Ultracold electron source for a compact X-ray source

In numerous fields, X-rays play a key role in terms of experimentation and measurement. For instance, in materials science, X-rays are used to investigate the structure and composition of new materials at the atomic and molecular level.

In biomedical research, x-ray imaging techniques such as x-ray crystallography and microscopy are used in the development of drugs and to understand the diseases they should fight.

In addition, environmental sciences, energy and fundamental physics research, archaeology and cultural heritage, and advanced manufacturing all benefit greatly from the advantages of x-ray diagnostics.

Demand for X-rays

In this moment in time, X-rays needed for investigative and diagnostic techniques are only available at large complexes called synchrotrons and x-ray free-electron lasers. These are among the most complex and high-tech research facilities that have been devised by humans, but are extremely expensive to construct, to maintain, and to operate.

As a result, there are only around 70 such facilities operational around the world and they are in extremely high demand, possibly resulting in waiting times of months.

Smart*Light

For his thesis, Daniel Nijhof looked at the development of a compact system known as Smart*Light that can produce x-rays with similar properties as those produced in synchrotrons, albeit with a lower intensity.

Bringing these types of X-rays to labs, museums, and incorporating these systems in manufacturing plants could bring great advantages to the various disciplines.

The generation of x-rays in this new system is realized through the interaction between short, relativistic electron bunches and high-intensity laser pulses. In this interaction, the electrons in the bunch collide with the laser pulses and cause an upshift in their frequency, analogous to the frequency shift of an ambulance’s siren when it is approaching.

Even though this mechanism has been known for a century, only recently have the necessary advancements in accelerator and laser technology matured to the required level that producing x-rays in this fashion has become worthwhile.

Relativistic velocities

To generate enough x-rays for this system to be viable, the electrons must be accelerated to relativistic velocities.

This is achieved using a custom-made radio-frequency (RF) linear accelerator (LINAC), adapted from existing LINAC structures designed by CERN for the Compact Linear Collider project.

In this structure, the electron bunches enter with a velocity of 55% of the speed of light and exit with a velocity that is more than 99.9% of the speed of light. In this work, Nijhof and his colleagues realized the construction of the electron beamline and preparation of the RF LINAC. Furthermore, acceleration of the electron bunches to speeds greater than 99% the speed of light has been demonstrated.

Modularity

What sets the Smart*Light project apart from other compact X-ray devices is the modularity of the design. Additional LINAC sections can be added to further increase the final energy of the electron bunches, increasing the range of achievable x-ray energy with it.

Furthermore, the source of the electron bunches can be swapped with other sources that similarly produce electrons travelling at 55% the speed of light.

This constitutes another part of the work presented in the thesis of Nijhof, where the additional acceleration of electron bunches produced from a laser-cooled and trapped atomic gas was explored.

He demonstrated that electron bunches produced in this way at velocities of roughly 15% the speed of light can be further accelerated properly by RF techniques.

Based on this, a dedicated RF structure has been designed that should be capable of accelerating these bunches to the required velocity of 55% the speed of light. The prospect of combining such a source with the Smart*Light electron beamline is very interesting and fits with the Smart*Light philosophy of generating high quality X-rays at reduced intensities.

Title of PhD thesis: Ultracold electron source development and X-band acceleration for a compact ICS-based x-ray source. Supervisors: Jom Luiten and Peter Mutsaers.