摘要:

Previous experiments on the Electron Diffusion Gauge showed that the diocotron mode damping increases with higher neutral gas filling pressure. Yet the energy dissipated from a rotating plasma by collisions with neutrals is predicted to excite the mode. To resolve this, experiments have been conducted to examine the coupling between expansion and the m=1 diocotron mode. Results from recent experiments have shown interesting phenomena: 1) The degree and sensitivity of mode growth is observed to be strongly dependent on filament conditions. Mode growth rates of nearly 20 sec−1have been observed even with negligible resistive drive. Specifically, at low filament bias voltages (and correspondingly low electron densities ∼ 1–2 × 107electrons/cm), the mode growth is very sensitive to the heating voltage across the filament, even though changes in filament heating voltage barely affect the plasma expansion, the plasma density profile, the filament emission, or the resulting electron density. 2) At low neutral gas pressure (< 10−9Torr), the diocotron mode growth rate increases with neutral pressure. However, the growth rate is several orders of magnitude larger than theoretical predictions. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1635161

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

The diocotron instability in a low‐density non‐neutral electron plasma is examined via numerical simulations. The vortex method is used in the simulations. A special‐purpose computer, MDGRAPE‐2, accelerates the calculations of the Biot‐Savart integral in the vortex method. The diocotron modes reproduced by the simulations agree with the results obtained by the linear theory. This indicates that the simulation results are qualitatively correct. The linear growth rates of the diocotron instability in the simulations also agree with the theoretical ones. This implies that MDGRAPE‐2 gives the sufficiently accurate results for the calculations of the Biot‐Savart integral. A boundary effect from the conducting wall is discussed. It is concluded that the electric field induced by the conducting wall makes the nonlinear stage unstable and causes the clumps to merge. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1635162

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

The evolution of them&thgr;=1 diocotron mode for hollow density profiles has been an open problem of non‐neutral plasma physics for more than a decade. In fact, the classic 2D fluid model leads to results that are in substantial disagreement with the ones provided by experiments. In the present work, a simplified kinetic model is proposed, in order to provide a quantitative estimate of the role played by kinetic effects on the evolution of the m&thgr;=1 diocotron instability. A two‐fluid model is deduced and a linear analysis of stability is performed, showing that kinetic effects can play an important part in the description of the evolution of them&thgr;=1 diocotron mode. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1635163

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

The present work deals with two important theoretical aspects of the 2D fluid theory for non‐neutral plasmas: the study of the merger between plasma vortices and the non‐linear study of the evolution of the diocotron instability. In both cases, analytical and semi‐analytical models have been deduced and the results have been tested by means of a numerical code based on the contour dynamics technique developed by the Authors. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1635164

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

A vortex crystal is a quasi‐stationary, symmetric array of intense vortices (clumps). A low‐level of background vorticity is experimentally observed to assist three clumps in forming an equilateral triangle starting from initial positions on a linear array. The triangle constitutes a unit cell of a crystal in a many‐vortex system. The background vortex curbs the orbital motion of the clumps with unequal strengths to arrest them at the vertices of an equilateral triangle by wrapping them with different sized belts of depleted vorticity (ring holes). We characterize the contributions of a low‐level background vorticity distribution on the formation of ordered states of clumps. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1635165

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

Observations have shown that two discrete vortices (clumps) immersed in a broad profile of background vorticity merge quickly or form a binary state that lasts for a long period. The different paths of the vortical evolution critically depend on slight differences in the initial background vorticity distribution (BGVD). By a fine control of the BGVD the asymptotic inter‐clump distance is found to be a two‐valued function of∇&zgr;b/&zgr;b2, where &zgr;bis the BGV at the initial location of the clumps, corresponding to the merger and the binary state. The degeneracy is removed by considering the degree of depletion of BGVD between two clumps at the time of their proximity in the initial phase. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1635166

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

Production of antihydrogen atoms by mixing antiprotons with a cold, confined, positron plasma depends on parameters such as the plasma density and temperature. We discuss a non‐destructive diagnostic, based on an analysis of excited, low‐order plasma modes, that provides comprehensive characterization of the positron plasma in the ATHENA antihydrogen apparatus. The dipole and quadrupole modes of a spheroidal positron plasma are interpreted in the framework of a cold fluid theory. In particular, the excitation and detection of the dipole mode are analytically modeled considering the response of the center‐of‐mass to a resonant driving perturbation. The model is compared to, and validated by, numerical simulations with a particle‐in‐cell code. Measurements of the positron plasma properties are discussed. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1635167

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

We demonstrate three‐dimensional annihilation imaging of antiprotons trapped in a Penning trap. Exploiting unusual feature of antiparticles, we investigate a previously unexplored regime in particle transport; the proximity of the trap wall. Particle loss on the wall, the final step of radial transport, is observed to be highly non‐uniform, both radially and azimuthally. These observations have considerable implications for the production and detection of antihydrogen atoms. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1635168

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

Positron traps have been demonstrated to have the capability for producing high‐quality positron beams and intense positron pulses for a variety of scientific and technological applications including atomic physics experiments and probes for materials science. Techniques for accumulating positrons in traps have now advanced to the point where the development of commercial trap‐based positron beam systems is feasible. Trap‐based beams offer advantages over conventional positron beam systems, including the ability to produce ultra‐cold positron beams, giant positron pulses, and ultra‐short pulses. They also enable high‐efficiency brightness enhancement schemes. The design and performance of a new trap‐based positron beam system is described. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1635169

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

A new Penning‐Malmberg trap using a 5 tesla magnetic field and a cryogenic electrode structure (T∼10K) has been constructed with the goal of producing large (N ⩾ 1010), high‐density positron plasmas and cold positron beams (&Dgr;&Vegr; ∼ 1 meV). With background pressures ⩽ 10−11torr and rotating electric fields to counteract plasma expansion due to background asymmetries, this trap is designed to be a nearly ideal reservoir of positrons with very long confinement and annihilation times. This paper describes recent experiments using electron plasmas to optimize confinement and plasma compression, and minimizing the diameters of extracted beams. Further, it is shown that this trap will be an excellent device in which to study the physics issues associated with a recently proposed multi‐cell trap. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1635170

出版商:AIP    

年代:1903

数据来源: AIP