Ondrej Grover

The electron density at the separatrix ( n e , sep ) plays a central role in balancing energy confinement, detachment achievement, and edge-localized mode suppression in tokamaks, thereby influencing core-edge integration. To study what determines this key parameter, a database of H-mode separatrix density measurements from the Alcator C-Mod, ASDEX Upgrade, and JET tokamaks has been assembled using a consistent analysis method across all devices. This dataset is used to assess the validity of a physics-based predictive model and to derive a regression scaling expression for n e , sep , both requiring only engineering parameters as input. The theory-based expression is obtained by coupling two-point model equations with simple geometrical relations, and successfully reproduces experimental measurements across all three devices, with the exception of a common multiplicative constant. The regression confirms similar parameter dependencies, revealing a positive dependence on divertor neutral pressure and the ratio of the power entering the scrape-off layer to the major radius, a negative dependence on the toroidal magnetic field and minor radius, and no significant dependence on the plasma current. Both the resulting scaling and theory-based expressions predict n e , sep within a factor of 1.5 across the three machines, and provide projections to next-step devices (ITER, SPARC, DTT, JT-60SA and COMPASS-U) that are in agreement with available SOLPS simulations.

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.

The transition between the ELMy H-mode and the EDA H-mode is studied on Alcator C-Mod using an experimental database and recently-developed predictive pedestal models. High-resolution Thomson scattering measurements are used to inspect both the separatrix and pedestal operational space using two independent fit functions, each tailored to analyze either the pedestal or the separatrix and near-SOL profiles. The pedestal density, n _e ^ped , and the separatrix density, n _e ^sep are compared to main chamber neutral measurements. n _e ^ped is sensitive to neutral sources only in the ELMy H-mode regime and not in the EDA H-mode regime. Density fluctuation spectra reveal that quasi-coherent structures become stronger at higher densities and more coherent in the EDA relative to the inter-ELM phases of ELMy H-modes, before weakening again at the highest values of n _e ^ped . The Saarelma-Connor pedestal density prediction model is validated for ELMy H-modes up to n _e ^ped = 2.0×10 ²⁰ m ^-3 . An additional transport channel driven by resistive ballooning modes (RBM), D _RBM , scaling directly with α _t , a collisional turbulence control parameter, and inversely with k _RBM ² q _cyl , the product of the square of the characteristic RBM turbulence wavenumber and the cylindrical safety factor, is shown to improve the prediction for EDA H-modes, finding good model agreement up to n _e ^ped = 3.0×10 ²⁰ m ^-3 . For comparison, EPED scans in n _e ^ped are then performed at three values of n _e ^sep /n _e ^ped . Increasing this ratio moves the peeling-ballooning branch transition to lower n _e ^ped , increasing n ^ped in the peeling branch and decreasing it in the ballooning branch. Agreement is found for large ELM H-modes. SPARC pedestal density predictions for an ELMy and an EDA/QCE-like H-mode are performed using the extension to the standalone Saarelma-Connor model and are found consistent with earlier assumptions used in EPED modeling. Inclusion of D _RBM significantly weakens the density gradient near the separatrix, lowering n _e ^ped by approximately 20%.

The origin of the difference in the high-confinement ( H -mode) power threshold between favorable and unfavorable drift configurations in tokamaks—experimentally linked to a deeper radial electric field ( E r ) well in the former—remains unresolved. Using first-principles gyrokinetic simulations of edge and scrape-off-layer turbulence in the ASDEX Upgrade tokamak, we show that turbulence-driven poloidal flows generate this deeper E r well in the favorable configuration through enhanced nonlinear turbulence-mean flow energy transfer. This transfer is significantly weaker in the unfavorable case, yielding a shallower E r well, while turbulence intensity is simultaneously higher. Within the turbulence-flow shear suppression paradigm, the combination of stronger shear and reduced turbulence facilitates H -mode access in the favorable configuration. These results provide the first validated, self-consistent full- f gyrokinetic explanation of how drift configuration controls the nonlinear dynamics of profiles, E r , flows, and turbulence, thereby setting the H -mode power threshold.

The separatrix operational space (SepOS) model (Eich and Manz 2021 Nucl. Fusion 61 086017) is shown to predict the L–H transition, the L-mode density limit, and the ideal magnetohydrodynamic ballooning limit in terms of separatrix parameters for a wide range of Alcator C-Mod plasmas. The model is tested using Thomson scattering measurements across a wide range of operating conditions on C-Mod, spanning n ― e = 0.3–5.5 × 10 20 m⁻³, B t = 2.5 –8.0 T, and B p = 0.1–1.2 T. An empirical regression for the electron pressure gradient scale length, λ p e , against a turbulence control parameter, α t , and the poloidal fluid gyroradius, ρ s , p , is constructed for H-modes and found to require positive exponents for both regression parameters, indicating turbulence widening of near-scrape-off layer widths at high α t and an inverse scaling with B p , consistent with results on ASDEX Upgrade. The SepOS model is also tested in the unfavorable drift direction and found to apply well to all three boundaries, including the L–H transition as long as a correction to the Reynolds energy transfer term, α RS < 1 is applied. I-modes typically exist in the unfavorable drift direction for values of α t ≲ 0.3 . Finally, an experiment studying the transition between the Type-I ELMy and EDA H-mode is analyzed using the same framework. It is found that a recently identified boundary at α t = 0.55 excludes most EDA H-modes but that the balance of wavenumbers responsible for the L-mode density limit, namely k EM = k RBM , may better describe the transition on C-Mod. The ensemble of boundaries validated and explored is then applied to project regime access and limit avoidance for the SPARC primary reference discharge parameters.

