460
F. Bordry et al.
formed by a stack of high power ceramic-carbon resistor disks housed in a
coaxial metallic enclosure, which is the return path of the current, to minimize the
inductance and thus allow good matching over a wide range of frequencies. For
cooling and insulation purposes the resistors are usually immersed in oil. Where
available length for the kicker magnet has priority over rise time, a short circuit at
the pulse output end of the magnet has the advantage of doubling the kick strength,
for a given PFN voltage and system impedance. However the fill time of the magnet
is also doubled; to establish full field in the magnet the pulse has to travel to the
magnet output and then reflect back to the driven end of the magnet. In addition,
for a magnet terminated in a short-circuit, during the field pulse the magnet also
experiences both polarities of voltage [162].
8.7.1.2 Beam Coupling Impedance
Kicker magnets with ferrite or laminated steel yokes, directly exposed to the beam,
can contribute a major fraction of the longitudinal and transverse beam coupling
impedance of a machine [168]. The passage of the beam, without providing an
appropriate path for the beam image current, can also cause resonances within
the volume of the tank and lead to higher order mode losses. Since these effects
can results in beam instabilities their reduction is becoming increasingly important
with rising beam intensity. Electromagnetic coupling with high-intensity beams also
leads to heating of the ferrites, sometimes beyond their Curie temperature.
Where needed and possible these issues are being addressed in existing applications by various retrofitting techniques, thus avoiding a costly redesign of the whole
kicker system and possible implicit layout changes. The kicker magnet vacuum tank
can be shielded from the beam by incorporating metallic transition pieces between
the tank flanges and the kicker magnet [169] and between subsequent magnet
modules in a common tank, thus minimising changes in aperture dimensions and
avoiding some resonances.
To reduce the temperature rise of the ferrites, due to beam induced heating, indirect water cooling has been implemented with electrically insulating but thermally
conducting Aluminium-Nitride plates in contact with the ferrites [170]. Particular
precautions must be taken here because of the presence of the high voltage. To
reduce the coupling of the beam with the ferrite yoke, without loss of aperture,
two comb structures of silver paint stripes can be serigraphed onto a ferrite (Fig.
8.70, left) [171]. Each of the two sets of stripes is connected to a high voltage
plate; the capacitive coupling between the two sets of stripes, which is enhanced
by the permittivity of the ferrite, provides the path for the beam image current,
without creating eddy current loops which would adversely impact the field rise
time. Nevertheless the overlapping length of the two serigraphed combs should
be optimized to avoid resonances at undesirable frequencies [172]. The serigraphy
technique is less attractive for short cells because of both high voltage breakdown
issues and the relatively small capacitive coupling between the two sets of stripes.
F. Bordry et al.
formed by a stack of high power ceramic-carbon resistor disks housed in a
coaxial metallic enclosure, which is the return path of the current, to minimize the
inductance and thus allow good matching over a wide range of frequencies. For
cooling and insulation purposes the resistors are usually immersed in oil. Where
available length for the kicker magnet has priority over rise time, a short circuit at
the pulse output end of the magnet has the advantage of doubling the kick strength,
for a given PFN voltage and system impedance. However the fill time of the magnet
is also doubled; to establish full field in the magnet the pulse has to travel to the
magnet output and then reflect back to the driven end of the magnet. In addition,
for a magnet terminated in a short-circuit, during the field pulse the magnet also
experiences both polarities of voltage [162].
8.7.1.2 Beam Coupling Impedance
Kicker magnets with ferrite or laminated steel yokes, directly exposed to the beam,
can contribute a major fraction of the longitudinal and transverse beam coupling
impedance of a machine [168]. The passage of the beam, without providing an
appropriate path for the beam image current, can also cause resonances within
the volume of the tank and lead to higher order mode losses. Since these effects
can results in beam instabilities their reduction is becoming increasingly important
with rising beam intensity. Electromagnetic coupling with high-intensity beams also
leads to heating of the ferrites, sometimes beyond their Curie temperature.
Where needed and possible these issues are being addressed in existing applications by various retrofitting techniques, thus avoiding a costly redesign of the whole
kicker system and possible implicit layout changes. The kicker magnet vacuum tank
can be shielded from the beam by incorporating metallic transition pieces between
the tank flanges and the kicker magnet [169] and between subsequent magnet
modules in a common tank, thus minimising changes in aperture dimensions and
avoiding some resonances.
To reduce the temperature rise of the ferrites, due to beam induced heating, indirect water cooling has been implemented with electrically insulating but thermally
conducting Aluminium-Nitride plates in contact with the ferrites [170]. Particular
precautions must be taken here because of the presence of the high voltage. To
reduce the coupling of the beam with the ferrite yoke, without loss of aperture,
two comb structures of silver paint stripes can be serigraphed onto a ferrite (Fig.
8.70, left) [171]. Each of the two sets of stripes is connected to a high voltage
plate; the capacitive coupling between the two sets of stripes, which is enhanced
by the permittivity of the ferrite, provides the path for the beam image current,
without creating eddy current loops which would adversely impact the field rise
time. Nevertheless the overlapping length of the two serigraphed combs should
be optimized to avoid resonances at undesirable frequencies [172]. The serigraphy
technique is less attractive for short cells because of both high voltage breakdown
issues and the relatively small capacitive coupling between the two sets of stripes.