Conceptual design of a slow positron source based on a magnetic trap

Abstract

A unique 10.3 T superconducting wiggler was designed and manufactured at BINP SB RAS. The installation of this wiggler in the SPring-8 storage ring provides a possibility to generate a high-intensity beam of photons (SR) with energy above 1 MeV (Ando et al., J. Synchrotron Radiat. 5 (1998) 360). Conversion of photons to positrons on high- Z material (tungsten) targets creates an integrated positron flux more than 10 13 particles per second. The energy spectrum of the positrons has a maximum at 0.5 MeV and the half-width about 1 MeV (Plokhoi et al., Jpn. J. Appl. Phys. 38 (1999) 604). The traditional methods of positron moderation have the efficiency ε= N s/ N f of 10 −4 (metallic moderators) to 10 −2 (solid rare gas moderators) (Mills and Gullikson, Appl. Phys. Lett. 49 (1986) 1121). The high flux of primary positrons restricts the choice to a tungsten moderator that has ε≈10 −4only (Schultz, Nuc. Instr. and Meth. B 30 (1988) 94). The aim of our project is to obtain the moderation efficiency ε⩾10 −1. We propose to moderate the positrons inside a multi-stage magnetic trap based on several (3–6) electromagnetic traps that are connected in series. Magnetic field of the traps grows consecutively from stage to stage. We propose to release the positrons from the converter with the use of an additional relativistic electron beam passing in synchronism with the SR pulse in the vicinity of the converter. The average electron beam energy and current are 1–2 MeV and 100 mA, respectively. The electrical field of the beam is high enough to distort the positron paths by an amount comparable with the Larmor radius. The further drift of the positrons to the trap axis will occur due to the strengthening of the magnetic field. The magnetic field amplitude of adjacent traps varies in time in the antiphase and increases from 0.9 T in the first stage to 6 T in the last one. The positron transition from stage to stage takes place at the moment of the field equalization. The removal and longitudinal confinement of the positrons is provided by the electrostatic switches that separate the stages. Mixing of the positron velocity components occurs due to the scattering on the neutral gas. The gas pressure is sustained at a level of 10 −6 Torr inside the third stage of the trap and the positron energy decreases down to 1 keV here. The last stage of the device is the Malmberg–Penning trap with a 6 T stationary magnetic field (Malmberg and Driscoll, Phys. Rev. Lett. 44 (10) (1980) 654). The positrons accumulate inside it in 1 s approximately and cool down to energy less than 1 eV.