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Announcements Research Highlights Recent and Upcoming Disruption Experiments on Alcator C-Mod R.S. Granetz Summary of the Transient Events Workshop S.A. Sabbagh ITPA Update Schedule of Burning Plasma Events Contact and Contribution Information
Toki Conference Abstract Deadline Extended
New Deadline: August 7, 2015
The International Toki Conference is a topical meeting dedicated to plasma and fusion science. The series began in 1989 with the founding of the National Institute for Fusion Science and has been held on an annual basis ever since. The last 10 years have seen great advances in the plasma and fusion sciences based on progress in basic physics and technology. These developments also accelerate innovative applications of plasmas in industrial, biomedical, agricultural, and other fields. With this background, we are planning the 25th International Toki Conference to explore new aspects in creating the future through the innovative sciences of plasma and fusion.
MHD and Macroscopic Plasma Physics Topical Group, Leaders: R.S. Granetz and S.A. Sabbagh
This month’s Research Highlight is composed of two separate items written by the leaders of the BPO’s MHD and Macroscopic Plasma Physics Topical Group. In the first, written by R.S. Granetz, a summary of disruption experiments on Alcator C-Mod is presented with an emphasis on the particular experimental design and accompanying theory employed to address the ITER relevant scenarios in which tokamak disruption mitigation is affected by the pre-existing MHD in the plasma. The second part of the Highlight, written by S.A. Sabbagh, summarizes the Transient Events Workshop. This workshop is one of four called by the DOE Fusion Energy Sciences program to collect input for future planning activities.
Recent and Upcoming Disruption Experiments on Alcator C-Mod
Massachusetts Institute of Technology, Cambridge, MA, 02139, USA
Three different disruption-related experiments are being done during the FY2015 run campaign on Alcator C-Mod. The first one, which was carried out in June, involved studying disruption mitigation with massive gas injection (MGI) on plasmas that had locked modes. The other two experiments involve the study of highly relativistic runaway electrons, and will be done later in the campaign after the direction of the plasma current is reversed.
MGI relies on self-generated MHD modes, particularly n=1, to quickly transport the injected impurities into the core of the plasma, which is necessary to achieve successful mitigation of energy and halo current. ITER has pointed out that nearly all MGI experiments to date have been done on “healthy” plasmas, i.e. plasmas without any significant pre-existing MHD modes. But disruption mitigation on ITER, and future reactors, will be done on plasmas that are already in distress, and likely to have significant MHD activity going on. If the pre-existing MHD modes interact with the MGI self-generated MHD modes, it is conceivable that the effectiveness of MGI disruption mitigation might be affected. With that in mind, ITER has requested that MGI be tried on “sick” plasmas, i.e. plasmas with pre-existing MHD modes, to compare its mitigation effectiveness to MGI on healthy plasmas.
Motivated by ITER’s concern, we have chosen to study MGI on plasmas with m=2/n=1 locked modes on Alcator C-Mod, since locking can be reproducibly triggered with our external error field coils (“A-coils”). Over the course of two run days, the A-coils were run in three different configurations, producing m=2/n=1 locked modes with three different toroidal phasings. Two different gas valves, located on opposite sides of the torus, were used for the MGI. Our standard mixture of 15% argon and 85% helium was used for the MGI gas. Measurements of mitigation relevant parameters made on both locked and unlocked plasmas included fraction of energy converted to radiation, toroidally-resolved radiated power, energy deposited on the outer divertor, vertical displacement, halo currents to the outer divertor, Mirnov signals, etc. We are still in the process of analyzing this large set of data. Although we do observe some differences in Prad asymmetries, our preliminary conclusion is that MGI mitigation works equally well on plasmas with and without locked modes. The results will be presented in the ITER session of the upcoming APS-DPP conference.
Later in this year’s campaign, after we reverse the Ip direction, we will be running experiments to study runaway electrons (REs). In order to have reproducible, well-diagnosed conditions, we run very low density plasmas to study relativistic runaways during the flattop. In addition to standard hard x-ray diagnostics and video imaging of synchrotron emission, we also employ our MSE system to measure the polarization of the synchrotron emission, and two absolutely-calibrated visible spectrometers to measure the synchrotron emission spectrum. This spectrum contains information about the RE energy and pitch angle distributions. Recent theoretical work sug- gests that at C-Mod’s high magnetic field (same as ITER’s) the synchrotron emitted power can effectively limit the maximum RE energy and pitch angle, resulting in a mono-energetic bump in the distribution. In preparation for our upcoming experiments, we have been working with the theory group at Chalmers to simulate representative RE synchrotron spectra. Based on this modeling, we believe that our spectrometers will be able to distinguish between a mono-energetic distribution and a continuum distribution, and therefore help to confirm or reject the recent theoretical predictions.
Summary of the Transient Events Workshop
Columbia University, New York, NY 10027, USA
A community workshop was held under the direction of the Department of Energy Office of Fusion Energy Science (OFES) on the challenges and opportunities for controlling potentially damaging transient events in tokamak fusion reactors, in particular disruptions and edge-localized modes . From its inception, the workshop was organized into two Panels, the first on Preventing Device Damage From Disruptions and the second on Avoiding Deleterious Effects of ELMs in High Performance Plasmas. This process followed from the OFES study and associated 2014 Report on Strategic Planning that defined the highest priority Tier 1 initiative as: “Control of deleterious transient events: to understand highly damaging transients and minimize their occurrence in ITER-scale systems.” The logical continuation of this effort is to determine progress in these areas since the Research Needs Workshop (ReNeW) report of 2009, determining the remaining key challenges, and stating research plans that can satisfy the remaining challenges. The present workshop process started with a call for community input in the form of white papers (68 total) and talks (36 total)  presented at the Transient Events Community Input Workshop (held March 30 – April 1, 2015). This input was reviewed by the two Panels (each comprised of three Sub-Panels) and discussed over weeks of videoconferences. The combined input from the community and extensive discussion by the Panels was used to produce documents prepared by each Sub-panel addressing the challenges and opportunities for controlling disruptions and edge-localized modes in tokamaks.
