At ASDEX Upgrade Research Articles

Existence of a detachment cliff at ASDEX Upgrade

The detachment cliff is a bifurcative transition to partial detachment recently discovered at the DIII-D tokamak (McLean et al 2015 J. Nucl. Mater. 463 533–6). This work presents a database analysis of target parameters in L-mode and H-mode discharges to search for a detachment cliff at ASDEX Upgrade (AUG). Most of the transitions from attached to partially detached divertor conditions observed in H- and L-mode discharges in AUG show bifurcative-like characteristics that are consistent with the properties of the detachment cliff if the B×∇B drift is directed towards the active X-point. In the operational space of power and density, the bifurcative transitions identified during an L-mode discharge occur at injected power and density higher than a threshold value ( Ptot > 0.7 MW and n e > 1.6 × 10 19 m−3, respectively). Furthermore, the temperatures at which the transitions start are found to be insensitive to the injected impurity, the injected power and the value of the upstream density. Finally, the study of the evolution of the target parameters, of the intensity of the Dα line and of specific manometers and bolometer lines of sights shows that the physical process underlying the detachment cliff and the self-sustained divertor oscillations (Heinrich 2020 Nucl. Fusion 60 076013) might be the same.

On learning latent dynamics of the AUG plasma state

In this work, we demonstrate the utility of state representation learning applied to modeling the time evolution of electron density and temperature profiles at ASDEX-Upgrade (AUG). The proposed model is a deep neural network, which learns to map the high dimensional profile observations to a lower dimensional state. The mapped states, alongside the original profile's corresponding machine parameters, are used to learn a forward model to propagate the state in time. We show that this approach is able to predict AUG discharges using only a selected set of machine parameters. The state is then further conditioned to encode information about the confinement regime, which yields a simple baseline linear classifier, while still retaining the information needed to predict the evolution of profiles. We, then, discuss the potential use cases and limitations of state representation learning algorithms applied to fusion devices.

Admixed pellets for fast and efficient delivery of plasma enhancement gases: Investigations at AUG exploring the option for EU-DEMO

Gas and pellet injection are envisaged for particle fuelling in EU-DEMO. The gas system will provide edge and divertor fuelling and any further gas species required for operation. Pellets, mm-sized bodies formed from solid hydrogen fuel, are designed for efficient and fast core fuelling. However, they can also be employed for a more efficient delivery of plasma enhancement gases, by admixing them with the fuelling pellets. To check this option for EU-DEMO, explorative investigations have been performed at ASDEX Upgrade (AUG).The AUG system produces ice in a batch process sufficient for about 100 pellets, initially designed for operation with pure H2 or D2. On a trial basis, pellet formation was tested using an H2/D2 mixture and admixtures containing small amounts (up to 2 mol%) of N2, Ar, Kr or Xe in the D2 host. A homogeneous and reproducible ice composition was found for the H2/D2 = 1:1 case. For all the admixed gases, a depletion of the admixture in the ice with increasing atomic number is observed. Nevertheless, the fast and efficient delivery of admixed pellets was clearly demonstrated in dedicated plasma experiments at AUG. Detailed investigations showed that the Ar supplied via admixed pellets has a higher radiation efficiency and a faster radiation rise than an Ar/D2 gas puff. Furthermore, Ar density measurements in a discharge with admixed pellet injection show reasonable agreement with findings of a fading admixed species’ concentration along the ice rod and assumptions on the pellet ablation location in the plasma. Investigations performed at the Oak Ridge National Laboratory with a large batch extruder using up to 2 mol% Ne in D2 confirmed that production of much larger ice quantities can be achieved.These initial explorative investigations clearly reveal the great potential of admixed pellets, although they also demonstrate that further technology efforts are required before their benefits can be utilized.

