Backing Roughing Pumps

The vacuum chamber pumping system consists of a 6-in. oil diffusion pump with both liquid-nitrogen-and water-cooled baffles. The diffusion pump is backed by a 15-cfm mechanical roughing pump. The chamber can be isolated from the pumping system by a 2-in., high-vacuum gate valve. 

Another drawback of oil-sealed pumps is the back-streaming of oil vapour into the roughing line, which may occur at low pressure. Contamination by back-streaming oil can be drastically reduced by using proper traps like molecular sieve traps with zeolite (see Section 1.6.4). 

The UHV pumping systems are mounted similarly in an open glove box frame and connected with the roughing pumps by vacuum lines containing absolute filters to keep back potential radioactive contaminants. 

Such pumps are gas-transfer pumps. They are used extensively both to back high-vacuum pumps and to act as devices for achieving pressures in the rough-medium range in activities such as distillation, drying, etc. 

Figure 3.8 Backing-pressure requirements in the changeover from roughing- (hacking) to high-vacuum pumps...

Operations that require you to cycle your vacuum line back and forth from atmospheric to low pressure can put significant strain on the fore pump and its oil, so a roughing pump should again be considered. 

Rotating anode X-ray generators, one of which is shown in Figure 7.3, have an arrangement of cathode and anode essentially the same as for the sealed tubes described above. The tubes, however, are not evacuated and sealed, but continuously pumped by a diffusion pump backed by a conventional roughing pump. The environment is therefore the same as the inside of the sealed tube. The second important feature of the rotating anode device is exactly that implied by its name. The anode, which is a hollow disc of copper (usually) several inches in diameter, is rotated at 3000 to 6000 rpm by an external motor so that the beam of electrons emitted from the cathode falls on a continuously changing surface. Water,... 

In ICP-MS (Fig. 112) the ions formed in the ICP are extracted with the aid of a conical water-cooled sampler into the first vacuum stage where a pressure of a few mbar is maintained. A supersonic beam is formed and a number of collision processes take place as well as an adiabatic expansion. A fraction is sampled from this beam through the conical skimmer placed a few cm away from the sampler. Behind the skimmer, ion lenses focus the ion beam now entering a vacuum of 10-5. This was originally done with the aid of oil diffusion pumps or cryopumps, respectively, but very quickly all manufacturers switched to turbomolecular pumps backed by roughing pumps. 

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