Session QI3: High Energy Density Physics II

Breakdown of Fermi-degeneracy in the simplest liquid metal

The phase diagram of Fermi-matter can be demarcated by regions where quantum or classical effects are uniquely dominant, as determined by the temperature of the system relative to its Fermi temperature, T_F. Studies of this cross-over have been limited to dilute systems such as liquid ³He or ultracold alkali gases. Here, we report the observation of the breakdown of Fermi degeneracy in the simplest element: deuterium shock compressed to the metallic state. Above the insulator–metal transition, the optical reflectance shows the distinctive temperature-independent saturation, which is prescribed by the Mott minimum scattering limit, in agreement with previous experiments. However, at T > 0.4 T_F, the reflectance of metallic deuterium rises with a temperature-dependent slope that is characteristic of a classical Landau–Spitzer plasma. The onset of the crossover coincides with the fluid transitioning from a strongly coupled metal to a moderately coupled plasma. Our results provide an invaluable benchmark for quantum statistical models of coulomb systems, over a wide range of temperature relevant to dense astrophysical objects and ignition physics.    

Solving the Polarity Riddle in Dense Plasma Focus

Dense plasma focus (DPF) devices are conventionally operated with a polarity such that the inner electrode (IE) is the anode. It has been found that interchanging the polarity of the electrodes (i.e. IE as cathode) causes an order of magnitude decrease in the neutron yield when using deuterium as a fill gas. This polarity riddle has previously been studied empirically through several experiments, and is yet not well understood. To understand the puzzle, we use the hybrid particle-in-cell (PIC) code Chicago to simulate both polarities and are able to reproduce this drastic reduction in yield. The PIC method captures electric fields and the acceleration of deuterons that produce neutrons in a DPF. We find that when using reverse polarity ions are still accelerated and, in fact, attain similar energy spectra as in the standard polarity case. The difference is that the fields are flipped and thus ions are accelerated in the opposite direction. So in the reverse polarity case, the majority of the “plasma target” (formed by the imploding plasma) is in the opposite direction of the beam, and, thus, the beam hits the IE and produces few neutrons. This knowledge is used to design a better inner electrode configuration, which allows reverse polarity to create a high-quality ion beam as well as a high-density target. Such a design is shown to generate yield comparable to standard polarity. Further, the results suggest that for some applications, making the inner electrode negative can be advantageous.   

Direct Measurements of Nonlocal Heat Fux by Thomson Scattering

In diverse fields of plasma physics including astrophysics, inertial confinement fusion, and magnetohydrodynamics, classical thermal transport provides the foundation for calculating heat flux. The classical theories of thermal transport [e.g., Spitzer–Härm (SH) and Braginskii] break down in the presence of large temperature gradients, turbulence, or return current instabilities: they do not include nonlocal effects where energetic electrons travel distances comparable with the temperature scale length (L_T) before colliding. Nonlocal theories have improved upon and established the limits of classical transport (λ_ei/L_T∼10^-2) by accounting for the velocity dependence of the mean free path (λ_ei). These limits were confirmed by a novel Thomson-scattering technique that provided the first direct measurement of nonlocal heat flux. The heat flux was measured directly from the amplitudes of the Langmuir fluctuations and indirectly through the electron temperature and density profiles (q_SH =-κ∇T_e), and were enabled by setting the Thomson-scattering geometry to probe Langmuir fluctuations with phase velocities near the range of the heat-carrying electrons (v_φ∼3.5_th). The measured heat flux agreed with classical SH values when λ_ei/L_T<10^-2, but in the opposite limit, the differences were as large as a factor of 2. Vlasov–Fokker–Planck simulations self-consistently calculated the electron distribution functions used to reproduce the measured Thomson-scattering spectra and to determine the heat flux. The multigroup nonlocal Schurtz–Nicolaï–Busquet (SNB) model overpredicted for all values of λ_ei/L_T, demonstrating the need to include physics often missing from computationally expedient nonlocal models.