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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.
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... The code is based on the model that is described in details elsewhere [11]. The NAM-ECRIS is a Particlein-Cell Monte-Carlo Collisions code that traces a movement of macro-particles representing ions and atoms in ECRIS plasma. ...
... To see the gas-mixing effect, the model should be modified compared to the version described in [11]. We assume that the ion motion is affected by a dip (Δφ) in the positive plasma potential. ...
... In the following, we present the data obtained with the fixed power of 500 W. The selection is a rather arbitrary: the calculated extracted current of O 6+ ions is at the level of around 1 mA at this power, close to what is measured with the DECRIS-SC2 source when injected microwave power is 600 W. As it is discussed in [11], the calculated value of the coupled power as it is used in our model can substantially differ from the experimentally measured injected power both due to the incomplete microwave absorption in the plasma and deviations of the electron energy distribution function from the Maxwell-Boltzmann one. ...
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The particle-in-cell MCC code NAM-ECRIS is used to simulate the ECRIS plasma sustained in a mixture of Kr with O2, N2, Ar, Ne and He. The model assumes that ions are electrostatically confined in ECR zone by a dip in the plasma potential. Gain in the extracted krypton ion currents is seen for the highest charge states; the gain is maximized when oxygen is used as the mixing gas. A special feature of oxygen is that most of singly charged oxygen ions are produced after dissociative ionization of oxygen molecules with the large kinetic energy release of around 5 eV per ion. Increased loss rate of energetic lowly charged ions of the mixing element requires building up of the retarding potential barrier close to ECR surface to equilibrate electron and ion losses out of the plasma. In the mixed plasmas, the barrier value is large (~1 V) compared to the pure Kr plasma (~0.01 V), with the longer confinement times of krypton ions and with the much higher ion temperatures.
... As the 1+ intensities yielded by the ISOL method are typically low, the charge breeder efficiency needs to be as high as possible to realize this. Fundamental nuclear physics research hence benefits from the continued improvements in ECRIS technology, which in turn requires the development of state-of-the art plasma diagnostics methods for R&D purposes and benchmarking of simulations [9][10][11] . ...
... has an infinitude of solutions, and the acceptable solution set is obtained by defining a penalty function (10) and minimizing it as a function of E e in a dense array of n e points. Here the dependence on E e comes from the ionization rate coefficients, defined as ...
... Refs. 10,12 ) and k-α measurements (e.g. Ref. 42 ). ...
Article
The consecutive transients (CT) method is a plasma diagnostic technique of charge breeder electron cyclotron resonance ion source plasmas. It is based on the short-pulse injection of singly charged ions and the measurement of the resulting transients of the extracted multi-charged ion beams. Here, we study the origin of the large uncertainty bounds yielded by the method to reveal avenues to improve its accuracy. We investigate effects of the assumed electron energy distribution (EED) and the uncertainty inherited from the ionization cross section data of K4+–K12+ ions on the resulting plasma electron density ne, average energy ⟨Ee⟩, and the characteristic times of ion confinement τq, electron impact ionization τinzq, and charge exchange τcxq provided by the CT method. The role of the EED was probed with Kappa and double-Maxwellian distributions, the latter resulting in a shift of the ne and ⟨Ee⟩ distributions. The uncertainty of the ionization cross section σq→q+1inz was artificially curtailed to investigate its impact on values and uncertainties of the plasma parameters. It is demonstrated that the hypothetical perfect knowledge of σq→q+1inz significantly reduces the uncertainties of τq, τinzq, and τcxq, which motivates the need for improved cross section data.
... A series of investigations are devoted to describe electron cyclotron resonance ion sources (ECRIS). Because of the very low pressure and the large electron energy (up to several keV) particle methods such as particle in cell (PIC) or Monte Carlo have been used [20][21][22]. Partly, the particle methods are applied to the ion component only and electrons are taken as a neutralizing background with a given mean energy [21]. ...
... Because of the very low pressure and the large electron energy (up to several keV) particle methods such as particle in cell (PIC) or Monte Carlo have been used [20][21][22]. Partly, the particle methods are applied to the ion component only and electrons are taken as a neutralizing background with a given mean energy [21]. Recently, very time-consuming 3D PIC simulations of the electron component assuming ions as background have been published [22]. ...
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A self-consistent fluid model has been successfully developed and employed to model an electron cyclotron resonance driven hydrogen plasma at low pressure. This model has enabled key insights to be made on the mutual interaction of microwave propagation, power density, plasma generation, and species transport at conditions where the critical plasma density is exceeded. The model has been verified by two experimental methods. Good agreement with the ion current density and floating potential - as measured by a retarding energy field analyzer - and excellent agreement with the atomic hydrogen density - as measured by two-photon absorption laser induced fluorescence - enables a high level of confidence in the validity of the simulation.
