Anomalous Diffraction in Cold Magnetized Plasma

Cold magnetized plasma possesses an anisotropic permittivity tensor with a unique dispersion relation that for adequate electron density and magnetic field results in anomalous diffraction of a right-hand circularly polarized beam. In this work, we demonstrate experimentally anomalous diffraction of a microwave beam in plasma. Additionally, decreasing the electron density enables observation of the transition of the material from a hyperbolic to a standard material. Manipulation of the control parameters will enable plasma to serve as a reconfigurable metamaterial-like medium.

Figure 1

Top: Comparison between EFCs for different electron densities. These EFCs were calculated for a frequency of 7.9 GHz, an external magnetic field of 0.35 T, and an electron density of (a) $1\times {10}^{12}{\mathrm{cm}}^{-3}$ and (b) $1\times {10}^{11}{\mathrm{cm}}^{-3}$. (a) A hyperbolic EFC. Note that for (b) the EFC is similar to the EFC in vacuum, allowing propagation at all angles. In this case, the plasma will not exhibit anomalous diffraction. Bottom: Poynting vector amplitude obtained by numerical comsol simulations of a RHCP beam propagating from a slit for plasmas with the same densities as in (a) and (b). A cross section (dashed line) of the contour map is displayed for each.

Figure 2

Schematic of the experimental setup.

Figure 3

Comparison of the measured signals for a $100\mu \mathrm{s}$, 7.9 GHz pulse, for the setup with plasma (blue, solid line) and without ionization (red, dotted line). Ionization begins at $t=0$. A substantial increase can be observed in the measured signal in the case of plasma compared to nonionized gas. Inset: Measurement-based electron density profile.

Figure 4

Received signal of a 7.9 GHz, 10 ms pulse with the receiving antenna fixed on the main axis of the plasma chamber. Blue solid line, experimental results; red dotted line, simulation results. For comparison, the data were normalized to the experimental vacuum transmission level. Black dashed line, experimental magnetic field. (1) Transmission begins prior to ionization, to provide the vacuum signal for comparison. (2) At $t=0$, ionization commences and the plasma becomes opaque, i.e., the cutoff region. The electron density for most of this period is above $1\times {10}^{13}{\mathrm{cm}}^{-3}$. (3) Once the magnetic field builds up and the electron density decreases, transmission of the wave is possible. A signal increase is obtained immediately after cutoff. (4) The density decreases to a point that the effect no longer occurs and there is a return to previous transmission levels. Inset: Density profile used for the simulations.

Figure 5

Received signal of a 7.9 GHz, 10 ms pulse with the receiving antenna displaced 6 cm off the chamber main axis. Blue solid line, experimental results; red dotted line, simulation results. For comparison, the data were normalized to the experimental vacuum transmission level. Black dashed line, experimental magnetic field. Note that a significant signal increase is observed for the main axis results (Fig. 4) while the displaced results display no substantial signal increase.