Electron Cyclotron Resonance Ion Source Plasma Research Articles

Design and preliminary tests of an Active Plasma Chamber for ECR Ion Sources

An innovative plasma chamber for Electron Cyclotron Resonance Ion Sources (ECRIS) has been developed at INFN and will soon be installed and tested with the AISHa (Advanced Ion Source for Hadrontherapy) ion source. It consists in inserting a particular liner into the existing chamber, which allows an electrical segmentation of the internal walls of the chamber. The purpose of this system is to reduce the ion losses induced by the anisotropic diffusion mechanism, to improve the plasma confinement and thus to increase the overall performance of the ion source. In fact, in ECRIS plasmas, electrons mostly diffuse along magnetic field lines while ions mostly leak across the same lines. In particular, the inner walls of the plasma chamber are covered with 30 tiles, each one polarized to a proper positive voltage. The tiles are made of Al-6082 and anodized except for the surface directly facing the plasma. The anodizing process makes each tile electrically insulated from the others and from the plasma chamber while preserving the correct operation of the cooling system. The tiles are wrapped by 2 half-cylinders made of Al-6082 acting as shells. Some tiles are equipped of a temperature sensor and machined to allow the wiring of the entire system.In this work the results of the preliminary tests of the thermal and electrical behaviour of the active chamber and the future perspectives are presented.

Effects of magnetic configuration on hot electrons in a minimum-B ECR plasma

To investigate the hot electron population and the appearance of kinetic instabilities in highly charged electron cyclotron resonance ion source (ECRIS), the axially emitted bremsstrahlung spectra and microwave bursts emitted from ECRIS plasma were synchronously measured on SECRAL-II (Superconducting ECR ion source with Advanced design in Lanzhou No. II) ion source with various magnetic field configurations. The experimental results show that when the ratio of the minimum field to the resonance field (i.e. Bmin/Becr) is less than ∼0.8, the bremsstrahlung spectral temperature Ts increases linearly with the Bmin/Becr–ratio when the injection, extraction and radial mirror fields are kept constant. Above this threshold Ts saturates and the electron cyclotron instability appears simultaneously. This study has also demonstrated that Ts decreases linearly with the increase of the average gradient over the ECR surface when the on-axis gradient and hexapole field strengths are constant. In addition, it is found that Ts decreases with the increase of the gradient at the resonance zone (∇Becr on axis and <∇Becr>) at relatively low mirror ratio and is insensitive to the gradient at high mirror ratio when Bmin is constant. Compared to a recent study taken on a fully superconducting ECRIS (Benitez et al 2017 IEEE Trans. Plasma Sci. 45 1746–54), in which it was concluded that Ts is only sensitive with Bmin, this article shows different results discussing the mechanisms behind the correlation of magnetic field parameters to Ts.

ECRIS plasma spectroscopy with a high resolution spectrometer.

Electron Cyclotron Resonance Ion Source (ECRIS) plasmas contain high-energy electrons and highly charged ions implying that only noninvasive methods such as optical emission spectroscopy are reliable in their characterization. A high-resolution spectrometer (10 pm FWHM at 632 nm) enabling the detection of weak emission lines has been developed at University of Jyväskylä, Department of Physics (JYFL) for this purpose. Diagnostics results probing the densities of ions, neutral atoms, and the temperature of the cold electron population in the JYFL 14 GHz ECRIS are described. For example, it has been observed that the cold electron temperature drops from 40 eV to 20 eV when the extraction voltage of the ion source is switched off, accompanied by two orders of magnitude decrease in Ar9+ optical emission intensity, suggesting that diagnostics results of ECRIS plasmas obtained without the extraction voltage are not depicting the plasma conditions of normal ECRIS operation. The relative changes of the plasma optical emission and the ion beam current have been measured in CW and amplitude modulation operation mode of microwave injection. It is concluded that in the CW mode, the ion currents could be limited by diffusion transport and electrostatic confinement of the ions rather than beam formation in the extraction region and subsequent transport. The high resolution of the spectrometer allows determining the ion temperature by measuring the Doppler broadening of the emission lines and subtracting the wavelength dependent instrumental broadening. The measured ion temperatures in the JYFL 14 GHz ECRIS are between 5 and 28 eV, depending on the plasma species and charge state. Gas mixing is shown to be an effective method to decrease the ion temperature of high charge state argon ions from 20 eV in pure argon discharge to 5 eV when mixed with oxygen.

