl Marmar (MIT)||David Newman (U Alaska)|
|David Maurer (Auburn)||David Pace (GA)|
|David Pace (GA)||Juergen Rapp (ORNL)|
|Juergen Rapp (ORNL)||Susana Reyes (LLNL)|
|Susana Reyes (LLNL)||Jim Terry (MIT)|
|Jim Terry (MIT)||François Waelbroeck (U Texas)|
|Dan Thomas (GA)||TBD|
You may notice that we will once again have thirteen Council members. This reflects a change in our bylaws to address a slight discrepancy. The bylaws state that there should be twelve members, each member has a three-year term, and the chair and vice chair have two-year terms with their Council membership terms extended if needed. But we realized that this would push us away from our current practice of replacing four Council members each year, so some years we might only replace three and some years five. To fix this, the bylaws have been amended to allow the Council to grow above twelve members to accommodate an extended term of the chair and/or vice chair.
At this time, the leaders of five of our Topical Groups are nearing the end of their terms. We are in the process now of selecting new leaders and deputy leaders for each of these groups. In some cases, the outgoing deputy leaders will become the new leaders, but this is not automatic. Either way, we are at a minimum seeking candidates for deputy leaders for each of these five groups. If you would like to volunteer yourself or a friend, please contact the outgoing leader (or Amanda Hubbard or me).
You should hear more about this in the coming weeks.
|Topical Group||Outgoing Leader||Deputy Leader|
|Confinement and Transport||Gary Staebler (GA)||Saskia Mordijck (W&M)|
|Diagnostics||Ted Biewer (ORNL)||Brent Stratton (PPPL)|
|Integrated Scenarios||Chris Holcomb (LLNL)||Francesca Poli (PPPL)|
|MHD and Macroscopic Plasma Physics||Bob Granetz (MIT)||Steve Sabbagh (Columbia)|
|Modeling and Simulation||Xianzhu Tang (LLNL)||Lang Lao (GA)|
Virtual Laboratory for Technology
We are pleased to note the recent activities of the Virtual Laboratory for Technology (VLT) under newDirector, Phil Ferguson. USBPO Deputy Director Amanda Hubbard attended their kick-off meeting in June, and we look forward to increased communication and coordination between VLT and USBPO, and more generally between the fusion physics and technology communities
Activities at the APS-DPP Conference
The USBPO is organizing two events during the upcoming annual meeting of the APS Division of Plasma Physics, in San Jose October 31-November 4.
The USBPO will host a town meeting on ITER on Thursday evening, November 3, at 7:30PM. The featured speaker will be David Campbell, Director of the ITER Science and Operations Department. Dr. Campbell will speak about Progress in ITER Construction and Strategy Towards the Operations Phase.
The ninth annual Research in Support of ITER contributed oral session will be held. This years speakers are as follows (not in order):
|T. Abrams||ORAU||Characterizing the intra-ELM tungsten erosion profile in the DIII-D divertor in different ELM regimes|
|Larry Baylor||ORNL||ELM Mitigation in Low-rotation ITER Baselin Scenario Plasmas on DIII-D with High Frequency Deuterium Pellet Injection|
|D. Brennan||Princeton U||Progress and challenges in predictive modeling of runaway electron generation in ITER|
|M. Brookman||U of Texas||Experimental Evidence of ECH Deposition Broadening on DIII-D|
|W. Choi||Columbia||Feed-back control of 2/1 locked mode phase: experiment on DIII-D and modeling for ITER|
|L. Cui||PPPL||Transport modeling of DIII-D RMP ELM controlled plasmas|
|A. Hassanein||Purdue||Secondary radiation damage effects during transient events on ITER divertor and nearby|
|E. Kolemen||Princeton U||Transient-Free Operations With Physics-Based Real-time Analysis and Control|
|J. McGlenaghan||ORAU||Extrapolations of the high betaP scenario to ITER using TGYRO modeling|
|y.-K. Oh||NFRI||Highlights of the KSTAR Research relevant to ITER|
|F. Poli||PPPL||EC power management for NTM control in ITER|
|P. Snyder||GA||Prediction and OPtimization of the ITER Pedestal|
|W. Suttrop||IPP Garching||AUG/DIII-D RMP ELM Suppression Similarity Experiment|
|A. Tinguely||MIT||Analysis of Runaway Electron Synchroton Emission in Alcator C-Mod|
|J. Wright||MIT||Application of the three ion species ICRF scenario to ITER operations|
Plasma-wave Interactions Topical Group, Leaders: Robert Pinsker and Greg Wallace
Heating of magnetically-confined plasmas with waves in the Ion Cyclotron Range of Frequencies (ICRF) is a very successful technology and is used on many devices around the world, including ITER. However, significant challenges that are associated with the necessity of having the wave-launching structure (antenna) close to the plasma surface remain and are the topic of much ongoing work. At MIT, the Alcator C-Mod group has taken a unique approach to addressing ICRF-specific problems and has made significant progress, as discussed in this month’s Research Highlight.
