Critical issues highlighted by collective Thomson scattering below electron cyclotron resonance in FTU

Abstract

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.

Experimental characterization of anomalous strong scattering of mm-waves in TEXTOR plasmas with rotating islands

“…Finally, we note the similarities between our measurements in both the high-and low-density regimes and the reported anomalous signal from the FTU tokamak [42,43], LHD [9], and at ASDEX Upgrade both in ECRH heated plasmas [44] and during general CTS operation [39] where strong signals are observed in correlation with edge localized modes (ELMs).…”

Evolution of the millimeter-wave collective Thomson scattering system of the high-field tokamak Frascati Tokamak Upgrade

“…3. This is the strongest reason why, in CTS with f gyr Ͻ f ec , the transmitting antenna must be made a priori robust against the risk of gyrotron perturbation.…”

“…2 As discussed in a dedicated article, 3 the anomalous spectra in question were unambiguously interpreted as strayradiation spectra associated with perturbed gyrotron conditions caused by the back-reflection in the access port of the power fraction polarized in the extraordinary ͑X͒ mode in the gyrotron beam injected in the plasma to produce the scatter-ing. A strong impulse to the investigation of these spectra came from the consideration that, FTU being one of the very few high-field tokamaks ͑toroidal magnetic field: B T Յ 8 T͒ presently in operation, our experiment was the only one carried out in the propagation window of interest for the demonstration of a diagnostic of the confined alphas for the future International Thermonuclear Experimental Reactor ͑ITER͒.…”

Collective Thomson Scattering on FTU using a new ECRH launcher

“…he new set-up of CTS in FTU [4] will basically consist in a 140GHz GYCOM gyrotron with 400kW nominal power and pulse duration up to 0.5s , a high-power transmission line connecting the gyrotron to the outer mirror of one of the two lines of the new launcher (the transmitting antenna), and a low-power transmission line connecting the outer mirror of the second line of the same system (the receiving antenna) to a dedicated receiving line.…”

“…The experimentation was generally performed with B T ≤7.2T on the vessel axis, with the probing beam that had to cross the EC resonance and the nearby upper-hybrid (UH) cut-off for the X mode at a position inside the beam injection port. Clear evidences were found [3] that a local breakdown occurred near the critical layers, which in turn caused a back-reflection of beam power toward the gyrotron and a consequent perturbation of its spectrum, making the CTS measurements impossible [4]. This problem can be extended also to CTS in ITER, due to the similarity of conditions.…”