744
M. Dohlus et al.
For a given frequency the four-vane structure is the most power efficient, for
low current applications 4-rod structures may be cheaper to build. Currents of up to
100 mA can be handled with a single RFQ in the frequency range of 80–400 MHz,
while higher currents require beam from two parallel RFQs to be combined in a
funnelling section that operates at half the frequency of the main linac.
Drift Tube Linac
Once the beam has reached an energy of approximately 3 MeV, the efficiency of
the acceleration provided by an RFQ will drop to a level where it would be optimal
to use an alternative method of acceleration. In high-power proton linacs, a Drift
Tube Linac (DTL) is often chosen to succeed the RFQ due to its high efficiency, and
well-understood design principles.
A DTL is a large RF cavity through which the beam passes, absorbing power
from the accelerating field. The time taken for the beam to traverse this structure is
normally many tens of RF periods, and so metallic drift tubes are added to shield the
beam from the decelerating phase of the RF. The drift tubes must then be designed
to have the correct length and location for the beam to see the correct amplitude and
phase of the accelerating fields.
The DTL accelerates the bunched beam from the RFQ to energies between
50 and 100 MeV [121]. The accelerating efficiency of DTLs drops at energies
above 50 MeV and therefore a change of structure is required. The problem is
that the physics & engineering effort required to build a more efficient structure
costs more than any potential operational savings while increasing the risks. The
normal conducting alternatives are Separated DTLs (JPARC), Side Coupled DTLs
(Los Alamos and SNS), and Cavity coupled DTLs (LINAC4). Transverse focusing
is achieved by permanent or electromagnetic quadrupoles housed inside the DTL
drift tubes and/or between RF cavities.
Intermediate Energy Acceleration
In the energy range between approximately 100 MeV and 500 MeV, there tends to
be a split in the technologies chosen for beam acceleration.
For machines with a duty cycle of more than a few percent, the ohmic losses
due to the finite resistance of the metallic cavities is a significant driver of the
operational costs of the accelerator. For this reason, these machines tend to choose
superconducting structures in order to increase the fraction of the RF power
that accelerates the beam. Despite the running cost of the cryogenic facility,
superconducting facilities with significant duty cycles will cost substantially less to
operate than room-temperature machines. An emerging alternative to higher energy
DTL-variants is the superconducting spoke cavity technology. Spoke resonators
have a large transverse and longitudinal acceptance and are mechanically very stiff,
reducing their sensitivity to microphonics and to Lorenz force detuning compared
to elliptical resonators in this energy range.
For machines whose duty cycle is less than this cut-off, the long time taken
for filling the superconducting cavities with RF before beam arrival becomes
significant, thereby degrading the efficiency. For these machines, the higher RF
power costs are easily outweighed by the removal of the large cryogenic facility.
M. Dohlus et al.
For a given frequency the four-vane structure is the most power efficient, for
low current applications 4-rod structures may be cheaper to build. Currents of up to
100 mA can be handled with a single RFQ in the frequency range of 80–400 MHz,
while higher currents require beam from two parallel RFQs to be combined in a
funnelling section that operates at half the frequency of the main linac.
Drift Tube Linac
Once the beam has reached an energy of approximately 3 MeV, the efficiency of
the acceleration provided by an RFQ will drop to a level where it would be optimal
to use an alternative method of acceleration. In high-power proton linacs, a Drift
Tube Linac (DTL) is often chosen to succeed the RFQ due to its high efficiency, and
well-understood design principles.
A DTL is a large RF cavity through which the beam passes, absorbing power
from the accelerating field. The time taken for the beam to traverse this structure is
normally many tens of RF periods, and so metallic drift tubes are added to shield the
beam from the decelerating phase of the RF. The drift tubes must then be designed
to have the correct length and location for the beam to see the correct amplitude and
phase of the accelerating fields.
The DTL accelerates the bunched beam from the RFQ to energies between
50 and 100 MeV [121]. The accelerating efficiency of DTLs drops at energies
above 50 MeV and therefore a change of structure is required. The problem is
that the physics & engineering effort required to build a more efficient structure
costs more than any potential operational savings while increasing the risks. The
normal conducting alternatives are Separated DTLs (JPARC), Side Coupled DTLs
(Los Alamos and SNS), and Cavity coupled DTLs (LINAC4). Transverse focusing
is achieved by permanent or electromagnetic quadrupoles housed inside the DTL
drift tubes and/or between RF cavities.
Intermediate Energy Acceleration
In the energy range between approximately 100 MeV and 500 MeV, there tends to
be a split in the technologies chosen for beam acceleration.
For machines with a duty cycle of more than a few percent, the ohmic losses
due to the finite resistance of the metallic cavities is a significant driver of the
operational costs of the accelerator. For this reason, these machines tend to choose
superconducting structures in order to increase the fraction of the RF power
that accelerates the beam. Despite the running cost of the cryogenic facility,
superconducting facilities with significant duty cycles will cost substantially less to
operate than room-temperature machines. An emerging alternative to higher energy
DTL-variants is the superconducting spoke cavity technology. Spoke resonators
have a large transverse and longitudinal acceptance and are mechanically very stiff,
reducing their sensitivity to microphonics and to Lorenz force detuning compared
to elliptical resonators in this energy range.
For machines whose duty cycle is less than this cut-off, the long time taken
for filling the superconducting cavities with RF before beam arrival becomes
significant, thereby degrading the efficiency. For these machines, the higher RF
power costs are easily outweighed by the removal of the large cryogenic facility.