Basic Pumping Principles

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Basic Pumping Principles

• Conductance
• Pumping Speed
• Outgassing
• Leaks
• Virtual Leaks

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Conductance
Formal Definition of Conductance the ratio of
throughput, under steady-state conservation
conditions, to the pressure differential between
two specified isobaric sections inside the
pumping system. Throughput/pressure difference
The molecules/sec value is a measure of the
tube's conductance -- its ability to allow
molecules to pass from one end to the other. But
since we stated the pressure is fixed, like
pumping speed, conductance is quoted as a
volumetric flow, l/s.
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Units of Measure Although both are measured
in the same units of volume per unit time (for
example, liter/sec, cubic meters/hr, cubic
feet/min, etc.), the terms conductance and
pumping speed should not be used interchangeably.
Pumping speed applies to active devices that
permanently remove gas molecules from a system
(i.e. pumps or traps). Conductance applies to
passive devices that simply transmit the gas from
end to end (i.e. tubes, elbows, valves,
non-cooled baffles). However, it is very relevant
to ask how the pump's pumping speed is affected
by the conductances of ports, valves between pump
and chamber..
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Calculations

• Calculating Conductances The time to calculate
  conductances is before you buy any piece of
  vacuum equipment. You should know the approximate
  operating characteristics of your new or
  about-to-be-modified system while it is still at
  the scratch-pad stage. Since it is trivial matter
  to reduce conductance if you really need to,
  always attempt to maximize conductance. To do
  that in any high-vacuum situation, keep these
  points in mind
• Make all tubes and components as short as
  possible
• Make all tubes and components as open (large
  diameter) as possible.
• Use as few bends and angles as possible
• The part with the smallest conductance determines
  the maximum conductance.
• In high-vacuum or UHV applications, the statement
  that the conductance is too high' has no
  meaning.
• The statement that the conductance is 'too low'
  has very real (and expensive-to-fix) meaning.

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• The conductance of an orifice -- a hole in an
  infinitely thin plate -- is determined as
  follows
• Measure the orifice's radius in centimeters
• Enter the table at the appropriate 'a' (radius)
  row. Go right to the Fo column and read the
  conductance in L/sec
• A conductance value of a straight cylindrical
  tube is calculated as follows
• Measure the (overall) length of the tube in any
  convenient units.
• Measure the tube's internal diameter in the same
  units.
• Divide the internal diameter by 2 to give the
  radius.
• Divide the length by the radius.
• (This gives the 'L/a' ratio used in Dushman's
  table.)
• Convert the radius to centimeters.
• (This gives 'a (cm)' to use in Dushman's table.)
• Enter the table at the appropriate 'a' row. Go
  right until under the value of the calculated
  'L/a' ratio.If the exact number is not
  available, use the next larger 'L/a' value or
  interpolate.

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Combining Conductances in Series They are added
as reciprocals. (series capacitors.)
1/Ctotal           1/C1      1/C2      1/C3  
    1/C4      1/C5      1/C6
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Conductances in Parallel Using two conductances
simultaneously between two chambers is not a
common occurrence but we should understand how to
calculate the numbers when it does happen. The
arithmetic is even easier than above. As an
example, we will use two tubes in parallel, the
first 1800 L/sec and the second 2300 L/sec. The
total conductance is given by Ctotal           
C1      C2
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Combining Conductances and Pumping Speeds
(series) Since pumping speeds are also measured
in volumetric flow units, for conductance
calculations, a pump is indistinguishable from a
conductance. Indeed, the following table makes
much better sense if we assume that Conductance
C2 isn't a conductance at all but is a pump.
Now, the table shows what happens when we
attach successively higher pumping speed pumps to
a skinny 10 L/sec tube connected to the chamber.
The Total Conductance now becomes the effective
pumping speed from the chamber If the pump is a
small drag pump with 10 L/sec pumping speed, the
effective pumping speed from the chamber will be
half that of the pump's nominal values. If the
pump is a high priced turbo with a pumping speed
of 1,000 L/sec, the effective pumping speed from
the chamber is 9.9 L/sec. That skinny tube has
thrown away 99 of the nominal pumping speed.
The smallest conductance rules indeed!
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Let C1 be a Tube, and C2 be a pump. The total
conductance (and total pumping speed is
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Outgassing
So, what does outgassing rate mean? Well, its
name suggest a measurement of the amount of gas
that comes out of something in a given time.
Outgassing rate, then, is the amount of gas
leaving a surface under vacuum in unit time.
PRESSURE x VOLUME per unit AREA per unit TIME
Torr x Liters per Square Centimeter per
Second Pascal x Cubic Meter per Square Meter per
Second Inches of Water Gauge x Cubic Rod, Pole,
or Perch per Square Furlong per Fortnight
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Ultimate Pressure

