Balloon Tuning

Technology Summary

Before a particle accelerator cavity is installed, it is carefully tested and tuned using an automated machine that grasps the edges of each cell to make small, precise adjustments. The process continues until the cavity is adjusted so that, once the cavity is up and running inside an accelerator, it’s in the shape to produce the perfect electric field to propel charged particles. But before most cavities can be installed, they must also be fitted with a metal jacket so the cavity can be cooled to extremely low temperatures with liquid helium. After that, the only easy way to apply forces to the cells is to push or pull on the ends of the cavity, rather than targeting each cell individually. If a cavity becomes misshaped during or after the process of putting the jacket on, the traditional tuning method can’t be applied without cutting the metal jacket off — a laborious, time-consuming task.

The Invention

Fermilab has developed a way to tune “dressed” cavities by accessing them from the inside— without introducing contaminants or damaging the cavity walls— using specially-designed balloons. A pump fills each balloon with gas until it applies about two bars of pressure— a little less than what’s recommended for standard car tires. This isn’t enough pressure to reshape a cavity cell on its own, but that pressure can be used to influence which cell deforms when forces are applied to the ends of a cavity at room temperature. Balloons allow one to single out a particular cell, either stretching or squeezing it.

If a particular cell needs to be stretched, a balloon inflated inside it provides an extra nudge for it to expand as the flanges are pulled apart. Whereas if a cell needs to be squeezed, a series of balloons can support all the other cells as the two ends are pushed together.


Currently, this is the only straightforward way to tune dressed cavities other than cutting the helium vessel to access the outer surface of the cavity cells, tuning the bare cavity, and re-dressing the cavity. This significantly increases the cost and delays the schedule of large-scale particle accelerator projects that comprise hundreds of cavities.

Applications and Industries

The primary application for this technology is the particle accelerator industry, but any industry with components necessitating very specific internal geometries that cannot be easily accessed from the outside could potentially be “tuned” in this way.

Category: Engineering Technologies, Accelerator Technologies

Tags and keywords: Balloon tuning


Upper:To stretch one cell of an accelerator cavity, a balloon is placed inside it and inflated.
Lower: To squeeze a particular cavity cell, a balloon is placed inside the cells surrounding it. The balloons support the surrounding cells, resulting in the unoccupied cell being reshaped as forces are applied to each end of the cavity.

Invention Details

Patent Status: Patented

Aaron G Sauers, CLP
Fermilab, MS 312 – PO Box 500
Batavia, IL 60510