Posted: September 29, 2009

Miniature X-ray source with undulating electrons

(Nanowerk News) The potential of laser technology seems inexhaustible. Fresh proof has now been presented by an international team at Munich’s Cluster of Excellence “Munich Centre for Advanced Photonics” (MAP), in the Laboratory for Attosecond Physics (LAP) of Ludwig-Maximilians-Universität (LMU) in Munich and Max Planck Institute of Quantum Optics (MPQ) in Garching. Also involved were Dresden-Rossendorf research Centre and Oxford University’s Clarendon Laboratory (UK). The physicists are the first to succeed on the laboratory scale in producing soft X-radiation by means of laser light.
This is done by generating pulses of electrons with intense laser flashes. The same laser beam accelerates the electron pulses to approximately the velocity of light within a distance a thousand times shorter than required by previous techniques. These are then focused into a short undulator with magnetic fields inside. These force the electrons to take an undulating course, thus causing them to emit X-radiation. The experiment shows that it is possible to produce so-called brilliant X-radiation by means of light. In brilliant radiation a very large number of photons of the same wavelength are bunched to a beam. Such radiation affords many more applications than ordinary X-radiation. Hitherto, however, it could only be produced in accelerators kilometers long. LAP’s scientists have now opened the way to producing brilliant X-radiation in much more compact devices as well. This is reported in the online edition of Nature Physics ("Laser-driven soft-X-ray undulator source").
Since its discovery at the close of the 19th century, X-radiation has provided insights into worlds invisible to the naked eye. One could not imagine today’s medicine, physics, materials science and chemistry without this radiation. Until now it became possible to make structures visible that are no bigger than atoms. This calls for brilliant x-radiation. Today it is produced in expensive accelerators kilometers long, thus making it not generally accessible. There are just a few devices in the world that are capable of producing this brilliant X-radiation at great expense. Brilliant radiation bunches a very large number of photons (light particles) that also move in phase.
A team around Professor Florian Grüner and Professor Stefan Karsch at the Laboratory for Attosecond Physics aims to provide brilliant X-radiation inexpensively in compact devices. The physicists have now reached an important milestone: By means of intense laser light and a plasma composed of hydrogen atoms they have for the first time succeeded at a laboratory of LMU Munich and MPQ in producing X-radiation with a wavelength of about 18 nanometers (soft X-radiation). For this purpose the physicists used laser pulses lasting just a few femtoseconds, a femtosecond being a millionth of a billionth of a second. In this ultrashort time the light pulses attain powers of about 40 terwatts; in comparison, an atomic power plant generates about 1000 megawatts, which is 1000 times less.
The enormous power of the pulses are only made possible by their extreme shortness. The strong electric and magnetic fields of the light pulses separate electrons from hydrogen atoms and thus produce plasma. These electrons are accelerated with the same laser pulse to approximately the velocity of light, and within a distance of 15 mm at that, this being a thousand times shorter than needed by todays technologies.
The electrons then enter the undulator, a device 30 cm long and 5 cm wide. It produces a magnetic field on the inside that forces the electrons to take an undulating sinusoidal course. This accelerates the electrons back and forth, causing them to emit photons in the soft X-ray range. Until today, it has only been possible to generate light in the visible or infrared ranges, i.e. with much longer wavelengths than those of X-radiation. The aim to gain access to the shortest possible light wavelengths is to be sought in the laws of optics. It is stated that with light one can only image structures equivalent in size to its wavelength. That is to say, if an object is investigated withX-ray light with a wavelength of 18 nanometers, for example, it has to be at least as large in size in order to make it visible. Atoms and numerous molecules, however, are much smaller.
Reducing the wavelength of laser-produced X-radiation is the next objective of LAP’s scientists. “In principle, our experiment has demonstrated that it is possible to produce X-radiation in a university laboratory by means of ultrashort light pulses”, states Professor Florian Grüner. But the potential of undulator technology is much greater. “Our experiment paves the way to an inexpensive source of laser-driven X-radiation”, predicts Grüner.
The physicists’ next step is to further increase the energy of the electrons flying though the undulator. For this purpose the scientists will increase the energy of the light pulses producing the electrons. The prime objective of Florian Grüner’s group is to realize a laser-driven free-electron laser whose light is about a million times more brilliant than the undulator radiation now measured. The radiation should then have wavelength of just a few nanometres. It could generate completely new, detailed insights into the microcosm of nature. The radiation could likewise be applied in medicine, for example, to detect minute tumours before they spread. This would greatly enhance the chances of curing cancer.
Source: Ludwig-Maximilians-Universität München
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