468
F. Bordry et al.
Beam impact leads to heating of the septum, and the thermal expansion can
cause warping which increases the effective thickness seen by the beam (case of
foil septa). Once the deformation becomes too important the device needs to be
overhauled. Wires are typically spring-loaded such that damaged wires will be
retracted from the beam and high voltage regions and not get stuck in an unwanted
position. To further reduce deformations caused by thermal effects the septum
support can be made out of Invar which is most relevant for high intensity or high
energy extraction channels. The opposite electrode consists essentially of a bar made
out of stainless steel or titanium and is at high voltage (up to over 300 kV). Careful
preparation and high voltage conditioning of the surfaces exposed to the high field
are essential for reliable operation.
To allow high electric fields electrostatic septa are installed under vacuum;
the vacuum works as an insulator between septum and electrode. If the vacuum
requirements are not too severe (order of 10 −9 mbar) anodized aluminium can be
used to eliminate the dark current on the cathode electrode, thus allowing operation
with fields as high as 10 to 20 MV/m. To achieve a vacuum of up to 10 −12 mbar septa
may be designed to be bake-able at up to 300 ◦ C. Electrostatic septa are usually fitted
with remote positioning systems which allow alignment of the septum with respect
to the circulating beam to minimize losses, optimize the trajectories, and adjust the
gap width. This feature is also very helpful for HV conditioning purposes.
If the interaction with the beam is to be minimized, a wire septum is the preferred
choice. However, it allows some of the field to penetrate into the circulating beam
region, the degree of which depends on the wire diameter and spacing. Ions created
by ionisation of the residual gas by the beam can cross through the wire array
into the high field area and provoke HV breakdowns. To counteract this, clearing
electrodes (so called “ion traps”, visible in Fig. 8.75) are added which provide a
small electric field (few kV) in the circulating beam area and capture the ions before
they can escape. The price to pay for the reduced beam interaction with a wire
septum is the additional complexity of the clearing electrodes and their associated
power supplies and electronics. Foil septa do not need clearing electrodes; to avoid
ionised residual gas entering the high field region longitudinally, aluminium screens
are sometimes mounted at the entry and the exit of the extraction channel (septum).
Beam impact leads to activation of the septum (either foil or wires), and, due
to the scattered particles, also of the septum support and surrounding material and
equipment. The radiation level has an impact on the cool-down time needed before
maintenance or an intervention can be carried out on the equipment. To reduce the
amount of beam interacting with the septum, a passive diffuser can be installed
upstream of and in line with the septum [189, 190]. A diffuser is typically composed
of an array of wires with a thickness greater than the septum, to scatter away the
beam that otherwise would hit the septum. By scattering this finite part of the beam,
the beam density that hits the septum is reduced, reducing the overall activation of
both diffuser and electrostatic septum. Relevant for the effectiveness of a diffuser
is the angular spread of the beam at the septum, the distance between diffuser and
septum (phase advance), as well as the alignment of the diffuser with respect to the
septum [191]. To ease the alignment with the septum, the diffuser can be installed on
F. Bordry et al.
Beam impact leads to heating of the septum, and the thermal expansion can
cause warping which increases the effective thickness seen by the beam (case of
foil septa). Once the deformation becomes too important the device needs to be
overhauled. Wires are typically spring-loaded such that damaged wires will be
retracted from the beam and high voltage regions and not get stuck in an unwanted
position. To further reduce deformations caused by thermal effects the septum
support can be made out of Invar which is most relevant for high intensity or high
energy extraction channels. The opposite electrode consists essentially of a bar made
out of stainless steel or titanium and is at high voltage (up to over 300 kV). Careful
preparation and high voltage conditioning of the surfaces exposed to the high field
are essential for reliable operation.
To allow high electric fields electrostatic septa are installed under vacuum;
the vacuum works as an insulator between septum and electrode. If the vacuum
requirements are not too severe (order of 10 −9 mbar) anodized aluminium can be
used to eliminate the dark current on the cathode electrode, thus allowing operation
with fields as high as 10 to 20 MV/m. To achieve a vacuum of up to 10 −12 mbar septa
may be designed to be bake-able at up to 300 ◦ C. Electrostatic septa are usually fitted
with remote positioning systems which allow alignment of the septum with respect
to the circulating beam to minimize losses, optimize the trajectories, and adjust the
gap width. This feature is also very helpful for HV conditioning purposes.
If the interaction with the beam is to be minimized, a wire septum is the preferred
choice. However, it allows some of the field to penetrate into the circulating beam
region, the degree of which depends on the wire diameter and spacing. Ions created
by ionisation of the residual gas by the beam can cross through the wire array
into the high field area and provoke HV breakdowns. To counteract this, clearing
electrodes (so called “ion traps”, visible in Fig. 8.75) are added which provide a
small electric field (few kV) in the circulating beam area and capture the ions before
they can escape. The price to pay for the reduced beam interaction with a wire
septum is the additional complexity of the clearing electrodes and their associated
power supplies and electronics. Foil septa do not need clearing electrodes; to avoid
ionised residual gas entering the high field region longitudinally, aluminium screens
are sometimes mounted at the entry and the exit of the extraction channel (septum).
Beam impact leads to activation of the septum (either foil or wires), and, due
to the scattered particles, also of the septum support and surrounding material and
equipment. The radiation level has an impact on the cool-down time needed before
maintenance or an intervention can be carried out on the equipment. To reduce the
amount of beam interacting with the septum, a passive diffuser can be installed
upstream of and in line with the septum [189, 190]. A diffuser is typically composed
of an array of wires with a thickness greater than the septum, to scatter away the
beam that otherwise would hit the septum. By scattering this finite part of the beam,
the beam density that hits the septum is reduced, reducing the overall activation of
both diffuser and electrostatic septum. Relevant for the effectiveness of a diffuser
is the angular spread of the beam at the septum, the distance between diffuser and
septum (phase advance), as well as the alignment of the diffuser with respect to the
septum [191]. To ease the alignment with the septum, the diffuser can be installed on