ECR Plasma Research Articles

Characterization of Hydrogen Plasma in an ECR based Large Volume Plasma Chamber

Hydrogen plasma characterization was carried out in a large volume (dia∼ 1.0 m, h∼ 1.0 m) plasma chamber to evaluate the efficacy of production of uniform, large-area H- beam for fusion applications. Up to seven Compact ECR Plasma Sources (CEPS; Indian Patent #301583, Patentee: IIT Delhi) can be mounted on the top dome of the chamber (one in centre and six on a ∼60 cm dia circle). Axially poled permanent ring magnets provide the magnetic field for each CEPS and the total field in the chamber is the combined field of all CEPS. Separate experiments were conducted with: (i) six CEPS with opposite polarity on adjacent magnets, (ii) seven CEPS with identical polarity on all the magnets, and (iii) a single CEPS at the center. In case (ii) the field lines repel while for (i) a cusp field is formed between adjacent sources. For case (iii), one obtains a uniform plasma density over 35 cm radius (n ∼ 4 ϗ 1010 cm-3, Te ∼1-2 eV), ∼ 60 cm downstream (from central source mouth) with 400W power at ∼ 1-3 mTorr pressure, yielding ideal conditions for volume mode H- production over a large area. For case (ii), however, plasma from each source flows along its individual field lines, without forming uniform plasma down-tream. Whereas for case (i), because of cross-talk between adjacent sources, the system became unstable, giving oscillations in plasma formation and microwave reflected power. Hence, it appears that the single CEPS configuration is the most efficacious for large area H- generation.

Metal evaporation study using the GANIL high temperature oven for intense metal ion beams production

A new resistive oven dedicated to the production of metal ion beams using ECR ion sources has been developed at GANIL. It aims to produce beams such as 58Niq+, 50Tiq+, 50Vq+ or 238Uq+ with ECR4 ion source on cyclotrons injectors and with Phoenix V3 ion source on Spiral 2facility. Typical intensity about 1.2 1013 pps is requested for the Super Separator Spectrometer (S3). Evaporated atom fluxes and their angular distributions have been measured on a dedicated test bench thanks to a quartz microbalance. These investigations were carried out using three crucibles with various exit caps geometries for which the fluxes and angular distributions display distinct behaviors. Based on those results, crucible is designed to obtain a high flux of atoms overlapping the ECR plasma while minimizing the atoms losses on the ion source chamber.

Magnetically confined microwave ECR plasma assisted study on radiochemical reaction of tritium and carbon monoxide

Tritium induced radiochemical reaction between hydrogen and carbon monoxide was investigated with the aid of ECR plasma reaction. The results show that long-chain organics such as C4Q10, C5Q12 and C6Q14 were formed in DT-CO mixture without any catalyst, and short-chain organics were the reactants to long-chain organics. CH4, C2H2 and C2H4 were detected in the plasma reaction of hydrogen and carbon monoxide mixture, but no C3+ organics were found. This study revealed that the dominant process of the tritium induced reactions mainly caused by the free radicals produced by β-rays from tritium decay. The fracture of CH bonds promotes the carbon chain growth, while the breakage of CC bonds inhibits the growth of organic carbon chain.

X-ray diagnostics of ECR ion sources-Techniques, results, and challenges.

The high magnetic confinement provided by the minimum-B structure of electron cyclotron resonance ion sources (ECRIS) hosts a non-equilibrium plasma, composed of cold multi-charged ions and hot electrons whose energy can expand up to ≈1MeV. With a very limited accessibility, the ECR plasma is difficult to study. The x-ray photons generated by the interaction of the warm and hot electron populations within the plasma, as well as the photons generated by electrons lost to the chamber wall, are a signal of great interest as it gives an insight to the properties of the ECR plasma. After an introduction presenting the mechanism generating the x rays in ECRIS, this Review presents the methodology to measure x rays both for volumetric measurements and plasma imaging. The main insights of those measurements are presented. Prospects of x-ray measurements to better understand the plasma dynamics are finally highlighted.

