Marvelous: just like rasogolla made with brown sugar

Shekhar Mishra

Shekhar Mishra

The sun was about to rise in Kolkata, and morning news could not have been better for our Variable Energy Cyclotron Center (VECC) scientific colleagues in Kolkata, India. As part of the Indian Institutions and Fermilab Collaboration (IIFC), our colleagues designed, fabricated, electropolished, processed and tested their first superconducting radio-frequency (SRF) cavity, jointly with the Inter-University Accelerator Center (IUAC), Delhi, Fermilab and Argonne National Laboratory.

The accelerating cavity is intended to advance R&D for the future Fermilab PIP-II project. PIP-II, which stands for Proton Improvement Plan II, will bring Fermilab’s accelerator complex to deliver very high-power beams for the lab’s various experiments.

From left: Surajit Ghosh, Pranab Bhattacharya (VECC), Dmitri A. Sergatskov and Alex Melnitchouk (Fermilab) contributed to this project. Here they are pictured in the Vertical Test Stand Control Room at the Fermilab Technical Campus. Photo courtesy of Surajit Ghosh

From left: Surajit Ghosh, Pranab Bhattacharya (VECC), Dmitri A. Sergatskov and Alex Melnitchouk (Fermilab) contributed to this project. Here they are pictured in the Vertical Test Stand Control Room at the Fermilab Technical Campus. Photo courtesy of Surajit Ghosh

To everyone’s delight, the VECC cavity exceeded the design parameters of the PIP-II project. One parameter is the cavity’s accelerating gradient – a measure of the cavity’s strength in kicking forward the particle beam that travels through it. Another is its quality factor, a measure of power dissipation of the cavity. The VECC one-cell cavity’s measured accelerating gradient is about 30 million electronvolts per meter (MV/m) with a quality factor of about 1.5 x 1010.

Steve Holmes, the PIP-II project manager, remarked on the great progress our colleagues have made, saying, “These results get the VECC effort off to a very good start. We were particularly pleased that our two VECC colleagues could be here to participate in the processing and testing of these cavities.”

From left: Kishore Mistry (IUAC), Sumit Som (VECC) and P.N. Prakash (IUAC) inspect an accelerating cavity at the electron beam welding facility at the Inter-University Accelerator Center in Delhi, India. Photo courtesy of Sumit Som

From left: Kishore Mistry (IUAC), Sumit Som (VECC) and P.N. Prakash (IUAC) inspect an accelerating cavity at the electron beam welding facility at the Inter-University Accelerator Center in Delhi, India. Photo courtesy of Sumit Som

For one year, in the true spirit of collaboration, every Tuesday colleagues from VECC and Fermilab – 11 time zones apart – gathered over videoconference to discuss the progress and technical issues with the SRF cavities they have been jointly developing.

This is the first cavity of this type: designed by VECC and based on the specifications provided by Fermilab to efficiently accelerate protons with an average velocity of 61 percent of the speed of light. VECC fabricated the cavity using the electron beam welding expertise of the IUAC SRF facility. (The IUAC team has already established their leadership by fabricating two cavities of a different kind, called spoke resonator cavities, for PIP-II earlier this year.)

The VECC scientific staff worked with Fermilab colleagues in all aspects of cavity tuning, processing and measurements, while they worked with Argonne colleagues in cavity electropolishing — a technique that gives these metallic structures a mirror finish and boosts performance. The cavity was tested by immersing the evacuated cavity in liquid helium at a temperature of minus 271 degrees Celsius. The results from the measurement are shown in the plot. The cavity reached a gradient of up to 34.5 MV/m at maximum power.

This plot shows the VECC cavity quality factor (in blue) over a range of accelerating gradients. It exceeded PIP-II specifications (yellow box), achieving a quality factor of 1.5 x 1010 at 30 MV/m.

This plot shows the VECC cavity quality factor (in blue) over a range of accelerating gradients. It exceeded PIP-II specifications (yellow box), achieving a quality factor of 1.5 x 1010 at 30 MV/m.

Sumit Som, the subproject coordinator of IIFC–Department of Atomic Energy for this project and head of the Accelerator Technology Group, VECC, remarked on this achievement of IIFC: “This is the highest gradient attained so far in the world for this type of cavity. This highly encouraging result will motivate the team at VECC to perform even better in future.”

I tend to agree, and now VECC is fabricating a second one-cell cavity with improved design. They have also finished the preliminary design of a five-cell SRF cavity that will be used in PIP-II. The collaboration expects to finalize the design in early fall and start on fabrication soon after.

Shekhar Mishra is the PIP-II deputy project manager and spearheaded the IIFC.