ECR Plasma Research Articles

Novel modelling of metal atoms diffusion and ion transport in ECR plasma relevant to ion sources and in-plasma nuclear physics studies

Metals can be injected into electron cyclotron resonance ion sources (ECRIS) via different techniques, among which resistive ovens are used to vaporize neutral materials, later captured by the energetic plasma that will step-wise ionize them, hence giving multiply charged ion beams for accelerators. Recently, PANDORA, a novel ECR plasma trap, has been conceived to perform interdisciplinary research spanning from nuclear physics to astrophysics, where in-plasma high charge states of metallic species are demanded. However, a full knowledge on the vaporization method and on the coupling of neutral atoms with plasma and its overall dynamics is still not available. Simulations, hence, are of fundamental relevance to improve the overall efficiency, reduce consumption of rare expensive isotopes, and to improve the ion source performance. We present a numerical study about metallic species suitable for oven injection in ECRIS, focusing on metals diffusion, transport, and wall deposition under molecular flow regime. We studied the metal dynamics with and without plasma. Results underline the plasma role on a space-dependent conversion yield, reflecting the strongly inhomogeneous ECR plasma. The plasma and its parameters have been modelled using an established self-consistent particle-in-cell model. The numerical tool is conceived for the PANDORA plasma trap but can be extended to other ECR plasmas and traps. As test cases we studied the 134Cs and 48Ca radioisotopes, as metals of interest for the modern nuclear physics. A focus is given on the β-decaying 134Cs, as an application case for PANDORA, providing quantitative estimates of the γ-detection signal-poisoning effect by neutral metals deposition at the chamber wall.

For intense proton beam production with compact ion sources: the ALISES ion source family developed at CEA Saclay

The production of intense proton beams in CEA Saclay started in the 90’s with the development of the SILHI source to inject the IPHI accelerator. This ECR ion source is still in operation nowadays. ECR plasma is a well-known heating process and plasma chamber internal dimensions were investigated in order to reduce size and maintenance time. In 2012 R&D investigations on those simple plasma chamber parameters were started with the design of the first ALISES ion source that will give birth to a new source family still in progress today. This paper will summarize the different steps of this development.

3D Self-Consistent Full Wave – PIC Models for Investigating Space-Resolved ECR Plasma Properties

Simulations are a powerful method to study the correlation between output beams and internal dynamics of electron cyclotron resonance ion sources (ECRIS), which involve a complex interplay between injected power, RF frequency, gas type and pressure. We present here some details on 3D full-wave Particle-in-Cell (PIC) code suites that can simulate electron and ion dynamics self-consistently in an ECR plasma. Preliminary runs of the simulation show an encouraging match with experimental data which acts as a benchmark for the PIC codes and highlights its potential for fundamental and applied interdisciplinary plasma research.

Design of a novel ECR ion trap facility for nuclear physics and fundamental plasma processes studies

An innovative ECR ion trap facility, called PANDORA (Plasma for Astrophysics, Nuclear Decay Observation and Radiation for Archaeometry), was designed for fundamental plasma processes and nuclear physics investigations. The overall structure consists of three subsystems: a) a large (70 cm in length, 28 cm in inner diameter) ECR plasma trap with a fully superconducting B-minimum magnetic system (Bmax = 3.0 T) and an innovative design to host detectors and diagnostic tools; b) an advanced non-invasive plasma multidiagnostics system to locally characterize the plasma thermodynamic properties; c) an array of 14 HPGe detectors. The PANDORA facility is conceived to measure, for the first time, in-plasma β-decaying isotope rates under stellar-like conditions. The experimental approach consists in a direct correlation of plasma parameters and nuclear activity by disentangling - by means of the multidiagnostic system that will work in synergy with the γ-ray array - the photons emitted by the plasma (from microwave to hard X-ray) and γ-rays emitted after the isotope β-decay. In addition to nuclear physics research, fundamental plasma physics studies can be conducted in this unconventional ion source equipped with tens of detection and diagnostic devices (RF polarimeter, optical emission spectroscopy (OES), X-ray imaging, space and time-resolved spectroscopy, RF probes, scope), with relevant implications for R&D of ion sources for accelerator physics and technology. Several studies have already been performed in downsized nowadays operating ECRIS. Stable and turbulent plasma regimes have been described quantitatively, studying the change of plasma morphology, confinement, and dynamics of losses using space resolved X-ray spectroscopy.

