Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Plasma hotter than expected

21.04.2008
Scientists at MPQ discover unexpected properties of laser-induced plasmas using new method of electron deflectometry

Laser-induced plasmas are of great interest because they are suitable for a variety of applications such as nuclear fusion, acceleration of electrons and ions, and the generation of X-Ray sources and attosecond pulses in the extreme ultraviolet.

A detailed knowledge of the temporal evolution of this state is critical for optimizing the parameters of a given application. With the help of a new pump-probe technique Dr. Martin Centurion, Peter Reckenthäler, and Dr. Ernst Fill of the Attosecond and High-Field Physics Division (Director: Prof. Ferenc Krausz) at Max Planck Institute of Quantum Optics (MPQ) in Garching have now succeeded in observing plasma dynamics in real time (Nature Photonics, DOI 10.1038/nphoton.2008.77).

They discovered that against all expectations OFI (optical-field induced) plasmas build up high electric and magnetic fields. This knowledge may have a significant impact on a range of applications of laser-induced plasmas.

A plasma is a state of very hot and dense matter in which the bonds between the electrons of the atomic shell and the nucleus are broken apart such that positively charged ions and negatively charged electrons coexist independently.

According to standard theory the charges balance each other out, and as a consequence the interior of the plasma is free of electric fields. Charge fluctuations are supposed to occur only over very small distances that are of the order of the Debye length (0.1 micrometer). In contrast to these expectations the MPQ experiments have revealed a positively charged core and a cloud of electrons expanding far beyond the Debye length.

At MPQ the OFI plasma is generated by intense laser pulses with duration of 50 femtoseconds (1 fs=10 to the power of -15 sec) that are directed onto nitrogen streaming from a nozzle. Due to the high electric fields of the laser pulse the atoms get ionized and a plasma forms in the region of the focus of the laser beam. Now pulses of monoenergetic 20 keV electrons are sent through the plasma. Thereafter the electrons are detected on a screen. The influence of the plasma on the probe-beam of electrons is reflected in their spatial distribution. If the plasma were free of electric fields the electrons would be distributed homogenously and only be blocked by the gas nozzle. The experiments, however, show a very interesting and fast changing pattern on the screen.

In order to observe the temporal evolution of the plasma the time delay between laser pulse and probe pulse is varied. The images taken in a distance of a few picoseconds show the following behavior (see the figure below from left to right): In the very beginning - after about a few picoseconds - a "hole" appears in the electron beam in the area of the laser focus. The electrons missing in the depleted region have obviously wandered into two bright "lobes" on each side of the plasma region, which move away along the line of laser propagation in opposite directions.

This goes on for about 80 picoseconds, then the electrons accumulate in the central region to a density higher than in the original beam. After about 300 picoseconds this pattern gets washed out.

The scientists explain these observations with the following mechanisms: Shortly after the plasma is generated by a laser pulse a positively charged core is formed which is surrounded by a cloud of hot electrons. Due to this charge-separation electric and magnetic fields build up that deflect the electrons resulting in the distribution described above. After about 100 picoseconds the electron cloud expands beyond the original plasma region reaching a radius that is a thousand times larger than the Debye length. Under these conditions the probe beam now becomes focused onto the centre of the detector screen resulting in a bright spot.

Numerical simulations based on these assumptions are in very good agreement with the experimental data. The relevant physical parameters, such as the fields, the number of charges and the electron temperature can be deduced. The calculations also show that such a charge distribution is only possible if a small fraction of the electrons heats up to temperatures higher than the plasma temperature itself. This could possibly be caused by frequent recollisions of the electrons with the atomic nuclei.

