448
F. Bordry et al.
narrow cone around the angle of reflection for backward (vacuum to metal) OTR so
that if the foil is placed at 45 ◦ to the beam direction, the radiation produced is at 90 ◦
to the beam direction. In addition, forward OTR (metal to vacuum) is also produced
around the beam direction when the beam exits the foil. The angular distribution of
the emitted radiation has a central hole and a peak located at 1/γ . The higher the
value of γ the sharper the peaks and the more light can be collected, which is why
OTR is generally suited to lepton or high energy hadron machines. The imaging
can again be performed using simple optics followed by a CCD camera. See Fig.
8.64. Recent studies have demonstrated that OTR imaging systems are capable of
measuring a beam size below one micron [137, 138]. A non-invasive alternative to
OTR screens has been developed using diffraction radiation from slits [139], which
has also demonstrated good performance [140].
8.6.4.3 Wire Scanners
Of all the instruments used for measuring the emittance of circulating beams, wirescanners are considered to be the most trustworthy. They come in two different
types; rotative and linear. Rotative wire scanners can reach speeds of up to 20 ms −1
and consist of a thin wire (some tens of microns in diameter) mounted on a fork
which is attached to a rotating motor, while linear scanners use motors which
push/pull the wire across the beam. There are two ways of obtaining a beam profile
with wire scanners; by measuring the secondary emission current in the wire as
a function of wire position (similar to SEM-grid acquisition) or by measuring the
flux of secondary particles created as the beam interacts with the wire. This latter
technique is often used for high intensities, where the heating of the wire produces
thermal emission which falsifies the secondary emission results. It relies on the use
of radiation detectors, typically scintillators followed by photo-multipliers, placed
downstream of the wire scanner to detect the γ -radiation and secondary particles
produced when the wire intercepts the beam. To make the flux collected independent
of the wire position may require the summation of the signals from two or more
detectors positioned around the beam chamber. Fast wire scanners are nearly nondestructive over a wide range of energies. Their spatial resolution can reach the
micrometer range [141] and, with fast gated electronics, the profiles of individual
bunches can be observed. Their great sensitivity also allows them to be used for the
study of beam halo.
For H − and lepton accelerators the wire can be replaced by a laser beam, with
the detection of stripped electrons [142] or Compton scattered [143] electrons
respectively. This technique is ideally suited to high power machines, where a wire
would not survive, or to machines where the transverse beam size is extremely
small requiring a very thin ‘wire’ which is obtained by tightly focussing the laser
beam.
F. Bordry et al.
narrow cone around the angle of reflection for backward (vacuum to metal) OTR so
that if the foil is placed at 45 ◦ to the beam direction, the radiation produced is at 90 ◦
to the beam direction. In addition, forward OTR (metal to vacuum) is also produced
around the beam direction when the beam exits the foil. The angular distribution of
the emitted radiation has a central hole and a peak located at 1/γ . The higher the
value of γ the sharper the peaks and the more light can be collected, which is why
OTR is generally suited to lepton or high energy hadron machines. The imaging
can again be performed using simple optics followed by a CCD camera. See Fig.
8.64. Recent studies have demonstrated that OTR imaging systems are capable of
measuring a beam size below one micron [137, 138]. A non-invasive alternative to
OTR screens has been developed using diffraction radiation from slits [139], which
has also demonstrated good performance [140].
8.6.4.3 Wire Scanners
Of all the instruments used for measuring the emittance of circulating beams, wirescanners are considered to be the most trustworthy. They come in two different
types; rotative and linear. Rotative wire scanners can reach speeds of up to 20 ms −1
and consist of a thin wire (some tens of microns in diameter) mounted on a fork
which is attached to a rotating motor, while linear scanners use motors which
push/pull the wire across the beam. There are two ways of obtaining a beam profile
with wire scanners; by measuring the secondary emission current in the wire as
a function of wire position (similar to SEM-grid acquisition) or by measuring the
flux of secondary particles created as the beam interacts with the wire. This latter
technique is often used for high intensities, where the heating of the wire produces
thermal emission which falsifies the secondary emission results. It relies on the use
of radiation detectors, typically scintillators followed by photo-multipliers, placed
downstream of the wire scanner to detect the γ -radiation and secondary particles
produced when the wire intercepts the beam. To make the flux collected independent
of the wire position may require the summation of the signals from two or more
detectors positioned around the beam chamber. Fast wire scanners are nearly nondestructive over a wide range of energies. Their spatial resolution can reach the
micrometer range [141] and, with fast gated electronics, the profiles of individual
bunches can be observed. Their great sensitivity also allows them to be used for the
study of beam halo.
For H − and lepton accelerators the wire can be replaced by a laser beam, with
the detection of stripped electrons [142] or Compton scattered [143] electrons
respectively. This technique is ideally suited to high power machines, where a wire
would not survive, or to machines where the transverse beam size is extremely
small requiring a very thin ‘wire’ which is obtained by tightly focussing the laser
beam.