An international journal of news from the stellarator community Editor: James A. Rome Issue 166 August 2019 E-Mail: [email protected] Phone: +1 (865) 482-5643 On the Web at https://stelnews.info injector from Oak Ridge National Laboratory (ORNL) that Report from the 19th Coordi- is being brought to W7-X with additional support from EUROfusion and NIFS, Japan, and which implements nated Working Group Meeting technology planned for use on ITER. March 12–14, 2019, The scientific community is excited by early theoretical Helmholtz Society, Berlin, Germany ideas to explain the latest results from stellarator experi- ments. Worldwide groups, concentrated in research cen- Overview ters as well as in universities, are increasingly applying their expertise in the understanding of ground-breaking Worldwide stellarator research has gained substantial physics questions, such as turbulence in three-dimensional momentum: On Wendelstein 7-X (W7-X, Greifswald, (3D) fields, impurity transport, fast-ion confinement, and Germany), first campaigns with an uncooled divertor have plasma flows and currents. demonstrated the highest-ever fusion performance normal- ized to the plasma volume and at the same time, the lon- The above-mentioned aspects were the essence of a three- gest pulse lengths. Deuterium experiments in the Large day meeting held at the headquarters of the Helmholtz Helical Device (LHD, Toki, Japan) showed confinement Society, the funding agency of the German fusion pro- at higher performance than gyro-Bohm scaling—differ- gramme (see Fig. 1). The meeting was organized and ences in terms of engineering parameters are less clear. sponsored by the Max Planck Institute for Plasma Physics. Smaller devices are enabling experimental insights for Roughly forty on-site and ten remote participants provided physical understanding, such as turbulence studies in TJ-II reports on collaborations, grouped into seven topics. In an (Madrid, Spain), fueling studies in Heliotron-J (Kyoto, informal workshop format, the participants discussed pro- Japan), and the examination of flows in HSX (Wisconsin, posals for joint action and experiments, taking advantage USA). Progress in the refurbishment of Uragan 2-M of comparative studies in different devices. The CWGM is (Kharkiv, Ukraine) and new concept devices (CFQS, not a scientific conference, but rather a interactive work- Chengdu, China) complement the larger experiments, cre- ating new opportunities to assess aspects relevant to reac- tor-like operation and stellarator optimization. In this issue . Equally important, stellarators benefit from the latest cut- Report from the 19th Coordinated Working ting-edge developments in diagnostics, heating, and Group Meeting (CWGM) plasma fueling. Coordinated programs in the EU, the US, This annual meeting was held at the headquarters of and Japan link innovative developments [e.g., integrated the Helmholtz Society in Berlin March 12–14, 2019. systems for overload detection (EU consortium), x-ray Roughly forty on-site and ten remote participants pro- detectors (Princeton Plasma Physics Laboratory), super- vided reports on collaborations, grouped into seven fast surveillance cameras (EU around WIGNER Research topics that reviewed progress during the past year, Centre for Physics, Hungary) and phase-contrast imaging and planned for the future. The coordinated working (MIT)] to the stellarator programs with obvious mutual group actions (CWGA) serve as a basis to follow up synergies: the pace of experiments is accelerated by joint actions and were agreed to be followed up in the employing these unprecedented measuring capabilities. forthcoming meetings............................................... 1 More and more benefits for tokamaks are materializing: experience gained in long-pulse stellarator experiments contributes to large-scale devices such as JT-60SA and ITER. A select example is the steady-state fueling pellet All opinions expressed herein are those of the authors and should not be reproduced, quoted in publications, or used as a reference without the author’s consent. Fig. 1. Participants of the 19th CWGM in Berlin (March 12–14, 2019). shop wherein agreements on joint publications under the superimposed seeding of krypton and neon in low-density auspices of the IEA Technology Collaboration Program on discharges. In the seeding experiments, the edge electron Stellarators/Heliotrons are promoted to become the mea- temperature decreased about 50%, while the radiated surable outcome of the CWGM. A session on the program power was doubled compared with neon seeding. plans of the main contributors served to enable the At W7-X, a high-density campaign with an island divertor exchange of information, and the community was invited was conducted. As