The Accelerator
RHIC is an intersecting storage ring particle accelerator. Two independent rings (arbitrarily denoted as "Blue" and "Yellow" rings, see also the photograph) circulate heavy ions and/or protons in opposite directions and allow a virtually free choice of colliding positively charged particles (the eRHIC upgrade will allow collisions between positively and negatively charged particles). The RHIC double storage ring is itself hexagonally shaped and 3,834 m long in circumference, with curved edges in which stored particles are deflected and focused by 1,740 superconducting niobium-titanium magnets. The dipole magnets operate at 3.45 T. The six interaction points (between the particles circulating in the two rings) are at 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 PHENIX, are located at 6 and 8 o'clock respectively.
A particle passes through several stages of boosters before it reaches the RHIC storage ring. The first stage for ions is the Electron Beam Ion Source (EBIS), while for protons, the 200 MeV linear accelerator (Linac) is used. As an example, gold nuclei leaving the EBIS have a kinetic energy of 2 MeV per nucleon and have an electric charge Q = +32 (32 of 79 electrons stripped from the gold atom). The particles are then accelerated by the Booster Synchrotron to 100 MeV per nucleon, which injects the projectile now with Q = +77 into the Alternating Gradient Synchrotron (AGS), before they finally reach 8.86 GeV per nucleon and are injected in a Q = +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 p + p, d + Au, Cu + Cu, Cu + Au, Au + Au and U + U. The projectiles typically travel at a speed of 99.995% of the speed of light. For Au + Au collisions, the center-of-mass energy is typically 200 GeV per nucleon-pair, and was as low as 7.7 GeV per nucleon-pair. An average luminosity of 2×1026 cm−2s−1 was targeted during the planning. The current average luminosity of the collider is 30×1026 cm−2s−1, 15 times the design value. The heavy ion luminosity is increased by a factor of 2 through stochastic cooling.
One unique characteristic of RHIC is its capability to collide polarized protons. RHIC holds the record of highest energy polarized protons. Polarized protons are injected into RHIC and preserve this state throughout the energy ramp. This is a difficult task that can only be accomplished with the aid of Siberian snakes (in RHIC a chain 4 helical dipole magnets). Run-9 achieved center-of-mass energy of 500 GeV on 12 February 2009. In Run-12 the average p + p luminosity of the collider reached 105×1030 cm−2s−1, with a time and intensity averaged polarization of 52%.
The AC dipoles have been also used in non-linear machine diagnostics for the first time in RHIC.
Read more about this topic: Relativistic Heavy Ion Collider