Time resolved X-ray emission diagnostics in an axis-symmetric simple mirror trap

Time resolved characterization of plasma X-ray emission represents an innovative technique to characterize non-equilibrium phases in Electron Cyclotron Resonance Ion Sources (ECRIS). Indeed, although ECRIS plasmas are usually in dynamical equilibrium, many relevant phenomena are characterized by fast transients and require a dedicated diagnostics to be fully investigated. In particular, time resolved diagnostics can be applied to characterize the X ray emission during the plasma turn on and off and during the turbulence phenomena induced by Cyclotron-Maser instability. This paper presents the experimental set-up used during the last campaign at measurement carried out at the Istituto Nazionale Fisica Nucleare -Laboratori Nazionali del Sud (INFN-LNS), together with the main achieved results. Particular relevance will be given to the characterization of the afterglow phase, which highlighted the existence of two electron populations characterized by different mean lifetime. Evidences of X-ray bursts emission in the afterglow phase, compatible with Cyclotron Maser instability, will be also shown.

Modelling RF-plasma interaction in ECR ion sources

This paper describes three-dimensional self-consistent numerical simulations of wave propagation in magnetoplasmas of Electron cyclotron resonance ion sources (ECRIS). Numerical results can give useful information on the distribution of the absorbed RF power and/or efficiency of RF heating, especially in the case of alternative schemes such as mode-conversion based heating scenarios. Ray-tracing approximation is allowed only for small wavelength compared to the system scale lengths: as a consequence, full-wave solutions of Maxwell-Vlasov equation must be taken into account in compact and strongly inhomogeneous ECRIS plasmas. This contribution presents a multi-scale temporal domains approach for simultaneously including RF dynamics and plasma kinetics in a “cold-plasma”, and some perspectives for “hot-plasma” implementation. The presented results rely with the attempt to establish a modal-conversion scenario of OXB-type in double frequency heating inside an ECRIS testbench.

Numerical model of electron cyclotron resonance ion source

Important features of Electron Cyclotron Resonance Ion Source (ECRIS) operation are accurately reproduced with a numerical code. The code uses the particle-in-cell technique to model a dynamics of ions in ECRIS plasma. It is shown that gas dynamical ion confinement mechanism is sufficient to provide the ion production rates in ECRIS close to the experimentally observed values. Extracted ion currents are calculated and compared to the experiment for few sources. Changes in the extracted ion currents are obtained with varying the gas flow into the source chamber and the microwave power. Empirical scaling laws for ECRIS design are studied and the underlying physical effects are discussed.

Accessibility condition of wave propagation and multicharged ion production in electron cyclotron resonance ion source plasma.

A new tandem type source of electron cyclotron resonance (ECR) plasmas has been constructing for producing synthesized ion beams in Osaka University. Magnetic mirror field configuration with octupole magnets can be controlled to various shape of ECR zones, namely, in the 2nd stage plasma to be available by a pair mirror and a supplemental coil. Noteworthy correlations between these magnetic configurations and production of multicharged ions are investigated in detail, as well as their optimum conditions. We have been considering accessibility condition of electromagnetic and electrostatic waves propagating in ECR ion source plasma, and then investigated their correspondence relationships with production of multicharged ions. It has been clarified that there exits efficient configuration of ECR zones for producing multicharged ion beams experimentally, and then has been suggested from detail accessibility conditions on the ECR plasma that new resonance, i.e., upper hybrid resonance, must have occurred.

Suppression of cyclotron instability in Electron Cyclotron Resonance ion sources by two-frequency heating

Multiple frequency heating is one of the most effective techniques to improve the performance of Electron Cyclotron Resonance (ECR) ion sources. The method increases the beam current and average charge state of the extracted ions and enhances the temporal stability of the ion beams. It is demonstrated in this paper that the stabilizing effect of two-frequency heating is connected with the suppression of electron cyclotron instability. Experimental data show that the interaction between the secondary microwave radiation and the hot electron component of ECR ion source plasmas plays a crucial role in mitigation of the instabilities.

EXPERIMENTAL CHARACTERIZATION OF AN OVERDENSE PLASMA IN A COMPACT ION SOURCE

&lt;p&gt;Electron Cyclotron Resonance Ion Sources (ECRIS) are compact plasma-based machines able to feed particle accelerators with high intensity beams of multi-charged ions. ECRIS plasmas are density-limited, since they are sustained by E.M. wave propagation up to a cut-off density value. In the past, the only way to improve ECRIS performance was to increase the microwave frequency and the magnetic field strength to satisfy the ECR condition. A different plasma heating mechanism is being applied at INFN-LNS. It is based on Electron Bernstein Waves (EBW), i.e., electrostatic waves which do not suffer any density cut-off. Highlights concerning preliminary signatures of EBW formation and subsequent absorption are given here.&lt;/p&gt;

Visible Light Emission of Electron Cyclotron Resonance Plasmas

Electron cyclotron resonance (ECR) plasmas generated by microwave and confined by magnetic field are, in most cases, generated for highly charged ion production followed by injection of them into an accelerator. The ECR plasma itself usually cannot be seen because it is deeply hidden in the center of the facility. In the Institute of Nuclear Research (ATOMKI), the ECR ion source (ECRIS) operates as a stand-alone device, and this independence opens a possibility for us to look in the plasma chamber during an almost normal operation. High-resolution visible light photographs of the ATOMKI ECRIS plasma were taken and presented here.

