Healing plasma current ramp-up by Nitrogen seeding in the full Tungsten environment of WEST

Achieving a successful plasma current ramp-up in a full Tungsten tokamak can be challenging due to the large core radiation (and resulting low core temperature) that can be faced with this heavy metallic impurity if its relative concentration is too high. Nitrogen injection during the plasma current ramp-up of WEST discharges greatly improves core temperature and Magneto-Hydro-Dynamic stability. Experimental measurements and integrated simulations with the RAPTOR code complemented with the Qualikiz Neural Network for computing turbulent transport allow a detailed understanding of the mechanisms at play. Increased edge radiation during this transient phase is shown to improve confinement properties, driving higher core temperature and better MHD stability. This also leads to increased operation margins with respect to Tungsten contamination.

Beta-induced Alfvén eigenmodes and geodesic acoustic modes in presence of strong tearing activity during the current ramp-down on JET

The analysis of the current ramp-down phase of JET plasmas has revealed the occurrence of additional magnetic oscillations in pulses characterized by large magnetic islands. The frequencies of these oscillations range from 5 kHz to 20 kHz, being well below the toroidal gap in the Alfven continuum and of the same order of the low-frequency gap opened by plasma compressibility. The additional oscillations only appear when the magnetic island width exceeds a critical threshold, suggesting that the oscillations could tap their energy from the tearing mode (TM) by a non-linear coupling mechanism. A possible role of fast ions in the excitation process can be excluded, being the pulse phase considered characterized by very low additional heating. The calculation of the coupled Alfven-acoustic continuum in toroidal geometry suggests the possibility of beta-induced Alfven eigenmodes (BAE) rather than beta-induced Alfven acoustic eigenmodes (BAAE). As a main novelty compared to previous works, the analysis of the electron temperature profiles from electron cyclotron emission has shown the simultaneous presence of magnetic islands on different rational surfaces in pulses with multiple magnetic oscillations in the low-frequency gap of the Alfven continuum. This observation supports the hypothesis of different BAE with toroidal mode number n = 1 associated with different magnetic islands. As another novelty, the observation of magnetic oscillations with n = 2 in the BAE range is reported for the first time in this work. Some pulses, characterized by slowly rotating tearing modes, exhibit additional oscillations with n = 0, likely associated with geodesic acoustic modes (GAM), and a cross-spectral bicoherence analysis has confirmed a non-linear interaction among TM, BAE and GAM, with the novelty of the observation of multiple triplets (twin BAEs plus GAM), due to the simultaneous presence of several magnetic islands in the plasma.

Micro-features of ambipolar snapback behaviour under high current injection to design capacitorless memory device

This paper presents micro-features of capacitorless memory cells based on snapback phenomenon and modeling of space-charges. 2—Dimensional gate grounded NMOS structure is specified and its operational window of the memory cell is inspected using the Synopsys TCAD tool. This work examines snapback behaviour in one transistor DRAM memory cell in the absence of a storage capacitor under zero gate bias and applied ramp of high current at the drain terminal. Carrier electrostatics and memory cell mechanisms are also explored by adjusting the slope of the high current ramp. The process variation is examined for different parameters in the device. The current crowding phenomenon due to the injection of electrons and holes is investigated, giving rise to ambipolar behaviour. Due to the snapback, redistribution of electron and hole current is investigated. This work also evaluates the impact on electrostatic potential along channel and bulk under the snapback. It explains the dependency of snapback on potential build-up. Post-snapback electron current flipping presents the flow line near the gate region. The bipolar activity is manifested in surface and bulk regions to show its impact through analytics. The effect of gate biasing is also examined under the applied current ramp.