Student project at IRF Uppsala
| INSTITUTET FÖR RYMDFYSIK
| Swedish Institute of Space Physics
|| (59°50.272′N, 17°38.786′E)
Independent thesis, advanced level (15 c)/Studentprojekt (15 hp)
Adaption of Inertial Confinement Fusion Results to Spherical Plasma Expansion at Comets
Student: Viktor Sparrman,
Period: Spring 2022
Comet nuclei emit gas (mostly water vapour). As the comet gravity is weak this gas expands freely into space around it. Solar EUV radiation as well as energetic particles ionizes some of the gas molecules, providing the comet with an expanding ionosphere. This interacts with the solar wind and interplanetary magnetic field (IMF) giving a very complex and dynamic environment. However, for sufficiently active comets the IMF cannot penetrate the region closest to the nucleus. For this region an assumption of spherical symmetry is not unreasonable. This has been used in several previous fluid models of the inner coma, but such models can only represent the energy distribution of the electrons by a Boltzmann distribution. In reality, electric fields may trap part of the electrons, and to model such phenomena a kinetic approach is needed.
In inertial confinement fusion plasma physics, an expanding plasma bubble is created at a certain instant by bombardment of a small target with high power lasers. In contrast to the comet problem, with continuous outgassing and ionization, this is an initial value problem, but the two situations display obvious similarities. The problem of spherical symmetric plasma expansion is much studied in the inertial confinement context. Can anything of this be carried over to the comet plasma problem?
Recent missions to solar system comets, such as ESA's Rosetta mission, raise interest for models and descriptions of their plasma environment. The interaction with various space phenomena such as stellar wind make the construction of an analytical description difficult. Instead, a simplified view of the comet environment is considered where the effects of magnetism and departures from radial symmetry are neglected. This is done in an effort to construct an approximation of the comet plasma behaviour later to be compared against observational accounts to find which plasma features are dependent on more complex phenomena and which plasma features arise as a result of the simpler comet view. Several attempts are made to construct an analytical description of comet plasma as based on the description within another branch of plasma physics: fusion. Previous work regarding the vacuum expansion of plasma after a stationary target is rapidly ablated via high-intensity lasers appears promising for adaptation to the comet environment. Before the comet environment can be considered the different natures of the two problems have to be considered. For example, the comet case is a stationary expansion problem as opposed to fast-ignition fusion where the expansion is treated as an initial value problem. Having accounted for the problems' inherent differences, a few methods are proposed to convert solutions of lab fusion distribution functions to the comet case. Additionally, a numerical approach to calculate the distribution function of comet electrons is presented employing ergodic invariance. Lastly, a toy-model simulation of the timescale for variations in the potential show that the error in the ergodic invariance may in practice have a faster convergent timescale dependence than theoretical bounds suggest. Optimistically, this suggest the possibility of future use in numerical attempts at modelling comet plasma.
Figure 1: Comparison of comet plasma (row a) and idealized inertial confinement fusion (row b expansion cases. Ions (blue) and electrons (red) are individually indicated whereas the general motion of neutral gas is marked with radial arrows (black). a. Coma thickness corresponds to shell opacity in image. Creation of new ion-electron pairs (origin marked as a black dot) occurs continuously. Ion motion is radial and initial electron motion is isotropic with respect to ionization origin. Only newly created ions and electrons are shown. b. At the time of ionization (t=0), the initial solid target is confined within a small volume. After the ionization (t>0), the plasma radius is dependent on time since ionization and the plasma front velocity.