R. E. Marrs

The Electron‐Beam Ion Trap

The mention of few‐electron atoms usually brings to mind hydrogen, helium or other light elements in neutral form. However, these simple atoms are part of a sequence of ions having the same number of electrons but different nuclear charges. For example, the hydrogen‐like sequence spans neutral hydrogen through hydrogen‐like uranium, U91+. Both the atomic physics and the applications of the most highly charged ions in such isoelectronic sequences are receiving increasing attention. Recently the electronbeam ion trap has made it possible to produce and study any such ion in a modest‐sized apparatus (figure 1).

A high-energy electron beam ion trap for production of high-charge high-Z ions

Abstract We have developed a new high-energy electron beam ion trap, the first laboratory source of low-energy, few-electron, high- Z ions. We describe the device and report measurements of its performance, including the electron beam diameter, current density and energy, and measurements of the ionization balance for several high- Z elements in the trap. This device opens up a wide range of possible experiments in atomic physics, plasma physics, and nuclear physics.

Evaporative cooling of highly charged ions in EBIT: An experimental realization

Both the total number and trapping lifetime of near‐neon‐like gold ions held in an electron beam ion trap have been greatly increased by a process of ‘evaporative cooling.’ A continuous flow of low‐charge‐state ions into the trap cools the high‐charge‐state ions in the trap. Preliminary experimental results using titanium ions as a coolant are presented.

Projection x-ray microscope powered by highly charged ions

Recombination of slow highly charged ions at the surface of a target foil can be used as a source of x rays for a projection x-ray microscope. In a first test of this concept, a low emittance beam of Ar18+ and Ar17+ ions from an electron beam ion trap was focused with einzel lenses to a 20 μm full width at half maximum spot on a beryllium target foil. The 3 keV x rays from radiative deexcitation of the ions were used to obtain a magnified image of an electroformed nickel mesh with 20 μm resolution by projection onto a CCD camera. Prospects for substantial improvements in resolution and intensity are discussed.

Modeling the ion‐source performance of an electron‐beam ion trap (invited)

Several key features of the electron‐beam ion trap (EBIT) enable it to provide superior ion‐source performance for many applications requiring ultra‐high‐charged ions. This paper briefly reviews these features and the operating conditions in the existing EBITs. The present performance of the EBIT as an ion source is demonstrated by producing and extracting ions up to Th80+ and U70+ at microsecond‐wide ion beam pulses of about 104 ions per second. Using as examples the production of U92+, U90+, and Dy66+, modeling results are presented to show how the fundamental processes limit the quality and quantity of ions that can be obtained from an upscaled EBIT.

DIRECT MEASUREMENT OF TWO-ELECTRON CONTRIBUTIONS TO THE GROUND STATE ENERGY OF HELIUMLIKE HIGH-Z IONS

We report on a novel technique which exploitsradiative recombination transitions for a direct experimental determination of the two-electron contributions to the ground state energy in heliumlike high-Z ions. Results are presented of a first experiment which was conducted at an electron beam ion trap for various elements ranging from Z=32 to 83. The comparison with theoretical predictions demonstrates that the achieved precision already provides a sensitive test of second order many-body contributions and approaches the size of the two-electron (screened) Lamb shift. The potential of the new technique will be outlined and the capability of the ESR storage ring for future investigations will be emphasized.

System for Calibrating the Energy-Dependent Response of an Elliptical Bragg-Crystal Spectrometer

A multipurpose spectrometer (MSPEC) with elliptical crystals is in routine use to obtain x-ray spectra from laser produced plasmas in the energy range 1.0-9.0 keV. Knowledge of the energy-dependent response of the spectrometer is required for an accurate comparison of the intensities of x-ray lines of different energy. The energy-dependent response of the MSPEC has now been derived from the spectrometer geometry and calibration information on the response of its components, including two different types of detectors. Measurements of the spectrometer response with a laboratory x-ray source are used to test the calculated response and provide information on crystal reflectivity and uniformity.

A wire probe as an ion source for an electron beam ion trap

A thin wire probe inserted near the electron beam in an electron beam ion trap provides a source of ions for the trap. The wire can be plated with a small amount of source material permitting the use of rare or expensive materials. Here we present results on tests with probes of various materials to demonstrate the success of the technique. In one case a sample of approximately 100 ng of 233U was plated on a platinum wire tip and used to continuously fill the trap for 10 days.

EBIT Trapping Program

The LLNL electron beam ion trap provides the worlds only source of stationary highly charged ions up to bare U. This unique capability makes many new atomic and nuclear physics experiments possible.

X-ray measurement of the ionization balance in an electron beam ion trap

Abstract The charge-state abundances of bare, hydrogen-, helium-, lithium-, and beryllium-like iron in the Livermore electron beam ion trap have been measured with uncertainties ranging from 3 to 15% by monitoring the emitted X rays. The ionization balance is determined in two ways: (1) by observing radiative recombination photons with a solid-state detector, and (2) by observing line radiation with a Bragg crystal spectrometer. The results are in good agreement with an ionization balance model.