Editing Relativistic Heavy Ion Collider (section)

==The accelerator==
RHIC is an intersecting [[storage ring]] [[particle accelerator]]. Two independent rings (arbitrarily denoted as "Blue" and "Yellow") circulate heavy [[ion]]s and/or polarized [[proton]]s in opposite directions and allow a virtually free choice of colliding positively [[charged particle]]s (the [[#The future|eRHIC]] upgrade will allow collisions between positively and negatively charged particles). The RHIC double storage ring is [[hexagon]]ally shaped and has a circumference of {{val|3834|u=m}}, with curved edges in which stored particles are deflected and focused by 1,740 [[superconducting magnet]]s using [[niobium-titanium]] conductors. The [[dipole magnet]]s operate at {{val|3.45|ul=T}}.<ref>
{{cite web
 |author      = P. Wanderer
 |date        = 22 February 2008
 |title       = RHIC Project
 |url         = https://www.bnl.gov/magnets/RHIC-project.php
 |publisher   = [[Brookhaven National Laboratory]], Superconducting Magnet Division
 |access-date  = 2021-03-21
}}</ref> The six interaction points (between the particles circulating in the two rings) are in the middle of the six relatively straight sections, where the two rings cross, allowing the particles to collide. The interaction points are enumerated by clock positions, with the injection near 6 o'clock. Two large experiments, STAR and sPHENIX, are located at 6 and 8 o'clock respectively. The sPHENIX experiment is the newest experiment to be built at RHIC, replacing PHENIX at the 8 o'clock position.<ref>
{{cite web
 |title=RHIC Accelerators
 |url=http://www.bnl.gov/RHIC/complex.asp
 |publisher=[[Brookhaven National Laboratory]]
 |access-date=2010-02-16
}}</ref>

A particle passes through several stages of [[wikt:booster|booster]]s before it reaches the RHIC storage ring. The first stage for ions is the [[electron beam ion source]] (EBIS), while for protons, the {{val|200|ul=MeV}} [[linear accelerator]] (Linac) is used. As an example, gold nuclei leaving the EBIS have a [[kinetic energy]] of {{val|2|u=MeV}} per nucleon and have an electric charge ''Q''&nbsp;=&nbsp;+32 (32 of 79 electrons stripped from the gold atom). The particles are then accelerated by the Booster [[synchrotron]] to {{val|100|u=MeV}} per nucleon, which injects the projectile now with ''Q''&nbsp;=&nbsp;+77 into the [[Alternating Gradient Synchrotron]] (AGS), before they finally reach {{val|8.86|u=GeV}} per nucleon and are injected in a ''Q''&nbsp;=&nbsp;+79 state (no electrons left) into the RHIC storage ring over the AGS-to-RHIC Transfer Line (AtR).

To date the types of particle combinations explored at RHIC are {{nowrap|[[proton|p]] + [[proton|p]]}}, 
{{nowrap|[[proton|p]] + [[aluminum|Al]]}},  {{nowrap|[[proton|p]] + [[gold|Au]]}}, {{nowrap|[[deuteron|d]] + [[gold|Au]]}}, {{nowrap|[[helion (chemistry)|h]] + [[gold|Au]]}}, {{nowrap|[[copper|Cu]] + [[copper|Cu]]}}, {{nowrap|[[copper|Cu]] + [[gold|Au]]}}, {{nowrap|[[zirconium|Zr]] + [[zirconium|Zr]]}}, {{nowrap|[[ruthenium|Ru]] + [[ruthenium|Ru]]}}, {{nowrap|[[gold|Au]] + [[gold|Au]]}} and {{nowrap|[[uranium|U]] + [[uranium|U]]}}. The projectiles typically travel at a speed of 99.995% of the [[speed of light]]. For {{nowrap|Au + Au}} collisions, the [[center of mass|center-of-mass]] energy is typically {{val|200|u=GeV}} per [[nucleon]]-pair, and was as low as {{val|7.7|u=GeV}} per [[nucleon]]-pair. An average [[luminosity (scattering theory)|luminosity]] of {{val|2|e=26|u=cm<sup>−2</sup>⋅s<sup>−1</sup>}} was targeted during the planning. The current average {{nowrap|Au + Au}} luminosity of the collider has reached {{val|87|e=26|u=cm<sup>−2</sup>⋅s<sup>−1</sup>}}, 44 times the design value.<ref name="RHIC Run Overview">
{{cite web
 |title=RHIC Run Overview
 |url=http://www.agsrhichome.bnl.gov/RHIC/Runs/
 |publisher=[[Brookhaven National Laboratory]]
}}</ref> The heavy ion luminosity is substantially increased through [[stochastic cooling]].<ref>
{{cite journal
 |author=M. Blaskiewicz
 |author2=J. M. Brennan
 |author3=K. Mernick
 |year=2010
 |title=Three-Dimensional Stochastic Cooling in the Relativistic Heavy Ion Collider
 |journal=[[Physical Review Letters]]
 |volume=105 |issue=9 |page=094801
 |bibcode=2010PhRvL.105i4801B
 |doi=10.1103/PhysRevLett.105.094801
 |pmid=20868165
}}</ref>

