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The experimental mini-accelerator achieves record power.



The experimental mini-accelerator achieves record power.

The two-stage miniature accelerator is operated with terahertz radiation (shown here in red). In a first step (left) the groups of electrons (shown in blue) are compressed, in a second step (right) they are accelerated. The two individual elements each have approximately two centimeters in width. Credit: DESY, Gesine Born

DESY scientists have reached a new world record for an experimental type of miniature particle accelerator: for the first time, an accelerator with terahertz power more than doubled the energy of the injected electrons. At the same time, the configuration significantly improved the electron beam quality compared to previous experiments with the technique, as did Dongfang Zhang and his colleagues at the Center for Laser Science of Free Electrons (CFEL) in the DESY report on Magazine Optics. "We have achieved the best beam parameters so far for terahertz accelerators," Zhang said.

"This result represents a critical step for the practical implementation of accelerators with terahertz power", emphasized Franz Kärtner, who heads the ultra-fast optical and X-ray group at DESY. Terahertz radiation is among the infrared and microwave frequencies in the electromagnetic spectrum and promises a new generation of compact particle accelerators. "The wavelength of the terahertz radiation is approximately one hundred times shorter than the radio waves currently used to accelerate the particles," explained Kärtner. "This means that the accelerator components can also be built to be a hundred times smaller." The terahertz approach promises laboratory-sized accelerators that will allow completely new applications, for example, as compact X-ray sources for materials science and perhaps even for medical imaging. The technology is currently in development.

As terahertz waves oscillate so fast, each component and each step must be precisely synchronized. "For example, to achieve the best energy gain, the electrons have to hit the terahertz field exactly during its acceleration half cycle," Zhang explained. In accelerators, the particles do not usually fly in a continuous beam, but are packaged in clusters. Due to the rapid change of field, in terahertz accelerators these groups must be very short to guarantee uniform acceleration conditions throughout the group.

"In previous experiments, the electron clusters were too long," Zhang said. "Since the terahertz field oscillates so rapidly, some of the electrons in the group accelerated, while others even slowed down, so in total there was only a moderate average energy gain and, most importantly, a broad distribution of energy., resulting in what we call poor beam quality. " To make matters worse, this effect greatly increased the emission, a measure of how well a particle beam is transversely grouped. The tighter the better, the lower the emission.

To improve beam quality, Zhang and his colleagues built a two-step accelerator from a multipurpose device they had previously developed: the accelerator and segmented terahertz electron manipulator (STEAM) can compress, focus, accelerate and analyze packages of electrons with terahertz radiation. The researchers combined two STEAM devices online. First they compressed the groups of incoming electrons from approximately 0.3 millimeters in length to only 0.1 millimeters. With the second STEAM device, the compressed clusters accelerated. "This scheme requires control at the level of quadrillions of a second, which we achieved," said Zhang. "This led to a fourfold reduction in energy distribution and improved the emission six times, producing the best beam parameters of a terahertz accelerator so far."

The net energy gain of the electrons that were injected with an energy of 55 kiloelectron volts (keV) was 70 keV. "This is the first increase in energy over 100 percent in an accelerator with terahertz power," Zhang said. The coupled device produced an acceleration field with a maximum intensity of 200 million volts per meter (MV / m), near the most powerful and advanced conventional accelerators. For practical applications, this still needs to be significantly improved. "Our work shows that even three times the compression of the electron groups is possible, along with a higher terahertz energy, acceleration gradients in the Gigavolt-per-meter regime seem feasible," Zhang summarized. "The concept of teraherties seems to be increasingly promising as a realistic option for the design of compact electron accelerators."


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More information:
Dongfang Zhang et al, Phase control of femtoseconds in ultrafast electron sources driven by high field terahertz, Optics (2019). DOI: 10.1364 / OPTICA.6.000872

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The experimental mini-accelerator achieves record power (2019, July 11)
recovered on July 12, 2019
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