746
M. Dohlus et al.
HOM couplers typically have an inductive and a capacitive component, such
that there is a very sharp rejection at the frequency of the accelerating mode.
Although the addition of this filter successfully limits the impact on the accelerating
efficiency of the cavity, the requirement for a non-zero capacitance between the
central conductor and the cavity wall strongly increases the risk of a resonant
cascade of electrons being field-emitted from the metallic surfaces. This would
severely limit the successful operation of the cavity.
It has been shown [122] possible to instrument the signals excited on HOM
couplers, to act as a diagnostic device measuring the 4D transverse location of the
bunch (with position and angular resolutions of 4 μm and 140 μrad in FLASH),
as well as to measure the phase of the bunch with respect to the accelerating RF
(with a resolution of 0.08 ◦ at 1.3 GHz in FLASH). A determination of the cavitycavity transverse alignment using the excited HOMs has also been demonstrated.
The ability to instrument every accelerating cavity in this way is a major advantage
to the HOM coupling scheme.
11.4.3 Rapid Cycling Accelerators for Short Pulse Spallation
Neutron Sources
Accelerator schemes proposed for short pulse spallation neutron sources include:
1. Charge exchange injection from a pulsed H − linac to a fast cycling synchrotron
2. Charge exchange injection from a pulsed H − linac to a pulsed compressor ring
3. Direct proton injection from a pulsed proton linac to a fast cycling synchrotron
4. Direct proton injection from a pulsed proton linac to a pulsed compressor ring
5. Direct proton injection from a pulsed proton linac to a fast cycling FFAG ring
11.4.3.1 Charge Exchange Injection from a Pulsed H − Linac to a Fast
Cycling Synchrotron
A rapid cycling proton synchrotron (RCS) was the basis of a first, short pulse
source for neutron scattering research. It was proposed by J Carpenter in 1972, and
developed at ANL as the IPNS. The 450 MeV, 6.4 kW 30 Hz RCS source continued
successfully for 26 years [123] and other sources soon followed. In Japan, KENS
was developed at KEK, with a spallation target fed from their 20 Hz, 500 MeV,
3.5 kW proton RCS. ANL then studied higher power sources, but next came ISIS at
RAL, (1979–1984) with a 70 MeV H − linac and a 50 Hz, 800 MeV, 160 kW, RCS,
[124], exceeding the beam power of the IPNS by a factor of 25.
There followed a Central European Initiative for a spallation source in 1993–
1994, involving detailed feasibility studies [125] at both Vienna and CERN. Austron
proposed to use a 0.13 GeV, H − linac with a 1.6 GeV, 213 m circumference, proton
M. Dohlus et al.
HOM couplers typically have an inductive and a capacitive component, such
that there is a very sharp rejection at the frequency of the accelerating mode.
Although the addition of this filter successfully limits the impact on the accelerating
efficiency of the cavity, the requirement for a non-zero capacitance between the
central conductor and the cavity wall strongly increases the risk of a resonant
cascade of electrons being field-emitted from the metallic surfaces. This would
severely limit the successful operation of the cavity.
It has been shown [122] possible to instrument the signals excited on HOM
couplers, to act as a diagnostic device measuring the 4D transverse location of the
bunch (with position and angular resolutions of 4 μm and 140 μrad in FLASH),
as well as to measure the phase of the bunch with respect to the accelerating RF
(with a resolution of 0.08 ◦ at 1.3 GHz in FLASH). A determination of the cavitycavity transverse alignment using the excited HOMs has also been demonstrated.
The ability to instrument every accelerating cavity in this way is a major advantage
to the HOM coupling scheme.
11.4.3 Rapid Cycling Accelerators for Short Pulse Spallation
Neutron Sources
Accelerator schemes proposed for short pulse spallation neutron sources include:
1. Charge exchange injection from a pulsed H − linac to a fast cycling synchrotron
2. Charge exchange injection from a pulsed H − linac to a pulsed compressor ring
3. Direct proton injection from a pulsed proton linac to a fast cycling synchrotron
4. Direct proton injection from a pulsed proton linac to a pulsed compressor ring
5. Direct proton injection from a pulsed proton linac to a fast cycling FFAG ring
11.4.3.1 Charge Exchange Injection from a Pulsed H − Linac to a Fast
Cycling Synchrotron
A rapid cycling proton synchrotron (RCS) was the basis of a first, short pulse
source for neutron scattering research. It was proposed by J Carpenter in 1972, and
developed at ANL as the IPNS. The 450 MeV, 6.4 kW 30 Hz RCS source continued
successfully for 26 years [123] and other sources soon followed. In Japan, KENS
was developed at KEK, with a spallation target fed from their 20 Hz, 500 MeV,
3.5 kW proton RCS. ANL then studied higher power sources, but next came ISIS at
RAL, (1979–1984) with a 70 MeV H − linac and a 50 Hz, 800 MeV, 160 kW, RCS,
[124], exceeding the beam power of the IPNS by a factor of 25.
There followed a Central European Initiative for a spallation source in 1993–
1994, involving detailed feasibility studies [125] at both Vienna and CERN. Austron
proposed to use a 0.13 GeV, H − linac with a 1.6 GeV, 213 m circumference, proton