RFQ system to take Fermilab into the Intensity Frontier

Alluding to a quote from Winston Churchill, Fermilab scientist C.Y. Tan made this drawing of the new RFQ in April as a way to rally his Proton Source Department colleagues in the run-up to the device’s commissioning.

On Wednesday morning, employees from across Fermilab assembled in the Linac Gallery to observe the decommissioning of the Cockcroft-Walton generators, which provided beam to the lab’s accelerators for 40 years. At the beginning of September, members of the Proton Source Department will begin installing the Cockcroft-Waltons’ replacement, the radio-frequency quadrupole system.

The RFQ system will perform the same function as the Cockcroft-Walton generators but in a much smaller space and with fewer parts. Whereas the generators needed two massive rooms to operate, the RFQ is only about 3.5 meters long. In that space, it will ramp up the energy of hydrogen ions to 750 keV for entrance into the Linac, just as the Cockcroft-Walton generators did.

“With the RFQ, we’ll have better beam quality to start with so there are fewer losses down the line,” said AD operations specialist Pat Karns, who is writing his master’s thesis on the installation and commissioning of the RFQ.

Previously, beam started from two sources – one in each Cockcroft-Walton generator – and had to be bent to maneuver the particles into the buncher cavity ahead of the Linac. This caused a loss of a significant portion of the beam and meant that the bunching – or clustering of hydrogen ions within the beam – was inconsistent.

“The RFQ will do the acceleration and bunching in one device, preserving the bunching,” said Bill Pellico, who leads the Proton Improvement Plan team. The beam’s small cross section will help provide more intense and reliable beam for Intensity Frontier experiments such as Mu2e, NOvA and Project X.

The PIP team looked to Brookhaven National Laboratory’s RFQ system for inspiration. AD scientist C.Y. Tan, who led the design effort, based the Fermilab design on the BNL design, adding several unique features. One of them is the use of an Einzel lens as a beam chopper, designed by retired scientist Jim Lackey. This system uses solid-state switches to pulse a voltage across the lens. At high enough voltage, the lens will not allow beam into the RFQ, but at lower voltage, the beam is transmitted. Another feature is a sliding rail, designed by engineer Kevin Duel. The rail allows the two hydrogen ion beam sources, designed by source expert Dan Bollinger, to be switched if one degrades, which happens every three to nine months.

Tan also added that, in general, the RFQ system would be more accessible for repairs than the Cockcroft-Walton generators were.

The PIP team has been preparing the RFQ for installation for several months. In early September, the Linac Gallery will start to undergo a series of modifications for the installation process, which is due to be complete in November. The beam will need to be running in December for the Neutron Therapy Facility, said Tan.

Karns said that system tuning will continue until the end of shutdown in March 2013. He was enthusiastic about the future of the RFQ.

“It’s a more modern system,” he said. “Hopefully we’ll get to be the experts on it for the next 20 years.”

Joseph Piergrossi