Exponential frequency spectrum and Lorentzian pulses in magnetized plasmas

Abstract

Two different experiments involving pressure gradients across the confinement magnetic field in a large plasma column are found to exhibit a broadband turbulence that displays an exponential frequency spectrum for frequencies below the ion cyclotron frequency. The exponential feature has been traced to the presence of solitary pulses having a Lorentzian temporal signature. These pulses arise from nonlinear interactions of drift-Alfvén waves driven by the pressure gradients. In both experiments the width of the pulses is narrowly distributed resulting in exponential spectra with a single characteristic time scale. The temporal width of the pulses is measured to be a fraction of a period of the drift-Alfvén waves. The experiments are performed in the Large Plasma Device ͑LAPD-U͒ ͓W. Gekelman et al., Rev. Sci. Instrum. 62, 2875 ͑1991͔͒ operated by the Basic Plasma Science Facility at the University of California, Los Angeles. One experiment involves a controlled, pure electron temperature gradient associated with a microscopic ͑6 mm gradient length͒ hot electron temperature filament created by the injection a small electron beam embedded in the center of a large, cold magnetized plasma. The other experiment is a macroscopic ͑3.5 cm gradient length͒ limiter-edge experiment in which a density gradient is established by inserting a metallic plate at the edge of the nominal plasma column of the LAPD-U. The temperature filament experiment permits a detailed study of the transition from coherent to turbulent behavior and the concomitant change from classical to anomalous transport. In the limiter experiment the turbulence sampled is always fully developed. The similarity of the results in the two experiments strongly suggests a universal feature of pressure-gradient driven turbulence in magnetized plasmas that results in nondiffusive cross-field transport. This may explain previous observations in helical confinement devices, research tokamaks, and arc plasmas.

Convective transport by intermittent blob-filaments: Comparison of theory and experiment

“…Somewhat similar localized bloblike temperature filaments have been created in the linear plasma device LAPD. 217 Such external control of blobs has not been possible in fusion devices, mainly due to the large energy and particle flux already existing in normal conditions in these devices.…”

Permutation entropy and statistical complexity analysis of turbulence in laboratory plasmas and the solar wind

“…The LAPD edge fluctuations have the highest level of complexity of the three measured data sets and occupy a middle region in terms of the permutation entropy. Previous work has shown that the LAPD drift-wave turbulence may be dominated by nonlinear interactions of relatively small numbers of modes, and thus tend to exhibit more chaotic, complex behavior [16]; thus, its coordinates occupy a position closest to known chaotic maps. Finally, the SSX fluctuations exhibit a level of complexity in between the other two plasmas.…”

“…The solar wind is highly variable but there are two broad types: fast wind (V > 600 km/s) which is emitted from open coronal field lines and is typically low density (<5 protons/cm 3 ), has few large scale structures and has high amplitude but less developed turbulence, and slow wind, (V < 500 km/s) which is typically found in the ecliptic plane and originates from more complex coronal magnetic topology and is denser and more structured than the fast wind with more evolved but lower amplitude turbulence [26,27]. Here we use multiday long intervals of a fast wind stream (January [14][15][16][17][18][19][20][21]2008) and a slow wind stream (January [24][25][26][27][28][29]2010) with large scale magnetic fluctuations on the order of 10 nT.…”

Observation of exponential spectra and Lorentzian pulses in the TJ-K stellarator

“…However, the log-log display format compresses a large dynamic range of the data and thus it can obscure important underlying features of the fluctuations, as has been recently found in experiments performed in a linear device. 24 The studies in the large plasma device (LAPD) by Pace et al 24 considered two different situations in which crossfield pressure gradients were created in a magnetized plasma column. One was a pure temperature gradient consisting of a hot electron channel surrounded by a cold plasma.…”