Striking while the ion is hot!

An international collaboration of researchers led by Trinity physicists Dr Sam Vinko and Professor Justin Wark has shown how ultra-fast, femtosecond ionization dynamics can be directly investigated in dense plasmas for the first time, by means of an Auger clock. The results are reported in Nature Communications (DOI: 10.1038/ncomms7397).

Using the brightest X-ray source on the planet, the LCLS X-ray free-electron laser in California, the team successfully created a very hot and dense plasma, at electron temperatures and densities comparable to those found half-way into the centre of the Sun. In these extreme hot-dense conditions, collisions between electrons and ions govern much of the dynamics, and understanding the underlying physics of this process is crucial to our understanding of the behaviour of the plasma as a whole.

Collisions between electrons and ions in dense plasmas are extremely fast, and cannot be measured directly. They occur, on average, in less than a femtosecond – a million-billionth of a second. To understand how short this is, there are more femtoseconds in each minute than there have been minutes since the beginning of the universe.

However, while such rates are far too large to be investigated directly, the team managed to capture this ultra-fast process for the first time using a clever trick: they timed the ultra-fast collisions to another ultra-fast process inherent to the atoms – Auger decay, using it as an ultra-fast chronometer.

These first results reported from the experiment indicate that the collisions actually occur even faster than expected from standard theories, suggesting plasmas might be able to ionize and equilibrate quicker than thought possible at these high densities and temperatures. The measurement will help improve our understanding of the physics in extreme conditions, and will have an impact upon the modelling capabilities of systems relevant both to astrophysics and inertial confinement fusion research.

Posted: 10 March 2015