Ion Storage Ring TSR Research Articles

ELECTRON-ION RECOMBINATION OF Fe12 +FORMING Fe11 +: LABORATORY MEASUREMENTS AND THEORETICAL CALCULATIONS

We have measured dielectronic recombination (DR) for Fe12 + forming Fe11 + using the heavy ion storage ring TSR located at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany. Using our results, we have calculated a plasma rate coefficient from these data that can be used for modeling astrophysical and laboratory plasmas. For the low temperatures characteristic of photoionized plasmas, the experimentally derived rate coefficient is orders of magnitude larger than the previously recommended atomic data. The existing atomic data were also about 40% smaller than our measurements at temperatures relevant for collisionally ionized plasmas. Recent state-of-the-art theory has difficulty reproducing the detailed energy dependence of the DR spectrum. However, for the Maxwellian plasma rate coefficient, recent theoretical results agree with our measurements to within about 30% for both photoionized and collisionally ionized plasmas.

Few-Body Dynamics studied with the In-Ring Reaction Microscope at the TSR of MPIK

A Reaction Microscope was implemented into the ion storage ring TSR. Experiments have been performed studying the collision dynamics in ionization and electron loss in ion-atom collisions. An additional MCP detector was inserted, aiming to detect photons in coincidence emitted in processes such as radiative electron capture.

Resonant structure of low-energyH3+dissociative recombination

High-resolution dissociative recombination rate coefficients of rotationally cool and hot ${\mathrm{H}}_{3}^{+}$ in the vibrational ground state have been measured with a 22-pole trap setup and a Penning ion source, respectively, at the ion storage-ring TSR. The experimental results are compared with theoretical calculations to explore the dependence of the rate coefficient on ion temperature and to study the contributions of different symmetries to probe the rich predicted resonance spectrum. The kinetic energy release was investigated by fragment imaging to derive internal temperatures of the stored parent ions under differing experimental conditions. A systematic experimental assessment of heating effects is performed which, together with a survey of other recent storage-ring data, suggests that the present rotationally cool rate-coefficient measurement was performed at 380${}_{\ensuremath{-}130}^{+50}$ K and that this is the lowest rotational temperature so far realized in storage-ring rate-coefficient measurements on ${\mathrm{H}}_{3}^{+}$. This partially supports the theoretical suggestion that temperatures higher than assumed in earlier experiments are the main cause for the large gap between the experimental and the theoretical rate coefficients. For the rotationally hot rate-coefficient measurement a temperature of below 3250 K is derived. From these higher-temperature results it is found that increasing the rotational ion temperature in the calculations cannot fully close the gap between the theoretical and the experimental rate coefficients.

STORAGE RING CROSS SECTION MEASUREMENTS FOR ELECTRON IMPACT IONIZATION OF Fe11+FORMING Fe12+AND Fe13+

We report ionization cross section measurements for electron impact single ionization (EISI) of Fe^11+$ forming Fe^12+ and electron impact double ionization (EIDI) of Fe^11+ forming Fe^13+. The measurements cover the center-of-mass energy range from approximately 230 eV to 2300 eV. The experiment was performed using the heavy ion storage ring TSR located at the Max-Planck-Institut fur Kernphysik in Heidelberg, Germany. The storage ring approach allows nearly all metastable levels to relax to the ground state before data collection begins. We find that the cross section for single ionization is 30% smaller than was previously measured in a single pass experiment using an ion beam with an unknown metastable fraction. We also find some significant differences between our experimental cross section for single ionization and recent distorted wave (DW) calculations. The DW Maxwellian EISI rate coefficient for Fe^11+ forming Fe^12+ may be underestimated by as much as 25% at temperatures for which Fe^11+ is abundant in collisional ionization equilibrium. This is likely due to the absence of 3s excitation-autoionization (EA) in the calculations. However, a precise measurement of the cross section due to this EA channel was not possible because this process is not distinguishable experimentally from electron impact excitation of an n=3 electron to levels of n > 44 followed by field ionization in the charge state analyzer after the interaction region. Our experimental results also indicate that the double ionization cross section is dominated by the indirect process in which direct single ionization of an inner shell 2l electron is followed by autoionization resulting in a net double ionization.