Ion beam properties for ECR ion source injector systems

Electron Cyclotron Resonance (ECR) ion sources are essential components of heavy-ion accelerators due to their ability to produce the wide range of ions required by these facilities. The ever-increasing intensity demands have led to remarkable performance improvements of ECR injector systems mainly due to advances in magnet technology as well as an improved understanding of the ECR ion source plasma physics. At the same time, enhanced diagnostics and simulation capabilities have improved the understanding of the injector beam transport properties. However, the initial ion beam distribution at the extraction aperture is still a subject of research. Due to the magnetic confinement necessary to sustain the ECR plasma, 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, which leads to emittance growth. This paper will focus on the beam properties of ions extracted from ECR ion sources and diagnostics efforts at LBNL to develop a consistent modeling tool for the design of an optimized beam transport system for ECR ion sources.

Numerical simulations of the ion capture process for laser-generated plasmas interacting with electron cyclotron resonance ion sources

The most effective devices able to feed modern accelerators with heavy ions (either in terms of reliability or performance) are electron cyclotron resonance ion sources (ECRIS), which are able to generate intense currents of multiply charged ions. One of the disadvantages of the ECRIS consists of the limitation in the production of metallic ions (usually obtained by means of ovens and sputtering techniques), especially in the case of refractory elements. Laser ablation is an effective method for such elements. The coupling between a laser ion source (LIS) and an ECRIS has been studied at some facilities (INFN-LNS, RIKEN, University of Frankfurt, ANL): the experiments were successful but an adequate modeling and additional experimental activities are mandatory. Preliminary calculations have shown that the ion energy from the LIS should be lower than a few hundreds of eV in order to permit an effective coupling of LIS ions with the ECRIS plasma. A detailed investigation of the ion capture by the ECRIS plasma has now been carried out by means of numerical simulations based on the Monte Carlo method. The energy distribution function of the LIS plasma has been taken into account, as well as the density and temperature distribution of the ECRIS plasmas and the pattern of the magnetic field in the resonance region. The captured fraction of LIS ions strongly depends on the temperature (the lower energy component of the ion energy distribution function is easily absorbed by the ECRIS plasma). The effect of negatively biased targets has been also investigated: simulations demonstrate that for negative potentials of the order of 0.4 kV the captured fraction abruptly increases, and in some particular cases it exceeds 50% of the total ion current.

Simulation of charge breeding of rubidium using Monte Carlo charge breeding code and generalized ECRIS model

A Monte Carlo charge breeding code (MCBC) is being developed by FAR-TECH, Inc. to model the capture and charge breeding of 1+ ion beam in an electron cyclotron resonance ion source (ECRIS) device. The ECRIS plasma is simulated using the generalized ECRIS model which has two choices of boundary settings, free boundary condition and Bohm condition. The charge state distribution of the extracted beam ions is calculated by solving the steady state ion continuity equations where the profiles of the captured ions are used as source terms. MCBC simulations of the charge breeding of Rb+ showed good agreement with recent charge breeding experiments at Argonne National Laboratory (ANL). MCBC correctly predicted the peak of highly charged ion state outputs under free boundary condition and similar charge state distribution width but a lower peak charge state under the Bohm condition. The comparisons between the simulation results and ANL experimental measurements are presented and discussed.

Nb 3 Sn 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.

Perspectives for the ECLISSE method with third generation ECRIS

The ECLISSE experiment performed at INFN-LNS in 2002 with the superconducting ECR ion source SERSE proved the possibility to couple a laser ion source (LIS) with an electron cyclotron resonance ion source (ECRIS) in order to obtain cw ion beams from refractory metals with characteristics that are not achievable with standard LIS. The oven that is used for the production of vapors inside the ECRIS fails with refractory elements and the sputtering method is not adequate for the production of beams with intensity above a few tens of micro ampere. So the use of a LIS inside the ECRIS plasma chamber may permit to obtain the advantage of a LIS in terms of beam current with the typical features of an ECRIS (high charge states, cw mode operation, relatively low emittance). One of the limiting factors of the previous experiment was the relatively low plasma density of the previous generation ECRIS, working at a frequency between 14 and 18 GHz. Nowadays the availability of third generation ECRIS (operating at 28 GHz or higher, i.e. with a plasma density four times larger) may extend the capabilities of this method, as the denser plasma may catch ions from LIS with energies up to about 1 keV per charge state, much higher than it is possible for sources operating at lower frequency (e.g. about 200 eV per charge state for the SERSE source). The perspectives opened by the availability of MS-ECRIS source operating at 28 GHz will be described along with some details on the proposed experimental programme, to be carried out in 2008 at GSI, Darmstadt.

