Axially Symmetric Divertor Experiment Research Articles

Multi-machine benchmark of the self-consistent 1D scrape-off layer model DIV1D from stagnation point to target with SOLPS-ITER

This paper extends a 1D dynamic physics-based model of the scrape-off layer (SOL) plasma, DIV1D, to include the core SOL and possibly a second target. The extended model is benchmarked on 1D mapped SOLPS-ITER simulations to find input settings for DIV1D that allow it to describe SOL plasmas from upstream to target—calibrating it on a scenario and device basis. The benchmark shows a quantitative match between DIV1D and 1D mapped SOLPS-ITER profiles for the heat flux, electron temperature, and electron density within roughly 50% on: (1) the Tokamak Configuration Variable (TCV) for a gas puff scan; (2) a single SOLPS-ITER simulation of the Upgraded Mega Ampere Spherical Tokamak; and (3) the Upgraded Axially Symmetric Divertor EXperiment in Garching Tokamak (AUG) for a simultaneous scan in heating power and gas puff. Once calibrated, DIV1D self-consistently describes dependencies of the SOL solution on core fluxes and external neutral gas densities for a density scan on TCV whereas a varying SOL width is used in DIV1D for AUG to match a simultaneous change in power and density. The ability to calibrate DIV1D on a scenario and device basis is enabled by accounting for cross field transport with an effective flux expansion factor and by allowing neutrals to be exchanged between SOL and adjacent domains.

Development of a Scalable MMC Pulsed Power Supply through HIL Methodology

Nuclear fusion experiments are becoming one of the most interesting focuses of research, given the hope of generating programmable, safe, and green energy. Among them, ASDEX (axially symmetric divertor experiment) upgrade has been operating at the Max Planck Institute for Plasma Physics (IPP) research center since 1991. To ignite and confine the plasma, several coils must be supplied through controllable high-current pulsed power supplies. The toroidal field magnets are here considered and a modular multilevel converter (MMC)-like system was designed and tested thanks to a small-scale prototype in previous works. The MMC-like topology, consisting of full-bridge submodules (SMs) with IGBTs and supercapacitor and exploitable also for other industrial applications, was chosen because of its modularity, redundancy, fault tolerance, and large amount of stored energy. The prototype, made of four SMs, was necessary to highlight critical key points in the design process. However, its scalability must be further tested before building a full-scale power supply, meant to reach almost 2400 SMs to guarantee the energy required by the load. This paper aims at validating hardware-in-the-loop (a powerful, safe, and relatively inexpensive real-time simulation environment that enables testing with real control boards) as a useful technology for power supply scalability studies and not only for control strategy tests. The results obtained previously from the prototype will allow us to finally increase the number of SMs and test the MMC-like scalability.

Development of a MMC demonstrator for nuclear fusion devices power supplies

The modular multilevel converter (MMC) has become one of the most attractive converters for high-power applications such as high voltage DC (HVDC) converters, but also for fusion devices power supplies. The combination of this technology with a high power density energy storages such as supercapacitors (SC) represents a promising alternative to power the toroidal field (TF) magnets of the ASDEX (Axially Symmetric Divertor Experiment) Upgrade experiment operated at the Max-Planck-Institute for Plasma Physics (IPP) research center. After a first feasibility study, a single MMC submodule (SM) composed by four insulated-gate bipolar transistors (IGBT) forming a 4QC (4-quadrant chopper), a SC module and a power stage filter has been developed and successfully tested. Therefore, three additional identical modules were built to test their series and parallel operation in order to prove their capability to be scaled up.This work shows the step-by-step development of the MMC demonstrator, highlighting the results of the SM synchronized operation which is fundamental for the scalability of the system. The outcome of these experiments is relevant not only for the specific application of ASDEX Upgrade TF coils, but also for many other applications due to the flexible four-quadrant operation of the converter.

Full-Bridge Submodule Development of an MMC-Like Topology for ASDEX Upgrade Toroidal Field Coils Power Supply

The modular multilevel converter (MMC) has become one of the most attractive converters for high-power applications such as high-voltage dc (HVDC) converters, but also fusion devices’ power supplies. This converter, thanks to the discrete-leveled output voltage and its identical submodules (SMs) by which it is composed, represents a promising alternative to replace the flywheel generator (FG) that actually provides electrical power to Axially Symmetric Divertor Experiment (ASDEX) Upgrade device’s toroidal field (TF) coils. Due to the pulsed dc operation of these coils and their high-power needs (up to 150 MW) for each experiment, a small-scale version of MMC is under development with some differences compared with conventional ones: SM’s capacitors have been replaced with supercapacitor (SC) modules to increase the amount of available stored energy, while SMs belonging to different “rows” are interconnected to simplify their control and increase the reliability of the converter. This article shows the development and the operation of a single IGBT full-bridge (FB) SM highlighting advantages of this configuration and challenges of a full-scale converter: a proper power stage filter has been designed to protect SCs from voltage spikes and high-frequency current content generated during IGBT switching events, while SM’s control has been realized, thanks to the EtherCAT protocol, which ensures synchronization in case of SMs’ parallel operation and reliability of the future full-scale converter.

