4 Impedance and Collective Effects
147
also exponentially increasing when moving to SEYs below 1.3–1.4. The technique
described here is what we call ‘beam induced machine scrubbing’ and machine like
the SPS and LHC fully rely on it to run successfully with 25 ns spaced beams. While
beam induced scrubbing is an important option for already built machines, coatings
with intrinsically low SEY materials can be envisaged at the design stage to limit
the creation of an electron cloud in future machines. A well established method
to reduce multipacting is coating with TiN, a material whose secondary emission
yield becomes quickly low after some conditioning (through illumination under
synchrotron light). The thickness of the coating must be of the order of a μm, such
as not to alter the resistive impedance seen by the beam. A more favorable getter
material made from TiZrV, called Non-Evaporable Getter (NEG), was developed
at CERN. This getter material is characterized by its greater structural stability
than TiN, its pumping capability and its low activation temperature. The warm
sections of the LHC, about 10% of the circumference, have been coated with NEG.
The NEG coating was also tested at several light source insertion devices, where
circumstantial evidence suggests an increase in the effective impedance, presumably
due to a larger surface roughness and low conductivity. The additional contributions
to the ring impedance from the surface roughness and low conductivity impedance
of the coating layer is of no concern for the longer proton bunches in the LHC, but
could significantly affect the stability of the short positron beams in the Damping
Rings of a future linear collider. From 2007 on, new efforts have been put on the
search for coating materials that do not require high temperature activation and do
not suffer from aging. In particular, amorphous carbon (a-C) thin films, deposited
with d.c. magnetron sputtering, have shown to possess all these qualities. Besides,
their maximum secondary emission yields, measured in the lab, reach values even
below one and the films are also stable against mechanical stress. Testing of a-C
coating in accelerator environments (SPS and Cesr-TA) has demonstrated all these
features. Another method to reduce the secondary emission yield of a surface is
to use a naturally rough material. Here the SEY reduction is a geometrical effect
due to the high probability of quick re-absorption of the electrons emitted with low
energy.
Multipacting can also be suppressed by solenoids, though one should pay
attention to the possibility of exciting undesired cyclotron resonances. Electric
clearing fields are an efficient cure, as shown both in simulations and measurements
of electron cloud in the CERN PS. They were already used to cure electron-proton
instabilities for the coasting proton beams in the CERN ISR during the early 70s.
At the SNS operating with long proton bunches all BPMs can be biased with a
clearing voltage of 1 kV. To be effective for the multipacting experienced by short
bunches with close spacing, the clearing electrodes must be mounted all around the
ring, in distances of a few tens of cm and voltages of the order 1 kV are probably
required. The impedance introduced by many such devices could be prohibitive, as
it appeared to be the case in the DANE positron ring with the very first clearing
electrode design. Other options for a practical implementation of electric clearing
fields may be splitting the beam pipe into a top and bottom half, isolated from
each other and held at different potential. Biasing the two jaws of a collimator
147
also exponentially increasing when moving to SEYs below 1.3–1.4. The technique
described here is what we call ‘beam induced machine scrubbing’ and machine like
the SPS and LHC fully rely on it to run successfully with 25 ns spaced beams. While
beam induced scrubbing is an important option for already built machines, coatings
with intrinsically low SEY materials can be envisaged at the design stage to limit
the creation of an electron cloud in future machines. A well established method
to reduce multipacting is coating with TiN, a material whose secondary emission
yield becomes quickly low after some conditioning (through illumination under
synchrotron light). The thickness of the coating must be of the order of a μm, such
as not to alter the resistive impedance seen by the beam. A more favorable getter
material made from TiZrV, called Non-Evaporable Getter (NEG), was developed
at CERN. This getter material is characterized by its greater structural stability
than TiN, its pumping capability and its low activation temperature. The warm
sections of the LHC, about 10% of the circumference, have been coated with NEG.
The NEG coating was also tested at several light source insertion devices, where
circumstantial evidence suggests an increase in the effective impedance, presumably
due to a larger surface roughness and low conductivity. The additional contributions
to the ring impedance from the surface roughness and low conductivity impedance
of the coating layer is of no concern for the longer proton bunches in the LHC, but
could significantly affect the stability of the short positron beams in the Damping
Rings of a future linear collider. From 2007 on, new efforts have been put on the
search for coating materials that do not require high temperature activation and do
not suffer from aging. In particular, amorphous carbon (a-C) thin films, deposited
with d.c. magnetron sputtering, have shown to possess all these qualities. Besides,
their maximum secondary emission yields, measured in the lab, reach values even
below one and the films are also stable against mechanical stress. Testing of a-C
coating in accelerator environments (SPS and Cesr-TA) has demonstrated all these
features. Another method to reduce the secondary emission yield of a surface is
to use a naturally rough material. Here the SEY reduction is a geometrical effect
due to the high probability of quick re-absorption of the electrons emitted with low
energy.
Multipacting can also be suppressed by solenoids, though one should pay
attention to the possibility of exciting undesired cyclotron resonances. Electric
clearing fields are an efficient cure, as shown both in simulations and measurements
of electron cloud in the CERN PS. They were already used to cure electron-proton
instabilities for the coasting proton beams in the CERN ISR during the early 70s.
At the SNS operating with long proton bunches all BPMs can be biased with a
clearing voltage of 1 kV. To be effective for the multipacting experienced by short
bunches with close spacing, the clearing electrodes must be mounted all around the
ring, in distances of a few tens of cm and voltages of the order 1 kV are probably
required. The impedance introduced by many such devices could be prohibitive, as
it appeared to be the case in the DANE positron ring with the very first clearing
electrode design. Other options for a practical implementation of electric clearing
fields may be splitting the beam pipe into a top and bottom half, isolated from
each other and held at different potential. Biasing the two jaws of a collimator