Dirk Hartmann

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.

A microwave reflectometer has been integrated into the ion cyclotron resonance heating (ICRH) antenna system of the Wendelstein 7–X (W7-X) stellarator to measure electron density profiles in the edge region of the plasma. The diagnostic operates in X-mode and is specifically designed to probe the plasma directly in front of the ICRH antenna, where local density conditions critically influence radio-frequency power coupling. Dedicated calibration procedures were developed to improve the data collection and analysis. Electron density profiles, reconstructed from the corrected beat frequencies, show good agreement with measurements from Thomson scattering and Alkali beam emission spectroscopy, validating the reflectometry measurement. The edge density profiles can be measured when ICRH power is injected and exhibit the expected response to global density ramp-up experiments. These results demonstrate the capability of the microwave reflectometer to provide reliable edge density measurements and establish a basis for future studies of ICRH plasmas in W7-X.

The Wendelstein 7-X stellarator has completed two consecutive experimental campaigns (OP 2.2 and OP 2.3) under a new operational strategy enabling more than one year of uninterrupted device availability. This approach, supported by exceptionally high subsystem reliability, allowed sustained high- efficiency plasma operations with up to 80–100 discharges per day across a broad range of magnetic configurations. Several key technical upgrades—most notably the first operation of a 1.5 MW class steady-state gyrotron, a new steady-state pellet injector, and advanced real-time feedback control systems significantly enhanced heating, fueling, and plasma control capabilities. Together, these improvements enabled major advances in long-pulse performance, high-β operation, and confinement optimization. Long-pulse discharges achieved 1.8 GJ of injected energy under fully detached divertor conditions, while reduced-field scenarios facilitated record volume-averaged β values approaching 3%. High- performance plasmas with centrally peaked density profiles, created via NBI or sustained pellet fueling, demonstrated strongly reduced turbulent transport and stellarator-record fusion triple products. Complementary studies of power exhaust and divertor heat loads revealed the role of scrape-off-layer drift physics in shaping strike-line patterns under attached conditions. Together, the results from OP 2.2 and OP 2.3 significantly expand the operational space of W7-X and strengthen its role as a leading platform for steady-state stellarator research and reactor-relevant plasma scenarios.

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 .

Plasma production in stellarators with ion cyclotron heating is complementary to the widely used electron cyclotron heating (ECRH). The prospective light ion minority scenario developed and tried on Uragan-2M and Large Helical Device (LHD) stellarators had been reexamined at LHD. The distinctive feature of this attempt is that the parameters of plasma produced are better than before and are comparable with those of plasma produced by the ECRH. This new possibility of ion cyclotron heating, if it becomes practical, motivates rethinking of the role of ion cyclotron heating in stellarator machines.

A spontaneous reduction in anomalous particle transport in the plasma core is seen experimentally in reproducible, purely neutral beam heated plasma phases at Wendelstein 7-X (W7-X). Heating and fueling the plasma exclusively with the neutral beam injection system for several seconds leads to continuously peaking plasma density profiles with strong gradients inside mid minor radius. A significant acceleration of the density peaking occurs after a certain onset time and is examined with a detailed particle transport analysis in several discharges. By invoking the particle continuity equation, the total experimental radial electron flux is deduced from the time evolution of the electron density profile and the radially resolved particle sources. Subtracting the modeled neoclassical particle flux contribution gives the anomalous particle flux. Exploiting the evolving plasma conditions, anomalous diffusion and convection coefficients are computed from the flux variation with density and density gradients. In several discharges a significant and consistent change of the anomalous transport coefficients is seen when crossing a specific normalized density gradient length.

In the Wendelstein 7-X (W7-X) stellarator, turbulence is the dominant transport mechanism in most discharges. This leads to a ‘clamping’ of ion temperature over a wide range of heating power, predominantly flat density profiles where hollow profiles driven by neoclassical thermo-diffusion would be expected and by rapid impurity transport in injection experiments. Significantly reduced turbulent transport is observed in the presence of strong core density gradients found transiently after core pellet injection and irregularly after boronisation or boron pellet injection. Density peaking is also achieved in a controlled manner in purely neutral beam heated discharges where particle transport analysis reveals an abrupt reduction in the main-ion particle flux leading to significant density profile peaking not explained by the NBI particle source alone. The plasmas exhibit a heat diffusivity of around χ = 0.25 ± 0.1 m 2   s − 1 at mid radius, a factor of around 4 lower than ECRH dominated discharges. Despite the improved confinement, the achieved ion temperature is limited by broader heat deposition and the lower power-per-particle given the higher density. This is overcome with limited reintroduction of ECRH power, where the low heat diffusivity diffusivity is maintained, the density rise supressed and ion temperatures above the clamping limit are achieved. The applicability of these plasmas for a high performance scenario on transport relevant time scales is assessed, including initial predictions for planned heating upgrades of W7-X, based on a range of assumptions about particle transport.

A new neutral beamline (OPTEMIST) uniquely capable of exploring the predicted improvement of fast ion confinement in Wendelstein 7-X (W7-X), which comes with increasing plasma beta, is proposed. As the plasma beta increases in the W7-X device, the high mirror magnetic configuration has drift orbits that begin to close, enhancing the confinement of the deeply trapped particles. The existing neutral beam system is found to produce particle populations that do not adequately probe the deeply trapped orbits. Fast tritons generated by thermal deuterium–deuterium fusion reactions are found to probe the necessary conditions for demonstrating this effect. However, it is found that diagnostically measuring this effect will be difficult. A scoping study of a neutral beamline that directly populates the trapped orbits is performed. It is found that a monoenergetic population of 120 kV injected protons provides the largest confinement enhancement in the fast ion population as the plasma beta is increased. The necessity to raise plasma density to increase plasma beta results in blinding of spectroscopic beam measurements by bremsstrahlung. An array of novel fast ion loss detectors that would adequately assess the confinement of these particles is proposed.

The effect of magnetic islands in the core region of Wendelstein 7-X (W7-X) on fast ion confinement is explored through simulations with the BEAMS3D code. A magnetic configuration where the n / m = 5 / 5 island chain is shifted to r / a ∼ 0.7 allows the exploration of core island physics in W7-X. The control coil system on W7-X allows the tuning of the island size either increasing the island width or decreasing it. A coupling of the BEAMS3D code to the FIELDLINES code provides a versatile mechanism for incorporating magnetic islands and stochastic regions into the BEAMS3D code. Collisionless simulations suggest that the presence of core islands degrade the confinement of passing particles in the region of the island chain. Full neutral beam simulations of W7-X show a similar behavior with confinement decreasing as the island width is increased. Comparisons between a vacuum magnetic field and low beta HINT2 simulation are made showing similar fast ion behavior. Measurements of lost fast ions in W7-X confirm this trend with the control coil suppressed island configuration showing lower losses than that with no control coils applied. Simulations of fast ion wall loads are performed suggesting no drastic change in loss pattern and a slight reduction in losses with minimized islands.