424
F. Bordry et al.
Fig. 8.56 Typical pump
down curves observed in
accelerator beampipes [b]
air at room temperature is given by:
λ air [m] =
6.67 × 10
−3
P [Pa]
; λ = 67 m at 10
−4 Pa
10
−6 mbar
,
(8.49)
In steady-state or equilibrium conditions, the throughput (quantity of gas per unit
of time), Q, is conservative: the same at one end of a vacuum system as it is at the
other.
Q =
P V
t
= P
V
t
= P · S,
(8.50)
S being defined as the pumping speed [m 3 /h], often in l/s.
In reality, Eq. (8.49) becomes Eq. (8.50) where, P 0 , is defined as the ultimate pressure. In general, S varies in the range of three orders of magnitude
(~1 → 1000 l s −1 ) while Q can extend over more than 10 orders of magnitudes: ~10 −5 mbar·l·s −1 ·cm −2 for plastics → 10 −15 mbar·l·s −1 ·cm −2 for metals [95]. The right choice of materials and treatments is compulsory in the
design of vacuum systems especially those for accelerators. In this respect the
measurement of outgassing rates is a basic activity for an ultra-high vacuum
expert.
Equation (8.50) shows that the ultimate pressure which is intrinsic to the system
dominates at low pressure regimes. At a given stage, increasing the pumping speed
will not lead to the expected pressure decrease.
P =
Q
S
+ P 0 ,
(8.51)
F. Bordry et al.
Fig. 8.56 Typical pump
down curves observed in
accelerator beampipes [b]
air at room temperature is given by:
λ air [m] =
6.67 × 10
−3
P [Pa]
; λ = 67 m at 10
−4 Pa
10
−6 mbar
,
(8.49)
In steady-state or equilibrium conditions, the throughput (quantity of gas per unit
of time), Q, is conservative: the same at one end of a vacuum system as it is at the
other.
Q =
P V
t
= P
V
t
= P · S,
(8.50)
S being defined as the pumping speed [m 3 /h], often in l/s.
In reality, Eq. (8.49) becomes Eq. (8.50) where, P 0 , is defined as the ultimate pressure. In general, S varies in the range of three orders of magnitude
(~1 → 1000 l s −1 ) while Q can extend over more than 10 orders of magnitudes: ~10 −5 mbar·l·s −1 ·cm −2 for plastics → 10 −15 mbar·l·s −1 ·cm −2 for metals [95]. The right choice of materials and treatments is compulsory in the
design of vacuum systems especially those for accelerators. In this respect the
measurement of outgassing rates is a basic activity for an ultra-high vacuum
expert.
Equation (8.50) shows that the ultimate pressure which is intrinsic to the system
dominates at low pressure regimes. At a given stage, increasing the pumping speed
will not lead to the expected pressure decrease.
P =
Q
S
+ P 0 ,
(8.51)