374
F. Bordry et al.
that can be rapidly tuned. In these cases moderate or even extremely small Q’s can
become optimum.
In addition to the fundamental mode, the field distribution of which is normally
optimized for acceleration, other modes exist in the cavity, which can (and will)
also interact with the particle beam. Even if not actively driven by an amplifier,
these so-called higher order modes (HOM’s) still present their impedance to the
beam and may lead to instabilities and consequently have to be considered in the
design. They are normally selectively coupled out and damped using external loads
(HOM damper), thus reducing their Q.
8.2.2 The RF Cavity as Part of the System
The RF cavity is only one part of an RF system for a particle accelerator; the
complete RF system typically consists of the following elements: (1) a master RF
signal generator, controlled to have the correct frequency, phase and amplitude for
acceleration, (2) the RF amplifier chain amplifies this signal to often very large
power, (3) this power is then fed through the power coupler into the RF cavity, in
which it leads to the desired large electromagnetic RF field, designed to optimally
interact with the particle beam. As explained above, the cavity often uses resonance
to build up large fields with relatively modest powers. In modern RF systems, the
behaviour of the beam is constantly monitored and a multitude of feedback and
feed-forward loops is constantly correcting the phase and amplitude of the RF; this
latter is generally referred to as (4) low-level RF system (LLRF). Ancillary systems
assure the correct tune of the resonance frequency (see below), correct vacuum and
temperature conditions and interlocks for safety and protection.
8.2.3 Ferrite Cavities
In relatively small synchrotrons for protons and heavier ions, the speed of the particles is still changing substantially during acceleration. This requires the frequency to
be swept over a large range during the acceleration cycles. For the CERN PS Booster
e.g., which accelerates protons from to 50 MeV to 1.4 GeV, the proton velocity and
consequently the revolution frequency varies by roughly a factor 3 in about 500 ms.
In order to still take advantage of a resonance phenomenon, the resonance frequency
must be varied simultaneously with the ramping of the magnetic field and the
acceleration of the particles; in the PS Booster the corresponding frequency swing
for the h = 1 system (where h denotes the harmonic number) is 0.6 MHz to 1.8 MHz,
which was implemented as a ferrite cavity system with magnetic tuning [36].
A typical ferrite cavity is a double coaxial resonator with the accelerating gap in
the middle, half of which is conceptually sketched in Fig. 8.17. The inner conductors
of the coaxial are the beam pipes—the outer conductor is the cavity housing. Ferrite
F. Bordry et al.
that can be rapidly tuned. In these cases moderate or even extremely small Q’s can
become optimum.
In addition to the fundamental mode, the field distribution of which is normally
optimized for acceleration, other modes exist in the cavity, which can (and will)
also interact with the particle beam. Even if not actively driven by an amplifier,
these so-called higher order modes (HOM’s) still present their impedance to the
beam and may lead to instabilities and consequently have to be considered in the
design. They are normally selectively coupled out and damped using external loads
(HOM damper), thus reducing their Q.
8.2.2 The RF Cavity as Part of the System
The RF cavity is only one part of an RF system for a particle accelerator; the
complete RF system typically consists of the following elements: (1) a master RF
signal generator, controlled to have the correct frequency, phase and amplitude for
acceleration, (2) the RF amplifier chain amplifies this signal to often very large
power, (3) this power is then fed through the power coupler into the RF cavity, in
which it leads to the desired large electromagnetic RF field, designed to optimally
interact with the particle beam. As explained above, the cavity often uses resonance
to build up large fields with relatively modest powers. In modern RF systems, the
behaviour of the beam is constantly monitored and a multitude of feedback and
feed-forward loops is constantly correcting the phase and amplitude of the RF; this
latter is generally referred to as (4) low-level RF system (LLRF). Ancillary systems
assure the correct tune of the resonance frequency (see below), correct vacuum and
temperature conditions and interlocks for safety and protection.
8.2.3 Ferrite Cavities
In relatively small synchrotrons for protons and heavier ions, the speed of the particles is still changing substantially during acceleration. This requires the frequency to
be swept over a large range during the acceleration cycles. For the CERN PS Booster
e.g., which accelerates protons from to 50 MeV to 1.4 GeV, the proton velocity and
consequently the revolution frequency varies by roughly a factor 3 in about 500 ms.
In order to still take advantage of a resonance phenomenon, the resonance frequency
must be varied simultaneously with the ramping of the magnetic field and the
acceleration of the particles; in the PS Booster the corresponding frequency swing
for the h = 1 system (where h denotes the harmonic number) is 0.6 MHz to 1.8 MHz,
which was implemented as a ferrite cavity system with magnetic tuning [36].
A typical ferrite cavity is a double coaxial resonator with the accelerating gap in
the middle, half of which is conceptually sketched in Fig. 8.17. The inner conductors
of the coaxial are the beam pipes—the outer conductor is the cavity housing. Ferrite