Warm Coupler Conditioning starts in the ESS Superconducting Linac

Cecilia Maiano SRF
Accelerator Engineer Cecilia Maiano, thrilled to see the first RF pulse to a cryomodule cavity in the tunnel. Image: Ulrika Hammarlund/ESS

The first in-situ conditioning of a Spoke Cryomodule in the ESS Superconducting Linac marks a significant step towards preparation to the full accelerator operation and the Beam on Dump milestone.

MCR action

Action in the ESS Main Control Room as the warm conditioning got started.

On Friday, June 28, a significant milestone was reached in the ESS superconducting linac. For the first time, warm coupler conditioning in a Spoke Cryomodule was performed in-situ, in the Accelerator tunnel. The achievement, which saw a Radio-frequency (RF) pulse transmitted from an RF power station in the Gallery building through the RF distribution system to a power coupler in a  fully installed Spoke Cryomodule, marks the beginning of a thorough conditioning process that will encompass all accelerating components along the superconducting linac, preparing their operation. This crucial step, executed successfully, involved collaborative efforts from the RF, Superconducting RF and Power Converters teams, ICS controls, Radiation Protection, the ESS vacuum team and the ESS Main Control Room. 

Conditioning screenshot

A graph showing the first RF wave measured in the cavity of the cryomodule.

Benjamin Bolling/ESS

At midday on Thursday, a sense of excitement filled the air as operators, accelerator technicians, engineers, and scientists witnessed the first RF power ramp-up sequence deployed for a power coupler in a spoke cryomodule in the tunnel. This significant achievement follows months of meticulous preparation, which included setting up and testing the RF power sources, connecting and testing the cryomodule and its infrastructure, and deploying the necessary controls. The milestone is particularly impressive given that detailed preparations began nearly two years ago, aimed at implementing mitigating measures to safely carry out this work without restricting worker access to the whole tunnel. This accomplishment marks a crucial step towards achieving the ESS Beam on Dump (BoD) milestone.

Emanuele Laface

Accelerator Physicist Emanuele Laface setting up the parameters for the conditioning.

During the power coupler conditioning stage, RF power is gradually sent to the system systematically increasing its average power (by regulating RF pulse length, peak power, and repetition rates), in order to gradually prepare the RF surfaces to sustain high power operation and desorb the gases from the surfaces. RF pulses start with the shortest possible pulse and increase in power until the maximum is reached. This process is repeated with several pulse lengths until the nominal ESS RF pulse length is achieved. A similar process will be performed after the accelerator cooldown this coming autumn, tuning the cavities to their nominal frequency and gradually exposing the surfaces to RF operation and bringing each cavity to its nominal operating point, ready to accelerate the beam.

Spoke Linac

The Spoke Linac at ESS, with 13 double-cavity cryomodules.

The warm coupler conditioning of the first cryomodule was completed on Tuesday July 2, setting the stage for the serial conditioning of all cavities along the superconducting linac, which is set to start in August. To date, 26 out of the 27 cryomodules needed for BoD and Beam on Target (BoT) are in place in the tunnel, with the final cryomodule scheduled for installation in mid-July.

MCR team

Team celebrations in the ESS Main Control Room.

Learn more about cryomodules

What is a cryomodule?

A cryomodule is a large tank with thermal shielding which contains a series of superconducting radio frequency (SRF) cavities. The cavities are cooled down to 2 Kelvin (-271 degrees Celsius) using a cryogenic system distributing cold helium.

The SRF cavities inside the cryomodule are designed to create an alternating electromagnetic field that accelerates protons as they pass through the string of connected cavities. By using a superconducting material (niobium) and cooling the cavities to extremely low temperatures, the cavities will allow for very high acceleration of the particles. At ESS the final proton speed will exceed 95% the speed of light.


Cross-section of ESS spoke and elliptical cryomodules with two and four superconducting cavities.

Who delivers the cryomodules?

The spoke cryomodules, a French in-kind contribution to ESS, were designed and manufactured by IJC Lab, a research unit of the French National Centre for Scientific Research (CNRS). Initial testing, including conditioning and cooldown of the modules, has been carried out at the FREIA lab, Uppsala University, Sweden.

The elliptical cryomodules are engineered, assembled, and supplied by French CEA, with medium beta cavities provided by INFN, Italy, and high beta cavities from STFC, UK. Testing is carried out in TS2 at ESS.