Taming the ICRF Antenna –Plasma Edge Interaction: ICRF Actuator Development at Alcator C-Mod
1 1 MIT Plasma Science and Fusion Center, Cambridge, Massachusetts, 02139, USA
Figure 1. Unrolled view of the C-Mod outer wall shows the field aligned (FA) and toroidally aligned (TA) antennas. The FA antenna is rotated such that the antenna straps are perpendicular to the total magnetic field.
For future fusion reactors, radio frequency (RF) techniques will
be necessary to sustain the plasma current and heat the plasma
to thermonuclear conditions. Experience with RF actuators in
present tokamaks indicates that high power density RF can be
difficult to couple through the boundary plasma. If not properly
designed, RF actuators can enhance the sputtering of first-wall
surfaces, leading to contamination of the hot core plasma. Additionally,
the RF power can interact with the plasma edge and exacerbate
already harsh conditions leading to enhanced heat loads,
erosion, and sputtering. In addition to these challenges, future
fusion reactors will present a still more severe environment due
to radiation and nuclear heating on RF heating and current drive
Ion cyclotron range of frequency (ICRF) heating and current drive physics has been experimentally established and scales favorably to reactor plasmas. However, maintaining high coupled power through plasma variations including edge localized modes (ELMs) and confinement transitions is extremely challenging. Additionally ICRF interaction with the edge plasma, particularly impurity contamination and enhanced localized heat loads, often limits plasma performance. In a reactor, the antennas will require high power density to minimize the antenna surface area and further complications arise from the restricted choice of materials viable in a reactor environment. Here, we report recent highlights in developing an ICRF actuator that scales more favorably towards reactors and characterizing the antenna plasma interaction.
In C-Mod, a field aligned (FA) ICRF antenna has been developed . The distinguishing feature of a FA antenna is that the current straps and antenna structure are perpendicular to the total magnetic field rather than to the toroidal direction as in a conventional toroidally aligned (TA) ICRF antenna. Alignment to the total magnetic field allows integrated Ek (electric field along a magnetic field line) to be minimized through strap symmetry.
Figure 2. For an ELM-ing discharge, the eld aligned and toroidally aligned antenna reflection coecient amplitude and phase are shown in a polar plot, illustrating that the FA antenna has lower reflection coecient variation.
