We present a novel concept to tackle the power exhaust challenge of a magnetically confined fusion plasma. It relies on the prior establishment of an $X$-point radiator that dissipates a large fraction of the exhaust power before it reaches the divertor targets. Despite the spatial proximity of the magnetic $X$ point to the confinement region, this singularity is far away from the hot fusion plasma in magnetic coordinates and therefore allows the coexistence of a cold and dense plasma with a high potential to radiate. In the compact radiative divertor (CRD) the target plates are placed close to this magnetic $X$ point. We here report on high performance experiments in the ASDEX Upgrade tokamak that indicate the feasibility of this concept. Despite the shallow (projected) field line incidence angles of the order of ${\theta }_{\perp }=0.2°$, no hot spots were observed on the target surface monitored by an IR camera, even at a maximum heating power of $P_{\mathrm{heat}}=15\mathrm{MW}$. And even with the $X$ point located exactly on the target surface and without density or impurity feedback control, the discharge remains stable, the confinement good ($H_{98,y2}=1$), hot spots absent, and the divertor in a detached state. In addition to its technical simplicity, the CRD scales beneficially to reactor-scale plasmas that would benefit from an increased volume of the confined plasma, more space for breeding blankets, smaller poloidal field coil currents, and—potentially—an increased vertical stability.

In this work, we explore the pedestal properties of negative triangularity discharges with upper triangularity of δ u ≈ − 0.35 and with both ion ∇ B drift directions. In all cases, the discharges undergo a transition to H-mode with accompanying edge-localized modes (ELMs) that are not explained by the peeling-ballooning stability analysis alone. A variation in the ion ∇ B drift direction is observed. A lower pressure gradient, shallower radial electric field well and increased temperature fluctuations are measured in the favorable case. In addition, irregular ELMs are present. This difference is more pronounced in electron cyclotron resonance heating (ECRH) plasmas compared to plasmas with combined neutral beam injection and ECRH. A comparative analysis between two discharges, featuring similar plasma parameters but varied shaping, suggests that an interplay between other parameters, such as magnetic shear, the timing of the auxiliary heating and shaping, might play a strong role in low- to high-confinement transitions. While the low shaping discharge maintained L-mode, the high shaping discharge entered H-mode with ELMs, contrary to expectations.

This letter reports on the efforts carried out at the ASDEX Upgrade tokamak to integrate I-mode plasmas with pellet fueling and to increase the I-mode Greenwald fraction f G W , two important requirements for any DEMO operational scenario. For the first time, stationary I-mode plasmas have been achieved with pellet fueling and the core Greenwald fraction has been increased up to 0.8. Larger f G W were not achieved due to technical constraints rather than to a physics-based limit. Pellet-fueled I-mode plasmas exhibit enhanced core and edge density, while core and edge temperature are reduced. Nonetheless, edge normalized gradients remain I-mode-like, namely shallow for the density and steep for the temperature. The I-mode energy confinement time is found to obey two distinct density dependencies: for f G W < 0.4 it rises with increasing f G W , while for f G W > 0.4 the energy confinement time plateaus with increasing f G W , or even decreases with f G W for the pellet-fueled plasmas. Similarities with Ohmic and L-mode energy confinement time dependency on density are discussed.

In the tokamak ASDEX Upgrade, Integrated Data Analysis (IDA) is used to infer plasma quantities, such as electron density, using heterogeneous data sources. Essential is forward modeling from the parameter space into the data space with physically reasonable models for probabilistic evaluation. This paper presents a new forward model for O-mode profile reflectometry, a necessary prerequisite for Bayesian inference and inclusion in IDA. An efficient forward model based on the analytic solution for a piece-wise linear density description allows IDA to overcome problems associated with the established determination of cut-off locations via Abel inversion and Bottollier-Curtet’s method. Instead of using a hard-coded initialization for densities below the first measured cut-off density, other diagnostics, such as the lithium beam, are used to analyze the shape of the initial part of the profile. Error propagation from the measured data, and other uncertain sources, to the uncertainties in the density profile and also its gradient is an intrinsic property of the probabilistic approach, which benefits from the joint analysis. Missing or ambiguous data do not prevent the profile evaluation, but only increase the uncertainty for densities in the affected range. Density profiles together with their uncertainties are determined by the joint analysis of complementary diagnostics, with the newly added reflectometry closing a gap in the outer core region. A stand-alone inversion based on the new forward model, including uncertainty quantification, is introduced, optionally providing an n(R) profile with uncertainties and a gradient. This method is a candidate for real-time analysis, providing error bars.

Recent experiments conducted during the 2025 campaign in ASDEX Upgrade have pushed the limits of negative triangularity shaping, achieving top triangularity values of δ top ∼ − 0.55 while keeping bottom triangularity around 0 in diverted plasmas. Guided by ideal-MHD stability calculations using the linear MHD BALOO solver and supported by TCV results, these experiments have demonstrated a transition to an edge that is more stable against peeling-ballooning modes, leading to ELM-free plasmas. Those ELM-free plasmas also exhibit a dithering behavior similar to limit-cycle oscillations. Linear MHD modeling showed that the achieved shapes remained marginal with respect to the second stability region for ballooning modes suggesting that with minor further shaping, a robust NT edge could be achieved. The associated energy confinement is generally reduced, with clear signs of strong power degradation. In addition, a high fraction of fast-ion energy is observed, particularly in high-power/low density discharges. An approximately 15% improvement in normalized energy confinement is obtained when seeding nitrogen. As this effect comes from the edge, it highlights a path towards pedestal tailoring for further confinement optimization and supporting negative triangularity as a potentially viable reactor-relevant operational scenario in full-metal-wall devices.