Neutral Penetration Research Articles

An analytic solution for the distribution of neutral particles in a Maxwellian plasma using the method of singular eigenfunctions

The method of singular eigenfunctions is used to obtain a solution to the problem of penetration of neutrals into a dense plasma with a realistic Maxwellian ion distribution. A general solution is obtained and explicit results given for models of the reflection processes at the wall. These include the neutral density distribution, the emergent neutral velocity distribution and the neutral particle and energy fluxes to the wall. The analytic results indicate the validity of the diffusion approximation and provide a boundary condition for such a treatment.

Calculation of the transport of neutral atoms in highly ionized plasmas using neutron transport methods

The linear Boltzmann equation for the uansport of neuual atoms in highly ionized plasmas is compared with the neutron transport equation. It is shown that, when the plasma is isotropic, the neutral transport equation can be solved with neutron uansport methods. Two formulations of the neutral transport equation that are amenable to solution by existing multigroup neutron uansport codes are given – the multigroup formulation and the multispecies approximation. In the first, the solution provides the neuttal energy (as well as spatial and angular) disuibution, whereas in the latter, the specuum of each neutral species is assumed to be known and fixed. As an illustration, a problem of neutral penetration to and interaction with the plasma of the ST Tokamak is solved. The solution is obtained for a two-species model using the discrete ordinates code ANISN.

Screening of a high-density plasma from neutral gas penetration

In several experiments on a magnetically confined plasma and under certain conditions of a fusion reactor in stationary operation, neutral gas will be present in the regions surrounding the plasma. The inflow of neutrals and the outflow of plasma by diffusion or free streaming is studied in this paper, both for closed and open-ended systems: In a closed bottle neutral gas will not be able to penetrate through the cool boundary layers into the hot core of a high-density plasma. At plasma densities above 1021 m−3 the energy losses from interaction with the neutrals become negligible compared to the thermonuclear power production, but not at densities below 1019 m−3. This is due to the fact that the penetration depth of the neutrals depends on the ionization rate and becomes about 10−3 and 0.1 m at the densities 1021 and 1019 m−3. Further, the neutral gas inflow will be delayed by diffusion through a cool boundary region of sufficiently high density. In stationary operation the plasma density will be determined by the magnetic field strength and the density of the surrounding neutral gas. A condition for stable heat balance in a boundary region of finite thickness is deduced in this paper. As an alternative to a fusion device where the plasma is surrounded by an ultra-high vacuum region, it is therefore possible to surround a high-density plasma by a neutral gas blanket. The latter should be dense enough for released wall impurities to diffuse only slowly towards the plasma boundary. With a superimposed stream of neutral gas along the plasma boundary, the impurities can then be removed before reaching the plasma. In an open-ended system, thin layers of neutral gas are formed by plasma particles which escape along the magnetic field and recombine at the end walls. The theory agrees with measurements of the density of a fully ionized plasma as a function of the applied magnetic field and the density of the surrounding neutral gas. It also suggests that Bohm diffusion has not been present in earlier experiments with rotating plasmas where the density was found to decay at a very slow rate during free-wheeling.

A Study of Penetrating Cosmic-Ray Showers in Water

Penetrating showers produced locally by non-ionizing radiation were studied using counter techniques. The collision length in water of the non-ionizing component which is capable of producing penetrating showers was found to be (98\ifmmode\pm\else\textpm\fi{}13) g/${\mathrm{cm}}^{2}$. Comparison of this collision length with that expected from the geometrical cross section seems to indicate that the hydrogen nuclei in water have a rather small cross section for the production of penetrating showers by neutral radiation. Results obtained at two elevations were used to calculate the absorption length in air of the neutral penetrating shower producing component. The absorption length measured, (115\ifmmode\pm\else\textpm\fi{}19) g/${\mathrm{cm}}^{2}$, agrees with that obtained by others for the total penetrating shower producing component.