ASDEX Upgrade Plasmas Research Articles

10 MVA thyristor converters for the active damping of generator shaft oscillations at ASDEX Upgrade

Over the last years, ASDEX Upgrade (AUG) improved its fast plasma feedback control. This had a direct implication on the magnetic coils power supplies. During a discharge, very fast changes of the active power occur. This results in torsional oscillations in the flywheel generators shaft lines that feed the power supplies. These oscillations speed up the aging of the generator assemblies. Therefore an active control system providing electromagnetic damping for the generator shaft oscillations has been set up. To optimize this system, two new thyristor converters have been installed and commissioned. Especially the new “design features” of the thyristor converter Groups 8 and 9 with generator torque reference processing and protection will be explained in the paper. Further on, the paper will present the various modes of operation, analyses the results of measurements obtained during commissioning, compares them to the calculated (design) values and reports on the performance achieved during AUG plasma experiments.

Evidence for Strong Inversed Shear of Toroidal Rotation at the Edge-Transport Barrier in the ASDEX Upgrade

The toroidal rotation of H-mode plasmas in ASDEX Upgrade is studied in the outermost 5 cm of the confined plasma. The projection of the rotation velocity along the line of sight (approximately toroidal) is measured using charge exchange recombination spectroscopy, with a radial resolution of up to 3 mm and a temporal resolution of 1.9 ms. At about 1 cm inside the separatrix the rotation exhibits a local minimum. From there, the rotation in codirection increases towards the plasma center and towards the separatrix. The latter increase is the focus of this work. It is situated in the region of the edge transport barrier and amounts to 10-20 km/s. It is observed for D+, He2+, B5+, and C6+. The described rotation feature at the edge is not visible during an ELM crash and is probably connected to the occurrence of steep gradients in this plasma region.

Role of shielding in modelling cryogenic deuterium pellet ablation

For the better characterization of pellet ablation, the numerical LLP code has been enhanced by combining two relevant shielding mechanisms: that of the spherically expanding neutral cloud surrounding the pellet and that of the field elongated ionized material forming a channel flow. In contrast to our expectation the presence of the channel flow can increase the ablation rate although it reduces the heat flux travelling through it. The contribution of the different shielding effect in the ablation process is analysed for several pellet and plasma parameters and an ablation rate scaling is presented based on simple regression in the ASDEX Upgrade pellet and plasma parameter range. Finally the simulated results are compared with experimental data from typical ASDEX Upgrade discharges.

TAE internal structure through high-resolution soft x-ray measurements in ASDEX-Upgrade

The internal structure of toroidal Alfvén eigenmodes (TAEs) excited in ASDEX-Upgrade plasmas with ion cyclotron resonance heating (ICRH) has been measured through a high-resolution multichord soft x-ray diagnostic. The radial profiles of multiple TAEs are compared with predictions of the linear resistive magnetohydrodynamic code CASTOR and the nonperturbative gyrokinetic code LIGKA and reasonably good agreement is found. The evolution of the TAE radial profiles could be followed in time, both across sawteeth in pure ICRH discharges and in discharges with different types of transitions from L-mode to H-mode obtained with a combination of ICRH and neutral beam injection.

2D fluid modeling of the ASDEX upgrade scrape-off layer up to the first wall

We present an application to the full ASDEX upgrade edge plasma of a novel method for 2D fluid modeling, including for the first time a realistic representation of the First Wall. Two independent edge plasma codes are coupled to this purpose: B2 (with a detailed physics content but intrinsic geometrical limitations due to the 5-point computational scheme) for the inner region of the Scrape-Off Layer (the so-called near SOL) and ASPOEL (with simplified physics content but larger geometrical flexibility thanks to the Control Volume Finite Element CVFE scheme) for the outer region (the far SOL). The two codes share information across an interface magnetic surface, representing the outer boundary for B2 and the inner boundary for ASPOEL. An iterative procedure is developed, ensuring the continuity of profiles and fluxes at the interface. The radial profiles of density and temperature computed at the outboard mid-plane across the complete SOL, up to the first wall, are in good agreement with experimental data.

