Compact synchrotron source: The electron laser for the laboratory bench

Synchrotron radiation is an indispensable tool for researchers in all disciplines who want to elucidate the structure and properties of materials and material samples.

It is electromagnetic radiation with wavelengths from infrared to X-rays with high luminosity and brilliance.

It is usually generated by electrons that are forced into curved paths, for example when circulating in a storage ring or through a slalom course made up of dipole magnets of alternating polarity arranged one behind the other – so-called undulators.

Regardless of how synchrotron light is generated, all methods require high-energy electron beams that have to be revved up with large ring-shaped or linear particle accelerators.

Manfred Lindinger

Editor in the department "Nature and Science".

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With a length of 3.4 kilometers, the free-electron laser XFEL in Hamburg is the world's largest facility for synchrotron radiation in the X-ray range.

The associated particle accelerator alone extends over a length of around two kilometers.

Because of the size, and above all because of the high costs of modern synchrotron sources, ways are being sought to generate synchrotron light with less effort, especially with significantly smaller systems.

As a result, the coveted radiation could ultimately also be generated in the laboratory and made available, for example, to material researchers or biologists for their on-site analyses.

The so-called plasma accelerator opens up one possibility.

This promising approach is based on theoretical considerations from the 1970s.

Here, electrons are not accelerated with strong and high-frequency electric fields, but with plasma waves.

In this way, compact particle accelerators can be realized that actually fit comfortably on a laboratory bench.

The plasma itself is usually generated and excited via a gas discharge and an intense laser beam that is aimed at gas atoms.

High-power lasers are usually used for this, which again increases the technical effort.

However, the plasma can also be ignited more simply via a gas discharge and excited with electrons.

An Italian research group from the Laboratori Nazionali di Frascati is taking this route.

The heart of their plasma accelerator is a three centimeter long and two millimeter thin tube filled with hydrogen gas.

A short, high voltage pulse of several thousand volts tears the atoms into their component parts, and a hot plasma of electrons and protons is created.

A radiated green laser beam from an Nd-YAG laser stabilizes the ionized gas.

Slalom through a magnetic field course

Two bunches of electrons, which are ejected from a copper electrode by short laser pulses, arrive in quick succession inside the tube after a flight distance of a few centimeters with an initial energy of almost 88 billion electron volts (MeV).

The first bunch of electrons acts as a kind of driver and causes the freely moving electrons of the plasma to oscillate.

In the process, strong electric fields build up.

These give the incoming electrons of the second bunch an additional boost of a good six MeV along the way.

The result is an electron pulse of around 94 MeV.

For comparison: Common particle accelerators with their high-frequency cavity resonators only manage 30 to 100 MeV per meter of acceleration distance.

As Riccardo Pompili and his colleagues report in the journal "Nature", they were able to get two problems of plasma accelerators under control: the energy stability and the quality of the accelerated electron beams.

The average energy of the particles fluctuated by only 0.2 percent over 500 shots.

The reason for the fluctuation was not the fluctuating plasma, as is often the case, but the varying time interval between the electron bunches.

In their most recent experiment, Pompili and his colleagues have shown that the bundles of electrons can already be used to generate coherent synchrotron radiation, as in a classic free-electron laser, due to their high quality.

They let the charged particles run in a slalom through five undulators about two meters long.

With every change in direction of the electrons, induced by the changing magnetic dipole fields, infrared synchrotron light was created.

The researchers working with Pompili bundled it into an intense beam with a wavelength of 820 nanometers and examined it with a spectrometer.

With electron energies of a few hundred electron volts - which are definitely possible with laser-plasma technology - the Italian researchers want to advance into the blue and ultraviolet spectral range.

For this purpose, the plasma accelerator is to be optimized.

Their goal is electric fields in the hydrogen plasma of one gigavolt per meter.

They want to achieve this with a plasma tube that is forty centimeters long.

It is uncertain whether the structure developed by the researchers from Frascati can ever compete with the large synchrotron facilities.

To do this, it would have to be able to run around the clock and generate higher pulse rates.

This is a major challenge for all researchers working on plasma accelerators, but not an unsolvable problem, as physicists from the Desy research center in Hamburg recently showed.

They've managed to keep a plasma-powered particle accelerator running non-stop for thirty hours.