First they start by using regular vacuum pumps, then switch to oil diffusion pumps, or turbomolecular pumps. In the case of turbomolecular pumps the molar mass of the gas you evacuate matters, it could make sense, to flush the tube with a dry high mw-gas before evacuating to the final pressure. Usually pressures in these tubes are in the range of 0.1 to 0.005 Pa.
Dear Satish,
Patrick already has given a sufficient answer.
My remark is related to your above mentioned number of 10-12 mmHg of pressure. This is a quite over shoot for an x-ray tube.
Maximum ultra high vacuum will be more useful in X rays tubes and will prolong the life of the tubes
Usually the leftover gas in the tube only provokes Lilienfeld-radiation, which corresponds to a loss of energy in the bremsstrahlung [close to the anode]. At high energies (10-100 kV) sputtering is a much bigger problem, especially with static anodes, as well as self-absorption of emitted photons close to the anode. The pressure of the tube corresponds to its lifetime, but only in theory. In reality there is no big difference.
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