Spontaneous Thermal Waves in a Magnetized Plasma

Coherent temperature oscillations corresponding to thermal (diffusion) waves are observed to be spontaneously excited in a narrow temperature filament embedded in a large, but colder, magnetized plasma. The parallel and transverse propagation properties of the waves satisfy the predictions of the classical transport theory based on Coulomb collisions. The frequency of the oscillations meets the conditions for a quarter-wave thermal resonator. This is the plasma version of thermal resonators used in the study of other states of matter.

Figure 1

Time evolution of electron temperature, $T_e$, at a position 224 cm axially away from the beam source at the filament center. (a) Development over the full beam-pulse. (b) The spontaneous coherent thermal oscillation over a 2.2 ms interval. Solid curves are experimental measurements and dashed curves are predictions of the transport code. The code result in (a) does not include a driven thermal wave, while in (b) a modulation at 5.1 kHz is included. The bottom trace in (a) is the injected beam current.

Figure 2

Solid curve shows the radial dependence of the normalized amplitude of the coherent thermal oscillations at $z=544\mathrm{cm}$. Dashed curve is the prediction of the transport code. The top dashed-dotted curve is the radial profile of the electron temperature filament.

Figure 3

Axial dependence of resonator. Curve labeled $T_e$ is the steady-state axial temperature profile predicted by transport code. Solid blue curve is the axial phase of a temperature wave at 5.1 kHz also obtained from the code. Dashed green line is a WKB phase calculation using the dispersion relation of Eq. (1). Red line corresponds to a wave phase of $\pi /2$. Intercept of curves shows system behaves as a quarter-wave thermal resonator.