No abstract

Extraction of large-scale coherent structure from plasma turbulence using rake probe and wavelet analysis in a tokamak Rev. Sci. Instrum. 77, 063505 (2006); Vortex-like coherent structures are observed in the edge plasma of ohmically heated ADITYA tokamak ͓ Phys. Rev. Lett. 69, 1375 ͑1992͔͒. The structures are observed on statistical basis when the floating potential fluctuations are analyzed using conditional averaging technique. The structures, which have dipole nature, experience stretching until their radial isolation across the limiter is destroyed. The potential fluctuation also shows non-Gaussian statistics indicating intermittency in broadband turbulence of the edge plasma.

Investigation of highly complex electromagnetic phenomena responsible for fundamental processes in geospace plasma calls for a laboratory device, large enough to provide minimum boundary effects on the electromagnetic waves under concern. The production of large volume, uniform, and quiescent plasma of density ∼1012 cm−3 requires a suitable large area plasma source. The influence of energetic electrons on the phenomena under study can be eliminated using pulsed, highly reproducible discharges so that the afterglow, devoid of energetic electrons, becomes the experimental plasma. Measurement over a large volume with high spatial and temporal resolution, acquisition, and storage of voluminous data form other difficult tasks. The requirement to have tasks of plasma generation, wave excitation, and data acquisition automatically sequenced, also demands a sophisticated control system. A large volume plasma device (LVPD), possessing the following unique features, has been developed in our laboratory: (i) a large area multifilamentary source, (ii) a pulsed power system (width ⩽15 ms and turn off time ≈10 μs), (iii) a three-axes probe drive, (iv) data acquisition and control system with 1 GS/s sampling rate and ≈20 GB memory, and (v) an extended magnet coil system giving desired field profile. The achieved plasma parameters include, ne∼1012 cm−3, Te≈8 eV in the main glow, and ne≈3×1011 cm−3 and Te≈2 eV in the after glow, and plasma uniformity within 5% over 1 m in axial and radial extents. The present article describes the LVPD device.

The spatial and temporal structures of magnetic signal in the tokamak ADITYA is analysed using recently developed singular value decomposition (SVD) technique. The analysis technique is first tested with simulated data and then applied to the ADITYA Mirnov coil data to determine the structure of current peturbation as the discharge progresses. It is observed that during the current rise phase, current perturbation undergoes transition fromÑ poloidal structure to Ñ and then to Ñ ¿ . At the time of current termination, Ñ ¾ perturbation is observed. It is observed that the mode frequency remains nearly constant ( 10 kHz) when poloidal mode structure changes from Ñ to Ñ ¾ . This may be either an indication of mode coupling or a consequences of changes in the plasma electron temperature and density scale length.

No abstract

This paper discusses a large area multifilamentary plasma source used in the large volume plasma device. This source, based on directly heated filaments, is simple in design and produces quiescent (δn/n ≈ 1%) plasmas of high density ( 10 18 m −3 ), low temperature (∼1-2 eV), over a large area (≈1.1 m 2 ) and a large volume (≈1.6 m 3 ). With the investment of ≈40 kW (1350 A, 30 V) power, the filaments are heated to ≈2000 K to yield emission current density ∼1 A cm −2 at the filament surface. Experiments demonstrate that this source is suitable for carrying out electromagnetic wave excitation studies in the electron magnetohydrodynamics regime. There are certain inherent difficulties associated with direct heating which sets a maximum limit to the filament length and with the requirement of field tailoring. As far as the present study is concerned, these difficulties are acceptable in comparison with the distinct advantages the source possesses, in terms of its low cost and technical features, making it user-friendly.

The spectral characteristics of fluctuation induced particle transport in the scrape-off layer region (SOL) of the ADITYA tokamak are measured. The transport is found to be inward at frequencies lower than 20 kHz and outward at higher frequencies. The sense of the observed inward particle transport is in agreement with that predicted by ionization driven drift wave turbulence theory. This observation suggests a very interesting method of controlling the particle confinement time by partial control of the long wavelength ionization instability

The wavelet bicoherence technique is used to examine floating potential fluctuations in ADITYA tokamak edge plasma, which have shown vortex-like coherent structures on statistical basis [Proceedings of the 15th International Conference on Plasma Physics and Controlled Nuclear Fusion, Seville (International Atomic Energy Agency, Vienna, 1995), Vol. 1, p. 583]. On a single discharge basis, the data exhibit intermittent episodes of quadratic interaction in which low and high frequency components are coupled. The broadband nonlinear phase coupling is used as a diagnostic signature of localized coherent structure. It is observed that the wavelet bicoherence is high in the presence of sparse structures and small when the structures are closely packed. The turbulent edge plasma has regions of packed Gaussian structures and sparse non-Gaussian structures.