The turbomolecular pump was developed and patented at Pfeiffer Vacuum in 1958 by Dr. W. Becker. Turbomolecular pumps belong to the category of kinetic vacuum pumps. Their design is similar to that of a turbine. A multi-stage, turbine-like rotor with bladed disks rotates in a housing. The blades of a turbine or a compressor are referred to collectively as the blading. Interposed mirror-invertedly between the rotor disks are bladed stator disks having similar geometries.
Bearings
Mounting the shaft of a turbopump rotor by means of two ball bearings requires arrangement of both bearings on the fore-vacuum side due to the lubricants in the bearings. This results in a unilateral (cantilever) support of the rotor with its large mass.
Hybrid bearing support offers advantages in this regard with respect to rotor dynamics. Hybrid bearing designates the use of two bearing concepts in one single pump. In this case, an oil-lubricated ball bearing is mounted on the end of the shaft on the fore-vacuum side, and the high vacuum side is equipped with a maintenance-free and wear-free permanent magnetic bearing that centers the rotor radially. The oil for lubricating the fore-vacuum side bearing is contained in an operating fluid reservoir. A small dry safety bearing is arranged within the magnetic bearing stator. During normal operation, a journal rotates freely within this bearing. In the event of strong radial shocks, the safety bearing stabilizes the rotor and rotates only briefly. If the rotor is out of balance, the bearings on both ends of the shaft will generate significantly lower bearing-stressing vibration forces than in the case of a floating bearing. The magnetic bearing on the high vacuum side is totally insensitive to vibration. Only very small vibration forces are transferred to the housing as a result. Moreover, this eliminates the need for the larger of the two bearings in a cantilever concept, whose size limits rotational speed.
Large pumps from a flange diameter of 100 mm alternatively use bearings known as 5-axis magnetic bearings [24]. The rotor is levitated through digital electronic control via distance sensors and electromagnets. The degrees of freedom of the movement of a turborotor are continuously monitored and readjusted in real time. The absence of mechanical contact between the rotor and housing keeps the vibration generated by the pump low. The rotor revolves around its own axis of inertia. Any imbalance due to one-sided coating or erosion (such as in plasma etching) is counteracted within broad limits.
In addition to the absence of oil on the backing-vacuum side, freedom from wear and maintenance is another advantage. In the event of a power failure, the magnetic bearings are supplied with electricity through the rotational energy of the pump. This enables power failures to be easily bridged for several minutes. Should the power failure be of longer duration, the rotor will safely come to a stop at a very low speed through the use of an integrated safety bearing. During system malfunctions, the safety bearing shuts down the rotor to avoid any damage to the pump.
Motors / Drives
Brushless DC motors that afford rotational frequencies of up to 1,500 Hz (90,000 rpm) are used to drive the rotors. This enables the blade velocities that are necessary for pumping the gases.
Today, the drives are typically attached directly to the pumps. The power supply is with 24, 48 or 72 volt direct current, generated by external power supply packs or ones that are integrated in the electronic unit of the pump.
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Figure 4.21: Degrees of freedom of a turbo-rotor
4.9.1.1 Turbomolecular pump operating principle
The pumping effect of an arrangement consisting of rotor and stator blades is based upon the transfer of impulses from the rapidly rotating blades to the gas molecules being pumped. Molecules that collide with the blades are adsorbed there and leave the blades again after a certain period of time. In this process, blade speed is added to the thermal molecular speed. To ensure that the speed component that is transferred by the blades is not lost due to collisions with other molecules, molecular flow must prevail in the pump, i. e. the mean free path must be greater than the blade spacing
In the case of kinetic pumps, a counterpressure occurs when pumping gas; this causes a backflow. The pumping speed is denoted by S0. The volume flow rate decreases as pressure increases and reaches a value of 0 at the maximum compression ratio K0.
