Radial compression and torque-balanced steady states of single-component plasmas in Penning-Malmberg traps

Penning-Malmberg traps provide an excellent method to confine single-component plasmas. Specially tailored, high-density plasmas can be created in these devices by the application of azimuthally phased rf fields (i.e., the so-called “rotating wall” technique). Recently, we reported a regime of compression of electron (or positron) plasmas in which the plasma density increases until the $E\times B$ rotation frequency, ${\omega }_E$ (with ${\omega }_E\propto$ plasma density), approaches the applied frequency, ${\omega }_{\mathrm{RW}}$ . Good compression is achieved over a broad range of rotating wall frequencies, without the need to tune to a mode in the plasma. The resulting steady-state density is only weakly dependent on the amplitude of the rotating-wall drive. Detailed studies of these states are described, including the evolution of the plasma temperature, peak density, and density profiles during compression; and the response of the plasma, once compressed, to changes in frequency and rotating-wall amplitude. Experiments are conducted in a $4.8\mathrm{T}$ magnetic field with $\sim {10}^9$ electrons. The plasmas have initial and final temperatures of $\sim 0.1\mathrm{eV}$ . They can be compressed to steady-state densities $>{10}^{10}{\mathrm{cm}}^{-3}$ and plasma radii $<200\mu \mathrm{m}$ . The outward, asymmetry-driven plasma transport rate, ${\Gamma }_o$ , of the compressed plasmas is independent of density, $n$ , in contrast to the behavior at lower densities where ${\Gamma }_o\propto n^2$ . The implications of these results for the creation and confinement of high-density electron and positron plasmas and the creation of finely focused beams are discussed.