Bunched Ion Beams Research Articles

Acceleration and focusing of intense ion beams in rf undulator structures

Bunching, acceleration, and transverse focusing of intense ion beams in an undulator linac are considered. Such an accelerator features the absence of an rf field harmonic synchronous with the beam. A 3D equation of motion in the Hamiltonian form is derived in the smooth approximation, and the general conditions for ion beam acceleration and transverse focusing in the undulator linac are formulated. Basic analytical results are compared with the results of numerical simulation of the beam dynamics in the polyharmonic field of an accelerating cavity.

Preparation of cooled and bunched ion beams at ISOLDE-CERN

At ISOLDE a new RadioFrequency Quadrupole ion Cooler and Buncher (RFQCB) is being constructed to improve ion optical properties of low-energy RIBs. The new features of the mechanical design and the status of the test bench, which will serve to test the device, will be presented in this contribution.

Laser spectroscopy with cooled beams

This contribution describes the programme being carried out by a collaboration involving the Universities of Birmingham, Jyväskylä and Manchester, using collinear laser spectroscopy to measure isotope shifts and hyperfine structures of radioisotopes. The work benefits from the unique combination of an ion guide isotope separator (IGISOL) and an RFQ ion cooler-buncher, allowing measurements on ion fluxes as low as 150 s −1 . Recent studies include Zr and Ce isotopes produced by proton-induced fission of uranium, neutron deficient isotopes of Ti, and K = 8 isomers in 130 Ba and 176 Yb. Development work on a more sensitive technique of Collinear Resonance Ionization Spectroscopy (CRIS) with bunched ion beams is also described.

Linear radio frequency quadrupole for the cooling and bunching of radioactive ion beams

A linear radio frequency quadrupole has been built for the transport, cooling, and bunching of radioactive ions extracted from an ECR source. The device uses the buffer gas cooling technique and was designed such as to extend the technique for the cooling of very light ions using H2 as buffer gas. We describe here the technical specifications of the device and present results of the first tests concerning the cooling and bunching of stable ions.

Electron cooling of bunched ion beam at NIRS-HIMAC

Electron-cooling experiments have been carried out at the HIMAC-synchrotron in order to provide high-quality beams for medical and related research. For investigating the behavior of the free radicals, especially, the beam bunch was compressed by electron cooling with a triangle-wave RF-field. The dependences of the bunch length on the electron energy and the RF voltage were investigated under various intensities of Ar18+ beam. As a result, the bunch length was limited by around 20ns due to the space-charge effect under an intensity of 1.33×106ions/bunch and an RF amplitude of 340V.

Nonlinear stationary waves and solitons in ion beam‐plasma configuration

A fully nonlinear treatment for stationary waves in a cold plasma with an ion beam propagating parallel to the magnetic field is carried out in the framework of Hall MHD. It is shown that the linearly unstable system can evolve into a steady nonlinear configuration in which both ion populations undergo a coherent helical motion guided by the magnetic field. The complete system of equations can be reduced to two coupled differential equations for the transverse proton (or beam ion) speed and the phase difference between the wave motion of both ion fluids. In the case of a weak beam these equations possess a phase‐portrait integral which describes the main dynamical features of the system. A soliton‐type solution with a core filled by smaller‐scale oscillations is found for the transverse components of the ion velocities and magnetic field. It is suggested that gyrophased bunched ion beams associated with large‐amplitude quasi‐monochromatic ULF waves often observed in the ion foreshock are a manifestation of these fully nonlinear structures. A driver for such oscilliton‐like structures is provided by a linear instability in which the wave is exponentially growing. The momentum exchange between two different ion populations, mediated by the magnetic field stresses, leads to the formation of a steady nonlinear wave‐oscilliton.

Structural changes in bunched crystalline ion beams

Measurements of the spatial distribution of bunched crystalline ion beams in the radio frequency quadrupole storage ring PALLAS are presented for different ratios of the longitudinal and the transverse confinement strengths. The length of highly elongated crystalline ion bunches and its dependence on the bunching voltage is compared to predictions for a one-dimensional ion string and three-dimensional space-charge-dominated beams. The length is found to be considerably shorter than that predicted by the models. Furthermore, the scaling of the length with the bunching voltage is shown to differ from the expected inverse cube root scaling. These differences can partially be attributed to the formation of a mixed crystalline structure. Additionally, a concise mapping of the structural transition from a string to a zig-zag configuration as a function of the ratio of the confinement strengths is presented, which in a similar way deviates from the predictions.