Experimental study of the edge radial electric field in different drift configurations and its role in the access to H-mode at ASDEX Upgrade

The formation of the equilibrium radial electric field (Er) has been studied experimentally at ASDEX Upgrade (AUG) in L-modes of “favorable” (ion ∇ B-drift toward primary X-point) and “unfavorable” (ion ∇ B-drift away from primary X-point) drift configurations, in view of its impact on H-mode access, which changes with drift configurations. Edge electron and ion kinetic profiles and impurity velocity and mean-field Er profiles across the separatrix are investigated, employing new and improved measurement techniques. The experimental results are compared to local neoclassical theory as well as to a simple 1D scrape-off layer (SOL) model. It is found that in L-modes of matched heating power and plasma density, the upstream SOL Er and the main ion pressure gradient in the plasma edge are the same for either drift configurations, whereas the Er well in the confined plasma is shallower in unfavorable compared to the favorable drift configuration. The contributions of toroidal and poloidal main ion flows to Er, which are inferred from local neoclassical theory and the experiment, cannot account for these observed differences. Furthermore, it is found that in the L-mode, the intrinsic toroidal edge rotation decreases with increasing collisionality and it is co-current in the banana-plateau regime for all different drift configurations at AUG. This gives rise to a possible interaction of parallel Pfirsch–Schlüter flows in the SOL with the confined plasma. Thus, the different H-mode power threshold for the two drift configurations cannot be explained in the same way at AUG as suggested by LaBombard et al. [Phys. Plasmas 12, 056111 (2005)] for Alcator C-Mod. Finally, comparisons of Er profiles in favorable and unfavorable drift configurations at the respective confinement transitions show that also the Er gradients are all different, which indirectly indicates a different type or strength of the characteristic edge turbulence in the two drift configurations.

Electron temperature fluctuation measurements with Correlation Electron Cyclotron Emission in L-mode and I-mode plasmas at ASDEX Upgrade

The Correlation Electron Cyclotron Emission (CECE) diagnostic at ASDEX Upgrade (AUG) is used to investigate the features of outer core and pedestal (ρpol = 0.85-1.0) turbulence across confinement regime transitions. The I-mode confinement regime is a promising operational scenario for future fusion reactors because it features high energy confinement without high particle confinement, but the nature of the edge and pedestal turbulence in I-mode plasmas is still under investigation. The edge Weakly Coherent Mode (WCM) appears in the I-mode pedestal and may play a role in transport. In this work we explore electron temperature (Te) fluctuations in the plasma outer core and pedestal using a 24-channel high radial resolution CECE radiometer. CECE measurements provide turbulence information including the Te fluctuation amplitude, turbulent spectra, and radial localization of turbulent features. With CECE measurements we show that the WCM is localized in the pedestal region in both L-mode and I-mode and is measured in optically thick plasmas with a Te fluctuation amplitude of 2.3%. Broadband drift wave turbulence is measured in the outer core with a Te fluctuation amplitude of <1%. A second CECE system recently installed at AUG allowed for non-standard fluctuation measurements during L-mode and I-mode experiments. The second CECE system was toroidally separated from the primary system, allowing measurements of the long-range toroidal correlation of the WCM indicating its low toroidal mode number. A reflectometer sharing a line of sight with the second CECE system enabled density-temperature cross-phase (αne Te ) measurements. The WCM αne Te changes between L-mode and I-mode as the Te gradient steepens.

Overview of L- to H-mode transition experiments at ASDEX Upgrade

This paper presents an overview of results from L–H transition experiments that were performed at ASDEX Upgrade (AUG) with the aim of identifying the underlying mechanisms leading to H-mode confinement. With a broad variety of experiments and new diagnostic techniques, as well as modeling efforts, AUG has contributed substantially to improving our understanding of the L–H transition over the past years. In this review, the important roles of the ion heat channel and the edge radial electric field (E r ) in the L–H transition physics are brought into context with known dependencies of the H-mode power threshold (), such as the impact of wall material, magnetic perturbations, and the magnetic configuration. Furthermore, experimental and theoretical results obtained at AUG on the L-mode edge turbulence are connected to the mean-field E r and its related shear flow. This led to a deeper understanding of the I-phase plasma regime, has resolved the so-called isotope effect of , and led to the development of a semi-analytical model that can describe AUG’s experimental observations of the L–H transition together with the L- and H-mode density limits.

