Time-resolved emission spectra from liquid hydrogen fuel pellets injected into the LHD machine provide quantitative data from a unique high-density plasma in a magnetic fusion plasma environment.
The spectra show a mysterious feature: the line-width and continuum lowering increase in time, over some 200 microseconds, as if the electron density were increasing by about a factor two. Since the pellet begins as a cryogenic liquid and is heated by the kilovolt LHD plasma, common sense would argue that the pellet should rapidly expand and its density should rapidly decrease.
The observed rising electron density near the emitting atoms can be understood as a consequence of heat conduction from the hot exterior during the hydrodynamic expansion of the pellet. Hydrogen in the pellet core is too cold to radiate and the exterior is too hot to emit line radiation. The emission comes from an intermediate region whose density would decrease in time if hydrodynamic flow were dominant, but which increases because the heat conduction is (evidently) more rapid than the hydrodynamic flow.
Our numerical simulation of the pellet heating includes flux-limited heat conduction and volume heating by high-energy ions from NBI. Hydrogen pressure and energy are obtained from an atomic model including molecules, positive and negative ions and a range of excited states of neutral H. The populations are determined by equilibrium statistical mechanics.
This experiment gives a spectroscopic test of heat conduction in high-density plasma. Since there is a long history of controversy about flux-limited and anomalous heat conduction in comparable laser-produced plasmas, finding a satisfactory numerical model for this relatively well-characterized plasma dynamics has broad scientific interest.