Characterization and modeling of plasma sheath in 2.45 GHz hydrogen ECR ion sources

In this article, we present a multi-fluid numerical model developed for application on electron cyclotron resonance ion sources (ECRIS). The 1D-model is matured to compute the density of the ion species in the plasma sheath in the presence of an inhomogeneous magnetic field of a 2.45 GHz ECRIS. The multi-fluid model in cylindrical coordinates is focused on solving the continuity and momentum equations of hydrogen plasma particles to characterize their sheath properties. In addition, 28 important processes, including volume and surface collisions, have been included in the COMSOL Multiphysics package to simulate the ECR plasma. We study the elementary processes containing electron–atom, electron–molecule, atom–molecule, molecule–molecule, and particle–wall interactions. Then, the results of the model and the simulation of a 2D-hydrogen plasma are reported, and future perspectives are discussed throughout the paper.

Highly Conductive Metal Film Dry Etching via ECR Plasma Source

Highly Conductive Metal Film Dry Etching via ECR Plasma Source

Volumetric Recombination in the Linear ECR Plasma Device NUMBER

Volumetric Recombination in the Linear ECR Plasma Device NUMBER

Improvement of miniaturized 2.45 GHz ECR plasma flood gun at PKU

In an ion implanter, plasma flood gun (PFG) is used to provide electrons to neutralize the accumulated charge on the wafer surface to avoid breakdown damage. With the development of ion implantation technology, four key requirements have been put forward for PFG. They are simple structure, plasma with high density and low electron temperature, no metal contamination and long life. The existing PFG, such as the filament type PFG, can hardly meet the above requirements at the same time. 2.45 GHz ECR ion source with the advantages of high beam density, high stability, long life time and no filament metal contamination, has shown great potential to work as PFG. Recently, a miniaturized 2.45 GHz permanent magnet electron cyclotron resonance PFG (PMECR-PFG) has been developed at Peking University (PKU). In our previous test, 8.8 mA electron extraction current was obtained with argon gas. In this work, by optimizing the magnetic field configuration to a resonant configuration, the performance of this ECR-PFG was greatly improved. With 100 W microwave power, an 80 mA electron current load was obtained under the extraction voltage of 0.1 kV. To minimize the metal contamination, a three-slit graphite plasma electrode was fabricated and a 50 mA load was generated at only 30 W RF power. During all tests, the gas consumption rate is lower than 0.6 sccm, which is beneficial to maintain the vacuum of implanter beamline.

Comparison of End-Point Energies of Bremsstrahlung Photons from 28- GHz ECR Ion Source in Three Measurement Locations

Bremsstrahlung photons measurement show elevated energy emission beyond critical energy in 28-GHz ECR and its nature is not revealed. Thus, the aim of this study is to unveil the nature of high end-point energies at the three measurement locations. Bremsstrahlung photons from a 28-GHz ECR ion source were measured at Busan center of Korean Basic Science Institute (KBSI). The gamma-ray detection system consists of three round type NaI(Tl) scintillation detectors placed 62 cm radially from the beam axis at the three measurement locations namely injection, centre and extraction sides of an ECR ion source, and a NaI(Tl) scintillation detector placed at the view port for monitoring photon intensity along the beam axis. Such design of the experiment was based on the inner structure of the ECR ion source and the shape of the ECR plasma. Also, bremsstrahlung photons were measured by exchanging the detector positions for studying systematic uncertainties. Monte Carlo simulation based on the Geant4 package was performed to take geometrical acceptance and the energydependent detection efficiency into account due to large non-uniformity in the material budget. True bremsstrahlung energy spectra from the 28-GHz ECR ion source were extracted using an inverse matrix unfolding method. The measured high end-point energies at the three measurement locations were 1.68 MeV, 1.72 MeV and 2.04 MeV for the injection, centre and extraction sides, respectively. The high end-point energy values of the measured bremsstrahlung photons beyond the theoretical energy value of 1.33 MeV were found to be correlated with the structure of the ECR ion source and the shape of the ECR plasma. Furthermore, the high end-point energy values resulting from the bremsstrahlung photons measurements in the 28-GHz ECR plasma are due to the higher number of unconfined electrons arriving at the chamber wall easily, and producing higher bremsstrahlung photons. The unconfined electrons are the results of imperfect magnetic confinement that cause electrons to escape from the plasma volume.

Structure of intermittent variation of floating potential in a converging field following a magnetic beach ECR plasma source

In the converging field configuration of cylindrical plasma following an ECR plasma source, intermittent variation of floating potential, which is a negative spike with about a ten microsecond duration, is observed. The event is ubiquitous; independent of measurement position and magnetic field. Cross-correlation analysis for two floating potentials at separate positions reveals that a strong correlation exists within the event timescale. The correlation along the magnetic field line suggests an axially elongated structure. The radial extent of the structure is clarified by the radial correlation profile. Different FWHM of the radial profile at the plasma production region and the downstream converging field region is essentially explained by field line tracing.

