J. R. Danielson

Plans for the creation and studies of electron–positron plasmas in a stellarator

Electron-positron plasmas are unique in their behavior due to the mass symmetry. Strongly magnetized electron-positron, or pair, plasmas are present in a number of astrophysical settings, such as astrophysical jets, but they have not yet been created in the laboratory. Plans for the creation and diagnosis of pair plasmas in a stellarator are presented, based on extrapolation of the results from the Columbia Non-neutral Torus stellarator, as well as recent developments in positron sources. The particular challenges of positronium injection and pair plasma diagnostics are addressed.

Dependence of positron―molecule binding energies on molecular properties

Positron annihilation on many molecular species occurs via capture into vibrational Feshbach resonances. The study of the downshifts in the energy of these resonances from the vibrational modes in the molecule using a tunable, high-resolution positron beam provides a measure of the positron‐molecule binding energy. Regression analysis on data for 30 molecules is used to identify the molecular properties that affect these binding energies. One parameterization that fits the data well involves a linear combination of the molecular dipole polarizability, the permanent dipole moment and the number of π bonds in aromatic molecules. The predictions of this empirical model are compared with those from positron‐molecule binding energy calculations. They are also tested in cases where other experimental evidence indicates that molecules do and do not bind positrons. Promising candidate molecules for further experimental and theoretical investigation are discussed. (Some figures in this article are in colour only in the electronic version)

Radial compression and torque-balanced steady states of single-component plasmas in Penning-Malmberg traps

Penning-Malmberg traps provide an excellent method to confine single-component plasmas. Specially tailored, high-density plasmas can be created in these devices by the application of azimuthally phased rf fields (i.e., the so-called “rotating wall” technique). Recently, we reported a regime of compression of electron (or positron) plasmas in which the plasma density increases until the E×B rotation frequency, ωE (with ωE∝ plasma density), approaches the applied frequency, ωRW. Good compression is achieved over a broad range of rotating wall frequencies, without the need to tune to a mode in the plasma. The resulting steady-state density is only weakly dependent on the amplitude of the rotating-wall drive. Detailed studies of these states are described, including the evolution of the plasma temperature, peak density, and density profiles during compression; and the response of the plasma, once compressed, to changes in frequency and rotating-wall amplitude. Experiments are conducted in a 4.8T magnetic fiel...

Plasma manipulation techniques for positron storage in a multicell trap

New plasma manipulation techniques are described that are central to the development of a multicell Penning trap designed to increase positron storage by orders of magnitude (e.g., to particle numbers N⩾1012). The experiments are done using test electron plasmas. A technique is described to move plasmas across the confining magnetic field and to deposit them at specific radial and azimuthal positions. Techniques to fill and operate two in-line plasma cells simultaneously, and the use of 1kV confinement potentials are demonstrated. These experiments establish the capabilities to create, confine, and manipulate plasmas with the parameters required for a multicell trap; namely, particle numbers >1010 in a single cell with plasma temperature ⩽0.2eV for plasma lengths ∼10cm and radii ⩽0.2cm. The updated design of a multicell positron trap for 1012 particles is described.

Extraction of small-diameter beams from single-component plasmas

A nondestructive technique is described to extract small-diameter beams from single-component plasmas confined in a Penning-Malmberg trap following radial compression using a rotating electric field. Pulsed beams with Gaussian radial profiles and diameters as small as 50μm are extracted from electron plasmas initially 2mm in diameter. A simple theory for the beam diameter predicts 4λD (full width to 1∕e), where λD is the Debye length, in good agreement with experimental measurements on electron plasmas. Applications and extensions of this technique to create bright, finely focused beams of positrons and other scarce particles are discussed.

Electrostatic beams from tailored plasmas in a Penning-Malmberg trap

In recent work, a technique was developed to extract high quality beams from single-component plasmas confined in a Penning-Malmberg trap in a 4.8 T magnetic field. In this paper, a procedure is developed to extract these beams from the confining magnetic field and then focus them to create especially tailored electrostatic beams. Electron beams are extracted from the field in two stages: they are first transported to a region of reduced field (1 mT), and then taken to zero field with a nonadiabatic, fast extraction. Once in the field-free region, the beams are focused using an Einzel lens. Experimental results and numerical simulations are presented to illustrate the extraction and focusing process. Theoretical expressions are developed to describe the modifications to the relevant beam energy and spatial distributions. Where possible, analytic expressions are presented for the case relevant here of beams with Gaussian radial profiles. Beam emittance considerations are discussed as well as prospects for further development of these techniques. Application of these techniques to provide high-quality positron beams is also discussed.

Creation of finely focused particle beams from single-component plasmas

In a recent communication [Danielson et al., Appl. Phys. Lett. 90, 081503 (2007)], a nondestructive technique was described to create finely focused beams of electron-mass, charged particles (i.e., electrons or positrons) from single-component plasmas confined in a Penning–Malmberg trap. This paper amplifies and expands upon those results, providing a more complete study of this method of beam formation. A simple model for beam extraction is presented, and an expression for a Gaussian beam profile is derived when the number of extracted beam particles is small. This expression gives a minimum beam diameter of four Debye lengths (full width to 1/e) and is verified using electron plasmas over a broad range of plasma temperatures and densities. Numerical procedures are outlined to predict the profiles of beams with large numbers of extracted particles. Measured profiles of large beams are found in fair agreement with these predictions. The extraction of over 50% of a trapped plasma into a train of nearly ide...

Positron cooling by vibrational and rotational excitation of molecular gases

Measurements of positron temperature as a function of time are presented when a positron gas, confined in an electromagnetic trap at an elevated temperature (⩾1200 K), is cooled by interactions with the molecular gases CF4, N2 or CO at 300 K. A simple model describing positron thermalization by coupling to vibrational and rotational modes is presented, with cooling-rate predictions calculated in the Born approximation. Comparisons to the measured positron cooling-rate curves permit estimates of the magnitudes of the relevant cross sections. The results are compared with experiment for the case of vibrational excitation, where direct measurements exist; and they provide estimates of the rotational excitation cross sections where direct measurements are not currently possible. Positron cooling rates are compared for these gases at 300 K, and estimates of their effectiveness in cooling positrons to cryogenic temperatures are discussed.

Positron binding to alcohol molecules

Results are presented for positron binding to a selection of molecules containing the hydroxyl functional group. These molecules, which span in the range of carbon atoms from 1 (methanol) to 4 (1-butanol), have moderate permanent dipole moments ranging from about 1.4 to 2.4?D. The dependence of the binding energy on the magnitude of the molecular dipole polarizability and static dipole moment is studied. An effect that appears to be due to the localization of the bound positron is discussed.

Measuring positron-atom binding energies through laser-assisted photorecombination

Described here is a proposed experiment to use laser-assisted photorecombination of positrons from a trap-based beam and metal atoms in the gas phase to measure positron-atom binding energies. Signal rates are estimated, based in part upon experience studying resonant annihilation spectra using a trap- based positron beam.