Saikat Chakraborty Thakur

Suppression of drift wave turbulence and zonal flow formation by changing axial boundary conditions in a cylindrical magnetized plasma device

For drift wave turbulence, due to charge conservation, the divergence of the parallel current is coupled to the divergence of the perpendicular polarization current, which determines the effective radial momentum flux, i.e., the Reynolds stress. Changes in the current flow patterns also affect the nonlinear energy transfer from smaller to larger scales. Here, we show that by changing the end plate boundary conditions in a cylindrical plasma device, the radial currents through the plasma and hence the net momentum transport and the nonlinear coupling for the inverse energy transfer are strongly modified. The transition to drift wave turbulence and the formation of low frequency zonal flows can be either suppressed with conducting boundaries or enhanced with insulating boundaries.

Threshold for formation of a stable double layer in an expanding helicon plasma

We report detailed studies of a threshold antenna frequency for the creation of an ion beam due to the formation of a stable electric double layer (DL) in an expanding, low pressure, argon helicon plasma. Mutually consistent measurements of ion beam energy and density obtained with a retarding field energy analyzer and laser-induced-fluorescence indicate that a stable ion beam of approximately 15 eV appears for antenna frequencies above 11.5 MHz. At lower antenna frequencies, for which the rf coupling to the plasma improves, large electrostatic instabilities appear downstream of the expansion region and a well-formed ion beam is not observed. Further studies of the low frequency (∼17.5 kHz) electrostatic fluctuations suggest that they arise from beam-driven, ion acoustic instabilities. We also observe a sharp increase in the upstream density at the same threshold antenna frequency, confirming a theoretical model that predicts an increase in the upstream density due to enhanced ionization resulting from electrons accelerated upstream by the DL. (Some figures in this article are in colour only in the electronic version)

Measurements of neutral helium density in helicon plasmas.

Laser-induced-fluorescence (LIF) is used to measure the density of helium atoms in a helicon plasma source. For a pump wavelength of 587.725 nm (vacuum) and laser injection along the magnetic field, the LIF signal exhibits a signal decrease at the Doppler shifted central wavelength. The drop in signal results from the finite optical depth of the plasma and the magnitude of the decrease is proportional to the density of excited state neutral atoms. Using Langmuir probe measurements of plasma density and electron temperature and a collisional-radiative model, the absolute ground state neutral density is calculated from the optical depth measurements. Optimal plasma performance, i.e., the largest neutral depletion on the axis of the system, is observed for antenna frequencies of 13.0 and 13.5 MHz and magnetic field strengths of 550-600 G.

Continuous wave cavity ring down spectroscopy measurements of velocity distribution functions of argon ions in a helicon plasma

We report continuous wave cavity ring down spectroscopy (CW-CRDS) measurements of ion velocity distribution functions (VDFs) in low pressure argon helicon plasma (magnetic field strength of 600 G, T(e) ≈ 4 eV and n ≈ 5 × 10(11) cm(-3)). Laser induced fluorescence (LIF) is routinely used to measure VDFs of argon ions, argon neutrals, helium neutrals, and xenon ions in helicon sources. Here, we describe a CW-CRDS diagnostic based on a narrow line width, tunable diode laser as an alternative technique to measure VDFs in similar regimes but where LIF is inapplicable. Being an ultra-sensitive, cavity enhanced absorption spectroscopic technique; CW-CRDS can also provide a direct quantitative measurement of the absolute metastable state density. The proof of principle CW-CRDS measurements presented here are of the Doppler broadened absorption spectrum of Ar II at 668.6138 nm. Extrapolating from these initial measurements, it is expected that this diagnostic is suitable for neutrals and ions in plasmas ranging in density from 1 × 10(9) cm(-3) to 1 × 10(13) cm(-3) and target species temperatures less than 20 eV.