Slow Extraction of Charged Ion Pulses from the REXEBIS

The Isotope mass Separator On-Line DEvice (ISOLDE) facility located at CERN, produces and transports Radioactive Ion Beams (RIBs) at low or high energy through the REX/HIE-ISOLDE linear accelerator, for nuclear physics, astrophysics, solid-state physics and applied-physics purposes. Increasing the charge state of the ions is a prerequisite for efficient acceleration and is accomplished by an Electron Beam Ion Source (REXEBIS). For more effective event discrimination at the experimental detectors, such as the MINIBALL spectrometer, it is advantageous to increase the pulse width of extracted ions from this EBIS. A Slow Extraction scheme is presented which uses a function comprised of discrete voltage steps to apply the extraction potential to the EBIS trap barrier. This function effectively stretches the pulse length of both stable and radioactive ion beams, with different mass-to-charge ratios and provides for extracted pulse widths in the millisecond range. Key operational parameters of the EBIS impacting the average ionic temperature and its axial energy spread are discussed, in order to anticipate changes in the resulting ion pulse time structures during experimental runs.Comment: 17th International Conference on Ion Sources (ICIS17, October 2017, Geneva

Transverse Emittance Measurements of the Beams Produced by the ISOLDE Target Ion Sources

The Isotope mass Separator On-Line DEvice (ISOLDE) is a Radioactive Ion Beam (RIB) facility based at CERN where rare isotopes are produced from 1.4 GeV-proton collisions with a target. The different types of targets and ion sources, operating conditions and ionization schemes used during the physics campaign results in extracted beams with various emittances. Characterizing the beam emittance allows deducing the transport efficiency to low-energy experimental stations (up to 60 keV) and the mass resolving power of the separators. We report on emittance measurements for different beams of stable elements extracted from surface and plasma ion sources. The dependence of the emittance on the different conditions of operation of the ion sources is investigated and the results are compared to previous measurements

Characterization of Very Low Intensity Ion Beams from the REX/HIE-ISOLDE Linear Accelerator at CERN

The Isotope Separator On-Line Device (ISOLDE) at CERN is one of the world-leading research facilities in the field of nuclear physics. Radioactive Ion Beams (RIBs) are produced when 2.0 GeV protons, driven from the Proton Synchrotron Booster (PSB), impact onto a target. The RIB of interest is extracted and transported to experimental stations either directly or following an acceleration to higher energies. Before meeting the requirements for acceleration, the continuous low-energy RIB is accumulated and cooled into the REX-TRAP Penning trap and transferred toward the REXEBIS charge breeder. The ion beam extracted from the REXEBIS is pulsed and multi-ionized to reach a mass-to-charge ratio (A/q) within the acceptance of the linac. After that, the RIB is accelerated (up to 9.7 MeV/u at A/q = 3.1) through the Radioactive EXperiment normal-conducting linac injector and the recently completed High Intensity and Energy superconducting extension (REX/HIE-ISOLDE) and finally transported to one of the three experimental stations located at the end of High Energy Beam Transfer (HEBT) lines. The manipulation and precise knowledge of the ion beam properties are of primary importance for the experimental apparatus, as well as for the calibration and efficiency of the detection systems. The study of ion beam properties mainly concerns the dynamics over time and space distributions of density, motion and energy, correlated with beam physics, that is to say: the purity of the beam of interest by considering potential sources of contamination, the ion-pulse time-structure, the transversal and longitudinal properties (Courant-Snyder parameters and energy distribution). The core of the thesis aims at defining instrumental methodologies that allow for the determination of RIBs properties from the REX/HIE-ISOLDE linear accelerator, at intensities often too weak to make use of conventional beam-line monitoring detectors (for instance, Faraday cups). We propose new approaches appropriate for very low-intensity ion beams, using a silicon detector as a beam collector. In general, the demonstration of a new measurement concept suitable for sub-femto-Ampere ion beams is corroborated with ordinary measurement techniques at ion beam currents higher than several pico-Amperes. The capability to measure absolute beam intensities over three orders of magnitude is illustrated with a practical use during beam operation, tuning the Radio-Frequency Quadrupole structure. Then, we measure and study the quality of the beam produced from REXEBIS in the scope of evaluating the electron gun's performance and the general charge-breeding conditions. An effort dedicated to optimizing the ion pulse time distribution extracted from an EBIS, called Slow Extraction, led us to use measurements of axial energy distributions further. By justifying the collisional aspects of the plasma, we infer a correlation between the measured axial energy distributions and the temperature of ions. Key indicators of the extracted beam's quality can then be deduced from the ion temperature using Boltzmann distributions, such as the emittance. We illustrate the studies related to the charge-state and the energy dynamics with the case of a multi-charged xenon beam. The question of beam purity is assessed by demonstrating the possibility to map a spectrum of the contamination from residual gas ions on a wide A/q-range. Furthermore, we characterize the beam dynamics in the HEBT lines after acceleration through REX/HIE-ISOLDE linac. The principles behind the identification of the transverse and the longitudinal beam properties are mainly based on multiple acquisitions of the respective trace-space projections. In the case of transverse beam properties, we correlate measurements using the quadrupole-scan method and the double-slit method using a $^{39}$K$^{10+}$ beam of less than one femto-Ampere current, at an energy of 3.8 MeV/u. We show the uncertainties associated with the evaluation of the emittance and Twiss parameters for each method. We then explain the choice made for measuring the beam energy and the analysis of results using RIB. The complete longitudinal beam properties are measured using a superconducting quarter-wave resonator as a buncher, and measuring the beam energy and time-of-flight distributions