X-ray spectra emitted by Cl 14+ ions in ECRIS plasmas

We study the contribution of the most important processes leading to the creation of excited states of Cl 14+ ions from the ground configurations of Cl ions in an Electron Cyclotron Resonance Ion Source (ECRIS), which lead to the emission of K X-ray lines. Theoretical values for inner-shell excitation, K and KL ionization cross-sections, and energies and transition probabilities for the de-excitation processes are calculated in the framework of the Multi-Configuration Dirac–Fock (MCDF) method. With reasonable assumptions about the electron energy distribution, a theoretical Kα X-ray spectrum is obtained, which reproduces closely a recent experimental result.

Ion beam capture and charge breeding in electron cyclotron resonance ion source plasmas

Beam capture of injected ions and charge breeding in electron cyclotron resonance (ECR) charge breeder ion source plasmas has been investigated utilizing an ECR plasma modeling code, the generalized ECR ion source model, and a Monte Carlo beam capture code. Beam capturing dynamics, charge breeding in the plasma, and the extracted charged ion states are described. Optimization of ion beam energy is performed for (1) high beam capture efficiency and (2) high charge state ion beam extractions. A sample case study for ANL-ECR has been performed. Ions entering ECR ion source plasma are slowed down mostly by the background ions. Assuming Maxwellian plasma ions, maximum beam energy loss occurs when the beam velocity is around the background thermal velocity in magnitude. It is also found that beam capture location affects charge state distribution. For instance, with a majority of beam ions captured near the middle of the device higher currents for higher charge states are obtained. The beam ions captured near the entry have a higher probability of backstreaming after they are captured. For this reason, the optimum beam energy of the injected Ar(+) beam ions for charge breeding is generally higher than the optimum input beam energy for maximum beam energy loss.

Investigation of the density and temperature of electrons in a compact 2.45 GHz electron cyclotron resonance ion source plasma by x-ray measurements

X-ray measurements of the plasma of a compact 2.45 GHz electron cyclotron resonance ion source (ECRIS) are performed to determine the temperature and density of the electrons heated resonantly in the ECRIS. The x-ray detector used to investigate the plasma consists of a small silicon (Si-PIN) photodiode to detect photons in the energy range of 1–100 keV. The detector has an energy resolution of 180 eV at 5.9 keV that allows us to record detailed x-ray spectra. Assuming two temperature electron populations, both Maxwellian distributed, the analysis of the x-ray spectra shows a temperature of about 2 keV for the hot electron fraction in addition to the population of cold electrons at less than 2 eV. The fraction of the hot electrons amounts to 1–10%. We present a description of the x-ray detector set-up as well as x-ray spectra and calculations for the temperature and density of the electrons in the ECRIS plasma.

Electron cyclotron resonance ion source one-dimensional fluid modeling

A one-dimensional (1D) fluid computer model for multiple ion species in an electron cyclotron resonance ion source (ECRIS) plasma has been developed. The ions species are assumed to be highly collisionally coupled and are treated using 1D fluid equations. The non-Maxwellian anisotropic electron distribution function is modeled by a 1D bounce-averaged Fokker–Planck code. ECR heating is included in the model as a quasilinear rf-diffusion term including relativistic detuning, rf pitch-angle scattering, and multiple resonance frequencies/locations. In a typical ECRIS, the electrons are very noncollisional and confined magnetically. The ions follow this electron confinement via the electrostatic potential. The 1D axial electrostatic potential profile predicted by the model shows an ion confining core electrostatic well as expected in ECRIS plasmas. Modeling results for the Argonne National Laboratory ECR-I ECRIS configuration are presented along with a discussion of the difficulties in benchmarking the model with Faraday cup measurements.

Design of a two-ion-source (2-IS) beam transport line for the production of multi charged radioactive ion beams

A “two-ion-source” beam transport line between a surface ionization source and a 6.4 GHz on-line Electron Cyclotron Resonance Ion Source (ECRIS) for the production of multi-charged radioactive ions has been designed. The 1 + ions from the surface ionization source are decelerated and focused onto the ECRIS plasma so that they can be efficiently trapped there and further ionized to charge state q>1 +. A scheme for stepwise and gradual deceleration of the 1 + ion beam consisting of a multi-electrode decelerator and a tuning electrode placed before the ECRIS has been optimized. The beam dynamics calculations show that the 1 + beam decelerated to energies of 20–50 eV could be focused to a spot size smaller than the radial dimensions of the ECR plasma zone.