Penning–Malmberg trap

The Penning–Malmberg trap (PM trap), named after Frans Penning and John Malmberg, is an electromagnetic device used to confine large numbers of charged particles of a single sign of charge.

Much interest in Penning–Malmberg (PM) traps arises from the fact that if the density of particles is large and the temperature is low, the gas will become a single-component plasma.

is applied to inhibit positron motion radially, and voltages are imposed on the end electrodes to prevent particle loss in the magnetic field direction.

This is due to the fact that, for a sufficiently strong magnetic field, the canonical angular momentum

of the charge cloud (i.e., including angular momentum due to the magnetic field B) in the direction

is the cyclotron frequency, with particle mass m and charge e. If the system has no magnetic or electrostatic asymmetries in the plane perpendicular to

The PM traps are typically filled using sources of low energy charged particles.

[5] This can be accomplished by applying a torque on the plasma using rotating electric fields [the so-called "rotating wall" (RW) technique],[6][7][8] or in the case of ion plasmas, using laser light.

[9] Very long confinement times (hours or days) can be achieved using these techniques.

Particle cooling is frequently necessary to maintain good confinement (e.g., to mitigate the heating from RW torques).

This can be accomplished in a number of ways, such as using inelastic collisions with molecular gases,[2] or in the case of ions, using lasers.

[9][10] In the case of electrons or positrons, if the magnetic field is sufficiently strong, the particles will cool by cyclotron radiation.

Figure 2 shows an early study of confinement in a PM trap as a function of a background pressure of helium gas.

The latter (“anomalous transport”) mechanism has been shown to be due to inadvertent magnetic and electrostatic asymmetries and the effects of trapped particles.

1) is replaced by a series of coaxial cylinders biased to create a smoothly varying potential well (a “multi-ring PM trap”).

[12] One fruitful area of research arises from the fact that plasmas in PM traps can be used to model the dynamics of inviscid two-dimensional fluid flows.

[14][15][16][17] PM traps are also the device of choice to accumulate and store anti-particles such as positrons and antiprotons.

[21] The PM traps also provide an excellent source for cold positron beams.

Fig. 1. Schematic diagram of a Penning–Malmberg trap biased to confine positively charged particles in a set of three cylindrical metal electrodes (green and blue). Due to the particles’ charge, there is a radial electric field which causes the plasma to rotate about the magnetic field direction with angular velocity ω r . See Ref. [ 2 ] and for details.
Fig. 2. Decay time of the central density of a pure electron plasma as a function of helium gas pressure at magnetic fields of (□) 0.07, (⋄) 0.02, and (○) 0.004 tesla. Adapted from Ref. [ 13 ]