Development status of a next generation ECRIS: MARS-D at LBNL.

To demonstrate a Mixed Axial and Radial field System (MARS) as the best magnet scheme for future ECRISs, MARS-D, a demonstrative ECRIS using a NbTi MARS magnet is progressing at Lawrence Berkeley National Laboratory. An optimized MARS design can use either NbTi or Nb3Sn coils with reduced engineering complexities to construct the needed high-field magnets. The optimized magnet design could enhance MARS-D to a next generation ECRIS by producing minimum-B field maxima of 5.6 T axially and 3.2 T radially for operating frequencies up to 45 GHz. In-progress test winding has achieved a milestone demonstrating the fabrication feasibility of a MARS closed-loop coil.

Investigation on the electron flux to the wall in the VENUS ion source.

The long-term operation of high charge state electron cyclotron resonance ion sources fed with high microwave power has caused damage to the plasma chamber wall in several laboratories. Porosity, or a small hole, can be progressively created in the chamber wall which can destroy the plasma chamber over a few year time scale. A burnout of the VENUS plasma chamber is investigated in which the hole formation in relation to the local hot electron power density is studied. First, the results of a simple model assuming that hot electrons are fully magnetized and strictly following magnetic field lines are presented. The model qualitatively reproduces the experimental traces left by the plasma on the wall. However, it is too crude to reproduce the localized electron power density for creating a hole in the chamber wall. Second, the results of a Monte Carlo simulation, following a population of scattering hot electrons, indicate a localized high power deposited to the chamber wall consistent with the hole formation process. Finally, a hypervapotron cooling scheme is proposed to mitigate the hole formation in electron cyclotron resonance plasma chamber wall.

Development of a new superconducting Electron Cyclotron Resonance Ion Source for operations up to 18 GHz at LBNL.

A new superconducting Electron Cyclotron Resonance Ion Source (ECRIS) is under development at LBNL to harness the winding techniques of a closed-loop sextupole coil for the next generation ECRIS and to enhance the capability of the 88-in. cyclotron facility. The proposed ECRIS will use a superconducting closed-loop sextupole coil to produce the radial field and a substantial portion of the axial field. The field strengths of the injection, central and extraction regions are adjusted by a three solenoids outside the closed-loop sextupole coil. In addition to maintaining the typical ECRIS magnetic field configuration, this new source will also be able to produce a dustpan-like minimum-B field to explore possible ECRIS performance enhancement. The dustpan-like minimum-B field configuration has about the same strengths for the maximum axial field at the injection region and the maximum radial pole fields at the plasma chamber walls but it can be substantially lower at the extraction region. The dustpan-like minimum-B will have a field maximum Bmax ≥ 2.6 T for operations up to 18 GHz with a ratio of Bmax/Bres ≥ 4 and higher ratios for lower frequencies. The field maxima of this new source can reach over 3 T both at the injection and the plasma chamber walls which could also support operation at 28 GHz. The source will be built of cryogen-free with the magnets directly cooled by cryo-coolers to simplify the cryostat structure. The source design features will be presented and discussed.

Production of high intensity 48Ca for the 88-Inch Cyclotron and other updates.

Recently the Versatile ECR for NUclear Science (VENUS) ion source was engaged in a 60-day long campaign to deliver high intensity (48)Ca(11+) beam to the 88-Inch Cyclotron. As the first long term use of VENUS for multi-week heavy-element research, new methods were developed to maximize oven to target efficiency. First, the tuning parameters of VENUS for injection into the cyclotron proved to be very different than those used to tune VENUS for maximum beam output of the desired charge state immediately following its bending magnet. Second, helium with no oxygen support gas was used to maximize the efficiency. The performance of VENUS and its low temperature oven used to produce the stable requested 75 eµA of (48)Ca(11+) beam current was impressive. The consumption of (48)Ca in VENUS using the low temperature oven was checked roughly weekly, and was found to be on average 0.27 mg/h with an ionization efficiency into the 11+ charge state of 5.0%. No degradation in performance was noted over time. In addition, with the successful operation of VENUS the 88-Inch cyclotron was able to extract a record 2 pµA of (48)Ca(11+), with a VENUS output beam current of 219 eµA. The paper describes the characteristics of the VENUS tune used for maximum transport efficiency into the cyclotron as well as ongoing efforts to improve the transport efficiency from VENUS into the cyclotron. In addition, we briefly present details regarding the recent successful repair of the cryostat vacuum system.

Recent progress on the superconducting ion source VENUS.

The 28 GHz Ion Source VENUS (versatile ECR for nuclear science) is back in operation after the superconducting sextupole leads were repaired and a fourth cryocooler was added. VENUS serves as an R&D device to explore the limits of electron cyclotron resonance source performance at 28 GHz with its 10 kW gryotron and optimum magnetic fields and as an ion source to increase the capabilities of the 88-Inch Cyclotron both for nuclear physics research and applications. The development and testing of ovens and sputtering techniques cover a wide range of applications. Recent experiments on bismuth demonstrated stable operation at 300 eµA of Bi(31+), which is in the intensity range of interest for high performance heavy-ion drivers such as FRIB (Facility for Rare Isotope Beams). In addition, the space radiation effects testing program at the cyclotron relies on the production of a cocktail beam with many species produced simultaneously in the ion source and this can be done with a combination of gases, sputter probes, and an oven. These capabilities are being developed with VENUS by adding a low temperature oven, sputter probes, as well as studying the RF coupling into the source.

