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Drift-Type Waves in Rotating Tokamak Plasma
Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Physics, Department of Astronomy and Space Physics.
2003 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The concept of energy production through the fusion of two light nuclei has been studied since the 1950’s. One of the major problems that fusion scientists have encountered is the confinement of the hot ionised gas, i.e. the plasma, in which the fusion process takes place. The most common way to contain the plasma is by using at magnetic field configuration, in which the plasma takes a doughnut-like shape. Experimental devices of this kind are referred to as tokamaks. For the fusion process to proceed at an adequate rate, the temperature of the plasma must exceed 100,000,000C. Such a high temperature forces the plasma out of thermodynamical equilibrium which plasma tries to regain by exciting a number of turbulent processes. After successfully quenching the lager scale magnetohydrodynamic turbulence that may instantly disrupt the plasma, a smaller scale turbulence revealed itself. As this smaller scale turbulence behaved contrary to the common theory at the time, it was referred to as anomalous. This kind of turbulence does not directly threaten existents of the plasma, but it allows for a leakage of heat and particles which inhibits the fusion reactions. It is thus essential to understand the origin of anomalous turbulence, the transport it generates and most importantly, how to reduce it. Today it is believed that anomalous transport is due to drift-type waves driven by temperature and density inhomogeneities and the theoretical treatment of these waves is the topic of this thesis.

The first part of the thesis contains a rigorous analytical two-fluid treatment of drift waves driven solely by density inhomogeneities. Effects of the toroidal magnetic field configuration, the Landau resonance, a peaked diamagnetic frequency and a sheared rotation of the plasma have been taken into account. These effects either stabilise or destabilise the drift waves and to determine the net result on the drift waves requires careful analysis. To this end, dispersion relations have been obtained in various limits to determine when to expect the different effects to be dominant. The main result of this part is that with a large enough rotational shear, the drift waves will be quenched.

In the second part we focus on temperature effects and thus treat reactive drift waves, specifically ion temperature gradient and trapped electron modes. In fusion plasmas the α-particles, created as a by-product of the fusion process, transfer the better part of their energy to the electrons and hence the electron temperature is expected to exceed the ion temperature. In most experiments until today, the ion temperature is greater than the electron temperature and this have been proven to improve the plasma confinement. To predict the performance of future fusion plasmas, where the fusion process is ongoing, a comprehensive study of hot-electron plasmas and external heating effects have been carried out. Especially the stiffness (heat flux vs. inverse temperature length scale) of the plasma has been examined. This work was performed by simulations done with the JETTO code utilising the Weiland model. The outcome of these simulations shows that the plasma response to strong heating is very stiff and that the plasma energy confinement time seems to vary little in the hot-electron mode.

Place, publisher, year, edition, pages
Uppsala: Institutionen för astronomi och rymdfysik , 2003. , p. 34
Series
Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1104-232X ; 837
Keywords [en]
Space and plasma physics, fusion, plasma, tokamak, rotation, drift, ship, ITG, TE, Landau, Te/Ti, hot-electron, confinement, stiffness, Weiland
Keywords [sv]
Rymd- och plasmafysik
National Category
Fusion, Plasma and Space Physics
Research subject
Space and Plasma Physics
Identifiers
URN: urn:nbn:se:uu:diva-3400ISBN: 91-554-5625-1 (print)OAI: oai:DiVA.org:uu-3400DiVA, id: diva2:162669
Public defence
2003-06-05, Polhemsalen, Ångström Laboratory, Uppsala, 10:00
Opponent
Supervisors
Available from: 2003-05-15 Created: 2003-05-15Bibliographically approved
List of papers
1. Ship-Wave Eigenmodes of Drift Type in Rotating Tokamak Plasmas
Open this publication in new window or tab >>Ship-Wave Eigenmodes of Drift Type in Rotating Tokamak Plasmas
2000 (English)In: Physica scripta. T, ISSN 0281-1847, Vol. 62, no 2-3, p. 169-176Article in journal (Refereed) Published
Abstract [en]

Ship waves of drift type in rotating plasma of axisymmetric, large aspect-ratio tokamaks with concentric, circular magnetic surfaces are investigated. Plasma rotation is driven by an electrostatic radial electric field and the waves under consideration may be excited by plasma flow past some static obstacle. The analysis performed is based on rigorously derived eigenmode equations coupled in poloidal mode numbers through toroidal effects. The existence of two qualitatively different types of ship wave eigenmodes is demonstrated. Namely (i) global modes that have a structure of quasimodes localised in both radial and poloidal directions and correspond to the bounded states in a potential well which are marginally stable and (ii) propagating modes that experience shear convective damping. The dispersion relations for both types of eigenmodes are obtained both in the weak and the strong coupling approximation.

