Q – cavity quality factor

| 23 August 2012

Rings a bell? This one's Q factor could be better. Image: Tony Fischer Photography

Physicists need to understand each accelerator cavity individually before assembling a collider. One of the cavity characteristics physicists measure is called the cavity quality factor, Q factor for short.

One way to understand the Q factor is to pick up a bell. Let the clapper tap the bell edge once. You’ll notice that just one tap is enough to leave a long-lived sound. How long that resonating tone lasts is a measure of the bell’s quality factor.

Like a bell, an accelerator cavity also resonates. A cavity, explains SLAC physicist Marc Ross, is essentially a hollow metal ball, with a vacuum inside, made of sheet metal. “There are certain frequencies which resonate with that particular size ball in a particular way,” Ross says. Just as a shape can resonate mechanically with sound, a metal shape can also resonate with an electromagnetic field.

One way to measure quality factor is to shut off power and observe how long it takes the electromagnetic field to decay within the cavity. The longer the cavity can hold on to that power, the higher its Q factor. Cavities with a high quality factor lose less energy as heat, which means keeping the cavity cool is also easier.  Characterising a cavity’s Q factor allows physicists to understand how well it can store up energy—and in turn how efficiently it gives that energy to particles.

ILC cavities must have a nominal Q greater than 8 billion, meaning that after at least 8 billion cycles of radiofrequency accelerating power, 99.5% of the cavity’s stored energy will have decayed.

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