Ksenia Aleynikova

We report ion cyclotron emission (ICE) in Wendelstein 7-X during discharges heated purely by electron cyclotron resonance heating that lack any fast-ion population, a phenomenon we term thermal population ICE (TPICE). TPICE is detected on the unshielded ICRF antenna strap but not on B-dot probes, indicating a predominantly electrostatic nature. A central finding is that ICE is not exclusively a fast-ion phenomenon: the thermal plasma can excite the same edge eigenmodes that fast ions drive. The evidence strongly suggests that the electrostatic component of fast-ion-induced ICE (FI ICE) and TPICE originate from the same instability, supported by the nearly identical spectra of TPICE and electrostatic FI ICE at spatially separated detectors; matching FI ICE peaks from neutral beam injected sources that are injected in opposite directions relative to the magnetic field; and a discharge where a nearly pure helium plasma produces neither TPICE nor helium FI ICE, yet yields clean hydrogen FI ICE upon a hydrogen NBI blip. Kolesnichenko's model, the only theory developed explicitly for TPICE, predicts an electrostatic instability with a strictly positive frequency detuning of 2–5 MHz above the cyclotron frequency, and an electromagnetic instability with a very small detuning of either sign. Only an electrostatic signal is observed, at a frequency closer to the electromagnetic prediction. This disagreement is attributed to the instability likely originating outside the last closed flux surface, where profile data are unavailable. The model is inapplicable to helium discharges, where the signal peaks at 41 MHz rather than at any predicted cyclotron harmonic.

The influence of magnetic shear on kinetic ballooning modes (KBMs) and electromagnetic turbulence is reported in the Wendelstein 7-X (W7-X) and Heliotron-J (H-J) stellarators. Gyrokinetic simulations at finite normalized plasma pressure β reveal that sub-threshold KBMs (stKBMs) are present in all configurations, i.e. KBMs that are resonantly destabilized far below the iMHD limit. A reduced KBM model shows that boosting magnetic shear and weakening bad curvature stabilizes (st)KBMs in stellarator geometry. In nonlinear simulations, instead of undergoing nonlinear electromagnetic stabilization, turbulent fluxes in W7-X increase with β when stKBMs are destabilized. Lower-magnetic-shear configurations of W7-X are found to produce lower transport, due to the generation of stronger zonal flows. In H-J, turbulent fluxes reduce with β —despite hosting stKBMs—due to a boosted zonal-flow response at higher β . This highlights the importance of accounting for both linear and nonlinear dynamics when aiming to improve reactor performance.

Transient phenomena and their control are of high relevance in magnetic confinement fusion plasmas to guarantee a stable and safe plasma operation. Interpretative simulations can maximize the insights gained from experiments on present machines and predictive simulations can help in the preparation of design, mitigation techniques and operational scenarios for future devices. In this article, we provide an overview of recent advances and novel scientific results obtained with the 3D non-linear hybrid fluid-kinetic code JOREK, covering physics of plasma transients from the core to the scrape-off layer (SOL) both for tokamak and stellarator devices. Substantial progress was made in the physics understanding, model validation with experiments and experiment interpretation, thus, giving confidence for predictions to devices like DTT, ITER and DEMO. The topics addressed comprise a wide range: the edge physics of new operation scenarios and edge localized mode suppression; major disruptions with a focus on runaway electrons and vertical displacement events as well as disruption mitigation by shattered pellet injection; the physics mechanisms and operational limits of the flux pumping regime for sawtooth control; MHD limits of stellarators and work towards incorporating advanced edge/SOL/exhaust dynamics; continuing improvements of the code for more efficient hybrid simulations on conventional and accelerated high performance computing architectures.

A new method is presented for the analysis of coherent mode activity, using tomographic reconstruction of the soft x-ray emissivity of the plasma. The analysis includes the determination of the mode frequency, the poloidal mode number, the radial location, the propagation direction, and the ballooning character. The parameters of a constructed mode are fitted to the line-integrated frequency-filtered soft x-ray signals of the 360 photodiodes installed in the Wendelstein 7-X (W7-X) stellarator. Artificial line-integrated photodiode signals, based on experiment-relevant parameters, are used to explain the workflow of the mode analysis. It is demonstrated that the mode parameters can be successfully reconstructed, even for modes at or near the noise level of the measurements. The method is applied to experimental measurements on W7-X, where strong coherent mode activity has been observed in the frequency space during a phase with a core plasma beta of β0 = 4.8%. The mode analysis indicates an m = 14 ± 2 mode whose characteristics agree with kinetic ballooning modes that can be destabilized at high β.

