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We first describe the improved receiving system of the diagnostic experiment of millimeter-wave collective Thomson scattering being run on the Frascati Tokamak Upgrade (FTU), and then discuss some peculiar problems and new operating procedures related to the investigation of strong anomalous spectra of nonthermal origin, many-orders-of-magnitude stronger than the ion thermal feature merged in them, systematically observed in the experimentation, and finally ascribed to a perturbation of the gyrotron that generates the probing beam. Arguments in favor of a more general valence of the solutions actuated for the specific case of FTU are finally given. (c) 2007 American Institute of Physics.

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Steady internal transport barriers (ITBs) are obtained in FTU at ITER-relevant magnetic field and density (ne0 1.3 × 1020 m-3) in almost full non-inductive discharges, sustained by lower hybrid (LH) and electron cyclotron (EC) RF waves sources. Similarly to ITER, only electrons are directly heated which in turn heat ions via collisions and no momentum is injected. Collisions do not affect the mechanisms of turbulence suppression and energy transport. At the highest densities the ion thermal conductivity remains the ohmic level, while the energy confinement time exceeds the ITER 97-L scaling by about 1.6 times. The ITB radius can be varied in the range 0.2 r/a 0.65 modifying the radial profile of the LH driven current, acting mainly on the safety factor q. A liquid lithium limiter (LLL) of innovative design, composed of a mesh of porous capillaries, has been tested successfully for the first time on a medium size tokamak. The LLL surface showed no damage up to the maximum thermal load of 5MWm-2. With LLL cleaner plasmas are obtained and the particle recycling strongly drops; new interesting regimes of particle transport arise at high density, with highly peaked profiles. Significant progress in disruption mitigation by means of EC power has shown that they can be avoided when absorption occurs directly on the MHD islands driving the disruption. Feedback control/suppression of MHD tearing modes (TM, m = 2) with EC waves has been achieved relying on a real-time detection of the TM and of its radial location. Testing the collective Thomson scattering in ITER-relevant configuration has stressed that avoiding backscattered radiation to the source is very crucial. The theory of the evolution of fishbone-like instabilities driven by LH generated supra-thermal electrons in FTU is outlined, and its relation to the trapped ? particles dynamics is stressed.

Strong anomalous spectra were systematically observed in collective Thomson scattering (CTS) at 140 GHz in the high-field tokamak FTU, a proof-of-principle experiment of CTS on thermal density fluctuations performed with propagation below electron cyclotron (EC) resonance with the final aim of demonstrating high-density CTS in the extraordinary mode. Following the results of two experimental campaigns expressly performed to investigate them, these spectra are ascribed to a gyrotron perturbation caused by a back-reflected signal originating in the beam injection port, where the electron cyclotron layer, the upper-hybrid layer and the right-handed cutoff layer unavoidably crossed by the probing beam are activated by a breakdown plasma sustained by the beam itself. The degree of generality ascribable to our results and the constraints they set on present and future CTS experiments with propagation below electron cyclotron resonance are discussed. A viable solution in terms of a transmit antenna robust against the risk of gyrotron perturbation is suggested.

We first describe the improved receiving system of the diagnostic experiment of mm-wave Collective Thomson Scattering being run on the Frascati Tokamak Upgrade (FTU) and then discuss some peculiar problems and new operating procedures related to the investigation of strong anomalous spectra of non thermal origin, many orders-of-magnitude stronger than the ion thermal feature, systematically observed in the experimentation and finally ascribed to a perturbation of the gyrotron that generates the probing beam. Arguments in favor of a possible more general valence of the solutions actuated for the specific case of FTU are finally given.

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The design of diagnostics for the Frascati Tokamak Upgrade (FTU) is challenging because of the compact-ness of the machine (8-cm-wide ports) and the low op-erating temperatures requiring the presence of a cryostat. Nevertheless, a rather complete diagnostic system has been progressively installed. The basic systems include a set of magnetic probes, various visible and ultraviolet spectrometers, electron cyclotron emission (ECE) for elec-tron temperature profiles measurements and electron tails monitoring, far-infrared and CO2interferometry, X-ray (soft and hard) measurements, a multichord neutron di-agnostics (with different type detectors), and a Thomson scattering system. Some diagnostics specific to the FTU physics program have been used such as microwave re-flectometry for turbulence studies, edge-scanning Lang-muir probes for radio-frequency coupling assessment, oblique ECE, and a fast electron bremsstrahlung (FEB) camera for lower hybrid current drive-induced fast elec-tron tails. These systems are briefly reviewed in this paper. Further developments including a scanning CO2 laser two-color interferometer, two FEB cameras for tomo-graphic analysis, a motional Stark effect system, and a collective Thomson scattering system are also described.