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 F.M. Poli Schedule of Burning Plasma Events Contact and Contribution Information
Extraordinary Meeting of the ITER Council
The ITER Council met this week in Paris to hear the results of an independent assessment of the proposed schedule and resource estimate. The panel found that these were credible and complete, and provide “a good starting point for a revised schedule based on credible estimates of cost and human resources, taking into consideration the financial constraints of the Members and relying on effective sharing of staff resources between the Domestic Agencies and the ITER Organization.” Following this good news, the ITER Council will consider a resource-loaded schedule up to first plasma and additionally from first plasma to DT operation during its next regularly scheduled meeting in June. More information about the findings of the independent review panel can be found at http://www.iter.org/newsline/-/2443.
In a related development, Secretary of Energy Ernest Moniz is expected to inform Congress of his recommendation on whether the US should remain a partner in the ITER project in the coming days.
The ITER Council met in Paris on April 27 to receive the report of interior of the IC Independent Review Group (ICRG; Photo © ITER Organization).
Upcoming ITER Science and Technology Advisory Committee meeting
Preparations are also underway for next month’s STAC-20 (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 three charges (condensed version) will be addressed
with STAC’s findings being reported at the upcoming ITER Council meeting in June:
1. Assess progress in the design and R&D for the in-vessel coils (Vertical Stabilization and ELM control coils)
2. Review the development of disruption mitigation scenarios with the support of the ITER Members’ community of experts and progress on the design of the DMS including specifications, manufacturing and commissioning plan for possible testing in JET, etc.
3. Review the status of the vacuum vessel pressure suppression system design.
The first two charges are heavily informed by research performed at US facilities on RMP ELM control (charge 1) and disruption mitigation (charge 2). With regard to charge 1, the community should give itself a pat on the back for rapidly responding to requests made last fall for research concerning the need to rotate the resonant magnetic perturbation (RMP) to spread the heat load arising from magnetic asymmetries. Once again, the US STAC participants will be myself, Rob Goldston, Earl Marmar, Juergen Rapp, and Jim Van Dam.
Jobs at ITER
There are currently 29 positions posted at the ITER Jobs website (http://www.iter.org/jobs) including several engineering and physics positions that might be of interest to some of our readers. The application deadlines of all of these positions are rapidly approaching, so if you have an interest in working in France on one of the world’s leading scientific endeavors, apply quickly!
APS-DPP Contributed Oral Session on “Research in Support of ITER”
For the eighth time, last year’s APS Division of Plasma Physics annual meeting included a contributed
oral session on Research in Support of ITER, which included 15 talks from US and foreign participants.
These sessions have become quite popular, and are always well attended.
The US Burning Plasma Organization is organizing a similar session for the 58th Annual Meeting of
the Division of Plasma Physics, which will take place in San Jose, California, on October 31 through
November 4. Once again, we are looking for talks on research that has been done specifically to address
ITER design, operation, or physics issues. These brief talks are “standard” contributed orals: 10 minutes
in duration, followed by a 2 minute discussion period. We hope to have broad participation once again, so
we can highlight the breadth of this work and the institutions performing it, both US and international.
Please watch next month’s column for information how to request a slot in this session. Note that there are only 15 slots, and we usually get too many requests. So please understand if we can’t fit your presentation.
We are also hoping to once again organize a Town Meeting on ITER as we have in the past. We are not always able to do this due to competition from the November ITER Council meeting and IAEA (in even years). Watch for an announcement later.
Integrated Scenarios Topical Group, Leaders: Chris Holcomb and Francesca Poli
Experimental operation of NSTX-U is now beginning, and one of the main goals of the program is to demonstrate non-inductive start-up of a spherical tokamak. The simulations presented in this month’s research highlight discuss ways this can be accomplished.
Simulations towards the achievement of non-inductive current ramp-up and sustainment in NSTX-U
1 Princeton Plasma Physics Laboratory, PO Box 451, Princeton, NJ 08543-0451, USA.
The Spherical Torus (ST) is a leading candidate for a Fusion Nuclear
Science Facility (FNSF) due to its compact size and modular
configuration. The high neutron wall load, the high values of plasma
to magnetic pressure ratio achievable and the ease of maintenance make
the ST an attractive option for nuclear component test facilities
[1-4]. Critical elements of ST research in support of steady-state
operation and demonstration of the viability of the ST as a fusion
power plant include sustainment of fully non-inductive current with
large bootstrap current fraction, non-solenoidal start-up and ramp-up
and operation at high βN and high confinement with Resistive Wall Mode stabilization.
Future ST-FNSF are projected to rely on Neutral Beam Injection (NBI) to sustain about 50% of the plasma current, with the remainder provided by the self-generated bootstrap current. Although NBI will also provide heating and current drive for non-inductive current ramp-up, up to 50% of the injected power can be shined-thru at the low densities typical of start-up plasmas. At these densities Radio-Frequency waves have instead good accessibility to the core and can effectively heat the plasma to Neutral Beam injection. On NSTX-U, radio-frequency waves at high harmonics of the ion cyclotron frequency are launched at 30 MHz with a large 12 strap antenna array that spans 90 degrees toroidally around the outside of the torus . Spectra corresponding to parallel wavevectors of 3, 8, 13 m−1 can be selected by feeding the antenna with six decoupled sources. For deuterium ions and BT = 1 T, the harmonic resonances inside the last close flux surface range from the 2nd to the 5th .
In support of the NSTX-U research plan and in order to minimize failures and reduce operational costs, extensive effort is ongoing to self-consistently simulate the synergy of RF sources and Neutral Beams in sustaining a fully non-inductive, MHD stable plasma . Non-inductive ramp-up is challenging for several reasons. Most obviously, one must provide sufficient current to replace the inductive current contribution. Contrary to an inductive discharge, where current diffuses from the outside, in a non-inductive ramp-up the externally driven current profiles are determined by the characteristics of the external sources.
