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RHIC Serves as World’s First & Only Collider
of Polarized Protons for ‘Spin’ Physics

Accelerating & Colliding High-Energy Polarized Protons at RHIC

Novel Accelerator Hardware

These world-record accomplishments were made possible by partnering with RIKEN, which is the Japanese Institute of Physical and Chemical Research and the advanced development and deployment of five novel types of accelerator hardware:

Anatoly Zelenski is pictured with the optically pumped, polarized-ion source that he designed and built for spin physics at RHIC.

OPPIS: is an optically pumped, polarized-ion source that produces a direct current beam of 1-milliampere negative hydrogen ions (H-) with 75 percent polarization in 200 microsecond pulses at a repetition rate of 7.5 hertz (Hz). This corresponds to a polarized proton beam of 4 x 1011 after the H- beam is accelerated through a radio-frequency quadrupole and the 200 million-electron-volt linear accelerator, and strip-injected into the 1.5 GeV booster. After acceleration within the booster, the beam is then transferred to RHIC’s injector, the Alternating Gradient Synchrotron (AGS).

Partial & full Siberian snakes: are chains of vertical and horizontal dipole magnets or a single solenoid magnet, originally invented in Novosibirsk in the late 1970s. They are used to overcome spin-misaligning magnetic effects, called depolarizing spin resonances, within circular accelerators and colliders, such as, respectively, the AGS, and RHIC itself. With a corkscrew-like design that causes the polarized particles to have a snake-like trajectory, Siberian snakes work by rotating the beam’s spin direction, thus keeping the spin precession frequency constant.

Accelerating polarized protons since 1984, the AGS now employs a partial Siberian snake, which is a 2-tesla, 2.3-meter-long solenoid non-superconducting, or warm, magnet module located in the I10 straight section. This 5-percent snake rotates the spin by 9 degrees, which, in conjunction with a spin flipper, is sufficient to avoid most beam depolarization up to the required RHIC transfer energy of approximately 25 GeV.

Not having the space limitations of the AGS, RHIC utilizes two pairs of full Siberian snakes, one pair for each of its two rings, with one snake located at 3 o’clock, the other at 9 o’clock in each ring. Each 100-percent Siberian snake consists of four 4-tesla, 2.4-meter long superconducting, 360-degree-twist helical dipole magnet modules able to rotate the spin by 180 degrees to counteract depolarizing resonances.

Spin flippers: are radio frequency ac dipole magnets used to produce oscillating magnetic fields, which not only work to minimize depolarizing resonances in conjunction with the partial Siberian snake in the AGS, but also allow the polarization direction to be reversed during a store in RHIC. To cause a full, 180-degree flip of the spin, these dipoles produce a strong, artificial spin resonance excited coherently for the entire beam. The AGS spin flipper is a 100-kHz, 20-Gauss-meter warm vertical dipole. Located at IP4 and common to both beams, RHIC’s spin flipper is a 40-kHz, 100-Gauss-meter dipole.

p-C CNI polarimeter: employs proton-carbon elastic scattering in the Coulomb nuclear scattering region (p-C CNI) to measure beam polarization. With one located at C15 in the AGS and one within RHIC’s IP12 region, each p-C CNI polarimeter employs an ultra-thin carbon ribbon as the target for p-carbon elastic scattering. To identify the recoil carbon ions, which arrive out of time with the prompt background from the target, silicon strip detector measure simultaneously the particles’ energy and time of flight. With minimal beam perturbation, the p-C CNI polarimeters work accurately, with 5 percent relative error, and quickly, within 1 minute in RHIC and 5 minutes in the AGS.

Reaching Design Polarization

For the next run in 2004, the intermediate goal is to increase beam polarization to 50 percent at 100 GeV. In 2005-06, the polarization is expected to reach the design goal of 2 x 1011 protons per bunch with 70 percent at 250 GeV. To reach the intermediate and design goals, polarization losses occurring within the AGS will be overcome by replacing the existing partial solenoidal Siberian snake with helical versions that can avoid more of the depolarizing spin resonances.

The first substitution will be a 5-percent, or 9-degree, 1.5-tesla warm helical magnet assembly, which is now being built in Japan by RIKEN and is expected to be delivered this fall. The helical magnet will avoid the coupling depolarizing resonances that the present solenoid snake produces. The ultimate partial snake for the AGS will be a superconducting helix that is five times stronger. The 25-percent, or 45-degree, 3-tesla superconducting helix is now being constructed within Brookhaven’s magnet division, with commissioning scheduled for 2005.

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