Tailoring of microwave power density in an ECR ion source using an optimized ridge coupler

One of the interesting areas in the Electron Cyclotron Resonance Ion Source (ECR-IS) design which requires further exploration is the microwave power launching scheme and the power coupling optimization with the plasma chamber. The electron heating efficiency and thereby the plasma density highly depends on the electric field distribution inside the plasma chamber; therefore, it is important to optimize the coupling of microwave power to the plasma chamber, to maximize the electric fields in the plasma chamber for a given microwave power. For this, a single-step quarter wavelength ridge coupler design study has been carried out using CST MW Studio Suite along with the plasma chamber and ridge wave guide designs. The experimental measurements of electric and magnetic field profiles in the plasma chamber assembly with different coupling configurations have been done using an innovative bead pull technique and a magnetic field probe. The experimental results match well with the simulation results and the comparative studies of different coupling configurations reveal that the single step ridge coupler based scheme improves the electric field inside the plasma chamber to at least five times than the conventional ridge waveguide. To further improve the E-field amplitude in the plasma chamber by another 40%, a novel tuning scheme for the coupler has been introduced. Preliminary plasma studies have been carried out with the optimized coupler on a multicusp ECR plasma source and the plasma density measurements performed using a microwave cut-off probe. The plasma measurements show that even at low input microwave powers (∼300 W) the plasma density is comparable with standard ECR-IS. The present study thus, sheds light on the coupling configuration of microwave to plasma chamber with experimental measurements of cavity mode and fields in the cavity which will be useful for high intensity accelerator applications in understanding the plasma evolution, beam parameters and its dependence on different operating parameters.

Development of capacitive‐coupled hall‐type MHD generator

AbstractWe have demonstrated a capacitively coupled Hall‐type MHD generator using ECR plasma. To clarify the characteristics of the fabricated MHD generator, we measured the power generation characteristics as a function of magnetic field strength using a DC Hall‐type MHD power generation experiment. The results showed that the output power decreased due to magnetic pressure at the higher magnetic field. However, the output power corresponded to the theoretical value at the lower magnetic field. An AC Hall‐type MHD power generation experiment was conducted using an AC magnetic field. As a result, full‐wave rectification voltage was observed as per theory. Finally, capacitively coupled Hall‐type MHD power generation experiments were conducted, and full‐wave rectified waveforms were observed as in AC Hall‐type MHD power generation. These waveforms were similar to the output waveforms predicted from theory. These results show that the capacitively coupled Hall‐type MHD generator is feasible.

Laboratory generation of hazes in Titan's upper atmosphere using ECR plasma

Laboratory generation of hazes in Titan's upper atmosphere using ECR plasma

Development of Capacitive-coupled Hall-type MHD Generator

We have demonstrated a capacitively coupled Hall-type MHD generator using ECR plasma. To clarify the characteristics of the fabricated MHD generator, we measured the power generation characteristics as a function of magnetic field strength using a DC Hall-type MHD power generation experiment. The results showed that the output power decreased due to magnetic pressure at the higher magnetic field. However, the output power corresponded to the theoretical value at the lower magnetic field. An AC Hall-type MHD power generation experiment was conducted using an AC magnetic field. As a result, full-wave rectification voltage was observed as per theory. Finally, capacitively coupled Hall-type MHD power generation experiments were conducted, and full-wave rectified waveforms were observed as in AC Hall-type MHD power generation. These waveforms were similar to the output waveforms predicted from theory. These results show that the capacitively coupled Hall-type MHD generator is feasible.

Overview of Numerical Simulations for Calculating In-Plasma β-Decay Rates in the Framework of PANDORA Project

β-decay rates are essential inputs in stellar nucleosynthesis models to explain observed nuclear abundances. While current models continue to use terrestrial values, experiments in storage rings indicate strong divergence between decay rates of neutral and ionised atoms [1], necessitating renewed investigations into stellar decay rates. The PANDORA project aims at measuring lifetimes of specific radio-isotopes trapped in an ECR plasma (which mimics astrophysical environments to some extent) and compare them with theoretical predictions [2], consequently verifying the models and allowing decay rate estimation for any isotope in the stellar interior. We present here a simulation scheme to characterise the space-resolved charge state and level population distribution of buffer and radio-isotope ions in ECR ion sources in order to calculate in-plasma decay rates. The algorithm is based on a Particle-in-Cell Monte Carlo (PIC-MC) routine that simultaneously models charge transport with collision-radiative processes. Preliminary results from the simulation are also shown, along with important takeaways for code-optimisation.