Developing a physics understanding of the quasi-continuous exhaust regime: pedestal profile and ballooning stability analysis

It has been experimentally observed that at ASDEX Upgrade (AUG) plasmas at relatively high shaping, an increase of gas fuelling corresponds to an increase of the frequency and intensity of the type-I edge localised modes (ELMs). At high enough fuelling, the plasma enters the quasi continuous exhaust (QCE) regime. We have performed ideal ballooning n → ∞ stability analysis on four AUG discharges, comparing the type-I ELM dominated phases, with the phases that are in the QCE regime. The results of this study show that as the gas puff increases, the plasma gets more ballooning unstable in the pedestal region, especially very close to the separatrix, at the pedestal bottom. On the contrary, in the middle of the pedestal, the discharges are more ballooning stable. Here the locally negative magnetic shear has a stabilising effect on ballooning modes, allowing access to the second stability region. Our analysis of the ballooning stability and the confinement factor H98 suggest that with optimisation of the pedestal shape, good confinement without type-I ELMs can be achieved. Necessary ingredients are that the region of the highest pressure gradient is not ideal ballooning limited, while the pedestal bottom is ballooning unstable. Ideal stability analysis of 36 simulated ITER profiles shows that, similarly to the experimental cases from AUG, a high pedestal top pressure can be maintained concomitant with a ballooning instability at the pedestal bottom, making QCE a promising scenario.

Experimental investigation of L- and H-mode detachment via the divertor Thomson scattering at ASDEX Upgrade

The capabilities of the newly installed divertor Thomson Scattering (DTS) diagnostic (Kurzan et al 2021 J. Inst. 16 C09012) at ASDEX Upgrade (AUG) have been demonstrated by measuring 2D electron densities n e and temperatures T e from attached to fully detached divertor conditions in L- and H-mode. The collected dataset is a breakthrough for divertor studies at AUG in which such measurements have been so far missing. Besides highlighting the strengths and limits of the DTS system, this work provides confirmations and new insights into detachment physics. The transition between partial and pronounced detachment correlates with a 2D redistribution of the electron density from the inner to the outer divertor in both L- and H-mode. In pronounced detachment, a strong parallel pressure gradient could be confirmed throughout the complete SOL. Finally, measurements of n e and T e of a stable X-point radiator revealed local temperatures as low as 1 eV within the confined plasma, whereas the pressure is shown to be constant on closed field lines.

In-out charge exchange measurements and 3D modelling of diagnostic thermal neutrals to study edge poloidal impurity asymmetries

A new method was developed to model the neutral population produced by the gas puff based charge exchange recombination spectroscopy systems at ASDEX Upgrade (AUG). With this method, the edge impurity density on the high field side (HFS) and low field side (LFS) can be obtained without the need to apply a neutral beam injection system. The neutral penetration needed for the calculation of the impurity density is obtained with a new gas puff module implemented in the FIDASIM code. The LFS impurity density profile evaluated with the new gas puff module matches the impurity density calculated with standard beam-based charge exchange diagnostics. Impurity temperature, rotation and density profiles at the HFS and LFS of an AUG H-mode discharge are presented. Edge impurity toroidal and poloidal flows show asymmetric structures. The impurity density asymmetries obtained with the new gas puff module are consistent with the observed flow structure.

The impact of inverted density gradients on density profiles measured by reflectometry: an experimental and numerical investigation at ASDEX Upgrade

The high-field side high-density (HFSHD) region at ASDEX Upgrade (AUG) is a well-documented phenomenon leading to a dense plasma in the inner divertor region that expands upwards to the midplane, resulting in poloidally asymmetric scrape-off layer density profiles. This work investigates, via simulation and experiment, whether the HFSHD at the midplane leads to hollow density profiles at the high-field side. Using the frequency-modulated continuous-wave O-mode reflectometer at AUG, experimental evidence has been found of reflection patterns compatible with a hollow density profile that are reproduced by 1D full-wave simulations. Furthermore, this work assesses the uncertainties in the density profile reconstruction as a consequence of the inverted gradient, showing that the presence of an HFSHD may lead to an overestimation of the density in the confined region.

