Storage Ring TSR Research Articles

Test of special relativity in an ion storage ring

An accurate measurement of the Doppler effect in collinear laser spectroscopy has been performed at the TSR storage ring with electron cooled7Li+ ions atΒ=0.064. This experiment is a sensitive test of theγ=(1−Β2)−1/2 factor(Β=v/c) in the special theory of relativity. The Doppler shifted frequencies of the moving7Li+ ions are compared with calibrated molecular lines at rest. The frequencies at rest for the7Li+ ions are known from independent measurements. The Doppler shifted frequencies in the collinear experiment have been measured with a precision ofδv/v=6×10−9, mainly limited by the signal width of the resonance. A corresponding upper limit of 8×10−7 is deduced for any deviation of the time dilatation factorγ in special theory of relativity. A still higher accuracy is expected with a laser cooled ion beam. If such a beam is simultaneously subjected to RF bunching, the particle velocity and phase are exactly known. With an additional high resolution laser frequency measurement an improvement of at least one order of magnitude may be achieved.

Recombination of electrons with highly charged heavy ions at very low energies

Recombination of highly charged ions with free electrons is studied in merged-beams experiments at the UNILAC accelerator in Darmstadt and at the heavy-ion storage ring TSR in Heidelberg. Unexpected high recombination rates are observed for a number of ions at very low energiesEcm in the electron-ion center-of-mass frame. In particular, theoretical estimates for radiative recombination are dramatically exceeded by the experimental recombination rates of U28+ ions nearEcm=0 eV. The observations point to a general phenomenon in electron ion recombination depending onEcm, on the ion charge state, and possibly also on electron density, electron beam temperature, and strength of external magnetic fields.

Measurements of long atomic lifetimes at an ion storage ring

Millisecond lifetimes of excited levels in multicharged ions may be measured at ion storage rings. At the Heidelberg ion storage ring TSR, particle detection after collisions between the circulating ions and electrons of the cooler has been used to obtain dielectronic recombination spectra as well as time curves of ionization and recombination for four-electron ions, with the aim of measuring transition probabilities for magnetic dipole and for electric dipole intercombination decays. The measurements on the 2s2p3P1o level in the Be-like ions of N and O indicate lifetime values of Τ=(1.6±0.2) ms for N3+ and of Τ=(0.5±0.1) ms for O4+. Problems of these experiments are discussed as well as the prospects for improved measurements on other such ions of astrophysical interest, planning for state-specific optical detection.

Fast scintillators as radiation resistant heavy-ion detectors

Three promising scintillation materials, namely GSO(Ce), YAP(Ce), and CsI (pure), were investigated under 50MeV 12C and 100MeV 32S heavy-ion irradiation. The radiation resistance, light output and time structure of the scintillation were measured. The measurements were complemented by studies of the transmittance and thermoluminescence of the irradiated crystals. GSO(Ce) and CsI(pure) show an excellent radiation resistance together with a high light output. Maintaining a time resolution in the nanosecond regime, their applicability for heavy-ion detection with accumulated doses up to 5 × 10 13 ions/cm 2 was demonstrated. This enables their continuous operation in counting applications with high average ion rates occurring, e.g., at heavy-ion storage rings. A CsI(pure) scintillation detector realized on the basis of these measurements for electron-ion recombination experiments at the Heidelberg heavy-ion storage ring TSR is described.

Electron cooling and recombination experiments with an adiabatically expanded electron beam

Magnetically guided electron beams with transverse temperatures reduced with respect to the cathode temperature by a factor of more than 7 were realized in the electron cooling device of the heavy-ion storage ring TSR and the effect of the reduced transverse temperature in recombination and electron cooling experiments was studied. Measured dielectronic recombination resonances at low relative energy and spectra of laser-stimulated recombination indicate that transverse electron temperatures of about 17 meV have been obtained at cathode temperatures of about 110 meV. The temperature dependence of the spontaneous electron-ion recombination rate during electron cooling was investigated and found to follow the inverse square-root law expected from the theory of radiative recombination, although the measured absolute rates are higher than predicted. A new method based on analyzing the intensity of the fluorescence light emitted during simultaneous laser and electron cooling is used to measure the longitudinal electron cooling force in a range of relative velocities extending over two orders of magnitude (10 5–10 7cm/s). The results confirm the occurrence of ‘magnetized electron cooling’ also at the reduced transverse temperature and show that, compared to earlier measurements at the high transverse temperature, the cooling force increases by about a factor of 2; a considerably larger increase by a factor of ≈5 would be expected if ‘magnetized electron cooling’ would not exist.

