Impact of ICRF on the scrape-off layer and on plasma wall interactions: From present experiments to fusion reactor

Recent achievements in studies of the effects of ICRF (Ion Cyclotron Range of Frequencies) power on the SOL (Scrape-Off Layer) and PWI (Plasma Wall Interactions) in ASDEX Upgrade (AUG), Alcator C-Mod, and JET-ILW are reviewed. Capabilities to diagnose and model the effect of DC biasing and associated impurity production at active antennas and on magnetic field connections to antennas are described. The experiments show that ICRF near-fields can lead not only to E x B convection, but also to modifications of the SOL density, which for Alcator C-Mod are limited to a narrow region near antenna. On the other hand, the SOL density distribution along with impurity sources can be tailored using local gas injection in AUG and JET-ILW with a positive effect on reduction of impurity sources. The technique of RF image current cancellation at antenna limiters was successfully applied in AUG using the 3-strap AUG antenna and extended to the 4-strap Alcator C-Mod field-aligned antenna. Multiple observations confirmed the reduction of the impact of ICRF on the SOL and on total impurity production when the ratio of the power of the central straps to the total antenna power is in the range 0.6 < P-cen / P-total < 0.8. Near-field calculations indicate that this fairly robust technique can be applied to the ITER ICRF antenna, enabling the mode of operation with reduced PWI. On the contrary, for the A2 antenna in JET-ILW the technique is hindered by RF sheaths excited at the antenna septum. Thus, in order to reduce the effect of ICRF power on PWI in a future fusion reactor, the antenna design has to be optimized along with design of plasmafacing components.

Study of plasma fluctuations in new closed flux-surface configurations of the TORPEX experiment

Fabio Avino, Ambrogio Fasoli, Ivo Furno, Sébastien Jolliet, Joaquim Loizu Cisquella, Annamaria Mosetto, Paolo Ricci

The TORoidal Plasma EXperiment (TORPEX) is a Simple Magnetized Torus (SMT) where open helical magnetic field lines are obtained from the superposition of a small vertical component on the main toroidal field. The SMT configuration features the main driving mechanisms of plasma instabilities characterising the Scrape-Off Layer (SOL) of larger tokamak devices, making TORPEX a very attractive machine to perform fusion oriented studies of plasma turbulence. A new experimental set-up has been recently developed to generate a poloidal magnetic field. It consists in a toroidal copper wire suspended in the middle of the vacuum vessel by seven 1mm stainless steel wires, three vertical and four lateral, plus the vertical electrical coaxial feed-through. A current up to 1kA can be driven in the system, giving us the possibility to include a rotational transform and to deal with a SOL much closer to the one of a fusion device. Moreover plasma core and closed-to-open field line region studies could be performed. The technical details of the set-up will be given together with the experimental results obtained during the first campaigns with Hydrogen plasmas. A magnetic configuration with almost closed flux surfaces has been chosen, resulting from the poloidal magnetic field together with a small vertical component. The radial profiles of the main plasma parameters will be presented, such as plasma density n_e, plasma temperature T_e and plasma potential V_pl. The spectral properties of plasma dominant coherent modes at the position of maxima fluctuations will be provided too, including power spectral density estimates and measurements of vertical and toroidal wave-numbers obtained for different values of poloidal magnetic field. This allows us to calculate the poloidal and toroidal mode-numbers which can be used together with fluid simulations to asses the kind of instability we are dealing with. An overview of the new accessible magnetic field configurations will be showed as well, ranging from Single Null X-points to magnetic Snowflakes.

2013

Model-based electron density estimation using multiple diagnostics on TCV

Federico Alberto Alfredo Felici, Cristian Galperti, Francesco Pastore, Olivier Sauter, Anna Ngoc Minh Trang Vu

Estimation of the dynamic evolution of electron plasma density during a tokamak discharge is cru- cial since it directly affects the plasma performance, confinement and stability, therefore it needs to be monitored and controlled. Knowledge of the density profile can also be used to control in a more direct way the desired aspects of the plasma density, for example choosing to control the core, volume averaged or edge density, replacing control methods that rely e.g. on a single line-averaged electron density from a specific interferometer chord. The reconstruction of the density profile can be performed with the RAPDENS code [1], employing the Extended Kalman Filter technique. The code collects the electron plasma density measurements from the available real-time diagnostics and uses them to constrain the solution of a predictive model that describes the 1D particle transport equation for the electron plasma density. Following recent improvements to the code for use on ASDEX-Upgrade [2], we report on the applica- tion of this method for routine reconstruction of density profiles in the TCV tokamak. In particular the transport coefficients of the electron diffusion equation, as well as the ionization depth of the fueling gas, have been tuned heuristically with the available off-line diagnostics data for higher fi- delity reconstruction of density profiles in L-mode, H-mode and negative triangularity discharges in TCV. We also present an extensive comparison of real-time estimated density profiles with the off-line Thomson scattering measurements across a wide range of experimental conditions. Finally, we report the first results on the upgraded detection and correction of interferometer errors with a model-based approach relying on the observer. References: [1] T. C. Blanken et al. “Control-oriented modeling of the plasma particle density in tokamaks and application to real-time density profile reconstruction”. In: Fusion Engineering and Design 126 (2018). issn: 09203796. doi: 10.1016/j.fusengdes.2017.11.006. [2] T.O.S.J. Bosman et al. “Kalman filter density reconstruction in ICRH discharges on ASDEX Upgrade”. In: Fusion Engineering and Design 170 (Sept. 2021). issn: 09203796. doi: 10.1016/ j.fusengdes.2021.112510.1

2022

Analysis and modelling of momentum transport based on NBI modulation experiments at ASDEX Upgrade

Basil Duval, Emiliano Fable, Giovanni Tardini

The prediction of plasma rotation is of high interest for fusion research due to the effects of the rotation upon magnetohydrodynamic (MHD) instabilities, impurities, and turbulent transport in general. In this work, an analysis method was studied and validated to reliably extract momentum transport coefficients from neutral beam injection (NBI) modulation experiments. To this end, a set of discharges was created with similar background profiles for the ion and electron temperatures, the heat fluxes, the electron density, and the plasma rotation that, therefore, should exhibit similar momentum transport coefficients. In these discharges, a range of temporal perturbations were imposed by modulating and varying the power deposition of the NBI, electron-cyclotron-resonance heating (ECRH), and ion-cyclotron-resonance heating (ICRH). The transport model including diffusion, convection, and residual stress was implemented within the ASTRA code. The Prandtl number Pr = chi(phi)/chi(i) was assessed via the GKW code. A convective Coriolis pinch was fitted and the intrinsic torque from the residual stress was estimated. The obtained transport coefficients agree within error bars for sufficiently small imposed temperature perturbations, as would be expected, from the similar background profiles. This successful validation of the methodology opens the door to study the parametric dependence of the diffusive and convective momentum transport of the main ions of the plasma as well as the turbulent intrinsic torque in a future work.