RECRIS-Romanian ECR ion source: Performances and experimental developments

The major infrastructures of nuclear physics in Europe adopted the technology of electron cyclotron resonance (ECR) ion sources for the production of heavy-ion beams. Most of them use 14GHz electron cyclotron resonance ion sources (ECRISs), except at INFN-LNS, where an 18GHz superconducting ECRIS is in operation. In the past five years it was demonstrated, in the frame of the EU-FP5 RTD project called “Innovative ECRIS,” that further enhancement of the performances requires a higher frequency (28GHz and above) and a higher magnetic field (above 2.2T) for the hexapolar field. Within the EU-FP6 a joint research activity named ISIBHI has been established to build by 2008 two different ion sources, the A-PHOENIX source at LPSC Grenoble, reported in another contribution, and the multipurpose superconducting ECRIS (MS-ECRIS), based on fully superconducting magnets, able to operate in High B mode at a frequency of 28GHz or higher. Such a development represents a significant step compared to existing devices, and an increase of typically a factor of 10 for the intensity is expected (e.g., 1emA for medium charge states of heavy ions, or hundreds of eμA of fully stripped light ions, or even 1eμA of charge states above 50+ for the heaviest species). The challenging issue is the very high level of magnetic field, never achieved by a minimum B trap magnet system; the maximum magnetic field of MS-ECRIS will be higher than 4 or 5T for the axial field and close to 2.7T for the hexapolar field. The detailed description of the MS-ECRIS project and of its major constraints will be given along with the general issues of the developments under way.

Recent development of electron cyclotron resonance (ECR) ion sources in China is reviewed. Emphasis is put on high charge state ECR ion sources which have been mainly developed in China by Institute of Modern Physics (IMP). Presently two ECR ion sources built by IMP for highly charged ion beams are put into operation for cyclotrons and atomic physics research. The development of high charge state ECR ion sources at IMP has progressed with a new magnetic field configuration, better condition for extraction of highly charged ions, high mirror magnetic field, large plasma volume, and special techniques to provide extra cold electrons. These techniques greatly enhance the production of highly charged ions from IMP ECR ion sources. So far more than 185 eμA of Ar11+ and 50 eμA of Xe26+ were produced by the IMP ECR ion sources. The metallic ion beam production was tested and the first beam Ca11+40 was provided to the cyclotrons at IMP. The beam intensity of Ca11+40 could reach 130 eμA. The next part of this article will report the latest progress of 2.45 GHz ECR ion sources in China. A 2.45 GHz compact permanent magnet proton ion source was designed and constructed by IMP. A new microwave feeding system is applied on this ion source. The ion source is able to deliver 90 mA of mixed ion beam (H1++H2++H3+) after preliminary commissioning. The article also mentions a small 2.45 GHz ECR ion source which was built by Sichuan University and used for industry applications. The dependence of plasma density and electron temperature on radio frequency power, neutral gas pressure, and different microwave windows was measured by a Langmuir probe on this ion source.

In order to enhance the output of highly charged ions from an electron cyclotron resonance ion source (ECRIS), several techniques like wall coating, biased disk, electron gun have been proposed and are meanwhile employed as standard tools at most of the existing installations. Although the detailed mechanisms are not clear, it has become evident that the additional injection of electrons into the plasma chamber of an ECRIS considerably improves its performances. Depending on the special conditions of the source these additional electrons can either compensate for losses of plasma electrons or even may change global plasma parameters (e.g. plasma potential) and hence positively influence the extraction at high rates of highly charged ions. The first experiments on biased disk were carried out at CEA, Grenoble and KVI, Groningen in 1990-91 and since then, this method is used by almost any ECRIS in the world. Wall coating is not so widely used but it has been proved to be effective not only for ECRIS but also for microwave discharge ion source (MDIS). Other methods to provide electrons to the plasma will be described in the following. In particular, at the Institute of Physics and Nuclear Engineering (IFIN-HH), Bucharest, special metal-dielectric (MD) structures of Al-Al2O3 have been developed. They are characterized by their very high secondary electron emission coefficients and therefore they seem to be ideal to be used in an ECRIS. Preliminary tests at IK in Frankfurt and at KVI have given interesting results. In the following an analysis of the possibilities opened by this method and other methods involving electron donors in ECRIS and MDIS field is carried out.

VENUS is a third generation electron cyclotron resonance (ECR) ion source, which incorporates a high field superconducting NbTi magnet structure, a 28 GHz gryotron microwave source and a state of the art closed cycle cryosystem. During the decade from initial concept to regular operation, it has demonstrated both the feasibility and the performance levels of this new generation of ECR ion sources and required innovation on magnet construction, plasma chamber design, and beam transport. In this paper, the development, performance, and major innovations are described as well as a look to the potential to construct a fourth generation ECR ion source.