Resonant electron impact ionization and recombination of Li-like Cl 14+ and Si 11+ at the Heidelberg Test Storage Ring

Absolute cross sections for dielectronic recombination and electron-impact ionization of Li-like Cl 14+ and Si 11+ ions have been measured with high energy resolution at the Heidelberg heavy ion storage ring TSR, using the electron cooler as a free electron target. Recombination and ionization rates were recorded simultaneously for the whole energy range from 0 up to the second ionization threshold. We discuss excitation-autoionization steps, dielectronic ionization and dielectronic recombination resonances associated with 1s → 2 l and 1s → 3 l excitations. The results agree well with theory, but the distorted wave calculations overestimate direct ionization by ∼ 20%.

Ionization and recombination in highly charged ion-electron collisions

Merged beams of electrons and accelerated highly charged ions have been employed to study resonant and non-resonant excitation phenomena in electron-ion collisions. In particular, dielectronic capture processes were observed with high collision-energy resolution both in the channels of net ionization and recombination. Ionized and recombined product ions were simultaneously detected downstream from the electron cooler in the Heidelberg test storage ring TSR with stored Li-like Si 11+, Cl 14+ ions as well as Na-like Cl 6+, Fe 15+ and Se 23+ ions. Absolute cross sections and rates for dielectronic and radiative recombination and for single ionization have been measured for electron-ion center-of-mass energies E cm ranging from 0 eV up to several keV. In separate experiments at the GSI accelerator facilities absolute rates for recombination of Au 76+ and U 28+ were determined. Detailed spectroscopic information on the structure and stability of multiply excited states of highly charged ions were obtained.

Electron impact ionization and dielectronic recombination of sodium-like iron ions

Absolute rates and cross sections for dielectronic recombination and ionization of Na-like Fe 15+ (1s 22s 22p 63s) ions were measured at electron impact energies between 0 and 1030 eV using the Heidelberg heavy ion storage ring TSR with the cooling device as an electron target. In the region where excitation-autoionization and related resonant processes are relevant, variations of the ionization cross sections below 0.5% were observed with an energy resolution of 4 to 5 eV FWHM. The theoretical calculations are in good overall agreement with the measured cross sections but do not reproduce all the details of the rich resonance structures revealed by our experiment. In the lower energy range we made use of the magnetically expanded electron beam with a transverse temperature corresponding to only kT ⊥ = (15 ± 3) meV. This resulted in an excellent energy resolution in the measurements of dielectronic recombination associated with 3s → 3p, 3d, 4 l core excitations. Here, theory and experiment are in very good agreement.

A test of special relativity with stored lithium ions

Laser spectroscopy at the heavy ion storage ring TSR in Heidelberg allows for precision experiments testing the limits of the special theory of relativity. With an opticalΛ-type three-level system of7Li+ the Doppler shift has been measured by saturation spectroscopy as a test of the time dilatation factor γ = (1 −β 2)−1/2 at an ion velocity ofυ = 6.4% c. A precision ofΔν/ν < 9 × 10−9 has been obtained, which sets a second-order limit of 1.1 × 10−6 for any deviation from the time dilatation factor. The fourth-order limit of this deviation is set below 2.7 × 10−4 by the present experiment. These limits are given at a 1 σ confidence level.

Molecular physics in a storage ring: dissociative recombination and excitation of cold HD +

As a first example of a molecular physics experiment in a storage ring we describe measurements with a HD + beam stored in the Heidelberg Test Storage Ring TSR with a lifetime of 5 s. The transverse degree of freedom could be electron cooled within 10 s. Deexcitation of high vibrational states was observed within ≅ 300 ms. With this internally cold ion beam dissociative recombination and dissociative excitation with free electrons at an energy of 0.3–40 eV were measured. The resonances and the threshold behavior of the cross section depend sensitively on the internal excitation. Future applications for molecular physics in a storage ring are discussed.

Dielectronic recombination from the ground state of heliumlike carbon ions.

The cross section for dielectronic recombination of ${\mathrm{C}}^{4+}$ ions in the 1${\mathit{s}}^{2}$ ground state has been measured with an energy resolution of \ensuremath{\approxeq}2 eV at the heavy-ion storage ring TSR (at Heidelberg) up to the 1s2s${(}^{1,3}$S) and 1s2p${(}^{1,3}$P) excitation thresholds just below 310 eV. Resonance energies and absolute cross sections are found to be well reproduced by theoretical calculations except in a region extending \ensuremath{\approxeq}5 eV below the almost degenerate excitation thresholds 1s2s${(}^{1}$S) and 1s2p${(}^{3}$P). These discrepancies imply that theory overestimates the cross section, energy-integrated over all dielectronic resonances, by \ensuremath{\approxeq}10%.