Non-adiabatic passing electron response and outward impurity convection in gyrokinetic calculations of impurity transport in ASDEX Upgrade plasmas

Gyrokinetic calculations with the GS2 code of impurity transport in ASDEX Upgrade H-mode plasmas are presented. A method to separate the diagonal and off-diagonal terms in the quasi-linear flux computed by the gyrokinetic code for a trace impurity is introduced and applied. It is shown that in the experimental conditions of strong central electron cyclotron heating, unstable modes rotating in the electron diamagnetic direction are excited in the central region of the plasma. These are generated by the non-adiabatic response of passing electrons and feature extremely elongated eigenfunctions along the field line. The related fluctuations in the electrostatic potential generate an outward convection of the impurities. These theoretical findings are in agreement with the outward convection of laser ablated Si measured in these experimental conditions. In contrast, in the outer region of the plasma, as well as everywhere in the case of discharges without central electron heating, ion temperature gradient modes produce an impurity pinch directed inwards, which is also in agreement with the experimental observations.

A pellet launcher tool optimized for the control of edge localized modes in ASDEX Upgrade H-mode plasmas

A main challenge for fusion research oriented pellet launcher systems is the control of the plasma edge region, especially of the possibly dangerous edge localized modes (ELMs) observed in high performance H-mode plasmas. In order to improve the experimental data base required for extrapolation, a novel cryogenic multi-pellet injection system was developed for ASDEX Upgrade, which is entirely dedicated to control and mitigation of type-I ELMs. Based on the blower gun principle, it is capable of combining high repetition rates up to 143 Hz and low pellet sizes and velocities. Expected benefits are an enhanced tokamak operational area and the option for more sophisticated experiments investigating the underlying physical mechanisms. This paper reports on design criteria, construction, extensive laboratory tests and first injection into ASDEX Upgrade plasmas.

Molecular contribution to the Dα emission in the divertor of the ASDEX Upgrade experiment

Spectral line emission from the deuterium Balmer α line and from molecular deuterium in the Fulcher band, d 3 Π u→a 3 ∑ g +, was studied in the outer divertor region of the ASDEX Upgrade plasma with a pair of relatively calibrated spectrometers. A dependence of the signals on the proximity of the plasma strike point and on divertor gas puffing was noted. The rotational spectrum of the lowest diagonal vibrational molecular bands indicated rotational temperatures of ca. 1000 K and hence, through calculation of relative upper state populations, a ground state vibrational temperature of ca. 4000 K. Calculating the particle fluxes, the molecular deuterium was found to contribute up to 1% of the Dα intensity, the relative importance of the molecules at some divertor heights was affected by the position of the strike point.

Linear gyrokinetic stability calculations of electron heat dominated plasmas in ASDEX Upgrade

Linear gyrokinetic calculations of low density electron heat dominated plasmas in ASDEX Upgrade [O. Gruber, H.-S. Bosch, S. Günter et al., Nucl. Fusion 39, 1321 (1999)] are presented. It is found that the dominant mode is a trapped electron mode, which under the experimental conditions is sensitive to the density gradient, collisions, and magnetic shear. The quasilinear heat flux scales in good agreement with the experimental observed heat conductivity. Many experimentally observed phenomena can be explained on the basis of the presented calculations: the existence of a threshold, the fast propagation of the heat wave, and the moderate stiffness of the electron channel. The implications of the calculations for the empirical investigations are discussed.

Pedestal Characteristics of H-Mode Plasmas in JT-60U and ASDEX Upgrade

The role of the pedestal structure in ELMy H-mode plasmas for the core energy confinement and for the ELM energy losses were investigated in two tokamak-type devices, JT-60U and ASDEX Upgrade. The confinement degradation seen at higher densities is attributed to the reduction of the pedestal temperature limited by the ELM activities and the stiffness of the temperature profiles. In high triangularity or impurity seeded H-modes, in which higher energy confinement is generally achieved, a higher pedestal temperature is obtained by improving the edge MHD stability or the density profile peaking, respectively. The upper bound of the ELM energy loss is characterized by the pedestal energy. The ELM energy loss can become smaller at fixed pedestal energy when the pedestal collisionality is raised. Raising the pedestal collisionality also enhances the inter-ELM perpendicular transport loss and reduces the ELM loss power fraction. In ASDEX Upgrade, the continuous pellet injection is shown to effectively mitigate ELM losses while preserving the core confinement quality. In JT-60U, the operation regime of grassy ELM is investigated and shown to achieve highly integrated performance of high energy confinement with small ELMs in the low collisionality regime.

