- Number 293 |
- August 17, 2009
Removing accelerators’ limits
Gianluigi Ciovati is fascinated with failure: He studies failure in accelerator cavities, the structures inside an accelerator that “accelerate” particles such as protons and electrons.
In particular, he wants to know why accelerator cavities fail due to a condition known as Q-drop. During Q-drop, a cavity reaches a threshold at which the more energy that is forced into it, the more that energy is dissipated in the cavity wall, resulting in less energy available to be transferred to the beam.
"My work has been to devise some models to try to test and identify what are the causes of this Q-drop and eventually how to resolve it," Ciovati explained.
Ciovati’s research, conducted at DOE’s Jefferson Lab, builds on his experience designing accelerator cavities at INFN-Milan (Italy's National Institute of Nuclear Physics). At INFN, Ciovati worked on the team that designed the accelerator cavities used in the Spallation Neutron Source at ORNL.
An accelerator cavity is a resonator. By ringing like a bell, it stores the energy pumped into it, which can then be absorbed by a charged-particle beam, thus accelerating it.
His research at Jefferson Lab focuses on magnetic vortices inside the niobium metal skin of cavities. These vortices are like little swirls of magnetic field that can interfere with the energy being pumped into the cavity.
In collaboration with Ganapati Myneni at Jefferson Lab and North Carolina State University, he is studying how to prevent these vortices from being trapped near the inner surface of the niobium cavity.
Currently, the magnetic vortices are thought to be trapped in contaminants, such as hydrogen and oxygen clusters, sitting just under the surface of the niobium metal that has undergone standard cavity processing. There is a potential for trapping the vortices any time the metal is cooled below the superconducting critical temperature in the presence of a residual magnetic field (such as the Earth’s magnetic field) or when there is a large range of temperatures along the cavity.
To reduce the amount of hydrogen in niobium, and the potential for trapping these magnetic vortices in the cavities, a high-temperature heat treatment in a vacuum furnace is already part of the standard cavity processing procedure. But impurities can also be introduced through this process.
"When the furnace cools down, niobium is a very good getter for these kinds of impurities. So there are residual gases that are left in the furnace, such as hydrogen and oxygen, that will eventually be re-absorbed on the surface," he said.
Ciovati aims to build a clean furnace, where he can control the gases inside as the niobium cools down. If the idea results in fewer cavities with Q-drop, it could benefit accelerators worldwide.For his work, Ciovati will soon be recognized with a 2009 Presidential Early Career Award for Scientists and Engineers. This is the highest honor bestowed by the U.S. government on outstanding scientists and engineers who are early in their independent research careers.
Submitted by DOE's Jefferson Lab