... So far, our NAM-ECRIS model [12,13] was applied for simulations of ECRIS in assumption that all electrons are warm. The electron temperature Tew was taken as a free parameter for the code, being connected to the coupled microwave power by assumption that the mean energy of electrons that are lost out of the plasma is 3/2Tew. ...
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Electron dynamics in Electron Cyclotron Resonance Ion Source is numerically simulated by using Particle-In-Cell code combined with simulations of the ion dynamics. Mean electron energies are found to be around 70 keV close to values that are derived from spectra of X-ray emission out of the source. Electron life time is defined by losses of low-energy electrons created in ionizing collisions; the losses are regulated by electron heating rate, which depends on magnitude of the microwave electric field. Changes in ion confinement with variations in the microwave electric field and gas flow are simulated. Influence of electron dynamics on the afterglow and two-frequency heating effects is discussed.
... The characteristics of installations of this type over the past three decades have improved due to increased knowledge in the field of physics of processes occurring in the plasma of the ion source, the emergence of new technologies, and the use of superconductors. Research in the field of physics and technology of ECR ion sources is carried out in many scientific centers in the United States [1], China [2], Japan [3], France [4], Germany [5], Russia [6], etc. ...
Article
We review the current understanding of high charge state ion confinement and ion temperature in Electron Cyclotron Resonance Ion Sources (ECRIS). Experiments probing the ion confinement time of various charge states strongly favour a confinement scheme where the high charge state ions are trapped in a local dip of the ambipolar plasma potential. The electrostatic confinement permits ion confinement times of 10 ms order-of-magnitude. The dwelling time of the ions, undergoing stepwise ionisation from neutrals to high charge states, is long enough for the energy transfer from the plasma electrons to heat the ions to 10-15 eV while the energy exchange in ion-ion collisions results in all charge states having essentially the same temperature. We then describe a technique, using the ion temperatures obtained through optical emission spectroscopy and afterglow transient beam currents, to estimate the magnitude of the potential dip. In our example, measured with a 14 GHz ECRIS the value of the potential dip is 1.3-1.9. We demonstrate that the temporal characteristics of the afterglow transient occurring in 1 ms scale can be estimated by assuming that the afterglow peak of high charge state ion currents is caused by a change of the ion confinement scheme from electrostatic trapping to random walk diffusion resulting in order-of-magnitude reduction of the ion confinement time.
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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.
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We establish multicomponent 1+ injection into a charge breeder electron cyclotron resonance ion source and an associated computational procedure as a noninvasive probe of the electron density ne, average electron energy 〈Ee〉, and the characteristic times of ionization, charge exchange, and ion confinement of stochastically heated, highly charged plasma. Multicomponent injection allows refining the ne, 〈Ee〉 ranges, reducing experimental uncertainty. Na/K injection is presented as a demonstration. The 〈Ee〉 and ne of a hydrogen discharge are found to be 600−300+600eV and 8−3+8×1011cm−3, respectively. The ionization, charge exchange, and confinement times of high charge state alkali ions are on the order of 1 ms–10 ms.
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Lifetimes of radioactive nuclei are known to be affected by the level configurations of their respective atomic shells. Immersing such isotopes in environments composed of energetic charged particles such as stellar plasmas can result in β-decay rates orders of magnitude different from those measured terrestrially. Accurate knowledge of the relation between plasma parameters and nuclear decay rates are essential for reducing uncertainties in present nucleosynthesis models, and this is precisely the aim of the PANDORA experiment. Currently, experimental evidence is available for fully stripped ions in storage rings alone, but the full effect of a charge state distribution (CSD) as exists in plasmas is only modeled theoretically. PANDORA aims to be the first to verify these models by measuring the β-decay rates of select isotopes embedded in electron cyclotron resonance (ECR) plasmas. For this purpose, it is necessary to consider the spatial inhomogeneity and anisotropy of plasma ion properties as well as the non-local thermodynamic equilibrium (NLTE) nature of the system. We present here a 3D ion dynamics model combining a quasi-stationary particle-in-cell (PIC) code to track the motion of macroparticles in a pre-simulated electron cloud while simultaneously using a Monte Carlo (MC) routine to check for relevant reactions describing the ion population kinetics. The simulation scheme is robust, comprehensive, makes few assumptions about the state of the plasma, and can be extended to include more detailed physics. We describe the first results on the 3D variation of CSD of ions both confined and lost from the ECR trap, as obtained from the application of the method to light nuclei. The work culminates in some perspectives and outlooks on code optimization, with a potential to be a powerful tool not only in the application of ECR plasmas but for fundamental studies of the device itself.
Article
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.