reported by Marcin Jakubowski, this to provide feedback to programmatic considerations. led to the first achievement in stellarators of a high recy- China’s quickly developing stellarator program, with a cling regime, where downstream densities are signifi- sound balance of sustainable build-up of know-how and cantly higher than upstream densities. This allowed scientifically interesting new concepts, namely the outline reaching stable detachment of the plasma with plasma of a quasiaxially symmetric device, attracted great interest. duration of up to 30 s. It is possible to reduce divertor The sessions were led by colleagues serving as coordina- power loads with the help of impurity seeding. Florian tors. The remainder of this report gives brief summaries of Effenberg showed that with neon as the seed, global reduc- each session. tion of divertor loads by a factor of 4 could be achieved at W7-X. Seeding with N2 leads to more subtle influence on Divertor and Edge Physics in Stellarators power loads. Nitrogen seeding results in lower recycling (M. Jakubowski) and requires a long puff duration in order to establish a Two major experimental campaigns were recently con- significant enhancement of radiated power. Nitrogen is ducted that aimed at investigating important aspects of therefore a promising candidate for radiated power control divertor physics in helical devices. At LHD, results of the using a feedback system. first deuterium campaigns were reported by Suguru Masu- As is the case in LHD, edge transport is observed to be zaki. There is no clear difference between hydrogen (H) affected by drift effects in W7-X, e.g., EB drifts. Ken and deuterium (D) plasmas in terms of divertor physics. Hammond presented results from experiments with posi- The asymmetries in divertor particle fluxes, that arise due tive and negative directions of the main magnetic field. to edge drifts look identical in both plasmas. The same is Victoria Winters discussed experiments on the influence true for divertor load patterns. Additional non-evaporative of carbon impurities on radiation patterns. During methane getter pumps installed under the dome structure of the puffs, the percentage increase of C-VI radiation is the helical divertor allowed for increasing of the pump speed same at different density levels, which probably means by about 25%. Stable detachment was achieved using Stellarator News -2- August 2019 that transport processes are similar. Grzegorz Pełka pre- Furthermore, Gates presented a brief comparison of the sented first results for modeling of W7-X limiter plasmas density limit in stellarators and tokamaks and introduced with a new code FINDIFF. It solves fluid equations for fundamental differences and similarities. It was stressed main ions and neutrals in curvilinear coordinates. The that there was seemingly no interest in the tokamak com- code is still not in good enough shape to be able to repro- munity to tackle this issue at the moment, and it was pro- duce experimental data; nevertheless, significant progress posed that the stellarator community should lead an effort has been made in the last year. to verify the presented models experimentally. Several areas in which joint research could be performed Joint actions agreed to in this session are: at LHD and W7-X were identified. They include the A comparison of operational limits for different stel- investigation of stable detachment, the role of plasma larators including, but not limited to, the three largest drifts at the edge, changes in divertor structures due to ones: LHD, TJ-II, and W7-X. plasma dynamics, and 3D migration patterns in helical An investigation on whether identified instabilities devices. More specific actions will be defined in the are generic to stellarators or depend on the specific course of 2019. details of a particular magnetic configuration. Scaling and operation limits (G. Fuchert) An assessment of the density limit with special focus on a comparison with tokamaks. Knowledge about operational limits and scalings of global confinement parameters is crucial for efficient scenario 3D Fast Ion Physics (S. Lazerson) development in stellarators. In this session, three main top- The study of fast ions in stellarators speaks to a fundamen- ics were discussed: The scaling of the energy confinement tal issue of nuclear fusion, specifically the nuclear fusion time, MHD and reconnection instabilities, and the radia- reaction of hydrogen isotopes not only to generate neu- tive density limit. trons but also to sustain the plasma parameters necessary Concerning the energy confinement time scaling, Yamada for the reaction by heating the plasma with the fusion-pro- presented experimental data on isotope effects in LHD. In duced alpha particles While