Molecular drag pump

The earliest molecular drag pump was created by Wolfgang Gaede, who had the idea of the pump in 1905, and spent several years corresponding with Leybold trying to build a practical device.[5] The first prototype device to meet expectations was completed in 1910, achieving a pressure of less than ${\displaystyle 10^{-6}}$ mbar.[6] By 1912, twelve pumps had been created, and the concept was presented to the meeting of the Physical Society in Münster on 16 September of that year, and was generally well received.[5]

Gaede published several papers on the principles of this molecular pump,[7][8] and patented the design.[9] The working principle is that the gas in the chamber is exposed to one side of a rapidly spinning cylinder. Collisions between the gas and the spinning cylinder gives the molecules of gas momentum in the same direction as the surface of the cylinder, which designed to turn away from the vacuum chamber and toward a fore-line. A separate backing pump is used to lower the pressure at the fore-line (output of the molecular pump), since in order to function, the molecular pump needs to operate under pressures low enough that the gas inside is in free molecular flow. One important measure of the pump is the compression ratio, ${\displaystyle K}$. This is the ratio of the pressure of the vacuum, ${\displaystyle P_{vac}}$ to the pressure to the outlet, ${\displaystyle P_{fore}}$ and is roughly constant across different pressures, but depends on the individual gas.[10]

${\displaystyle {\frac {P_{fore}}{P_{vac}}}=K}$

The compression ratio can be estimated using the kinetic theory of gases by calculating the flow due to collisions with the rotating surfaces, and rate of diffusion in the reverse direction.[11] The compression ratio tends to be better for heavy molecules, since the thermal velocity of lighter gasses is higher and speed of the rotating cylinder has a less effect on these faster moving, lighter gasses.

This "Gaede molecular pump" was used in an early experiment testing vacuum gauges.[12]

The improved Holweck design was invented in the early 1920s by Fernand Holweck[13][14] as part of his apparatus for his work in studying soft X-rays. It was manufactured by French scientific instrument maker, Charles Beaudouin.[15] He applied for a patent on the device in 1925.[16] The main difference from the Gaede pump was the addition of a spiral, cut into either to the spinning cylinder, or to the static housing. Holweck pumps have been frequently modeled theoretically.[2][17][18] Holweck's classmate and collaborator, H. Gondet, would later suggest other improvements to the design.[5][19]

Another design was given by Manne Siegbahn.[20] He had produced a pump which was used in 1926.[21] About 50 of Siegbahn's pumps were made from 1926 to 1940.[5] These pumps were generally slower than comparable diffusion pumps, so were rare outside of Uppsala University. Larger, faster pumps of the Siegbahn type began to be made around 1940 for use in a cyclotron.[22] In 1943, Seigbahn published a paper regarding these pumps, which were based on a rotating disk.[23]