An intense accelerator-based D-D/D-T neutron generator, with neutron yields of 6 × 1010 n/s due to the D-D reaction and 6 × 1012 n/s due to the D-T reaction, is being developed at Lanzhou University in China. The 2.45 GHz electron cyclotron resonance (ECR) ion source and the pre-analysis system, as the front injector for the accelerator tube of the neutron generator, are designed to provide a 35 mA deuterium beam for neutron production and a 50 mA proton beam for commissioning. The pre-analysis system consists of a solenoid, a steering magnet, a vacuum chamber, a dipole magnet, three quadrupole magnets, a beam dump, and a Faraday cup. The ion source and pre-analysis system were completed a trial operation, and the intensity of the proton beam measured at the end of the pre-analysis system was higher than 50 mA when the hydrogen ion beam current extracted from the ECR ion source was greater than 70 mA.

Electron cyclotron resonance (ECR) ion sources were first proposed by R. Geller in 1965 and H. Postma using their experience with mirror machines for fusion plasma studies. ECR ion sources for the generation of multiply charged ions need the highest possible plasma confinement to reach a high value of the product of electron density and confinement time, net, necessary for step-by-step ionization of the ions. This ECR ion source, called MAFIOS, delivered just moderate charge states as from a penning ionization gauge ion source because ion lifetime in the plasma was short. In 1975 Geller reported on a new ECR ion source design using a two-stage arrangement called SuperMAFIOS. In the presence of a sufficiently high magnetic field, electron cyclotron resonance heating of the electrons will occur. The higher magnetic field improves the confinement of the ions and in this way improves the charge state distribution.

Superconducting electron cyclotron resonance ion source with advance design in Lanzhou (SECRAL) is an 18-28 GHz fully superconducting electron cyclotron resonance (ECR) ion source dedicated for highly charged heavy ion beam production. SECRAL, with an innovative superconducting magnet structure of solenoid-inside-sextupole and at lower frequency and lower rf power operation, may open a new way for developing compact and reliable high performance superconducting ECR ion source. One of the recent highlights achieved at SECRAL is that some new record beam currents for very high charge states were produced by 18 GHz or 18+14.5 GHz double frequency heating, such as 1 e microA of (129)Xe(43+), 22 e microA of (209)Bi(41+), and 1.5 e microA of (209)Bi(50+). To further enhance the performance of SECRAL, a 24 GHz/7 kW gyrotron microwave generator was installed and SECRAL was tested at 24 GHz. Some promising and exciting results at 24 GHz with new record highly charged ion beam intensities were produced, such as 455 e microA of (129)Xe(27+) and 152 e microA of (129)Xe(30+), although the commissioning time was limited within 3-4 weeks and rf power only 3-4 kW. Bremsstrahlung measurements at 24 GHz show that x-ray is much stronger with higher rf frequency, higher rf power. and higher minimum mirror magnetic field (minimum B). Preliminary emittance measurements indicate that SECRAL emittance at 24 GHz is slightly higher that at 18 GHz. SECRAL has been put into routine operation at 18 GHz for heavy ion research facility in Lanzhou (HIRFL) accelerator complex since May 2007. The total operation beam time from SECRAL for HIRFL accelerator has been more than 2000 h, and (129)Xe(27+), (78)Kr(19+), (209)Bi(31+), and (58)Ni(19+) beams were delivered. All of these new developments, the latest results, and long-term operation for the accelerator have again demonstrated that SECRAL is one of the best in the performance of ECR ion source for highly charged heavy ion beam production. Finally the future development of SECRAL will be presented.

To reveal the argon plasma characteristics within the entire region of an electron cyclotron resonance (ECR) ion source, the plasma parameters were diagnosed using a bended Langmuir probe with the filament axis perpendicular to the diagnosing plane. Experiments indicate that, with a gas volume flow rate and incident microwave power of 4 sccm and 8.8 W, respectively, the gas was ionized to form plasma with a luminous ring. When the incident microwave power was above 27 W, the luminous ring was converted to a bright column, the dark area near its axis was narrowed, and the microwave power absorbing efficiency was increased. This indicates that there was a mode transition phenomenon in this ECR ion source when the microwave power increased. The diagnosis shows that, at an incident microwave power of 17.4 W, the diagnosed electron temperature and ion density were below 8 eV and 3 × 1017 m−3, respectively, while at incident microwave power levels of 30 W and 40 W, the maximum electron temperature and ion density were above 11 eV and 6.8 × 1017 m−3, respectively. Confined by magnetic mirrors, the higher density plasma region had a bow shape, which coincided with the magnetic field lines but deviated from the ECR layer.

