A new principle for bolometric measurements has been invented at JET¹. Its main feature is that a change in resonant frequency is measured rather than voltage levels. As a result, it should be virtually immune to the type of noise that is encountered in fusion research.

Bolometers involve the use of a temperature sensitive element, usually a resistor. On a tokamak like JET a number of problems arise, most of which are inherent to the current design: large number of cables and vacuum feedthroughs that can lead to connection problems; electro-magnetic pick-up and interference². In future devices remote handling operations require much fewer cables. Also the cable length will increase and the measuring environment become more severe, leading to decreasing signal to noise ratios. Radiation hardness is a further important requirement, that could probably not be met by the existing detectors.

The sensitive element of the new design is a temperature sensitive ferroelectric capacitor within a tuning circuit (TC). When its capacitance changes the resonant frequency of the TC is shifted. This shift is tracked by phase-locked loop electronics (PLL), which is connected by matched transmission lines. The new system can resolve very low power levels, due to the stronger temperature effect of the ferroelectric compared to the resistor, making it an ideal high performance sensor. Capacitance measurements are insensitive to electro-magnetic pick-up and the cables used guarantee sound connections. The TC operates up to very high temperatures and is radiation hard. Another important advantage derives from measuring in the frequency regime: The PLL circuitry is being designed to allow separation of the frequency in question from the rest of the band, so that multiple signals can be sent down a single transmission line.

¹UK Patent Application "Ferroelectric Bolometer", Provisional No: 9623139.4, Nov. 1996

²R. Reichle, "Bolometer for ITER", Varenna Conference 1995