One unique characteristic of RHIC is its capability to collide polarized protons. RHIC holds the record of highest energy polarized proton beams. Polarized protons are injected into RHIC and preserve this state throughout the energy ramp. This is a difficult task that is accomplished with the aid of corkscrew magnetics called 'Siberian snakes' (in RHIC a chain 4 helical [[dipole]] magnets). The corkscrew induces the magnetic field to spiral along the direction of the beam <ref>{{cite journal
 |date=22 March 2002
 |title=Snake charming induces spin-flip
 |url=http://www.cerncourier.com/main/article/42/3/2
 |journal=[[CERN Courier]]
 |volume=42
 |issue=3
 |page=2
 |access-date=13 September 2006
 |archive-date=5 December 2008
 |archive-url=https://web.archive.org/web/20081205081201/http://www.cerncourier.com/main/article/42/3/2
 |url-status=dead
 }}</ref> 
Run-9 achieved center-of-mass energy of {{val|500|u=GeV}} on 12 February 2009.<ref>
{{cite web
 |title=RHIC Run-9
 |url=http://www.agsrhichome.bnl.gov/AP/Spin2009/
 |publisher=[[Brookhaven National Laboratory]]/[[Alternating Gradient Synchrotron]]
 |access-date=2010-02-16
}}</ref> In Run-13 the average {{nowrap|[[proton|p]] + [[proton|p]]}} luminosity of the collider reached {{val|160|e=30|u=cm<sup>−2</sup>⋅s<sup>−1</sup>}}, with a time and intensity averaged polarization of 52%.<ref name="RHIC Run Overview"/>

AC dipoles have been used in non-linear machine diagnostics for the first time in RHIC.<ref>
{{cite journal
 |author=R. Tomás
 |display-authors=etal
 |date=2005
 |title=Measurement of global and local resonance terms
 |journal=[[Physical Review Special Topics: Accelerators and Beams]]
 |volume=8 |issue=2 |page=024001
 |bibcode=2005PhRvS...8b4001T
 |doi=10.1103/PhysRevSTAB.8.024001
|doi-access=free
 }}</ref>

<gallery class="center" widths="200px" caption="Accelerator components">
File:Helium refrigeration system at Relativistic Heavy Ion Collider (RHIC).jpg|The 25&nbsp;MW Helium refrigeration system that cools the superconducting magnets down to the operating temperature of 4.5&nbsp;K<ref>{{cite web|title=Cryogenic Systems Group, Photo Gallery|url=https://www.bnl.gov/cad/cryo/photoGallery.asp|publisher=Brookhaven National Laboratory|access-date=7 August 2017}}</ref>
File:Arc dipole magnet of Relativistic Heavy Ion Collider (RHIC).jpg|An arc dipole magnet. Electrical bus slots (top and bottom) and beam tube (middle) at the top section of the vacuum shell<ref>{{cite web|title=RHIC Project|url=https://www.bnl.gov/magnets/RHIC-project.php|publisher=Brookhaven National Laboratory|access-date=7 August 2017}}</ref>
File:Curvature of beam tube of Relativistic Heavy Ion Collider arc dipole magnet.jpg|Curvature of beam tube seen through the ends of an arc dipole magnet
File:Two main accelerator rings inside the Relativistic Heavy Ion Collider tunnel.jpg|Two main accelerator rings inside the RHIC tunnel
File:STAR Detector at Relativistic Heavy Ion Collider.jpg|STAR detector
File:A Forward Silicon Vertex Detector (FVTX) sensor on a microscope.jpg|A Forward Silicon Vertex Detector (FVTX) sensor of PHENIX detector on a microscope<ref>{{cite journal|last1=Kapustinsky|first1=Jon S|title=Sensors/FPHX Readout Chip WBS 1.4.1/1.4.2|date=17 November 2010|url=https://www.phenix.bnl.gov/WWW/publish/brooks/silicon/reviews/Nov10/talks/Sensors_FPHX_11_10.pdf|access-date=7 August 2017}}</ref>
</gallery>