Aninhado a 30 pés de profundidade em Menlo Park, Califórnia, um trecho de 800 metros de comprimento de túnel é agora mais frio do que a maior parte do universo. Ele abriga um novo acelerador de partículas supercondutoras, parte de um projeto de atualização para o laser de elétrons livres de raios-X Linac Coherent Light Source (LCLS) no Laboratório Nacional de Aceleradores SLAC do Departamento de Energia.
As equipes resfriaram com sucesso o acelerador a menos 456 graus Fahrenheit - ou 2 Kelvin - uma temperatura na qual ele se torna supercondutor e pode impulsionar os elétrons para altas energias com quase zero de energia perdida no processo. É um dos últimos marcos antes que o LCLS-II produza pulsos de raios X que são 10.000 vezes mais brilhantes, em média, do que os do LCLS e que chegam até um milhão de vezes por segundo - um recorde mundial para o X-X mais poderoso de hoje. fontes de luz de raios.

"Em apenas algumas horas, o LCLS-II produzirá mais pulsos de raios X do que o laser atual gerou em toda a sua vida útil", diz Mike Dunne, diretor do LCLS. "Dados que antes levavam meses para serem coletados podem ser produzidos em minutos. Levará a ciência dos raios X para o próximo nível, abrindo caminho para toda uma nova gama de estudos e avançando nossa capacidade de desenvolver tecnologias revolucionárias para abordar alguns dos os desafios mais profundos que a nossa sociedade enfrenta."

Com esses novos recursos, os cientistas podem examinar os detalhes de materiais complexos com resolução sem precedentes para impulsionar novas formas de computação e comunicação; revelar eventos químicos raros e fugazes para nos ensinar como criar indústrias mais sustentáveis ​​e tecnologias de energia limpa; estudar como as moléculas biológicas realizam as funções da vida para desenvolver novos tipos de produtos farmacêuticos; e espie o mundo bizarro da mecânica quântica medindo diretamente os movimentos de átomos individuais.

Um feito arrepiante

O LCLS, o primeiro laser de elétrons livres de raios X rígidos (XFEL) do mundo, produziu sua primeira luz em abril de 2009, gerando pulsos de raios X um bilhão de vezes mais brilhantes do que qualquer coisa que havia antes. Ele acelera os elétrons através de um tubo de cobre à temperatura ambiente, o que limita sua taxa a 120 pulsos de raios X por segundo.

Em 2013, o SLAC lançou o projeto de atualização LCLS-II para aumentar essa taxa para um milhão de pulsos e tornar o laser de raios X milhares de vezes mais poderoso. 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. + Explorar mais

Upgraded X-ray laser shows its soft side