UC San Diego
Tailored charged particle beams from single-component plasmas
- Author(s): Weber, Tobin Robert
- et al.
There are currently many uses of positrons as well as a strong potential for novel applications on the horizon. Due to the scarce nature of antimatter, positron research and technology is frequently limited by the ability to collect, confine, and manipulate antiparticles. Trapping large numbers of positrons as nonneutral plasmas has proven ideal in this endeavor. This thesis focuses on exploiting the attractive properties of single-component positron plasmas to develop new tools for antimatter research. A Penning-Malmberg trap is used to confine single component electron (used for increased data rate) plasmas. The trap consists of a cylindrical electrode structure, centered in the bore of a superconducting magnet. The superconducting magnet supplies a uniform 5 tesla field that provides radial confinement, while voltages applied to both ends of the electrode structure confine the plasma axially. The trap exhibits long confinement times (̃ days) and low plasma temperatures (T < 20 meV). The focus of this thesis is the development of a nondestructive technique to create narrow beams with narrow energy spreads and transverse spatial widths from single-component plasmas in a Penning-Malmberg trap. This technique is valuable for effectively and efficiently utilizing trapped positrons. Beams are extracted by carefully lowering the confining trap potential on one end to some extraction voltage. Due to the plasma space charge, beam pulses ([Delta]t < 10 [mu]sec) emerge from near the plasma center with radii as small as [Rho]b = 2[lamba]D (HW 1/e, and energy spreads [Delta]E ̃ T. Through cyclotron radiation and the rotating wall, the plasma temperature and density is tailored such that beams are narrow ([Rho]b = 2[lamba]D) and ([Delta]E ̃ T) cold, resulting in quality beams of a low emittance. A simple nonlinear model is used to derive equations predicting a wide range of beam properties from only the plasma parameters and the extraction voltage. An expression is first derived for the radial profile of the beam [Sigma]b (r). A relation for the total number of escaping particles as a function of the extraction voltage is developed. From this expression, the full energy distribution of the beam is obtained, as a function of the kinetic energies in direction parallel and perpendicular to the magnetic field. The equations are generically written in terms of the scaled beam number, extraction voltage, and electrode radius, only. The resulting expressions are verified experimentally over a wide range of beam number and electrode radius. General trends in the RMS energy spread of the beams [Delta]E are discussed. The extraction of more than 50 % of a trapped plasma into a train of nearly identical beams is demonstrated. The techniques described above result in beams in a high (e.g., several tesla) magnetic field. However for many applications, such as atomic-physics scattering experiments and the creation of microbeams by electrostatic focusing and remoderation, beams in a magnetic-field free region are desired. For these applications, a technique is described to create high- quality electrostatic beams by extracting the initial beam from the confining magnetic guide field. The beam is first adiabatically transferred to a low field region, then brought through a magnetic shield to a region of zero field by means of a nonadiabatic fast extraction. Once in this zero field region, the beam is focused to smaller transverse dimensions using an electrostatic (Einzel) lens. This technique is shown to produce quality electrostatic beams in an efficient and reproducible way. Potential applications and the possibilities for further advances in magnetically guided and electrostatic beams are discussed. Finally, results of RW compression in low fields (B=2 T) are briefly reported. Difficulties encountered while operating the RW in low fields (e.g., B=2 T) are presented and discussed