10 The Largest Accelerators and Colliders of Their Time
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so-called strong, strong model, where each beam is described by a large number
of super-particles and the thin lens beam-beam lenses are split into many beambeam-lens slices. The individual particle’s trajectories are impacted by the collective
electro-magnetic forces induced by the particles of the opposite beam, respectively.
This consitutes a fully dynamical model of the beam-beam interactions. The result
of this study was that this effect indeed exists and would affect the performance of
the EIC if not avoided by a careful choice of the machine parameters. The threshold
in proton bunch intensity for this catastrophic instability is a factor of two above the
chosen eRHIC design values of proton bunch intensity [177].
10.5.2.2 Beam-Gas Backgrounds of the Colliding Beam Detectors
The experience with beam induced backgrounds at HERA is very important for the
design of the interaction regions and the collision detectors of the EIC or LHeC.
The combination of synchrotron radiation emitted by the electron beam in the
separator- and the focusing fields when entering the interaction region and the
corresponding strong sources of desorbed gases in the vicinity of the collision
detectors caused initially strong hadron-beam-gas induced detector backgounds.
These backgrounds improved slowly by conditioning the surfaces of masks and
absorbers by the presence of electron beam and scattered synchroton photons. Eventually a very high dynamical vacuum quality in the order of 1 nbar with full beam
intensity was achieved. This required a installation of a large pumping speed and
good vacuum conductance with integrated NEG pump in the IR quadrupole magnets
and Titanium supplimation pumps. The cold beam pipe of the superconducting IR
magnets (at a temperature of 80 K) acted as a cryopump and helped to improve the
vacuum. But once the surface was saturated, the magnes needed to be warmed up
and the accumulated gas gas molecules on the cold surfaces needed to be removed
from the IR vacuum system using turbo pumps. Each warm-up and cooldown cycle
marked a step in background improvement.
Another important lesson learned from HERA IR operations was that the vacuum
system must be designed such that IR vacuum leaks are unlikely and that venting of
the IR and detector vacuum for maintenance and repair purposes must be avoided
by design.
The analysis of HERA backgrounds [178] shows the importance of extremely
good vacuum quality in the beam vacuum chamber inside the central colliding beam
detector. This makes high vacuum pumping speed and high vacuum conductance
within the detector beam pipe mandatory. Techniques such as in situ bakeout and
NEG coating of the detector beam chamber might be unavoidable to overcome this
difficulty.
This experience is exploited in the design of the interaction regions of the EIC,
in particular at eRHIC. First background simulations indicate that difficult initial
background conditions as experienced in HERA can be avoided taking the lessons
learned into account.
637
so-called strong, strong model, where each beam is described by a large number
of super-particles and the thin lens beam-beam lenses are split into many beambeam-lens slices. The individual particle’s trajectories are impacted by the collective
electro-magnetic forces induced by the particles of the opposite beam, respectively.
This consitutes a fully dynamical model of the beam-beam interactions. The result
of this study was that this effect indeed exists and would affect the performance of
the EIC if not avoided by a careful choice of the machine parameters. The threshold
in proton bunch intensity for this catastrophic instability is a factor of two above the
chosen eRHIC design values of proton bunch intensity [177].
10.5.2.2 Beam-Gas Backgrounds of the Colliding Beam Detectors
The experience with beam induced backgrounds at HERA is very important for the
design of the interaction regions and the collision detectors of the EIC or LHeC.
The combination of synchrotron radiation emitted by the electron beam in the
separator- and the focusing fields when entering the interaction region and the
corresponding strong sources of desorbed gases in the vicinity of the collision
detectors caused initially strong hadron-beam-gas induced detector backgounds.
These backgrounds improved slowly by conditioning the surfaces of masks and
absorbers by the presence of electron beam and scattered synchroton photons. Eventually a very high dynamical vacuum quality in the order of 1 nbar with full beam
intensity was achieved. This required a installation of a large pumping speed and
good vacuum conductance with integrated NEG pump in the IR quadrupole magnets
and Titanium supplimation pumps. The cold beam pipe of the superconducting IR
magnets (at a temperature of 80 K) acted as a cryopump and helped to improve the
vacuum. But once the surface was saturated, the magnes needed to be warmed up
and the accumulated gas gas molecules on the cold surfaces needed to be removed
from the IR vacuum system using turbo pumps. Each warm-up and cooldown cycle
marked a step in background improvement.
Another important lesson learned from HERA IR operations was that the vacuum
system must be designed such that IR vacuum leaks are unlikely and that venting of
the IR and detector vacuum for maintenance and repair purposes must be avoided
by design.
The analysis of HERA backgrounds [178] shows the importance of extremely
good vacuum quality in the beam vacuum chamber inside the central colliding beam
detector. This makes high vacuum pumping speed and high vacuum conductance
within the detector beam pipe mandatory. Techniques such as in situ bakeout and
NEG coating of the detector beam chamber might be unavoidable to overcome this
difficulty.
This experience is exploited in the design of the interaction regions of the EIC,
in particular at eRHIC. First background simulations indicate that difficult initial
background conditions as experienced in HERA can be avoided taking the lessons
learned into account.