Studies of drift waves in a toroidal heliac

Abstract

Low frequency, coherent density fluctuations have been studied for three typical magnetic configurations in the helical axis stellarator SHEILA using Langmuir probe techniques. The parametric dependence, the threshold magnetic field, the frequency spectrum and the spatial structure of the fluctuations are measured experimentally. Mode ana!yses are made in a magnetic coordinate system. Both the mode numbers thus obtained and the smallness of the directly measured values of k,, ( k i l k , -100) indicate

A review of experimental drift turbulence studies

“…The interested reader is referred to these original papers [223][224][225][226][227][228] or to textbooks for a discussion of the various classes of electron pressure gradient driven linear drift instability. We also note that similar studies of the onset of coherent collisional drift waves have been provided in a toroidal heliac device [229], and gave good agreement with a finite beta collisional drift wave theory developed for a high aspect ratio circular cross-section torus [230].…”

Toroidal currents and radial transport driven by ponderomotive forces of fluctuations in a plasma

“…This is connected with the fact that the azimuthal wavenumber m of these fluctuations is positive, m > 0. Our results are in agreement with measurements by Prater et al (1978), where the plasma confinement was improved in some cases after the onset of fluctuations of a similar structure to those measured by Shi et al (1989).…”

“…Even in the complicated geometry of the SHEILA heliac, the measurements of fluctuating fields by Shi et al (1989) were consistent with the theoretical model of linear drift-wave theory (Frederickson & Bellan 1985), which was developed in a cylindrical geometry for a large-aspect-ratio tokamak. This allowed use of the cylindrical model of ponderomotive force theory in computations of j Qz and V Mr according to (12) and (13).…”

“…Now let us present estimates of the toroidal current j Oz and the radial transport velocity V Mr according to (10) and (13), using experimentally measured electromagnetic fields of fluctuations in the SHEILA heliac, as published by Shi et al (1989).…”

“…The major radius of SHEILA is 0-19 m and the averaged minor radius is 0-035 m. The magnetic field B o is up to 0-18 T in a pulse of 55 ms. For further details, we refer to Blackwell et al (1989). Shi et al (1989) studied low-frequency and highly coherent fluctuations. The fluctuating fields have been identified as drift waves, with characteristic frequencies of the order of 10 4 Hz, with very long parallel wavelengths of about 10 m, or & n « 0-6 m" 1 , and with characteristic poloidal wavenumbers between m = + 1 and + 3 , depending on the details of the magnetostatic field configuration in SHEILA.…”