We find the analytical solutions of the dispersion relations and the regions of their existence which are defined, mainly, by the value and direction of the poloidal rotation velocity. An accumulation point for global ship eigenmodes is determined. For propagating ship waves the real and the imaginary parts of the poloidal wavenumber are found. It is shown that the imaginary part always corresponds to convective damping of these waves

Keywords
Ship waves of drift type in rotating plasma of axisymmetric, large aspect-ratio tokamaks with concentric, circular magnetic surfaces are investigated. Plasma rotation is driven by an electrostatic radial electric field and the waves under consideration ma
National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-90344 (URN)10.1238/Physica.Regular.062a00169 (DOI)
Available from: 2003-05-15 Created: 2003-05-15 Last updated: 2017-12-14Bibliographically approved
2. Localising Effects of a Peaked Diamagnetic Frequency on Drift Modes in Rotating Tokamak Plasmas
Open this publication in new window or tab >>Localising Effects of a Peaked Diamagnetic Frequency on Drift Modes in Rotating Tokamak Plasmas
2002 (English)In: Physica scripta. T, ISSN 0281-1847, Vol. T98, no -, p. 151-154Article in journal (Refereed) Published
Abstract [en]

In this paper we investigate the effects of a diamagnetic frequency peaking and velocity shear on a two-dimensional drift mode structure. Previous study made by Horton et al. showed that a strong diamagnetic frequency peaking can trap the wave energy both radially and along the magnetic field lines. We show that the localising effect of the diamagnetic frequency peaking can be suppressed by a strong velocity shear in the edge plasma. The same phenomenon is present in the bulk plasma, but due to the velocity shear not being as pronounced there, the effect is nominal.

National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:uu:diva-90345 (URN)10.1238/Physica.Topical.098a00151 (DOI)
Available from: 2003-05-15 Created: 2003-05-15 Last updated: 2017-12-14Bibliographically approved
3. Stability of the Landau Resonance for Drift Modes in Rotating Tokamak Plasma
Open this publication in new window or tab >>Stability of the Landau Resonance for Drift Modes in Rotating Tokamak Plasma
2003 (English)In: Journal of Plasma Physics, ISSN 0022-3778, E-ISSN 1469-7807, Vol. 60, no 5, p. 371-Article in journal (Refereed) Published
Abstract [en]

The linear stability of drift waves in a poloidally rotating tokamak plasma is considered. The derived dispersion relation features a peaking of the diamagnetic frequency which gives the drift modes an irreducible two-dimensional character. We then show that inverse Landau damping can be suppressed and even stabilized, if the flow's shear is strong. Even though the instability, excited by the Landau resonance, is stronger at a high velocity shear for positive rotation velocities, effects due to the rotation of the plasma can reverse the sign and induce damping of the two-dimensional drift modes. This stabilizing mechanism works only for positive rotation velocities. For negative rotation velocities, we show that only modes with high poloidal mode numbers are unstable.

National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:uu:diva-90346 (URN)10.1017/S0022377803002253 (DOI)
Available from: 2003-05-15 Created: 2003-05-15 Last updated: 2017-12-14Bibliographically approved
4. JETTO Simulations of Te/Ti Effects on Plasma Confinement
Open this publication in new window or tab >>JETTO Simulations of Te/Ti Effects on Plasma Confinement
Show others...
Manuscript (Other academic)
Identifiers
urn:nbn:se:uu:diva-90347 (URN)
Available from: 2003-05-15 Created: 2003-05-15 Last updated: 2010-01-13Bibliographically approved

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