Niché à 30 pieds sous terre à Menlo Park, en Californie, un tronçon de tunnel d'un demi-mile de long est maintenant plus froid que la majeure partie de l'univers. Il abrite un nouvel accélérateur de particules supraconductrices, qui fait partie d'un projet de mise à niveau du laser à rayons X à rayons X de la source de lumière cohérente Linac (LCLS) du laboratoire national des accélérateurs SLAC du ministère de l'Énergie.

Les équipages ont réussi à refroidir l'accélérateur à moins 456 degrés Fahrenheit - ou 2 Kelvin - une température à laquelle il devient supraconducteur et peut propulser les électrons à des énergies élevées avec une énergie presque nulle perdue dans le processus. Il s'agit de l'une des dernières étapes avant que le LCLS-II ne produise des impulsions de rayons X 10 000 fois plus brillantes, en moyenne, que celles du LCLS et qui arrivent jusqu'à un million de fois par seconde, un record mondial pour le X- le plus puissant d'aujourd'hui. sources de rayons lumineux.

"En quelques heures seulement, le LCLS-II produira plus d'impulsions de rayons X que le laser actuel n'en a généré pendant toute sa durée de vie", déclare Mike Dunne, directeur du LCLS. "Des données qui auraient autrefois pris des mois à être collectées pourraient être produites en quelques minutes. Cela fera passer la science des rayons X à un niveau supérieur, ouvrant la voie à une toute nouvelle gamme d'études et faisant progresser notre capacité à développer des technologies révolutionnaires pour répondre à certaines des les défis les plus profonds auxquels notre société est confrontée."

Grâce à ces nouvelles capacités, les scientifiques peuvent examiner les détails de matériaux complexes avec une résolution sans précédent pour piloter de nouvelles formes d'informatique et de communication ; révéler des événements chimiques rares et éphémères pour nous apprendre à créer des industries plus durables et des technologies énergétiques propres ; étudier comment les molécules biologiques remplissent les fonctions de la vie pour développer de nouveaux types de produits pharmaceutiques; et découvrez le monde bizarre de la mécanique quantique en mesurant directement les mouvements d'atomes individuels.

Un exploit effrayant

LCLS, le premier laser à rayons X durs à électrons libres (XFEL) au monde, a produit sa première lumière en avril 2009, générant des impulsions de rayons X un milliard de fois plus lumineuses que tout ce qui avait précédé. It accelerates electrons through a copper pipe at room temperature, which limits its rate to 120 X-ray pulses per second.

In 2013, SLAC launched the LCLS-II upgrade project to boost that rate to a million pulses and make the X-ray laser thousands of times more powerful. For that to happen, crews removed part of the old copper accelerator and installed a series of 37 cryogenic accelerator modules, which house pearl-like strings of niobium metal cavities. These are surrounded by three nested layers of cooling equipment, and each successive layer lowers the temperature until it reaches nearly absolute zero—a condition at which the niobium cavities become superconducting.

"Unlike the copper accelerator powering LCLS, which operates at ambient temperature, the LCLS-II superconducting accelerator operates at 2 Kelvin, only about 4 degrees Fahrenheit above absolute zero, the lowest possible temperature," said Eric Fauve, director of the Cryogenic Division at SLAC. "To reach this temperature, the linac is equipped with two world-class helium cryoplants, making SLAC one of the significant cryogenic landmarks in the U.S. and on the globe. The SLAC Cryogenics team has worked on site throughout the pandemic to install and commission the cryogenic system and cool down the accelerator in record time."

One of these cryoplants, built specifically for LCLS-II, cools helium gas from room temperature all the way down to its liquid phase at just a few degrees above absolute zero, providing the coolant for the accelerator.

On April 15, the new accelerator reached its final temperature of 2 K for the first time and today, May 10, the accelerator is ready for initial operations.

"The cooldown was a critical process and had to be done very carefully to avoid damaging the cryomodules," said Andrew Burrill, director of SLAC's Accelerator Directorate. "We're excited that we've reached this milestone and can now focus on turning on the X-ray laser."

Bringing it to life

In addition to a new accelerator and a cryoplant, the project required other cutting-edge components, including a new electron source and two new strings of undulator magnets that can generate both "hard" and "soft" X-rays. Hard X-rays, which are more energetic, allow researchers to image materials and biological systems at the atomic level. Soft X-rays can capture how energy flows between atoms and molecules, tracking chemistry in action and offering insights into new energy technologies. To bring this project to life, SLAC teamed up with four other national labs—Argonne, Berkeley Lab, Fermilab and Jefferson Lab—and Cornell University.

Jefferson Lab, Fermilab and SLAC pooled their expertise for research and development on cryomodules. After constructing the cryomodules, Fermilab and Jefferson Lab tested each one extensively before the vessels were packed and shipped to SLAC by truck. The Jefferson Lab team also designed and helped procure the elements of the cryoplants.

"The LCLS-II project required years of effort from large teams of technicians, engineers and scientists from five different DOE laboratories across the U.S. and many colleagues from around the world," says Norbert Holtkamp, SLAC deputy director and the project director for LCLS-II. "We couldn't have made it to where we are now without these ongoing partnerships and the expertise and commitment of our collaborators."

Toward first X-rays

Now that the cavities have been cooled, the next step is to pump them with more than a megawatt of microwave power to accelerate the electron beam from the new source. Electrons passing through the cavities will draw energy from the microwaves so that by the time the electrons have passed through all 37 cryomodules, they'll be moving close to the speed of light. Then they'll be directed through the undulators, forcing the electron beam on a zigzag path. If everything is aligned just right—to within a fraction of the width of a human hair—the electrons will emit the world's most powerful bursts of X-rays.

This is the same process that LCLS uses to generate X-rays. However, since LCLS-II uses superconducting cavities instead of warm copper cavities based on 60-year-old technology, it can can deliver up to a million pulses per second, 10,000 times the number of X-ray pulses for the same power bill.

Once LCLS-II produces its first X-rays, which is expected to happen later this year, both X-ray lasers will work in parallel, allowing researchers to conduct experiments over a wider energy range, capture detailed snapshots of ultrafast processes, probe delicate samples and gather more data in less time, increasing the number of experiments that can be performed. It will greatly expand the scientific reach of the facility, allowing scientists from across the nation and around the world to pursue the most compelling research ideas. + Explorer plus loin

Upgraded X-ray laser shows its soft side