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

Abstract

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×B rotation frequency, ωE (with ωE∝ plasma density), approaches the applied frequency, ωRW. 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 4.8T magnetic field with ∼109 electrons. The plasmas have initial and final temperatures of ∼0.1eV. They can be compressed to steady-state densities >1010cm−3 and plasma radii <200μm. The outward, asymmetry-driven plasma transport rate, Γo, of the compressed plasmas is independent of density, n, in contrast to the behavior at lower densities where Γo∝n2. 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.