Magnetizing a Complex Plasma without a Magnetic Field

We propose and demonstrate a concept that mimics the magnetization of the heavy dust particles in a complex plasma while leaving the properties of the light species practically unaffected. It makes use of the frictional coupling between a complex plasma and the neutral gas, which allows us to transfer angular momentum from a rotating gas column to a well-controlled rotation of the dust cloud. This induces a Coriolis force that acts exactly as the Lorentz force in a magnetic field. Experimental normal mode measurements for a small dust cluster with four particles show excellent agreement with theoretical predictions for a magnetized plasma.

Figure 1

Estimates of ${\omega }_{\mathrm{c}}/{\omega }_0$ in various SCPs: White dwarf stars [7] (${\rho }_m\gtrsim {10}^9\mathrm{kg}{\mathrm{m}}^{-3}$, $B\lesssim {10}^5\mathrm{T}$), neutron stars [14] (${\rho }_m\sim {10}^7\dots {10}^{14}\mathrm{kg}{\mathrm{m}}^{-3}$, $B\lesssim {10}^{11}\mathrm{T}$), ions in Penning traps [13, 16] ($n\sim {10}^7\dots {10}^8{\mathrm{cm}}^{-3}$, fields of several Tesla), and complex plasmas [8, 10, 11]. The method presented in this Letter (rotating complex plasmas) covers a wide range of magnetizations at room temperature.

Figure 2

Left: Variation of the effective magnetization, the coupling parameter, screening parameter, and damping constant with the rotation frequency in a finite two-dimensional system. Right: Scaling of $\Gamma$ and $\kappa$ as a function of the effective magnetization $2\Omega /{\overline{\omega }}_{\perp }$ for typical complex plasma conditions.

Figure 3

Experimental setup: Dust particles are confined in a layer approximately 1 cm above the driven electrode. The electric field within the plasma sheath compensates gravity and provides vertical confinement, while horizontal confinement is realized by a cylindrical cavity of 20 mm in diameter and 2 mm in depth. The particles are illuminated from the side by a laser fan and can be observed from the bottom through a transparent electrode coated with indium tin oxide (ITO). The top electrode is rotatable.

Figure 4

Comparison of experiment and theory: Frequencies of the seven nontrivial normal modes of $N=4$ dust particles as a function of the scaled rotation frequency. Symbols denote experimental results (${\omega }_{\perp }/2\pi =2.52{\mathrm{s}}^{-1}$), lines are the theoretical eigenfrequencies with $\kappa (0)=0.7$. The eigenvectors in the unmagnetized limit [36] ($\Omega =0$) are sketched for four particular modes. Note that the two modes (iii) are degenerate in this case. The breathing mode and the two center-of-mass modes are indicated by “br” and “${\mathrm{cm}}^{\pm }$,” respectively. The effective magnetization $2\Omega /{\overline{\omega }}_{\perp }$ is shown on the upper axis [Eq. (1)].