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Register a new userRadial Current and Rotation Profile Tailoring in Highly Ionized Linear Plasma Devices
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In a rotating magnetized plasma cylinder with shear, cross-field current can arise from inertial mechanisms and from the cross-field viscosity. Considering these mechanisms, it is possible to calculate the irreducible radial current draw in a cylindrical geometry as a function of the rotation frequency. The resulting expressions raise novel possibilities for tailoring the electric field profile by controlling the density and temperature profiles of a plasma.
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The calculation presented in Rotation Velocities and Radial Electric Field in the Plasma Edge by W. M. Stacey [Contrib. Plasma Phys. 46, (2006)] contains an inconsistent treatment of the electrostatic potential. Comparing the expressions for the potential associated with the radial electrostatic field with that associated with the poloidal electrostatic field reveals the inconsistency. A field-theoretic perspective implies that the electrostatic field must vanish in a model based upon the physics of a neutral, conducting fluid.
Here we study the structure of a highly ionizing shock wave in a gas of high atmospheric pressure. We take into account the gas ionization when the gas temperature reaches few orders of an ionization potential. It is shown that after gasdynamic temperature-raising shock and formation of a highly-ionized nonisothermal plasma $T_e>>T_i$ only the solitary ion-sound wave (soliton) can propagate in this plasma. In such a wave the charge separation occurs: electrons and ions form the double electric layer with the electric field. The shock wave form, its amplitude and front width are obtained.
We measured the parameter reproducibility and radial electron density profile of capillary discharge waveguides with diameters of 650 um to 2 mm and lengths of 9 to 40 cm. To our knowledge, 40 cm is the longest discharge capillary plasma waveguide to date. This length is important for >= 10 GeV electron energy gain in a single laser driven plasma wakefield acceleration (LPA) stage. Evaluation of waveguide parameter variations showed that their focusing strength was stable and reproducible to <0.2% and their average on-axis plasma electron density to <1%. These variations explain only a small fraction of LPA electron bunch variations observed in experiments to date. Measurements of laser pulse centroid oscillations revealed that the radial channel profile rises faster than parabolic and are in excellent agreement with magneto-hydro-dynamic simulation results. We show that the effects of non-parabolic contributions on Gaussian pulse propagation were negligible when the pulse was approximately matched to the channel. However, they affected pulse propagation for a non-matched configuration in which the waveguide was used as a plasma telescope to change the focused laser pulse spot size.
We present experiments and numerical simulations which demonstrate that fully-ionized, low-density plasma channels could be formed by hydrodynamic expansion of plasma columns produced by optical field ionization (OFI). Simulations of the hydrodynamic expansion of plasma columns formed in hydrogen by an axicon lens show the generation of unit[200]{mm} long plasma channels with axial densities of order $n_e(0) = 1 times 10^{17} cm^{-3}$ and lowest-order modes of spot size $W_M approx 40 mu m$. These simulations show that the laser energy required to generate the channels is modest: of order 1 mJ per centimetre of channel. The simulations are confirmed by experiments with a spherical lens which show the formation of short plasma channels with $1.5 times 10^{17}cm^{-3} lesssim n_e(0) lesssim 1 times 10^{18} cm^{-3}$ and $61 mu m gtrsim W_M gtrsim 33 mu m$. Low-density plasma channels of this type would appear to be well-suited as multi-GeV laser-plasma accelerator stages capable of long-term operation at high pulse repetition rates.
We present the first study of the formation and dissipation of current sheets at electron scales in a wave-driven, weakly collisional, 3D kinetic turbulence simulation. We investigate the relative importance of dissipation associated with collisionless damping via resonant wave-particle interactions versus dissipation in small-scale current sheets in weakly collisional plasma turbulence. Current sheets form self-consistently from the wave-driven turbulence, and their filling fraction is well correlated to the electron heating rate. However, the weakly collisional nature of the simulation necessarily implies that the current sheets are not significantly dissipated via Ohmic dissipation. Rather, collisionless damping via the Landau resonance with the electrons is sufficient to account for the measured heating as a function of scale in the simulation, without the need for significant Ohmic dissipation. This finding suggests the possibility that the dissipation of the current sheets is governed by resonant wave-particle interactions and that the locations of current sheets correspond spatially to regions of enhanced heating.
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