Baking and degassing in ultra-high vacuum system (3)

    The unbaked part at the inlet of the pump is an important factor affecting the ultra-high vacuum. The area of the part that cannot be baked in the system is related to the type of vacuum pump used. The ion pump (with the magnet removed) and the molecular sieve pump can be baked as a whole , Turbomolecular pumps and refrigerator cryopumps can be partially baked at 100℃, while diffusion pumps cannot be baked at all.

    The baking curve shows that the system pressure increases sharply as the heating temperature rises after the heating starts, and the pressure also rises to the maximum when the maximum baking temperature is reached. During the subsequent period of maintaining the baking temperature unchanged, the pressure decreases slowly with the pumping time, and the heating is stopped. After that, the pressure drops rapidly, and the inflection point appears at room temperature. After that, the system pressure decreases with the pumping time at room temperature.

    When the ultra-high vacuum system performs baking and degassing, the uniformity of the baking temperature is more important than the level of the baking temperature. It is not easy to achieve the uniform baking temperature of each part in a complex metal system. The quality of each part of the material is different, the heat capacity of various materials, the difference in the directionality of the thermal conductivity, and the distance from the heating body, etc., will affect the uniformity of heating.

    Therefore, in order to obtain a more uniform baking temperature, it is better to use a baking furnace to cover the entire vacuum container than to use a winding heating wire. Especially for large-mass metal flanges, it is difficult to simultaneously heat by heat conduction, and only by heat radiation. The temperature rises slowly, and the radiant heat is a function of the fourth power of the heating body temperature, so it is very necessary to use a higher heating temperature and a sufficiently long heating time as much as possible.

    Nevertheless, to achieve uniform heating of the system, the thick flange of the large ultra-high vacuum system should also be heated by auxiliary heating plates. The power of the auxiliary heating plates can be calculated according to the heat capacity of the material. To obtain the best results, experiments should be conducted determine.

(Huiputech)