Profile dependent signature of the linear MHD spectrum
- FOM Institute for Plasmaphysics, Nieuwegein (Netherlands)
- JET Joint Undertaking, Abingdon (United Kingdom); and others
We have been studying in detail the spectrum of linear MHD waves in tokamak plasmas using the CASTOR code. This spectrum consists of global modes like Toroidicity induced Alfven Eigenmodes (TAE`s) that show up as resonances in active excitation experiments, and continua of modes that are localised on flux surfaces, in particular the Alfven continuum. From our numerical studies we have found that several aspects of the spectrum can be directly related to properties of the plasma equilibrium. From a single measurement it is possible to relate the continuum response to the plasma safety factor and density profiles near the edge. The dependence of TAE frequencies and damping rates on equilibrium profiles is more difficult to understand, however the dynamical behavior of the TAE`s can reveal changes in the safety factor profile in the center and at the edge of the plasma. For some parameter regimes the signature of the Alfven spectrum changes dramatically. We find that for an equilibrium with a non-monotonic safety factor profile a series of global Alfven modes appears. These modes are localized in the plasma center and have an increasing number of radial nodes with decreasing frequency. If there is sufficient information on the mode structure of the excited waves this can give additional information on equilibrium parameters. This work, that has been given the name MHD-spectroscopy, demonstrates the possible use of Alfven waves, that can be excited in a straightforward way by a set of external coils in tokamak experiments and measured by using, for example, the magnetic pick-up coils. The TAE measurements done in the JET tokamak using the saddle coils to excite Alfven waves at small amplitudes are especially suited for comparison with linearised MHD calculations. In this paper we will compare the results of our numerical calculations with active Alfven measurements performed at JET.
- OSTI ID:
- 489519
- Report Number(s):
- CONF-960354--
- Country of Publication:
- United States
- Language:
- English
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