We have performed for the first time precision spectroscopy on a coasting fast7Li+ ion beam in a storage ring. The ion beam moving with 6.4% speed of light was first electron cooled and then merged with two counterpropagating laser beams acting on two different hyperfine transitions sharing a common upper level (λ-system). One laser was frequency locked to thea3127J2 hfs frequency component established as a secondary frequency standard at 514 nm. The second laser was tuned over theλ-resonance, which was recorded relative to127J2 hfs components. This experiment is sensitive to the time dilation in fast moving frames and will lead to new limits for the verification of special relatively. The present status of the experiment and perspectives in accuracy are discussed.

Since the first observation of laser cooling of fast, stored ions,1 quantitative investigations of the cooling process and its limits have been undertaken. To overcome the limited interaction time when cooling ions at velocities of approximately 5% of the velocity of light, the ions are trapped in the Heidelberg test storage ring (TSR). There, they can interact repeatedly with a copropagating laser beam in one of the four straight sections of the TSR.

At the TSR cooler ring at Heidelberg, laser studies were carried out using singly charged lithium and beryllium ions. Laser spectroscopy of relativistic lithium ions (v=0.04c) yielded signals with a narrow linewidth, suitable for an experimental test of special relativity. A dramatic reduction of the beam temperature, as defined by the longitudinal velocity spread, was achieved via laser cooling in both cases. At the ion energies available at ESR it will become possible to prepare and store bare ions up to U92+. Electron cooling was succesfully demonstrated for hydrogen-like Bi82+ ions, where a laser experiment is scheduled to study the ground-state hyperfine splitting.

Laser cooling of ions at relativistic energies was first observed at the TSR storage ring in Heidelberg. A 7Li+ ion beam moving at 6.4% the speed of light was overlapped with resonant co‐ and counter‐propagating laser beams. The longitudinal temperatures were found to pass below 190 mK. Limits and applications of laser cooled relativistic ion beams are discussed. Laser cooling and electron cooling of the ion beam were combined.

Experiments with relativistic ions at the test storage ring TSR [P. Baumann et al., Nucl. Instr. and Meth. A268 (1988) 531] demonstrate the potential of the interaction of laser light with energetic stored ions for spectroscopic purposes as well as for manipulation of the ion velocity. Latest results for Li+ ions are reported. At the ion energies available at ESR [B. Franzke, Nucl. Instr. and Meth. B24B25 (1987) 19] it will become possible to prepare and store bare ions up to U92+. Experiments using these exotic beams are discussed and an outlook to the situation at even higher energies is given.

The first successful laser cooling of ions at relativistic energies was observed at the Heidelberg TSR storage ring. A $^{7}\mathrm{Li}^{+}$-ion beam of 13.3 MeV was oberlapped with resonant copropagating and counterpropagating laser beams. The metastable ions were cooled from 260 K to a longitudinal temperature of below 3 K and decelerated by several keV. The longitudinal velocity distribution was determined by a fluorescence method. After laser cooling a strongly enhanced narrow peak appeared in the Schottky noise spectrum in addition to the uncooled ion distribution.

An overview of atomic physics experiments at the heavy ion Test Storage Ring (TSR) is given. Highly charged ions up to fully stripped silicon have been stored at energies between 4 and 12 MeV/u. The enhancement of the beam intensity by stacking, the beam lifetime, and electron cooling of these ion beams are discussed. Radiative and state‐selective dielectronic recombination rates of hydrogen‐like oxygen ions with free electrons from the electron cooler were measured. Beam noise spectra are being investigated with regard to collective effects caused by the Coulomb interaction in the cold ion beams. Resonance fluorescence from stored single‐charged ions was observed using tunable narrow‐band lasers. First indications of laser cooling in a storage ring were seen.

The Heidelberg heavy ion test storage ring TSR started operation in May 1988. The lifetimes of the ion beams observed in the first experiments can be explained by interactions with the residual gas. Multiple Coulomb scattering, single Coulomb scattering, electron capture and electron stripping are the relevant processes. Electron cooling of ions as heavy as O8+ has been observed for the first time. With increasing particle number, the longitudinal Schottky noise spectrum becomes dominated by collective waves for cooled beams, allowing a determination of velocities of sound. After correcting for these coherent distortions fo the Schottky spectrum, the longitudinal beam temperature could be extracted. The observed longitudinal equilibrium beam temperatures increase strongly with the charge of the ions. For a cooled C6+ beam, temperatures a factor of 120 higher were measured compared to a proton beam with the same particle number. The shrinking of the beam diameter due to electron cooling was observed with detectors which measured the profile of charge-changed ions behind a bending magnet. A strong laser-induced fluorescence was detected when storing metastable 7Li+ ions in the ring. Via the Doppler effect a very accurate measurement of the ion velocity profile could be performed. First attempts to observe laser cooling failed, probably due to heating effects from intrabeam scattering and a coupling between longitudinal and transversal motion in the beam. Several experiments under preparation are outlined.

Publisher Summary This chapter presents a clear understanding of laser-ion interactions under storage ring conditions to prepare the basis of laser cooling of fast-stored ion beams. In addition, the method of laser-induced fluorescence provides precise data for beam properties such as absolute velocity, momentum spread, and lifetime. 9Be+ ions stored in a heavy-ion storage ring are a promising species for laser cooling down to temperatures several orders of magnitude less than those reached for protons by electron cooling at the Novosibirsk ring. Short cooling times and microkelvin temperatures can be envisaged, where the structure of the ion beam is dominated by Coulomb repulsion. The chapter presents a schematic view of an experimental configuration where the light beam from a homemade frequency-doubled free-jet ring dye laser with typically 30 mW power in the ultraviolet propagated parallel to the ions. Two cylindrical high-voltage (HV) electrodes were installed in a 1.3 m long zone in the center of the straight ring section. The laser-ion interaction was monitored via the fluorescence emanating from inside the first HV cylinder, using a cooled photomultiplier operated in the single-photon counting mode. The experimental results indicated the production of a Bennett hole in the ion velocity distribution, thus providing an important step towards laser cooling of fast Be+ ions stored in a ring.

Laser cooling and spectroscopy in traps have reached impressive perfections. This chapter discusses laser cooling and partially Doppler-suppressed spectroscopy on stored ions at 5.4% speed of light in the test storage ring (TSR) heavy ion storage ring in Heidelberg. It discusses the requirements for saturation spectroscpy with two counterpropagating collinear laser beams. In contrast to Penning and RF-traps, the ions in a storage ring move at a high longitudinal velocity with small transverse harmonic oscillations around the central orbit with just a few times the orbiting frequency in the case of strong focussing. However, the injection of the high velocity beam introduces large betatron oscillations, which influence the laser interaction considerably. A proper tuning of the storage ring, electron cooling, and laser cooling should condense the ion beam to a long lived beam with Δp/p well below 10-6. Using a simple description of the ions motion in a storage ring, the basic properties of the observed signals can reproduced, and stringent conditions for high resolution spectroscopy testing special relativity by measuring the one way speed of light can be constructed.