Tuning methods for multigap drift tube linacs

Multigap cavities are used extensively in linear accelerators to achieve velocities up to a few percent of the speed of light, driving nuclear physics research around the world. Unlike for single-gap structures, there is no closed-form expression to calculate the output beam parameters from the cavity voltage and phase. To overcome this, we propose to use a method based on the integration of the first and second moments of the beam distribution through the axially symmetric time-dependent fields of the cavity. A beam-based calibration between the model's electric field scaling and the machine's rf amplitudes is presented, yielding a fast online energy change method, returning cavity amplitude and phase necessary for a desired output beam energy and energy spread. The method is validated with 23Na6+ beam energy measurements.

Investigation of ion beam space charge compensation with a 4-grid analyzer

Experiments to investigate the space charge compensation of pulsed high-current heavy ion beams are performed at the GSI ion source text benches with a 4-grid analyzer provided by CEA/Saclay. The technical design of the 4-grid analyzer is revised to verify its functionality for measurements at pulsed high-current heavy ion beams. The experimental investigation of space charge compensation processes is needed to increase the performance and quality of current and future accelerator facilities. Measurements are performed directly downstream a triode extraction system mounted to a multi-cusp ion source at a high-current test bench as well as downstream the post-acceleration system of the high-current test injector (HOSTI) with ion energies up to 120 keV/u for helium and argon. At HOSTI, a cold or hot reflex discharge ion source is used to change the conditions for the measurements. The measurements were performed with helium, argon, and xenon and are presented. Results from measurements with single aperture extraction systems are shown.

The proton injector for the accelerator facility of antiproton and ion research (FAIR)

The new international accelerator facility for antiproton and ion research (FAIR) at GSI in Darmstadt, Germany, is one of the largest research projects worldwide and will provide an antiproton production rate of 7 × 10(10) cooled pbars per hour. This is equivalent to a primary proton beam current of 2 × 10(16) protons per hour. For this request a high intensity proton linac (p-linac) will be built with an operating rf-frequency of 325 MHz to accelerate a 35 mA proton beam at 70 MeV, using conducting crossed-bar H-cavities. The repetition rate is 4 Hz with beam pulse length of 36 μs. The microwave ion source and low energy beam transport developed within a joint French-German collaboration GSI/CEA-SACLAY will serve as an injector of the compact proton linac. The 2.45 GHz ion source allows high brightness ion beams at an energy of 95 keV and will deliver a proton beam current of 100 mA at the entrance of the radio frequency quadrupole (RFQ) within an acceptance of 0.3π mm mrad (norm., rms).

Demonstration of charge breeding in a compact room temperature electron beam ion trap

For the first time, a small room-temperature electron beam ion trap (EBIT), operated with permanent magnets, was successfully used for charge breeding experiments. The relatively low magnetic field of this EBIT does not contribute to the capture of the ions; single-charged ions are only caught by the space charge potential of the electron beam. An over-barrier injection method was used to fill the EBIT's electrostatic trap with externally produced, single-charged potassium ions. Charge states as high as K(19+) were reached after about a 3 s breeding time. The capture and breeding efficiencies up to 0.016(4)% for K(17+) have been measured.

The design of an electron gun switchable between immersed and Brillouin flow

An electron gun, which can be switched from immersed flow to Brillouin flow during operation, may have advantages for charge breeders as well as for electron beam ion sources and traps (EBISTs). For EBISTs this allows to change the current density according to the repetition frequency and charge state, for charge breeders and EBISTs a lower current density in immersed flow provides higher acceptance for injected ions, while the higher current density in Brillouin flow results in shorter breeding times and a lower emittance for the extracted beam. Therefore, we have designed such a gun for an EBIS with 5 T central magnetic field and without the use of iron and moving the gun. The gun was placed in the axial fringing field of the 5 T solenoid in such a position that a gate valve can be placed between the gun and the cryostat to allow for simple maintenance. The field at the cathode surface turned out to be only 0.05 T, which is not enough to focus 50 A∕cm(2) at a few kV. However, if a small normal conducting solenoid is placed over the vacuum tube in position of the gun, a field of 0.1 T may be obtained. With this the use of LaB(6) as cathode material results in a magnetic compression of 44 and therewith in a focused current density in the trap region of more than 2000 A∕cm(2). By reversing the current in the gun solenoid the cathode field can easily compensated to zero. By proper design of the electrodes and the compression region, the gun will be able to deliver a beam in Brillouin flow. While this is interesting by itself--remember the "super-compression" reported on CRYEBIS-I--any magnetic field between zero and the value for immersed flow will result in an electron beam with a wide range of adjustable high current densities. The design tools used have been INTMAG(C) for the calculation of magnetic fields, EGN2(C) for the simulation of the gun and ANALYSE(C) for detailed analysis of the results (for more information see www.egun-igun.com).