Long discharges in a steady state with D2 and N2 on the actively cooled tungsten upper divertor in WEST

Nitrogen (N2) will be used in ITER to enhance the radiative fraction to ∼90%, thereby cooling the edge plasma and preventing damage to the plasma-facing components. However, the reactivity of N2 with hydrogen isotopes can lead to the formation of tritiated ammonia (NT3). This should be considered in terms of the in-vessel tritium inventory, the regeneration of the cryo pumps, and the processes in the ITER de-tritiation plant. In the ‘W’ Environment in Steady-state Tokamak (WEST), a series of long L-mode discharges (∼50 s), with a constant N2 seeding from the outer strike point region has been performed on the upper actively cooled divertor. In the absence of active pumping, the N2 balance shows steady-state retention during plasma discharge, and is partially (∼35%) released in between discharges. Although a significant amount of N2( 18.65 Pa m3) has been injected, the wall still exhibited N2 pumping capabilities. Under these conditions, as long as this N2 reservoir is not saturated, there is not enough N available for the detectable threshold of ND3 formation to be reached. In these WEST experiments, no ammonia is detected during the pulse or after the pulse in the outgassing phase. These results are consistent with and complementary to the N2 seeded experiments performed in the Joint European Torus (JET) with its ITER-like wall and in the Axially Symmetric Divertor Experiment (ASDEX) upgrade.

An ionization pressure gauge with LaB6 emitter for long-term operation in strong magnetic fields.

We report here on a potentially significant improvement in the design of neutral pressure gauges of the so-called ASDEX-type which were first used in the Axially Symmetric Divertor EXperiment (ASDEX). Such gauges are considered state-of-the-art and are in wide use in fusion experiments, but they nonetheless suffer from a relatively high failure rate when operated at high magnetic field strengths for long times. This is therefore a significant concern for long-pulse, high-field experiments such as Wendelstein 7-X (W7-X) and ITER. The new design is much more robust. The improvement is to use a LaB6 crystal instead of a tungsten wire as the thermionic emitter of electrons in the gauge. Such a LaB6 prototype gauge was successfully operated for a total of 60 h in B = 3.1 T, confirming the significantly improved robustness of the new design and qualifying it for near-term operation in W7-X. With the LaB6 crystal, an order of magnitude reduction in heating current is achieved, relative to the tungsten filament based gauges, from 15-20 A to 1-2 A. This reduces the Lorenz forces and the heating power by an order of magnitude also and is presumably the reason for the much improved robustness. The new gauge design, test environment setup at the superconducting magnet, and results from test operation are described.

On the interpretation of tungsten emission spectra in fusion devices

Atomic spectra emitted by fusion plasmas are generally contaminated by ions originating from plasma erosion of material walls. These ions may be present in several charge states and the radiation they emit falls in the x-ray to vacuum ultraviolet regions, making them atomic fingerprints used as a diagnostic tool. This work reports on recent achievements on the interpretation of specific tungsten spectra from the Axially Symmetric Divertor Experiment (ASDEX) Upgrade tokamak and the Large Helical Device (LHD) stellarator.

Addendum to papers from Axially Symmetric Divertor Experiment (ASDEX) Upgrade Team, published in Proceedings of the 18th Topical Conference on High-Temperature Plasma Diagnostics, Wildwood, New Jersey, May 2010

Addendum to papers from Axially Symmetric Divertor Experiment (ASDEX) Upgrade Team, published in Proceedings of the 18th Topical Conference on High-Temperature Plasma Diagnostics, Wildwood, New Jersey, May 2010

Addendum to papers from Axially Symmetric Divertor Experiment (ASDEX) Upgrade Team, published in Review of Scientific Instruments

Addendum to papers from Axially Symmetric Divertor Experiment (ASDEX) Upgrade Team, published in Review of Scientific Instruments

Addendum to papers from the Axially Symmetric Divertor Experiment (ASDEX) Upgrade Team, published in Review of Scientific Instruments