Validation of low-Z impurity transport theory using boron perturbation experiments at ASDEX upgrade

An experimental technique has been developed at ASDEX upgrade (AUG) to separately identify the diffusive and convective components of the boron particle flux. Using this technique a database of B transport coefficients has been assembled that shows that the normalized ion temperature gradient () is the strongest organizing parameter for both the B diffusion and convection and large is a necessary ingredient to obtain hollow B density profiles in AUG. This database also shows that large changes in the applied neutral beam injection (NBI) have a relatively small impact on impurity transport compared to similar changes in electron cyclotron resonance heating (ECRH). Even low levels of ECRH power dramatically increase both the diffusive and convective fluxes and lead to peaking of the impurity density profile. Comparisons to a combination of neoclassical and quasi-linear gyrokinetic simulations show good agreement in the measured and predicted diffusion coefficients. The outward convection measured in NBI dominated plasmas, however, is not well captured by the simulations, despite the inclusion of fast ions. In contrast, the convection is reasonably well reproduced for plasmas with flat or peaked boron density profiles. This dataset provides an excellent experimental validation of the non-monotonic, predicted, convective-particle-flux created by the combination of pure-pinch, thermo-diffusion, and roto-diffusion. In addition, this dataset demonstrates a non-monotonic dependence of the experimental particle diffusivity to ion heat conductivity (D/χ i) in qualitative agreement with theoretical predictions.

Adoption of the ASDEX Upgrade pumping to hydrogen released by an in-vessel cryopump

Detached divertor scenarios at ASDEX Upgrade (AUG) need high gas fluxes, which are mainly pumped with the gas binding in-vessel cryopump (CP). To cope with the large quantities of hydrogen released from the CP during regeneration, a hydrogen compatible pumping system (HCPS), which is built to comply with safety regulations for explosion protection, has been built up in parallel to the already existing gas transfer pumping system (TPS). The amount of gas injected in the vacuum vessel (VV) is limited by an additional volume added to the gas inlet system. A fully automated procedure has been implemented for comprise with safety regulations. This includes daily tests of the components and communications with other relevant systems.

Developing fuel sources for a steady-state multi-purpose pellet launching system

Like for any power plant, refuelling will be one key issue in a future fusion reactor. It has to be safe and efficient; due to the operational principle, it also needs to occur in steady state. From previous investigations, it became clear that injection of pellets - mm-size solid bodies produced from frozen fuel – is the only possible and technically realistic solution for this task. Beyond their capability for fuelling, pellets have been found potentially useful as well for other purposes, e.g. to control edge localised modes. Therefore, efforts are required to develop a pellet launching system (PLS) capable to meet all basic reactor requirements.To foster this kind of technology evolution and to enhance the PLS applied at ASDEX Upgrade (AUG), a collaborative investigation between IPP and ORNL has been initiated. This aims to replace the existing pellet source able to deliver to a centrifuge injector up to 130 stored pellets produced in a batch process with repetition rates up to 80 Hz by a steady state source. The new source is based on the ORNL extruder types and capable of delivering either in true steady state or at least from a reservoir with sufficient solid fuel to produce up to 2000 pellets. Experiments extruding ice with different square cross sections showed a maximum throughput of about 300 mm3/s for Protium (H) and 215 mm3/s for Deuterium (D) with a 1.9 mm square cross section. Ice temperatures of 9 K for H and 16 K for D were found to be optimum. For D, extrusion rates correspond to a 1.9 mm pellet delivery rate of about 25 Hz. Doping the ice with up to 2% Neon was achieved at the expense of moderately reduced extrusion speeds. A new scheme has been designed to inject pellets in parallel from multiple sources for further enhancing the delivery rate.

Advances in turbulence measurements using new Correlation ECE and nT-phase diagnostics at ASDEX Upgrade