Experimental characterization of thrust production mechanisms in a magnetic nozzle ECR thruster

Direct thrust measurements have been performed on a low-power (30 W) ECR plasma thruster operated with xenon. The total thrust, the force exerted on the walls of the source (thermal thrust), and the force exerted on the permanent magnet (magnetic thrust) have been measured separately for various operating conditions (power and flowrate). The results are compared for two magnetic field topologies (with a different magnetic field gradient and divergence). The magnetic thrust dominates thermal thrust for most operating conditions, and its contribution to total thrust can be as high as 80%. A saturation of magnetic thrust with increasing power has been observed and is correlated with a decrease in thrust efficiency. A high degree of electron anisotropy can be inferred from the thrust measurements.

Possibility of Using the 140 GHz Frequency for ECR Plasma Heating in the T-15MD Tokamak

Possibility of Using the 140 GHz Frequency for ECR Plasma Heating in the T-15MD Tokamak

Design study of a HPGe detector array for β-decay investigation in laboratory ECR plasmas

In the frame of the PANDORA project, a new experimental approach aims at measuring in-plasma β-decay rate as a function of thermodynamical conditions of the environment, namely a laboratory magnetized plasma able to mimic some stellar-like conditions. The decay rates (expected to change dramatically as a function of the ionization state) will be measured as a function of the charge state distribution of the in-plasma ions. The new experimental approach aims at correlating the plasma environment and the decay rate. This can be performed by simultaneously identifying and discriminating—through an innovative multi-diagnostic system working synergically with a γ-ray detection system —the photons emitted by the plasma and γ-rays emitted after the isotope β-decay. In this study, the numerical simulations supporting the design of the γ-ray detector array, including a statistical significance study to check the feasibility of measuring the in-plasma decay rates, are presented. Geant4 simulations focused on the design of the array of γ-ray detectors and the evaluation of total efficiency depending on the detector type and the optimal displacement of detectors around the trap (including collimation systems and shielding). The simulation results showed that, due to technical limitations in the number of apertures that can be created in the magnetic trap, the best compromise is to use 14 HPGe (70% of relative efficiency) detectors surrounding the magnetic trap. The HPGe detectors were chosen for their excellent energy resolution (0.2% @ 1 MeV), since the harsh environment (the background is represented by the intense plasma self-emission) strongly affects the signal-to-background ratio. Once determined the total photopeak efficiency (0.1–0.2%), the sensitivity of the PANDORA experiment was checked in a “virtual experimental run,” by exploring the measurability of isotope decay rates for the first three physical cases of PANDORA: 176Lu, 134Cs and 94Nb. The preliminary results demonstrated the feasibility of the measurement in terms of the signal-to-background ratio and significance that it is possible to reach. The results indicated that experimental run durations could take from several days to 3 months, depending on the isotope under investigation, thus shedding new light on the role of weak interactions in stellar nucleosynthesis.

Some aspects of the plasma potential in 3D simulation of ECRIS operation

A 3D particle-in-cell plus Monte Carlo collision (PIC/MCC) code is developed for the simulation of electron cyclotron resonance ion source (ECRIS). The self-consistent interaction between the plasma and the potential field is taken into account, as well as Coulomb collisions, stepwise ionization, and charge exchange collisions between particles. In addition, a precalculation module based on a single-particle approach is introduced to speed up simulations. The stable distributions of the high-energy electrons are obtained and then sent to the subsequent simulation of ECRIS operation as the well-confined warm and hot electrons. An implicit electrostatic PIC model in this simulation self-consistently describes the evolution of the ECR plasma. The results are obtained for the plasma potential in a steady state, including the global amplitude and distribution profiles. The potential distribution of the ECR plasma is characterized by magnetic fields. These results, together with those for the charge density, are analyzed and discussed from the perspective of plasma diffusion.

Energy distribution and bremsstrahlung spectra of energetic electrons escaping from the ECR plasma with high energy input

Electron energy distribution function (EEDF) is an important characteristic that defines the processes of plasma confinement and ionization. Understanding such processes is crucial for the proper tuning of ion sources. In the ECR plasma, the EEDF is significantly non-Maxwellian, and the exact shape remains still unknown. As the initial step towards the EEDF resolving, the energy distribution of the electrons lost from the magnetic mirror trap was directly measured in a wide range of neutral gas pressures and gyrotron powers along with bremsstrahlung spectra. A series of experiments were performed on the newly constructed Gasdynamic Ion Source for Multipurpose Operation (GISMO) facility allowing record-breaking specific energy input into the plasma. Obtained distributions showed unconventional behavior as the function of source parameters. The EEDF shape and the bremsstrahlung spectra showed a threshold qualitative change with the gyrotron power presumably as a result of the development of kinetic instabilities.