First Beam in Run 8
By Fulvia Pilat
Together with the glorious foliage and the kids happily back in school, autumn this year, brought back another cycle of RHIC operations, as it has been possible to start Run-8 even in the presence of a budget continuing resolution. This is extremely beneficial to the effective running of operations in that it avoids last year conflicting needs of progressively delaying the schedule while keeping the accelerators ready to go.
The essential ‘ingredients’ of the RHIC startup are: the systematic checkout of systems and software prior to beam, a final system readiness integration test (“dry-run”), the cryogenics cool-down of the rings to the operating temperature of 4.5K, the concurrent set-up of the injectors (this run with the added challenge to prepare both deuteron and gold beams) and finally the RHIC setup with beam. The script of RHIC beam set-up has been developed in over 8 years of operational experience: after set-up at injection energy for systems and instrumentation, we enter the ramp development phase. When the ramp transmission for both rings is ~85-90% we set up collisions for the experiments to start tuning the detectors overnight. Luminosity development then starts with the progressive increase of bunch intensity and number of bunches until it is adequate to start the around-the-clock running for physics.
While the overall organization of the startup is by now well-established, new developments and improvements are introduced every run to increase overall accelerator performance. Run-8 is not an exception: we are running this year with a new lattice with higher phase advance in the arcs for the yellow ring – the so called IBS-suppression lattice: this was developed and tested last run in the framework of the APEX program and demonstrated its effectiveness in counteracting intra-beam scattering, the main mechanism for emittance growth with heavy ions. For the blue ring, this year hosting the deuteron beam, the lattice is very similar to what we had in Run-7 since IBS is negligible for deuterons. For the yellow ring we also count on an improved stochastic cooling system to contain the longitudinal beam distribution. With respect to the beam parameters in Run-3 (our previous deuteron-gold run) we can expect higher performance and luminosity from higher bunch intensity in gold, and an increase in the number of bunches from 56 up to 111. With a total of 532m of NEG pipes installed since Run-3 we expect that the machine will not be limited by dynamic pressure rise and will then be able to accommodate the higher beam intensities. Moreover we are running with stronger focusing at the interaction points, with a β of 1m, half the value of Run-3, capitalizing on the successful experience of running with lower β* in ion operations during Run-5 and Run-7.
While large gains in the startup time are not likely to be found after a number of years of operation, improvements are still possible. One example has been this year's performance of the cryogenics system in speeding up the ring cool down. The RHIC cool-down has in fact been accomplished in record time: first of all the rings were kept at 80K throughout the summer shut-down, and pre-cooling the beam to 45K the week before November 1, the planned start of the liquid helium cool down, made the latter extremely fast. In an unprecedented 6 days both rings have been brought from 45K to the operating temperature of 4.5 K.
The RHIC set-up with beam is also close to optimal, however we could still possibly gain some time (of the order of 1-2 days) by taking advantage earlier of tune and coupling feedback during ramp development. Another obviously large factor determining the length of he startup is system failure and associated machine uptime. Improving machine availability, affecting not only the startup but also the overall delivered integrated luminosity for the run, has been one of the main topics discussed at the RHIC Retreat this summer and one of the goals for Run-8.
Work with beam in the injectors started on October 22, the cool down started on November 1. At writing time we are well into the phase of RHIC set-up with beams, with most of the ramping development being done and collision with 56 bunches established on November 18. We are ready to provide collisions for the set-up of the experiments over night and the rate of progress is consistent with going into physics on December 1st, as planned, or earlier. We are all looking forward to another year of fruitful operations at RHIC!