Experimental study of fluctuations excited by a narrow temperature filament in a magnetized plasma

Abstract

A systematic study is made of the spontaneous growth of fluctuations in temperature, density, and magnetic field in a narrow ͑on the order of the electron skin depth͒ field-aligned temperature filament embedded in a large magnetized plasma. Two broad classes of fluctuation ͑''low'' and ''high'' frequency modes͒ have been identified and studied in detail. A high-frequency drift-Alfvén mode grows at frequencies about one tenth the ion gyrofrequency in the region of the filament where the temperature gradient is large. The measured radial profiles of the density and magnetic field fluctuations associated with this mode agree well with theoretical predictions. The high-frequency mode has been observed to exhibit several interesting nonlinear features, including steepening wave form, progression in azimuthal mode number, coupling to the low frequency mode with subsequent sideband generation, and eventually a transition to broad band turbulence. The nature of the low-frequency mode which has frequencies about one fiftieth of the ion gyrofrequency is less certain, but it has been identified as a spatially localized, azimuthally symmetric mode consisting primarily of temperature fluctuations. Both the high and low-frequency modes give rise to electron heat transport at rates in excess of the classical values.

Exponential frequency spectrum and Lorentzian pulses in magnetized plasmas

“…18 of Ref. 36, is observed to occur simultaneously with the appearance of exponential power spectra of the type shown in Fig. 6.…”

“…These oscillations correspond to coherent drift-Alfvén waves driven unstable by the electron temperature gradient 35 and have been described in detail in a previous publication. 36 A third stage appears after 5 ms of beam injection. At this later time low-frequency oscillations mixed with incoherent high-frequency oscillations are observed.…”

“…The largest temperature gradient is achieved at a radial position about 3 mm from the center of the beam injector. The radial eigenfunction 35,36 of the high-frequency oscillations ͑i.e., the drift-Alfvén waves͒ peaks near this location. Accordingly, in Fig.…”

Chaos in magnetic flux ropes

“…The entropy and complexity are calculated for a time length spanning several rotation periods of the ropes; therefore temporal/spatial smearing is anticipated. High complexities around the flux ropes close to the source (where the rotation and smearing is negligible) could indicate that the chaotic signatures are due to gradient driven modes as in [44,45]. The C-H diagrams display entropy as well as complexity and entropy is absent from figure 12.…”

“…The first application of the permutation analysis methodology to plasma physics was done by Maggs and Morales [43]. The data was time series of the electron temperature measured in an unstable filament of heat that was generated in a magnetoplasma [44]. The data indicate that the filament which is stable for a short time (roughly an ion gyro-period) exhibits chaotic behavior when it becomes unstable.…”