• The ultimate pressure obtainable is a
  competition between outgassing rate and pumping
  speed.

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Sample Calculation
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• To measure the outgassing rate of some specific
  material
• you do exactly the same test, only twice. First,
  with the chamber empty to find the background
  outgassing rate, then with a known area of the
  material in the chamber. The material's
  outgassing rate is the difference between these
  rates divided by the total area of the material.
  Note that in a fixed time period, the second
  pressure rise should always be greater than the
  first since the new material adds to the existing
  chamber rate.

The microscopic cleanliness and condition of a
surface has a large effect on its outgassing
rate. The surface's vacuum history is very
significant to its outgassing rate.
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Total Gas Load

• What is Gas Load?  When discussing pressures and
  pumping, we are really speaking of the effects of
  gas-phase molecules. These are the only ones we
  can measure or pump. Unfortunately, if we remove
  all gas-phase molecules, an extremely low
  pressure does not results. The reason is simple
  all the gas-phase molecules removed by the
  pumping mechanism are replaced by molecules from
  other sources. We can summarize those other
  sources as
• real leaks at welds, flanges or porous
  construction materials
• virtual leaks trapped volumes at welds, screw
  threads or two mating surfaces
• outgassing which includes gas or vapor...
• desorbing from the wall surfaces
• diffusing from the wall matrix
• evaporation of materials with high vapor pressure
  
• permeation through elastomeric gaskets
• permeation through the glass or metal walls
• backstreaming gases from the pump
• backstreaming oil vapor from an oil-sealed pump
• condensable vapors (eg solvents) coming out of
  solution in the pump oil
• desorbing gas from a saturated trap
• desorbing gas from a cryogenic trap with a
  falling cryogen level
• In a tight vacuum system, a major contributor to
  gas-phase molecules is wall desorption. For
  example, a spherical chamber, 1 ft diameter at 1
  x 10-6 torr with its inner wall covered with one
  monolayer of adsorbed water vapor, has 7000
  times more molecules on the surface than in the
  gas-phase. And it is a reasonable assumption that
  a real chamber at 1 x 10-6 torr has a surface
  coating thicker than one monolayer.

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Leaks and Virtual Leaks

• Leaks something not sealed or flow through seal
• Virtual Leak air trapped within inner chamber
  volume

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Pumping Speed
Formal Definitions of Pumping Speed the
volumetric rate at which gas is transported
across a plane. the ratio of the throughput of
a given gas to the partial pressure of that gas
at a specific point near the inlet port of the
pump. The first definition makes clear its
dependence on volumetric rate, emphasizing that
pumping speed is not about the quantity of gas
measured by pressure x volume per unit time or
number of molecules per unit time.
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Practical Interpretation of Pumping Speed You
often hear talk about a pump sucking gas from a
chamber. This is nonsense! If gas molecules are
(magically) removed from one section of an
enclosed volume, molecules in the remaining
sections, in their normal high-speed flight,
randomly collide and bounce off walls to fill the
total volume at a lower pressure. For pumps
this means -- until a gas-phase molecule
propelled by collisions enters the pump's
mechanism, that molecule cannot be removed from
the chamber. A pump is more like a one-way valve
into which gas molecules may enter but do not
leave. Pumping speed measures the pump's
ability to remove gas from its inlet in some
stated time period. But the measurement uses the
gas volume, not the amount of gas. It might also
be called a measure of the pump's quality. With
any one type of pump, the higher this quality,
the more money you are asked to pay for it.
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At any fixed pressure at the pump's inlet, there
is a direct relationship between the volume
pumped and the number of molecules entering the
'one-way valve' in unit time. The table shows a
pump with a 1000 L/sec pumping speed throughout
its useable range. (For simplicity, atoms and
molecules are lumped together and called
'atoms.')
If we could watch the molecules at an ideal
pump's entrance, the movement would be in one
direction only - into the one-way valve. As the
inlet pressure reduced, the number entering would
reduce, but the (volumetric) pumping speed
remains constant.
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Vacuum Chambers
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Mechanical Transfer Pumps
Rotary Vane Pump
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Rotary Vane Properties

