In Pumpdown 1, you used the mechanical pump and bypass line to pump down the  reaction chamber. This has two limitations:

• Typically the bypass line has a low conductance (to prevent particle generation), so it will take a long time to approach base pressure of the reaction chamber, as needed to fully evacuate reactive contaminant species (e.g., water or hydrocarbons from previous air exposure).
• The base pressure achievable with only the mechanical pump is not low enough to remove sufficient reactive species and achieve low enough base pressure in the reaction chamber to produce high quality materials.

A more complex pumping system is needed, utilizing a second pump capable of attaining much lower pressure than can a mechanical pump alone.

Turbo pump
A turbomolecular pump (usually called simply a turbo pump) is shown above the mech pump in the simulator window.  Turbo pumps utilize a stack of turbine blades which rotate at very high speed (of order 50,000 rpm) to move gas from the inlet port (shown here on top) to the exhaust port (shown here on the bottom).  Turbo pumps are very effective at low pressures (<10 torr), essentially in the molecular flow regime in which gas densities are so low that the molecules collide with chamber walls far more often than with each other.  As a result,  turbo pumps can achieve chamber base pressures of 10^-9 torr or below.

However, the high packing of fan blades and the high rotation speed of the turbo pump make it ineffective at higher pressures, where fluid (viscous) flow dominates. Powering a turbo pump alone at atmospheric pressure will barely cause the blades to rotate.

Turbo pump system
Accordingly, a mechanical pump is required in combination with the turbo pump, making the turbo pump system (or pump stack, i.e., turbo + mech pumps) capable of evacuating systems starting at 1 atm and achieving very low base pressures of order 10^-9 torr.  The turbo pump system in the simulator consists of the following components, depicted from top to bottom in the simulator window:

• gate valve V2 (between turbo pump and reaction chamber)

This gate valve connects or isolates the turbo pump system to/from the reaction chamber.  When open, its large conductance (wide opening) permits the full pumping capacity of the turbo pump system to evacuate the chamber to very low pressure.  When closed, it isolates the pump system from the chamber, allowing the chamber to be vented while keeping the pump stack at low pressure.
• turbo pump

The turbo pump removes gas from the chambers connected to its inlet (shown on top) down to a very low pressure (of order 10^-9 torr), provided a mech pump is connected to the turbo pump exhaust port to remove this gas from the entire system and maintain a relatively low pressure (<10 torr) at the turbo exhaust position.
• cutoff valve V1 (between mech and turbo pumps)

The cutoff valve between mech and turbo pumps connects or isolates the mech pump from the turbo pump.  When open, it permits the mech pump to pump on the exhaust side of the turbo pump, ultimately reducing the pressure throughout the turbo pump enough (<10 torr) that the turbo may function effectively. Furthermore, as the turbo achieves its high rotation speed, the gas it pumps from inlet (top) to exhaust (bottom) is then removed by the mech pump, so that the mech pump functions as a backing pump for the turbo.  The conductance of V1 when open must be sufficient so as not to limit the pumping effectiveness of the mech pump.

When the cutoff valve V1 is closed, the same mech pump may be used to evacuate the reaction chamber through the bypass line. (For modest time periods, e.g. a few minutes, V1 may be closed without degrading the turbo pump vacuum conditions appreciably.)

• mechanical pump

The mechanical pump pumps down the turbo pump region to <10 torr and maintains this low pressure condition so that the turbo may operate effectively.

Exercises

Exercise 1: Pump down the reaction chamber through the turbo pump, with just the mechanical pump.  Previously you pumped down the reaction chamber using the mech pump and the bypass line.  Another path is to use the mechanical pump to pump the reaction chamber through the turbo pump.  If VB and VV are closed, opening V1 and V2 connects the mechanical pump to the volumes in the turbo pump and in the reaction chamber.  Therfore, the turbo pump system can evacuate the reaction chamber if only the mech pump is powered. To do this:  Start/restart the simulator (stop if already running).  Turn on the mechanical pump .  Observation: The mechanical pump pressure decreases.   Open V1, the cutoff valve separating the mechanical pump from the turbo pump.  Observation: The turbo pump pressure decreases continously. The mechanical pump pressure first increases, then decreases.  Recall that the turbo pump pressure reflects pressure at the turbo pump inlet.  Since the turbo pump is also a turbo chamber, the mech pump evacuates gas not only from the exhaust side of the turbo pump, but also from the inlet side directly through the turbo blades, even when they are not moving. The mech pump pressure initially increases, since when V1 is opened, the gas initially in the turbo pump chamber expands into the mech pump chamber, increasing its pressure; then the mech pump reduces its pressure as it pumps down the turbo pump chamber as well.   Open V2, the gate valve separating the turbo pump from the chamber (after both mechanical and turbo pumps attain equilibrium).  Observation: When V2 is opened, the reaction chamber pressure decreases continuously, but more slowly than did the mech pump or turbo pump pressures.  The mech pump and turbo pump pressures first increase, then decrease.  Now the mech pump is connected to the reaction chamber as well as the turbo pump (and turbo pump chamber), so opening V2 causes the mech pump to pump down the reaction chamber.  Since its volume is considerably larger than that of either the turbo pump or mechanical pump, its pressure decreases more slowly.   The mech and turbo pump inlet pressures both increase initially when V2 is opened, since the gas in the reaction chamber quickly expands into the turbo and mech pump chambers, increasing their pressures.  As the mech pump works, it then reduces pressures in the reaction chamber, turbo pump chamber, and mech pump chamber.    Questions   What base pressure does the reaction chamber reach?  Roughly how long after opening valve V2 does it take to reach approximate base pressure?  Answers

 

Summary
The turbo pump system includes a turbo pump (to achieve very low pressures), together with a mech pump (to initiate pumpdown at higher pressures and to back the turbo pump), and valves to isolate the individual pumps.

With the turbo pump power off, the mech pump can be used to pump down the turbo pump chamber and the reaction chamber to the base pressure of the mech pump.