A long confinement time of electron plasma, approaching magnetic pumping transport limit, has been observed in SMARTEX-C (a small aspect ratio partial torus with Ro/a∼1.59). Investigations of the growth rate reveal that they are governed by instabilities like resistive wall destabilization, ion driven instabilities, and electron-neutral collisions. Successful confinement of electron plasmas exceeding >1×105 poloidal E→×B→ rotations lasting for nearly 2.1±0.1 s is achieved by suppressing these instabilities. The confinement time has been estimated in two ways: (a) from the frequency scaling of the linear diocotron mode launched from sections of the wall that are also used as capacitive probes and (b) by dumping the plasma onto a charge collector at different hold times.

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The spectral analysis of floating potential fluctuations measured in the edge plasma region (0.87 < r/a < 1.0) of Saha Institute for Nuclear Physics tokamak (SINP-tokamak) using Langmuir probes reveals the existence of a highly coherent mode with a frequency in the range of 15–21 kHz. Long range correlations in poloidal and toroidal directions are observed over a wide range of plasma discharges having different values of the edge safety factor, from very low qedge (<2) to high qedge (>3). These coherent modes are simultaneously observed in density and radial electric field fluctuation spectra as well. These coherent modes are identified as geodesic acoustic modes (GAMs) having different characteristics over the entire qedge range. In discharges with qedge greater than 3, the local wave number spectra of the mode exhibit the properties of continuum GAM with the observed poloidal and toroidal mode numbers of m ∼ 0 and n ∼ 0, and the mode is radially localized. The observed frequency and its variation with the safety factor for qedge > 3 closely agree with the theoretical predictions using the measured values of temperature. In contrast, for qedge < 3.0, the GAM nature changes from continuum to the Eigenmode as the associated GAM frequency remained uniform at ∼13–17 kHz over the q edge range of 1.5 to 3 and ceased to depend on local temperature. Furthermore, the poloidal wave number of the coherent mode no longer remains zero and is observed to increase when qedge falls below 2.5. Coherent modes in magnetic fluctuations having similar frequencies to those of electrostatic fluctuations are also observed in the discharges with the q edge below 2.5. The coupling of these electrostatic and magnetic modes may be responsible for triggering the Eigenmode GAM.

Experiments with trapped electron plasmas in a SMall Aspect Ratio Toroidal device (SMARTEX-C) have demonstrated a flute-like mode represented by oscillations on capacitive (wall) probes. Although analogous to diocotron mode observed in linear electron traps, the mode evolution in toroids can have interesting consequences due to the presence of in-homogeneous magnetic field. In SMARTEX-C, the probe signals are observed to undergo transition from small, near-sinusoidal oscillations to large amplitude, non-linear “double-peaked” oscillations. To interpret the wall probe signal and bring forth the dynamics, an expression for the induced current on the probe for an oscillating charge is derived, utilizing Green's Reciprocation Theorem. Equilibrium position, poloidal velocity of the charge cloud, and charge content of the cloud, required to compute the induced current, are estimated from the experiments. Signal through capacitive probes is thereby computed numerically for possible charge cloud trajectories. In order to correlate with experiments, starting with an intuitive guess of the trajectory, the model is evolved and tweaked to arrive at a signal consistent with experimentally observed probe signals. A possible vortex like dynamics is predicted, hitherto unexplored in toroidal geometries, for a limited set of experimental observations from SMARTEX-C. Though heuristic, a useful interpretation of capacitive probe data in terms of charge cloud dynamics is obtained.

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