It is the owl shift on a Wednesday in the Relativistic Heavy Ion Collider (RHIC)/AGS Main Control Room (MCR). Instead of the usual quiet work of the night operation crew, who efficiently attend to the needs of the accelerators and the RHIC physics experiments, the MCR is bursting with activity. It is an accelerator-physics experiments (APEX) night: teams of physicists, supported by machine specialists and operators collaborate on testing the latest and newest beam techniques proposed for RHIC. Ever since its first years of operation, RHIC has not only fulfilled its main purpose of delivering luminosity to its physics experiments (PHENIX, STAR, BRAHMS and PHOBOS), but has also supported an organized beam-experiments programme.
The goals of the machine experiments are to explore new and advanced beam techniques, to foster machine performance, including development and testing of new diagnostics, and to widen the experimental understanding of accelerator physics. Beam experiments entail a longer time scale than the day-to-day optimization of machine performance: it typically takes a year for a technique successfully demonstrated in beam experiments to become fully operational.
The accelerator-physics programme for an upcoming run at RHIC is formulated and discussed at a regular summer workshop, and beam-experiment proposals are submitted in the autumn and reviewed and prioritized by an internal committee before the start of the run, typically in winter. Twelve hours a week for beam experiments are scheduled during the physics running time, with enough flexibility to minimize the impact on the overall programme.
The accelerator-physics topics that have so far received the most attention and effort are those aimed at overcoming limitations on the performance of RHIC, namely intra-beam scattering and the electron-cloud-induced pressure rise that occur during ion operations, and beam-beam effects and polarization during operations with polarized protons. Many of these performance issues at RHIC are common to other high-energy colliders.
Collaboration with other institutions and laboratories is a natural and strong component of the RHIC beam-experiments programme, and accelerator physicists from other laboratories actively participate in experiments of mutual interest. Colleagues from CERN in particular have actively contributed to non-linear studies and the development of correction techniques that have eventually led to the reduction of the beam size at the experimental interaction point, resulting in increased luminosity. The development of new instrumentation and diagnostic techniques is another example of collaboration with CERN. These include the feasibility test of tune feedback during acceleration and the precise observation of electron-cloud formation - issues that are very relevant to the future performance of the Large Hadron Collider.
Future beam-experiments work at RHIC will focus on supporting and preparing for the planned RHIC upgrades: a ten-fold increase in luminosity in RHIC heavy-ion collisions, made possible by a full-energy electron-cooling system planned for early in the next decade, and the eventual addition of an electron beam for electron-ion collisions. Near-term activities to support future projects include the development of a new magnet lattice with higher focusing to reduce the effect of intra-beam scattering, and tests of how the machine operates with an increased number of bunches.
With Run 6 scheduled to start at RHIC in February 2006, after a couple of weeks of machine set-up and performance optimization, we are looking forward not only to delivering polarized-proton collisions to our physics experiments, but also to APEX with polarized proton beams.