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10-15 May 2021
Virtual Event
Europe/Vienna timezone
The Conference will be held virtually from 10-15 May 2021

First neutral beam experiments on Wendelstein 7-X

13 May 2021, 14:00
4h 45m
Virtual Event

Virtual Event

Regular Poster Magnetic Fusion Experiments P6 Posters 6

Speaker

Dr Samuel Lazerson (Max-Planck-Institut für Plasmaphysik)

Description

The Wendelstein 7-X (W7-X) experiment commissioned the first of two neutral beam boxes [1] in the previous divertor campaign, providing 3.6 MW of heating power, achieving of densities above $2\times10^{20}$ $m^{-3}$, and providing the first initial assessment of fast ion confinement in the device. Demonstration of the confinement of fast ions is key to forwarding the stellarator concept as a nuclear fusion reactor. Experiments exploring the interplay between electron-cyclotron resonance heating (ECRH) and neutral beam injection (NBI) were performed through a series of discharges varying the ratio of NBI to ECRH power.  It was found that even a small amount of ECRH was enough to arrest a continuous density rise during NBI operation. Discharges solely heated by NBI featured a continuous density rise with strong density peaking in the core of the plasma. In these discharges, densities above $2\times10^{20}~ m^{-3}$ were achieved, opening the possibility to explore OXB ECRH operation. Infrared camera images suggest fast ion wall loads which are consistent with numerical predictions. In general, discharges were free from the presence of Alfvénic activity suggesting future upgrades to assess the triggering these modes. These experiments provide data for future scenario development and initial assessment of fast-ion confinement in a drift optimized stellarator.

A small amount of ECRH (500kW) is added to a NBI discharge halting the density rise (20181009.34).  A discharge without ECRH is shown for comparison (20181009.43).

The NBI system on W7-X is designed to inject neutral hydrogen at 55 keV providing 1.8 MW of heating per source for up to 5 s (60 keV, 2.5MW, and 10 s for Deuterium), thereby providing particles which mimic fusion alphas (gyro-radius scaling) in a larger Helias reactor.  The injection geometry is neither radial, nor tangential, but rather populates both the trapped and passing particles, allowing the assessment of fast ion confinement across the trapped passing boundary. The experiments conducted drove a beam current in a direction which lowers the overall rotational transform consistent with beam line geometry.  Discharges solely heated by NBI indicate a drastically different character than those with a combination of ECRH and NBI. A continuous density rise over the discharge was found to be arrested by even a small amount of ECRH consistent with results from Wendelstein 7-AS (figure 1). In addition to acting as a heating/fueling source, the beam-plasma interaction enables spectroscopic measurements of beam attenuation, density and power as well as impurity densities, ion temperature, and rotation measurements, which are key for validation of equilibrium, transport and fast ion codes. The first experiments on W7-X with NBI successfully demonstrated the system and helped to access parameter regimes as yet unaccessible with ECRH operation alone.

Electron density measured by Thomson scattering for a discharge solely supported by NBI (20181009.43).  Strong peaking of the profile is clearly present.

Discharges heated solely by NBI demonstrated a continuous density rise achieving densities above $2\times10^{20}~m^{-3}$ [2].  These discharges had density profiles which peaked inside of $r/a\sim0.6$ with little change in density outside this radius (figure 2).  Temperatures were relatively modest around 1.0 keV with broad shapes in these discharges. Discharges with similar levels of ECRH and NBI indicated a small density rise, no meaningful peaking of the density profile, and a small change in ion temperature.  Even a small amount of ECRH introduced into a NBI discharge was enough to arrest the continuous density rise, with reduction in density peaking in the core of the plasma. Achievement of these high densities suggests the future possibility of operating with OXB ECRH heating above the O-2 cutoff.

The assessment of fast ion confinement in W7-X is key topic in the overall experimental program and NBI is envisioned as the primary method by which to achieve this goal.  The particles injected by the NBI system scale, in normalized gyro-radius, to fusion alphas in a larger HELIAS type reactor. It has been predicted that as plasma beta increases, fast-ion confinement improves [3]. These experiments have provided data to help validate our numerical models for fast ion confinement. Predictions of wall overloads [4,5], beam deposition [6], and radial electric fields [7] have already been validated against experimental data provided by the NBI system. These models are now being used in the development of fast-ion diagnostics for future campaigns [8,9].

The first experiments on W7-X with NBI have provided a wealth of information for more detailed studies to come in future campaigns.  The ability of the system to sustain plasmas for 5 s, thereby achieving high density operation was demonstrated. In the next campaign, a second beam box with two sources will be brought into operation, doubling the heating power and fueling of the system.