This process culminated in a workshop (held June 8 – 11, 2015 at General Atomics) to discuss and finalize the preparation of the Sub-panels documents, along with further input from the community . The reports show that significant progress has been made in both the understanding and control of transient phenomena since ReNeW, but additional challenges remain to develop robust disruption and ELM control solutions for ITER and next step reactors. The final combined report will document the findings and recommendations to address the research gaps and needs for the US fusion program to maintain leadership in the rapidly evolving areas of disruption prevention and edge localized mode control, to meet challenges in time for ITER operation, and to develop the physics basis to inform the design of future tokamaks beyond ITER.
More information concerning the ITPA may be found at the Official ITPA Website.
Transport and Confinement Topical Group
Contributed by Paola Mantica, Chair, ITPA Transport and Confinement Topical Group
The ITPA Transport & Confinement group spring meeting took place from 5th to 7th of May at the ITER Organization and dealt with the following topics: Heavy impurity transport, Advanced Core Modeling, L-H transition physics (including isotope effects) and threshold scaling with metallic wall, Discussion on the new Joint Experiments (JEXs) on particle transport and update on on-going JEXs, 3D effects, and preparatory talks for the next meeting in October. In addition, the group discussed two items as requested by the ITPA Coordinating Committee, namely the possible updates of the transport and confinement part of the Nuclear Fusion volume on “Progress in the ITER physics basis” and the revision of the ITER H98(y,2) scaling. To address these two ITPA CC requests a bullet-type survey of progress in core transport since 2007 has been launched, and a new joint activity (TC-28: Revision of ITER scalings) has been started with a small initial group of experienced people and considering, initially, only the JET, AUG, DIII-D and C-Mod data. The topics for the October 2015 meeting were discussed and agreed: Intrinsic rotation physics and use of the new database, Status of edge shortfall & fixes to quasi-linear models, Stiffness, Dependence of momentum/particle pinch on collisionality, and the Update of the strategy to address the ITER energy confinement revision and for future joint studies of 3-D effects on ITER plasmas.
Excellent progress was reported in validating models of W turbulent and neoclassical transport taking into account poloidal asymmetries on JET and AUG data. ICRH is an important tool for controlling neoclassical W transport in JET due to its central deposition and the favourable effects of ICRH-accelerated minorities. Simulations of W transport in ITER baseline scenarios with similar tools to those applied to experiments suggest favorable conditions for weak W peaking due to the low fueling provided by the negative NBI in ITER. Development of physics-based transport modelling suites fast enough for real time control was reported through the application of neural networks to the Qualikiz gyrokinetic quasilinear modelling results, with a similar approach being developed by GA.
Continuing with the validation of models for core transport against the experiment, a large effect of ExB shear was identified in TGLF simulations of JET Hybrid plasmas. This is at variance with previous GENE simulations that showed a dominant effect of non-linear electromagnetic stabilization and a joint activity has been started to investigate the reasons for such discrepancy. A new dataset on L-H threshold in presence of metallic wall with data from AUG, JET and C-Mod has been created and will be used to analyze the impact of metallic wall as well as other factors such as divertor geometry on the power required to access the H-mode in present experiments. Further work was presented aimed at addressing the collisionality dependence of core R/Lne which has been found in JET, AUG, and C-Mod but not in DIII-D. Gas puff modulation experiments showed that the NBI particle source plays a significant role in JET and thus part of the collisionality dependence of R/Lne may be due to the resulting larger central NBI source in the low collisionality conditions at JET.
The results of the study of the effects of 3-D fields on particle transport in several devices were presented. The changes to particle transport (particularly the density pump-out) in H-modes is found to be associated with changes in the edge electric field (probably due to the formation of an ergodic layer at the plasma) and edge rotational shear and not to the increase of particle outflux due to the increase in ELM frequency which can occur when 3-D fields are applied. Studies of the effect of 3-D fields on plasma rotation were also discussed including the effects of the JxB torque and neoclassical toroidal viscosity (NTV). Due to the strong dependence of these effects on the plasma response to the applied fields it is not possible to validate the existing predictions without good modelling of plasma response, as shown in JET. Modelling of 3-D effects on plasma rotation in ITER with the MARS code indicates that for n = 3 and n = 4 and currents up to 45 kAt in the coils a rather small effect on edge rotation is seen in general, with the exception of n=3 when peeling modes are unstable. On KSTAR the impressively low levels of error field and TF ripple, which are consistent with the large value of the toroidal plasma flow velocity, allow a very fine control of the external 3-D fields applied to the plasma making it an ideal device where to study the effects of these fields on the H-mode plasmas. The use of TGLF+NEO for the validation of theory predictions of toroidal and poloidal flows against the ITPA intrinsic rotation database was also proposed.
2015— NSTX-U First Plasma —
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