R. G. Greaves

Creation and uses of positron plasmas

Advances in positron trapping techniques have led to room‐temperature plasmas of 107 positrons with lifetimes of 103 s. Improvements in plasma manipulation and diagnostic methods make possible a variety of new experiments, including studies just being initiated of electron–positron plasmas. The large numbers of confined positrons have also opened up a new area of positron annihilation research, in which the annihilation cross sections for positrons with a variety of molecules have been measured, as well as the energy spread of the resulting gamma rays. Such measurements are of interest for fundamental physics and for the modeling of astrophysical plasmas.

Emerging science and technology of antimatter plasmas and trap-based beams

Progress in the ability to accumulate and cool positrons and antiprotons is enabling new scientific and technological opportunities. The driver for this work is plasma physics research—developing new ways to create and manipulate antimatter plasmas. An overview is presented of recent results and near-term goals and challenges. In atomic physics, new experiments on the resonant capture of positrons by molecules provide the first direct evidence that positrons bind to “ordinary” matter (i.e., atoms and molecules). The formation of low-energy antihydrogen was observed recently by injecting low-energy antiprotons into a cold positron plasma. This opens up a range of new scientific opportunities, including precision tests of fundamental symmetries such as invariance under charge conjugation, parity, and time reversal, and study of the chemistry of matter and antimatter. The first laboratory study of electron-positron plasmas has been conducted by passing an electron beam through a positron plasma. The next maj...

Antimatter plasmas and antihydrogen

Recent successes in confining antimatter in the form of positron and antiproton plasmas have created new scientific and technological opportunities. Plasma techniques have been the cornerstone of experimental work in this area, and this is likely to be true for the foreseeable future. Work by a number of groups on trapping antimatter plasmas is summarized, and an overview of the promises and challenges in this field is presented. Topics relating to positron plasmas include the use of positrons to study the unique properties of electron–positron plasmas, the interaction between positrons and ordinary matter, and the laboratory modeling of positron-annihilation processes in interstellar media. The availability of cold, trapped antiprotons and positrons makes possible the production of neutral antimatter in the form of antihydrogen. This is expected to enable precise comparisons of the properties of matter and antimatter, including tests of fundamental symmetries and the measurement of the interaction of ant...

Creation of a monoenergetic pulsed positron beam

We have developed a versatile, pulsed source of cold (ΔE=0.018 eV), low-energy positrons (E≈0–9 eV). Multiple pulses of 105 positrons, each 10 μs in duration, are extracted from a thermalized, room temperature positron plasma stored in a Penning trap. The frequency, duration, and amplitude of the pulses can be varied over a wide range.

New source of ultra-cold positron and electron beams

Abstract We have developed a new and versatile source of cold ( ΔE≈0.018 eV ) , low-energy ( E≈0−9 eV ) , magnetized positrons and electrons. Particles are extracted from thermalized, room-temperature, single species plasmas confined in a Penning trap. Typically, the trap contains from 107 to 109 particles, which can be released either in the form of a quasi steady-state beam or as a pulsed beam. Of the order of 100 pulses of 105 positrons, each 10 μs in duration, have been achieved. Size, duration and frequency of pulses within a pulse train are easily variable over a wide range. Cold quasi steady-state electron beams with a diameter of ≈3 mm have also been achieved. Limited only by the total charge contained in the trap, electron currents of 0.1 μA for several milliseconds have been generated routinely. This source represents a combination of attractive features not previously available using any single technique.

Radial compression and inward transport of positron plasmas using a rotating electric field

It has recently been demonstrated that positron plasmas confined in a Penning-Malmberg trap can be compressed radially by applying a rotating electric field [Phys. Rev. Lett. 85, 1883 (2000)]. A more complete description of the original experiments is presented, together with the results of new measurements. Good coupling of the rotating electric field is observed over a broad range of frequencies. The heating caused by the rotating field is counteracted by cooling using a polyatomic gas. Rapid compression rates ṅ/n∼15 s−1 can be achieved, with central density increases of a factor of 20 or more. The good coupling and high compression rates can be explained in terms of excitation of heavily damped Trivelpiece–Gould modes, or alternatively as coupling directly to particle bounce resonances. Potential improvements and applications are discussed, including the production of high-density positron plasmas and brightness-enhanced positron beams.

Low‐order longitudinal modes of single‐component plasmas

The low‐order modes of spheroidal, pure electron plasmas have been studied experimentally, both in a cylindrical electrode structure and in a quadrupole trap. Comparison is made between measurements of mode frequencies, recent analytical theories, and numerical simulations. Effects considered include trap anharmonicity, image charges, and temperature. Quantitative agreement is obtained between the predictions and these measurements for spheroidal plasmas in the quadrupole trap. In many experiments on single‐component plasmas, including antimatter plasmas, the standard diagnostic techniques used to measure the density and temperature are not appropriate. A new method is presented for determining the size, shape, average density, and temperature of a plasma confined in a Penning trap from measurements of the mode frequencies.

A multicell trap to confine large numbers of positrons

Abstract The design of a multicell Penning–Malmberg trap capable of confining 10 15 positrons is described. The motivation for choosing a multicell design is discussed, and key factors determining performance are identified. Specific issues for further research and development and possible extensions of this type of design are also discussed.

Positron trapping and the creation of high-quality trap-based positron beams

This paper describes studies of positron accumulation and cooling using molecular gases. The production of high-quality, bright positron beams from trapped positron plasmas is also discussed. Trapping efficiency and cooling-rate measurements are presented for a number of gases. Results are presented for the radial compression of magnetized positron plasmas using a rotating electric field and molecular gas cooling. A technique to exploit plasma space-charge effects to extract the central portion of trapped plasmas is also described. The use of both techniques to produce bright positron beams in new regimes of parameter space is discussed.

Stored positrons for antihydrogen production

The production of antihydrogen is examined in the light of recent experimental results on a technique for the efficient accumulation, manipulation, and storage of positrons. From these data, we argue that this high-efficiency positron trapping technique could be adapted for the production of antihydrogen and would offer significant advantages over other positron trapping techniques currently being proposed for this purpose.