Cooling and bunching of ion beams for collinear laser spectroscopy

A greatly increased sensitivity in collinear laser spectroscopy experiments has been achieved by the application of new on-line ion cooling and bunching techniques. Cooling of a low-energy ion beam to low emittance and low velocity spread is shown to increase the peak efficiency while bunching the beam results in highly efficient background suppression.

A very large Paul trap ion gun for delivering bunched low energy ion beams

Ions extracted from mass separators normally have energies in the range of tens of kilovolts and poor emittances, and are thereby not suited for soft landing, especially in ion deposition applications. In this article a system is described that has been developed for in-flight capture of continuous 60 keV ion beams and delivering them as low energy beam pulses of less than 1 μs duration and energies of less than 5 keV. They are also cooled so that their emittances and energy spreads are considerably reduced from those of the original beam. The basis of the system is a very large Paul trap as a collection system, together with a pulse-down cavity for modifying the energy of the delivered beam. The performance of the system has been analyzed using time-of-flight of the extracted ions. Experimental results are presented, together with numerical simulations of the results that lead to an understanding of the spatial and momentum properties of the ion collection in the trap. The results show how such collection systems can be designed for specific purposes, such as high-current implantation or precise implantation of smaller collections. Such a system can also be used where the ultimate destination is storage in an electromagnetic trap at ground potential.

A linear radiofrequency quadrupole ion trap for the cooling and bunching of radioactive ion beams

A linear radio-frequency quadrupole ion guide and beam buncher has been installed at the ISOLTRAP mass spectrometry experiment at the ISOLDE facility at CERN. The apparatus is being used as a beam cooling, accumulation, and bunching system. It operates with a buffer gas that cools the injected ions and converts the quasicontinuous 60-keV beam from the ISOLDE facility to 2.5-keV beam pulses with improved normalized transverse emittance. Recent measurements suggest a capture efficiency of the ion guide of up to 40% and a cooling and bunching efficiency of at least 12% which is expected to still be increased. The improved ISOLTRAP setup has so far been used very successfully in three on-line experiments.

Bunched Crystalline Ion Beams

We report on the realization of bunched crystalline ion beams in a tabletop quadrupole storage ring. The phase transition of the beam, laser-cooled at an energy of 1 eV, is observed as a sudden drop in the transverse beam size. The crystalline beam evolves from a linear string of ions to a vertical zigzag band when the bunch is longitudinally compressed. Bunched and coasting beams show similar properties close to the phase transition. Here, the spatial beam profile exhibits an unexpected two-component distribution, possibly due to a phase mixing process.

Accurate masses of neutron-deficient nuclides close to [formula omitted

Mass measurements with the Penning-trap mass spectrometer ISOLTRAP at ISOLDE/CERN are extended to nonsurface ionizable species using newly developed ion-beam bunching devices. Masses of 179–197Hg, 196,198Pb, 197Bi, 198Po and 203At were determined with an accuracy of 1×10 −7 corresponding to δm≈20 keV. Applying a resolving power of up to 3.7×10 6 ground and isomeric states of 185,187,191,193,197Hg were separated. First experimental values for the isomeric excitation energy of 187,191Hg are obtained. A least-squares adjustment has been performed and theoretical approaches are discussed to model the observed fine structure in the binding energy.

A linear radiofrequency ion trap for accumulation, bunching, and emittance improvement of radioactive ion beams

An ion beam cooler and buncher has been developed for the manipulation of radioactive ion beams. The gas-filled linear radiofrequency ion trap system is installed at the Penning trap mass spectrometer ISOLTRAP at ISOLDE/CERN. Its purpose is to accumulate the 60-keV continuous ISOLDE ion beam with high efficiency and to convert it into low-energy low-emittance ion pulses. The efficiency was found to exceed 10% in agreement with simulations. A more than 10-fold reduction of the ISOLDE beam emittance can be achieved. The system has been used successfully for first on-line experiments. Its principle, setup and performance will be discussed.

