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Pressures and Pumps
1 atmosphere ~ 1 bar ~ 760 mm Hg ~ 760
~ 100,000 PaIon gauges read in mbar, i.e. 1x10-10 mbar = 1x10-13 atm.Sometimes ion gauges read in torr but ours are set to mbar(1 mbar is ~1 torr)
Ultra High Vacuum
Molecular mean free path λ N2 at 295 KMonolayer time (to 1 Langmuir)
3.4 x 104 km
3.4 x 10
km42 years10-19Interstellar space2.3 x 103 AU4.2 x 105 years
1 Langmuir = 1x10-6 torr for 1 s
Viscous vs. Molecular Flow Regimes
The gas in a vacuum system can be in a
in a molecular state, or an intermediate state between the two.The mean free path of the gas molecules is very small at atmospheric pressure so that the flow of the gas is limited by its viscosity. At low pressures where the mean free path of the molecules is similar to the dimensions of the vacuum enclosure, the flow of the gas is governed by viscosity as well as by molecular phenomena; this is the intermediate flow. At very low pressures where the mean free path is much larger than the dimensions of the vacuum enclosure, the flow is molecular.
Viscous > 10
-4 Molecular < 10-6
a mechanism to repeatedly expand a cavity, allow gases to flow in from the chamber, seal off the cavity, and exhaust it to the atmosphere.Momentum transfer pumps: also called molecular pumps, use high speed jets of dense fluid or high speed rotating blades to knock gas molecules out of the chamber.Entrapment pumps: capture gases in a solid or adsorbed state. This includes cryopumps, getters (TSPs), and ion pumps. Only works at already low pressures!
Turbo pumps utilize a stack of turbine blades which rotate at very high speed
to move gas from the inlet port to the exhaust port. Turbo pumps can achieve chamber base pressures of 10-9 torr or below, depending on chamber geometry (conductance). 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. THEREFORE TURBOS ARE BACKED BY ROTARY PUMPS
ion pumps operate by ionizing gas within a magnetically
confined cold cathode
discharge. Very similar to cold cathode sputter gun!The events that combine to enable pumping of gases under vacuum are: Entrapment of electrons in orbit by a magnetic field. Ionization of gas by collision with electrons. Sputtering of titanium by ion bombardment. Active gases stick to titanium. Cannot pump at high pressures or collector becomes saturated3 -7 kV range, we use 5 kV. (Higher voltage means greater pumping)No moving parts or oil: no maintenance or vibration
Titanium Sublimation Pumps (TSPs)
Resistively heat Ti metal
Thin layer of Ti on chamber walls
Gases in chamber stick to the Ti, thereby pumping the chamberN.B. Sample areas must be shielded!
Titanium Sublimation Pumps (TSPs)
Can’t pump un-reactive gases: Noble gases,
Pumping ability of TSPs also depend on gas composition Great for pumping waterPressure(mbar)
60 m (1 h)
-9400 m (~7 h)
600 m (~13 h)
Gases can physisorb to the walls of the chamber if they are cold
2(L) – 80 K He(L) – 5 KOvertime the surface area becomes saturated and pumping effect is diminishedSystems needs to be recharged by warming and pumping the outgas by other meansDuring He(L) fills in the LT the temperature is temporarily increased, resulting in the outgassing of mostly of
H2 and CO
Bake out and outgassing
with the appropriate pumps,
you still cannot achieve UHV. After pumping down from atmosphere there are a lot of gas molecules adsorbed to the walls of the chamber. These molecules slowly desorb and get pumped away but this is a very slow process. To accelerate this (exponentially!) is to increase the temperature of the entire chamber, called a bakeout.