Kredit:SLAC National Accelerator Laboratory
Inbäddat 30 fot under jorden i Menlo Park, Kalifornien, är en halv mil lång tunnelsträcka nu kallare än större delen av universum. Den rymmer en ny supraledande partikelaccelerator, en del av ett uppgraderingsprojekt till Linac Coherent Light Source (LCLS) röntgenfri elektronlaser vid Department of Energys SLAC National Accelerator Laboratory.
Besättningar har framgångsrikt kylt acceleratorn till minus 456 grader Fahrenheit - eller 2 Kelvin - en temperatur vid vilken den blir supraledande och kan öka elektroner till höga energier med nästan noll energi som går förlorad i processen. Det är en av de sista milstolparna innan LCLS-II kommer att producera röntgenpulser som är 10 000 gånger ljusare i genomsnitt än LCLS och som kommer upp till en miljon gånger per sekund – ett världsrekord för dagens mest kraftfulla röntgenpulser. strålljuskällor.
"På bara några timmar kommer LCLS-II att producera fler röntgenpulser än vad den nuvarande lasern har genererat under hela sin livstid", säger Mike Dunne, chef för LCLS. "Data som en gång kan ha tagit månader att samla in skulle kunna produceras på några minuter. Det kommer att ta röntgenvetenskapen till nästa nivå, bana väg för ett helt nytt utbud av studier och främja vår förmåga att utveckla revolutionerande teknologier för att hantera några av de djupaste utmaningarna som vårt samhälle står inför."
Med dessa nya funktioner kan forskare undersöka detaljerna i komplexa material med oöverträffad upplösning för att driva nya former av datoranvändning och kommunikation; avslöja sällsynta och flyktiga kemiska händelser för att lära oss hur man skapar mer hållbara industrier och ren energiteknik; studera hur biologiska molekyler utför livets funktioner för att utveckla nya typer av läkemedel; och kika in i kvantmekanikens bisarra värld genom att direkt mäta enskilda atomers rörelser.
En skrämmande bedrift
LCLS, världens första hårda röntgenfrielektronlaser (XFEL), producerade sitt första ljus i april 2009 och genererade röntgenpulser en miljard gånger starkare än något som hade kommit tidigare. Den accelererar elektroner genom ett kopparrör vid rumstemperatur, vilket begränsar dess hastighet till 120 röntgenpulser per sekund.
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. + Utforska vidare