Progress towards the development of a realistic electron cyclotron resonance ion source extraction model.

In this paper, an ongoing effort to provide a simulation and design tool for electron cyclotron resonance ion source extraction and low energy beam transport is described and benchmarked against experimental results. Utilizing the particle-in-cell code WARP, a set of scripts has been developed: A semiempirical method of generating initial conditions, a 2D-3D hybrid method of plasma extraction and a simple beam transport deck. Measured emittances and beam profiles of uranium and helium beams are shown and the influence of the sextupole part of the plasma confinement field is investigated. The results are compared to simulations carried out using the methods described above. The results show that the simulation model (with some additional refinements) represents highly charged, well-confined ions well, but that the model is less applicable for less confined, singly charged ions.

Nb3Sn superconducting magnets for electron cyclotron resonance ion sources.

Electron cyclotron resonance (ECR) ion sources are an essential component of heavy-ion accelerators. Over the past few decades advances in magnet technology and an improved understanding of the ECR ion source plasma physics have led to remarkable performance improvements of ECR ion sources. Currently third generation high field superconducting ECR ion sources operating at frequencies around 28 GHz are the state of the art ion injectors and several devices are either under commissioning or under design around the world. At the same time, the demand for increased intensities of highly charged heavy ions continues to grow, which makes the development of even higher performance ECR ion sources a necessity. To extend ECR ion sources to frequencies well above 28 GHz, new magnet technology will be needed in order to operate at higher field and force levels. The superconducting magnet program at LBNL has been developing high field superconducting magnets for particle accelerators based on Nb(3)Sn superconducting technology for several years. At the moment, Nb(3)Sn is the only practical conductor capable of operating at the 15 T field level in the relevant configurations. Recent design studies have been focused on the possibility of using Nb(3)Sn in the next generation of ECR ion sources. In the past, LBNL has worked on the VENUS ECR, a 28 GHz source with solenoids and a sextupole made with NbTi operating at fields of 6-7 T. VENUS has now been operating since 2004. We present in this paper the design of a Nb(3)Sn ECR ion source optimized to operate at an rf frequency of 56 GHz with conductor peak fields of 13-15 T. Because of the brittleness and strain sensitivity of Nb(3)Sn, particular care is required in the design of the magnet support structure, which must be capable of providing support to the coils without overstressing the conductor. In this paper, we present the main features of the support structure, featuring an external aluminum shell pretensioned with water-pressurized bladders, and we analyze the expected coil stresses with a two-dimensional finite element mechanical model.

Development of a pepper-pot device to determine the emittance of an ion beam generated by electron cyclotron resonance ion sources.

This paper describes the recent development and commissioning of a pepper-pot emittance meter at the Lawrence Berkeley National Laboratory (LBNL). It is based on a potassium bromide (KBr) scintillator screen in combination with a charged coupled device camera. Pepper-pot scanners record the full four-dimensional transverse phase space emittances which are particularly interesting for electron cyclotron resonance ion sources. The strengths and limitations of evaluating emittances using optical pepper-pot scanners are described and systematic errors induced by the optical data acquisition system will be presented. Light yield tests of KBr exposed to different ion species and first emittance measurement data using ion beams extracted from the 6.4 GHz LBNL electron cyclotron resonance ion source are presented and discussed.

Measurement of the high energy component of the x-ray spectra in the VENUS electron cyclotron resonance ion source.

High performance electron cyclotron resonance (ECR) ion sources, such as VENUS (Versatile ECR for NUclear Science), produce large amounts of x-rays. By studying their energy spectra, conclusions can be drawn about the electron heating process and the electron confinement. In addition, the bremsstrahlung from the plasma chamber is partly absorbed by the cold mass of the superconducting magnet, adding an extra heat load to the cryostat. Germanium or NaI detectors are generally used for x-ray measurements. Due to the high x-ray flux from the source, the experimental setup to measure bremsstrahlung spectra from ECR ion sources is somewhat different from that for the traditional nuclear physics measurements these detectors are generally used for. In particular, the collimation and background shielding can be problematic. In this paper, we will discuss the experimental setup for such a measurement, the energy calibration and background reduction, the shielding of the detector, and collimation of the x-ray flux. We will present x-ray energy spectra and cryostat heating rates depending on various ion source parameters, such as confinement fields, minimum B-field, rf power, and heating frequency.

High intensity production of high and medium charge state uranium and other heavy ion beams with VENUS.

The next generation, superconducting electron cyclotron resonance (ECR) ion source VENUS (versatile ECR ion source for nuclear science) started operation with 28 GHz microwave heating in 2004. Since then it has produced world record ion beam intensities. For example, 2850 e microA of O(6+), 200 e microA of U(33+) or U(34+), and in respect to high charge state ions, 1 e microA of Ar(18+), 270 e microA of Ar(16+), 28 e microA of Xe(35+), and 4.9 e microA of U(47+) have been produced. A brief overview of the latest developments leading to these record intensities is given and the production of high intensity uranium beams is discussed in more detail.