Addendum to papers from the Axially Symmetric Divertor Experiment (ASDEX) Upgrade Team, published in <i>Review of Scientific Instruments</i>

Present Status of the New Multifrequency ECRH System for ASDEX Upgrade

A new multifrequency electron cyclotron resonance heating system is under construction for the Axially Symmetric Divertor Experiment (ASDEX) Upgrade tokamak experiment. For the first time in a fusion device, this system employs multifrequency gyrotrons that are step-tunable in the range 105-140 GHz. In its final stage the system will consist of four gyrotrons with a total power of 4 MW and a pulselength of 10 s. The first two gyrotrons, working at 105 and 140 GHz, were installed and tested. Transmission line elements such as corrugated waveguides, polarizer mirrors and vacuum windows are designed to cope with this frequency band. The system includes fast steerable launchers at the front end that will allow for localized feedback-controlled power deposition in the plasma.

Stochastization as a possible cause of fast reconnection in the frequently interrupted regime of neoclassical tearing modes

The role of stochastization of magnetic field lines in fast reconnection phenomena occurring in magnetized fusion plasma is analyzed. A mapping technique is applied to trace the field lines of toroidally confined plasma where the perturbation parameter is expressed in terms of experimental perturbation amplitudes determined from the Axially Symmetric Divertor Experiment (ASDEX) Upgrade tokamak [S. Günter, C. Angioni, M. Apostoliceanu et al., Nucl. Fusion 45, S98 (2005)]. It is found that fast reconnection observed during amplitude drops of the neoclassical tearing mode instability in the frequently interrupted regime can be related to stochastization.

Autoregressive moving average model for analyzing edge localized mode time series on Axially Symmetric Divertor Experiment (ASDEX) Upgrade tokamak

An approach to analysis of time series of edge localized modes (ELMs) is proposed. It is based on the use of the autoregressive moving average model, which decomposes time series into deterministic and noise components. Despite the inclusion of nonlinearity in the model, the resulting deterministic equations for the ELM time series measured on Axially Symmetric Divertor Experiment Upgrade tokamak turn out to be linear. This contrasts with the findings on JAERI tokamak (JT-60U) and tokamak à configuration variable that ELMs exhibit features of chaotic dynamics, namely, the presence of unstable periodic orbits. This methodology for distinguishing chaotic behavior is examined, and found to be susceptible to misinterpretation.

Applying the CO2 laser collective Thomson scattering results from JT-60U to other machines

A collective Thomson scattering (CTS) system based on a pulsed CO2 laser is being developed to demonstrate the feasibility of alpha-particle diagnostics. Tests on this system are being conducted on the JT-60U tokamak. The system consists of a pulsed laser (15 J in 1:s at 10.6:m) and a wide band (∼8 GHz) heterodyne receiver with a quantum-well infrared photodetector [Liu et al., IEEE Electron Device Lett. 16, 253 (1995)]. Stray light is reduced by a notch filter containing hot CO2 gas. The heterodyne receiver is absolutely calibrated using a large area blackbody radiation source [Bennett et al., Appl. Opt. 27, 3324 (1988)]. Results from the tests on JT-60U indicate a larger than expected noise background, especially for the lower velocity measurements determined by the smaller frequency shifts from the laser line. These results are used to estimate the signal-to-noise ratio that would be expected for a CTS diagnostic installed on other devices; ITER (“the way” in Latin), Joint European Torus-Enhanced Performance JET-EP, and axially symmetric divertor experiment (ASDEX). Calculations and possible system improvements are presented.

High resolution edge density measurements in ASDEX upgrade H-mode discharges with broadband reflectometry

In high-confinement (H-mode) plasmas with edge localized modes (ELMs), the energy stored at the edge pedestal region directly affects the global plasma confinement. The measurement of the edge pedestal evolution and the study of the collapse mechanism of the pedestal structure due to ELMs is, therefore, of great importance to understand plasma confinement. In Axially Symmetric Divertor Experiment (ASDEX) Upgrade, information about the edge density profile has been obtained in H-mode plasmas both with Thomson scattering and by combining lithium beam and interferometry measurements. However, information around the edge pedestal region is not routinely available. Here we use microwave reflectometry to obtain high resolution measurements of the edge transport barrier and pedestal region, namely the pedestal position and density, the pedestal width, and the edge density gradient. The evolution of the pedestal density is obtained automatically with an algorithm applied directly to the group delay. The other parameters are derived from the corresponding density profiles. Measurements performed in the presence of type I ELMs reveal that the edge density profile modifications occur in the same time scale of the Dα radiation at the divertor. The collapse and recovery of the pedestal structure during each ELM is resolved. The values of the pedestal width (around 4–5 cm) measured in-between ELMs are similar to those obtained previously in similar ASDEX Upgrade discharges. Also, the evolution of the pedestal density given by reflectometry is found to be in good agreement with the changes of the line integrated density measured by the interferometer.

