Fusion Programme - ENEA - Fusione

Fusion Programmewas not affected by plasma exposition.Moreover, the probe measurements did notshow clear evidence of the uncorrelated largeevents observed in 2006. A possibleinterpretation suggested was that lithization ofthe walls during the summer campaign led toan increase in plasma temperature in the SOLand, consequently, to vaporisation of dustparticles.The feasibility of introducing aerogel samplesin the SOL to capture hypervelocity dustparticles was investigated. Aereogels arehighly porous, very low density material(1 mg cm -3 ) that allow hypervelocity particlesFig. 1.9 - Aerogel sample used for outgassing tests to be captured without destroying them andare already widely employed to study cosmicdust. Preliminary outgassing tests wereperformed on aereogel samples (fig. 1.9) and possible sputtering was evaluated.The Thomson scattering (TS) diagnostic was used to measure the dust present in the FTU plasmadischarges after disruptions [1.20]. Elastic scattering signals, due to the presence of dust particles, wereoften detected at the end of runaway discharges. It should be noted that the edge lines of sight of the TSdo not cover the SOL region, where usually laser elastic scattering by dust is observed in other tokamaks.The average density of the dust particles after disruptions was about 10 7 m -3 . Assuming a Rayleigh regimeof scattering, the size of the particles did not exceed the order of 0.1 μm, with the size distribution roughlyfollowing a power law.From theory predictions, it is expected that in standard FTU operating conditions (B T =5.3 T,Experimental I p =0.5 MA, =0.5×10 20 m -3 ) fishbone-like modes are destabilised by coupling asearch for relatively high LH power (P LH ≥1.5 MW) in the presence of a q min =1 layer located off-axis ofthe plasma column. Experiments aimed at optimising plasma operations for excitingelectronfishbone-like internal kink instabilities driven by electrons have recently been performed infishbones FTU with moderate LH power (0.5 MW). The plasma current density radial profile evolutionin realistic experimental conditions was modelled with the JETTO code, including the earlyphase of the plasma current ramp-up. The experiment was performed with the lithizatedvessel, which provided a broader T e profile as well as a lower Z eff than in the case of the standardboronised vessel. As a consequence, slower current diffusion is produced, which is more suitable forexploring optimal destabilisation conditions for the fishbone-like mode.2007 Progress Report18During this experiment, MHD precursor events of the fishbone-like instability are observed with LH poweras low as 0.5 MW (see fig. 1.10), i.e., lower than the expected threshold for strong excitation of thefishbone-like instability (≈1.5 MW) [1.21]. During the time window in which q min ≥1, just before the sawtoothonset, this precursor consists of an m=1, n=1 mode occurring at t ≥0.32 s and localised around r/a=0.3.[1.20] E. Giovannozzi et al., Dust measurements withThomson scattering in FTU, Presented at theInternational Workshop on Burning PlasmaDiagnostics (Varenna 2007), to be published inPlasma Phys.