Development of seven-gap resonators for the Heidelberg high current injector

A high current injector for the heavy ion storage ring TSR in Heidelberg is under construction. As a part of the injector eight seven-gap resonators with high shunt impedance are being developed. These resonators ( ƒ 0 = 108.48 MHz ) are designed for the synchronous velocities of β s = 3.7, 4.5, 5.1 and 5.7%. Low power models with scaling factors of 1:2.5 were built in order to study the characteristics of these new resonators. Following low level measurements to optimize the voltage distribution and eigenfrequency, a first power resonator was built and successfully tested at 80 kW (duty cycle of 25%). At this power, the resonator generated a maximum voltage (summation of all gap amplitude voltages) of 1.75 MV. This paper describes the design of the resonators and gives some details of the measurements.

Upgrading of the Heidelberg accelerator facility with a new high current injector

The existing accelerator facility at the Max-Planck-Institute in Heidelberg consists of a 12 MV tandem Van de Graaff accelerator, a post-accelerator and a heavy ion cooler storage ring TSR. Many experiments at the TSR, especially laser cooling, are limited by the low current delivered by the MP tandem. A new injector consisting of a high current source, two RFQs and eight seven-gap resonators will increase the currents for singly charged ions by up to three orders of magnitude. In a second phase, an ECR source for highly charged ions will be added and the high current injector will be used in combination with the Heidelberg heavy ion post-accelerator. This new system will deliver beams up to uranium with energies above the Coulomb barrier of the heaviest elements. In this paper the design and the status of the project are presented.

High-resolution measurement of dielectronic recombination of lithiumlike Cu26+

Energies and absolute cross sections for dielectronic recombination of lithiumlike ${\mathrm{Cu}}^{26+}$ via \ensuremath{\Delta}N=0 and \ensuremath{\Delta}N=1 resonances have been measured at the Heidelberg heavy-ion storage ring TSR employing the electron cooler as a target of free electrons. In particular, resonances associated with 2s\ensuremath{\rightarrow}2p and 2s\ensuremath{\rightarrow}3l excitations have been observed. Resonances due to the doubly excited states 1${\mathit{s}}^{2}$2${\mathit{p}}_{1/2}$nl and 1${\mathit{s}}^{2}$2${\mathit{p}}_{3/2}$nl could be resolved up to n=30. Fine-structure components are partly resolved for the 1${\mathit{s}}^{2}$3l3l' resonances. The results are compared with theoretical calculations and show good agreement.

An induction accelerator for the Heidelberg test storage ring TSR

An induction accelerator has been installed in the heavy ion test storage ring TSR in Heidelberg. It allows for constant acceleration or deceleration of stored coasting ion beams without affecting their velocity profile and is well suited for ion beam manipulation in cooling experiments and for measurements of velocity dependent cooling forces. The design and operation of the device and first applications to laser cooling and to measurements of laser and electron cooling forces are described.

New physics with cooled ion beams

We review new possibilities for heavy ion cooler rings in different fields of physics and describe present experiments at the Heidelberg Test Storage Ring TSR.

Laser spectroscopy and laser cooling of relativistic stored ion beams

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. B24 B25 (1987) 19] it will become possible to prepare and store bare ions up to U 92+. Experiments using these exotic beams are discussed and an outlook to the situation at even higher energies is given.

First laser cooling of relativistic ions in a storage ring.

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.

One year of operation at the Heidelberg TSR

After one year of operation the heavy ion storage ring TSR at the Heidelberg Max-Planck Institut für Kernphysik has reached full performance. As designed 1000 turns are accumulated by a combination of multiturn and rf stacking. Due to phase space compression by an electron cooler the momentum spread of the beams is Δ p/ p = 10 −5 −10 −4 depending on the heating by intrabeam scattering. The cooled beam lifetime is pushed to the limits set by charge exchange processes as electron capture for bare nuclei and electron stripping for incompletely stripped ion beams. As the vacuum pressure is P ≤ 10 −10 Torr at present, beam lifetimes range from τ = 36 h for 21 MeV protons to 20 s for 7 MeV Be +. Intensities of up to 18 mA (3 × 10 10 particles) C 6+ beam have been stacked by applying phase space cooling during injection. For these high intensities the splitting of the longitudinal Schottky noise signal showed irregular behaviour with respect to the expected Δƒ ∼ I 1 2 scaling law.

First experiments with the heidelberg test storage ring TSR

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 O 8+ 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 C 6+ 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.