Initial commissioning results with the NSCL Electron Beam Ion Trap

The ReA reaccelerator is being added to the National Superconducting Cyclotron Laboratory (NSCL) fragmentation facility in order to provide exotic rare-isotope beams, not available at the Isotope Separation On-Line facilities, in the several-MeV/u energy range. The first stage of the NSCL reaccelerator complex, consisting of an EBIT charge breeder, a room-temperature radiofrequency quadrupole (RFQ) accelerator, and superconducting linear accelerator modules, has been completed and is being put into operation. Commissioning of the EBIT has started by extracting charge-bred residual gas ions, ions created from a Ne gas jet directed across the EBIT's electron beam and ions captured from an external test ion source. Charge-bred ions from the Ne gas jet have been extracted as a pulse and accelerated through the RFQ and the two cryomodules.

The NSCL electron beam ion trap for the reacceleration of rare isotopes coming to life: first extraction tests with a high-current electron gun

NSCL is currently constructing the ReA3 reaccelerator, which will accelerate rare isotopes obtained from gas stopping of fast-fragment beams to energies of up to 3 MeV/u for uranium and higher for lighter ions. A high-current charge breeder, based on an electron beam ion trap (EBIT), has been chosen as the first step in the acceleration process, as it has the potential to efficiently produce highly charged ions in a single charge state. These ions are fed into a compact linear accelerator consisting of a radio frequency quadrupole structure and superconducting cavities. The NSCL EBIT has been fully designed with most of the parts constructed. The design concept of the EBIT and results from initial commissioning tests of the electron gun and collector with a temporary 0.4 T magnet are presented.

0(gs)+ -->2(1)+ transition strengths in 106Sn and 108Sn

The reduced transition probabilities, B(E2; 0(gs)+ -->2(1)+), have been measured in the radioactive isotopes (108,106)Sn using subbarrier Coulomb excitation at the REX-ISOLDE facility at CERN. Deexcitation gamma rays were detected by the highly segmented MINIBALL Ge-detector array. The results, B(E2;0(gs)+ -->2(1)+)=0.222(19)e2b2 for 108Sn and B(E2; 0(gs)+-->2(1)+)=0.195(39)e2b2 for 106Sn were determined relative to a stable 58Ni target. The resulting B(E2) values are approximately 30% larger than shell-model predictions and deviate from the generalized seniority model. This experimental result may point towards a weakening of the N=Z=50 shell closure.

The MAXEBIS at GSI as a test ion source for charge breeding and for HITRAP

The electron beam ion source MAXEBIS, developed and built at the University of Frankfurt, has been installed at GSI to serve as an offline test ion source for the HITRAP project and for use as a test setup for charge breeding explorations. The setup has been equipped with new diagnostics and beam optics devices. Two ion sources dedicated to the production of singly charged ions for external ion injection into the MAXEBIS have been included. First time of flight (TOF) spectra with external injected, charge-bred argon ions were taken. However, these spectra indicated beam losses in the beam transport from the multipassage spectrometer (MPS) to the MAXEBIS. In order to understand the losses, the beam transport has been simulated with the SIMION code and the beam line has been modified accordingly. Measurements of the MAXEBIS' beam emittance, using the "nondestructive" beam profile method, have been performed as well as measurements of the electron current density via charge state analysis of the TOF spectra. The injection and breeding efficiency of the MAXEBIS have been determined for the first time. The results of the measurements and the planned experiments will be discussed.

Investigation of an electron string ion source with field emission cathode

The string mode of operation for an electron beam ion source uses axially oscillating electrons in order to reduce power consumption, also simplifying the construction by omitting the collector with cooling requirements and has been called electron string ion source (ESIS). We have started a project (supported by INTAS and GSI) to use Schottky field emitting cathode tips for generating the electron string. The emission from these specially conditioned tips is higher by orders of magnitude than the focused Brillouin current density at magnetic fields of some Tesla and electron energies of some keV. This may avoid the observed instabilities in the transition from axially oscillating electrons to the string state of the electron plasma, opening a much wider field of possible operating parameters for an ESIS. Besides the presentation of the basic features, we emphasize in this paper a method to avoid damaging of the field emission tip by backstreaming ions.

