A schematic diagram of the experimental setup. Both the anode plate and the end plate have multicusped magnetic mirrors mounted on them and they are electrically grounded along with the vacuum vessel. The side view (looking from narrow source end) shows the poloidal ion and electron drift directions along with a cross-sectional view of electric and magnetic probes.
(a) A typical probe characteristics of Langmuir probe obtained in the LVPD plasma at a location, , , and , (b) electron current after subtracting the ion current from the total probe current, and (c) the selected region of the bias voltage that gives straight line fit for determining electron temperature, .
Three traces each of: (a) discharge currents, , (b) ion saturation currents, , and (c) floating potential, , when the LVPD is operated with the narrow source. The negative during turn off is an artifact of droop characteristics of the CT and it does not indicate any plasma feedback. The ion saturation current exhibits fluctuation for plasma density exceeding . The traces in (b) and (c) are obtained at the location , , and .
The induced voltage, when pickup coil is rotated from to 360° with respect to the direction of . Here, represents the angle between the coil-normal and the magnetic field.
(a) The ensemble average of 100 discharges (smooth) and a single discharge (with fluctuations) data for (a) ion saturation current and (b) the diamagnetic loop pickup voltage. (c) The magnetic field in presence of plasma was obtained from the ensemble-averaged signal of (b). All measurements were at .
Evolution of plasma density profile during discharge pulse. The evolution is broadly divided into three parts, i.e., the formation, the steady state, and the decay phases. The steady state phase is attained during 6–9 ms in the discharge.
Evolution of floating potential profile during the discharge. The profile shows absence of electric field in the core region during the steady state phase.
Radial profiles of plasma parameters obtained at and during steady state. (a) Plasma electron density, , (b) electron temperature, , (c) plasma potential, , and (d) net magnetic field after diamagnetic expulsion when the LVPD is operated with the narrow source.(e)–(h) represents similar measurements for the broad source.
Radial profiles of fluctuations measured at a location and . (a) Axial magnetic field, and (b) ion saturation current, . The data for the narrow source are represented by the bullets (with large error bars) and by the open circles when the LVPD is operated at 6 and 11 G, respectively. The data for the broad source (at 6 G) are represented by “” symbols. The dashed line in (a) represents capacitive noise of the pickup coils. A significant suppression of fluctuations is observed for the broad source.
Normalized density and magnetic field fluctuations measured at a location, , , and for different applied axial magnetic field. Since plasma density and electron temperature do not change significantly, the -axis can be converted into plasma beta, .
(a) Time series of simultaneously measured from the probe array and (b) cross-correlation functions between various Langmuir probe pairs (with increasing separation). The estimated poloidal phase velocity (between probe 1 and 5) , (c) cross power spectrum, and (d) coherency spectrum of fluctuations signifies the existence of low frequency mode.
Radial profile of azimuthal phase velocity, , of fluctuation (bullets) and the drift velocity as determined from the fitted form of the measured plasma potential. The corrected azimuthal phase velocity, , is shown by cross symbol.
(a) Power spectra for ion current fluctuation at a location , , and , (b) radial profile of electron temperature shows a gradient region in the core plasma, and (c) contour plot of autopower with frequency and radial distance. The spectral power is concentrated in the core region and exhibits broad peak.
Contour plot of joint wave number-frequency spectrum, for (a) and . The spectrum also indicates that fluctuations in both propagate in electron diamagnetic drift direction.
[(a), (b), (d), and (e)] Time series of different fluctuation data from the same plasma discharge, (c) cross-correlation function of and data, and (f) cross-correlation function of and data. The data presented here are for probe location, , , and .
The axial variation in (a) fluctuations, three regions are demarcated as A, B, and C. They represent the cusp region, cathode region and the uniform plasma region, (b) fluctuations. Fluctuations are maximizing much away from the cusp region. Observation rules out the origin of instability near the mirrors or near the filaments.
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