A FA antenna has several features that make it attractive for a long pulse, high coupled power mission. Perhaps foremost is resilient antenna coupling over a wide variation of plasma conditions including confinement transitions and ELMs. The FA antenna reflection coefficient has significantly reduced variation in both magnitude and phase over a range of plasma conditions, as shown in Figure 2. We speculate the reduced variation in reflection coefficient for the FA antenna is a result of reduced mutual coupling of neighboring straps via slow waves . In addition to resilient coupling, the FA antenna has low RF enhanced impurity source at the antenna. The impurity source at the FA and TA antennas is monitored spectroscopically. In Figure 3, the RF power is increased in three steps: 1 MW (0.6-0.725 s) to 1.75 MW (0.725-0.875 s) to 2.5 MW (0.875-1 s). The blue trace in the first panel in Figure 3 is the molybdenum I signal (386.4 nm) from a view of the TA antenna with the TA antenna powered and the red trace is molybdenum I signal of the FA antenna when the FA is powered. These signals are referenced to the molybdenum I signal when the antenna in the view is off for a similar discharge (same injected power, B-field, density, and plasma current), shown in the black trace. The second panel shows the RF power waveform. For the TA antenna view, the molybdenum I signal responds strongly when the TA antenna is powered (blue trace) and is proportional to the injected power as expected. The molybdenum I signal approximately doubles for each 0.75 MW step. The impurity sources measured at the antenna are nearly eliminated for the FA antenna compared to the TA antenna. The design implication is that the antenna can be made from robust materials that have high strength at high temperature and the antenna would not require low Z coatings that have poor thermomechanical properties and a potential to disintegrate.
Figure 3. Molybdenum source measured at the antennas in identical dis- charges except for the active antenna. The reference is measured at the FA antenna for an identical discharge with the plasma being heated with the TA antenna.
Figure 4. Comparison of nitrogen puff with and without RF showing the increase in core N emission proportional to ICRF injected power.
RF enhanced heat flux to the antenna structure during operation has been observed on a number of experiments and can limit the antenna coupled power. For the FA antenna, the heat flux to the FA antenna is reduced to a level similar to that observed for identical discharges heated by the TA antenna with the FA antenna not powered. The estimated total power deposited upon the antenna limiter tiles as a fraction of the total coupled RF energy is ∼ 6 kJ for 1.5 MJ injected or 0.4%. For comparison, JET  and Tore Supra  have reported 2-10% and ∼ 3.5% respectively and the ITER design target is 0.625%.
All the news presented so far has been positive. However, field-alignment does not completely resolve all ICRF challenges. For improved antenna performance, the primary tradeoff is that the power density for a FA antenna is 25-30% higher than for a TA antenna due to the helical geometry. Despite reduction of the impurity source at the antenna, residual core impurity contamination associated with RF power (i.e., an rf-specific impurity source) is still present. This suggests the possibility of an increased source away from the antenna or that transport is modified in the presence of RF. In addition, the RF enhanced plasma potentials that are associated with the FA antenna are similar to those excited by the TA antenna.
Figure 5. With subtraction of the background core N emission that is proportional to ICRF injected power, the impurity penetration is unaffected by the presence of ICRF power.
To investigate whether the residual impurity contamination is a result of modified impurity penetration or transport, we injected known amounts of nitrogen impurity from four locations that have different mappings to an ICRF antenna, both at locations where convection cells are formed and at locations far away from convection cells, energized to up to 1 MW. Core levels of low-Z impurities are characterized by measuring emission from H-like charge states using a radially-viewing vacuum ultraviolet (VUV) spectrometer. Plasma impurity content is comprised of intrinsic sources and puffed nitrogen. Due to the partially recycling nature of nitrogen, some remains from previous discharges leading to an intrinsic source prior to injection. We found that the core N evolved as shown in Figure 4, consisting of an increase that follows the ICRF power and one that tracks the injection rate. Compensating for the increase in background by subtracting a background proportional to the RF power, the impurity penetration was found to be the same with and without ICRF power as shown in Figure 5. The response to the N injection was found not to depend on magnetic mapping. In L-mode discharges, we also compared the measured impurity confinement time for discharges with and without ICRF power. The impurity confinement time for L-mode discharges, measured by monitoring the decay of Be-like Ca emission from discrete CaF2 injections, is 20-25 ms independent of RF power. We conclude that the increased impurity contamination is due to an increased source rather than modified transport or penetration.
