Electrostatic Turbulence in magnetised plasmas

Research project P 14545 Electrostatic Turbulence in magnetised plasmas Roman SCHRITTWIESER 09.10.2000 The investigation of fusion plasma phenomena in order to realise the future production of energy in a fusion reactor is one, of the most important goals of plasma physics Here it is especially the study of the radial loss of particles from a magnetically confined hot fusion plasma which poses a great challenge to the field of physics. In spite of strong theoretical and experimental efforts the reasons for the inexplicable high particle loss, which impairs the confinement and, impedes the attainment of the breakeven condition, are still far from being comprehensively understood. An important loss mechanism is due to strong electrostatic fluctuations in the scrape off layer of a magnetically confined plasma. These fluctuations of the electric field lead to turbulent transport in the edge region of a plasma column or plasma ring, the mechanism of which needs thorough investigations not only, in fusion plasma devices but also in laboratory plasma machines. For this purpose it is very important to determine the dc and the ac components of the electric field, and to measure the cross-correlation between its radial and poloidal fluctuating components. This is particularly important for the physics of the L-H transition (i.e. the transition from the low density to the high density mode) in a tokamak plasma. Moreoever, sheared poloida I flows have a stabilising effect for turbulent transport in, fusion plasmas, and one of the mechanisms to produce such a shear is a radially varying Reynolds. stress. The main purpose of the project is to determine this Reynolds stress in a number of different plasma devices, and this shall be done in collaboration between two European fusion plasma laboratories (Lisbon and Madrid) and three European laboratory plasma groups, the applicant`s group (the Innsbruck Experimental Plasma Physics Group EPPG) and the groups in Iasi, Romania, and in Ljubljana, Slovenia. To determine the electric field, a plasma diagnostic tool, novel for fusion machines, shall be used,, namely electron emissive probe according to the special design of the IEPPG. Only with such type of probes the plasma potential (and thereby the electric field by using several probes) can be determined without strong systematic errors. In the framework of the project, also the improvement of emissive probes for fusion devices is planned. In addition, the radial transport shall be measured and correlated with the experimentally determined Reynolds stress. In this way, new important insights into the fluctuation phenomena in the edge region of various magnetised plasmas and into the formation and effects of turbulence shall be won.

In addition to (i) edge plasma turbulence, the main subject of this research project, two closely related topics of plasma physics were also treated: (ii) the investigation and further development of electric plasma probes and (iii) the formation of complex space charge structures. In all three subjects the progress and success of the project was extraordinary. Electric plasmas are ionised gases and are also called the fourth state of matter. Modern plasma physics is essential for nuclear fusion, technical and biophysical applications and space plasmas. There are numerous industrial processes which use plasmas. On the other hand fusion energy is becoming a feasible alternative as virtually inexhaustible energy supply for the future. Electrostatic turbulence is a common phenomenon of plasmas. Turbulence appears especially at edges where density and electric potential of the plasma show strong gradients. There, even in the case of a magnetically confined plasma, turbulence can lead to an enhanced transport of particles and thus to a loss of plasma. This is especially unfavourable in the case of fusion plasma, which has to be confined long enough at very high densities and temperatures in order to produce more energy by fusion processes than had to be invested to push it to these conditions. Our investigations were part of the Association of the Austrian Academy of Sciences with the EURATOM fusion programme. The experiments were carried out in European toroidal fusion machines in Portugal, Spain, Czech Republic and Germany. We have succeeded to measure directly essential edge plasma parameters for the first time simultaneously, in particular the radial fluctuation-induced particle flux and the Reynolds stress. We found experimental evidence for the correctness of the theoretical presumption that a radial gradient of the Reynolds stress reduces the particle flux. This is a very important finding for magnetic fusion experiments with a direct bearing to the future large fusion experiment ITER (International Tokamak Experimental Reactor). In order to measure the above mentioned plasma parameters directly we had partially to develop new plasma diagnostic tools. These were special plasma probes which are able to emit an electron current into the plasma. In this way a direct determination of the plasma potential became possible. We were worldwide the first group to use electron-emissive probes in fusion experiments and have thereby not only performed pioneering work in the investigation of edge plasma turbulence but also in the development of plasma diagnostic tools. Our most recent development was a laser-heated emissive probe. The third topic of the project was concerned with the investigation of complex space charge structures which can appear in all types of plasmas under the influence of strong currents or partcle beams. The most important subjects of our research were so-called fireballs in front of an additional anode in a discharge plasma. Also in this case we have made very essential new discoveries which led to a new and better understanding of such phenomena. Our group was one of the first to observe multiple fireballs which can either appear as concentric shells or as adjacent structures near each other.