10 The Largest Accelerators and Colliders of Their Time
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threshold can be increased by lowering the peak current during transition crossing.
This instability has a growth rate faster than the synchrotron period and is similar
to a beam break-up instability. The instability is also enhanced by the presence of
electron clouds [74].
Deuteron-Gold Operation
Colliding 100 GeV/nucleon deuteron beam with 100 GeV/nucleon gold beam will
not produce the required temperature to create a new state of matter and therefore
serves as an important comparison measurement to the gold-gold collisions. The
rigidity of the two beams is different by about 20%, which results in different
deflection angles in the beam-combining dipoles on either side of the interaction
region. This requires a non-zero angle at the collision point, which slightly reduces
the available aperture.
The injection energy into RHIC was also the same for both beams requiring
the injector to produce beams with different rigidity. With same energy beams
throughout the acceleration cycle in RHIC the beams can remain cogged. Without
this the effect of the time-modulated long-range beam-beam interaction in the
IRs would lead to rapid beam loss. The typical bunch intensity of the deuteron
beam was about 1 × 10 11 with a transverse rms emittances of about 2 μm and a
total longitudinal emittance of 0.3 eVs/nucleon. The gold beam parameters were
similar to the gold-gold operation described above. Recently the ring with the
gold beam was operated with increased transverse focusing to reduce the effect of
intra-beam scattering on the transverse emittance. As a beneficial side effect the
two beams crossed transition at different times, which reduced the development
of the fast transverse instability. A peak luminosity of 27 × 10 28 cm −2 s −1
(100 × 10 30 cm −2 s −1 nucleon-pair luminosity) and store-averaged luminosity of
14 × 10 28 cm −2 s −1 was reached at the IRs with a 0.85 m betastar.
Polarized Proton Operation
Figure 10.6 shows the layout of the RHIC accelerator complex highlighting the
components required for polarized beam acceleration. The ‘Optically Pumped
Polarized Ion Source’ [75] is producing about 10 12 polarized protons per pulse. A
single source pulse is captured into a single bunch, which is sufficient beam intensity
to reach in RHIC the nominal bunch intensity of 2 × 10 11 polarized protons.
In the AGS two partial Siberian snakes are installed, an iron-based helical dipole
that rotates the spin around the longitudinal direction by 11 ◦ and a superconducting
helical dipole that can reach a 3 T field and a spin rotation of up to 45 ◦ . A view down
the magnet gap is shown in Fig. 10.7. With the two partial snakes placed with one
third of the AGS ring between them all vertical spin resonances are avoided up to
the required RHIC transfer energy of 23.8 GeV as long as the vertical betatron tune
is placed at 8.98, very close to an integer [76]. With an 80% polarization from the
source 65% polarization was reached at AGS extraction. The remaining polarization
loss in the AGS comes from weak spin resonances driven by the horizontal motion
of the beam [77]. It is planned to overcome them by quickly shifting the betatron
tune during resonance crossing.
605
threshold can be increased by lowering the peak current during transition crossing.
This instability has a growth rate faster than the synchrotron period and is similar
to a beam break-up instability. The instability is also enhanced by the presence of
electron clouds [74].
Deuteron-Gold Operation
Colliding 100 GeV/nucleon deuteron beam with 100 GeV/nucleon gold beam will
not produce the required temperature to create a new state of matter and therefore
serves as an important comparison measurement to the gold-gold collisions. The
rigidity of the two beams is different by about 20%, which results in different
deflection angles in the beam-combining dipoles on either side of the interaction
region. This requires a non-zero angle at the collision point, which slightly reduces
the available aperture.
The injection energy into RHIC was also the same for both beams requiring
the injector to produce beams with different rigidity. With same energy beams
throughout the acceleration cycle in RHIC the beams can remain cogged. Without
this the effect of the time-modulated long-range beam-beam interaction in the
IRs would lead to rapid beam loss. The typical bunch intensity of the deuteron
beam was about 1 × 10 11 with a transverse rms emittances of about 2 μm and a
total longitudinal emittance of 0.3 eVs/nucleon. The gold beam parameters were
similar to the gold-gold operation described above. Recently the ring with the
gold beam was operated with increased transverse focusing to reduce the effect of
intra-beam scattering on the transverse emittance. As a beneficial side effect the
two beams crossed transition at different times, which reduced the development
of the fast transverse instability. A peak luminosity of 27 × 10 28 cm −2 s −1
(100 × 10 30 cm −2 s −1 nucleon-pair luminosity) and store-averaged luminosity of
14 × 10 28 cm −2 s −1 was reached at the IRs with a 0.85 m betastar.
Polarized Proton Operation
Figure 10.6 shows the layout of the RHIC accelerator complex highlighting the
components required for polarized beam acceleration. The ‘Optically Pumped
Polarized Ion Source’ [75] is producing about 10 12 polarized protons per pulse. A
single source pulse is captured into a single bunch, which is sufficient beam intensity
to reach in RHIC the nominal bunch intensity of 2 × 10 11 polarized protons.
In the AGS two partial Siberian snakes are installed, an iron-based helical dipole
that rotates the spin around the longitudinal direction by 11 ◦ and a superconducting
helical dipole that can reach a 3 T field and a spin rotation of up to 45 ◦ . A view down
the magnet gap is shown in Fig. 10.7. With the two partial snakes placed with one
third of the AGS ring between them all vertical spin resonances are avoided up to
the required RHIC transfer energy of 23.8 GeV as long as the vertical betatron tune
is placed at 8.98, very close to an integer [76]. With an 80% polarization from the
source 65% polarization was reached at AGS extraction. The remaining polarization
loss in the AGS comes from weak spin resonances driven by the horizontal motion
of the beam [77]. It is planned to overcome them by quickly shifting the betatron
tune during resonance crossing.