8 Accelerator Engineering and Technology: Accelerator Technology
425
8.5.2.4 Outgassing of Materials
The outgassing is the spontaneous evolution of gas from solids or liquids. To be
compared with the degassing which is the deliberate removal of gas from a solid or
a liquid and with the desorption which is the stimulated release of adsorbed chemical
species from the surface of a solid or liquid.
The intrinsic outgassing rate is the quantity of gas leaving per unit time and per
unit of exposed geometric surface, or per unit of mass, at a specified time after
the start of the evacuation. The geometric surface is the visible surface without
correction for roughness or open porosity. The measured outgassing rate is the net
difference between the intrinsic outgassing rate and the rate of re-adsorption in the
test chamber. The re-adsorption rate depends on test chamber and on method of
measurement.
For unbaked metals, water is the main gas desorbed. The outgassing rate of water
decreases following a 1/t law. The outgassing of unbaked metals is not an intrinsic
value. However, measurements have shown that the water outgassing does not
depend significantly on the nature of metals, on surface treatments and on operating
temperature for temperatures lower than 110 ◦ C. At present no methods, except
heating (bake-out), exist to quickly remove water from unbaked metals. The fact
that desorption rates depend exponentially on temperature allows a faster reduction
of outgassing through heating (bake-out procedure). If a heat of adsorption of
15 kcal/mol and a bake-out temperature of 150 ◦ C are assumed, the rate of gas
removal will be increased by a factor of roughly 5000 relative to the value at room
temperature.
The outgassing of baked systems is dominated by H 2 diffusing out of the metallic
walls. The remaining outgassing species CH 4 and CO are believed to be related
to the presence of H 2 and its recombination with the surface contaminants. These
species will decrease if the H 2 level is reduced. Contrary to water outgassing,
the value of the outgassing rate of hydrogen is stable at room temperature. When
the thermal history is known, the outgassing of hydrogen is an intrinsic property
of metals. The diffusion model predicts values for the hydrogen outgassing that
are in agreement with experimental observations. For the vacuum-fired chambers
(950 ◦ C × 2 h, 10 −3 Pa H 2 ), the outgassing rate is limited by the background
signal induced by the gauges. The upper limit at room temperature is 10 −14
mbar·l s −1 ·cm −2 . In UHV systems, the outgassing of the beampipes can often be
compared to the outgassing of the vacuum instrumentation. Therefore, the outgassing of the instrumentation has to be considered in the design of an accelerator,
in particular close to the detectors in the experimental areas or in regions where
non-evaporable getter (NEG) coatings are used.
8.5.2.5 Kinetic Theory of Gasses
The gas density velocity distribution (mass of gas m) at a temperature, T, in a stationary large volume and at equilibrium follows a Maxwell-Boltzmann distribution
law:
425
8.5.2.4 Outgassing of Materials
The outgassing is the spontaneous evolution of gas from solids or liquids. To be
compared with the degassing which is the deliberate removal of gas from a solid or
a liquid and with the desorption which is the stimulated release of adsorbed chemical
species from the surface of a solid or liquid.
The intrinsic outgassing rate is the quantity of gas leaving per unit time and per
unit of exposed geometric surface, or per unit of mass, at a specified time after
the start of the evacuation. The geometric surface is the visible surface without
correction for roughness or open porosity. The measured outgassing rate is the net
difference between the intrinsic outgassing rate and the rate of re-adsorption in the
test chamber. The re-adsorption rate depends on test chamber and on method of
measurement.
For unbaked metals, water is the main gas desorbed. The outgassing rate of water
decreases following a 1/t law. The outgassing of unbaked metals is not an intrinsic
value. However, measurements have shown that the water outgassing does not
depend significantly on the nature of metals, on surface treatments and on operating
temperature for temperatures lower than 110 ◦ C. At present no methods, except
heating (bake-out), exist to quickly remove water from unbaked metals. The fact
that desorption rates depend exponentially on temperature allows a faster reduction
of outgassing through heating (bake-out procedure). If a heat of adsorption of
15 kcal/mol and a bake-out temperature of 150 ◦ C are assumed, the rate of gas
removal will be increased by a factor of roughly 5000 relative to the value at room
temperature.
The outgassing of baked systems is dominated by H 2 diffusing out of the metallic
walls. The remaining outgassing species CH 4 and CO are believed to be related
to the presence of H 2 and its recombination with the surface contaminants. These
species will decrease if the H 2 level is reduced. Contrary to water outgassing,
the value of the outgassing rate of hydrogen is stable at room temperature. When
the thermal history is known, the outgassing of hydrogen is an intrinsic property
of metals. The diffusion model predicts values for the hydrogen outgassing that
are in agreement with experimental observations. For the vacuum-fired chambers
(950 ◦ C × 2 h, 10 −3 Pa H 2 ), the outgassing rate is limited by the background
signal induced by the gauges. The upper limit at room temperature is 10 −14
mbar·l s −1 ·cm −2 . In UHV systems, the outgassing of the beampipes can often be
compared to the outgassing of the vacuum instrumentation. Therefore, the outgassing of the instrumentation has to be considered in the design of an accelerator,
in particular close to the detectors in the experimental areas or in regions where
non-evaporable getter (NEG) coatings are used.
8.5.2.5 Kinetic Theory of Gasses
The gas density velocity distribution (mass of gas m) at a temperature, T, in a stationary large volume and at equilibrium follows a Maxwell-Boltzmann distribution
law: