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A design study has been performed for a positron beam with an energy of 500 MeV to be realized at the applied physics area of the Mainz Microtron MAMI. Positrons will be created after pair conversion of bremsstrahlung, produced by the 855 MeV electron beam at MAMI in a tungsten converter target. From the two conceivable geometries (1) pair conversion in the bremsstrahlung converter target itself, and (2) bremsstrahlung pair conversion in a separated lead foil, the former was considered in detail. Positrons will be energy selected within an outside open electron beam-line bending magnet, and bent back by an additional sector magnet. Magnetic focusing elements in between are designed to prepare in a well shielded positron target chamber about 6 m away from the target a beam with horizontal and vertical emittances of ϵ v = 0.055 π mm mrad (1 σ ), and ϵ h = 0.12 π mm mrad (1 σ ), respectively, for a 10 μ m thick amorphous tungsten target and negligible momentum spread. At an accepted positron band width of 1 MeV, spots are expected vertically with an angular spread of 0.064 mrad and a size of 5.0 mm (FWHM), and horizontally with an angular spread of 0.64 mrad and a size of 7.7 mm (FWHM). The positron yield amounts to 13.1 per second, 1 MeV positron energy band width, and 1 nA electron beam current. [ABSTRACT FROM AUTHOR]

The nonlinear features of Ion Acoustic (IA) waves are studied in a fully relativistic three-dimensional (3-D) plasma system with consideration of effect of both positron beam and trapped electrons. We consider a set of 3-D magnetised hydrodynamic equations with pressure expansion for our plasma model along with kinetic Vlasov equation for electrons. Applying the perturbative expansion technique, a Modified Korteweg-de Vries (m-KdV)-like equation is derived, exhibiting the evolution of small amplitude IA waves in plasma. The modified coefficient of nonlinear term in K-dV equation has arrived due to impact of vortex-like distribution of electrons. An analytical and numerical investigation of the nonlinear evolution equations is exhibited with external magnetic field effects, the time derivative pressure expansion as well as other parameters like relativistic effect, mass variation, beam velocity and temperature effect have been taken into consideration. The presence of vortex like trapped electron distribution and positron beam governs the influence of soliton structure quite significantly. The present result should help us to understand the experiments that involve particle trapping and also the salient features of astrophysical environment like ionospheric plasma together with situations in plasma describing the electrostatic solitary structures usually seen in antimatter-related environment in interplanetary region. [ABSTRACT FROM AUTHOR]