Funneling experiments

High intensity rf-accelerator concepts for Heavy Ion Inertial Fusion (HIIF) injectors require small emittance, high current and high energy beams. The high brightness in such a driver linac is achieved by several funneling stages at low energies, in which two identically bunched ion beams are combined into a single beam with twice the frequency, current and brightness. For the first stage of a Heavy Ion Driver we propose the use of a two-beam accelerator structure which provides two beams within one cavity and a single rf-deflector structure which bends the two beams to one common axis. The progress of the experiment and first beam-test results will be presented.

Triple drift ion beam bunching systems for enhanced bunching efficiency

To obtain high efficiency in bunching ions one generally uses a multiharmonic buncher or a double drift buncher. Here we have explored the possiblity of using a triple drift bunching system to obtain still higher bunching efficiency. It consists of three bunchers separated in space. The first buncher is excited with the fundamental rf wave whereas the other two bunchers are driven by the second harmonic. We have optimized the parameters of such a system and have shown that by using only two rf frequencies one can obtain bunching efficiency higher than what can be obtained with a multiharmonic buncher consisting of nine harmonics.

Progress of the two-beam funneling experiment

The injector for a heavy ion inertial fusion (HIIF) driver has to provide a high current and small emittance ion beam which cannot be provided by one single ion source. The required beam will be reached by several funneling stages, where two identically bunched ion beams are combined into a single beam with twice the frequency, current and brightness. For the most critical first funneling stage a two-beam radio-frequency quadrupole (RFQ), where two beams are bunched and accelerated in a single RF cavity, and a novel scheme for an RF funneling deflector, operating at low voltages, has been developed. With the use of convergent incoming beams, the funneling structure will be placed around the beam crossing position. The progress of the experiment, consisting of a combination of a two-beam RFQ with an RF deflector for funneling of two He +-beams at low energies will be presented.

Laser spectroscopy of the 1s HFS of H-like ions at a bunched beam in the storage ring ESR

The investigation of the 1s HFS provides a good possibility for testing QED effects in a combination of a strong electric and magnetic field. Here, we report about the laserspectroscopic measurements of the ground state hyperfine splitting in 207Pb81+. To handle this M1-transition in the infrared optical regime with its long lifetime, we developed a new detection technique using a bunched ion beam. For the observation of fluorescence light, a new mirror system is adapted to the emission characteristics from an ion beam at relativistic velocities.

Laser preparation of bunched ion beams

Laser ionization at ISOLDE is performed with high repetition laser beams in the ion source unit and for daughter elements with low repetition lasers synchronized with laser desorption from a secondary target. We are preparing an implementation of thermal adsorption followed by laser desorption and laser ionization in the target ion source unit of ISOLDE. The combination of resonance ionization with thermal bunching allows the preparation of bunched and chemically pure ion beams. Bunched ion beams of Mg or Ca are prepared for post-acceleration at REX-ISOLDE. In order to investigate the process of adsorption and pulsed laser desorption, experiments are performed with Ba vapour. In a test setup Ba is effusing from an oven and adsorbed on a cooled surface, which can be heated by a pulsed Nd: YAG laser beam. The adsorption and bunched release of Ba atoms is detected via resonance absorption of the 6s 2 1S 0 → 6s6p 1P 1 atomic line at λ = 553 nm with a single mode dye laser.

A cylindrical Penning trap for capture, mass selective cooling, and bunching of radioactive ion beams

A Penning trap ion accumulator, cooler, and buncher for low-energy ion beams has been developed for the ISOLTRAP mass spectrometer at ISOLDE/CERN. A cylindrical electrode configuration is used for the creation of a nested trapping potential. This is required for efficient accumulation of externally produced ions and for high-mass selectivity by buffer gas cooling. The design goal of a mass resolving power of about 1 × 10 5 has been achieved. Isobar separation has been demonstrated for radioactive rare-earth ion beams delivered by the ISOLDE on-line mass separator.

Transverse laser cooling of a radio-frequency bunched ion beam in the storage ring TSR

We report on the observation of the indirect transverse laser cooling effect in a radio-frequency bunched beam of 7.3 MeV 9Be + ions, stored in the Heidelberg Test Storage Ring and subject to direct longitudinal laser cooling. This bunched scheme offers particular advantages for producing ultracold beams with unprecedented phase-space densities.