Centrifugal Separation

Due to the plasma rotation, ion species with different charge-to-mass ratios tend to centrifugally separate in a Penning trap [30, 31]. If ions with different q/m rotate about the trap axis at the same radius, they will tend to rotate with different rates because of different centrifugal forces. Collisional drag will cause a radial drift of the lighter ions inward, and the heavier ions outward, if the ions have the same charge. The different species will separate and the whole plasma will come to thermal equilibrium and rotate at uniform ur as a rigid body [31]. At low temperatures the density inside either species is constant, and drops to zero at the species boundaries within a distance on the order of the temperature-dependent Debye length. Therefore, trapped positrons, if cooled, will move to smaller radii than the 9Be+ ions. In the limit of zero temperature, the edges of each plasma will be sharp (Debye length-» 0), and the plasmas will completely separate, with the positrons forming a column of uniform density along the trap axis. If the 9Be+ plasma density is significantly below the Brillouin limit, the e+ and 9Be+ densities

Frequency (kHz)

Frequency (kHz)

Figure 3. Positron cyclotron resonance. (a)Resonance curve while stepping the microwave frequency across the resonance. b)9Be+ fluorescence signal when the applied microwaves are chopped at 22 Hz. The curve in (a) is a B-spline through the data points obtained by measuring peak-to-peak values of (b). For each step the data in (b) were averaged on a multichannel scaler for > 5 min.

Figure 3. Positron cyclotron resonance. (a)Resonance curve while stepping the microwave frequency across the resonance. b)9Be+ fluorescence signal when the applied microwaves are chopped at 22 Hz. The curve in (a) is a B-spline through the data points obtained by measuring peak-to-peak values of (b). For each step the data in (b) were averaged on a multichannel scaler for > 5 min.

are expected to be approximately equal and the plasma separation quite small [7, 31].

Figure 4 shows an image of a 9Be+ - e+ plasma along with the radial dependence of the fluorescence signal. The 9Be+ ion density no is calculated from the rotation frequency wr set by the rotating wall. With approximately equal density for both species, the number of positrons in the "dark" column of the plasma image is no x V, where V is the volume of the "dark" region.

If any ions with a charge-to-mass ratio greater than 9Be+ are created during the positron accumulation, they will also centrifugally separate and contribute to the size of the non-fluorescing column in the plasma center. With the 22Na source blocked, we deliberately created singly charged light-mass ions by ionizing background gas with a ~15 eV

jl i—■—i—■—i—•—i—»—i—■■ ■■ i ■•I—i—i—i—1—i

Figure 4• Two species 9Be+ - e+ plasma: a) camera image and b) radial variation of fluorescence signal integrated over z.

electron beam. Prom the volume of the central dark region as a function of time, the lifetime of the light-mass ions was measured to be less than 10 hours. Similar measurements were performed after accumulating positrons and are discussed in more detail in the next section. In this case very little change in the volume of the central dark region was observed after the 22Na source was blocked for 12 hours. This indicates that most of the dark central region in Fig. 4 is due to positrons rather than impurity ions of light mass.

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