Energetic particles driven modes are one of the concerns for burning plasmas. On Tore-Supra, fast ions and electrons are generated by the RF heating systems and fast particles driven modes are detected with ECE and reflectometry diagnostics. In ICRH heated plasmas, modes are observed in the acoustic frequency range 30–70 kHz. The observed frequency agrees with the frequency predicted for both geodesic acoustic modes and beta Alfvén eigenmodes (BAE), but their structure and their excitation by fast ions advocate for an identification as BAE. Experimental analysis displays the existence of an excitation threshold depending on various parameters such as the ICRH power, the minority fraction and the density, in agreement with the theoretical prediction resulting from a balance between the fast ion drive and Landau damping by thermal ions. In lower hybrid current drive plasmas, electron fishbones have been detected below 20 kHz. Evolution of the fishbone modes with LH power was studied. Precession fishbones are observed at moderate power to be replaced by a mode at diamagnetic frequency at higher power. Precession fishbone frequency exhibits periodical jumps (0.1 s). These jumps are linked to spontaneous transitions between modes at different wavenumbers and a redistribution of the fast electrons in resonance with these modes. The number of excited modes and their wave number change with the LH power.
The highest concentration of cold seep sites worldwide has been observed along convergent margins, where fluid migration through sedimentary sequences is enhanced by tectonic deformation and dewatering of marine sediments. In these regions, gas seeps support thriving chemosynthetic ecosystems increasing productivity and biodiversity along the margin. In this paper, we combine seismic reflection, multibeam and split-beam hydroacoustic data to identify, map and characterize five known sites of active gas seepage. The study area, on the southern Hikurangi Margin off the North Island of Aotearoa/New Zealand, is a well-established gas hydrate province and has widespread evidence for methane seepage. The combination of seismic and hydroacoustic data enable us to investigate the geological structures underlying the seep sites, the origin of the gas in the subsurface and the associated distribution of gas flares emanating from the seabed. Using multi-frequency split-beam echosounder (EK60) data we constrain the volume of gas released at the targeted seep sites that lie between 1,110 and 2,060 m deep. We estimate the total deep-water seeps in the study area emission between 8.66 and 27.21 × 106 kg of methane gas per year. Moreover, we extrpolate methane fluxes for the whole Hikurangi Margin based on an existing gas seep database, that range between 2.77 × 108 and 9.32 × 108 kg of methane released each year. These estimates can result in a potential decrease of regional pH of 0.015–0.166 relative to the background value of 7.962. This study provides the most quantitative assessment to date of total methane release on the Hikurangi Margin. The results have implications for understanding what drives variation in seafloor biological communities and ocean biogeochemistry in subduction margin cold seep sites.
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