Traditional proton accelerators, like the Large Hadron Collider at CERN, rely on strong radio frequency waves to increase particle speed. But the method used at HZDR is different. By using ultra-bright light pulses, protons receive a significant boost. The technique involves shooting intense, short laser pulses at thin metal foils, which then heat the material so intensely that electrons are emitted in vast numbers, leaving the heavy atomic nuclei behind. The resulting strong electric field between the negatively charged electrons and positively charged nuclei can then produce a powerful proton pulse over a minimal distance.
"The benefit of laser acceleration is that it enables us to concentrate a significant number of particles into a single proton bunch," notes HZDR physicist Dr. Karl Zeil. "Such an approach could have notable applications in the radiation therapy of tumors."
However, the existing approach of using metal foils as laser targets presents certain challenges. Not only is it challenging to generate multiple proton pulses per second, as the foil gets destroyed by a single laser shot and needs frequent replacements, but the acceleration process itself is complex and challenging to manage. As Zeil explains, the protons that are to be accelerated come from hydrocarbons that accumulate as contaminants on the metal foils, making precise control of the experiment problematic.
To address these issues, Zeil and his German-American research team proposed an alternative. "We decided to use a fine, strongly cooled hydrogen jet instead of a metal foil," Zeil explains. The hydrogen gas, cooled within a copper block until it liquefies, flows through a nozzle into a vacuum chamber where it cools further, solidifying into a micrometer-thin filament that acts as a target for the laser pulses. Since the hydrogen filament constantly renews itself, the laser consistently has a new, intact target for every shot.
This setup also promotes a more beneficial acceleration mechanism. Rather than merely heating the material, the laser pulses exert radiation pressure to expel the electrons from the hydrogen, generating the intense electric fields required to accelerate the protons. The team found that they could optimize this process by sending a weak, short light pulse ahead of the main laser pulse. This pre-heats the frozen hydrogen filament, causing it to expand and increase in cross-section size, thereby enhancing the acceleration distance and process efficiency.
"We were able to bring protons up to an energy of 80 MeV," Zeil reports, "close to the previous record for laser proton acceleration. Yet, unlike earlier facilities, our technique could generate several proton bunches per second." In addition, simulating the acceleration process with hydrogen targets proved easier using high-performance computing. The team also harnessed the computational power of the Center for Advanced Systems Understanding (CASUS) at HZDR to improve their understanding of the laser-material interaction, with future plans to employ AI algorithms to increase the accuracy of the laser pulses hitting the frozen hydrogen jet.
The research offers promising implications for radiation therapy, potentially increasing dosage while reducing irradiation time. Furthermore, according to an HZDR study, this method could better protect healthy tissue surrounding tumors, marking a significant step forward in cancer treatment.
Research Report:Ultra-short pulse laser acceleration of protons to 80 MeV from cryogenic hydrogen jets tailored to near-critical density
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