The Academy of Sciences of the Czech rebublic - ASCR


2mm microwave interferometer

Measured quantities:

Plasma electron density averaged along the tokamak diameter

Spatial resolution:

Line averaged value

Temporal resolution:

20 μs

Responsible person:

J. Zajac, F. Žáček

Collaboration:

Culham Centre for Fusion Energy, Abingdon, United Kingdom


Diagnostic description:
 
The phase shift generated by COMPASS plasma on the frequency 130 GHz reaches commonly several tens of fringes (jumps of measured phase if the real phase goes through the integral multiple of 2π; the number of jumps depends on the plasma density and plasma dimension). Utilization of two near frequencies 131 GHz (interferometer 1) and 133 GHz (interferometer 2), propagating through plasma in opposite directions, see Fig. 1, enables to avoid the fringes (to operate so called unambiguous interferometer). Namely, output without fringes is necessary condition for control density feed-back system. For this purposes, not only the phase difference between the reference a probing wave is determined (as it is done in the case of both interferometer 1 and 2), but the mutual phase difference between the both probing waves is registered as well. This phase difference is 65-times smaller than output of the both interferometers (given by the relative frequency difference of both probing waves). By mixing of the both wave frequencies, the circuit first IF down-conversion is done to the frequency 2&nbsnbsp;GHz. The second down-converted IF, where the carrying wave phase caused by plasma is evaluated (phase detectors, see figure), is 455 kHz. In this way the system delivers three independent outputs, two with phase jumps, one without jump.

 


Scheme of the 2 mm microwave interferometer

 

Fig. 1: Scheme of the 2 mm microwave interferometer.