L. Gruber

Formation of Strongly Coupled Plasmas from MultiComponent Ions in a Penning Trap

This report reviews the production of strongly-coupled, multi-component, non-neutral, highly-charged ion plasmas in a cryogenic Penning trap. A unique ion source–Penning trap system has been developed and used an Electron Beam Ion Trap-Source (EBIT-S), able to produce ions up to U92+, has been combined with a cold-bore Penning ion trap. The developed experimental techniques such as the ion production, extraction, bunching, multi-ion species trapping and storage (ions up to Th80+), detection, cooling, and imaging are presented. The produced mixed plasmas, consisting of several ion species [e.g. Be+, Xeq+ (charge q = 34 ... 44)], are cooled to a temperature of about 1 degree Kelvin and below. These low temperatures were obtained by applying a sypathetic cooling scheme, using laser cooled Be+ ions. The determination of the temperature and the density from the laser resonance width and the fluorescence imaging of the Be+ clouds respectively, yield a Coulomb coupling parameter for the xenon ion plasma of over 1000, large enough for crystallization. Crystallization can be achieved at these relatively high temperatures since the Coulomb coupling parameter scales with q2. Ion species with different mass to charge ratios centrifugally separate at low temperatures in the trap, and the plasma consists of concentric spheroidal shells of single component plasmas. But even in this case excellent thermal coupling between ion species takes place due to the long range Coulomb interactions. This is demonstrated by molecular dynamics simulations of the ion mixtures, which show ordered structures, indicating evidence for crystallization of the highly charged xenon ions. During the investigation of highly charged ion Coulomb crystallization in mixed strongly coupled plasmas, several enabling techniques were developed that are applicable to various areas of physics research Analogous thermodynamics (Coulomb matter) of white dwarf astrophysical plasmas can experimentally be investigated, high precision laser spectroscopy is made possible due to reduced Doppler broadening effects, cold ion sources can be developed and coherent quantum control experiments can be further explored.

RETRAP: An ion trap for laser spectroscopy of highly-charged ions

The possibility of highly-charged ions, captured and stored in an ion trap, and cooled by elastic collisions with confined Be ions, has been achieved in RETRAP; a cryogenic Penning trap system coupled to the Electron Beam Ion Trap (EBIT), which was used as a source of highly-charged ions. Be+ cooling in RETRAP has been carried out with a combination of resistive damping of the axial motion of the ions by a tuned circuit, and laser cooling. Spectroscopic goals of the research include metastable level lifetime measurements and precision laser spectroscopy on magnetic dipole transitions of selected highly-charged ions. Potential measurements of the hyperfine structure splitting, level lifetime, and bound state g-factor of a high-Z hydrogen-like ion 165Ho66+ already studied by emission spectroscopy in Super-EBIT, are discussed with relation to current progress.

Cold highly charged ions in a Penning trap: Experiment and simulation

Using the LLNL EBIT/RETRAP system non-neutral plasmas of highly charged ions were produced and cooled to temperatures around one Kelvin. These strongly coupled plasmas can model white dwarf astrophysical plasmas in the laboratory. These systems may also have potential application to quantum computation. The experimental results from the last operations of the trap at Livermore are discussed. Molecular dynamics simulation results are discussed as a guide to past and future experiments. The status and future plans for RETRAP at LBNL’s 88 inch Cyclotron are discussed.

Molecular Dynamics Simulations of Collisional Cooling and Ordering of Multiply Charged Ions in a Penning Trap

Molecular dynamics simulations are used to help design new experiments by modeling the cooling of small numbers of trapped multiply charged ions by Coulomb interactions with laser-cooled Be{sup +} ions. A Verlet algorithm is used to integrate the equations of motion of two species of point ions interacting in an ideal Penning trap. We use a time step short enough to follow the cyclotron motion of the ions. Axial and radial temperatures for each species are saved periodically. Direct heating and cooling of each species in the simulation can be performed by periodically rescaling velocities. Of interest are Fe{sup 11+} due to a EUV-optical double resonance for imaging and manipulating the ions, and Ca{sup 14+} since a ground state fine structure transition has a convenient wavelength in the tunable laser range.

Highly charged ion Coulomb crystallization in mixed strongly coupled plasmas

The investigation of highly charged ion Coulomb crystallization in mixed strongly coupled plasmas is of interest in many areas: white dwarf astrophysical plasmas are believed to have very similar thermodynamic properties, cold highly charged ions can be used as an object for high precision laser spectroscopy of fine and hyperfine transitions in the visible due to the absence of Doppler broadening and, an entirely new area of research is the potential application to highly charged ion based quantum computing schemes.We report the formation of such plasmas in a cryogenic Penning trap. These plasmas consisting of many species including Be+ and Xe44+ or Be+ and Xe15+ ions, are formed at a temperature of less than 4 K. The temperatures were obtained by applying a laser based sympathetic cooling scheme. The determination of the temperature and density from the laser resonance width and the fluorescence imaging of the Be+ clouds, respectively, yields a Coulomb coupling constant for the centrifugally separated Xe plasma high enough for crystallization.A molecular dynamics code, developed just for this purpose, was run to clarify the understanding of these plasmas and it was possible to show consistency between experiment and simulation.

Highly charged ion trapping and cooling

In the past few years a cryogenic Penning trap (RETRAP) has been operational at the Electron Beam Ion Trap (EBIT) facility at Lawrence Livermore National Laboratory. The combination of RETRAP and EBIT provides a unique possibility of producing and re-trapping highly charged ions and cooling them to very low temperatures. Due to the high Coulomb potentials in such an ensemble of cold highly charged ions the Coulomb coupling parameter (the ratio of Coulomb potential to the thermal energy) can easily reach values of 172 and more. To study such systems is not only of interest in astrophysics to simulate White Dwarf star interiors but opens up new possibilities in a variety of areas (e.g. laser spectroscopy, cold highly charged ion beams).

Collisions and spectroscopy of low-energy highly-charged ions using an ion trap

Electron transfer from H2 to highly-charged Xeq+ (q=35, 43–46) and Thq+ (q=73–80) ions at center-of-mass energies near 6 eV has been studied using the RETRAP system at Lawrence Livermore National Laboratory. The ions were produced in the Electron Beam Ion Trap and retrapped in the Penning ion trap. Initial cross section data are in reasonable accord with the predictions of the absorbing sphere model, and true double capture is found to be about 25% of the total. The development with time of the charge of a single ion undergoing collisions has been observed non-destructively. Certain spectroscopic measurements are planned, following cooling of the stored ions to cryogenic temperatures.