Session NP15: Poster Session: Magnetic Confinement: Conventional Tokamaks (9:30am - 12:30pm)

Electron temperature fluctuation measurements in the I-mode pedestal at ASDEX Upgrade

I-mode is a naturally ELM-free improved confinement regime, which exhibits high energy confinement without high particle confinement. The role of edge turbulence in heat and particle transport and the role of the Weakly Coherent Mode (WCM) in decoupling these transport channels are open questions. Measurements of electron temperature fluctuations can be obtained using a Correlation Electron Cyclotron Emission (CECE) diagnostic, which measures long wavelength electron temperature fluctuation amplitudes, spectra, and correlation lengths. This work presents edge and pedestal ($\rho_{\mathrm{pol}}=$0.9-1.0) temperature fluctuation measurements in L-mode and I-mode at ASDEX Upgrade from a 24-channel CECE radial comb. In this work, edge modes are localized and the structure of broadband turbulence is compared between the different confinement regimes. In addition to CECE measurements, this work presents linear stability analysis using the gyrofluid code TGLF to characterize the instabilities present and their dependence on radius and confinement regime.   

Changes in Electron Temperature Fluctuations and Transport with Isotropic Mass in L-mode Plasmas at ASDEX-Upgrade

Recent experiments at ASDEX Upgrade (AUG) were performed to study the differences in turbulent transport between plasmas with varying ion masses, hydrogen and deuterium. These measurements are the first step in a rigorous gyrokinetic model validation effort, actively underway at AUG, to understand the effect of ion mass on turbulence and transport in the core of tokamaks. A 24-filter radial comb Correlation Electron Cyclotron Emission (CECE) has been recently upgraded with a new antenna that allows a beam radius of $\sim$1.5cm at the resonance, enabling $\delta$Te measurements with k$_\perp<$2cm$^{-1}$ ($k_\perp\rho_s<0.36$) resolution between $\rho_{Tor}$=0.65-0.8. Fluctuation measurements have been performed in hydrogen and deuterium ECH-heated L-mode plasmas (B$_T$=2.37T, I$_P$=1MA, 600kW ECH, $\bar{n_e}$=2.5e19 m$^{-3}$). The n$_e$, T$_e$, T$_i$, and v$_{Tor}$ profiles are well matched between the discharges, within error. The shape of the $\delta$Te/Te fluctuation spectra differ significantly, and the total fluctuation level (integrated between 0-100kHz) shows lower fluctuation levels in hydrogen compared to deuterium, in contrast to energy confinement scaling expectations. $\delta$Te correlation lengths and linear stability analysis will also be presented.   

Improved vertical control by optimal actuator selection on TCV

High performance elongated plasmas are prone to the vertical instability which requires feedback control for stable operation, a task routinely performed in tokamaks but not optimized for the TCV case, where only a subset of the available poloidal field coils is used for this purpose. In this work a new general algorithm for synthesizing an optimized vertical controller is presented and its application on TCV is discussed. The design is based on a linearized model for the tokamak plasma-vessel-coils electromagnetic dynamics, which is used to determine the optimal linear combination of control currents mostly coupled with the unstable vertical mode of the system. In this way, it is possible to assign all relevant coils to vertical stabilization, while the remaining directions can be used for position and shape control. This approach, combined with structured H-infinity synthesis, determines the best trade-off between stability margin and input request for stabilization, effectively removing the need for shot-by-shot tuning. Dedicated TCV experiments confirm the efficacy of the new controller with a lower voltage requirement for stabilizing the same plasma, thus reducing the risk of power supply saturation and the consequent loss of vertical control.   

Stability of Ohmic Discharges with Hollow Current Profiles in the HL-2A Tokamak

New HL-2A Tokamak discharge data shows that equilibrium configurations with hollow current density profiles improve plasma confinement and stability. Stability of the ohmic discharges with hollow current density profiles on the HL-2A tokamak was analyzed and show improved confinement over for a range of plasma poloidal beta $\beta_p$, the axial and edge safety factors $q_0$ and $q_s$, as well as the minimum safety factor $q_{min}$. The results determine the restriction to the parameters of discharges with hollow current density profiles that is stricter than that of conventional discharges with peaked current density profiles. The hollow current profile plasma are more stable that the monotonic decreasing current profiles when $\beta_p$ is higher, indicating that the discharges with hollow current density profiles can achieve higher $\beta_p$ operations. There are restrictions, however, that both $q_0\gtrsim2$ and $q_s-q_0\gtrsim0.7$ are must hold together with $q_0-q_{min}\gtrsim0.7$, for the stronger negative central shear to become a strong stabilizing factor. We use simulations together with 3D simulations to confirm the important role of the discharges with hollow toroidal current densities to achieve advanced tokamak (AT) operations.