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 1x10-8 mbar Rough Vacuum High Vacuum Ultra High Vacuum

Pressure Pressure (mbar) Molecular mean free path λ N2 at 295 K Monolayer time (to 1 Langmuir) 10-4 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 AU is astronomical unit (approx. earth to sun)

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 Fluids cannot be pulled, so it is technically impossible to create a vacuum by suction. Suction is the movement of fluids into a vacuum under the effect of a higher external pressure, but the vacuum has to be created first. The easiest way to create an artificial vacuum is to expand the volume of a container. For example, the diaphragm muscle expands the chest cavity, which causes the volume of the lungs to increase. This expansion reduces the pressure and creates a partial vacuum, which is soon filled by air pushed in by atmospheric pressure. To continue evacuating a chamber indefinitely without requiring infinite growth, a compartment of the vacuum can be repeatedly closed off, exhausted, and expanded again. This is the principle behind positive displacement pumps, like the manual water pump for example. Inside the pump, a mechanism expands a small sealed cavity to create a deep vacuum. Because of the pressure differential, some fluid from the chamber (or the well, in our example) is pushed into the pump's small cavity. The pump's cavity is then sealed from the chamber, opened to the atmosphere, and squeezed back to a minute size. 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 In a momentum transfer pump, gas molecules are accelerated from the vacuum side to the exhaust side (which is usually maintained at a reduced pressure by a positive displacement pump). Momentum transfer pumping is only possible below pressures of about 0.1 kPa. Matter flows differently at different pressures based on the laws of fluid dynamics. At atmospheric pressure and mild vacuums, molecules interact with each other and push on their neighboring molecules in what is known as viscous flow. When the distance between the molecules increases, the molecules interact with the walls of the chamber more often than the other molecules, and molecular pumping becomes more effective than positive displacement pumping. This regime is generally called high vacuum. Molecular pumps sweep out a larger area than mechanical pumps, and do so more frequently, making them capable of much higher pumping speeds. They do this at the expense of the seal between the vacuum and their exhaust. Since there is no seal, a small pressure at the exhaust can easily cause backstreaming through the pump; this is called stall. In high vacuum, however, pressure gradients have little effect on fluid flows, and molecular pumps can attain their full potential. The two main types of molecular pumps are the diffusion pump and the turbomolecular pump. Both types of pumps blow out gas molecules that diffuse into the pump by imparting momentum to the gas molecules. Diffusion pumps blow out gas molecules with jets of oil or mercury, while turbomolecular pumps use high speed fans to push the gas. Both of these pumps will stall and fail to pump if exhausted directly to atmospheric pressure, so they must be exhausted to a lower grade vacuum created by a mechanical pump. As with positive displacement pumps, the base pressure will be reached when leakage, outgassing, and backstreaming equal the pump speed, but now minimizing leakage and outgassing to a level comparable to backstreaming becomes much more difficult. 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 ionization pumps, which use strong electrical fields to ionize gases and propel the ions into a solid substrate. 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! Entrapment pumps may be cryopumps, which use cold temperatures to condense gases to a solid or adsorbed state, chemical pumps, which react with gases to produce a solid residue, or

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) Entrapment pumps may be cryopumps, which use cold temperatures to condense gases to a solid or adsorbed state, chemical pumps, which react with gases to produce a solid residue, or

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 Entrapment pumps may be cryopumps, which use cold temperatures to condense gases to a solid or adsorbed state, chemical pumps, which react with gases to produce a solid residue, or Cryopumpingl is the removal of gas from a system by condensation onto a cold plate. The phenomenon of cryopumping is not new, but it has received much emphasis in the last few years because of the need for very great pumping speeds in space simulation work.2 As cryogenic liquids and low temperature refrigerators become more readily available, cryopumps are used in more and more facilities not related to space simulation. The use of liquid helium and liquid hydrogen in particle accelerator programs makes the use of cryopumping quite natural in these facilities

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. After initial pump down Chamber Wall keeps P high During Bakeout Chamber Wall Desorbs into gas To Pump After Bakeout Chamber Wall Lower ultimate P