11 Application of Accelerators and Storage Rings
767
11.5.2.2.3 Heavy Ion-Induced Desorption
During high-intensity, heavy-ion operation of several particle accelerators worldwide, dynamic pressure build-ups of several orders of magnitude have been
observed. The pressure increase is caused by lost beam ions that impact the
vacuum chamber walls at a grazing angle. Ion-induced desorption, which has been
observed at BNL, CERN, and GSI, can seriously limit beam intensity and lifetime
in synchrotrons or storage rings, because mainly heavy gas molecules are desorbed,
resulting in large cross-sections for projectile charge changing processes. In the past
several years, experiments have been performed at several laboratories to study the
observed dynamic vacuum degradations; it is important to understand and overcome
this problem for present and future heavy ion accelerators [211]. The ion-induced
desorption yield is defined as
η =
#desorped molecules
#incident ions
.
(11.91)
Experimental studies of the desorption yield for energetic ions on stainless steel
at room temperature indicate a scaling law η ∼ (dE/dx)
n
el , where (dE/dx) el ~Z 2 /A
is the electronic energy loss, Z is the charge number and A the mass number. In the
5–100 MeV/u energy range and for perpendicular incidence, n ≈ 3 was obtained for
uranium ions. Unfortunately, the measured desorption yields at 100 MeV/u were
η ≈ 100 for uranium and η ≈ 4 for argon (Fig. 11.45) [212]. As a consequence,
uncontrolled beam loss must be minimized to avoid pressure bumps caused by
lost heavy ions. Ions lost due to collisions with the residual gas should hit only
low-desorption materials or dedicated collimators and catchers [213]. Vacuum
pressures in the range of 10 −11 mbar and lower are required for highly charged and
partially ionized heavy ion beams in order to control these processes. Therefore, the
vacuum requirements and beam-loss collimation issues in high-intensity heavy ion
synchrotrons and storage rings differ greatly from those in proton facilities.
11.5.2.3 Ion Accelerator Facilities
In this article is not possible to describe all existing accelerator facilities for stable
or radioactive ion beams, or those under construction. A few examples have been
selected and described. The focus will be on facilities that are considered by the
scientific community especially important for heavy ion nuclear physics in the long
term, as described in the IUPAP Report 41 and the NuPECC Long Range Plan 2010
[157, 214]. The facilities described here have been categorized on the basis of the
main production methods for radioactive ions: In-flight and ISOL facilities.
767
11.5.2.2.3 Heavy Ion-Induced Desorption
During high-intensity, heavy-ion operation of several particle accelerators worldwide, dynamic pressure build-ups of several orders of magnitude have been
observed. The pressure increase is caused by lost beam ions that impact the
vacuum chamber walls at a grazing angle. Ion-induced desorption, which has been
observed at BNL, CERN, and GSI, can seriously limit beam intensity and lifetime
in synchrotrons or storage rings, because mainly heavy gas molecules are desorbed,
resulting in large cross-sections for projectile charge changing processes. In the past
several years, experiments have been performed at several laboratories to study the
observed dynamic vacuum degradations; it is important to understand and overcome
this problem for present and future heavy ion accelerators [211]. The ion-induced
desorption yield is defined as
η =
#desorped molecules
#incident ions
.
(11.91)
Experimental studies of the desorption yield for energetic ions on stainless steel
at room temperature indicate a scaling law η ∼ (dE/dx)
n
el , where (dE/dx) el ~Z 2 /A
is the electronic energy loss, Z is the charge number and A the mass number. In the
5–100 MeV/u energy range and for perpendicular incidence, n ≈ 3 was obtained for
uranium ions. Unfortunately, the measured desorption yields at 100 MeV/u were
η ≈ 100 for uranium and η ≈ 4 for argon (Fig. 11.45) [212]. As a consequence,
uncontrolled beam loss must be minimized to avoid pressure bumps caused by
lost heavy ions. Ions lost due to collisions with the residual gas should hit only
low-desorption materials or dedicated collimators and catchers [213]. Vacuum
pressures in the range of 10 −11 mbar and lower are required for highly charged and
partially ionized heavy ion beams in order to control these processes. Therefore, the
vacuum requirements and beam-loss collimation issues in high-intensity heavy ion
synchrotrons and storage rings differ greatly from those in proton facilities.
11.5.2.3 Ion Accelerator Facilities
In this article is not possible to describe all existing accelerator facilities for stable
or radioactive ion beams, or those under construction. A few examples have been
selected and described. The focus will be on facilities that are considered by the
scientific community especially important for heavy ion nuclear physics in the long
term, as described in the IUPAP Report 41 and the NuPECC Long Range Plan 2010
[157, 214]. The facilities described here have been categorized on the basis of the
main production methods for radioactive ions: In-flight and ISOL facilities.