Speaker
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.
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.
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 |
