Pulsed Beam Production

Several research groups have been investigating the use of Penning traps for various aspects of beam formation and handling. Penning traps are currently employed to capture positron pulses from LINACS for pulse-stretching applications [27, 28]. The capture and cooling of positrons from a radioactive source using laser-cooled ions in a Penning trap is being investigated for the production of ultra-cold positron beams [22, 23]. For the experiments described here, the high efficiency buffer gas technique described in Sec. 2 is used.

High quality positron beams can be produced from the trapped positrons by releasing them in a controlled manner from the trap. This is accomplished using the axial potential profile shown in Fig. 2(a). An asymmetrical well is created to ensure that positrons exit the trap in one direction only. The exit gate potential is held constant to fix the beam energy. The positrons are ejected from the trap by reducing the depth of the potential well. This is carried out either in a series of steps to create pulsed beams, or as a steady ramp to produce a continuous beam. Using this technique, positrons with axial and radial energy spreads as low as 18 meV have been created [13]. These are the coldest positron beams that have been created to date using any technique. These beams are currently being used to measure positron-molecule and positron-atom cross sections in the largely unexplored energy regime below 1 electron volt [14, 15].

The pulses produced by this technique are of the order of the bounce time of positrons in the trap, which is typically ~0.1-1 /j,s. These pulses are suitable for a variety of applications but for some applications such as positron annihilation lifetime spectroscopy (PALS) [1], subnanosecond pulses are required. Pulses of this duration can be produced using the more advanced technique shown in Fig. 2(b). The positrons are dumped from the trap by applying a quadratic potential profile to the entire positron flight path, leading to spatial and temporal focusing at the target [5, 29].

To first order, the pulse width is independent of the length of the positron cloud and is given approximately by [30]:

where e and m are the charge and mass of the positron, respectively, Vo is the magnitude of the applied potential, AE is the energy spread of the positrons, and zq is the length of the buncher. In practice, one might have V0 = 500V, z0 = 0.1 m and AE = 0.025 eV, yielding At ~ 150 ps, which would be suitable for PALS. To achieve this performance in a conventional beam line would require multiple stages of rf bunching.

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