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Figure 4.22: Operating principle of the turbomolecular pump
Compression ratio
The compression ratio, which is denoted K0K0, can be estimated according to Gaede [25]. The following applies for a visually dense blade structure (Figure 4.22):
Formula 4-8: Turbopump compression ratio
Mean molecule velocity[m · s1]
v Circumferential speed[m · s-1]
The geometric ratios are taken from Figure 4.22. The factor g is between 1 and 3 [26]. From the equation, it is evident that K0 increases exponentially with blade velocity vv as well as with √M because
Consequently, the compression ratio for nitrogen, for example, is significantly higher than for hydrogen.
Volume flow rate (pumping speed)
Pumping speed S0 is proportional to the inlet area A and the mean circumferential velocity of the blades v, i. e. rotational speed. Taking the blade angle α into account vields:
Formula 4-9: Turbopump pumping speed
Taking into account both the entry conductance of the flange
(Formula 1-24)
as well as the optimal blade angle of 45°, produces the approximate effective pumping speed Seff of a turbopump for heavy gases (molecular weight > 20) in accordance with the following formula:
Formula 4-10: Turbopump effective pumping speed
Dividing the effective pumping speed by the bladed entry surface of the uppermost disk and taking the area blocked by the blade thickness into consideration with factor df≈ 0.9, yields the specific pumping speed of a turbopump for nitrogen, for example (curve in Figure 4.23):
Formula 4-11: Specific pumping speed
On the Y axis in Figure 4.23 the specific pumping speed is plotted in l · s-1 · cm-2 and the mean blade velocity
is plotted on the X axis. Moving up vertically from this point, the point of intersection with the curve shows the pump’s maximum specific pumping speed SA. Multiplying this value by the bladed surface area of the inlet disk:
, obtains the pumping speed of the pump and enables it to be compared with the catalog information.
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Figure 4.23: Specific turbopump pumping speeds
The points plotted in Figure 4.23 are determined by Pfeiffer Vacuum on the basis of the measured values of the indicated pumps. Points far above the plotted curve are physically not possible.
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Figure 4.24: Pumping speed as a function of relative molecular mass
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Figure 4.25: Pumping speed as a function of inlet pressure
The pumping speeds thus determined still tell nothing about the values for light gases, e.g. for hydrogen. If a turbopump is designed for a low ultimate pressure, pump stages with various blade angles are used and the gradation is optimized for maximum pumping speeds for hydrogen. This produces pumps with sufficient compression ratios for both hydrogen (approximately 1,000) and nitrogen, which should be 109 due to the high partial pressure of nitrogen in the air. In the case of pure turbomolecular pumps, backing-vacuum pressures of approximately 10-2 mbar are required due to their molecular flow.
4.9.1.2 Holweck stage operating principle
A Holweck stage (Figure 4.26) is a multi-stage Gaede type molecular pump having a helical pump channel. Due to the rotation of the rotor, gas molecules entering the pump channel receive a stimulus velocity in the direction of the channel. Backflow losses occur through gaps between the barriers that separate the Holweck channels from each other and the rotor. The gap widths must be kept small to minimize backflow. Cylindrical sleeves (1) that rotate about helical channels in the stator (2) are used as Holweck stages. Arranging stators both outside as well as inside the rotor enables two Holweck stages to be easily integrated within one and the same pump. This means that the displaced gas particles are transported outside the rotor through the stator channel and then inside the rotor through further stator channels until they are conveyed back up to the backing pump through a collecting channel. Some modern turbopumps have several of these ”pleated“ Holweck stages.
The pumping speed S0 of the Holweck stages is equal to:
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Figure 4.26: Operating principle of a Holweck stage
Formula 4-12: Holweck stage pumping speed
Where b⋅h is the channel cross section and v⋅cosα the velocity component in the channel direction.
The compression ratio increases exponentially as a function of channel length L and velocity v⋅cosα [4]:
Formula 4-13: Holweck stage compression ratio
The values yielded with this formula are not attained in real Holweck stages since backflow over the barrier from the neighboring channel dramatically reduces the compression ratio, and this influence is not taken into account in Formula 4-13.
