APEX: Accelerator Physics Experiments at RHIC

by Fulvia Pilat

It is late at night on a Tuesday or early at sunrise on a Wednesday and the RHIC Main Control Room (MCR) is getting ready for an APEX shift: instead of the usual quiet work of the evening or owl operation crews who efficiently attend to the need of the accelerators and the RHIC physics experiments, teams of accelerator physicists start to show up. Accelerator physicists, supported by machine and system specialists as well as 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, but also supported an organized accelerator physics experiments program.

Some of the members of the APEX team in MCR

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 in general. The program provides also an opportunity to train and strengthen the experimental skills of accelerator physics post-docs, graduate students, and accelerator personnel. Beam experiments entail a longer time scale than the day-to-day optimization of machine performance while the typical turnover time for a technique successfully demonstrated in AP experiments to become fully operational is of the order of one year. The accelerator physics program for the upcoming run is formulated and discussed at a regular summer workshop and beam experiment proposals are submitted in the fall and reviewed and prioritized by an internal committee before the start of the RHIC run. Twelve hours per week for beam experiments are scheduled during the physics running time, with enough flexibility to minimize the impact on the overall program.

The accelerator physics topics that have so far received the most attention and effort are those aimed at overcoming fundamental RHIC performance limitations: intra-beam scattering and electron cloud induced pressure rise during ion operations, beam-beam effects and polarization during polarized protons operations. Many of the RHIC performance issues are common to other high-energy colliders. Collaboration with other institutions and laboratories, in particular with teams from FNAL and CERN, is therefore natural and a strong component of the RHIC beam experiments program.

This year with gold operations we have been focusing on operational corrections and studies aimed at reducing intra-beam scattering, the main limitation to RHIC performance with heavy ions. Operational corrections included the commissioning of the nonlinear correction system: additional independently powered sextupole circuits allowed to reduce the second order dependence of tunes on momentum deviations by at least a factor 5. The nonlinear correction is now used in regular operations. Another issue we are presently actively working on is the compensation at injection of orbit, tunes and chromaticity drifts. The persistent current effect in super-conducting magnets causes the magnet field to vary exponentially on the injection plateau after a down ramp. The drift becomes negligible after 20-25 minutes at injection, but since we are trying to reduce the turn-around time between stores, every minute counts. This compensation allows having the rings ready to fill and ramp immediately after the ramp down. We hope to be able to implement this correction in operation before the end of Run-7.

Measured tunes and chromaticity drifts at injection due to persistent currents in the SC magnets

Compensation of tune drifts in the first 1/2 hour at injection after ramp-down by varying the current in the main quadrupoles

We are also working on assessing the feasibility of a different machine lattice with higher phase advance and tighter dispersion that holds the promise of reducing the effect of intra-beam scattering. If the ramp with this lattice can be developed, it is a strong candidate for future ion operations. Although the beam-beam effect is not the leading limitation for ion operations, it is the leading one for protons. A system of wires parallel to the beams has been installed in one of the interaction regions, as a novel way to compensate the long-range beam-beam force. This is relevant to protons but testing and preparation works toward this correction has been done with gold, with the goal of gathering data and comparing the measurements with models being developed in preparation for proton operations at RHIC and at the LHC. Beam experiments work at RHIC in the next few years will be focused 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 (eRHIC).