The production of runaway electrons (REs) in fusion reactors based on the tokamak concept is a serious issue regarding the integrity of the vacuum vessel components . In ITER, REs are expected to develop during plasma disruptions  and the avalanche mechanism could let up to 12 MA RE beams, with energies up to the order of several MeV . To avoid deep melting of the plasma facing components, suitable mitigation techniques are necessary, such as pellet injection and magnetic control of RE current, extensively pursued in current tokamaks . A natural mechanism that might reduce the growth of REs is provided by the micro-instabilities. These are driven by the source of free energy inherent to the electron distribution function in the presence of REs. In particular, for relatively strong magnetic field (ω_pe≪Ω_ce, where ω_pe is the electron plasma angular frequency and Ω_ce is the electron cyclotron frequency), two lower frequency branches have been considered, limiting the analysis to cold plasma waves driven by the anomalous Doppler resonance, namely the magnetized plasma oscillations  and the whistler modes . The latter can be also driven by Cherenkov resonance and might be the most unstable modes for ITER-relevant parameters . The interaction of REs with micro-instabilities results in RE energy losses with formation of a plateau of the distribution function  and pitch-angle scattering, with (limited) energy losses by synchrotron radiation. In this vein, it has been proposed an external coupling of whistler waves to suppress the acceleration of REs . In addition, the detection of the waves driven by REs is important as a possible diagnostic tool. Experimental evidence of RE driven instabilities has been documented since the early ’70 , e.g. by oscillations in the loop voltage, longitudinal magnetic flux and microwave phase shifts signals vs. time. The onset of these instabilities is accompanied by jumps in the perpendicular energy of the electrons as detected by non-thermal electron cyclotron emission (ECE) . However, direct observation of radio-frequency emission in the 100-200 MHz frequency range, interpreted as originated by whistler waves driven by REs, has been only recently achieved for the first time in the DIII-D tokamak . It has been shown, in particular, that the frequencies of the radio emissions are proportional to the magnetic field, as required by the dispersion equation of the whistler waves (the magnetized plasma oscillations do not exhibit such behavior). During the last experimental campaign of the FTU tokamak, a diagnostic system able to detect radio-emission in runaway plasma discharges has been developed. A wideband antenna was installed in front of a glass viewport mounted on a vertical port with perpendicular section 40 cm × 16 cm , which is utilized to transmit the laser beam of the Thomson scattering diagnostic towards a light trap. The relevant cut-off for transmission of electromagnetic signals across the port has been estimated as f_co≅375 MHz. A double-shielded 50 Ohm coaxial cable, with 10 dB/100 m attenuation at 800MHz, is used to transmit the signal detected by the antenna to a fast sampling digitizer. The system has been exploited at a reduced sampling rate capability for detection of radio-emissions, acquiring datasets as large as 625 MS with 0.16 ns or 0.32 ns sampling time, corresponding respectively to 0.1 s or 0.2 s of recorded time intervals. During plasma discharges with REs, high frequency radio emissions, up to few GHz, were observed in the presence of pitch angle scattering instabilities as revealed by ECE, HXR and Cherenkov probe diagnostics. The temporal structure of the radio emissions is not stationary, with intense and relatively short bursts on top of a constant or slowly varying baseline level. Amplitude peaks, as estimated by standard deviation on short time (1.0 μs) segments, are generally coincident with the occurrence of pitch angle scattering instability events. Quite different shapes can be seen in the signals, with either rapid or relatively slow front end and with either monotonic or oscillating tail. Spectral analysis indicates the occurrence of fluctuations extending over a wide range of frequencies as well as isolated line spectra.
A general discussion of the experimental observations will be proposed elsewhere . We focus the present study on the time structure of the amplitude peaks of the radio emissions. These can be analysed with 100 ns time resolution, providing unprecedented information on the time development of the instability. In particular, a bicoherence analysis will be performed to detect system non-linearities by means of second-order statistics. The weak turbulence theory will be then used to estimate the characteristic time scales involved in the non-linear mode coupling for comparison with the observed time scales.
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|Affiliation||ENEA, Fusion and Nuclear Safety Depatment, C. R. Frascati, via E. Fermi 45 00044, Frascati, Roma (Italy)|
|Country or International Organization||Italy|