The long confinement times of a non-neutral plasma in a cylindrically symmetric trap depend on the conservation of the total angular momentum. In practice, background neutral gas and asymmetries inherent in the trap construction break the cylindrical symmetry and exert a drag on the non-neutral plasma. This drag causes a lost of the angular momentum and eventually radial particle loss. In general, the radial expansion and loss can be counteracted by applying an external torque from rotating fast waves [1,2].
We report an experimental study focused on controlling the density distribution of a pure electron plasma by using a radio-frequency (RF) field. A rotating electric field is applied across the plasma by transmitting 90-degree-shifted sinusoidal voltages to four wall segments azimuthally separated (m_θ=±1 mode). The plasma is compressed to increase the on-axis density by a factor of 5 within 0.8 s with minimum loss (< 3%) of total particles. The optimum driver frequency lies in 0.4 - 1.2MHz, corresponding to about ωp/30π. Phase-locked frequency ramp-up is more effective than fixed-frequency drive.
We carry out analyses of the momentum transfer in terms of wave exitation and propagation. The signal is detected by listening to the azimuthal wall sectors on the other side of the launched segments with an oscilloscope. The strong enhancement in the received power is observed intermittently during the on-axis density increases. That is to say the coupling between the launched wave and the plasma dynamically changes during the exited frequency ramp-up. We shall extend our analyses to the space-time resolved velocity distribution correlated with nonlocal distribution of the wave field. This experiment has significance in understanding the wave-induced transport as well as in developing effective technique for controlling vortex dynamics.

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