MXP – examples for application

The MXP testbed is used to produce RF breakdowns in controlled condition, for example for optical detection of arcs or tests of a guided radar arc detector.

Optical detection of arcs
Experiments have been carried out by a team led by Ecole Royal Militaire in Brussels to investigate the feasibility of an arc detector. The idea is to capture the light emitted by an arc by an optical fiber, with a filter tuned to the proper gas and a light detector.

The conclusions of the study:

  • Arcs have to be in the line of sight of the detector; therefore, the coverage of only single components (vacuum windows, T-junctions) is possible.
  • Optical detectors cannot work too near from the plasma because of the optical pollution from the plasma.
  • They cannot discriminate arcs from strong multipactors: this may be a problem for conditioning.
  • There are problems of maintenance because of dust deposition and irradiation of optical fibers which require regular cleaning and replacement.
  • To finish, it is an intrusive system with a complex design and problems of reliability.

Test of the GUIDAR (Guided Radar) detector

The GUIDAR arc detector is a new concept developed by Politecnico di Torino, based on the used of a UWB radar with encoded signal: It combines the advantage of short pulses (which have an excellent accuracy but are unfortunately sensitive to the background noise and jitter) with a pulse compression scheme, which uses a train of pulses with phase coding, increasing thus the signal energy and making it less sensitive to the noise.

As for the practical implementation of the system in a real ICRF system, it is necessary to have the best signal to noise ratio during the injection and reception of the signal in the transmission line where there is a high level of ICRF power. Therefore, studies were conducted on the MXP testbed to analyse the feasibility and to optimize the interface between the GUIDAR and the ICRF system. The method used directional couplers to avoid big, uneasy to handle, septate couplers. The experiments concluded on a promising detection of high voltage arcs in real conditions.

The figure gives an example of arc detection: It represents the magnitude of correlation between the injected and received signal – the peak thus corresponds to the presence of an arc on the signal path.

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