About the Author

Wolfram Fischer is the Deputy Head of the Accelerator Division within Brookhaven's Collider-Accelerator Department, and the Accelerator Physics Leader in the U.S. LHC Accelerator Research Program.

U.S. LARP at RHIC

By Wolfram Fischer

You may have wondered what the label “LARP” on RHIC’s integrated luminosity chart means. LARP stands for “LHC Accelerator Research Program” and the luminosity delivered at IP10 was to test a luminosity monitor for the LHC. The monitor was built at LBNL, tested at BNL, and will be shipped to CERN. Within the Collider-Accelerator Department, A. Drees was responsible for the test.

Figure 1. Delivered integrated luminosity in Run-7. STAR and PHENIX are the large RHIC experiments, MONOp is a test apparatus for monopole search, and LARP an LHC luminosity monitor test.

Four US laboratories are involved in LARP: BNL, FNAL, LBNL and SLAC. These laboratories work with CERN to make more LHC luminosity earlier, collaborate in an LHC interaction region upgrade, and develop US accelerator capabilities in the process. About half of the LARP resources go into the development of advanced magnets; the other half is for accelerator systems. Brookhaven’s Superconducting Magnet Division is also part LARP’s magnet effort.

In the Collider-Accelerator Department our involvement in LARP concentrates on efforts that are beneficial to both RHIC and the LHC. Both machines are hadron colliders, operating with protons and heavy ions, and many beam dynamics problems are the same. In some cases RHIC can be a test bed for the LHC. A number of people are working part of their time for LARP. Besides the luminosity monitor test, a few more examples may illustrate our work in LARP.

Tune, coupling and chromaticity control is important in all accelerators. In RHIC, polarized proton operation is especially demanding, in the LHC it is the need to keep beam losses at an absolute minimum. Over the last few years P. Cameron and collaborators have developed a tune and coupling feedback at RHIC, which is now tested at CERN’s SPS. It is planned to use this feedback in the LHC. Chromaticity feedback is still under study.

The beam-beam interaction is one of the most restrictive luminosity limits in colliders. RHIC’s beam-beam interactions are dominated by head-on collisions, where the particles in one beam see the electro-magnetic field of the other beam when 2 bunches collide. Both RHIC and LARP are now investigating a compensation scheme using an electron lens, an electron beam that would create a force of opposite sign of that created by a proton or hadron beam. Y. Luo, G. Robert-Demolaize, N. Abreu and the author are working on this problem.

The LHC beam-beam interaction will be dominated by so-called long-range interactions, where the beams pass each other with some separation but still experience the electro-magnetic field of the other beam. In RHIC this effect is relevant during the energy ramp, and in certain upgrade scenarios. For the LHC a compensation of the long-range beam-beam effect has been proposed using a wire parallel to the beam. The field of the wire, not too far away from the long-range interaction, would be of opposite sign to the field created by the other beam. To study the long-range beam-beam interactions in an operating collider, and to test a long-range compensation, 2 wires were installed in each of the RHIC rings, and experiments were done in the previous run.

Since the LHC design beam has 350 MJ of stored energy, even small beam losses can create severe problems. This requires a very efficient collimation system, and makes the predictability of loss locations extremely valuable. Simulation programs developed at CERN are currently benchmarked with actual RHIC loss data by G. Robert-Demolaize.

An A/C dipole has been operated in RHIC for a few years. With such a device, coherent dipole oscillation can be executed without increasing the beam size, which makes it an excellent diagnostic device to measure the basic machine properties. M. Bai, who is responsible for this device at RHIC, now works with colleagues at Fermilab and CERN towards an A/C dipole in the LHC.

Electron clouds are formed in all machines that operate with high-intensity hadron beams. In RHIC electron clouds caused dynamic pressure rises and instabilities at transition. In the LHC the design bunch spacing is only 25 ns (compared to 108 ns in RHIC), with which electron cloud effects can be expected. RHIC is one of the first operating machines with a large-scale installation of NEG coated beam pipes, one of the main counter measures against electron cloud effects. The NEG (=non-evaporable getter) technology for beam pipes had been developed at CERN for the LHC. RHIC has also installed an electron detector from CERN to investigate electron cloud effects.