Experimental Observation of Convective Cell Formation due to a Fast Wave Antenna in the Large Plasma Device

An experiment in a linear device, the Large Plasma Device, is used to study sheaths caused by an actively powered radio frequency (rf) antenna. The rf antenna used in the experiment consists of a single current strap recessed inside a copper box enclosure without a Faraday screen. A large increase in the plasma potential was observed along magnetic field lines that connect to the antenna limiter. The electric field from the spatial variation of the rectified plasma potential generated $\overrightarrow{\mathit{E}}\times {\overrightarrow{\mathit{B}}}_0$ flows, often referred to as convective cells. The presence of the flows generated by these potentials is confirmed by Mach probes. The observed convective cell flows are seen to cause the plasma in front of the antenna to flow away and cause a density modification near the antenna edge. These can cause hot spots and damage to the antenna and can result in a decrease in the ion cyclotron range of frequencies antenna coupling.

Figure 1

(a) A schematic of the experiment, not to scale. The coordinate system of the experiment is $z=0$ at the fast wave antenna and $x=y=0$ is along the LAPD axis. Two plasma sources are in operation during the experiment. The limiters block a direct connection between the antenna and the discharge circuitry along ${\mathbf{B}}_{\mathbf{0}}$ field lines. (b) Diagram of the fast wave antenna with one wall of the enclosure removed. All conductors are copper, the coaxial antenna feed is shown on the center left, and the strap is visible within the box. The side slots are visible on the back wall of the enclosure.

Figure 2

(a) Top panel: Time trace of the rf current on the antenna. Lower panel: representative time trace of plasma potential at $x=-14$, $y=-10\mathrm{cm}$. Potential increases of up to 90 V are observed after the antenna is powered up. (b) Plasma profile during the rf pulse taken at $z=65\mathrm{cm}$. The outline of the antenna box is shown in solid lines. The locations of highest $V_p$ are at the top and bottom of the ICRF antenna.

Figure 3

(a) The vectors show the $\overrightarrow{\mathit{E}}\times {\overrightarrow{\mathit{B}}}_0$ drifts calculated from $V_p$ measurements using the emissive probe. The color shows the magnitude of the electrostatic electric field in $\mathrm{V}/\mathrm{cm}$. This is also the $\overrightarrow{\mathit{E}}\times {\overrightarrow{\mathit{B}}}_0$ flow speed in $\mathrm{km}/\mathrm{s}$ (${\overrightarrow{\mathit{B}}}_0=0.1\mathrm{T}$). (b) A vector field of the flow measured by the Mach probe. The color map shows the mach speed calculated from the measurements. The location of the antenna is shown on the left as well as the outline of the limiter at $z=3.6\mathrm{m}$.

Figure 4

Five snapshots of the modification of the ion saturation current profile are shown. Substantial modifications are seen in front of the antenna with a decrease near the top and increase near the bottom, most dramatic at $t=80\mathrm{us}$. The rf switches on at $t=0$. Overplotted are vectors representing the $\overrightarrow{\mathit{E}}\times {\overrightarrow{\mathit{B}}}_0$ flow deduced from plasma potential measurements. The modifications of the profiles are consistent with the $\overrightarrow{\mathit{E}}\times {\overrightarrow{\mathit{B}}}_0$ flow patterns. Bottom right panel shows the time evolution of the ion saturation current near $x=-9$ and $y=0\mathrm{cm}$ as a function of antenna power, exhibiting a threshold at low powers and saturation at higher powers.