ASYMMETRY-INDUCED TRANSPORT

External azimuthal asymmetries provide a torque that can change the total angular momentum of our cylindrical plasmas. Due to the fact that the angular momentum is predominantly electromagnetic rather than mechanical, changes in angular momentum are brought about by the radial transport of particles.

Static Asymmetries, such as inherent trap asymmetries, drag on our rotating plasmas, decreasing the angular momentum and causing a radial expansion of particles toward the trap walls. This is the primary source of particle loss in most cylindrical Penning-Malmberg traps.

Non-static Asymmetries, such as the "rotating wall", can spin faster than the plasma, increasing the angular momentum and causing a compression of particles toward the trap axis.

Static Asymmetries

From measurments of transport due to static asymmetries we find that the transport properties of our non-neutral plasmas depend upon the plasma Rigidity R=f_b/f_E and Collisionality.

LOW COLLISIONALITY

Most experiments have measured transport in low collisionality electron plasmas, where the electron-electron collision frequency is much less than the axial bounce frequency (i.e. v_ee < f_b/100). In this regime, we divide the transport up into two sub-regimes:

Low-Rigidity (R<10) or "floppy" regime

High-Rigidity (R>20) or "rigid" regime

In the two different regimes, the transport is found to scale differently with asymmetry strength and plasma parameters (see abstract below).

HIGH COLLISIONALITY

Some experiments conducted on high-collisonality (i.e. v_ee > f_b/100) pure-ion and pure-electron plasmas have measured rates that are independent of changes in plasma temperature (unlike low-collisionality plasmas). This transport regime has not been well investigated and only consists of a few measurements of inherent asymmetry transport in cold and dense (floppy) plasmas.

Abstract from talk given by Jason Kriesel at the 1999 Non-Neutral Plasma Workshop in Pricenton NJ:

In a cylindrical trap, azimuthally asymmetric electric or magnetic fields (such as inherent trap asymmetries) cause the cross-magnetic-field transport of particles, leading to bulk radial expansion and eventually to particle loss at the trap walls. Experiments with applied electrostatic asymmetries identify two different transport regimes, ``slightly-rigid" and ``highly-rigid". Here the plasma rigidity, R = f_b/f_E, is the ratio of the axial bounce frequency to the azimuthal ExB rotation frequency. In the slightly-rigid regime (1 < R <10), the transport scales as V_a R^-2, where V_a is the applied asymmetry strength. This R^-2 scaling is proportional to L²/B², which has previously been observed for transport due to inherent trap asymmetries. The R^-2 mechanism appears to ``turn-off" as the rigidity is increased into the range R > 10. In the highly-rigid regime (R > 20), the transport is roughly independent of the rigidity and scales as V_a².
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