Figure 1. Left panel. Simulations with NBI. (a) NBI power waveform, (b) shine-thru and orbit loss power, (c) power absorbed on the electrons and on the ions, (d) central electron and ion temperature, (e) central and line averaged density, (f) plasma current waveform (black, thick) and contributions to the total current. Central panel. Simulations with HHFW. (a) Injected power (black) and power absorbed on the electrons (red) and on the ions (blue). (b) central value of electron (red) and ion (blue) temperature. (c) total requested current (thick), bootstrap (red), FW (blue) ohmic (black) current contribution. Right panel. Simulations with EC, HHFW and (a) EC waveform (magenta), HHFW injected power and power absorbed to the electrons (red), to the ions (blue) and to the fast ions (green). (b) Neutral Beam injected (black) power, and absorbed by the electrons (red) and by the ions (blue). (c) Central value of electron and ion temperature. (d) current waveform (thick black) and contributions: FWCD (blue), beam current (green), ohmic (black) and bootstrap (red), the total non-inductive current is also shown for comparison (magenta).
Figure 1 shows the plasma response to the use of
respectively NBI and HHFW, and to a combination of EC, HHFW and
NBI. We note that the simulations are using a reference waveform
for the plasma current, with a fast ramp to 300 kA in 50 ms and
then a slower ramp to 900 kA in 2.5 s. This is done to maintain a
target plasma current that we know to be MHD stable and then model
the plasma evolution towards this target.
In the simulation with NBI only the three neutral beam sources at low-tangency radius (first beamline) are injected from 25 ms, and one source at low energy and highest tangency radius after 50 ms. Different combinations of sources in the first beamline result in variations in the driven current only within 10% of the values shown. In addition, over half of the injected power is lost via shine-through.
In contrast to the NBI, HHFW power is effective at heating the plasma at low density, both in the electron and the ion channel. The HHFW system can potentially couple more than 4 MW, stepped-up virtually instantaneously. In practice, a significant fraction of the power is lost to the scrape-off layer. The phasing of the antenna used here corresponds to a parallel wavenumber of 8 m−1, the most favorable for driving current in the start-up plasma. As shown in Fig.1, the driven current has a maximum in the first 200 ms of discharge, then it drops after the H-mode transition as the current drive efficiency is proportional to temperature and inversely proportional to density. After 300 ms of discharge, the electron absorption also decreases, contributing to the drop in the calculated direct driven current. The NBI is more effective in H-mode, where the higher density reduces shine-thru and losses, while the HHFW is more effective at driving current in L-mode and in the early H-mode phase, where the density is lower.
A megawatt-level 28 GHz electron cyclotron heating system is currently planned as an upgrade in NSTX-U in 2017-18 . The gyrotron can deliver up to 1 MW of power to the plasma over a pulse length of 1-5 s. The EC waves have a cutoff for the O-mode at a density of 9.72×1018 m−3, which limits the application of EC heating to the early ramp-up phase.
As shown in Fig.1, EC waves are a very effective means for heating the NSTX-U start-up plasma to high temperature in a short time, because of the good accessibility at low density. EC waves heat the plasma very effectively to prepare a target where HHFW can be absorbed more favorably. An interesting synergy is observed between EC and HHFW for k=3m−3. Although this antenna phasing favors ion absorption, a ignificant fraction of the power is predicted to be absorbed on the electrons because of the large electron temperature. Basically, the combination of EC and HHFW with this phasing re-creates in the startup plasma conditions typical of flattop, with Te ∼ Ti. This results in a significant increase in the driven current during the EC phase. As little as 1-2 MW of absorbed HHFW power are sufficient to drive 300 kA non-inductively, a significant improvement over using HHFW alone. Ideally, a fully non-inductive start-up and ramp-up should use all three sources to maximize benefits: the EC to pre-heat the CHI start-up plasma, the HHFW to maximize the non-inductive current at low density and then the NBI to ramp the current after the L-H transition, when the current drive efficiency of the HHFW is reduced.
EC waves might be the only effective way of optimizing current drive at start-up. By heating plasma electrons, the EC heating prepares a background plasma where HHFW power can effectively drive current. Ongoing work is looking into the possibility of EBW startup and ramp-up to optimize the non-inductive current and to offer additional options for non-inductive operation.
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2016 — 10th Anniversary of USBPO Formation —
|May 10-11||ITER STAC Meeting||Caderache, France|
|May 30-June 3||PSI Conference||Rome, Italy|
|June 5-9||High Temperature Plasma Diagnostic Conference||Madison, Wisconsin, USA|
|June 19-23||International Conference on Plasma Sciences, ICOPS||Banff, Alberta, Canada|
|June 27-July 1||18th International Conference on Plasma Physics (ICPP2018)||Kaohsiung, Taiwan|
|July 4-8||European Physical Society Conference on Plasma Physics (EPS)||Leuven, Belgium|
|September 4-8||Joint EU-US Transport Task Force Meeting||Leysin, Switzerland|
|October 13-15||ITER STAC Meeting||Kizu, Japan|
|October 17-22||26th IAEA Fusion Energy Conference||Kyoto, Japan|
|October 24-26||T&C ITPA meeting JAEA||Naka, Japan|
|October 31-November 4||58th APS Division of Plasma Physics||San Jose, California, USA|
|December 13-14||37th Fusion Power Associates Annual Meeting and Symposium, Fusion Power Development: An International Venture|
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 (email@example.com)