This new technique of electron deflectometry is able to capture changes in the plasma evolution with a spatial resolution of 30 microns on a picosecond scale. The sensitivity is due to the fact that even small charge imbalances within the plasma are observable as distortions in the spatial profile of the electron beam. The new method will lead to a better understanding of laser plasmas and may have the potential to improve electron and ion acceleration techniques that are based on plasmas. [O.M.]
Original publication:
M. Centurion, P. Reckenthaler, S. Trushin, F. Krausz, and E. Fill
"Picosecond electron deflectometry of optical-field ionized plasmas"
Nature Photonics, DOI 10.1038/nphoton.2008.77
Contacts:
Dr. Ernst Fill
Max Planck Institute of Quantum Optics
Hans-Kopfermann-Straße 1
85748 Garching
Phone: +49 (0) 89 / 2891 4110
Fax: +49 (0) 89 / 2891 4141
e-mail: ernst.fill@mpq.mpg.de
Dr. Olivia Meyer-Streng
Press & Public Relations
Max Planck Institute of Quantum Optics
Phone: +49 (0) 89 / 32905 213
Fax: +49 (0) 89 / 32905 200
e-mail: olivia.meyer-streng@mpq.mpg.de

Dr. Olivia Meyer-Streng | Max-Planck-Institut
Further information:
http://www.mpq.mpg.de

More articles from Physics and Astronomy:

nachricht Tangled magnetic fields power cosmic particle accelerators
14.12.2018 | DOE/SLAC National Accelerator Laboratory

nachricht In search of missing worlds, Hubble finds a fast evaporating exoplanet
14.12.2018 | NASA/Goddard Space Flight Center

All articles from Physics and Astronomy >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Data use draining your battery? Tiny device to speed up memory while also saving power

The more objects we make "smart," from watches to entire buildings, the greater the need for these devices to store and retrieve massive amounts of data quickly without consuming too much power.

Millions of new memory cells could be part of a computer chip and provide that speed and energy savings, thanks to the discovery of a previously unobserved...

Im Focus: An energy-efficient way to stay warm: Sew high-tech heating patches to your clothes

Personal patches could reduce energy waste in buildings, Rutgers-led study says

What if, instead of turning up the thermostat, you could warm up with high-tech, flexible patches sewn into your clothes - while significantly reducing your...

Im Focus: Lethal combination: Drug cocktail turns off the juice to cancer cells

A widely used diabetes medication combined with an antihypertensive drug specifically inhibits tumor growth – this was discovered by researchers from the University of Basel’s Biozentrum two years ago. In a follow-up study, recently published in “Cell Reports”, the scientists report that this drug cocktail induces cancer cell death by switching off their energy supply.

The widely used anti-diabetes drug metformin not only reduces blood sugar but also has an anti-cancer effect. However, the metformin dose commonly used in the...

Im Focus: New Foldable Drone Flies through Narrow Holes in Rescue Missions

A research team from the University of Zurich has developed a new drone that can retract its propeller arms in flight and make itself small to fit through narrow gaps and holes. This is particularly useful when searching for victims of natural disasters.

Inspecting a damaged building after an earthquake or during a fire is exactly the kind of job that human rescuers would like drones to do for them. A flying...

Im Focus: Topological material switched off and on for the first time

Key advance for future topological transistors

Over the last decade, there has been much excitement about the discovery, recognised by the Nobel Prize in Physics only two years ago, that there are two types...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

VideoLinks
Industry & Economy
Event News

ICTM Conference 2019: Digitization emerges as an engineering trend for turbomachinery construction

12.12.2018 | Event News

New Plastics Economy Investor Forum - Meeting Point for Innovations

10.12.2018 | Event News

EGU 2019 meeting: Media registration now open

06.12.2018 | Event News

 
Latest News

Data use draining your battery? Tiny device to speed up memory while also saving power

14.12.2018 | Power and Electrical Engineering

Tangled magnetic fields power cosmic particle accelerators

14.12.2018 | Physics and Astronomy

In search of missing worlds, Hubble finds a fast evaporating exoplanet

14.12.2018 | Physics and Astronomy

VideoLinks
Science & Research
Overview of more VideoLinks >>>