Guided by predictions from nonlinear gyrokinetic simulations, two new turbulence diagnostics were designed and installed at ASDEX Upgrade (AUG) to probe the fundamentals of ion-scale turbulent electron heat transport. The first, a 30-channel correlation ECE (CECE) radiometer (105-128 GHz, 2nd harmonic X-mode), introduces a novel channel comb arrangement. This allows measurements of high radial resolution profiles (0:5 < r/a < 0:8) of low-k (k⍬⍴s < 0:3) temperature fluctuation amplitudes, frequency spectra and radial correlation length profiles in unprecedented detail. The second diagnostic is formed by the addition of two W-band and one V-band X-mode reflectometers on the same line of sight as the CECE to enable measurements of the phase angle between turbulent density and temperature fluctuations. Historically, the radial alignment between reflectometer and radiometer has been a challenge due to the requirement that alignment is achieved within a radial correlation length (< 5 10 mm). This challenge is significantly alleviated by using the CECE channel comb arrangement and the maximal coherence between reflectometer and radiometer can be unambiguously captured. Measurements of these quantities have been made in an AUG L-mode plasma, at the same radial location and have provided simultaneous quantitative constraints on realistic gyrokinetic simulations [Physics of Plasmas 25, 055903 (2018)] using the gyrokinetic code GENE. Here we present diagnostic detail for this study.

Beam tracing study for design and operation of two-pass electron cyclotron heating at ASDEX Upgrade

The electron cyclotron resonance heating system at ASDEX Upgrade (AUG) is currently being extended to eight similar Gyrotrons in total. Each Gyrotron operates at 105 and 140 GHz and is designed for up to 1 MW millimetre wave output power. A substantial part of the AUG program will focus on experimental conditions, where the plasma density may be above the X-2 cut-off density at 140 GHz. In order to cope with the high density, the heating system will operate in the O-2 mode scheme with potentially incomplete absorption in the first pass. Reflecting gratings installed into the heat shield on AUG’s inner column allow for a controlled second pass of the beam’s unabsorbed fraction. Thermocouple measurements serve to control the beam position on the grating. The beam geometry is being finalized for the launchers #1-4. Beam propagation is simulated with the TORBEAM code and previous high density experiments are used as a database. The geometry is optimized using three criteria: central deposition, high absorption and robustness of the beam dump after the second pass. The experimental conditions, and the plasma electron density in particular, may vary such that the Gaussian beam parameters of the incoming beam on the grating deviate from the design values. It is proposed to model the effect of the grating with an equivalent ellipsoidal mirror. Laboratory measurements are shown, which support this model.

Diagnostic setup for the divertor manipulator at wendelstein 7-X

The investigation of plasma wall interactions in terms of erosion processes, fuel retention and new plasma facing components exhibits a scientific gap as discharges in tokamaks and stellarators last only seconds up to a minute. Currently, linear plasma machines are bridging this gap, while Wendelstein 7-X (W7-X) will provide steady-state discharges of up to 30 min in its second operation phase (OP2), in which a divertor manipulator is envisaged to expose samples to reactor-relevant plasma fluxes and fluences. With a design following the limiter lock system at TEXTOR and the DIM-II at ASDEX Upgrade (AUG), the divertor manipulator at W7-X aims to study plasma surface interactions ex-situ by daily/weekly probe exchange and in-situ by embedded diagnostics and an observation system.The design, layout and capabilities of the embedded and observing diagnostics determine the accessible parameters for the in-situ investigation of the exposed probe materials, ranging from plasma edge measurements in the manipulator shaft to intersecting the magnetic island structure on the manipulator head. A versatile laser and observation system adds active surface analysis techniques. To prevent local overheating and excess thermal stress, essential diagnostics for temperature control are identified and combined with first considerations of the manipulator head shaping, based on the field line tracing tool of the W7-X web-service.

Evaluation of impurity densities from charge exchange recombination spectroscopy measurements at ASDEX Upgrade

At ASDEX upgrade (AUG) a new framework for the evaluation of impurity densities based on measurements from charge exchange recombination spectroscopy (CXRS) diagnostics has been developed. The charge exchange impurity concentration analysis code, or CHICA, can perform these calculations for all of the beam-based CXRS diagnostics at AUG and is equipped with the atomic data for all of the regularly measured charge exchange spectral lines (He, Li, B, C, N, O, and Ne). CHICA includes four different methods for the evaluation of the neutral density populations, which feature different implementations and contain varying levels of sophistication. These methods have been thoroughly benchmarked against one another, enabling the important processes for the evaluation of neutral densities to be identified as well as the neutral populations that are most critical to the accurate interpretation of the measured CXRS intensities. For the AUG neutral beams, charge exchange with the ground state of the first energy component is typically the dominant contribution to the measured CXRS intensities, but emission from reactions with the n = 2 beam halo population can contribute up to 35% to the total signal and must be included in the analysis. Neglect of this population leads to incorrect magnitudes and incorrect profile shapes of the calculated impurity density profiles. The edge lines of sight (LOS) of the core CXRS diagnostics at AUG intersect the edge pedestal inside of the neutral beam volume. Therefore, the impurity density is not constant along the LOS, complicating the interpretation of the measured intensities. Within CHICA a forward model for the edge impurity densities has been implemented, enabling the reconstruction of accurate edge profiles.

