8 Accelerator Engineering and Technology: Accelerator Technology
441
• Multiplexed
Each electrode is multiplexed in turn onto the same, single acquisition
electronics chain. This eliminates channel to channel variations, giving very good
accuracy. However, since the switching is generally quite slow such an acquisition
only tends to be used in circulating machines where the average position is of
importance.
• Sigma/Delta
A passive hybrid is generally used to produce sum () and difference ()
from opposite electrodes. The resulting signals are then directly digitised, or
downmixed to lower frequency before digitisation using homodyne detection
(self-mixing the signal) or heterodyne detection (external mixing using a clock
related to the accelerator radio-frequency). Once digitised, the position is computed as K×/ with K a scaling factor dependent on the pick-up geometry
(typically close to ¼ of the aperture).
• Individual Treatment
Each electrode is acquired separately but in parallel either by direct digitisation
or logarithmic amplifiers. Position is calculated by difference over sum or
logarithmic ratio normalisation by the central processing unit of the acquisition
system.
• Passive Normalisation
Such electronics convert the amplitude difference from opposite electrodes
into a phase or time difference, which can then be detected using standard phase
detection or time to digital conversion techniques. The position is extracted from
this measurement using amplitude to phase or time normalisation as described
above.
8.6.2 Beam Current and Intensity Measurement
The measurement of beam current or bunch intensity is one of the most basic
measurements performed at any accelerator [124]. This is usually done by means
of a Faraday Cup or Beam Current Transformer (BCT).
8.6.2.1 Faraday Cup
The Faraday cup technique relies on measuring the charge deposited in a metallic
cup when intercepting the whole beam. This requires careful design, often using
a bias voltage or magnetic field at the entrance of the cup, to avoid the secondary
charges created by the interaction of the beam with the metal from escaping. They
are usually used for relatively low current measurements due to problems related to
the heat-load for higher intensity beams. Such devices act as an absolute calibration
standard and are capable of providing fA resolution [125].
441
• Multiplexed
Each electrode is multiplexed in turn onto the same, single acquisition
electronics chain. This eliminates channel to channel variations, giving very good
accuracy. However, since the switching is generally quite slow such an acquisition
only tends to be used in circulating machines where the average position is of
importance.
• Sigma/Delta
A passive hybrid is generally used to produce sum () and difference ()
from opposite electrodes. The resulting signals are then directly digitised, or
downmixed to lower frequency before digitisation using homodyne detection
(self-mixing the signal) or heterodyne detection (external mixing using a clock
related to the accelerator radio-frequency). Once digitised, the position is computed as K×/ with K a scaling factor dependent on the pick-up geometry
(typically close to ¼ of the aperture).
• Individual Treatment
Each electrode is acquired separately but in parallel either by direct digitisation
or logarithmic amplifiers. Position is calculated by difference over sum or
logarithmic ratio normalisation by the central processing unit of the acquisition
system.
• Passive Normalisation
Such electronics convert the amplitude difference from opposite electrodes
into a phase or time difference, which can then be detected using standard phase
detection or time to digital conversion techniques. The position is extracted from
this measurement using amplitude to phase or time normalisation as described
above.
8.6.2 Beam Current and Intensity Measurement
The measurement of beam current or bunch intensity is one of the most basic
measurements performed at any accelerator [124]. This is usually done by means
of a Faraday Cup or Beam Current Transformer (BCT).
8.6.2.1 Faraday Cup
The Faraday cup technique relies on measuring the charge deposited in a metallic
cup when intercepting the whole beam. This requires careful design, often using
a bias voltage or magnetic field at the entrance of the cup, to avoid the secondary
charges created by the interaction of the beam with the metal from escaping. They
are usually used for relatively low current measurements due to problems related to
the heat-load for higher intensity beams. Such devices act as an absolute calibration
standard and are capable of providing fA resolution [125].