Space Charge Effects in Cyclotron Gas Stopper

The cyclotron gas stopper is a newly proposed device to stop energetic rare isotope ions from projectile fragmentation reactions in a helium-filled chamber. The radioactive ions are slowed down by collisions with a buffer gas inside a cyclotron-type magnet and are extracted via interactions with a Radio Frequency (RF) field applied to a sequence of concentric electrodes (RF carpet). The present study focuses on a detailed understanding of space charge effects in the ion extraction region. The space charge is generated by the ionized helium gas created by the stopping of the ions and eventually limits the beam rate. Particle-in-cell simulations of a two-component (electron-helium) plasma interacting via Coulomb forces were performed in the space charge field created by the stopping beam.Comment: 9 pages, 2 tables, 8 figure

Dynamics of Planetary Systems within Star Clusters: Aspects of the Solar System’s Early Evolution

Most planetary systems—including our own—are born within stellar clusters, where interactions with neighboring stars can help shape the system architecture. This paper develops an orbit-averaged formalism to characterize the cluster's mean-field effects, as well as the physics of long-period stellar encounters. Our secular approach allows for an analytic description of the dynamical consequences of the cluster environment on its constituent planetary systems. We analyze special cases of the resulting Hamiltonian, corresponding to eccentricity evolution driven by planar encounters, as well as hyperbolic perturbations upon dissipative disks. We subsequently apply our results to the early evolution of our solar system, where the cluster's collective potential perturbs the solar system's plane, and stellar encounters act to increase the velocity dispersion of the Kuiper Belt. Our results are twofold. First, we find that cluster effects can alter the mean plane of the solar system by ≾1° and are thus insufficient to explain the ψ ≈ 6° obliquity of the Sun. Second, we delineate the extent to which stellar flybys excite the orbital dispersion of the cold classical Kuiper Belt and show that while stellar flybys may grow the cold belt's inclination by the observed amount, the resulting distribution is incompatible with the data. Correspondingly, our calculations place an upper limit on the product of the stellar number density and residence time of the Sun in its birth cluster, η τ ≾ 2 × 10⁴ Myr pc⁻³

Undulator Background in the Final Focus Test Beam Experiment with Polarized Positrons

In the proposed E-166 experiment at SLAC, 50 GeV electrons pass through a helical undulator, and produce circularly polarized photons, which interact with a tungsten target and generate longitudinally polarized positrons. The background is an important issue for an experiment under consideration. To address this issue, simulations were performed with the code GEANT3 to model the production of secondary particles from high-energy electrons hitting an undulator. The energy density of photons generated at the target has been analyzed. Results of the simulations are presented and discussed