Advanced Tokamak Investigations in Full-Tungsten ASDEX Upgrade

The tailoring of the $q$-profile is the foundation of Advanced Tokamak (AT) scenarios. It depends on low collisionality $\nu^*$ which permits efficient external current drive and high amounts of intrinsic bootstrap current. At constant pressure, lowering $n_\mathrm{e}$ leads to a strong decrease of $\nu^* \sim {T_\mathrm{e}}^{-3}$.\\ After the conversion of ASDEX Upgrade to fully W-coated plasma facing components, radiative collapses of H-modes with little gas puffing due to central W accumulation could only be avoided partially with central ECRH. Also, operation at high $\beta$ with low $n_\mathrm{e}$ presented a challenge for the divertor. Together, these issues prevented meaningful AT investigations.\\ To overcome this, several major feats have been accomplished: Access to lower $n_\mathrm{e}$ was achieved through a better understanding of the changes to recycling and pumping, and optionally the density pump-out phenomenon due to RMPs. ECRH capacities were substantially expanded for both heating and current drive, and a solid W divertor capable of withstanding the power loads was installed. A major overhaul improved the reliability of the current profile diagnostics.\\ This contribution will detail the efforts needed to re-access AT scenarios and report on the development of candidate steady state scenarios for ITER/DEMO. Starting from the `hybrid scenario,' a non-inductive scenario ($q_{95}=5.3$, ${\beta_\mathrm{N}}=2.7$, ${f_\mathrm{bs}}>40\%$) was developed. It can be sustained for many $\tau_\mathrm{E}$, limited only by technical boundaries, and is also independent of the ramp-up scenario. The $\beta$-limit is set by ideal modes that convert into NTMs. The $T_\mathrm{i}$-profiles are steeper than predicted by TGLF, but nonlinear electromagnetic gyro-kinetic analyses with GENE including fast particle effects matched the experimental heat fluxes. We will also report on scenarios at higher $q_{95}$, similar to the EAST/DIII-D steady state scenario. The extrapolation of these scenarios to ITER/DEMO will be discussed.