To set up a turbo pumping station with diaphragm pumps with a final pressure of between 0.5 and 5 hPa, turbopumps are today equipped with Holweck stages. These kinds of pumps are termed turbo drag pumps. Since only low pumping speeds are required to generate low base pressures due to the high pre-compression of the turbopump, the displacement channels and, in particular, both the channel height as well as the clearances to the rotors can be kept extremely small, thus still providing a molecular flow in the range of 1 hPa. At the same time, this increases the compression ratios for nitrogen by the required factor of 103. The shift of the compression ratio curves to higher pressure by approximately two powers of ten can be seen from Figure 4.27.
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Figure 4.27: Compression ratios of pure turbopumps and turbo drag pumps
For turbopumps which are designed for high gas throughput, a compromise is made where the gas throughput, fore-vacuum compatibility and particle tolerance are concerned and the distance between the gaps in the Holweck stages is increased.
4.9.1.3 Turbopump performance data
Gas loads
that can be displaced with a turbomolecular pump increase proportionally to pressure in the range of constant volume flow rate. In the declining branch of the pumping speed curve the maximum displaced gas loads can continue to rise but they will reach thermal limits that depend on the size of the backing pump. The maximum permissible gas loads also depend on the pump temperature (cooling and/or heated pump) and the type of gas in question. Displacing heavy noble gases is problematic, because they generate a great deal of dissipated energy when they strike the rotor, and due to their low specific heat, only little of it can be dissipated to the housing.
Measurement of the rotor temperature by the manufacturer enables gas type-dependent process windows to be recommended for safely operating the turbopumps. The technical data for the turbopumps specify the maximum permissible gas loads at nominal rpms for hydrogen, helium, nitrogen, argon and CF4. A reduction in the rotation speed allows higher gas throughputs.
Pumps in the HiPace series with pumping speeds > 1,000 l · s-1 are equipped with rotor temperature monitoring and protect themselves from overheating.
Critical backing pressure
Critical backing pressure is taken to mean the maximum pressure on the backing-vacuum side of the turbomolecular pump at which the pump’s compression decreases. This value is determined as part of the measurements for determining the compression ratio in accordance with
ISO 21360-1:2012 by increasing the backing pressure without gas inlet on the intake side. In the technical data for turbomolecular pumps, the maximum critical backing pressure is always specified for nitrogen.
Base pressure, ultimate pressure, residual gas
In the case of vacuum pumps, a distinction is made between ultimate pressure and base pressure (see also Section 4.1.3). While the pump must reach base pressure pb within the prescribed time under the conditions specified in the measurement guidelines, the ultimate pressure pe can be significantly lower. In the HV range, base pressure is reached after 48 hours of bake-out under clean conditions and with a metallic seal. What is specified as the base pressure for pumps with aluminum housings is the pressure that is achieved without bake-out and with clean FKM seals.
Corrosive gas-version pumps have a higher desorption rate which can temporarily result in higher base pressures due to the coating on the rotor surface.
Dividing the backing pressure by the compression ratio yields the ultimate pressure.
Formula 4-14: Ultimate pressure
Whether ultimate pressure will be achieved will depend upon the size and cleanliness of both the equipment and the pump, as well as upon the bake-out conditions. After extreme bakeout (to over 300 °C) only H2, CO and CO2 will be found in the residual gas. These are gases that are dissolved in the metal of the vacuum chamber and continuously escape. A typical residual gas spectrum of clean, baked out equipment is shown in Figure 4.28.
In the backing pump used, the gas ballast should be switched on at regular intervals to prevent the accumulation of hydrogen in the fore-vacuum area. In many cases, the actual ultimate pressure will be a factor of the desorption conditions on the high vacuum side of the turbopump and its pumping speed, and not the compression ratios of the pumps.
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Figure 4.28: Typical UHV residual gas spectrum (turbopump)