The electron densities in the argon plasmas of the ORNL 6 GHz electron cyclotron resonance (ECR) ion source with a flat central magnetic field have been deduced from the ion branches of the electric probe current-voltage curves measured in the edge region of the plasmas. To overcome the difficulties due to unknown velocities of multiply charged ions at the sheath edge, a modified generalized Bohm criterion for the ion sheath velocity is introduced and the mean velocity of all ionic charge states at the sheath edge is assumed to be equal to the sound velocity of the system of particles. The calculated electron densities and temperatures for different plasmas optimized for four charge state distributions are discussed.

A 28 GHz electron cyclotron resonance (ECR) ion source is being developed for use as an injector for the superconducting linear accelerator of the Rare Isotope Science Project. Beam extraction from the ECR ion source has been simulated using the KOBRA3-INP software. The simulation software can calculate charged particle trajectories in three dimensional complex magnetic field structures, which in this case are formed by the arrangement of five superconducting magnets. In this study, the beam emittance is simulated to understand the effects of plasma potential, mass-to-charge ratio, and spatial distribution. The results of these simulations and their comparison to experimental results are presented in this paper.

Advances in metal ion sources

A metal vapor vacuum arc (Mevva) ion source was finally installed and operated inside the electron cyclotron resonance (ECR) ion source Alice; Mevva cathode was made of Al, while ECR buffer gas was nitrogen, and among residuals, argon. Installation, hardware control, and improvements of diagnostic are discussed. Magnetic analysis of the ECR emitted current shows the presence of an Al3+ peak of about 5–10 μA following Mevva pulse end after 15 μs, against a steady state current of 10 nA. Peak width is about 5 μs, which is the limit of our current amplifier. Peak remains when ECR is turned off. Application of a bias potential of ±46 V to the Mevva anode shows fairly small effects on time of flight, which may indicate a large kinetic energy of Mevva ions (280 eV) at the end of Mevva shot. Bias voltage and ECR operation proved compatible only at low power, in present installation; then, some or most of Mevva ions are not trapped inside the ECR.

The versatility of ECR (Electron Cyclotron Resonance) ion sources makes them the injector of choice for many heavy ion accelerators. However, the design of the LEBT (Low Energy Beam Transport) systems for these devices is challenging, because it has to be matched for a wide variety of ions. In addition, due to the magnetic confinement fields, the ion density distribution across the extraction aperture is inhomogeneous and charge state dependent. In addition, the ion beam is extracted from a region of high axial magnetic field, which adds a rotational component to the beam. In this paper the development of a simulation model (in particular the initial conditions at the extraction aperture) for ECR ion source beams is described. Extraction from the plasma and transport through the beam line are then simulated with the particle-in-cell code WARP. Simulations of the multispecies beam containing Uranium ions of charge state 18+ to 42+ and oxygen ions extracted from the VENUS ECR ion source are presented and compared to experimentally obtained emittance values.

A tandem-type electron cyclotron resonance (ECR) ion source (ECRIS) has been constructed for synthesizing, extracting, and analyzing ions. The feasibility and realization of the device which has rangy operation window in a single device are investigated to produce many kinds of ion beams like universal source based on ECRIS. The ECR plasmas are considered to be necessary to be available to coexist and to be operated individually with different plasma parameters. Both of analysis of ion beams and investigation of plasma parameters will be conducted on produced plasmas. In this article the experimental study concentrates on improvement of producing and extracting multicharged ion beams from the second stage of the tandem-type ECRIS under lower magnetic mirror field with octupole magnets. The assembly of the extractor is modified and their positions, gap, and potentials are investigated against each ion spices. We succeeded in producing, extracting multicharged ion currents and improving ion extraction, while the magnetic field is about 60% of the previous magnetic field strength. We will present obtained evidences experimentally in detail.

A linear accelerator, called RAON, is being developed as a part of the Rare Isotope Science Project (RISP) at the Institute for Basic Science (IBS). The linear accelerator utilizes an electron cyclotron resonance (ECR) ion source for providing intense highly-charged ion beams to the linear accelerator. The 28-GHz ECR ion source can extract heavy-ion beams from protons to uranium. The superconducting magnet system for the 28-GHz ECR ion source is composed of hexapole coils and four solenoid coils made with low-Tc superconducting wires of NbTi. An electromagnetic force acts on the superconducting magnets due to the magnetic field and flowing current in the case of not only the normal state but also the quench state. In the case of quench on hexapole coils, an unbalanced flowing current among the hexapole coils is generated and causes an unbalanced electromagnetic force. Coil motions and coil strains in the quench state are larger than those in the normal state due to the unbalanced electromagnetic force among hexapole coils. Therefore, an analysis of the electromagnetic characteristics of the superconducting magnet for the 28-GHz ECR ion source on series resistance of the protection circuit in the case of quench should be conducted. In this paper, an analysis of electromagnetic characteristics of Superconducting hexapole coils for the 28-GHz ECR ion source according to the series resistance of the protection circuit in the case of quench performed by using finite-elements-method (FEM) simulations is reported.