RHIC Run 10: The Push to Ultra-High 200 GeV Au Au Luminosity

By Kevin Brown, RHIC Run’10 Coordinator

What is the highest luminosity we can achieve for gold on gold collisions in RHIC? This is the question we accelerator physicists have been asking for many years and will continue to ask into the future. To answer this question we have to understand what the luminosity limitations are in RHIC. This has been studied in beam experiments and in simulations. The results of these experiments and simulations are used to modify and make improvements to the configuration of the accelerators.

In this year’s RHIC run, we will be commissioning a number of new ideas that will lead to the highest luminosities ever achieved in gold on gold collisions. These ideas push the limits of our understanding in accelerator physics and technology. The most significant improvement involves the implementation of bunched beam stochastic cooling, both longitudinal and transverse. Other improvements involve changes to the optics of the accelerators to reduce intra-beam scattering (IBS) and achieve smaller beams at the collision points. All of these improvements attempt to make the beams smaller and pack as many particles as we can into each collision.

Stochastic cooling isn’t a new idea. The technique was first developed at CERN (earning S. van der Meer a Nobel prize) and used at Fermilab to cool antiproton beams. But nobody has ever been successful using stochastic cooling to cool bunched beams at high energies in a collider, until recently, when the technique was developed at RHIC. This year we will be implementing longitudinal cooling in both RHIC rings and transverse cooling in the vertical plane of both rings.

The luminosity improvement from longitudinal stochastic cooling is about 20%, but the improvement from transverse cooling is potentially much greater, as much as a factor of 4. But optics modifications that allow for a smaller beam at the collision points (this optics parameter is known to us as the beta-star) can improve the luminosity by about 15%. Overall, in Run 10, we are looking to achieve about a factor of 2 increase in average store luminosity beyond the maximum average achieved in the last gold on gold run in FY’07.

There are a few other improvements that have been made which at first may not seem to offer any increase in luminosity. Although the luminosity that is achieved while beams are in collisions is important, so is the reliability and reproducibility of the accelerators, improving the amount of time beams are in collision.

Three fairly major improvements were made prior to this run. The most significant, in terms of accelerator operations, is new acceleration system controls in RHIC – something we call the low level RF – which is being commissioned this run. Another modification involves changes made to allow helium gas to be vented more safely in certain areas. This reduces the number of areas that have to be classified as ODH1, easing the safety requirements to be in those areas. Finally a third modification was in the regulation of the RHIC main magnet power supply systems, improving reproducibility. Of course, the staff at the Collider Accelerator Department has been very busy prior to this run making improvements to instrumentation, cryogenics, power supply reliability, and controls.

The process of turning on for Run 10 began in November. The final cool-down of RHIC to 4.5 K began on December 1. Everyone at C-AD has been working all through December to get systems turned on and checked out, and preparing the beams so that STAR and PHENIX can begin collecting events. Over Christmas weekend we put beams into collisions for the first time. Over New Years weekend we began providing PHENIX and STAR with collisions of sufficient quality for doing physics. We are all looking forward to the next couple of months; with record breaking luminosities and new and interesting results coming out of RHIC.

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