Large Hadron Collider (LHC) restarts with a "bang" after being shut down for over a year
David R. Winn, Ph.D., professor of physics and chair of the Department of Physics at Fairfield University: This is truly a technical tour de force, and was not expected to be achieved quite so quickly after first injecting beams into the collider rings last week.
What: CERN has announced the first collisions of proton beams in the center of the four main LHC experiments.
When: Monday, Nov. 23, following a shut down of the experiment for more than 12 months caused by a faulty copper splice.
What It Means: A commentary follows by Dr. David Winn, chair of the Physics Department at Fairfield University, who was responsible for inventing and designing a prototype for a section of the Compact Muon Solenoid (CMS) called a "quartz fiber calorimeter," which measures the energy particles that emerge from the LHC collisions.
"The accelerator physicists have done a remarkable job in controlling and aligning the magnetic "optics" which steer and confine the counter-rotating beams of high energy particles (much like a system of lenses and mirrors might make a periscope or camera lens to control or steer light beams). Not only do the orbits of the particles around the 27 kilometer circumference ring have to not vary turn-to-turn by more than about 2x the width of a human hair (a beam diameter/width of about 0.18 mm or less), but must close on themselves also at that accuracy - meaning that the beams of particles must be no more than that fraction of a mm wide at the collision points.
Moreover, the 2 independent 0.18 mm diameter beams must be made to hit each other dead center within about 0.01 mm, after travelling in opposite directions for 27 km, and in time to within 0.1 ns (billionths of a second), again after travelling 27 km. This is truly a technical tour de force, and was not expected to be achieved quite so quickly after first injecting beams into the collider rings last week.
The beam positions are dynamically corrected in real time - in a manner similar to the dynamic stability that you instinctively use to balance a tennis racquet grip down on end, on your upturned palm, by rapidly moving your hand sideways back-and-forth underneath it - dynamic stability: as the racquet tips/begins to fall in one direction, you move your hand underneath it in that direction to provide instantaneous support. It then often tips in the opposite direction as you overcorrect, and the process repeats. In the Large Hadron Collider, measuring the beam positions and correcting their motion happens at light speeds, and the over-corrections must be predicted in advance and eliminated."
Posted on November 25, 2009
Vol. 43, No. 135