With the repair and reinstallation of the cryomodule known as CM2, researchers at Fermilab, US, are back on the road towards achieving the International Linear Collider’s R&D goal (named task force “S1”): operating a cryomodule at ILC gradient specifications.
Originally installed in May 2012 as a part of the Advanced Superconducting Test Accelerator (ASTA) in Fermilab’s NML facility, CM2 is the first ILC-type cryomodule built in the United States with cavities expected to meet the ILC’s specifications. But a helium leak detected in the cryomodule put the project on hold until April 2013, when CM2 was reinstalled at NML.
“We expect to be ready for cooldown of CM2 this month, ” said project engineer Jerry Leibfritz. “Then we’ll start testing the cavities after that.”
In order to meet the ILC programme’s S1 goal, the average accelerating gradient over each of the cryomodule’s eight superconducting radiofrequency (SRF) cavities will need to reach at least 31.5 megavolts per metre (MV/m) after installation and cooldown.
A GDE team made an attempt to meet this goal in October 2010 at KEK, in a global collaboration known as the S1-Global experiment. They combined components from Fermilab, DESY (Germany), INFN (Italy), KEK (Japan) and SLAC (US) to build two short 4-cavity cryomodules that were then combined into an eight-cavity cryomodule. The cavities in this cryomodule fell slightly short of the goal, achieving an average gradient of 30.0 MV/m before installation and 26.0 MV/m for simultaneous operation of seven cavities after cooldown, as one cavity did not work properly. S1-Global showed that it is possible to operate a cryomodule close to ILC specifications, and Fermilab researchers hope CM2 will take the achievement one step further by meeting the requirements.
Earlier that year, CM2’s cavities had already met the gradient goal individually, as eight industry-made nine-cell 1.3-gigahertz (GHz) SRF cavities were selected based on their capability to reach the 31.5- MV/m gradient in preinstallation tests. Horizontal tests, the final individual-cavity tests, reached 35 MV/m, according to Fermilab engineering physicist Elvin Harms, who is leading the effort to make CM2 operational. After eight good cavities were chosen, the Fermilab team strung the cavities together inside the cryomodule, which was then placed in Fermilab’s NML facility.
However, when the helium leak was discovered, the entire cryomodule had to be removed from the NML facility and taken to the lab’s Industrial Building Complex for repair. Fortunately, the Fermilab team discovered the leak before attempting cooldown.
“If we had tried to cool it down, it would have leaked helium into the insulating vacuum, and you wouldn’t be able to keep it cold,” Leibfritz said. “You just couldn’t operate it.”
Since CM2’s return to the NML facility, Fermilab engineers have aligned it with the eventual path of the accelerator’s electron beam and have completed cleanroom vacuum work to prepare the SRF cavities for operation. All cavities have undergone warm coupler conditioning, which involves testing the devices that will provide radiofrequency power to the cavities after cooldown. All that remains is to finish the welding and pressure testing of the cryogenic pipes, which supply the helium and nitrogen that will cool CM2 to its operating temperature of 2 kelvins.
Once cold, the cavities will be tested to see if they reach the expected S1 goal of 31.5 MV/m. The radiofrequency power for the tests will come from a single klystron source to all eight cavities simultaneously, through a wave guide system that was built at SLAC.
Harms said there may be degradation of a few percent from the 35 MV/m gradient measured prior to installation, though even a small decrease is not ideal. To test achievement of the S1 goal, the team will only measure the performance of the cavities up to 31.5 MV/m. After this testing is complete, they will consider increasing the power to determine the practical limit.
“We’re going to bring the gradient up very conservatively,” Harms said. “We don’t want to do anything to damage these cavities.”
CM2 is the second eight-cavity cryomodule to be built at Fermilab. The first cryomodule, CM1, allowed Fermilab researchers gain experience building and operating a cryomodule. It was replaced by CM2 last year.
“We always knew CM1 was not going to be high-gradient and that it was going to come out,” Leibfritz said. “We plan on keeping CM2 here. It will be the first cryomodule for our test accelerator.”
CM2 is just one part of Fermilab’s Advanced Superconducting Test Accelerator, a superconducting linear accelerator originally conceived as an ILC prototype.
Fermilab scientists and engineers are currently commissioning a photocathode electron gun for the test accelerator. They have recently generated the first photoelectrons that will eventually provide the beam for the accelerator.
“Our goal by the end of the year is to be able to send beam through the full injector up to the cryomodule,” Leibfritz said, “and then, hopefully, beam through the cryomodule next year.”
After S1, the ILC programme’s S2 goal is to test an entire ILC RF unit, which is composed of three cryomodules. Fermilab’s NML facility has the ability to house up to six cryomodules with the recently completed extension of the concrete cave which houses the linear accelerator.
“CM2 is really just one more step along the way towards realising the ILC,” Harms said. “It’s another data point that we can put multi-cavity cryomodules together and make them work.”