Experimental mini-accelerator achieves record energy

DESY scientists set a new world record for an experimental type miniature particle accelerator: for the first time, a terahertz-powered accelerator more than doubled the energy of the injected electrons. At the same time, the configuration has significantly improved the quality of the electron beam compared to previous experiments with this technique, as reported by Dongfang Zhang and colleagues at the Center for Free Electron Laser Science (CFEL) of DESY in the newspaper Optica. "We have achieved the best beam parameters ever obtained for terahertz accelerators," Zhang said.

"This result represents a breakthrough for the practical implementation of terahertz accelerators," said Franz Kärtner, director of the high-speed optical and X-ray group at DESY. The terahertz radiation falls between 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 about a hundred times shorter than the radio waves currently used to accelerate the particles," Kärtner explained. "This means that the accelerator components can also be designed to be about a hundred times smaller." The terahertz approach promises accelerators the size of a laboratory that will allow totally new applications, for example as compact X-ray sources for materials science and perhaps even for the first time. medical imaging. The technology is under development.

Since terahertz waves oscillate so fast, each component and each step must be synchronized precisely. "For example, to get the best energy gain, the electrons must hit the terahertz field exactly during its half cycle of acceleration," Zhang explained. In accelerators, particles do not usually fly in a continuous beam, but are packed in packets. Due to the rapid evolution of the field, in terahertz accelerators, these clusters must be very short to ensure uniform acceleration conditions along the cluster.

"In previous experiments, the electron packets were too long," Zhang said. "Since the terahertz field is oscillating so fast, some of the electrons in the group have been accelerated, while others have even been slowed down, so there was only a slight gain in the number of electrons in the group. Medium energy and, more importantly, a wide dispersion of energy, resulting in what we call poor beam quality. "To make matters worse, this effect has greatly increased the emittance, a measuring the quality of the transverse beam of a particle beam. The closer the tightening is, the better the emittance.

To improve the quality of the beam, Zhang and his colleagues built a two-step accelerator from a multifunctional device that they had previously developed: The segmented terahertz electron accelerator and manipulator. (STEAM) can compress, focus, accelerate and analyze electron packets with terahertz radiation. . The researchers combined two STEAM devices online. They first compressed the incoming electron packets from a length of about 0.3 millimeters to just 0.1 millimeters. With the second STEAM device, they accelerated the compressed clusters. "This scheme requires quadrillion level control of hundredths of a second, which we have achieved," said Zhang. "This led to a fourfold reduction in energy dispersion and a sixfold increase in the emittance factor, producing the best beam parameters of a terahertz accelerator."

The net energy gain of electrons injected with an energy of 55 kiloelectron volts (keV) was 70 keV. "It's the first 100% increase in energy in a terahertz-powered accelerator," Zhang said. The coupled device produced an acceleration field with a maximum force of 200 million volts per meter (MV / m) – close to the most powerful conventional accelerators on the market. For practical applications, this still needs to be significantly improved. "Our work shows that even more than three times stronger compression of electron packets is possible.With a higher terahertz energy, acceleration gradients in the regime of gigavolts per meter seem feasible Zhang summarized. "The concept of terahertz therefore appears more and more as a realistic option for designing compact electron accelerators.