A fast framing camera system for observation of acceleration and ablation of cryogenic hydrogen pellet in ASDEX Upgrade plasmas

An observation system using fast digital cameras was developed to measure a cryogenic hydrogen pellet’s cloud structure, trajectory, and velocity changes during its ablation in ASDEX Upgrade plasmas. In this article the system, the applied numerical methods, and the results are presented. The three-dimensional pellet trajectory and velocity components were reconstructed from images of observations from two different directions. Pellet acceleration both in the radial and toroidal directions was detected. The pellet cloud distribution was measured with high spatio-temporal resolution. The cloud surrounding the pellet was found to be elongated along the magnetic field lines. Its typical size is 5–7 cm along the field lines and 2 cm in the perpendicular directions. A cloud extension in the poloidal direction was also observed which may be related to the drift of the detached part of the cloud.

Chapter 6: H-Mode and Pedestal Physics in ASDEX Upgrade

Studies in ASDEX Upgrade of the phenomenology and scaling of the H-mode transition, of edge-localized modes (ELMs), and characterization of the H-mode edge transport barrier carried out in various experimental campaigns between 1996 and 2001 are described. The H-mode transition is recognized by formation of a radial electrical field at the plasma boundary, which in ASDEX Upgrade is detected by an associated increase of the neutral particle charge exchange flux from ripple trapped particles. A scaling for the critical local edge temperature for the H-mode transition threshold is found. Similarity experiments with ASDEX Upgrade and Joint European Torus plasmas for the H-mode transition indicate that the H-mode transition can be obtained at the same values of dimensionless parameters ρ*, ν*, and β at the plasma edge, indicating that the threshold scaling is normally not dominated by atomic physics processes. Energy losses due to ELMs are examined. Different types of ELMs can be obtained, depending on plasma edge temperature and magnetics configuration. An interesting regime is the type II ELMy H-mode for configurations near double null, where the peak heat flux to the target is much reduced compared to large type I ELMs. High-resolution Thomson scattering measurements show that the edge transport barrier width in ASDEX Upgrade shows only weak variations, while the pedestal top electron pressure and pressure gradient strongly depend on the plasma current, or value of Bt/q95.

Characterization of ion heat conduction in JET and ASDEX Upgrade plasmas with and without internal transport barriers

In ASDEX Upgrade and JET, the ion temperature profiles can be described by R/LTi which exhibits only little variations, both locally, when comparing different discharges, and radially over a wide range of the poloidal cross-section. Considering a change of the local ion heat flux of more than a factor of two, this behaviour indicates some degree of profile stiffness. In JET, covering a large ion temperature range from 1 to 25 keV, the normalized ion temperature gradient, R/LTi, shows a dependence on the electron to ion temperature ratio or toroidal rotational shear. In particular, in hot ion plasmas, produced predominantly by neutral beam heating at low densities, in which large Ti/Te is coupled to strong toroidal rotation, the effect of the two quantities cannot be distinguished. Both in ASDEX Upgrade and JET, plasmas with internal transport barriers (ITBs), including the PEP mode in JET, are characterized by a significant increase of R/LTi above the value of L- and H-mode plasmas. In agreement with previous ASDEX Upgrade results, no increase of the ion heat transport in reversed magnetic shear ITB plasmas is found in JET when raising the electron heating. Evidence is presented that magnetic shear directly influences R/LTi, namely decreasing the ion heat transport when going from weakly positive to negative magnetic shear.

Measurement of spurious impurity concentrations in ASDEX Upgrade by x-ray spectroscopy

A two channel multicrystal spectrometer of Bragg geometry was constructed for measurements of absolute photon fluxes of ASDEX Upgrade plasmas in the wavelength range from 0.5 up to 26 Å. The calibration of the spectrometer was done by an ab initio calculation using the absolute reflection data from literature of four crystals. The calibration was validated by the direct comparison with other absolute calibrated diagnostics at the ASDEX Upgrade as well as by using the impurity transport code STRAHL. The absolute impurity concentrations of elements from N up to Cu have been extracted. O and F appear to be main impurity components after C, which is routinely measured by an independent diagnostic. Despite the significantly lower concentration of Ar, it contributes measurably to Zeff and can account for main plasma impurities as well. Other impurities either have never been seen or appear only sporadically (like Fe and Cu) and normally do not contribute significantly to the plasma radiation in the soft x-ray range.

