Starting Up RHIC
By Michael Sivertz
The RHIC Run 08 has started running "Physics", beginning a few day early on 26 November 2007. Although I am the Scheduling Physicist for RHIC this year, I am not an accelerator physicist, so my view of the RHIC Start Up is more from the standpoint of a spectator.
All that really matters to the experiments is the luminosity, or the rate that the particles in the two beams collide. More luminosity is good. The luminosity depends only on a handful of variables; the intensity of the beams, the beam size, and the rate at which they collide. I will describe the long and tortuous path from Summer Shutdown to Luminosity.
The first activity in Start Up is to get the RHIC ring cold. Between runs the ring magnets and beam pipe are kept evacuated and cool, if 77K (-321 degrees Fahrenheit) is your idea of cool. The liquid Helium (LHe) used for final cooling is only rented, and is shipped back to the vendor over the summer, leaving only liquid Nitrogen in the ring. The first truckload of LHe arrived at the end of October, and was pumped into RHIC without liquefaction to bring the ring down to 45K. Cooling with this 45K wave for a week proved to be very efficient, so that on Nov 2 when the 4K wave began and LHe was pumped around the ring, the final cooling went very quickly. The LHe cools the RHIC magnets so that they can function as superconductors, reducing the power consumption. The LHe also provides vacuum pumping as the beam pipe is cooled to the point that essentially all residual gas in the beam pipe is frozen out. Unlike last year when the cryogenics struggled with oil contamination of a heat exchanger, this year the cooldown went very quickly and smoothly. This procedure will be used for future Start Ups.
Once the ring was made cold, we could start to bring beam through to RHIC. The first leg of this chain is the Tandem Van De Graaff. Since the NASA Space Radiation Lab (NSRL) had just completed its fall run, the Tandem was ready to prepare for injection immediately. The Run08 is deuterons (d) and gold (Au) circulating in the Blue and Yellow RHIC rings respectively. This means that we would require both of the two BNL Van De Graaff accelerators, called MP6 and MP7, to inject into RHIC. One would be for d and the other Au. Attempts were made to get a single Tandem to operate as a source for both, but cooling problems on the foils made this impossible for this year. By 24 Oct we had injected deuterons from the Tandems into the Booster, the second leg in the chain of accelerators feeding RHIC. On 26 Oct the deuterons were transferred from the Booster to the AGS with high efficiency, and on 31 Oct deuterons were extracted from the AGS to the W-dump in preparation for injecting into RHIC. On 2 Nov gold beam was brought through the Tandem and injected into the Booster, with transport through the AGS to the W-dump the following day.
On 6 Nov we finished the RHIC Radiation Checkoff List that allow us to enable the interlocks in the Safety System. The RHIC tunnel was swept (checked for people left in the ring) for the first time, and RHIC was ready to accept beam. The Safety System was upgraded during the shutdown, providing new iris scanners and key trees to the experiments. The iris scanner identifies all people making an access to the beam area, and if they have the training and authorization allowing for an access, a key is released from the key tree that unlocks the gate to the experimental area or the RHIC tunnel.
On 8 Nov deuterons were circulated in the Blue ring of RHIC at injection energy. Ramping the beam energy from injection energy up to colliding energy required the RF cavities in RHIC. Radio Frequency or RF cavities provide electric fields that act on the beam the way a mechanical rabbit acts on the dogs at the dog track, staying out in front of the dogs but just close enough to encourage them to run faster. This electric field gives a kick to the particles in the beam each time they pass through the cavity as they go around the RHIC ring, increasing the particle's energy with each kick, until the particles have 100 GeV of energy per nucleon. We have plans to push up the beam energy to 250 GeV with proton beams later this year. In addition to the RF cavities used to accelerate the beams, we make use of special RF cavities that function to squeeze the beam into shorter and shorter buckets or bunches. These cavities operate at higher frequencies than the acceleration cavities, so the trough of the electric field that the beam travels in is made shorter, squeezing the beam into tighter and tighter bunches.
Just like transferring beam from the Booster to the AGS, transfer from the AGS to RHIC requires that the two machines be "in synch". RHIC is the Master clock, and the AGS RF cavities are driven by a feedback system that keeps them in step with RHIC. Similarly, the Booster RF must follow the AGS RF when injecting from Booster to AGS. By 11 Nov we had successfully injected gold into the Yellow ring and circulated it with an excellent lifetime. Work progressed on ramping the gold beam to collision energy. Early on 15 Nov, both beams had been ramped up to "store".
By the morning of 17 Nov the injection and transport efficiency for both beams had been brought above 85%, and a first attempt at collisions succeeded with both STAR and PHENIX showing ~800 collisions per second.
To maximize the luminosity for the experiments, the beams need to be focused down to the smallest spot at the center of the experiments' interaction point. This year STAR and PHENIX are operating the focusing magnets at a 'beta*" or focusing strength of 1 meter. The smaller the spot, the greater the probability that the particles in one ring will interact with the particles going the other way. Just as the rebucketing squeezes the bunches along the beam, the focusing squeezes the bunches transverse to the beam. Fighting our attempts to make very small interaction points, the beams interact within themselves to blow up or expand their size. We have developed a feedback system that measures excursions of particles away from the core of the bunch, and delivers electrical kicks to push them back into the core. This system, called "stochastic cooling" helps us keep beam circulating in RHIC for much longer times. A typical store will be kept for 6 hours, meaning that particles originating in the Tandem, and accelerated through the Booster and AGS are placed into the RHIC ring where they go around and around for 6 hours, with a few of the particles in each bunch colliding at every intersection.
The last ingredient in increasing the luminosity is to increase the number of particles in the machine. We can accomplish this by adding more Gold and Deuterons to each bunch. Currently we are operating with about 1E11 Deuterons and 1E9 Gold ions in each bunch of the machine. This is close to where we expect to operate for the entire run. For most stores, we have been putting 56 bunches of each ion species in the ring. Over the next few weeks we hope to increase this to over 100.