A high-current electron beam ion trap as a charge breeder for the reacceleration of rare isotopes at the NSCL

Reacceleration of low-energy rare isotope beams available from gas stopping of fast-fragment beams or from an ISOL target station to energies in the range of 0.3-12 MeV/nucleon is needed for experiments such as low-energy Coulomb excitation and transfer reaction studies and for the precise study of astrophysical reactions. The implementation of charge breeding as a first step in a reaccelerator is a key to obtaining a compact and cost-efficient reacceleration scheme. For highest efficiency it is essential that single charge states are obtained in a short breeding time. A low-emittance beam must be delivered. An electron beam ion trap (EBIT) has the potential to meet these requirements. An EBIT-based charge breeder is presently under design and construction at the NSCL as part of the construction of a reaccelerator for stopped beams from projectile fragmentation. This new facility will have the potential to provide low-energy rare isotope beams not yet available elsewhere.

In-line ion detector

An in-line particle detector (IPD) uses secondary electrons for the detection of multiply charged ions with low to medium energy (10-10 keV). The ion detector does not physically intercept the ion beam line and is fully transparent to ions without applied voltages. The activation of the detector is performed by applying appropriate voltages to electrodes, which avoids any physical movement. Equipped with a channel electron multiplier, single particle counting is possible as well as measurement of currents. This detector therefore has a large dynamical range from about 10(-17) to 10(-3) A. The basic principle also allows for ion beam diagnostics.

Coulomb excitation of neutron-rich Zn isotopes: first observation of the 2(1)+ state in 80Zn

Neutron-rich, radioactive Zn isotopes were investigated at the Radioactive Ion Beam facility REX-ISOLDE (CERN) using low-energy Coulomb excitation. The energy of the 2(1)+ state in 78Zn could be firmly established and for the first time the 2+ --> 0(1)+ transition in 80Zn was observed at 1492(1) keV. B(E2,2(1)+ --> 0(1)+) values were extracted for (74,76,78,80)Zn and compared to large scale shell model calculations. With only two protons outside the Z=28 proton core, 80Zn is the lightest N=50 isotone for which spectroscopic information has been obtained to date. Two sets of advanced shell model calculations reproduce the observed B(E2) systematics. The results for N=50 isotones indicate a good N=50 shell closure and a strong Z=28 proton core polarization. The new results serve as benchmarks to establish theoretical models, predicting the nuclear properties of the doubly magic nucleus 78Ni.

Coulomb excitation of 68,70Cu: first use of postaccelerated isomeric beams

We report on the first low-energy Coulomb excitation measurements with radioactive Ipi=6- beams of odd-odd nuclei 68,70Cu. The beams were produced at ISOLDE, CERN and were post-accelerated by REX-ISOLDE to 2.83 MeV/nucleon. Gamma rays were detected with the MINIBALL spectrometer. The 6- beam was used to study the multiplet of states (3-, 4-, 5-, 6-) arising from the pi2p3/2 nu 1g9/2 configuration. The 4- state of the multiplet was populated via Coulomb excitation and the B(E2;6--->4-) value was determined in both nuclei. The results obtained illustrate the fragile stability of the Z=28 shell and N=40 subshell closures. A comparison with large-scale shell-model calculations using the 56Ni core shows the importance of the proton excitations across the Z=28 shell gap to the understanding of the nuclear structure in the neutron-rich nuclei with N approximately 40.

"Safe" Coulomb excitation of 30Mg

We report on the first radioactive beam experiment performed at the recently commissioned REX-ISOLDE facility at CERN in conjunction with the highly efficient gamma spectrometer MINIBALL. Using 30Mg ions accelerated to an energy of 2.25 MeV/u together with a thin (nat)Ni target, Coulomb excitation of the first excited 2+ states of the projectile and target nuclei well below the Coulomb barrier was observed. From the measured relative deexcitation gamma-ray yields the B(E2;0(+)gs-->2(+)1) value of 30Mg was determined to be 241(31)e2 fm4. Our result is lower than values obtained at projectile fragmentation facilities using the intermediate-energy Coulomb excitation method, and confirms the theoretical conjecture that the neutron-rich magnesium isotope 30Mg resides outside the "island of inversion."