RHIC Run 9 So Far
By Eduard Pozdeyev, CAD
This operational year (also known as "RHIC Run 9") began with a 250 GeV x 250 GeV (500 GeV) polarized proton run. It was the first year of physics running with polarized protons colliding with 500 GeV total energy. Steven Vigdor, Associate Laboratory Director for Nuclear and Particle Physics, outlined the scope of this milestone run (BNL Bulletin, “2009 RHIC Run”, Vol.63 – No.9): “Physicists are interested in this higher energy because they can make measurements of polarized proton spin over an extended range of momentum of the quarks and gluons inside the proton. In particular, RIKEN physicists are interested in it because it will help them to meet a performance milestone they set for 2011 — the first measurements of W boson production in polarized proton collisions.”
The cooldown of RHIC magnets began on Feb. 2nd, with beam injected in RHIC five days later on Feb. 7th. In a record time, only ten days later, first collisions were registered at 500 GeV. Finally, stable overnight beam stores for calibrating the STAR and PHENIX detectors started on February 24. While progress was hindered temporarily by a number of hardware failures, physics running was ultimately declared on March 17th. At this point the machine went from development to production mode, with most of the beam time spent providing collisions at the detectors. Both detectors took data until the very last moment of the 500 GeV run on April 13. STAR and PHENIX physicists think that they should have accumulated a few hundreds of W decays to electrons, but there is still much work ahead to calibrate all the detectors and analyze the data offline to make sure that the detectors were working as planned, and that there are no errors in the data analysis.
With the 500 GeV run, we also had our first peek into the future. A number of questions important for future high energy runs with polarized protons had to be answered: How do we preserve beam polarization during ramps of each beam to 250 GeV? Will the detectors be able to operate efficiently and safely at higher energies and luminosities? A significant effort, spearheaded by the 500 GeV run coordinator Mei Bai, was undertaken by the Collider-Accelerator Department (CAD) to understand the spin dynamics at higher energies. An average polarization of 40% and above was achieved for the 250 GeV beams. To gain understanding of detector backgrounds, and develop safe operational procedures, extensive studies were conducted by STAR and PHENIX.
After the 500 GeV run, the RHIC energy was lowered for a 100GeV x 100GeV (200 GeV) polarized proton run. The 200 GeV run began on April 15 after budget issues were favorably resolved in early April and we obtained sufficient funds to extend the operations through June. The transition from 500 GeV to 200 GeV was smooth and quick, with the first overnight store in RHIC on April 17. After a week of machine development, needed to setup reliable acceleration ramps and stores, RHIC was declared in the physics mode once again on April 21st.
The physics program for the 200 GeV run will require an integrated delivered luminosity of 100 pb-1 with a polarization of 60%. The so-called "figure of merit", a parameter which characterizes the effectiveness of the physics data, depends on the beam polarization to the fourth power: LP4, where L is luminosity and P is polarization. Therefore, sustaining a high degree of polarization (~60%) through the whole run is essential. In addition to the high degree of polarization, a high luminosity (4x1031 cm-2s-1) is required. Currently, the machine is operated with an average polarization of 55% at store. Hard work is ongoing to decrease polarization losses in AGS and RHIC and reach 60% at store. In parallel, an effort to increase the average luminosity is also underway. In theory, increasing the beam intensity, and reducing the beam dimensions, increases the luminosity. However, in practice, achieving both at the same time is difficult because high intensity beams tend to have larger sizes. In turn, a larger beam size can negatively affect polarization. Thus, a very accurate tuning of the whole chain of our accelerators, optimizing a number of beam parameters simultaneously, is needed. Christoph Montag, who is the run coordinator for the 200 GeV run, leads this work, which is going well.
In parallel to running the physics program, CAD is working on upgrades and improvements, vital for future successful operation of the machine. One example is a "spin flipper," consisting of four DC dipole magnets and two AC tunable-frequency magnets, designed to flip the spin of polarized protons. Multiple spin flips by 180° during physics stores can significantly reduce systematic errors, e.g. those caused by a dependence of polarization on beam intensity. The first spin flipper is already installed in RHIC and being commissioned. Another major system to be commissioned this year is a new digital system designed to control the RHIC accelerating radio-frequency cavities and to synchronize CAD accelerators. This system is more accurate, more flexible, and easier to maintain than the old analog system. A substantial portion of the digital system was successfully tested between the 500 GeV and 200 GeV runs in mid April, and we plan to fully commission the new system in June. Also, there is a plan to commission the AGS "tune jump" quadrupoles this June. These pulsed quadrupoles reduce the amount of time the beam spends at depolarizing resonances in AGS. As a result of this commissioning, we expect an increase of polarization by up to 5% in RHIC. While this increase seems small, it will significantly boost the figure of merit (since it depends on the polarization raised to the fourth power).
With these and other upgrades, we look forward to continued efforts alongside the RHIC experiments to further optimize the overall performance of the RHIC program.