Under strong thermal excitation conditions it is possible to create transient, highly non-equilibrium states of matter. If the heating occurs fast enough a non-equilibrium state can be reached in which the electrons in the material are thermally excited but the ions temporarily remain cold and retain their lattice structure. As reported by McKenna et al in a recent paper published in 'Physical Review Letters' [1], the use of high power, short pulse lasers to achieve such transient states of matter enables investigation of the effects of crystal structure on the conduction of energetic electrons.

The research, performed using the Vulcan petawatt laser at the CLF, involved an investigation of the effects of lattice structure on energetic electron transport using three different forms (allotropes) of carbon: single-crystal diamond, vitreous carbon, and pyrolytic carbon. The electron beam transport was found to be smooth in diamond, while beam filamentation was observed for the other less ordered forms of carbon. The result is explained by electron transport simulations (see figure) by Alex Robinson (CLF) and conductivity calculations by Mike Desjarlais (Sandia National Laboratories) which show that although diamond is an insulator at room temperature, when rapidly heated by ultra-intense picosecond laser pulses, it enters into a transient state in which its electrical conductivity is comparable to many metals at room temperature. Heating more disordered forms of carbon in the same way results in excited states with dramatically lower conductivity. Thus the ordered lattice structure of warm-dense diamond is found to be a key factor in defining its high conductivity and hence the properties of energetic electron beam transport.

The research was performed by a collaboration from the University of Strathclyde, the CLF, Sandia National Laboratories, Lund University and the Beijing National Laboratory of Condensed Matter Physics. The findings may have implications on the choice of materials used in advanced fusion targets.

[1] P. McKenna et al., Phys. Rev. Lett. 106, 185004 (2011) and link to Physics website (link opens in a new window).

Images and captions

• Image 1 - Results of QMD-DFT calculations of conductivity for diamond and glassy carbon. This shows that diamond has exceedingly high conductivity in 1-10eV range compared to glassy carbon. The difference being nearly a factor of 100. Only at very high temperatures (i.e. > 100eV) do the two materials exhibit similar conductivity, that is in the plasma regime.
  
  
• Image 2 - Fast electrons propagate smoothly through diamond due to its excellent conductivity at low temperatures, whereas a fast electron beam is strongly filamented in glassy carbon due to its poor conductivity.