2020 marks the centenary of the development of the Holweck pump by the French physicist Fernand Holweck. As well as a stand-alone pump the Holweck design is used as a compound pumping stage in the widely employed TURBOVAC, nEXT and STP pumps. In this respect the Holweck design has been utilised to make significant advancements and improvements to the development of turbomolecular pumps over the last half century.
Holweck was born in 1890 and made a significant impact on a range of subjects as well as the Holweck molecular vacuum pump. In 1912 he became an assistant to Marie Curie and played an intrinsic role in the development of the Curie institute. In 1922 Holweck received his Doctorate for his studies on soft x-rays, bridging the gap in understanding between the far ultraviolet region and x-rays. Holweck also developed the gravimetric pendulum (for surveying in for example oil and mineral exploration) and was the first to develop the focusing of electrons and electron optics – in this respect he was at the forefront of the development of television. In 1938 Holweck became the Research Director of the CNRS (The French National Centre for Scientific Research).
Holweck and the molecular drag pump
Holweck designed the Holweck molecular vacuum pump (also known as the Holweck Pump) in 1920. This is a type of vacuum pump that utilizes the drag of air molecules against a rotating surface and achieves vacuum levels of the order of 10-6 mbar. It was subsequently manufactured by the renown French scientific instrument maker, Chares Beaudouin.
In the Holweck pump the pumping action is produced by a rotor usually in the form of a smooth cylinder. The stator is provided with spiral guide grooves. The design of the construction can also be reversed, with the stator being smooth and the rotor having the guide grooves. The principle is one of the exploitation of the molecular drag pumping principle.
Sketch of the Holweck pump and pressure taps location including (a) 3D view of rotor and stator and (b) 2D unrolled stator representation. Source: Researchgate
The stator-rotor gap needs to be very small to minimise back-leakage. The pumping speed of the mechanism is proportional to the channel cross-section and the rotational velocity and compression is an exponential function of channel length and velocity. The small dimensions of the entrance to the channel of the screw means that the pumping speed is much lower than the diameter of the inlet would indicate. The maximum pumping speed of a Holweck pump with an inlet flange of 10 cm inner diameter is about 30 l/s equating to a pumping efficiency as low as 3%. Above about 10 mbar the compression begins to decrease as a result of the transition to laminar flow conditions in the pump channel near to the exhaust side.
Holweck stage in turbomolecular pumps
The Holweck mechanism in turbomolecular pumps is said to be the most efficient from a pumping performance point of view; a higher pumping speed and better compression than the Gaede design. The Holweck principle is now used mainly in combination with bladed turbomolecular pumps as hybrid/compound pumps whereby the Holweck stage facilitates gas exhaust to high backing pressures and gives high process gas throughout.
Cutaway view of a bladed turbomolecular pump in combination with Holweck stage. Courtesy of Leybold Vacuum.
These high process flow pumps can be up to 4 slm and play a key role in the vacuum techniques used in contemporary microelectronics production.
F Sharipov et. al. Numerical Modeling of the Holweck Pump, Journal of Vacuum Science and Technology, A 23, pp1331-1339 (2005)
K Jousten (ed.) Handbook of Vacuum Technology, 2nd edition, Wiley-VCH Verlag GmbH & Co. 2016 (ISBN 978-3-527-41338-6)
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