Microwave reflectometry for turbulence studies on ASDEX Upgrade

We study the evolution of turbulence at several radial plasma locations in the Axially Symmetric Divertor Experiment (ASDEX) Upgrade tokamak, using microwave reflectometry. We analyze two types of scenarios: H-mode plasmas with improved performance and plasmas with electron cyclotron resonance heating modulation. Profound modifications of the turbulence spectra, occurring at specific plasma regions, have been found in both cases, indicating the relation between turbulence changes and confinement. The results demonstrate the ability of the reflectometry system on ASDEX Upgrade to make localized measurements. Mapping the radial distribution of turbulence can help us to understand the links between plasma rotation, shear, E×B flows, and the plasma turbulence behavior.

Improvements in the evaluation of Thomson scattering data on ASDEX upgrade

The avalanche diodes used as light detectors in the Thomson scattering diagnostic on the Axially Symmetric Divertor Experiment (ASDEX) Upgrade tokamak show a reduced signal-to-noise ratio during plasma discharges with high neutron rates and during plasma edge measurements. This often makes it impossible to determine the electron temperature from the ratios of the scattering signals of the different spectral channels analyzing the Thomson scattering spectrum. With the use of the maximum likelihood method combined with linearized electron temperature characteristics the electron temperature and density are determined directly from the scattering signals without calculating ratios between the signals. With this method the scattering signals for nearly all timepoints can be evaluated and the statistical errors of the electron temperature and density are up to factors of 2 and 1.4, respectively, smaller than achieved with the previous method.

Fast Lithium Beam Edge Plasma Spectroscopy at IPP Garching—Status and Recent Developments

Impurity ion concentration and impurity ion temperature in the plasma gradient region as well as the scrape-off layer are essential parameters for understanding the physics of L- and H-mode transport and the transport barrier itself. To gain access to these properties, the well-established Li-beam diagnostic capabilities on both fusion devices at the Max-Planck-Institut für Plasmaphysik (IPP) Garching [WENDELSTEIN 7-AS stellarator and the Axially Symmetric Divertor Experiment (ASDEX) Upgrade tokamak] have been extended to include the measurement of radial profiles of impurity ion densities and temperatures by means of charge-exchange spectroscopy. This paper describes the experimental setups on both devices and presents typical results of impurity ion investigations. Electron density measurements show excellent agreement with other diagnostics. In addition, several LiI spectral lines (2p to 2s, 3d to 2p, 4s to 2p, and 4d to 2p) have been measured to check the collisional-excitation Li-beam modeling, especially for collision processes involving higher Li(nl) states (n ≥ 3). The underlying database has been augmented by extensive investigations of lithium excitation processes. Cross sections for Li(2s to 2p) excitation by various impurity ions as well as proton impact Li(2l to nl) excitation have been calculated and measured in detail.

Discharge Supervision Control on ASDEX Upgrade

The Axially Symmetric Divertor Experiment (ASDEX) Upgrade’s digital systems for plant operation and real-time discharge control are described. Experience gained during development and operation is used to derive the structure of a reference discharge control system that serves as a target system to assess the impact of enhancements in the existing system. Within this context, the extension of supervision control from technical synchronization of dedicated controllers to physics-oriented coordination is discussed, and examples of technical and physical applications are shown.

The effect of a radial electric field on ripple-trapped ions observed by neutral particle fluxes

The effect of a radial electric field on nonthermal ripple-trapped ions is investigated using toroidal Monte Carlo simulations for edge tokamak plasmas. The increase in the neutral particle flux from the ions trapped in local magnetic wells observed by the charge exchange (CX) detector at a low confinement to high confinement transition at ASDEX (Axially Symmetric Divertor Experiment). Upgrade tokamak [Proceedings of the 20th European Conference on Controlled Fusion and Plasma Physics, Lisbon (European Physical Society, Petit-Lancy, Switzerland, 1993), Vol. 17C, Part I, p. 267] is reproduced in the simulations by turning on a radial electric field near the plasma periphery. The poloidal and toroidal angles at which the CX detector signal is most sensitive to the radial electric field are determined. A fast response time of the signal in the range of 50–100 μs to the appearance of the electric field can be found in the simulations with a relatively large half-width of the negative electric field region.