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Brookhaven | Long Island, NY | USA

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Going in Circles

RHIC's main control room

If you were expecting this blog entry to be about the great song from the Friends of Distinction, it unfortunately won’t be the case. Instead, I’m taking you on the first trip this year of RHIC’s Yellow beam, one of two oppositely circulating particle beams that collide in the center of RHIC’s detectors. Why? Because the main goal of my first shift of Run11 was to get our particles to circulate in a closed orbit in the Yellow beamline. This will also give me the opportunity to get back to some of the technical terms I used in my previous post. So allow me to go over a little bit of accelerator physics theory for a minute.

RHIC, which stands for Relativistic Heavy Ion Collider, is the name given to the large circular collider at Brookhaven National Laboratory; in order for particles to actually reach the collider from their source, they need travel through a series of other accelerators:

  1. the LINAC for protons, the Tandem-to-Booster beamline for heavy ions
  2. the Booster synchrotron
  3. the Booster-to-AGS (BtA) line
  4. the Alternating Gradient Synchrotron (AGS)
  5. the AGS-to-RHIC (AtR) line

This is somewhat similar to the launch of a space shuttle: as it takes off, it goes through different stages of acceleration to get to outer space, using different fuel tanks along the way. The same thing goes for the RHIC injection chain: by the time they reach the end of the AtR line, the accelerated particles, traveling in packs of 100 billion also called “bunches,” have reached 99.995% of the speed of light. To get an idea of what this represents, I’ll use another “real life” comparison: the RHIC main ring is 3833.8452 meters long (yes, we do need that level of precision), which is about 2.38 miles or roughly the same size as the Indianapolis 500 racetrack. And while the fastest Indycar drivers can complete one lap around the track in about 40 seconds, RHIC bunches can do 80,000 laps in a single second. Pretty fast, right?

Over the course of my shift, the AtR line was set up for Yellow injection, meaning that the beamline transporting particles from the AGS to the Yellow ring in RHIC could send bunches at the right position and the right angle (The beamline that sends particles in the other direction is called the Blue ring). Run11 is planned to have 111 bunches in each of the Blue and Yellow rings, so we have to set up our injection system in such a way that it does not affect the bunches already circulating in the machine.

The Yellow beam is traveling from right to left in this picture. The peak around 3,700 meters is from our "fake" injector magnet. The peak around 3,750 meters matches the location of the corrector magnet I used to detect the beam.

This is done by adjusting the timing settings of the injection kicker magnet – a device that does pretty much what its name says, gives particle bunches a “kick” on their journey around the RHIC racetrack. But, before we can use the kicker, the particles must be able to complete one full turn. So at first, we use a small corrector magnet that is able to kick the incoming bunches in the same way our injector kicker would (the only difference is that the correct magnet is powered continuously). To make sure that we actually complete one full turn, we need to check where the beam is inside of the machine; usually, we use our beam position monitors (BPMs), but these need to have their timing set up, too, and – you guessed it – that also requires the beam to do one full turn first.

Instead, I set up another small corrector magnet just upstream of the one used for injection in such a way that, if the beam comes around, it would get lost at the location of that corrector. And voila! (See screenshot)

Having established the first turn, we were able to time in our BPMs and get a better signal of the Yellow beam going around the machine. (Screenshot below).

From then on, we stopped using the “fake” injector magnet and started setting up the timing of the “real” one. We got a closed orbit shortly after that, and our Yellow beam has been happily running around since.

That is it for the first shift report on this blog; next up, we’re going up, up, and away!



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