A Stroll Through the PEP Tunnel at SLAC
September 15, 2006

Firstoff, it might be a good idea to take a look at the PEP-II regions. I started my stroll in interaction region 2 (IR-2).

The southern side of BaBar - notice the beam injecting in the center.

... and the view just inside the PEP tunnel.

Southward down the PEP tunnel away from IR-2.

The beams of electrons and positrons and electrons are steered around PEP-II by magnets. Unlike charges in electricty no magnetic monopoles exist in nature, the simplest structure is a dipole. Charged particles have their tragectories bent by magnetic fields, so magnetic dipoles, quadrapoles, etc. can be used to steer them. Here are a few early examples:

A dipole. Notice the beampipe passing through the center.

A quadrapole.

This one was made in China.

Another quadrapole.

A bigger dipole.

Another view further down the tunnel.

There are two separate beams which collide in IR-2, a high energy electron one and a low energy positron one. The low energy ring (LER) is at 3.1 GeV whereas the high energy one (HER) is at 9 GeV. The LER is located above the HER in the PEP tunnel as shown in this photograph.

Being higher in energy, the HER beam needs somewhat larger dipoles like this one to steer it.

A sextapole.

As I snuck further into the tunnel, not sure if I was really allowed to be there, I decided my goal (and cover story) would be that I wated to seen an octople magnet since there weren't any right near IR-2. The PEP ring is currently under "permitted access" during the shutdown which means that there oughtn't to be any hazardous radiation, so I wasn't concerned about any danger, just stepping on the annoying safety bureaucracies toes. For example, for the LST installation we have to work under the detector where it is very dusty. It would be nice to wear a simple dust mask in such a situation, however, this requires training and medical lung capacity tests. Not having weeks to set up such things we decided to forgo the masks and deal with black boogers and a little dust in our lungs. Just an example of how the safety bureaucracy makes the place more dangerous. Trying to look official with my camera, none of the techs I encountered questioned my presense though. L'audace, toujour l'audace.

A view back toward IR-2. For some reason there is alwayss drippy water in HEP facilities.

A dipole, sextapole, quadrapole and dipole aligned in tandem to steer the beam.

Survey markers were placed periodically in the floor of the PEP tunnel.

The PEP tunnel was laced with graffiti:

IR-4 is a smaller scale replica of IR-2, but without the detector. Had the government deigned to fund them, several experimental detectors could have taken data on collisions at various points around the ring simultaneously. The LEP ring at CERN operated this way in the 90's and the LHC (built in the old LEP ring) will also run several detectors at once.

Note that the HER and LER rings pass in IR-4 at slightly different radii soas to facilite steering them into collision had a detector been built here. Also the concrete curtain wall remains in place to left.

Apparently there was some rare zealousness to prevent graffiti in this region.

An RF power device for creating the electomagnetic wave the electrons surf on. (The one for the positrons is identical).

Run 5b ended due to a fire which I think was either here

or here (but I'm not really sure).

Several places along the tunnel are exits like this one off to the left.

Shafts allow airflow from the surface, and radiation shielding protects some exits in sensitive areas.

A view down the tunnel into IR-6.

Inside IR-6.

Another kind of sextapole, but still no sign of the elusive octopole.

Posters on the wall in the PEP tunnel. (One day I need to learn not to be such a retard with the flash).

The concrete wall in the center shields large spare pieces of beampipe which are stored just inside the curtain wall.

Still further down the tunnel to go...

The LER beamdump ... I didn't linger.

Looking down from the end of the LINAC toward the LER and HER in the PEP ring. The beampipe in the foreground is the LER injector. Positrons are accelerated down the LINAC and the injected into the LER until it is full. When the LER and HER are full, then the magnets near IR-2 are tuned so that collisions take place inside BaBar.

More ring to go until one reaches the HER injector.

A spring in the wall.

The HER injector.

I decided to leave my own little bit of graffiti in the tunnel as well.

Almost back to IR-2 I finally encountered one of the elusive octopoles.

IR-2 is up ahead.

Tools for surgery on PEP-II.

Ah, the welcome sight of the DIRC on the north side of BaBar means I've completed all ? miles of the ring!

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Last ∆ on 15 September 2006 by Bradley James Wogsland .