The local ICRF convective cell is thought to play an important role in RF interaction with the scrape-off layer. The convective cell strength, characterized by the poloidal velocity, is expected to scale inversely with magnetic field. In C-Mod, we examined the poloidal velocity, hence the Er/B, dependence on magnetic field using a set of discharges spanning 2.7 to 7.9 T and sought to characterize the RF enhanced plasma potential. We utilized a variety of heating scenarios for each discharge: 2nd harmonic H minority at 2.7 T, H minority at 5.4 T, and He3 minority at 7.9 T. The single pass absorption is weaker for the 2nd harmonic H (2.7 T) and He3 (7.9 T) cases compared to the H minority (5.4 T) case. The RF power was increased in a staircase fashion up to 1.5 MW to map the plasma potential dependence on power. For each discharge, the plasma current was scaled to allow the plasma potential measurement to map to the same location on the antenna: 0.4 MA at 2.7 T, 0.8 MA at 5.4 T and 1.2 MA at 7.9 T. The line average density was 1.2 ×1020 m−3 for all discharges to maintain similar antenna loading. The measured poloidal velocity decreased by a factor of 1/2, which is less than expected, over the range of magnetic field. As shown in Figure 6, the RF enhanced plasma potential scales with RF power as expected. In addition, the RF enhanced plasma potential increased with magnetic field and plasma current. For each discharge, the applied RF fields are expected to be very similar since the injected power and coupling are similar for all cases. Further, the data is better correlated with plasma parameters than with RF parameters, suggesting the RF enhanced electric field is strongly influenced by the local scrape-off layer plasma conditions.
We have also recently investigated the dependence of the local impurity source and RF enhanced potential on the power balance between the center two straps, Pcent, and the outer two straps, Pout. This experiment was motivated by positive three-strap antenna results from ASDEX-U . With ∼ 30 dB decoupling, we scanned Pcent/Pout from zero to greater than 1000. Initial results look promising. A minimum in the RF enhanced potential and local impurity source is observed for Pcent/Pout greater than 1 and less than 4 with a gradual rise in impurity source for Pcent/Pout greater than 4. This minimum correlates where the image currents in the antenna limiters are expected to be smallest. A possible mechanism for the RF enhanced sheaths is that RF image currents are flowing along field lines and minimizing these currents minimizes the RF plasma edge interaction.
Figure 5. With subtraction of the background core N emission that is proportional to ICRF injected power, the impurity penetration is unaffected by the presence of ICRF power.
Looking towards future experiments and reactors, the path to
reliable, effective ICRF coupled power is high field side (HFS)
launch, which is the next logical step in developing an ICRF
actuator . Wave penetration remains excellent from the
HFS and the single pass absorption remains near 100% even in relatively low temperature startup plasmas.
As indicated by the residual impurity contamination and impurity penetration/transport experiments,
RF fields away for the antenna are contributing to an increased impurity source . With 100% single pass
absorption, ICRF fields would be significantly restricted. The high field side location offers a quiescent
scrape off layer with little or no edge localized modes . With a field aligned antenna, the antenna load
would have significantly less variation and tailoring the power distribution on the antenna would lead to
minimization of RF enhanced plasma potentials. In double-null and near double-null configurations, HFS
SOL profiles are controlled by magnetic topology . Furthermore, the high field side has significantly
improved impurity shielding compared to the low field side [10,11]. Furthermore, HFS launch would make
the ICRF actuator largely independent of the shape of the last closed flux surface which has a strong
influence on confinement. In a nuclear environment, locating the antenna off the midplane on the HFS
minimizes the impact on tritium breeding and reduces nuclear heating compared to low field side. Placing an antenna close to the plasma appears possible and practical if the proper location is chosen.
Acknowledgements Work supported by US DOE award DE-FC02-99ER54512.
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2016 — 10th Anniversary of USBPO Formation —
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.
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Editor: Saskia Mordijck (firstname.lastname@example.org)