• Used to back other, high or ultra high vacuum
  pumps
• Pressures down to the 10-4 range
• At high inlet pressure, the oil vapor
  backstreaming is usually acceptably low. The
  forward movement of the bulk gas through the
  inlet tube creates a barrier to backstreaming. As
  the inlet pressure reaches 1 torr, however, the
  backstreaming rate starts to climb. By 0.01 torr,
  the backstreaming rate might be 100 times greater
  than at 1 torr. The ease with which these
  conditions arise strongly suggests every vane
  pump should be equipped with a well-maintained
  Foreline Trap to stop oil vapor.

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Diaphragm Pump

• The diaphragm pump has one of the simpler
  operating mechanisms of any mechanical pump. A
  flexible metal or polymeric sheet material seals
  a small, usually cylindrical, chamber at one end.
  At the other end are two spring-loaded valves
  one opening when the chamber pressure falls below
  the pressure outside the valve and the other
  opening when the chamber pressure exceeds the
  pressure outside the valve. A cam on a motor
  shaft rapidly flexes the diaphragm, causing gas
  to transfer in through one valve and out through
  the other.
• Pressure down to 1 Torr range.
• No backstreaming of oil molecules.

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Capture Pumps (UHV)

• Diffusion Pump
• Turbomolecular Pump
• Ion Pump

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Diffusion Pump

• Diffusion pumps operate by boiling a fluid, often
  a hydrocarbon oil, and forcing the dense vapor
  stream through central jets angled downward to
  give a conical curtain of vapor. Gas molecules
  from the chamber that randomly enter the curtain
  are pushed toward the boiler by momentum transfer
  from the more massive fluid molecules. Water
  cooled diffusion pumps have coils through which
  cooling water circulates during operation.
• They tolerate operating conditions (e.g. excess
  particulates or reactive gases) that would
  destroy other pumps they have often very high
  pumping speeds, a relatively low cost, and are
  vibration- and noise-free.
• Some models operate at pressures as high as 5 x
  10-3 torr while others, combined with an
  exceptionally good LN2 trap and augmented by a
  titanium sublimation pump to remove residual
  hydrogen, have reached pressures below 1 x 10-10
  torr.

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Turbomolecular Pump

• For normal commercially-available pumps, pumping
  speeds range from approximately 20 L/s to 3,000
  L/s.
• If the nitrogen partial pressure in the foreline
  measures 1 x 10-4 Torr, the chamber may reach a
  partial pressure of 2.5 x 10-13 Torr (a factor of
  4 x 108 lower)
• However, if the foreline has hydrogen at the same
  partial pressure as nitrogen, the chamber will
  not reach less than 1.6 x 10-7 Torr of hydrogen.

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Turbomolecular Drag Pump

• First, a single-stage impeller, similar to the
  top row of blades in a turbomolecular pump
  ensures the highest possible pumping speed for
  the open area of the pump's inlet.
• In the second stage a high speed rotor spins
  between two closely-spaced, inner- and
  outer-cylindrical walls with helical grooves
  facing the rotor. When the rotor's tangential
  velocity approaches that of the average gas
  molecule's velocity, momentum transfer causes
  flow toward the exhaust port. To assist that
  flow, the spiral grooves on the outer wall have a
  downward shape while the ones on the inner wall
  have an upward shape.
• In the third stage, a dynamic seal allows the
  pump to operate with a high exhaust pressure.
• The drag pump accepts continuous inlet pressures
  below 0.1 Torr and discharges with a foreline
  pressure of 10 to 40 Torr. The latter pressure
  indicates that diaphragm pumps, not normally
  suitable for backing high vacuum pumps, could
  work in some applications with drag pumps.

