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At Cornell

Creating New Forms of Matter

SRF Gate Valve Control

Meet CESR in Action

David Rubin

David Rubin

Imagine a circular aluminum tube shaped like the inner tube of a bicycle tire. The tube is one-half mile in circumference, and its cross section is about two inches in diameter. The tube is evacuated and buried in a tunnel 50 feet below Alumni Fields. Now look down at the tube. Electrons circulate in a counterclockwise direction through the tube, which is surrounded by powerful magnets steering the electrons around the tube’s curvature. The energy of the electrons is 5.3 GeV. This is the energy that an electron would gain if accelerated through an electric field of 5.3 billion volts. Nearly 100 billion electrons are bunched together into a packet that is 2 millimeters wide, 0.2 millimeters tall, and 10 millimeters long. The electron packet, or bunch, travels at nearly the speed of light, corresponding to 400,000 revolutions around the ring per second.

Positrons, the antimatter counterpart of electrons, circulate in a clockwise direction around the ring. Electrons and positrons have the same mass, but opposite electric charges—electrons are negative and positrons are positive. In a magnetic guide field, trajectories for oppositely charged particles traveling in opposite directions have identical but time-reversed trajectories. At the interaction point (IP) in the experimental hall in Wilson Lab, superconducting quadrupole magnets focus the counter-rotating packets of electrons and positrons to a thread with a cross section of a human hair, and the packets collide. The objective is for an electron and positron to come so close together that they annihilate one another, and new forms of matter emerge from the intensely concentrated ball of energy. The particles are so small that, in spite of the tiny spot into which they are focused, annihilations are rare and may occur only once in every 100,000 passages of electron bunch through positron bunch. The annihilation rate depends on the luminosity, which scales with the number of particles in the bunch and inversely with the cross-sectional area of the bunch.

To Increase Luminosity

Very soon after the first beam was circulated in the storage ring in 1979, the LEPP accelerator group began to develop and implement strategies for increasing the luminosity. The strategies are simply stated: increase the number of particles by adding more packets; reduce the spot size, or IP, by making the final focus quadrupoles stronger and moving them nearer to the collision point; and reduce the length of the packet by increasing the accelerating voltage.


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