Cooling of electrically insulated high voltage electrodes down to 30 mK

AEgIS [1] is an antimatter experiment, using high voltage electrodes at 100 mK. In this work two possible principles to cool these electrodes with a dilution refrigerator are described: the Rod and the Sandwich. The metallic Rod is electrically insulated by a ceramic and connects a single electrode directly with a heat exchanger placed in the mixing chamber. The Sandwich consists of an elec-trically insulating sapphire plate, at both sides covered with indium. The total thermal resistivities of the Rod and of different Sandwich samples are measured between (30 and 130) mK. The lowest resistivity of the Sandwich is achieved with indium vapour deposited on polished sapphire (26 cm2K4/W at 30 mK). The resistivity of the Rod is significantly lower (0.5 cm2K4/W at 30 mK).