Zoran Andelkovic

Laser cooling of externally produced Mg ions in a Penning trap for sympathetic cooling of highly charged ions

We have performed laser cooling of Mg ions confined in a Penning trap. The externally produced ions were captured in flight, stored and laser cooled. Laser-induced fluorescence was observed perpendicular to the cooling laser axis. Optical detection down to the single ion level together with electronic detection of the ion oscillations inside the Penning trap have been used to acquire information on the ion storage time, ion number and ion temperature. Evidence for formation of ion crystals has been observed. These investigations are an important prerequisite for sympathetic cooling of simultaneously stored highly-charged ions and precision laser spectroscopy of forbidden transitions in these.

High precision hyperfine measurements in Bismuth challenge bound-state strong-field QED

Electrons bound in highly charged heavy ions such as hydrogen-like bismuth 209Bi82+ experience electromagnetic fields that are a million times stronger than in light atoms. Measuring the wavelength of light emitted and absorbed by these ions is therefore a sensitive testing ground for quantum electrodynamical (QED) effects and especially the electron–nucleus interaction under such extreme conditions. However, insufficient knowledge of the nuclear structure has prevented a rigorous test of strong-field QED. Here we present a measurement of the so-called specific difference between the hyperfine splittings in hydrogen-like and lithium-like bismuth 209Bi82+,80+ with a precision that is improved by more than an order of magnitude. Even though this quantity is believed to be largely insensitive to nuclear structure and therefore the most decisive test of QED in the strong magnetic field regime, we find a 7-σ discrepancy compared with the theoretical prediction.

An improved value for the hyperfine splitting of hydrogen-like 209Bi82+

We report an improved measurement of the hyperfine splitting in hydrogen-like bismuth (209Bi82+) at the experimental storage ring ESR at GSI by laser spectroscopy on a coasting beam. Accuracy was improved by about an order of magnitude compared to the first observation in 1994. The most important improvement is an in situ high voltage measurement at the electron cooler (EC) platform with an accuracy at the 10 ppm level. Furthermore, the space charge effect of the EC current on the ion velocity was determined with two independent techniques that provided consistent results. The result of nm provides an important reference value for experiments testing bound-state quantum electrodynamics in the strong magnetic field regime by evaluating the specific difference between the splittings in the hydrogen-like and lithium-like ions.

First observation of the ground-state hyperfine transition in 209Bi80+

The long sought after ground-state hyperfine transition in lithium-like bismuth 209Bi80+ was observed for the first time using laser spectroscopy on relativistic ions in the experimental storage ring at the GSI Helmholtz Centre in Darmstadt. Combined with the transition in the corresponding hydrogen-like ion 209Bi82+, it will allow extraction of the specific difference between the two transitions that is unaffected by the magnetic moment distribution in the nucleus and can therefore provide a better test of bound-state QED in extremely strong magnetic fields.

The HITRAP facility for slow highly charged ions

At the GSI accelerator complex, behind the universal linear accelerator UNILAC and the synchrotron SIS, highly charged ions up to are produced at 400 MeV/nucleon. When this beam is sent through a copper foil all or nearly all remaining electrons are stripped. The HITRAP facility, a combination of a linear decelerator and a cryogenic Penning trap, is built to decelerate those ions to almost rest and to provide them for the experiments. In a number of commissioning beam times, the deceleration in the ESR, the extraction, bunching and, finally, deceleration to 6 keV/nucleon has been shown. The remaining steps, being capture and cooling in a cryogenic Penning trap, are presently tested off-line.

Sympathetic cooling in two-species ion crystals in a Penning trap

Abstract We have studied the formation and properties of two-species ion Coulomb crystals in the Penning trap of the SpecTrap experiment. These crystals have been formed by injection of admixture ions from an external source into a previously confined and laser-cooled cloud of magnesium ions. This kind of study, performed over a range of the admixture ions’ charge-to-mass ratios, indicates the conditions for their sympathetic cooling and the formation of two-species ion crystals. This mechanism allows efficient cooling of the admixed species such as highly charged ions which do not feature suitable laser-cooling transitions, and thus make them accessible to high-resolution laser spectroscopy.

Laser spectroscopy of the ground-state hyperfine structure in H-like and Li-like bismuth

The LIBELLE experiment performed at the experimental storage ring (ESR) at the GSI Helmholtz Center in Darmstadt aims for the determination of the ground state hyperfine (HFS) transitions and lifetimes in hydrogen-like (209Bi82+) and lithium-like (209Bi80+) bismuth. The study of HFS transitions in highly charged ions enables precision tests of QED in extreme electric and magnetic fields otherwise not attainable in laboratory experiments. While the HFS transition in H-like bismuth was already observed in earlier experiments at the ESR, the LIBELLE experiment succeeded for the first time to measure the HFS transition in Li-like bismuth in a laser spectroscopy experiment.

A compact source for bunches of singly charged atomic ions

We have built, operated, and characterized a compact ion source for low-energy bunches of singly charged atomic ions in a vacuum beam line. It is based on atomic evaporation from an electrically heated oven and ionization by electron impact from a heated filament inside a grid-based ionization volume. An adjacent electrode arrangement is used for ion extraction and focusing by applying positive high-voltage pulses to the grid. The method is particularly suited for experimental environments which require low electromagnetic noise. It has proven simple yet reliable and has been used to produce μs-bunches of up to 10(6) Mg(+) ions at a repetition rate of 1 Hz. We present the concept, setup and characterizing measurements. The instrument has been operated in the framework of the SpecTrap experiment at the HITRAP facility at GSI/FAIR to provide Mg(+) ions for sympathetic cooling of highly charged ions by laser-cooled (24)Mg(+).

SpecTrap: precision spectroscopy of highly charged ions—status and prospects

We present the status of the SpecTrap experiment currently being commissioned in the framework of the HITRAP project at GSI, Darmstadt, Germany. SpecTrap is a cryogenic Penning trap experiment dedicated to high-accuracy laser spectroscopy of highly charged ions (HCI) near rest. Determination of fine structure and hyperfine structure splittings in HCI with an expected relative spectral resolution of 10−7 will offer the possibility to test quantum electrodynamics in strong fields with unprecedented accuracy. Recently, we have demonstrated trapping and laser Doppler cooling of singly charged magnesium ions in SpecTrap. We report on the status of the experimental apparatus, measurements and present the future program toward storage and cooling of HCI.

Hyperfine transition in 209Bi80+—one step forward

The hyperfine transitions in lithium-like and hydrogen-like bismuth were remeasured by direct laser spectroscopy at the experimental storage ring. For this we have now employed a voltage divider which enabled us to monitor the electron cooler voltage in situ. This will improve the experimental accuracy by about one order of magnitude with respect to our previous measurement using the same technique.