Ceramic Feedthroughs Research Articles

Determination of Residual Stresses in Ceramic-Metal Brazed Joint using Finite Element Analysis (FEA) and Experimental Validation of the Results

Ceramic vacuum feedthroughs are an inevitable requirement for any vacuum system which requires electrical feedlines to be inserted into the vacuum environment. These feedthroughs consist of metal-ceramic-metal transition and, therefore, require the brazing process as a joining technique. This process allows joining two base materials, i.e., Alumina and Kovar, for this case, which manifests different thermo-mechanical response. The difference between the coefficient of thermal expansion (CTE) of these materials causes the development of residual stresses during the cooling phase of the brazing process. Such residual stresses, if not addressed properly, can lead to the failure in the brazed joint even before the design limits. The purpose of this study is to assess these stresses by performing the thermo-mechanical analysis of the brazing process of ceramic-metal assembly through finite element analysis (FEA) technique. This study includes the assessment of non-linear behavior (due to temperature-dependent material properties) of Alumina and Kovar assembly. Further, X-ray diffraction (XRD) based residual stress measurement technique has been utilized to validate the FEA results. The paper shall present the FEA methodology (model, boundary condition, and results) followed by the experimental results and their comparison.

Steady-State Tests of High-Voltage Ceramic Feedthroughs and Coaxial Transmission Line for ICRF Heating System of the Large Helical Device

Steady-state ion cyclotron range of frequency (ICRF) heating technologies have been developed to heat plasma for >30 min in the Large Helical Device (LHD). Steady-state-operation tests of high voltages up to 40 kV0p for >30 min were carried out on radio-frequency (rf) vacuum feedthroughs and a coaxial transmission line in a test set. Four types of ceramic feedthroughs, each having a 240-mm diameter, were tested. Cone-type alumina ceramic and cylinder-type silicon nitride composite-ceramic feedthroughs produced good performances of 40 kV/30 min and 50 kV/10 s. The others had vacuum leaks when subjected to long-pulse duration. The temperature of the cone-type alumina ceramic feedthrough was measured during the ICRF operations. By using gas-cooling techniques, the temperature increase of the ceramic was substantially reduced and saturated within 20 min. Without any gas-cooling techniques, the temperature increased linearly and did not saturate. Therefore, this approach could not be used for steady-state operation. The rf dissipation on the ceramic was calculated using the ANSYS finite element computer code. It was found that damaged feedthroughs had local high heat spots, which could result in vacuum leaks. A 240-mm-diam water-cooled coaxial transmission line was designed and tested for steady-state operation. Specially designed connector components and Teflon insulator disks were tested. During the test operation, the insulation gases of nitrogen, sulfur hexafluoride, and carbon dioxide were used to compare their insulation capabilities for steady state. For the duration of a 10-s pulse, these gases performed well up to 60 kV0p. However, for steady-state operation, carbon dioxide gas could not withstand voltages >40 kV0p. The connector components of the transmission line performed without problems below 50 kV0p and 1 kA0p for 30-min steady-state operation. The performance of the feedthroughs and transmission line exceeded the specifications for steady-state heating in the LHD.

A superconducting coaxial transmission line for dilution refrigerators

A novel helium-gas-filled coaxial insert sealed with ceramic feedthroughs, which thermally grounds the superconducting niobium inner conductor of a coaxial transmission line at the 1.3-K pot of a dilution refrigerator, is described. This transmission line–insert combination overcomes the large heat leak associated with room temperature coaxial access to a dilution refrigerator and results in relatively unperturbed broadband transmission of radio-frequencies from room temperature to the mixing chamber, with a heat load of less than 1 μW. Two such lines are used to access a cw NMR system incorporated in an Oxford Instruments model 200 top-loading dilution refrigerator, resulting in a rise of the base temperature from 10 mK up to 12 mK. The thermal impedance of the insert with and without helium is analyzed between 1.5 and 6 K; it is shown that the heat leak can be further reduced by an order of magnitude with relatively minor modifications.

Failure analysis of ceramic feedthroughs used in traveling-wave tubes

The effective operation of a high-voltage vacuum device, such as a traveling-wave tube (TWT), is dependent on its ability to maintain a high vacuum environment. However, after the brazing process, cracks in thick-wall ceramic shells will cause feedthrough leakage and eventually will impair the operation of a TWT. To investigate the cause of cracking in a thick-wall ceramic shell and to collect reliable design information, the stress field obtained from a shrink-fit solid cylinder can be used as an approximation. The limitation of this approximation can be established. Due to the brittle character of ceramic structures, the theory of linear elastic fracture mechanics can be used to study the ceramic feedthrough structure. With the known stress fields and crack sizes, the stress intensity factors of a typical feedthrough structure can be evaluated. The design curves for allowable crack sizes under different radial pressures can be plotted as well.

New deflected-beam gauge for pressures below 10−12 Torr

Design and performance of a new hot-filament ionization gauge, specially developed for total pressure measurements down to 10−14 Torr, are described. The gauge features an electrostatically deflected ion beam which is directed at the funnel cathode of a small channel electron multiplier (CEM). The spurious signal due to electronic desorption of ions from the anode is extremely small, because these ions are typically of higher initial total energy than those from the residual gas, and may, therefore, be evicted from the high-energy side of the ion beam that is energetically dispersed by the deflecting electric field. The x-ray background corresponds to a pressure in the 10−14 Torr range, and since it can be measured just by inverting the deflector potential, it may easily be zeroed out electronically. Almost none of the secondary particles, released by x rays or ions from the deflecting electrodes, is counted, because the CEM cathode is operated at high negative potential and because most positive secondary ions are collected at the negative deflecting electrode, due to their small kinetic energy. The gauge’s sensitivity for nitrogen is about 1017 counts/s⋅Torr⋅A, corresponding to 105/Torr for a CEM gain of 107. It is mounted on a 113.5-mm metal flange with ceramic feedthroughs and may be baked up to about 300 °C.

A miniature extractor gauge for the UHV

The design and performance of a miniaturized hot filament ionization pressure gauge, specially developed for the CERN intersecting storage rings, is described. It is mounted on a standard metal flange with ceramic feedthroughs and fits into an aperture of 15.7 mm. The gauge is of the extractor type and has a sensitivity factor for nitrogen of about 5 Torr−1. Its low pressure limit, about 5×10−12 Torr nitrogen equivalent, is mainly due to x rays. These two characteristic figures have been obtained, in spite of the gauge's smallness, by an optimization of most of the geometrical and electronical parameters. Some problems, seriously affecting the gauge’s performance, and particularly arising from its size, such as space charge effects and thermal outgassing, are discussed.