High-resolution storage-ring measurements of the dissociative recombination ofH3+using a supersonic expansion ion source

We have performed measurements of the dissociative electron recombination (DR) of ${\mathrm{H}}_{3}^{+}$ at the ion storage ring TSR utilizing a supersonic expansion ion source. The ion source has been characterized by continuous wave cavity ring-down spectroscopy. We present high-resolution DR rate coefficients for different nuclear spin modifications of ${\mathrm{H}}_{3}^{+}$ combined with precise fragment imaging studies of the internal excitation of the ${\mathrm{H}}_{3}^{+}$ ions inside the storage ring. The measurements resolve changes in the energy dependence between the ortho-${\mathrm{H}}_{3}^{+}$ and para-${\mathrm{H}}_{3}^{+}$ rate coefficients at low center-of-mass collision energies. Analysis of the imaging data indicates that the stored ${\mathrm{H}}_{3}^{+}$ ions may have higher rotational temperatures than previously assumed, most likely due to collisional heating during the extraction of the ions from the ion source. Simulations of the ion extraction shed light on possible origins of the heating process and how to avoid it in future experiments.

Ab initio study of resonant dissociative recombination of He2+

Resonant dissociative recombination of the He2+ molecular ion is an important process in the ionosphere as well as in laboratory plasmas. Recently, rate coefficients have been measured for the dissociative recombination of He+2 using the ion storage ring TSR in Heidelberg, Germany. Previously, calculations have been reported for the cross section in the low energy region. We will report results in the 0 to 15 eV energy region, where the cross section is dominated by a series of resonances converging to the first excited state of the molecular ion. We will also discuss the resonances converging to the next series of excited states. The resonance parameters for this system are obtained from electron scattering calculations using the Complex Kohn variational method. These resonance parameters are used as input to a time-dependent wave packet calculation of the dissociation dynamics. The calculated cross sections and rates will be reported and compared to available experiment.

Towards state selective measurements of the H3+ dissociative recombination rate coefficient

Ion storage and trapping techniques in connection with efficient internal state control and diagnostic for molecular ions offer the potential of providing rate coefficients for the dissociative recombination of H3+ for well-defined initial rotational states, required for understanding the role of this ion in the interstellar medium and other cold environments. Information on the vibrational and rotational excitation in stored H3+ ion beams, as obtained from experiments at the ion storage ring TSR, is reviewed. In addition, the arrangement of the TSR injector trap, using buffer gas cooling in a cryogenic radiofrequency multipole structure to inject pulses of internally cold H3+ ions into the TSR via a high-energy accelerator, is outlined. An account is given of tests towards the in-situ diagnostic of rotational level populations, where laser transitions between low-lying rovibrational levels could be detected in dilute H3+ ion ensembles using chemical probing in the radiofrequency multipole ion trap.

Electron cooling experiments at the heavy ion storage ring TSR

At low relative velocities the transverse cooling rates of fast stored ions by cold electrons are usually smaller than the longitudinal ones. By dispersive electron cooling, however, it is possible to increase the horizontal cooling rate on the expense of the longitudinal rate. The cooling scheme requires a horizontal gradient of the longitudinal cooling force, which was achieved by displacing the electron beam relative to the ion beam, and a finite dispersion in the ion–electron interaction region. Moreover, electron cooling of very hot ion beams is usually rather slow. Two different methods were investigated to study the longitudinal cooling times. The first method uses an induction accelerator to push the ions into the effective velocity range of the longitudinal electron cooling force. In the second method the mean electron velocity is swept over the velocity distribution of the ion beam in order to decrease the collection time.

Dispersive electron cooling experiments at the heavy ion storage ring TSR

At low relative velocities, the transverse cooling rates of fast stored ions by cold electrons are usually smaller than the longitudinal ones. By dispersive electron cooling, however, it is possible to transfer heat from the horizontal into the longitudinal degree of freedom, thereby increasing the horizontal cooling rate on the expense of the longitudinal rate. The cooling scheme requires a horizontal gradient of the longitudinal cooling force which can be achieved by displacing the electron beam relative to the ion beam, and a finite dispersion in the ion–electron interaction region. Operating the heavy ion test storage ring TSR at a moderate dispersion of D S =1.81 m , we investigated the dispersive electron cooling process and found good agreement with the expectations from fundamental considerations.

Progress and upgrading of the Heidelberg high current injector

A specialized rf-accelerator system HSI consisting of two RFQ’s and 8 rf seven-gap cavities was built for injection of high intensities of singly charged heavy ions into the Heidelberg heavy ion storage ring TSR. With different ion sources, this system now is used to deliver positive or negative, atomic and molecular ion beams with energies between 150 keV/a.m.u. and 5.3 MeV/a.m.u. final energy.

