Stored Ion Beam Research Articles

Electron cooling of ion beam with large momentum spread

We investigate a scheme for electron cooling of ion beams with large momentum spread of ±(1%). An induction accelerator (IndAcc) is utilized to accelerate or decelerate the stored ion beam in order to shift its momentum distribution toward the sensitive region of the electron cooling force. This method will be applied for further cooling of phase rotated hot ion beams produced by a high-power short pulse laser, to be used as an injection beam for synchrotron. Also, an induction accelerator has been designed for construction at the HIMAC ring, which will be used to test the feasibility of the present sweep scheme experimentally.

Precise atomic lifetime measurements with stored ion beams and ion traps

For many years, atomic lifetime measurements on multiply-charged ions have been done almost exclusively by beam-foil spectroscopy. For low ion charges, however, spin-changing "intercombination" transitions have a rate that renders them too slow for traditional fast-beam techniques. Here ion traps and fast-ion beams have been combined in the concept of heavy-ion storage rings. These devices have permitted not only an extension of intercombination lifetime measurements down to singly charged ions, but they also facilitated similar measurements on electric-dipole forbidden transitions. The electron-beam ion trap (EBIT) complements the storage-ring work for work on highly charged ions. Achievements, technical issues, and prospects are outlined. PACS Nos.: 32.70Cs, 32.30Jc, 34.50Fa

Vibrational and rotational cooling ofH3+

The vibrational relaxation of ${\mathrm{H}}_{3}^{+}$ molecules from a conventional plasma ion source is studied performing Coulomb explosion imaging on the ions extracted from a storage ring after variable times of storage. Storage for 2 s is found sufficient for radiative relaxation of the breathing excitation and the fragment velocity distribution in the breathing coordinate then agrees well with simulations based on the calculated ground-state wave function. The radiative decay of the two lowest pure breathing levels ${(1,0}^{0})$ and ${(2,0}^{0})$ is seen to be considerably faster than expected from rotationless calculations. Assuming a high rotational excitation of the ${\mathrm{H}}_{3}^{+}$ ions, as suggested already in earlier experiments, the theoretical transition probabilities of the University College London line list for ${\mathrm{H}}_{3}^{+}$ [L. Neale, S. Miller, and J. Tennyson, Astrophys. J. 464, 516 (1996)] can explain the increase of the vibrational cooling rates and reproduce the observed decay curve for the lowest breathing-excited level, confirming the absolute transition probabilities of these line tables. The observations give evidence for a quasistable population of high-lying rotational levels in the stored ion beam, relevant for the interpretation of storage ring measurements on the rate coefficients for dissociative recombination of ${\mathrm{H}}_{3}^{+}$ ions with low-energy electrons.

Dissociative recombination of D+(D2O)2 water cluster ions with free electrons

Dissociative recombination (DR) of the water cluster ion D+(D2O)2 has been studied at the heavy-ion storage ring CRYRING (Manne Siegbahn Laboratory, Stockholm University). Cluster ions were injected into the ring and accelerated to an energy of 2.28 MeV. The stored ion beam was merged with an almost monoenergetic electron beam, and neutral fragments produced by DR were detected by an energy-sensitive surface barrier detector. The first experimental determinations of the absolute DR cross section and branching ratios for a cluster ion are reported. The cross section for the process D+(D2O)2+e− is large and reaches 6⋅10−12 cm2 at a low center-of-mass collision energy of 0.001 eV. The cross section has an E−1.19±0.02 dependence in the energy range 0.001–0.0052 eV, and a steeper slope with an E−1.70±0.12 dependence for E=0.052–0.324 eV. The general trends are similar to the results for previously studied molecular ions, but the cross section is higher in absolute numbers for the cluster ion. Thermal rate coefficients for electron temperatures of 50–2000 K are deduced from the cross section data and the rate coefficients are consequently also large. Branching ratios for the product channels are determined with a grid technique. Break-up into 2D2O+D is the dominating dissociation channel with a probability of 0.94±0.04. The channel resulting in the fragments D2O+OD+D2 has a probability of 0.04±0.02, and the probability for formation of D3O+D2O is 0.02±0.03. The results are compared with data for molecular ions, and the cluster dissociation dynamics are discussed.

Experimental and theoretical investigation of radiative decay rates of metastable levels in La II

An experimental and theoretical investigation of lifetimes of metastable levels in La II has been performed. The experimentally obtained results using the laser probing of a stored ion beam were $\ensuremath{\tau}=5.2\ifmmode\pm\else\textpm\fi{}0.2\mathrm{s}$ for the ${a}^{1}{G}_{4}$ and $\ensuremath{\tau}=2.1\ifmmode\pm\else\textpm\fi{}0.3\mathrm{s}$ for the ${b}^{1}{D}_{2}$ levels. Within the error bars, the results are in good agreement with the relativistic Hartree-Fock calculations including core polarization.

