Experimental Observation of Electron-Temperature-Gradient Turbulence in a Laboratory Plasma

We report the observation of electron-temperature-gradient (ETG) driven turbulence in the laboratory plasma of a large volume plasma device. The removal of unutilized primary ionizing and nonthermal electrons from uniform density plasma and the imposition and control of the gradient in the electron temperature ($\nabla T_e$) are all achieved by placing a large (2 m diameter) magnetic electron energy filter in the middle of the device. In the dressed plasma, the observed ETG turbulence in the lower hybrid range of frequencies $\nu =(1–80\mathrm{kHz})$ is characterized by a broadband with a power law. The mean wave number $k_{\perp }{\rho }_e=(0.1–0.2)$ satisfies the condition $k_{\perp }{\rho }_e\le 1$, where ${\rho }_e$ is the electron Larmor radius.

Figure 1

(a) The layout of the internal components of LVPD. The marked numbers represent, namely, (1) back plate, (2) end plate, (3) Langmuir probe, and (4) a pair of $B$-dot and Langmuir probes. (b),(c) The radial and axial magnetic field of EEF both along and across its axis and in cross-section view. (d) The dotted rectangular contour in this schematic shows the filament locations whereas vertical lines on it represent the coils of the solenoid.

Figure 2

$I\mathrm{\text{−}}V$ characteristics of Langmuir probe with EEF (w. EEF) and without EEF (w.o. EEF) are shown in (a) and a comparison of electron temperature in hollow and flat density plasma is made in (b).

Figure 3

Comparison of electron temperature fluctuations in hollow and flat density plasma is shown. A black dash-dotted line represents the noise.

Figure 4

(a) Frequency-power spectrum of $\delta n_e$, a similar spectra is seen for $\delta B_z$ and $\delta T_e$, and (b) the cross-correlation functions.

Figure 5

Contour plot of the joint wave number spectra for density fluctuations is shown with normalized parameters. This is at $y=30\mathrm{cm}$ and for $B_z=5\mathrm{G}$. A similar spectrum is seen for $\delta B_z$.

Figure 6

Normalized (a) $\widehat{\gamma }$ and (b) ${\widehat{\omega }}_r$ versus ${\widehat{k}}_{\perp }$ for ETG mode.

Figure 7

A comparison of the theoretically estimated and experimentally observed normalized fluctuations.