Helium cryopump for electron optical devices design and preliminary cryogenic testing

A helium cryopump for electron optical devices was designed and preliminary cryogenic parameters of the first pump sample were tested. Low LHe consumption should enable economical operation of the pump without recycling the evaporated He. The pumping cryopanel is cooled by normal LHe down to 4.2 K. A LN2 bath is utilized for the LHe bath thermal shielding. Two additional heat radiation shields cooled by the vapours of the consumed cryoliquids are installed to decrease the LHe and LN2 losses. Three chevron screens placed between the input office and the cryopanel are used to protect the internal parts against direct heat radiation from the parts at ambient temperature. The chevron systems were replaced by low emissivity screens in the preliniary tests. Predicted low evaporation rates of LHe and LN2 were achieved

Small Superconducting Magnet for Low Temperature Nuclear Orientation

A new low temperature nuclear orientation facility was installed in the Faculty of Mathematics and Physics at Charles University in Prague. The facility is based on the small dilution refrigerator 3He-Ne of the "French" type and principally used for solid state physics research. The refrigerator allows to cool the radioactive samples down to 10 mK in the magnetic field of superconducting solenoid up to 4 T. The gammaray anisotropy is observed exclusively in the direction of the applied vertical magnetic field which is mostly suffiecient for the solid state study

A new construction of rotor of helium expansion turbine

The aim of our work was to develop a rotor of the expansion turbine designed for a helium liquefier that would enable operation in different modes and achieving of the desired effectiveness at same time. For meeting theserequirements the rotor should enable precise operation of the turbine in the range of 140 000 to 350 000 rqm at minimum heat flow to the low-temperature part of the turbine. Considering the low thermal conductivity and a high strength, the low-temperature part was made of VT3-1 titanium alloy. This contribution is focused mainly on problems of creation of mechanically strong and thermally resistant joint of this part of the rotor with the part working at room temperature

Cryostat for optical measurements in temperature range 80 - 300 K

A liquid nitrogen bath cryostat of classical concept was designed, manufactured and tested. A 0.4 litres vacuum-insulated inner vessel is suspended on a thin-wall neck. The specimen folder equipped with a temperature sensor and a heating winding is connected to the bottom of the inner vessel by means of a low-conductivity stainless steel tube. Copper foil strips of different cross-section areas are used to span this tube so that the entire thermal resistance between the vessel and the specimen holder can be adjusted in wide range. The rate of the specimen cooling can be changed this way. Using methods that consider both the non-linear dependence of thermal properties of materials and the influence of heat radiation, evaluations of the specimen cooling rate for different magnitudes of the thermal resistance have been made. Nitrogen consumption in various cryostat regimes was calculated too. These predicted values were compared with the experimental ones

Simulation of chevron baffle system for small helium bath cryopump

The inlet of a small helium bath cryopump was simulated by Monte Carlo method. Baffle screens of chevron type or straight plates were taken into account. The pump speed and heat loads of baffle screens were evaluated for a real geometry and the acceptable baffle configuration was proposed