Steep density gradients generally lead to improved plasma performance in the neoclassically optimized stellarator W7-X. This is evident in the global energy confinement time as well as in the ion temperature and can be explained by a strong reduction of the ion temperature gradient turbulence. Such conditions can experimentally be realized by several methods: injection of cryogenic hydrogen pellets, appropriate combination of neutral beam and electron cyclotron resonance heating and, in some cases, with low power of electron cyclotron resonance heating after preconditioning of the first wall. The duration of the improved phases is determined by the ability to sustain the steep density gradient, by technical limitations of the involved systems and, eventually, by the plasma stability. This paper gives an overview of relevant experimental results and presents example discharges where the improved confinement conditions could be extended to multiple seconds: up to 4 s using neutral beam injection and from 14 to 40 s with steady state pellet injection. In these plasmas the turbulent thermal diffusivity is reduced by a factor of 3 to 4 in a broad radial range, which allows high ion temperatures of up to 3 keV at the densities of about 1.5 • 10 ²⁰ m ^-3 .

We present nonlinear magnetohydrodynamic (MHD) simulations of high-β Wendelstein 7-X plasmas using the stellarator extension of the M3D-C1 code, building on the recent work that shows benign saturation of ideal ballooning modes above the designed β limit in the standard configuration [Zhou et al., Phys. Rev. Lett. 133, 135102 (2024)]. First, we examine the results' sensitivity to the parallel thermal conductivity. It is found that while an increased parallel conductivity reduces the linear growth rate, the saturated pressure profile is barely affected. Second, we consider the dependence on the profile shape. It is shown that an equilibrium with a peaked pressure profile and lower β is subject to more significant change than a broad profile with higher β and a larger growth rate, suggesting that benign saturation, or nonlinear stability, is not guaranteed and not dictated by linear growth. Third, we study the influence of the magnetic configuration, with the equilibrium rotational transform varied by adjusting the planar coil current. With similar growth rates, similar magnitudes of profile change are found regardless of the presence of a low-order resonance, which implies that the saturation mechanism is not specific to a resonant or non-resonant mode. These results indicate that MHD stability should still be treated seriously in stellarator operation and design, for which nonlinear modeling using tools like M3D-C1 can play an instrumental role.

In the Wendelstein 7-X stellarator (W7-X), two related global low-frequency electromagnetic oscillations are observed on many diagnostic systems (e.g. plasma stored energy, Mirnov coils, Langmuir probes, etc). The activity is related to the presence of magnetic islands at certain radii. However, in contrast to initial analyses in earlier works, the mode activity is not localized at the magnetic islands, but rather of an m = 1 type in the plasma outer-core. We observe a strong cross-correlation between electron temperature fluctuations in the confined plasma region, measured with electron cyclotron emission radiometer, and scrape-off layer fluctuations measured with electric probes. Two limiting cases of the mode activity can be observed based on magnetic configuration. A quasi-continuous (QC) mode case, with frequencies of typically some 100 Hz , is found in configurations that have a 5 / 5 magnetic island chain at the last closed flux surface (LCFS). The mode frequency is found to increase linearly with the core electron temperature and decrease with increasing electron density. In contrast, for magnetic configurations with the 5 / 5 magnetic island chain just inside the LCFS, the mode activity becomes bursty and intermittent, exhibiting a crash-event like signature. Although these intermittent events are less frequent, more plasma stored energy is lost per event compared to a period of the QC oscillations. The intermittent oscillations are observable as a broadband modulation in turbulence spectra on most diagnostic systems. The presence of these intermittent bursts goes along with an improved energy confinement in the plasma, superficially resembling H-mode and Edge Localized Modes in tokamaks. A self-limiting process is proposed as an explanation of the intermittent oscillations based on observed ejection of particles during each crash event, which leads to steepening of density profiles and temporary suppression of ion temperature gradient turbulence and in turn suppression of turbulence driven m = 1 mode causing the crash.

Magnetic fluctuations affecting turbulence and transport, which are manifest at finite normalized plasma pressure β, pose a significant challenge to magnetic confinement fusion devices aiming to achieve high performance. Such regimes are not yet comprehensively understood in stellarator geometry. This work presents simulations of electromagnetic instabilities and high-β turbulence in the Wendelstein 7-X (W7-X) stellarator, showing how ion-temperature-gradient-driven (ITG) turbulence is enhanced by unconventional kinetic ballooning modes well below the ideal MHD threshold. These sub-threshold KBMs (stKBMs) become strongly excited in the turbulent state and enable higher fluxes via zonal-flow erosion. The threshold of stKBM impact on turbulent fluxes is heavily dependent on the pressure gradient, evidenced here by the enhanced destabilization and fluxes resulting from the inclusion of an electron temperature gradient. Understanding and controlling these stKBMs will be paramount for W7-X and potentially other stellarators to achieve optimal performance.