EX/P8-12 Fishbone-like internal kink instability driven by supra-thermal electrons on FTU generated by lower hybrid radiofrequency power

The fishbone-like internal kink instability driven by supra-thermal electrons generated by lower hybrid current drive (LHCD) is of strong interest for the burning plasma research, as the trapped particle averaged bounce characterising the interaction of trapped alpha particles with low frequency magnetohydrodinamic (MHD) modes in burning plasmas depends on energy, not on mass. The charged fusion product effects can be usefully modelled by the analogous effect induced by the fast electrons on the low frequency MHD modes. Fishbone-like internal kink instabilities driven by electrons were observed during experiment on FTU (Frascati Tokamak Upgrade) aimed at producing internal transport barrier by optimising the q-profile evolution by LHCD power (1.7 MW). By the available theory, the phenomenon was interpreted in terms of an oscillating “fixed point” activity followed by one of “limit cycle”, produced by suprathermal electrons in presence of a q-profile with qmin≈1. More recent experiments of FTU have been performed for fully assessing the behaviour of the electron fishbone, by utilising a more controlled qprofile evolution obtained by: i) useful plasma-wall conditions and LHCD power waveforms, ii) robust qprofile evolution modelling, iii) high time resolution data of the fast electron Bremsstrahlung (FEB) camera, and MHD supported by soft X-ray tomography. The preliminary results are very interesting: during typical fixed-point oscillations, a spatial redistribution of the fast electron population occurs at the same radial position of fishbones, with the same characteristic time (0.1 ms). Introduction The interaction of trapped alpha particles with low frequency magnetohydrodinamic (MHD) modes in burning plasmas is characterised by small dimensionless orbits similar to electrons, so that the trapped particle averaged bounce depends on energy, not on mass. Consequently, the fishbone-like internal kink instabilities driven by supra-thermal electrons generated by lower hybrid current drive (LHCD) and electron cyclotron resonant heating (ECRH) is of strong interest for the burning plasma research. The charged fusion product effects can be usefully modelled by the analogous effect induced by the fast electrons on the low frequency MHD modes [1]. Fishbone-like internal kink instabilities driven by electrons were observed for the first time on Doublet III-D during ECRH experiments, and attributed to barely trapped supra-thermal electrons characterised by drift reversal [2]. The relevant physics of this instability is based on modes propagating in the ion diamagnetic direction that can be destabilized in presence of an inverted spatial gradient of the supra-thermal electrons. During experiment on FTU (Frascati Tokamak Upgrade) aimed at producing internal transport barrier by optimising the q-profile evolution by LHCD power (1.7 MW), a typical electron fishbone-like activity was observed, which was interpreted by theory by hypothesizing a spatial redistribution of the suprathermal population occurring in presence of a suitable evolution of the plasma current density profile [1]. The modelling indicated that the level of LH power input would control the transition from nearly steady state to bursting electron fishbone oscillations, i.e, quasi steady state non linear oscillations (fixed point) are followed by regular bursting (consistent with the limit cycle). During the electron fishbone activity, a significant redistribution of the fast electron population is expected to occur, analogous to the fast ion losses expected when ion fishbones are excited. In Toresupra, the data of hard-X ray diagnostic (60-80 keV range) provided some indication of some spatial redistribution of the suprathermal electron population [3]. However, the time behaviour of this phenomenon with respect to the MHD time scale (0.1 ms) could not be assessed due to the insufficient time resolution of the diagnostic (16 ms). In order to achieve a fully clear picture of the electron fishbone phenomenology, dedicated experiments have been recently planned and performed on FTU, by utilising a high performance hard-X ray diagnostic based on the fast electron Bremsstrahlung (FEB) camera, which has 4 microseconds of time resolution. It is useful for exploring with the necessary detail the linked behaviour of the suprathermal electrons produced by the LH power and the relevant MHD activity. The MHD signal has been analysed with the support of a new tool based on the soft Xray tomography, which provides also information on the q-profile evolution [4]. In addition, a modelling of the q-profile evolution (described in the next paragraph) based on the kinetic and magnetic data of the experiment, has been produced for addressing the operations of experiment necessary for fishbone destabilisation, as indicated in the theory in Ref. 1. As preliminary condition, the fishbone-like modes should occur in presence of the q-profile approaching the condition qmin ≈ 1, and a sufficient population of fast electrons obtained by LHCD. For an accurate search of the fishbone features (e.g., threshold conditions, typical timing with respect to the q-profile evolution, etc.), a LH power (up to 0.5 MW) lower than that utilised in the aforementioned previous experiments of FTU has been performed. In order to produce the necessary q-profile in the experiment, the LH power is applied before the onset time of the sawtooth, so that the condition of qmin ≈ 1 is satisfied. The sawtooth onset time point is individuated by a companion discharge performed with same parameters but without LH. Standard FTU operating conditions (BT=5.3 T, IP=0.5MA, = 0.5 10 m, R = 0.93 m, a = 0.33 m) have been considered. In order to detect with the necessary precision the radiofrequency power threshold and the relevant timing of the q-profile evolution, a relatively slow evolving qprofile should be produced. This condition is obtained, with the help of a modelling of the qprofile evolution, by applying a LH power waveform starting with a relatively low power (0.2 MW). In addition, in the very early phase of discharge, when the LH power cannot be applied due to the still too high loop voltage, the useful slow current relaxation is still guaranteed by utilising the low recycling/high peripheral electron temperature operation, which is provided by the Lithizated vessel facility available in FTU (see below) [8]. As new results of the FTU experiment, i) a q-profile evolution has been performed, useful for fishbone occurrence, as indicated by the available theory, ii) typical fishbone activity has been observed with 0.5 MW of LH power, consisting in quasi steady state non linear oscillations (fishbone fixed point phase), iii) a spatial redistribution of the fast electron population, occurring with the same characteristic time of the MHD signal (0.1 ms), and in phase with it, is obtained by the analysis based on the available FEB camera data. Experiment parameters and q-profile evolution The Figure 1 shows the time evolution of the main plasma parameters of the reference experiment. The LH power (Fig 1 d) switch-on is performed 0.14 seconds after the end of the plasma current ramp-up (Fig 1a). The LH power waveform consists of three steps (respectively of 0.2 MW at 0.30 s, the second of 0.5 MW from t = 0.36 s and t = 0.80 s, the third again of 0.2 MW from t = 0.80 s to t = 0.85 s) with the aim of maintaining the condition qmin € > ̃ 1necessary for fishbone