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SRF Cavities
A Highly Prized Technology for Accelerators

Superconducting RF Cavities

An Energetic Kick

Hasan Padamsee

Hasan Padamsee

A key component of any modern particle accelerator is the electromagnetic cavity resonator. Inside the hollow resonator cavity, large-amplitude, high frequency electromagnetic waves repeatedly “kick” charged particles to increase their velocity and energy. Most particle accelerators use microwave energy to propel particles. LEPP has developed cavities spanning a wide range of resonant frequencies between 200 and 3000 megahertz (one million cycles per second). For nearly three decades LEPP has been a world leader in developing and applying microwave cavity technology for uses in a range of areas, including elementary-particle physics, nuclear physics, basic materials science, and biology.

As physicists have demanded higher accelerating fields, cavities with continuous wave operation (pulsed operation at high-duty cycles) made out of traditional copper materials are increasingly uneconomical, because electrical resistance in the walls squanders energy. As an alternative, resonant cavities can be built with superconducting materials that have near-zero resistance. The microwave surface resistance of a superconductor is nearly a million times less than that of copper. Pure niobium metal is the superconductor of choice for most applications, because it has the lowest losses at the highest accelerating voltages. A niobium cavity becomes a superconductor when cooled in a bath of liquid helium, operating anywhere between 2 and 4.5 degrees above absolute zero (-273ºC). Although many thousands of watts of power are delivered to the cavity, almost all of it goes into accelerating particles, and fewer than 100 watts of power are absorbed by the walls and dissipated into liquid helium.

Superconducting RF cavities (SRF,) are nearly perfect resonators, achieving a quality factor, “Q,” of one to ten billion. This means that if the microwave power source were turned off, the electromagnetic waves inside the SRF cavity would oscillate ten billion times before dying away. To appreciate how amazing this is, if one of the bells in Cornell’s McGraw Tower had a similar Q, it would ring continuously when struck for more than three years!


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