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Electron cyclotron resonance ion source (ECRIS) plasmas are prone to kinetic instabilities due to anisotropy of the electron energy distribution function stemming from the resonant nature of the electron heating process. Electron cyclotron plasma instabilities are related to non-linear interaction between plasma waves and energetic electrons resulting to strong microwave emission and a burst of energetic electrons escaping the plasma, and explain the periodic oscillations of the extracted beam currents observed in several laboratories. It is demonstrated with a minimum-B 14 GHz ECRIS operating on helium, oxygen, and argon plasmas that kinetic instabilities restrict the parameter space available for the optimization of high charge state ion currents. The most critical parameter in terms of plasma stability is the strength of the solenoid magnetic field. It is demonstrated that due to the instabilities the optimum B min-field in single frequency heating mode is often ≤0.8B ECR, which is the value suggested by the semiempirical scaling laws guiding the design of modern ECRISs. It is argued that the effect can be attributed not only to the absolute magnitude of the magnetic field but also to the variation of the average magnetic field gradient on the resonance surface.
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Experimental observation of cyclotron instabilities in a minimum-B confined electron cyclotron resonance ion source plasma is reported. The instabilities are associated with strong microwave emission and a burst of energetic electrons escaping the plasma, and explain the periodic ms-scale oscillation of the extracted beam currents. Such non-linear effects are detrimental for the confinement of highly charged ions due to plasma perturbations at shorter periodic intervals in comparison with their production time. It is shown that the repetition rate of the periodic instabilities in oxygen plasmas increases with increasing magnetic field strength and microwave power and decreases with increasing neutral gas pressure, the magnetic field strength being the most critical parameter. The occurrence of plasma turbulence is demonstrated to restrict the parameter space available for the optimization of extracted currents of highly charged ions.
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In order to improve the performance of the JYFL 14 GHz electron cyclotron resonance ion source (ECRIS) and initiate low energy beam transport (LEBT) upgrade at the University of Jyväskylä, Department of Physics (JYFL) accelerator laboratory, a new ion beam extraction system has been designed and installed. The development of the new extraction was performed with the ion optical code IBSimu, making it the first ECRIS extraction designed with the code. The measured performance of the new extraction is in good agreement with the simulations. Compared to the old extraction the new system provides improved beam quality, i.e. lower transverse emittance values and improved structure of beam profiles, and transmission efficiency of the LEBT and the JYFL K-130 cyclotron. For example, the transmission efficiencies of 40Ar8+ and 84Kr16+ beams have increased by 80 and 90%, respectively. The new extraction system is capable of handling higher beam currents than the old one, which has been demonstrated by extracting new 4He+ and 4He2+ record beam currents of 1.12 mA and 720 μA, exceeding the old records of the JYFL 14 GHz ECRIS by a factor of two.
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The open-ended trap is one of the installations used for the magnetic confinement of thermonuclear plasma. Open-ended traps have a number of important advantages as compared with other confinement systems: They are attractive from the engineering point of view, their magnetic field is efficiently used to confine the plasma, they can be operated under steady-state conditions, and there is no particular problem with removing thermonuclear reaction products and heavy impurities from the plasma. At the same time, it has long been considered that the open-ended trap has a doubtful future as a basis for a thermonuclear reactor because of the relatively high rate of loss of plasma along the magnetic lines of force. The situation has changed for the better during the last decade, and a number of improved traps, that are largely free from this defect, has been proposed. This review examines the physical principles of open-ended traps (ambipolar, centrifugal, multimirror, gas-dynamic, and so on), the present state of research into these systems, and their future prospects. The use of open-ended traps as high-flux generators of 14-MeV neutrons is also discussed.
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The open-ended trap is one of the installations used for the magnetic confinement of thermonuclear plasma. Open-ended traps have a number of important advantages as compared with other confinement systems: they are attractive from the engineering point of view, their magnetic field is efficiently used to confine the plasma, they can be operated under steady-state conditions, and there is no particular problem with removing thermonuclear reaction products and heavy impurities from the plasma. At the same time, it has long been considered that the open-ended trap has a doubtful future as a basis for a thermonuclear reactor because of the relatively high rate of loss of plasma along the magnetic lines of force. The situation has changed for the better during the last decade, and a number of improved traps, that are largely free from this defect, has been proposed. This review examines the physical principles of open-ended traps (ambipolar, centrifugal, multimirror, gas-dynamic, and so on), the present state of research into these systems, and their future prospects. The use of open-ended traps as high-flux generators of 14-MeV neutrons is also discussed.
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An all-permanent, 6 GHz ECR ion source has been constructed at the Holifield Radioactive Ion Beam Facility (HRIBF), Oak Ridge National Laboratory (ORNL), that permits configuration of the central magnetic field in either conventional parabolic or flat minimum-B profiles. The magnitude of the central flat field configuration extends over an axial region of ∼ 2 cm to form a large and uniformly distributed ECR volume. The capability of operating the source in either volume or surface modes permits direct comparison of the performances of each source type. The studies show that the volume ECR source produces higher charge-states and higher intensities within a particular charge-state than does the surface form of the source. The X-ray spectra derived during operation of the source also suggest that the enhanced performance of volume ECR source is attributable to its ability to accelerate a larger population of electrons to higher energies than its conventional counterpart.