[1] Rust, N. et al. (2011). W7-X neutral-beam-injection: Selection of the NBI source positions for experiment start-up. Fusion Engineering and Design, 86(6-8), 728–731. http://doi.org/10.1016/j.fusengdes.2011.03.054
[2] Wolf, R. C. et al. (2019). Performance of Wendelstein 7-X stellarator plasmas during the first divertor operation phase. Phys. Plasmas, 26, 082504. https://doi.org/10.1063/1.5098761
[3] Drevlak, M. et al. (2014). Fast particle confinement with optimized coil currents in the W7-X stellarator. Nuclear Fusion, 54(7), 073002. http://doi.org/10.1088/0029-5515/54/7/073002
[4] Äkäslompolo, S., et al. (2019). Validating fast-ion wall-load IR analysis-methods against W7-X NBI empty-torus experiment. Journal of Instrumentation, 14(07), P07018–P07018. http://doi.org/10.1088/1748-0221/14/07/P07018
[5] Äkäslompolo, et al. (2018). Modelling of NBI ion wall loads in the W7-X stellarator. Nuclear Fusion, 58(8), 082010–15. http://doi.org/10.1088/1741-4326/aac4e5
[6] Lazerson, S. et al. (2020) Validation of the BEAMS3D neutral beam deposition model on Wendelstein 7-X. (submitted)
[7] Ford, O. et al. (2020) Charge Exchange Recombination Spectroscopy at Wendelstein 7-X. (submitted)
[8] Lazerson, S. et al. (2019). Development of a Faraday cup fast ion loss detector for keV beam ions. Review of Scientific Instruments, 1–6. http://doi.org/10.1063/1.5111714
[9] Ogawa, K. et al. (2019). Energy-and-pitch-angle-resolved escaping beam ion measurements by Faraday-cup-based fast-ion loss detector in Wendelstein 7-X. Journal of Instrumentation, 14(09), C09021–C09021. http://doi.org/10.1088/1748-0221/14/09/C09021

Affiliation Max-Planck-Institut für Plasmaphysik
Country or International Organization Germany

Primary author

Dr Samuel Lazerson (Max-Planck-Institut für Plasmaphysik)

Co-authors

Dr Dirk Hartmann (Max-Planck Institut für Plasmaphysik) Dr Oliver Ford (Max-Planck Institut für Plasmaphysik) Peter Poloskei (Max-Planck Institut für Plasmaphysik) Anabell Spanier (Max-Planck Institut für Plasmaphysik) Lilla Vanó (Max-Planck Institut für Plasmaphysik) Dr Norbert Rust (Max-Planck Institut für Plasmaphysik) Dr Paul McNeely (Max-Planck Institut für Plasmaphysik) Dr Simppa Äkäslompolo (Aalto University) Dr Kunihiro Ogawa (National Institute for Fusion Science) Michael Drevlak (Max-Planck-Institut für Plasmaphysik) Dr Christoph Slaby (Max-Planck Institut für Plasmaphysik) Dr Yuriy Turkin (Max-Planck Institut für Plasmaphysik) Dr Sergey Bozhenkov (Max-Planck Institut für Plasmaphysik) Dr Tristan W. C. Neelis (Eindhoven University of Technology) Dr Nicolaas Harder (Max-Planck Institut für Plasmaphysik) Dr Bernd Heinemann (Max-Planck Institut für Plasmaphysik) Dr Dieter Holtum (Max-Planck Institut für Plasmaphysik) Dr Werner Kraus (Max-Planck Institut für Plasmaphysik) Dr Riccardo Nocentini (Max-Planck Institut für Plasmaphysik) Dr Guillermo Orozco (Max-Planck Institut für Plasmaphysik) Dr Rudolf Riedl (Max-Planck Institut für Plasmaphysik) Dr Christian Hopf (Max-Planck Institut für Plasmaphysik) Dr Jens Knauer (Max-Planck Institut für Plasmaphysik) Dr Kai Jakob Brunner (Max-Planck Institut für Plasmaphysik) Dr Matthias Hirsch (Max-Planck Institut für Plasmaphysik) Dr Pasch Ekkehard (Max-Planck Institut für Plasmaphysik) Dr Marc Beurskens (Max-Planck Institut für Plasmaphysik) Dr Hannes Damm (Max-Planck Institut für Plasmaphysik) Dr Golo Fuchert (Max-Planck Institut für Plasmaphysik) Philipp Nelde (Technische Universität Berlin) Dr Evan Scott (Max-Planck Institut für Plasmaphysik) Dr Novimir Pablant (Princeton Plasma Physics Laboratory) Dr Andreas Langenberg (Max-Planck Institut für Plasmaphysik) Dr Karsten Ewert (Max-Planck Institut für Plasmaphysik) Dr Peter Traverso (Auburn University) Dr Pranay Valson (Max-Planck Institut für Plasmaphysik) Dr Uwe Hergenhahn (Max-Planck Institut für Plasmaphysik) Dr Andrea Pavone (Max-Planck Institut für Plasmaphysik) Kian Rahbarnia (Max-Planck Institut für Plasmaphysik) Dr Tamara Andreeva (Max-Planck Institut für Plasmaphysik) Dr Jonathan Schilling (Max-Planck Institut für Plasmaphysik) Dr Christian Brandt (Max-Planck Institut für Plasmaphysik) Dr Ulrich Neuner (Max-Planck Institut für Plasmaphysik) Dr Henning Thomsen (Max-Planck Institut für Plasmaphysik) Neha Chaudhary (Max-Planck Institut für Plasmaphysik) Dr Udo Hoefel (Max-Planck Institut für Plasmaphysik) Dr Torsten Stange (Max-Planck Institut für Plasmaphysik) Dr Gavin Weir (Max-Planck Institut für Plasmaphysik) Dr Nikolai Marushchenko (Max-Planck Institut für Plasmaphysik) Dr Marcin Jakubowski (Max-Planck Institut für Plasmaphysik) Dr Adnan Ali (Max-Planck Institut für Plasmaphysik) Dr Yu Gao (Max-Planck Institut für Plasmaphysik) Dr Holger Niemann (Max-Planck Institut für Plasmaphysik) Dr Aleix Puig Sitjes (Max-Planck Institut für Plasmaphysik) Dr Ralf Koenig (Max-Planck Institut für Plasmaphysik) Dr Robert C. Wolf (Max-Planck Institut für Plasmaphysik)

Presentation Materials