Transcript Positive displacement pumps

Pressures and Pumps
Andy Therrien
1/9/17
Pressure
1 atmosphere ~ 1 bar ~ 760 mm Hg ~ 760 torr ~ 100,000 Pa
Ion 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)
Lower Pressure
1x10-4 mbar
Rough Vacuum
1x10-8 mbar
High Vacuum
Ultra High Vacuum
Pressure
Pressure
(mbar)
10-4
Molecular mean free Monolayer time
path λ N2 at 295 K
(to 1 Langmuir)
34 cm
0.013 s
10-8
3.4 km
2.2 m
10-12
3.4 x 104 km
15 days
10-15
3.4 x 107 km
42 years
10-19
Interstellar space
2.3 x 103 AU
4.2 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 viscous state, 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
Three main types of pumps
Positive displacement pumps: use 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!
Pumps
Pumps we use
http://www.aip.org/avsguide/refguide/workingpress.html
Rotary Pump
Check oil level once a month
Ensure the pump is able to cool
Mechanical pumps can introduce noise in STM
http://www.quorumtech.com/Products/RV5PUMP.jpg
Turbomolecular Pump
http://www.varianinc.com
http://www.pfeiffer-vacuum.com
Turbomolecular Pump
Turbo pumps utilize a stack of turbine blades which rotate at very high speed
(1000 Hz) 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
http://www.thermionics.com/ip_too.htm
Ion Pumps
Sputter 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 saturated
3 -7 kV range, we use 5 kV. (Higher voltage means greater pumping)
No moving parts or oil: no maintenance or vibration
http://www.thermionics.com/ip_too.htm
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 chamber
N.B. Sample areas must be shielded!
Titanium Sublimation Pumps (TSPs)
Can’t pump un-reactive gases: Noble gases, Argon
Pumping ability of TSPs also depend on gas composition
Great for pumping water
Pressure
(mbar)
Pumping Duration
10-6
10 m
10-8
60 m (1 h)
10-9
400 m (~7 h)
10-10
600 m (~13 h)
Cyropumping
Gases can physisorb to the walls of the chamber if they are cold
N2(L) – 80 K
He(L) – 5 K
Overtime the surface area becomes saturated and pumping effect is diminished
Systems needs to be recharged by warming and pumping the outgas by other
means
During 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
Even 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.
Desorbs into gas
Lower ultimate P
Chamber Wall
Chamber Wall
Chamber Wall
keeps P high
To Pump
After initial
pump down
During
Bakeout
After
Bakeout