Measurement of the complete core plasma flow across the LOC–SOC transition at ASDEX Upgrade

A newly installed core charge exchange recombination spectroscopy (CXRS) diagnostic at ASDEX Upgrade (AUG) enables the evaluation of the core poloidal rotation () through the inboard-outboard asymmetry of the toroidal rotation with an accuracy of 0.5 to 1 km . Using this technique, the total plasma flow has been measured in Ohmic L-mode plasmas across the transition from the linear to saturated ohmic confinement (LOC–SOC) regimes. The core poloidal rotation of the plasma around mid-radius is found to be always in the ion diamagnetic direction, in disagreement with neoclassical (NC) predictions. The edge rotation is found to be electron-directed and consistent with NC codes. This measurement provides as well the missing ingredient to evaluate the core velocity () from data only, which can then be compared to measurements of the perpendicular velocity of the turbulent fluctuations () to gain information on the turbulent phase velocity (). The non neoclassical from CXRS leads to good agreement between and indicating that is small and at similar values as found with gyrokinetic simulations. Moreover, the data shows a shift of in the ion-diamagnetic direction at the edge after the transition from LOC to SOC consistent with a change in the dominant turbulence regime. The upgrade of the core CXRS system provides as well a deeper insight into the intrinsic rotation. This paper shows that the reversal of the core toroidal rotation occurs clearly after the LOC–SOC transition and concomitant with the peaking of the electron density.

Gaseous electron multiplier-based soft x-ray plasma diagnostics development: Preliminary tests at ASDEX Upgrade.

A Gaseous Electron Multiplier (GEM)-based detector is being developed for soft X-ray diagnostics on tokamaks. Its main goal is to facilitate transport studies of impurities like tungsten. Such studies are very relevant to ITER, where the excessive accumulation of impurities in the plasma core should be avoided. This contribution provides details of the preliminary tests at ASDEX Upgrade (AUG) with a focus on the most important aspects for detector operation in harsh radiation environment. It was shown that both spatially and spectrally resolved data could be collected, in a reasonable agreement with other AUG diagnostics. Contributions to the GEM signal include also hard X-rays, gammas, and neutrons. First simulations of the effect of high-energy photons have helped understanding these contributions.

Nitrogen retention mechanisms in tokamaks with beryllium and tungsten plasma-facing surfaces

Global gas balance experiments at ASDEX Upgrade (AUG) and JET have shown that a considerable fraction of nitrogen injected for radiative cooling is not recovered as N2 upon regeneration of the liquid helium cryo pump. The most probable loss channels are ion implantation into plasma-facing materials, co-deposition and ammonia formation. These three mechanisms are investigated in laboratory and tokamak experiments and by numerical simulations. Laboratory experiments have shown that implantation of nitrogen ions into beryllium and tungsten leads to the formation of surface nitrides, which may decompose under thermal loads. On beryllium the presence of nitrogen at the surface has been seen to reduce the sputtering yield. On tungsten surfaces it has been observed that the presence of nitrogen can increase hydrogen retention. The global nitrogen retention in AUG by implantation into the tungsten surfaces saturates. At JET the steady state nitrogen retention is increased by co-deposition with beryllium. The tokamak experiments are interpreted in detail by simulations of the global migration with WallDYN. Mass spectrometry of the exhaust gas of AUG and JET has revealed the conversion of nitrogen to ammonia at percent-levels. Conclusions are drawn on the potential implications of nitrogen seeding on the operation of a reactor in a deuterium–tritium mix.