IPP

 IWV

 KFS

 ZEL

 ZAT

In charge: Prof. U. Samm, IPP, u.samm@fz-juelich.de

Aims and Objectives

The project participates in the further development of magnetic confinement concepts for the realization of nuclear fusion as a new primary energy source. The central facility is the tokamak TEXTOR, which is operated in collaboration with the partners from the Trilateral Euregio Cluster. With its highly flexible instrumentation TEXTOR is oriented towards the investigation of fundamental processes in fusion plasmas. Main element of reporting year 2003 was the commissioning of the newly installed Dynamic Ergodic Divertor (DED). This unique experiment serves, amongst other things, for the improvement of the heat load distribution on the wall of the plasma vessel and for the control of plasma confinement and stability. Additional emphasis is focussed on the participation in the scientific programme of the European fusion experiment JET, the construction and subsequent utilization of the stellarator Wendelstein 7-X, and on the planning of the next step tokamak experiment ITER, which for the first time shall demonstrate a burning fusion plasma.

Significant Results in 2003

The DED was successfully taken into operation with direct and alternating currents up to a value of 7 kA and with frequencies up to 7 kHz, producing rotating magnetic fields in the dynamic mode. With the DED, a novel and unique tool has been provided for dynamically influencing the magnetic field topology by ergodisation with external coils. In combination with specialised diagnostic methods already installed at TEXTOR, new chances and potentials for basic studies have thus been opened, addressing the enhanced optimisation of the heat and energy transport as well as the stability of magnetic confinement. The relevance of ergodic structures has also been shown during the reporting year by means of collaborative experiments at the DIII-D tokamak in the USA. Here, already by static ergodisation in a mode of operation also being planned for ITER, negative effects due to boundary instabilities (so-called ELMs or edge localized modes) could be reduced considerably. Now the challenge is to achieve a better understanding of the basic mechanisms and - as a result - to contribute to an improved operation of ITER.

The set-up of plasma diagnostic systems still was of major importance during the reporting year. First measurements in combination with the DED confirmed predictions of the heat load distribution on wall elements caused by the magnetic field perturbation. Beside the expected smearing and the subsequent reduction of the heat load, a manifold of modifications of the magnetohydrodynamic features of the plasma could be observed - even though this is not understood yet. The development of a 3-dimensional hydrodynamic Monte Carlo code for the description of plasma transport in the complex magnetic field geometry of the DED was completed. In collaboration with a group at the University of Marseille and the Max-Planck-Institut für Plasmaphysik first attempts have been made to incorporate the DED into turbulence models.

For the further development of the ITER reference scenario, the H-mode, comprehensive studies have been conducted at JET. In the field of plasma stability the project made important contributions to the control of neoclassical tearing modes, which in ITER would result in undesirable deterioration of the plasma confinement, and in particular to the understanding of the ELM physics. Here, new properties were discovered which are not in agreement with the presently accepted explanation of the ELM cycle. Based on earlier TEXTOR results, a new feedback control scheme was developed for the quasi-stationary sustainment of JET discharges at high density. However, theoretical modelling of these plasma conditions (with the RITM code) shows that in JET the improvement of confinement does not take place, which in TEXTOR was observed under similar conditions.

In order to achieve a sufficient life time for materials, which are in direct contact with the plasma, for their later use in ITER or a fusion reactor, extensive studies of the plasma wall interaction are necessary. To this end, in JET and TEXTOR the carbon erosion and re-deposition has been investigated in-situ. Numeric models - such as the ERO code - play an important role for the interpretation of the experimental data. The observed carbon balances can only be understood in case an unexpectedly high erosion rate is assumed for the freshly deposited layers. The mechanism still is unclear up to now. Furthermore, the review and improvement of the description of atomic and molecular processes play an important role.

An important spin-off has resulted from a unique cooperation between the IPP, the University of Düsseldorf and Philips Lightning. In this project, the similarities of the physical properties in the ITER edge plasma and gas discharge lamps are utilized. Measurements with gas discharge lamps serve as a validation of the Monte Carlo code EIRENE, developed in Jülich, which subsequently will be used to describe parts of the ITER edge plasma.

Emphasis is also put on the development of plasma diagnostics and heating methods, also for their later use in Wendelstein 7-X or ITER. Worth mentioning are the further development of vacuum-UV spectroscopy, charge exchange recombination spectroscopy and motional Stark effect diagnostics. The construction of a combined electron cyclotron emission imaging and microwave imaging reflectometer system has been completed. A new 140 GHz gyrotron with 3 s pulse length and a power of 800 kW for heating the plasma electrons successfully went into operation - thus enriching the spectrum of heating methods available at TEXTOR with a flexible new instrument.

For the characterization and test for materials for their use in future devices, such as Wendelstein 7-X and ITER, the electron beam test facility JUDITH was employed, providing thermal loads of up to 20 MW/m2. Various tests included the study of thermal expansion behaviour of graded materials, dust generation at intense transient heat loads, neutron induced degradation of the thermal expansion coefficients and different target designs after neutron irradiation with fluences of up to 1 dpa.

The project will also take over extensive work packages for the construction of the stellarator Wendelstein 7-X, consisting of the development of diagnostic systems, tasks concerning the design and construction of components for the superconducting coils and bus bar system, the electrical joints as well as supporting tasks in welding technology.