Observation of Magnetically Induced Transparency in a Classical Magnetized Plasma

We report the first demonstration of magnetically induced transmission in an opaque magnetized plasma. Magnetically induced transmission in a plasma is a classical analog to the electromagnetically induced transparency in atomic systems. The transmission of radiation through an axially magnetized plasma is obtained by applying an additional one dimensional transverse spatial periodic magnetic field. The transverse-periodic magnetic field uncouples the right-hand electromagnetic wave from interacting with plasma electrons, rendering the plasma band-stop transparent. This provides means to control the extent of absorption of electromagnetic radiation in magnetized plasma.

Figure 1

(a) The normal modes for a magnetized plasma with and without the wiggler magnetic field for ${\Omega }_0={\omega }_p$, ${\kappa }_w/k_c=0.5$ and ${\Omega }_w/{\Omega }_0=0.2$. (b) The group velocity for the same parameters and notations as in (a).

Figure 2

Schematic of the experimental system-side view. ${\mathrm{Tx}}_{1,2}$ and ${\mathrm{Rx}}_{1,2}$ are transmitting and receiving antennas, respectively. A RHCP wave is launched from ${\mathrm{Tx}}_1$ and received by ${\mathrm{Rx}}_1$. Cutoff measurements are performed using a linearly polarized wave that is launched from ${\mathrm{Tx}}_2$ and detected by ${\mathrm{Rx}}_2$.

Figure 3

$B$-dot probe measurement of wiggler magnetic field penetration inside the plasma. The $B$-dot probe was positioned on the system axis at mid-system length. The inset demonstrates the ringing of the wiggler field with respect to time. The time on the horizontal axis is relative to the turn on of the axial magnetic field.

Figure 4

Time-dependent transmission intensity for a $20\mu \mathrm{s}$ pulse RHCP wave incident on a finite magnetized plasma slab as measured in the experimental setup [Fig. 2]. Transmission intensity is normalized to the vacuum transmission. The overall noise level can be estimated by the transmission value through plasma when the wiggler is off. The measured and simulated [Eq. (5)] transmissions are for the following parameters: driving frequency $f_{\mathrm{rf}}=15.8\mathrm{GHz}$, plasma interaction-length $h_{\mathrm{slab}}=100\mathrm{cm}$, $n_e\approx 3.1\times {10}^{12}{\mathrm{cm}}^{-3}$, $B_0=0.4\mathrm{T}$, $B_w=0.1\mathrm{T}$, and ${\lambda }_w=12\mathrm{cm}$. The time on the horizontal axis is relative to the turn on of the axial magnetic field.

Figure 5

A scan of the time averaged transmission vs frequency detuning for the frequency range 15.0–16.0 GHz and the same experimental conditions as in Fig. 4. The frequencies are normalized to the electron plasma frequency $f_p=15.8\mathrm{GHz}$. The band gap in these normalized units is between 0.74 and 1.44, the electron cyclotron and the upper cutoff frequencies, respectively.