10 The Largest Accelerators and Colliders of Their Time
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absorbers. Also machine protection system, loss monitors, and beam abort system
had to evolve as the stored current increased.
10.4.6 Electron Cloud
Electron cloud was the one of the toughest issues for the asymmetric B-factories,
specifically on the accumulation of the positron beam. The electron cloud had been
known as a possible cause of beam instability in positive-charged beams since a
long time ago such as the ISR era. Its observation [135] had been made at the
Photon Factory (PF) of KEK and a theoretical explanation [136] had been done
well before the start of the B-factories. What was new at the B-factories was the
single-bunch instability induced by electron clouds [137]. The previous instability
observed at the PF had been interpreted as a coupled-bunch instability, which was
supposed to be cured by a bunch-by-bunch feedback. Thus at least KEKB was not
well prepared for the single-bunch phenomena which have much higher frequency
than the available feedback. Actually possibility of such a single-bunch effect had
been suggested [138] before the construction of the B-factories, it had not been,
however, well recognized. The single-bunch effect was experimentally confirmed at
KEKB [139] as well as at CesrTA.
The electron cloud blowed up the vertical beam size drastically, and the threshold
beam current was 0.4 A with four-bucket (2.4 m) bunch spacing at KEKB. The
electron cloud appeared more severely in KEKB than in PEP–II, as the former had
a round Cu beam pipe while the latter an Al antechamber with TiN coating. Thus
the initial startup of the luminosity at KEKB was slower than PEP–II.
By applying weak magnetic field at the beam pipe, the electron cloud was
removed at least in the free space. Either permanent magnets or solenoids were
installed at KEKB and PEP–II to cover almost all straight sections and inside of
some magnets such as quadrupoles and weak dipoles by 2004. The mitigation
worked as expected and the blowup became unnoticeable at least for threebucket spacing in the case of KEKB [140]. Beside the magnetic field, various
mitigation techniques have been developed and tested at the B-factories, against the
formation of the electron cloud, including antechambers [141], TiN or Diamond-like
carbon coatings, grooved surface pipes [142], and clearing electrodes [143]. Those
techniques will be effective for future super B-factories and damping rings of linear
colliders. Also several measurements of the cloud density have been carried out.
Although the density of the electron cloud became below the instability threshold
by magnetic field, the betatron tune shift due to the cloud still remained in the LER
at KEKB to make the tune variation along the bunch train. A mitigation for the tune
variation was making use of pulsed quadrupoles as done at KEKB [144].
625
absorbers. Also machine protection system, loss monitors, and beam abort system
had to evolve as the stored current increased.
10.4.6 Electron Cloud
Electron cloud was the one of the toughest issues for the asymmetric B-factories,
specifically on the accumulation of the positron beam. The electron cloud had been
known as a possible cause of beam instability in positive-charged beams since a
long time ago such as the ISR era. Its observation [135] had been made at the
Photon Factory (PF) of KEK and a theoretical explanation [136] had been done
well before the start of the B-factories. What was new at the B-factories was the
single-bunch instability induced by electron clouds [137]. The previous instability
observed at the PF had been interpreted as a coupled-bunch instability, which was
supposed to be cured by a bunch-by-bunch feedback. Thus at least KEKB was not
well prepared for the single-bunch phenomena which have much higher frequency
than the available feedback. Actually possibility of such a single-bunch effect had
been suggested [138] before the construction of the B-factories, it had not been,
however, well recognized. The single-bunch effect was experimentally confirmed at
KEKB [139] as well as at CesrTA.
The electron cloud blowed up the vertical beam size drastically, and the threshold
beam current was 0.4 A with four-bucket (2.4 m) bunch spacing at KEKB. The
electron cloud appeared more severely in KEKB than in PEP–II, as the former had
a round Cu beam pipe while the latter an Al antechamber with TiN coating. Thus
the initial startup of the luminosity at KEKB was slower than PEP–II.
By applying weak magnetic field at the beam pipe, the electron cloud was
removed at least in the free space. Either permanent magnets or solenoids were
installed at KEKB and PEP–II to cover almost all straight sections and inside of
some magnets such as quadrupoles and weak dipoles by 2004. The mitigation
worked as expected and the blowup became unnoticeable at least for threebucket spacing in the case of KEKB [140]. Beside the magnetic field, various
mitigation techniques have been developed and tested at the B-factories, against the
formation of the electron cloud, including antechambers [141], TiN or Diamond-like
carbon coatings, grooved surface pipes [142], and clearing electrodes [143]. Those
techniques will be effective for future super B-factories and damping rings of linear
colliders. Also several measurements of the cloud density have been carried out.
Although the density of the electron cloud became below the instability threshold
by magnetic field, the betatron tune shift due to the cloud still remained in the LER
at KEKB to make the tune variation along the bunch train. A mitigation for the tune
variation was making use of pulsed quadrupoles as done at KEKB [144].