Plasma hotter than expected

Researchers discover unexpected properties of laser-induced nitrogen plasmas

Pictures of Plasma Development © MPQ
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Laser-generated plasmas have numerous applications in nuclear fusion, particle acceleration, and in the generation of x-ray or attosecond extreme ultraviolet pulses. In order to set the optimal properties for each purpose, it requires a precise knowledge of the temporal evolution of this condition. Researchers have now followed the plasma dynamics in real time using a novel query technique.

As the scientists around Martin Centurion, Peter Reckenthäler and Ernst Fill from the Max Planck Institute for Quantum Optics in Garching report in the journal Nature Photonics, it turned out that through a so-called OFI (optical fields induced) plasma contrary to expectations high electrical and magnetic fields are built. This finding can have a major impact on many applications of laser-induced plasmas.

A plasma is a hot and dense matter state in which atoms have effectively dissolved into their constituents - cores and electrons - so that positively charged ions and negatively charged electrons co-exist side by side. According to popular theories, the inside of a plasma is a field-free space in which the electric charges are uniformly distributed. Only within the smallest dimensions, the so-called Debye length - about 0.1 micrometer - it should come to fluctuations of electric charges.

Gas jet bombarded with laser pulses

The investigations have shown, however, that in the center of an OFI plasma evidently a positively charged area is formed, which surrounds a well over the Debye length reaching out cloud of electrons. To generate an OFI plasma, the scientists let nitrogen flow from a nozzle. They bombard this gas jet with intense, only 50 femtoseconds (fs, a fs = 10 high -15 sec) laser pulses from the visible spectral range. The high field strengths within the pulses ionize the atoms and lead to plasma formation in the laser focal point.

This gas plasma is then bombarded with three picosecond (ps) pulses (one ps = 10 high -12 sec) of electrons that have an energy of 20 kilo-electron volts. The repetition rate of the laser and electron pulses is one kHz (= 1, 000 pulses per second). After passing through the plasma, the electron beam with a diameter of three millimeters is detected on a detector. display

"Hole" in the electron beam

The effect of the plasma on the "interrogation beam" of electrons is reflected in their distribution: For a field-free plasma one would expect that the electrons cover the detector evenly and are blocked only by the gas nozzle. However, the experiments showed that the detector has an interesting, rapidly changing pattern.

To track the development of plasma over time, the time between laser and interrogation pulse is varied. The images thus obtained at a distance of a few picoseconds show the following: first, after a few picoseconds, a "hole" is created in the electron beam in the region of the laser focal point. The missing electrons have evidently migrated into two lobe-shaped regions which propagate along the laser beam on each side of the plasma region.

This development lasts for about 80 picoseconds. Then the interrogation electrons raise to a bright "spot" in the center, so that their density is even larger here than in the original beam. After about 300 picoseconds, these patterns gradually become blurred.

A cloud of hot electrons

The scientists have the following explanation for these observations: Shortly after the generation of the plasma by the laser pulse, a positively charged area is formed in the center, surrounded by a cloud of hot electrons. As a result of this charge separation, electric and magnetic fields are created, which deflect the electrons of the "interrogation beam" in such a way that the distribution described above results. The electron cloud extends beyond the original plasma, after 100 picoseconds its radius is about 1, 000 times larger than the Debye length.

Under these conditions, the interrogation beam is now focused on the center of the detector, which explains the appearance of the bright spot. Numerical simulations based on these assumptions give a good representation of the experimental data and allow to calculate parameters such as field strengths, total charge and electron temperature. They show that the charge distributions described can only occur if some of the plasma electrons heat up extremely and become much hotter than the plasma itself.

Understand the physics of laser-generated plasmas better

One of the processes that can cause this is the return of the oscillating electrons with the atomic nuclei. The Deflectometry technique demonstrated here can detect changes in plasma evolution within a few picoseconds with a spatial resolution of 30 microns. Their high sensitivity is based on the fact that small charge shifts within the plasma are noticeable as disturbances in the spatial profile of the electron beam.

According to the scientists, the new method has the potential to better understand the physics of laser-generated plasmas and to selectively improve plasma-based electron and ion accelerators.

(idw - Max Planck Institute for Quantum Optics, 21.04.2008 - DLO)