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Turbo Drag Pump
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Ion Pump
All ion pumps have the same basic components a
parallel array of short, stainless steel tubes
two plates (typically titanium but occasionally
tantalum) spaced a short distance from the open
ends of the tubes a strong magnetic field
parallel to the tubes' axes and a high voltage
charge on either tubes or plates.
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The magnetic field constrains electrons released
from the negative plates into tight helical
trajectories in the positive tubes. When they
strike a neutral gas atom or molecule, ionization
occurs. Since the ion is formed in a highly
positive region (with respect to the end plates),
the ion's potential energy converts to kinetic
energy as it accelerates toward the plate. On
impact, it has sufficient energy to sputter
titanium which coats all unshielded surfaces (the
tubes and cathode plates) with a thin film.
Several pumping mechanisms may then occur
chemical reaction between active gas molecules
and the fresh titanium surface burial of the
original ion in the bulk titanium plate or
burial of gas atoms/molecules resident on the
surface as the film started to form. The last two
mechanisms account for the pump's ability to
"pump" inert gases.
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Types of Ion Pump
There are two basic ion pump structures, the
diode and triode (named for the number of
electrically separate elements in the pump). In
the diode pump (Fig. 1), a high positive voltage
is placed on the tubes and the titanium plates
are grounded. It has high pumping speed for H2,
O2, CO2, N2, CO and other 'getterable' gases. A
variant, the noble diode (Fig. 2), has the same
electrical structure but has a tantalum plate in
place of one titanium plate. This modification
gives higher speed and greater stability for
inert gases while reducing H2 pumping speed.
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The triode pump's structure (Fig. 3) differs in
three major ways from that of the diode each
plate is insulated from the pump wall a high
negative potential is imposed on the plate and
the plate has multiple slots through it. The
three electrodes are, therefore, the tubes and
pump walls (both grounded) and the plates
(negative voltage). The triode's pumping
mechanisms match the diode's but the triode's
slotted plates allow the sputtered titanium to
deposit on the pump walls behind the plate. In
this position, the deposited material is much
less likely to suffer ion bombarded even at high
pressures with high bombardment rates. Any inert
gas physically buried in these deposits is,
therefore, less likely to be released.
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Ion Pump Properties

• Pressures between 1 x 10-9 torr 1 x 10-11 torr.
• Their pumping speeds and abilities to permanently
  trap inert gases, particularly helium, are not
  wonderful.
• The stray fields from the pump magnets, while not
  large, must be considered when specifying these
  pumps for surface science experiments involving
  low energy electron analysis.
• Extremely clean pumping.

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Pressure Measurement

• High Pressure
• Low Pressure

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High Pressure (Atm 10-4 Torr)

• Thermocouple
• Pirani
• Diaphragm

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Thermocouple Gauge
To determine a chamber's pressure range between
10 and 10-3 Torr a gauge measures the voltage of
a thermocouple spot-welded to a filament exposed
to system gas. A constant current supply feeds
the filament, though the filament's temperature
depends on thermal losses to the gas. At higher
pressure, more molecules hit the filament and
remove more heat energy, changing the
thermocouple voltage.
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Pirani Gauge
In a Pirani gauge (see above), two filaments
(platinum alloy in the best gauges), act as
resistances in two arms of a Wheatstone bridge.
The reference filament is immersed in a
fixed-gas pressure, while the measurement
filament is exposed to the system gas. A current
through the bridge heats both filaments. Gas
molecules hit the heated filaments and conduct
away some of the heat. If the gas pressures (or
composition) around the measurement filament is
not identical to that around the reference
filament, the bridge is unbalanced and the degree
of unbalance is a measure of the pressure
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High and Ultra High Vacuum

• Ion Gauge
• Residual Gas Analyzer

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Ion Gauge
heated filaments biased to give thermionic
electrons of 70e -- energetic enough to ionize
any residual gas molecules during collisions.
The positive ions formed drift to an ion
collector held at about 150V. The current
measures gas number density, a direct measure of
pressure.
Bayard-Alpert Ion Gauge
pressures as low as 1 x 10-9 Torr and possibly to
10-11 Torr
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Residual Gas Analyzer

• Hot filament electron-impact ionization section
  (ionized any gas molecules in the chamber).
• Quadrupole mass spectrometer.
• Faraday cup or electrom multiplier.
• Computer interface and display.
• Partial pressures down to 10-14 range.

