Atomic ‘Trojan horse’ for a new generation of X-ray lasers

Provided by University of Strathclyde

An intense electron beam that could be used in the X-ray lasers of the future has been produced in research led at the University of Strathclyde.

The beam was created by the plasma photocathode method, in which electrons were released from neutral atoms inside plasma. This has produced a potentially much brighter, plasma-based electron source which could be used in more compact, more powerful particle accelerators.

The research has been carried out as part of the ‘E-210: Trojan Horse’ experiment at the Facility for Advanced Accelerator Experimental Tests (FACET) at the Stanford Linear Accelerator Center in California, and is now published in Nature Physics.

Professor Bernhard Hidding, of Strathclyde’s Department of Physics, principal investigator in the study, said: “Our experiment demonstrates the feasibility of one of the most promising methods for next-generation electron sources and could push the boundaries of today’s technology by orders of magnitude.”

According to calculations by Professor Hidding and his research colleagues, the Trojan Horse technique could make electron beams 100 to 10,000 times brighter than today’s most powerful beams. Brighter electron beams would also make future X-ray lasers brighter and further enhance their scientific capabilities.

In current state-of-the-art accelerators, electrons are generated by shining laser light onto a metallic photocathode, which displaces electrons from the metal. These electrons are then accelerated inside metal cavities, where they draw more and more energy from a radiofrequency field, resulting in a high-energy electron beam. In X-ray lasers, this electron beam drives the production of extremely bright X-ray light.


However, metal cavities can support only a limited energy gain over a given distance, or acceleration gradient, before breaking down, and therefore accelerators for high-energy beams become very large and expensive.

In recent years, scientists have looked into ways to make accelerators more compact. They demonstrated, for example, that they can replace metal cavities with plasma that allows much higher acceleration gradients, potentially shrinking the length of future accelerators by 100 to 1,000 times. This is a central thrust of the Scottish Centre for the Application of Plasma-based Accelerators (SCAPA) at the University of Strathclyde. The new paper expands the plasma concept to the electron source of an accelerator.

First, the research team sent laser light into a mixture of hydrogen and helium gas. The light had just enough energy to strip electrons off hydrogen, turning neutral hydrogen into plasma. It was not, however, energetic enough to do the same with helium, which has electrons that are more tightly bound than those of hydrogen, and so it stayed neutral inside the plasma.


The scientists then shot an electron bunch through the plasma, where it produced a plasma wake, much like a motorboat creates a wake when it glides through the water. Trailing electrons can ‘surf’ the plasma wake and gain tremendous amounts of energy.

A second laser pulse was then flashed into the plasma, this time intense enough to liberate electrons from helium. If the timing is right, the ultracold helium electrons from this plasma photocathode are then rapidly captured by the plasma wave and produce a new, much brighter beam of electrons.

The researchers have also developed several auxiliary techniques, which allow them now improving the quality and stability of their output beams, and to harness the technique for applications. A central experimental pathway for this will be the “E-310: Trojan Horse-II” flagship collaboration at SLAC’s follow-up facility FACET-II, complemented by R&D in the UK at SCAPA and Daresbury Lab’s CLARA, and in Europe in context of the H2020 effort EuPRAXIA.


In a forward-looking, complementary project funded by the Science and Technology Facilities Council, Professor Hidding and colleagues are already exploring the benefits to be expected from ultrabright and ultrashort electron beams for X-ray free electron lasers and other light sources. These may allow production of X-ray pulses short and bright enough to allow observation of electronic motion inside atoms and molecules on their natural timescale.

Other partners involved in the experimental collaboration were: the University of California, Los Angeles; RadiaBeam Technologies; RadiaSoft LLC; Tech-X; DESY; the University of Colorado Boulder; the University of Oslo; the University of Texas at Austin and the Cockcroft Institute.