Thermo-diffusion in inertially confined plasmas

Abstract

In a plasma of multiple ion species, thermodynamic forces such as pressure and temperature gradients can drive ion species separation via inter-species diffusion. Unlike its neutral mix counterpart, plasma thermo-diffusion is found comparable to, or even much larger than, baro-diffusion. It is shown that such a strong effect is due to the long-range nature of the Coulomb potential, as opposed to short-range interactions in neutral gases. A special composition of the tritium and 3 He fuel is identified to have vanishing net diffusion during adiabatic compression, and hence provides an experimental test in which yield degradation is minimized during ICF implosions.In inertial confinement fusion (ICF), the central "hotspot" plasma, assembled by laser-driven spherical implosion [1], contains multiple ion species. Common combinations include low-Z fuel mixtures such as deuterium (D)/tritium (T) and D 3 He with possible addition of high-Z pusher ions, such as carbon or silicon, due to plastic or glass shell mixing [2,3] into the gas fill. Sometimes high-Z gas dopants such as Ar [4] or Kr [5] are intentionally introduced for diagnostic purposes, as well as to specifically study the pre-mix effects [6,7]. The powerful thermodynamic forces (e.g. pressure [8][9][10] and temperature gradients [9,10]) in an imploding target can drive ion species separation via inter-species diffusion. Observation of the resulting fuel stratification in the DT implosion, which upsets the initially optimal arrangement of equal number densities of D and T, in experiments [11] and kinetic simulations [12][13][14][15] have recently been reported. The targets with high-Z dopants show a particularly strong yield anomaly [6,7,16], suggesting that even stronger fuel stratification may take place.Perhaps the most intriguing physics aspect of interion-species diffusion in a collisional plasma is the role of thermo-diffusion, which, as its name suggests, is driven by the gradients of ion and electron temperatures. The novelty comes through as a sharp contrast to the better-known case of a neutral mixture, where thermo-diffusion is substantially less important than baro-diffusion, though often counteracts it [9]. According to statistical physics, thermo-diffusion strongly depends on the details of the collisional exchange between and within the species [10]. Due to the long range nature of Coulomb collisions in plasmas, as opposed to short range collisions between neutral particles, one may expect thermo-diffusion in plasmas and neutral mixtures to be fundamentally different. This difference becomes particularly striking with the observation that plasma baro-diffusion ratio k p is identical to its neutral counterpart [17].

Influence of coupling on thermal forces and dynamic friction in plasmas with multiple ion species

“…1 of Ref. [7]. For mixtures with closer ion masses, such as DT, modifications to both distribution functions contribute to A 12 and, at least for one of the species, this modification is substantial for any given c. Consequently, for weakly asymmetric weakly coupled ionic mixtures A 12 < 1 for all c.…”

“…1 of Ref. [7]. For a BIM with disparate ion masses, such as DKr, further insight from Braginskii's consideration can be derived.…”

“…[30] and the transport coefficients are computed with formulas of the Appendix A. Also shown is the weakly coupled limit Γ → 0 considered in our earlier work [7].…”

Nonlinear Structure of the Diffusing Gas-Metal Interface in a Thermonuclear Plasma

“…The diffusive spreading and area growth of the interface are separate processes whose product is the mix volume as in Eq. (13). Some fraction of the stirred volume is not mixed, V mix < V stir .…”

“…There is general agreement on the form of these fluxes in Refs. [7][8][9][10][11][12][13][14][15], although only Ref. [15] contains the explicit analytic formulas used here.…”