Experimental characterization of the electron heat transport in low-density ASDEX upgrade plasmas.

The electron heat transport is investigated in ASDEX Upgrade conventional L-mode plasmas with pure electron heating provided by electron-cyclotron heating (ECH) at low density. Under these conditions, steady-state and ECH modulation experiments indicate without ambiguity that electron heat transport exhibits a clear threshold in inverted Delta T(e)/T(e) and also suggest that it has a gyro-Bohm character.

Radial Electric Field Shear Due to Neoclassical Effects in Transport Barriers

The Er×B flow shear is simulated with a fully kinetic five-dimensional neoclassical Monte Carlo simulation for JET and ASDEX Upgrade plasma edge conditions. Here, Er is the radial electric field and B is the magnetic field. It is shown that high enough shear for turbulence suppression can be driven at the Low (L) to High (H) transition conditions by pure neoclassical effects including ion orbit losses. Simulations indicate higher threshold shear for ASDEX Upgrade than for JET. Shear values are compared to different models and experimental results of critical shear, and isotope effect on shear is discussed. Formation of an internal transport barrier in FT-2 in the presence of lower hybrid waves is also studied.

ELM phenomenon as an interaction between bootstrap-current driven peelingmodes and pressure-driven ballooning modes

An ELMy ASDEX Upgrade plasma equilibrium is reconstructed takinginto account the bootstrap current. The peeling mode stability of theequilibrium is numerically analysed using the GATO [1] code, and it is foundthat the bootstrap current can drive the plasma peeling mode unstable. Ahigh-n ballooning mode stability analysis of the equilibria revealed that,while destabilizing the peeling modes, the bootstrap current has a stabilizingeffect on the ballooning modes. A combination of these two instabilities is apossible explanation for the type I ELM phenomenon. A triangularity scanshowed that increasing triangularity stabilizes the peeling modes and canproduce ELM-free periods observed in the experiments.

Multiprocessor systems for real-time data acquisition on the Asdex upgrade and future plasma experiments

In this paper we present our transputer-based multitop multiprocessor systems for data acquisition, which are currently used on the Asdex upgrade experiment. The bandwidth of these systems goes from low-speed like the calorimetry diagnostic up to highspeed and large data volume systems like the soft-X-ray and Mirnov diagnostics, which collect several hundreds of megabytes of data during a plasma discharge of ≈8 s. Further, we present the multitop-MX, a newly developed system based on transputers and powerPCs, which provides real-time facilities for analysing the acquired data, to generate necessary information for the dynamic adaptation of sample rates, and to deliver triggers when certain events in the plasma are detected. The algorithm running on the powerPCs performs a wavelet like time-frequency transform. In the last part we give an outlook how to build the next generation of data acquisition systems to be used on the future plasma experiments W7-X and ITER, but also on Asdex upgrade. The hardware of these new distributed systems should be mainly based on established industry standards like the VME-bus, PCI-bus and FiberChannel, but also emerging technologies like SCI (scalable coherent interconnect) should be considered. The systems software should be well designed with object oriented methods to simplify the maintenance process and to enable further expansions and adaptations to new problems in an easy way.

On the application of the Wigner–Ville distribution to broadband reflectometry

Herein we propose a data processing method to analyse broadband reflectometry signals based on the Wigner–Ville distribution (WVD). The method offers the advantage of good localization of the reflectometry signal energy in the time-frequency domain. Simulations are presented to test the ability of this technique to follow the average beat frequency that is the relevant quantity to invert the plasma profile. The method is compared with the short-time fourier transform (STFT) analysis both for simulated and for ASDEX Upgrade plasmas. It is shown that the WVD has potential advantages for the routine evaluation of density profiles. The method can also be useful to study fluctuations, namely when abrupt changes of turbulence occur at the L–H transition.

H-mode studies with microwave reflectometry on ASDEX Upgrade

The microwave reflectometry system on ASDEX Upgrade measures density profiles (in broadband swept operation) and plasma fluctuations (fixed-frequency operation) both at the high-field side (HFS) and low-field side (LFS). Densities up to can be probed. We analyse the evolution of turbulence at the L-H transition and during the ELMy phase of H-mode discharges. The detailed density profile evolution during type I ELMs is resolved and profile oscillations associated with ELM precursors are studied.