B. Streibl

Vertical displacement events and halo currents

This review examines results from all non-circular tokamaks with a distinct emphasis on investigations in ASDEX-Upgrade. There a major fraction of the experimental time has been dedicated studying vertical displacement events of single null plasmas over a large range of q-values in an attempt to obtain the scaling of both the displacement dynamics and the splitting of forces between those associated with poloidal and toroidal plasma currents as a function of q and Bt. These studies on different tokamaks are accompanied by simulations with-among other codes-the tokamak simulation code TSC, in a version where halo currents flowing in the plasma scrape-off layer (SOL) evolve self-consistently. The technical consequences of VDEs for the machine design, measures taken and first predictions are discussed. Safety setups that have been developed and possible avoidance strategies are briefly described.

Properties of the new divertor IIb in ASDEX Upgrade

A new divertor configuration (DIV IIb) has been implemented in ASDEX Upgrade. In order to accommodate a large variety of plasma shapes with bottom triangularities (δ bot ) up to 0.48, the outer strike point region was modified and the roof baffle was lowered and diminished at its outer part in comparison with the previous divertor (DIV II). The inner part of the divertor strike point module remains unchanged, but at the divertor entrance a smooth transition to the central column is provided to minimize local hydrogen recycling. According to experiences with power handling in DIV II, ordinary fine grain graphite has been chosen for the outer strike point and, as before the tiles are slightly tilted in toroidal direction to hide the leading edges. A first characterization of DIV IIb reveals that the beneficial behaviour of DIV II is essentially maintained. There is an increase of the power density due to geometrical reasons at the outer target, whereas the divertor radiation for similar magnetic configurations is unchanged. The pumping characteristics for D and He are almost retained, suggesting a large influence of the inner divertor leg, the configuration of which remains unchanged. A significant reduction (20%) of the L-H threshold is observed consistent with larger temperature gradients inside the separatrix just before the transition.

ASDEX-Upgrade Discharge Control and Shot Management

ASDEX Upgrades fully digital control system is described. Discharge control consists of 6 real time computers for discharge and system monitoring, position and shape control and extended plasma control, all synchronized by a supervisor for discharge phase switching. The timing system is integrated giving absolute time for discharge control and diagnostics. Event-dependent operation is supported. Hierarchically organized protection systems are closely coupled with the discharge and machine control systems. All systems run under a software platform for automated experiment operation.

Protection Strategy in the ASDEX Upgrade Control System

The global protection strategy of ASDEX Upgrade has three goals: protection of personnel, protection of the machine and termination of potentially dangerous discharges The new discharge termination system reacts to deviations from the discharge schedule before machine limits are violated. Its integration into the discharge control system allows for smooth termination under central control.

Discharge Supervision Control on ASDEX Upgrade

The Axially Symmetric Divertor Experiment(ASDEX) Upgrades 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 oftechnical and physical applications are shown.

Chapter 2: Machine Design, Fueling, and Heating in ASDEX Upgrade

Abstract The machine design, power supply, and machine protection, as well as the different heating systems installed at ASDEX Upgrade are discussed. The available auxiliary heating power of 30 MW, supplied by three different heating systems, allows the power deposition to be varied and species heated over a large range. These three heating systems — neutral beam, ion cyclotron, and electron cyclotron heating — are presented in detail. A description of the pellet refueling system is included, which is successfully used for density control.

Paralleling of two large flywheel generators for the optimisation of the ASDEX Upgrade power supply

Abstract The power supply of the ASDEX Upgrade (AUG) tokamak consists of three pulsed distribution systems (110–85 Hz), each supplied by a dedicated flywheel generator: EZ2 (1.45 GJ/167 MVA) which solely feeds the toroidal field coils, EZ3 (500 MJ/144 MVA) and EZ4 (650 MJ/220 MVA). For quasi-stationary advanced tokamak experiments with extended flat-top phase, the power systems of EZ3 and EZ4 must be connected in parallel, so that full advantage of the installed generator power and flywheel energy can be realised. The variable frequency of the pulsed network, fast load changes (up to 1000 MW/s), together with the different parameters of the generators and saturation effects during pulsed operation, require detailed numerical models for an investigation of the stability limits of a parallel operation of the two machines on one common 10.5 kV busbar. The paper describes the dynamic load requirements of a feedback controlled plasma experiment, the controller development for paralleling two large flywheel generators, the results of system studies with optimised power supply and implications of these results on the technical realisation.

Fabrication and testing of W7-X pre-series target elements

The assembly of the highly-loaded target plates of the WENDELSTEIN 7-X (W7-X) divertor requires the fabrication of 890 target elements (TEs). The plasma facing material is made of CFC NB31 flat tiles bonded to a CuCrZr copper alloy water-cooled heat sink. The elements are designed to remove a stationary heat flux and power up to 10 MW m−2 and 100 kW, respectively. Before launching the serial fabrication, pre-series activities aimed at qualifying the design, the manufacturing route and the non-destructive examinations (NDEs). High heat flux (HHF) tests performed on full-scale pre-series TEs resulted in an improvement of the design of the bond between tiles and heat sink to reduce the stresses during operation. The consequence is the fabrication of additional pre-series TEs to be tested in the HHF facility GLADIS. NDEs of this bond based on thermography methods are developed to define the acceptance criteria suitable for serial fabrication.

Completion of Assembly and Start of Technical Operation of ASDEX Upgrade

The last important steps of the tokamak machine assembly are described. A survey of precision and tolerances is given achieved for the vacuum vessel, the toroidal field (TF) magnet and the poloidal field (PF) coils. The first commissioning tests of vacuum vessel, TF magnet and PF coils are described together with certain features of the control and technical supervisory system. Remarks on improvements of the electric power supply system and the fast plasma control system follow. Finally the programme of the next future is briefly outlined.

Excitation of torsional oscillations in generator shaft lines by plasma feedback control

Abstract The ASDEX Upgrade (AUG) tokamak requires an electrical power up to a few hundred MVA for a time period of 10–20 s. The power and energy is provided by three separate networks based on flywheel generators. In 1999, during a routine check performed on generator EZ3, it was discovered that coupling bolts of the flywheel generator shaft were deformed. Given that the active load of the generator (∼100 MW) in service is well below the design value of the shaft (∼800 MW), the damages may only be explained by a torsional resonance of the shaft line, itself excited by active power transients from the converter loads. A value of 23.6 Hz was calculated for the first eigen-frequency of the shaft line. Frequencies between 10 and 30 Hz have been identified in the spectrum of the load curves. Since torsional shaft oscillations are characterised by very low damping, torsional resonance can become dangerous even for over-dimensioned generator shafts. Therefore, a novel ‘torque’ measurement system was installed. The paper presents results from calculations and measurements showing that devices capable to measure the stress in the shaft line are essential for generator protection in feedback controlled fusion experiments.