Low-Current, Vertical Blowup in a Stored Laser-Cooled Ion Beam

Using a novel technique for real-time transverse beam profile diagnostics of a stored ion beam, we have observed the transverse size of a stored laser-cooled ion beam. Earlier we observed that the density of the beam is independent of the beam current. At very low currents, we observe an abrupt change in this behavior: The vertical beam size increases suddenly by about an order of magnitude. This observation implies a sudden change in the indirect vertical cooling mediated by intrabeam scattering. Our results have serious implications for the ultimate beam quality attainable by laser cooling.

Rotational-temperature evolution of a N2+ ion beam in a storage ring probed by laser induced fluorescence spectroscopy

Laser induced fluorescence spectroscopy has been applied for the first time on a stored fast molecular ion beam. The rotational temperature of the N2+ ion beam was studied as a function of storage time. For up to 1 s of storage time, a temperature of 1040 K can be used to describe the rotational energy distribution.

Experimental Investigation of Radiative Decay Rates of Metastable Levels in Eu II

The radiative lifetimes of the metastable a 9DJ (J = 2–5) levels in Eu II have been measured by laser probing of a stored ion beam. The lifetimes of all four J levels are slightly above 1 s. A deviation from the expected energy dependence of the decay rates for electric quadrupole transitions is observed and discussed.

Lifetime measurements of metastable states in Fe+.

The lifetime of two metastable levels in Fe+ has been measured by laser probing of a stored ion beam. In the dense spectrum of Fe+, the metastable levels a (6)S(5/2) and b (4)D(7/2) were selected and their lifetimes were determined to be 230 +/- 30 and 530 +/- 30 ms, respectively. The lifetimes are compared with previous theoretical results. Metastable lifetime measurements of Fe+ are of great importance for interpretation of spectra from astronomical objects. The present experiment opens for the possibilities to investigate lifetimes of metastable states in complex atomic ions, which have, so far, been unexplored.

Laser cooling of fast stored ions in barrier buckets

We apply novel bunch forms for efficient laser cooling of fast stored ion beams at the Heidelberg Test Storage Ring. 9 Be + ions are longitudinally confined in barrier buckets providing free motion of the cooled ions inside the bucket (quasi-coasting beam). The implementation of laser cooling in barrier buckets is described, and longitudinal dynamics of laser cooling is compared to electron cooling. With laser cooling, longitudinal temperatures far below 1 K are reached, corresponding to a momentum spread of Δ p/ p<10 −6. When the thermal energy becomes sufficiently small, significant modifications of the ion distribution due to space-charge effects are observed.

Density Limitations in a Stored Laser-Cooled Ion Beam

We present a new technique for transverse beam profile diagnostics of a stored ion beam, imaging thefluorescence light from a laser-excited ion beam onto a high resolution charge-coupled device detector.This technique has much higher sensitivity and spatial resolution than conventional techniques. Usingthis method we have obtained evidence for space-charge-dominated behavior of a stored, laser-cooledbeam of

Dissociative recombination of vibrationally excitedHD+: State-selective experimental investigation

measured. The method is based on using merged electron and molecular ion beams in a heavy-ion storage ring together with molecular fragment imaging techniques which allow us to probe the vibrational-state population of the stored beam as a function of time as well as the final state of the dissociation. The initial vibrational distribution of the stored ion beam ~from a Penning ion source! is found to be in good agreement with a Franck-Condon model of electron impact ionization, apart from slightly larger experimental populations found for low vibrational states; its time evolution in the storage ring reflects the predicted vibrational level lifetimes. Dissociative recombination measurements were performed with the electron and ion beams at matched velocities ~corresponding to average collision energies of about 10 meV!, and at several well-defined collision energies in the range of 3‐11 eV. The obtained vibrational-state specific recombination rate coefficients are compared with theoretical calculations and show that, although an overall agreement exists between experiment and theory, large discrepancies occur for certain vibrational states at low electron energy. @S1050-2947~99!09411-1# PACS number~s!: 34.80.Gs

Lifetime Measurement of the Metastable4dD3/22Level inSr+by Optical Pumping of a Stored Ion Beam

