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Positrons in the energy range of 3-30 MeV, produced by x rays emitted by betatron motion in a plasma wiggler of 28.5 GeV electrons from the SLAC accelerator, have been measured. The extremely high- strength plasma wiggler is an ion column induced by the electron beam as it propagates through and ionizes dense lithium vapor. X rays in the range of 1-50 MeV in a forward cone angle of 0.1 mrad collide with a 1.7 mm thick tungsten target to produce electron-positron pairs. The positron spectra are found to be strongly influenced by the plasma density and length as well as the electron bunch length. By characterizing the beam propagation in the ion column these influences are quantified and result in excellent agreement between the measured and calculated positron spectra. High-energy physicists use electron-positron collisions to validate the predictions of various field theories. The positrons (e ) needed for the collisions are currently pro- duced by bombarding a high-Z, solid target that is several radiation lengths (X0) thick with a high-energy electron beam (1). The resulting interaction creates bremsstrahlung x-ray photons which can interact with the atomic nuclei of the target producing electron-positron pairs. These cur- rently used ''thick-target'', bremsstrahlung esources may well fail when scaled to meet the requirements of future linear colliders, due to thermal stress fatigue from large volumetric ebeam energy deposition into the target from ionization and multiple scattering. One way to reduce this problem is to produce the x rays separately from a thin (<X0), high-Z target where the x-ray conversion to e -e

Positron Production by X Rays Emitted by Betatron Motion in a Plasma Wiggler