How do researchers explore nature at its most basic level? They build "super microscopes" that can solve atomic and subatomic details. This does not work with visible light, but they can investigate the dimensions of the smallest matter with electron beams, either by using them directly in particle colliders or by converting their energy to light X-rays in X-ray lasers. At the heart of such scientific discovery machines are particle accelerators that first generate electrons at a source and then increase their energy in a series of accelerator cavities.
Now, an international team of scientists, including scientists from the Department of Energy's SLAC National Accelerator Laboratory, has shown a potentially much lighter plasma-based electron source that can be used in more compact, more powerful particle accelerators.
The method, in which the electrons for the beam are released from neutral atoms inside the plasma, is called the Trojan horse technique because it is reminiscent of how the ancient Greeks are said to have invaded the city of Troja by hiding their powerful soldiers (electrons) inside a wooden horse (plasma), which then was pulled into the city (accelerator).
"Our experiment shows for the first time that the Trojan horse method actually works," says Bernhard Hidding of the University of Strathclyde in Glasgow, Scotland, the principal investigator of a study published today in Nature Physics . "It is one of the most promising methods for future power sources and can push the limits of today's technology."
Replacement of metal with plasma
In current state-of-the-art accelerators, electrons are generated by shining laser light on a metallic photo cathode, which kicks electrons out of the metal. These electrons are then accelerated into metal cavities, where they draw more and more energy from a radio frequency field, resulting in a high-energy electron beam. In X-ray lasers, such as SLAC's Linac Coherent Light Source (LCLS), the beam drives the production of extremely strong X-rays.
But metal cavities can only support a limited energy gain over a certain distance, or acceleration gradient, before decomposing, and therefore high-energy beam accelerators become very large and expensive. In recent years, researchers at SLAC and elsewhere have investigated ways to make accelerators more compact. For example, they showed that they can replace plasma metal cavities that allow much higher acceleration gradients, potentially shrinking the length of future accelerators 1
The new paper extends the plasma concept of the electron source to an accelerator. .
"We have previously shown that plasma acceleration can be extremely powerful and efficient, but we have not yet been able to produce rays of sufficient quality for future applications," says co-author Mark Hogan of SLAC. "Improving radiation quality is a top priority for the coming years, and developing new types of electron sources is an important part of that."
According to earlier calculations by Hidding and colleagues, Trojan horse technology would make electron beams 100 to 10,000 times lighter than today's most powerful beams. Lighter electron beams would also brighten future X-ray lasers and further enhance their scientific capabilities.
"If we can marry the two big shocks – high plasma acceleration gradients and plasma radiation creation – we could build X-ray lasers that develop the same force over a distance of a few meters rather than kilometers," says co-author James Rosenzweig, the principal investigator for the Trojan horse project at the University of California, Los Angeles.
Producing superior electron beams
The researchers conducted their experiment at SLAC & # 39; s Facility for Advanced Accelerator Experimental Tests (FACET). The facility, which is currently undergoing a major upgrade, generates pulses of highly energetic electrons for research on next-generation accelerator technologies, including plasma acceleration.
First, the team flashed laser light to a mixture of hydrogen and helium gas. The light had just enough energy to remove electrons from hydrogen and turn neutral hydrogen into plasma. However, it was not enough energy to do the same with helium, whose electrons are more tightly bound than hydrogen, so it remained neutral within the plasma.
Subsequently, the researchers sent one of FACET's electron bundles through plasma, where it produced a plasma surge, just as a motor boat creates an awake as it glides through the water. Subsequent electrons can "surf" after the guard and receive huge amounts of energy.
In this study, the subsequent electrons came from within the plasma (see animation above and the film below). Just as the electron group and its wake passed, scientists dropped the helium in plasma with a second, tightly focused laser flash. This time, the light pulse had enough energy to kick electrons out of the helium atoms, and the electrons were then accelerated in the wake.
The synchronization between the electron group and rushing through plasma at almost the speed of light and the laser flash, which lasted only a few million million seconds, was particularly important and challenging, says UCLA's Aihua Deng, one of the study's lead authors: "If lightning comes too soon the electrons that it produces interfere with the formation of the Plasma Monitor. If it arrives too late, the Plasma Monitor has progressed and the electrons will not be accelerated. "
The researchers estimate that the brightness of the electron beam obtained with the Trojan horse method can already compete with the brightness of the existing mother electron sources.
"What makes our technology transformative is how the electrons are produced," said Oliver Karger, the second major author, who was at the University of Hamburg, Germany, at the time of study. When the electrons are removed from helium, they rapidly accelerate in the forward direction, which keeps the beam narrowly bound and is a prerequisite for lighter beams.
More R&D work ahead
But before applications such as compact X-ray lasers can become reality, much more research needs to be done.
Then the researchers want to improve the quality and stability of their beam and work with better diagnostics that allow them to measure the brightness of the actual beam, instead of estimating it.
This development will be made when the FACET upgrade, FACET-II, is complete. "The experiment is based on the ability to use a strong electron beam to produce plasma monitoring," said Vitaly Yakimenko, head of SLAC's FACET division. "FACET-II will be the only place in the world that will produce such rays of sufficient intensity and energy."
Other partners participating in the project were Sci-Tech Daresbury, UK; the German research center DESY; University of Colorado Boulder; University of Oslo, Norway; University of Texas at Austin; RadiaBeam Technologies; RadiaSoft LLC; and Tech-X Corporation. Much of this work was funded by the DOE Office of Science. LCLS and FACET are DOE Office of Science user facilities.
Citation: A. Deng, O. Karger, et al., Nature Physics, August 12, 2019 (10.1038 / s41567-019-0610-9)
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