Lifetime Measurement of the Metastable<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mn>4</mml:mn><mml:mi mathvariant="italic">d</mml:mi><mml:mi /><mml:mrow><mml:mmultiscripts><mml:mrow><mml:mi>D</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn><mml:mi>/</mml:mi><mml:mn>2</mml:mn></mml:mrow><mml:mrow /><mml:mrow /><mml:mrow /><mml:mprescripts /><mml:mrow /><mml:mrow><mml:mn>2</mml:mn></mml:mrow><mml:mrow /><mml:mrow /></mml:mmultiscripts></mml:mrow></mml:math>Level in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mi mathvariant="normal">Sr</mml:mi></mml:mrow><mml:mrow><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:mrow></mml:math>by Optical Pumping of a Stored Ion Beam

A technique for lifetime measurements of long-lived atomic states utilising laser excitation of stored ion beams

A laser probing technique for atomic lifetime measurements of metastable states using an ion storage ring is presented. The method benefits from the high spectral resolution of conventional fast ion beam laser spectroscopy, and is applicable to metastable systems in lowly charged ions with lifetimes in the 10 ms–1 s regime. Due to strong probing mechanisms, the technique can be applied to ion beams with low metastable fractions. A comprehensive description of the laser probing technique is given, with examples from recent experiments on singly charged calcium, strontium and xenon, and future prospects are discussed.

Three-dimensional laser cooling method based on resonant linear coupling

A three-dimensional (3D) laser cooling method of fast stored ion beams based on a linear coupling mechanism is explored. We extensively study two approaches proposed in previous publications, i.e., the dispersive coupling scheme and the coupling-cavity scheme, confirming how much one can improve the transverse cooling efficiency. A possible design of a coupling cavity is presented. We employ the tracking code SAD and the molecular dynamics code SOLID to carry out reliable numerical experiments where realistic lattice structures of storage rings and particle Coulomb interactions are taken into account. Through systematic simulations, it is demonstrated that resonant coupling remarkably enhances transverse cooling rates for any initial beams, making it feasible to reach an equilibrium temperature far below the current achievable level. We further emphasize the crucial importance of avoiding the Mathieu instability. We also discuss the minimum cooling power required for beam crystallization as well as on an interpretation of past experimental results in the TSR and ASTRID storage rings.

Lifetimes of the metastable and levels in measured by laser probing of a stored ion beam

A laser probing method has been used in an ion storage ring to measure the lifetimes of the metastable and levels in . The lifetimes obtained were s for and s for . The most accurate results from ion trap measurements differ by 10% from each other. Our value for supports the longer lifetime value reported from Werth's group. For the level, the new measurement is consistent with previous results.

Transverse Laser Cooling of a Fast Stored Ion Beam through Dispersive Coupling

Transverse laser cooling of a fast stored ${}^{9}{\mathrm{Be}}^{+}$ ion beam based on a single-particle force independent of the ion density is demonstrated at the Heidelberg Test Storage Ring. The cooling scheme exploits longitudinal-horizontal coupling through ring dispersion and the transverse intensity profile of the longitudinally merged laser beam. By linear betatron coupling the horizontal force is extended to the vertical degree of freedom resulting in true 3D laser cooling. The observed transverse-cooling mechanism represents an important step towards crystalline ion beams.

Rapid adiabatic passage in laser cooling of fast stored ion beams

We present a simple and efficient method for the broadband laser excitation of ions in a fast beam by optical rapid adiabatic passage. The method relies on the frequency chirp affecting the ions in continuous-wave fixed-frequency laser light as a result of the Doppler effect when they are accelerated in local electrostatic fields. We have studied this excitation scheme at the Heidelberg Test Storage Ring and demonstrated its particular application to overcome collisional losses in laser cooling of fast stored ion beams.

Dissociative recombination of HD+ in selected vibrational quantum states

Rate coefficients for dissociative recombination of HD+ in selected vibrational states have been measured by a combination of two molecular fragment imaging methods by using the heavy-ion storage ring technique. Recombination fragment imaging yields state-to-state reaction rates. These rates are converted to rate coefficients by using vibrational level populations of the stored ion beam, derived from nuclear coordinate distributions measured on extracted ions. The results show strongly increasing rate coefficients for high vibrational excitation, where additional dissociation routes open up, in agreement with a theoretical calculation. Very low rate coefficients are found for certain, isolated vibrational states.

“White-light” Laser Cooling of a Fast Stored Ion Beam

We report the experimental demonstration of ``white-light'' cooling of a high-velocity ${}^{7}{\mathrm{Li}}^{+}$ ion beam stored at 6.4% of the speed of light in a storage ring. In a direct comparison with single-mode laser cooling, we show that white-light cooling is much more efficient to counteract strong intrabeam heating and leads to lower longitudinal beam temperatures at higher ion densities, i.e., much higher densities in longitudinal phase space.