Experimental investigation of a probe-induced localized electron temperature elevation near electron-cyclotron resonance

A spatially localized electron temperature elevation is observed near the electron-cyclotron resonance region. The properties of this phenomenon are examined in detail. The dependence of the temperature elevation upon launched wave amplitude and frequency relative to the local electron-cyclotron frequency, plasma density, neutral pressure, microwave cavity plasma source tuning, and probe type is discussed. These results are combined with detailed wave electric and magnetic field, emissive probe and spectral measurements and the use of different Langmuir probe types and orientations to characterize the temperature elevation, background plasma, and wave properties. It is found that the magnetic field and wave absorption near the electron-cyclotron resonance, probe type, and probe tip input impedance as seen from the plasma have important influences on the temperature elevation process. Two-probe correlation measurements are used to show that the elevation process exists in the radial neighborhood (<1 cm) of the probe tip and along a flux tube parallel to the magnetic field. This process can be present in all Langmuir probe-diagnosed ECRH experiments and can be suppressed by appropriate probe design. Measurements suggest that the electron temperature elevation is due to the interaction of sufficiently strong wave electric fields near electron-cyclotron resonance with an appropriate probe tip input impedance as seen by the fields