Superconducting Radio Frequency Cavity Research

Background

Radio frequency (RF) cavities are used to accelerate charged particles to high speeds. The electric field inside the cavity kicks a charged particle passing through it. A large speed is attained after the particle travels through many cavities. A typical cavity generates a potential of over 1 million volts. If we tried to do the same thing using ordinary 9 volt batteries, for example, we would need more than 100,000 batteries.

A conventional copper cavity requires a lot of power to produce a high potential. The power being dissipated in the cavity walls can be as high as 1 million watts. As a result, copper cavities cannot operate continuously at high potential; superconducting radio frequency (SRF) cavities must be used for this purpose. The power dissipation in an SRF cavity is about 100,000 times smaller than in a copper cavity. Special materials and extremely cold temperatures are needed to make a cavity superconducting. An SRF cavity made of niobium is operated at a temperature of about -456 °F or 2 Kelvin (K).

The Role of NSCL

SRF research is aimed at improving the performance of SRF cavities and extending SRF technology to new applications. At NSCL, SRF research focuses on acceleration of particles from 5 percent to 80 percent of the speed of light. We are presently building prototype cavities for acceleration from 40 percent to 55 percent of the speed of light.

An SRF cavity is made in several steps. The desired shape is first formed from sheet niobium. We then remove the surface damage layer via chemical etching. Even microscopic particulates stuck to the surface of the cavity can degrade the high-field performance, so the final cleaning is done inside a clean room similar to those used by the semiconductor industry. We rinse the cavity with ultra-pure water at high pressure. The water jets knock the dust particles off the surface. We put the cavity in a cryostat (the equivalent of a large thermos bottle) and cool it to cryogenic temperatures with liquid helium. An RF power generator is used to set up the high electric field in the cavity. Most of the infrastructure for the above steps is now installed at NSCL.

SRF cavity research will be essential for future research in nuclear physics. A long chain of SRF cavities will be needed for the proposed Rare Isotope Accelerator (RIA). RIA will be be used to accelerate heavy ions to higher energies than ever before.