10 The Largest Accelerators and Colliders of Their Time
603
provides an extremely bright gold beam that can be captured and bunch-merged
to provide the necessary bright bunches of 1 × 10 9 Au ions with a normalized
transverse rms emittance of less than 2.5 μm and a total longitudinal emittance of
less than 0.3 eVs/nucleon. The final stripping to bare Au 79+ occurs on the way
to RHIC. Recently the Tandem van de Graff pre-injector has been replaced by an
Electron Beam Ion Source (EBIS) followed by an RFQ and IH-Linac. The EBIS
can produce very bright high-charge state heavy ion beams by accumulating and
stripping heavy ions with a 10 A space-charge neutralizing electron beam. The new
source can produce beams of many species including uranium.
The two RHIC rings, labeled blue and yellow, are intersecting at six interaction
regions (IRs). All IRs can operate at a betastar between 2 and 10 m. In two
interaction regions, occupied by the two main detectors STAR and PHENIX,
the quality of the triplet quadrupoles allows further reduction of betastar to less
than 1 m. Typically betastar is about 10 m at injection energy for all IRs and is
then squeezed during the acceleration ramp first to 5 m at the transition energy
(γ T = 26.7), which minimizes the momentum dependence of the transition energy
crossing, and then to less than 1 m for PHENIX and STAR. A typical acceleration
cycle consists of filling the blue ring with 111 bunches in groups of 4 bunches, filling
the yellow ring in the same way and then simultaneous acceleration of both beams
to storage energy. During acceleration the beam bunches are longitudinally aligned
but are separated vertically by 10 mm in the interaction regions to avoid beam losses
from beam-beam interaction.
The collision rate is measured using identical Zero Degree Calorimeters (ZDC)
at all detectors. The ZDC counters detect at least one neutron on each side from
mutual Coulomb and nuclear dissociation with a total cross section of about 10
barns. Typical stores in RHIC last about 4–5 h. Due to intra-beam scattering, which
is particularly important for the fully stripped, highly charged gold beams, the initial
normalized rms emittance of about 2.5 μm grows to about 5 μm at the end of the
store and a significant amount of beam is lost from the RF bucket. The resulting
luminosity lifetime is about 2.5 h.
The reduced number of gold ions compared to typical proton bunches makes it
possible to contemplate stochastic cooling of the 100 GeV/nucleon bunched beam.
A 6–9 GHz longitudinal and a 5–8 GHz vertical stochastic cooling system was
installed using novel high power multi-cavity kickers and the high energy bunched
beam was successfully cooled [71]. Figure 10.4 shows the successful cooling of the
transverse emittance in the yellow ring compared to the emittance growth in the
blue ring without cooling. Both the vertical and horizontal emittance are reduced
due to x-y coupling. Figure 10.5 shows the vertex distribution at the end of the
store with and without stochastic cooling. It clearly shows a significant increase in
luminosity in the central region. The modulation of the vertex distribution is a result
of the 200 MHz storage RF system that provides about 4 MV/turn to compress
the bunch length and consequently reduce the length of the vertex distribution
to better match the acceptance of the collider detectors. Again due to intra-beam
scattering the initially narrow distribution widens during the store. A new 2 MV,
56 MHz superconducting cavity will provide improved RF focusing that, together
603
provides an extremely bright gold beam that can be captured and bunch-merged
to provide the necessary bright bunches of 1 × 10 9 Au ions with a normalized
transverse rms emittance of less than 2.5 μm and a total longitudinal emittance of
less than 0.3 eVs/nucleon. The final stripping to bare Au 79+ occurs on the way
to RHIC. Recently the Tandem van de Graff pre-injector has been replaced by an
Electron Beam Ion Source (EBIS) followed by an RFQ and IH-Linac. The EBIS
can produce very bright high-charge state heavy ion beams by accumulating and
stripping heavy ions with a 10 A space-charge neutralizing electron beam. The new
source can produce beams of many species including uranium.
The two RHIC rings, labeled blue and yellow, are intersecting at six interaction
regions (IRs). All IRs can operate at a betastar between 2 and 10 m. In two
interaction regions, occupied by the two main detectors STAR and PHENIX,
the quality of the triplet quadrupoles allows further reduction of betastar to less
than 1 m. Typically betastar is about 10 m at injection energy for all IRs and is
then squeezed during the acceleration ramp first to 5 m at the transition energy
(γ T = 26.7), which minimizes the momentum dependence of the transition energy
crossing, and then to less than 1 m for PHENIX and STAR. A typical acceleration
cycle consists of filling the blue ring with 111 bunches in groups of 4 bunches, filling
the yellow ring in the same way and then simultaneous acceleration of both beams
to storage energy. During acceleration the beam bunches are longitudinally aligned
but are separated vertically by 10 mm in the interaction regions to avoid beam losses
from beam-beam interaction.
The collision rate is measured using identical Zero Degree Calorimeters (ZDC)
at all detectors. The ZDC counters detect at least one neutron on each side from
mutual Coulomb and nuclear dissociation with a total cross section of about 10
barns. Typical stores in RHIC last about 4–5 h. Due to intra-beam scattering, which
is particularly important for the fully stripped, highly charged gold beams, the initial
normalized rms emittance of about 2.5 μm grows to about 5 μm at the end of the
store and a significant amount of beam is lost from the RF bucket. The resulting
luminosity lifetime is about 2.5 h.
The reduced number of gold ions compared to typical proton bunches makes it
possible to contemplate stochastic cooling of the 100 GeV/nucleon bunched beam.
A 6–9 GHz longitudinal and a 5–8 GHz vertical stochastic cooling system was
installed using novel high power multi-cavity kickers and the high energy bunched
beam was successfully cooled [71]. Figure 10.4 shows the successful cooling of the
transverse emittance in the yellow ring compared to the emittance growth in the
blue ring without cooling. Both the vertical and horizontal emittance are reduced
due to x-y coupling. Figure 10.5 shows the vertex distribution at the end of the
store with and without stochastic cooling. It clearly shows a significant increase in
luminosity in the central region. The modulation of the vertex distribution is a result
of the 200 MHz storage RF system that provides about 4 MV/turn to compress
the bunch length and consequently reduce the length of the vertex distribution
to better match the acceptance of the collider detectors. Again due to intra-beam
scattering the initially narrow distribution widens during the store. A new 2 MV,
56 MHz superconducting cavity will provide improved RF focusing that, together