PXIE progress: LEBT successfully delivers maximum current

Lionel Prost of the Accelerator Division inspects the PXIE low-energy beam transport, which is now capable of delivering 10 milliamps of current. Photo: Reidar Hahn

Fermilab has paved the first few feet of the long road to a dramatic upgrade of its injection complex. Specifically, it’s 8 feet, the current length of PXIE, a test accelerator that prototypes the front end of the proposed upgrades known as PIP-II.

The project accomplished a significant step when researchers recently passed beam through the nearly complete low-energy beam transport with a current of 10 milliamps, PIP-II’s specifications.

“Meeting the specification is the result of the hard work of many people,” said Paul Derwent, head of the PIP-II Department within the Accelerator Division.

An important goal was to pass this current through the LEBT with losses of less than 10 percent. The PXIE team has achieved that with losses of less than two percent.

PXIE’s LEBT is a series of components that links a negative hydrogen ion source to a radio-frequency quadrupole, the beam’s first accelerating component. The LEBT focuses, shapes and directs a nominal 5-milliamp beam so as to fit the size, position and angle requirements for the RFQ. After achieving the 10-milliamp milestone, the PXIE research team now concentrates its efforts on other beam characteristics related to beam quality. Poor beam quality would lead to beam loss downstream, which in turn can cause a loss of superconductivity in a cryogenic section of the future accelerator.

Lionel Prost, lead scientist for the ion source and LEBT portions of PXIE, said that even though LEBTs are not new technology, building one is no small task.

Dozens of people from all over Fermilab have contributed to designing, building and commissioning the LEBT. Additional support came from Oak Ridge National Laboratory’s Spallation Neutron Source and Lawrence Berkeley National Laboratory, which made contributions to the LEBT design and beam instrumentation. Berkeley Lab is also building the RFQ, which is expected to be completed in the spring.

The final goal of PXIE is to address technical risks associated with the PIP-II plans and help establish a reliable budget for the full-scale project.

“The theory and the machine itself are not the same thing,” Prost explained. “You have to make it work. So you take measurements, analyze the data and try to figure out why it does not match your expectations — and repeat until you understand what you actually have in front of you rather than what you had drawn up!”

Ultimately, PXIE will accelerate negative hydrogen ions to 25 million electronvolts. The ions will be carefully packed in a complicated sequence of bunches, allowing the future accelerator to feed multiple experiments simultaneously.

PXIE, designed to operate as a continuous-wave machine, will also demonstrate pulsed-beam operation required for injection into the Booster.

The road ahead is long, but the first few feet look to be on solid ground.

Troy Rummler