Understanding the transport processes that determine the plasma profile widths in the scrape-off layer (SOL) and divertor region of tokamaks is crucial for successful power and particle exhaust management in future devices. Plasma transport from the SOL into the private flux region (PFR) broadens the profiles and could mitigate the power exhaust challenge. Analysis of ion current profiles, measured by Langmuir probes in the ASDEX upgrade (AUG) tokamak, shows that the ion current width in the PFR, normalized by the flux expansion between outer target and midplane, is about 1.2 mm in L-mode and 0.9 mm in inter-ELM H-mode plasmas. The measured widths agree within a factor of two with predictions from an analytical model, based on Pfirsch-Schlüter flows. According to the model, the ion PFR width increases with the distance ΔR between the outer target and X-point major radii, and scales inversely with the poloidal magnetic field B p . For the tokamaks ITER and SPARC the model predicts PFR widths below 0.3 mm.

The plasma electron density is measured with multiple diagnostics in the scrape-off layer of the ASDEX Upgrade tokamak. The recorded values are typically in good agreement in the case of exponentially decaying profiles at low densities, but they become inconsistent as soon as a density shoulder is formed, and up to a factor of 3 deviation is experienced at high densities. An idea is put forward; namely, that the local plasma-wall proximity has an influence on the local density and thus the measurement results depend on the local limiter-plasma and limiter-diagnostic distances at the same time. The effect of local wall clearance dependence is demonstrated by creating a plasma having an extremely high scrape-off layer density ( n e > 3 ⋅ 10 19 m − 3 ) after the H-L back transition and varying the position of the separatrix in different directions. As a possible physical process, neutral recycling at the plasma facing components is identified. Similar behaviour is also observed in the QCE regime, where the appearance of small ELMs barely impacts the scrape-off layer profiles. There are also indications that the local wall clearance does have an impact on the density profile not only in the scrape-off layer, but also in the confined region.

The highly radiating nitrogen-seeded H-mode plasmas in unfavorable B_T has been characterized in the ASDEX Upgrade tokamak (AUG). Three levels of nitrogen puffing rates have been injected into a fully detached H-mode plasma, which is run in lower single null configuration with the ion B × ∇ B drift away from the X-point. A cold ( ∼ 1 − 2 eV) highly radiating ( ∼ 13.0 MW m − 3 ) region forms close to the X-point immediately after nitrogen seeding, as evidenced by measurements of the divertor Thomson scattering (DTS) and the two-dimensional bolometry reconstructions. In addition, the radiator moves further upwards above the X-point along the separatrix at the high-field side (HFS) with increasing nitrogen puffing rates, as evidenced by the Absolute eXtended UltraViolet (AXUV) measurements. The formation of the highly radiating regime is closely correlated to the modifications of the divertor plasma conditions. Along the line of sight of the DTS measurement, the electron temperature reduces down to a few eV, which initials near the X-point and further extends to the HFS scrape-off layer (SOL) simultaneously with the upward movement of the radiator, however, the electron temperature sustains ( ∼ 30 − 50 eV) at the low-field side (LFS) SOL with slightly decreased electron density. The highly radiating regime shows LFS/HFS divertor asymmetry, in contrast to that for H-mode plasmas in favorable B_T , suggesting that the drifts play an important role for the formation of the highly radiating X-point regime at AUG. The neutral particle flux increases significantly (factor of ∼ 2 ) in the private flux region, while it increases slightly ( < 20 % ) in the main chamber, thus suggesting an enhanced sub-divertor neutral compression with the formation of the highly radiating regime. Furthermore, a degradation of the pedestal electron density was observed with an enhancement of the electron temperature further inside the pedestal, and complete divertor detachment was achieved by nitrogen seeding with sustained plasma confinement. Finally, particle sources and flow patterns of deuterium and nitrogen ions have been analyzed by SOLPS-ITER modelling, confirming that the E × B drift plays an important role for the formation of the highly radiating regime in unfavorable B_T at AUG.

This work presents the first analysis of the disruptive locked mode (LM) triggered by the dynamics of a confinement change. It shows that, under certain conditions, the LM threshold during the transient is significantly lower than expected from steady states. We investigate the sensitivity to a controlled n = 1 error field (EF) activated prior to the L-H transition in the COMPASS tokamak, at q ₉₅ ∼ 3, β _N ∼ 1, and using EF coils on the high-field side of the vessel. A threshold for EF penetration subsequent to the L-H transition is identified, which shows no significant trend with density or applied torque, and is an apparent consequence of the reduced intrinsic rotation of the 2/1 mode during this transient phase. This finding challenges the assumption made in theoretical and empirical works that natural mode rotation can be predicted by global plasma parameters and urges against using any parametric EF penetration scaling derived from steady-state experiments to define the EF correction strategy in the entire discharge. Furthermore, even at EFs below the identified penetration threshold, disruptive locking of sawtooth-seeded 2/1 tearing modes is observed after about 30% of L-H transitions without external torque.