Residual Gas Ions Characterization from the REXEBIS

The Isotope mass Separator On-Line DEvice (ISOLDE) is a user facility located at CERN where Radioactive Ion Beams (RIBs) are produced from proton collisions onto a target, mass separated and transported to user experimental stations either directly at low energy or after being post- accelerated, notably for nuclear physics studies. Prior to acceleration through the REX/HIE-ISOLDE linear accelerator, the ion beam is accumulated, bunched and cooled in a Penning trap (REXTRAP) and afterwards charge-bred in an Electron Beam Ion Source (REXEBIS). Multi-charged radioactive species of interest are then selected by a mass-to- charge (A/q) ratio separator dipole in the Low Energy Beam Transfer Line (LEBT). A method is presented to character- ize the Residual Gas Ion (RGI) background contamination for different operational conditions of the REXEBIS. More particularly, a discussion is held about the influence of the confinement time inside the charge-breeder on the residual gas spectrum. Finally, a method to identify sub-pico-Ampere contaminants is demonstrated

Characterization of REX/HIE-ISOLDE RFQ longitudinal acceptance and transmission

The Isotope mass Separator On-Line DEvice (ISOLDE) based at CERN, is a Radioactive Ion Beam (RIB) facility where rare isotopes are produced from 1.4 GeV-proton collisions onto a target then are manipulated and transported to user experimental stations for studies, notably in the domain of nuclear physics. The RIB of interest is delivered to a dedicated experimental station either at low (up to 60 keV) or high energy (MeV/u range) after acceleration through the recently completed REX/HIE-ISOLDE linac upgrade. The first stage of the linac consists of normal-conducting IH and spiral-resonators and is preceded by a Radio-Frequency Quadrupole (RFQ). A description of the experimental setup and the specifications of the RFQ will serve as an introduction for the presentation of recent results about the transmission efficiency of the RFQ. Furthermore, a newly developed technique will be demonstrated, that allow for the capture of ion beam intensities below the femto-Ampere range. In fine, a measurement of the longitudinal acceptance of the RFQ will be included

Phasing of superconductive cavities of the REX/HIE-ISOLDE linac

ISOLDE is a facility dedicated to the production of a large variety of Radioactive Ion Beams. The facility is located at the European Organization for Nuclear Research (CERN). In addition to two target stations followed by low energy separators, the facility includes a post-accelerating linac with both normal conducting (REX) and superconducting (HIE-ISOLDE) sections. The HIE-ISOLDE section consists of four cryomodules with five SRF cavities each that need to be phased individually. In this paper, we will describe the procedure and the software applications developed to phase each of the cavities as well as improvements that will be introduced in the near future to reduce the time it takes to complete the process

Characterization of the Beam Energy Spread at the REX/HIE-ISOLDE Linac

ISOLDE is an on-line radioactive isotope separator located at CERN that works by colliding protons accelerated in the PS Booster into a fixed target and by separating the resultant ionized isotopes using a magnetic separator. The completion of the HIE-ISOLDE superconducting linac allows the acceleration of these ions to energy levels that were not reachable before, opening the door to new experiments in different fields. These experiments often have special requirements in terms of beam intensity and purity, transverse emittance or energy spread. A possible way to reduce the energy spread of the beam delivered to the experimental stations is to use one or more of the superconducting cavities as bunchers. The main results of several tests conducted during the last beam commissioning campaign prove that this mode of operation is feasible and will be presented in this paper

Software applications for beam traceability and machine documentation at ISOLDE

The ISOLDE facility at CERN requires a wide variety of software applications to ensure maximum productivity. It will be further enforced by two new applications; Automatic Save After set uP (ASAP) and Fast Beam Investigation (FBI). ASAP saves crucial time for the engineers in charge (EIC) during the physics campaign. It automatizes and standardizes a repetitive process. For each new set up, the EIC is required to document the settings of all important elements before delivering beam to the users. FBI will be serving two different needs. First, it will be used as a beam traceability tool. The settings of every element of ISOLDE that could obstruct, stop or affect the beam will be tracked by the application. This will allow to understand better the presence of radioactive contaminants after each experiment at critical points in the facility. The second functionality will allow real time monitoring of the machine status during a physics run. FBI will be the most efficient way to visualize the status of the machine and find the reason that prevents the beam from arriving to the experimental station. Finally, an application has been developed to automatize with flexibility a sequence of pre-defined assignments, such as performing a measurement and setting a value to a device

Nonadiabatic electron gun at an electron beam ion source: Commissioning results and charge breeding investigations

The electron gun of the REXEBIS charge breeder at the REX/HIE-ISOLDE facility at CERN has been upgraded from a standard magneto-immersed type to a gun using a nonadiabatic magnetic element. The results from the cathode emission and electron beam propagation tests are presented, as well as the charge breeding efficiency for the new design. Complete mass-scans of the extracted beam have been performed from which the level of cathode-originating contaminations could be established, as well as partial pressures of the most abundant residual gases in the ion trapping region. Furthermore, optimal breeding times for a broad range of elements and charge states, either introduced as a gas or externally injected as singly charged ions into the trapping region, are given for different electron currents. From these values, effective electron current densities have been derived. Finally, the axial ion energy distributions of various elements and charge states were also measured, and the derived ion temperatures were correlated with the ion and electron beam overlap factors. This paper was published in PHYSICAL REVIEW ACCELERATORS AND BEAMS 25, 013402 (2022), DOI: 10.1103/PhysRevAccelBeams.25.01340