450
F. Bordry et al.
(MCP). The avalanche of electrons produced by the MCP are then collected on
strip detectors or hit a phosphor screen to give an image of the beam profile that
can be monitored using a CCD camera. Due to their rigidity, ions are less sensitive
to the distorting effects of the space charge from the circulating beam, but their
slow drift time, even with high bias voltages, means that they spend a long time
in this beam field, making it difficult to analyse beam dimensions smaller than one
millimetre. However, in order to use electrons to produce an image, a transverse
magnetic field needs to be added around which the electrons spiral on their way to
the MCP. This eliminates, to a large extent, the space charge effects of the beam
and allows sharper images to be produced than with ions. This additional magnetic
field is also seen by the beam and has to be compensated by two corrector magnets
either side of the ionisation profile monitor. Direct detection of ionized particles
using in-vacuum pixel sensors has recently been demonstrated as an alternative to
MCP detection [145], providing a more sensitive monitor and eliminating the ageing
effects associated with micro channel plates.
Luminescence monitors [144] also rely on the interaction of the beam with a gas
in the vacuum chamber. In this case electrons are excited from the ground state to
a higher energy level by the passing beam. Once the beam has passed the electrons
return to the ground state and emit photons. In the case of nitrogen the dominant
photon wavelength is 391.3 nm, corresponding to light at the lower end of the visible
range, for which many detectors are available. In general, the residual gas alone
does not produce enough photons for accurate imaging and hence a local pressure
bump is usually created by injecting a small amount of additional gas to enhance
the photon production.
Most users consider both the residual gas ionisation and luminescence profile
monitors to be semi-quantitative and not to be relied upon for absolute emittance
measurements, even after calibration against some other instrument such as a wire
scanner. Their virtual transparency for the beam, however, makes them useful for
the continuous on-line tracking of beam size.
8.6.4.5 Synchrotron Radiation Monitors
Synchrotron radiation monitors [146] are limited to highly relativistic particles
and offer a completely non-destructive and continuous measurement of the 2dimensional density distribution of the beam. These monitors make use of the light
produced when highly relativistic particles are deflected by a magnetic field. They
are therefore usually positioned to make use of parasitic light produced by a dipole
magnet in the machine or behind a purpose built “undulator” magnet in which the
beam is deflected several times to enhance the photon emission.
The most common way of measuring the beam size with synchrotron radiation
is to directly image the extracted light using traditional optics and a camera. The
spatial resolution for such systems is usually limited by diffraction and depth-offield effects which can be overcome using interferometers [147]. If the beam is
sufficiently relativistic then the photon emission extends into the hard X-ray region
F. Bordry et al.
(MCP). The avalanche of electrons produced by the MCP are then collected on
strip detectors or hit a phosphor screen to give an image of the beam profile that
can be monitored using a CCD camera. Due to their rigidity, ions are less sensitive
to the distorting effects of the space charge from the circulating beam, but their
slow drift time, even with high bias voltages, means that they spend a long time
in this beam field, making it difficult to analyse beam dimensions smaller than one
millimetre. However, in order to use electrons to produce an image, a transverse
magnetic field needs to be added around which the electrons spiral on their way to
the MCP. This eliminates, to a large extent, the space charge effects of the beam
and allows sharper images to be produced than with ions. This additional magnetic
field is also seen by the beam and has to be compensated by two corrector magnets
either side of the ionisation profile monitor. Direct detection of ionized particles
using in-vacuum pixel sensors has recently been demonstrated as an alternative to
MCP detection [145], providing a more sensitive monitor and eliminating the ageing
effects associated with micro channel plates.
Luminescence monitors [144] also rely on the interaction of the beam with a gas
in the vacuum chamber. In this case electrons are excited from the ground state to
a higher energy level by the passing beam. Once the beam has passed the electrons
return to the ground state and emit photons. In the case of nitrogen the dominant
photon wavelength is 391.3 nm, corresponding to light at the lower end of the visible
range, for which many detectors are available. In general, the residual gas alone
does not produce enough photons for accurate imaging and hence a local pressure
bump is usually created by injecting a small amount of additional gas to enhance
the photon production.
Most users consider both the residual gas ionisation and luminescence profile
monitors to be semi-quantitative and not to be relied upon for absolute emittance
measurements, even after calibration against some other instrument such as a wire
scanner. Their virtual transparency for the beam, however, makes them useful for
the continuous on-line tracking of beam size.
8.6.4.5 Synchrotron Radiation Monitors
Synchrotron radiation monitors [146] are limited to highly relativistic particles
and offer a completely non-destructive and continuous measurement of the 2dimensional density distribution of the beam. These monitors make use of the light
produced when highly relativistic particles are deflected by a magnetic field. They
are therefore usually positioned to make use of parasitic light produced by a dipole
magnet in the machine or behind a purpose built “undulator” magnet in which the
beam is deflected several times to enhance the photon emission.
The most common way of measuring the beam size with synchrotron radiation
is to directly image the extracted light using traditional optics and a camera. The
spatial resolution for such systems is usually limited by diffraction and depth-offield effects which can be overcome using interferometers [147]. If the beam is
sufficiently relativistic then the photon emission extends into the hard X-ray region