SLAC researchers have found a way to increase the brightness and power of X-ray lasers

Despite great advances in X-ray free-electron laser science over the past decade, future applications still require fully coherent, stable X-rays, which the existing X-ray FEL facilities have yet to demonstrate.

At first glance, it seems impossible to capture X-rays using a mirror. But with the necessary storage equipment and creative thinking, such a concept is feasible on high-repetition rate accelerators like LCLS-II.

The researchers studied cavity based X-ray free electron lasers. In this concept, incoherent X-ray pulses produced by accelerator facilities, such as SLAC's, are captured by hollow structures that may be hundreds of meters long or possibly more than a kilometer long.

Now, researchers from the Department of Energy's SLAC National Accelerator Laboratory have calculated how XFEL's X-ray pulses can be made brighter and more reliable by building special chambers and diamond mirrors around it.

The researchers used a complex system of crystal cavities and mirrors without the need for very long and complex cavities.

SLAC scientist and co-author Jingyi Tang said: "The motivation for producing coherent, higher-brightness X-rays is to study real-world materials and what happens to those materials under different conditions. We want to work on systems that are more dynamic and hard to capture."

Inside the cavity, X-rays are reflected off four diamond mirrors that transmit X-ray pulses in the form of rectangular rings. The next beam of electrons inside the accelerator travels towards them while pulses run through the cavity.

When the electron beam arrives, the bouncing X-ray pulse reacts, tightening it and lining it up. When twisted in the unshaker, this tighter clump of electrons produces more coherent, brighter X-rays below the accelerator.

Before the new calculations, experts believed that maintaining the power of X-ray pulses as they bounced around the cavity would require a closely spaced electron beam or a cavity several kilometers long, making the concept more difficult to implement.

SLAC scientist and co-author Zhen Zhang said, "We show that even with powerful XFEL operating at a lower repetition rate, high-quality cavity systems may only need to be 100-300 meters long, which means more space between electron beams."

The key to this novel design is what management experts call the cavity quality factor Q. The reflectivity of the mirror in the cavity is represented by a quality factor. A high Q value indicates a very high reflectivity, allowing the X-ray energy to be recirculated within the cavity with minimal loss. A low Q value indicates reduced reflectivity, which means that a significant portion of the X-rays leave the cavity and travel along the accelerator.

When the X-rays are recirculated in a shorter cavity without interacting with the electron beam, the Q value remains very high. When these X-rays come into contact with the incoming electron beam, the researchers can carefully adjust the amplified X-ray wavelength and spectrum to change the cavity Q value, a process called Q-switching. This means that when the X-rays have enough power to escape the cavity and travel along the accelerator to the experiment, they can reduce the Q value.

By adjusting the Q value, the researchers can circulate coherent X-ray pulses around the cavity and mirror system multiple times. Since coherent X-ray pulses can now travel through the system with minimal loss, they have more time to build up power, reduce the required cavity length and produce X-rays with high output power.

The scientists noted that "the initial goal of the experiment was to demonstrate the increased power of the X-rays after they were recirculated through the cavity and to observe the performance of the cavity." After the initial objectives of the experiment have been achieved, Q-switches can also be tested on such CBXFEL systems."

Source: Laser Network