Anomalous scattering of high power millimetre waves from gyrotrons at 140 and 110 GHz is investigated for plasma with rotating islands at TEXTOR. The magnetic field and plasma density influence the spectral content of the scattered waves and their power levels significantly. Anomalous strong scattering occurs in two density regimes, one at low densities and one at high densities, that also depend on the magnetic field. The two regimes are separated by a quiescent regime without anomalous scattering. Investigations suggest that scattering in the high-density regime is generated at the low-field side intersection of the gyrotron beam and the island position. The transition from the quiescent regime to the high-density regime occurs when the gyrotron frequency is twice the upper hybrid frequency at this position. There is some evidence that the scattering in the low-density regime is generated near the plasma centre. Under this assumption all the observed scattering is generated when the gyrotron frequency is near or below twice the upper hybrid frequency.
Bibliographical noteThis work, supported by the European Communities under the contract of Association between EURATOM and DTU, FOM and FZJ,was carried out within the framework of the European Fusion Programme. The contribution of Egbert Westerhof and Waldo Bongers has been performed in the framework of the NWO-RFBR Centre of Excellence on Fusion Physics and Technology (grant 047.018.002).
Nielsen, S. K., Salewski, M., Westerhof, E., Bongers, W., Korsholm, S. B., Leipold, F., ... Stejner Pedersen, M. (2013). Experimental characterization of anomalous strong scattering of mm-waves in TEXTOR plasmas with rotating islands. Plasma Physics and Controlled Fusion, 55(11), . https://doi.org/10.1088/0741-3335/55/11/115003