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RGA quadrupole MS
Quadrupoles are four precisely parallel rods with
a direct current (DC) voltage and a superimposed
radio-frequency (RF) potential. And by scanning a
pre-selected radio-frequency field one
effectively scans a mass range.
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Vacuum Materials
Mechanical Properties The material must be
capable of being machined and fabricated. It
must have adequate strength at maximum and
minimum temperatures to be encountered, and must
retain it's elastic, plastic, and/or fluid
properties over the expected temperature range.
Thermal Properties The material's vapor
pressure must remain low at the highest
temperature. Thermal expansion of adjacent
materials must be taken into account, especially
at joints. Gas Loading Materials must not be
porous. Materials must be free of cracks and
crevices which can trap cleaning solvents and
become a source of virtual leaks later on.
Surface and bulk desorption rates must be
acceptable at extremes of temperature and
radiation.
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Common Materials

• Austenitic Stainless Steel is the most commonly
  used metal for high and ultra-high vacuum
  systems,since it fulfills all of the requirements
  above. U.S. 321 , 347, and 304 are chosen most
  frequently for satisfactory argon-arc welding.
  321 is used when low magnetic permeability is
  required.
• Aluminum and Aluminum Alloys are very cheap,easy
  to machine, and have a low outgassing rate as
  long as the alloy does not have a high zinc
  content. They have the disadvantage of low
  strength at high temperatures and high distortion
  when welding.
• Ceramics - Fully vitrified electrical porclean
  and vitrified alumina are excellent insulator
  that have a low outgassing rate,low gas
  permeability, and can be used to 1500 Degrees C.
  There are also some machinable ceramics
  available. All ceramics are brittle and must be
  handled with care.
• Borosilicate Glass ,a.k.a. Pyrex , is often used
  for small systems and viewing windows. Glass can
  be obtained as components from stock,is easy to
  fabricate into components,and has high corrosion
  resistance.
• Nitrile Rubber a.k.a. Buna N t.m. is widely used
  in demountable seals,i.e. "O" rings.
• Viton is bakeable to 200 Degrees C. and more
  suitable at lower pressures. Viton does have a
  tendency to compression set.

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Plastics

• PTFE has self-lubricating properties, a
  relatively low outgassing rate, is a good
  electrical insulator, and can be used at higher
  temperatures than other plastics. High
  permeability makes PTFE unsuitable as part of the
  vacuum envelope.
• . Polycarbonates and Polystyrene have moderate
  outgassing rates and water adsorption
  characteristics and are good electrical
  insulators.
• Nylon has self lubricating properties but a high
  outgassing rate and a high adsorption rate for
  water.
• Acrylics have the same undesirable vacuum
  properties as nylon.
• PVC has a high outgassing rate but does find
  application for rough vacuum lines and temporary
  connections such as leak detectors.
• Polyethylene may be usable if well outgassed.

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Resins and Epoxies

• Varian Torr Seal is a solvent free epoxy resin
  that can be used at pressures of 10e-09 mbar and
  below at temperatures from -45 Degrees C. to bake
  temperatures of 120 Degrees C. Torr seal dries to
  a hard,white consistency, and tends to be a bit
  pricey.
• Kurt J. Lesker KL-325K ,a solvent free epoxy
  packaged in a divider pouch. KL-320K is useful in
  the 10e-05 to 10e-07 torr range. Lesker also sell
  a conductive epoxy ( KL-325K ) but it's not
  recommended for pressures below 10e-03 torr.
• Vacseal is a low vapour pressure silicon resin
  available in aerosol spray cans and brush-on
  applicator bottles. The manufacturer claims it's
  good for applications from liquid helium
  temperatures to 450 Degrees C.