Sensor for plasma density profile measurement in magnetic fusion machine

A method is presented for the measurement of the electron density in an experimental plasma fusion device by means of microwave reflectometry. Since microwaves with increasing frequency can penetrate deeper into the plasma, it is possible to sample the electron density distribution by frequency-sweep techniques. For the first time the system allows a non-contact measurement of the electron density profile in the outermost region of a magnetically confined plasma without separate calibration. The influence of strong electron density fluctuations of up to 50% with frequencies up to 500 kHz has been eliminated by a fast sweep from 60 to 80 GHz with a sweep time of about 150 μs. In connection with microwave plasma heating experiments at 2.45 GHz, the theoretically predicted dependence of the reflection coefficient of the heating wave on the electron density has been confirmed.