Low temperature tests of thermometer for very low temperature use

We will report the progress of trial manufacture of a cryogenic thermometers for very low temperature use currently in development at KEK. At the accelerator facilities in KEK, like SuperKEKB and other cERL and STF accelerators use the superconducting acceleration scheme. The electron beam is being accelerated by the superconducting accelerating cavities produced by Niobium which has a high transition temperature. Even though the Niobium's transition temperature is rather high, it is necessary to cool down to a temperature below 9 K. To achieve this low temperature, we have to provide 4 K liquid helium for SuperKEKB accelerator and 2 K super fluid for cERL and STF accelerators to make the cavities superconducting state. For stable operation of each of the above accelerator facilities, temperatures must be accurately measured in such a cryogenic environment, and feed back to the cryogenic control systems. So the thermometers which can work in the cryogenic environment are essentially necessary, and to develop those thermometers is the main task of this research. This thermometer consists of a temperature detection part using an optical fiber material and its jig part as a device to maintain the sensitivity in the extremely low temperature range. We are trying to utilize the fact that the specific heat of the metal crystal becomes extremely small, as suggested by Debye's model. We have examined the low-temperature characteristics of optical fibers with Bragg gratings and have confirmed the reflected light wavelength was changed according to the environmental temperature. By testing the actual combinations of bimetal alloys that assists the wavelength change in a low temperature environment, we aimed to put the thermometer into practical use. We will report the results of the ongoing tests. (author)