Experimental Evidence for Magnetic Field Effects on Dielectronic Recombination via High Rydberg States

We report the first experimental observation of magnetic field effects on dielectronic recombination (DR) via highly excited Rydberg levels. Crossed static electric and magnetic fields ${E}_{y}$ and ${B}_{z}$ were imposed on the collision region in high resolution DR measurements with Li-like ${\mathrm{Cl}}^{14+}$ ions at the heavy ion storage ring TSR in Heidelberg. Enhancement of DR rate coefficients $\ensuremath{\alpha}$ for the group of high Rydberg states attached to the ${2p}_{1/2}$ and ${2p}_{3/2}$ series limits was observed when motional electric fields ${E}_{y}$ up to 380 V/cm were introduced. The associated enhancement rate $d\ensuremath{\alpha}/{\mathrm{dE}}_{y}$ which we found to be constant at least for ${E}_{y}\ensuremath{\le}100\mathrm{V}/\mathrm{cm}$ decreased by almost a factor of 2 when the longitudinal field ${B}_{z}$ was increased from 20 to 69 mT.

Photorecombination of Sc3+ and Ti4+ Ions: Search for Interference Effects and Recombination at low Energies

In search for interference between radiative and dielectronic recombination (RR and DR) absolute recombination rate coefficients for Ar-like Sc3+ and Ti4+ ions have been measured at the Heidelberg heavy ion storage ring TSR. Whereas the Sc3+ experiment suffers from statistics too poor for a line shape analysis, the Ti4+ recombination spectrum clearly does not exhibit asymmetric lineshapes due to quantum mechanical interferences as recently predicted theoretically for Sc3+ ions. The broad Ti3+(3s23p53d2 2F) DR resonance expected on the basis of multi-configuration Hartree-Fock calculations at 3.0 eV with a width of 1.3 eV appears to be shifted towards zero center-of-mass (c. m.) energy, such that its width is larger than its distance to threshold. At zero c. m. energy an unexplained recombination rate enhancement of a factor of 2 beyond the sum of RR and DR rates is observed.

Recombination of F6+ with Free Electrons at Very Low Energies

Radiative and dielectronic recombination of F6+ have been studied at the heavy ion storage ring TSR in Heidelberg. For a detailed investigation of rate enhancement effects at very low electron-ion center-of-mass energies experimental parameters such as the magnetic guiding field, the electron density and the adiabatic expansion factor of the electron beam have been varied systematically. Whereas measurements at different electron densities show no influence on the enhancement and while a variation of the expansion factor evokes the predicted behaviour, we see an increase of the enhancement with increasing axial magnetic field between 20 mT and 70 mT.

Field Enhanced Dielectronic Recombination of Si11+ and Cl14+ Ions

The enhancement of dielectronic recombination of multiply charged ions by external electric fields has been studied under controlled conditions. The heavy ion storage rings CRYRING at Stockholm University and the heavy ion storage ring TSR at the Max-Planck-Institut in Heidelberg were used to store beams of Si11+ (ion energy 280 MeV) and Cl14+ (ion energies 250 MeV and 110 MeV), respectively. Recombination in the electron cooler has been measured over energy ranges covering all resonances due to 2s → 2p core exitation. External electric fields up to 183 V/cm at CRYRING and up to 379 V/cm at the TSR have been applied. In the TSR experiment the influence of different magnetic guiding fields as well as the influence of different electron densities in the electron cooler on the recombination rate was investigated. A significant rate enhancement was found for 1s22pnl Rydberg resonances with n > 25 for Si11+ ions and n > 20 for Cl14+ ions.

Coulomb explosion imaging at the heavy ion storage ring TSR

We present a novel experimental setup which combines the storage of molecular ions with subsequent Coulomb explosion imaging (CEI) for molecular structure studies. A new extraction system has been installed at the heavy ion storage ring TSR which allows slow extraction of ions out of the ring. Attached to the extraction beamline, a new detector system for Coulomb explosion imaging has been set up to analyze the extracted molecular ions. By combining the CEI technique with the storage ring, it is possible to observe the structure of small molecular ions while the vibrational relaxation is in progress or after the ions have reached thermal equilibrium with the environment (300 K), where typically only the vibrational ground state is populated. A detailed description of the facility installed at the TSR is given and the first CEI results for HD +, which are in very good agreement with the theoretical expectation, are discussed.

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.

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%.

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.

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.