Knocking on Heaven's Door - Lisa Randall [68]
The initial acceleration phase takes place in CERN’s linac, which is a linear stretch of tunnel along which radio waves accelerate protons. When the radio wave is peaked, the associated electric field accelerates the protons. The protons are then made to drift away from the field so they don’t decelerate when the field goes down. They subsequently return to the field when it peaks again so that they repeatedly accelerate from one peak to the next. Essentially the radio waves pulse the protons in the way you push a child on a swing. The waves thereby boost the protons, increasing their energy, but only a tiny amount in this first acceleration stage.
In the next stage, the protons are kicked via magnets into a series of rings where they are further accelerated. Each of these accelerators functions similarly to the linear accelerator described above. However, because these next accelerators are ring shaped, they can repeatedly boost the protons’ energies as they circle around thousands of times. These circular accelerators thereby transfer quite a bit of energy.
This “fellowship of the rings” that accelerates protons before they enter the large LHC ring consists of the proton synchrotron booster (PSB) that accelerates protons to 1.4 GeV, the proton synchrotron (PS) that brings them up to 26 GeV in energy, and then the super proton synchrotron (SPS) that raises their energy to the so-called injection energy of 450 GeV. (See Figure 25 to see a proton’s journey.) This is the energy the protons carry when they enter the last acceleration stage in the large 27 kilometer tunnel.
A couple of these accelerating rings are relics of previous CERN projects. The proton synchrotron, which is the oldest, celebrated its golden anniversary in November 2009, and the proton synchrotron booster was critical to the operation of CERN’s last major project—namely, LEP—in the 1980s.
After protons leave the SPS, their 20 minute long injection phase begins. At this point the 450 GeV protons that emerged from the SPS are boosted to their full energy inside the large LHC tunnel. The protons in the tunnel travel along two separate beams going in opposite directions through narrow three-inch pipes that extend on the 27 kilometers of the underground LHC ring.
[ FIGURE 25 ] The path a proton travels on when accelerated by the LHC.
The 3.8 meter (12 ft.) wide tunnel that was built in the 1980s but that now houses the proton beams in their final acceleration stage is well lit and air conditioned and large enough to comfortably walk around in, as I had the opportunity to do while the LHC was still in the construction phase. I took only a short stroll inside the tunnel on my LHC tour, but it still took me far longer to traverse my few steps than the 89 millionths of a second it takes for the accelerated highly energetic protons traveling at 99.9999991 percent of the speed of light to make it around.
The tunnel sits about 100 meters underground, with the precise depth varying from 50 to 175 meters. This shields the surface from radiation and also means CERN didn’t have to buy up (and destroy) all the farmland lying over the tunnel’s location during the construction phase. Property rights did, however, delay tunnel excavation back in the 1980s when it was originally constructed for LEP. The problem was that in France, landowners are entitled to the entire region to the Earth’s center—not just the farmland they plow. The tunnel could be dug only after the French authorities blessed the operation by signing a “Déclaration d’Utilité Publique,” thereby making the underlying rock—and in principle the magma underneath too—public property.
Physicists debate whether the reason for the tilt in the tunnel’s depth was geology or if it was done to further defl ect radiation, but the fact is the tilt helps with both. The uneven terrain was in fact an interesting constraint on the tunnel’s depth and location. The region lying under the CERN site is mostly a type of compact rock known as molasses, but underneath the