Direct measurement of electron Bernstein waves in Low Aspect ratio Torus Experiment (LATE)

By using a newly developed five-pin probe antenna and two-dimensional mechanical probe driving system, the 2D wave pattern of phase and amplitude has been directly measured in the Low Aspect ratio Torus Experiment (LATE) device, for an overdense ECR plasma with microwave obliquely injected. In the case of O-mode injection, an electron Bernstein wave (EBW)-like wave pattern has been detected for the first time, in a localized region near the upper hybrid resonance layer. The pattern has a short wavelength of about 2 mm and is also electrostatic and backward. In the case of X-mode injection, the 2D wave pattern is quite different and no EBW signal can be observed. By adjusting the toroidal magnetic field in O-mode injection, it is found that both the position and size of the EBW region have changed, which suggests the localized condition of efficient O-X-B conversion and high collisional damping rate of EBWs in these experiments.

Effects of gas adsorbed on solid surface during gas breakdown in electron cyclotron resonance discharges

The microwave breakdown power (P wb) in an ECR plasma source was not merely determined by pressure (gas flow rate), but found to vary with the time interval between two successive breakdowns. The measured P wb dropped rapidly from a high value at a short time interval to a low level at a long time interval. The obtained dependence of P wb on pressure (gas flow rate) exhibited distinct features: the normal monotonicity and abnormal non-monotonicity at the short and long time intervals, respectively. The effective zone in the antenna’s surface, bombarded by hot electrons heated in the ECR layer, was validated by (1) masking the antenna with a film having a variable radius; (2) calculating the distribution of the vertical component of the microwave electric field with respect to the static magnetic field; and (3) imaging glows of transient breakdown discharges with a fast camera. The reduction in P wb was mainly attributed to the enhanced emission of δ-electrons from the gas-adsorbed antenna under the bombardment of energetic electrons coming from the ECR layer. The correlation between the dynamic gas coverage and the emission coefficient of δ-electrons was established to understand the abnormal ECR breakdown features.

On the Issue of Existence of a Soft X-Ray Pit in the Ohmic and ECR Plasma Heating Regimes at the L-2M Stellarator

On the Issue of Existence of a Soft X-Ray Pit in the Ohmic and ECR Plasma Heating Regimes at the L-2M Stellarator

Investigation of N(4S) kinetics during the transients of a strongly emissive pulsed ECR plasma using ns-TALIF

A nanosecond-two-photon absorption laser-induced fluorescence (ns-TALIF) technique was employed to investigate the transients of a strongly emissive, pulsed microwave discharge. We analysed the conditions that have to be fulfilled in order to use the TALIF intensities measured with the laser central frequency tuned to the absorption peak instead of the fully integrated laser excitation spectrum absorption for measuring N-atom densities. We demonstrated the validity of the method under our conditions and applied it to the straightforward monitoring of N-atom densities during the transients of a pulsed ECR plasma. In particular, we demonstrated the existence of an unexpected increase in N(4S) atom density of about 10% at the early stage of the transition from high- to low-power phase. Using a self-consistent, quasi-homogenous plasma model encompassing a detailed state-to-state kinetics, a thorough analysis of the N-atom and N2(B) kinetics was carried out and enabled this enhancement to be attributed to surface de-excitation of N(2D) and N(2P) atoms. The rise of the N(4S) atom density can serve as an indicator of the concentration of N(2D) and N(2P) atoms in the discharge.

Plasma simulation and optimization for a miniaturized antenna ECR ion source

At Peking University (PKU), a miniaturized 2.45 GHz antenna permanent magnet electron cyclotron resonance (PMECR) ion source, capable of generating a several-mA H+ beam, was produced and tested. The dimensions of the plasma chamber were ∅30 mm ×40 mm, and the entire size of the ion source (including the extraction system) supply was ∅150 mm ×160 mm. A 4.5 mA CW H+ beam was extracted at 35 kV with 200 W of microwave power supplied by a compact microwave generator with a coaxial cable. For a better understanding and optimization of the miniaturized ion source, plasma simulations were conducted to research the electromagnetic coupling of the antennas and the generation of ECR plasmas. The experimental and simulation results of the H+, H2+, and H3+ fractions of cylindrical-type and “T”-type antenna microwave ion sources were presented and compared. Based on a series of comparisons between the experimental and simulation results, the impedance of the miniaturized antenna ECR ion source plasma and the real pressure in the plasma chamber were accurately calculated. An optimized microwave coupling system was designed for the new miniaturized antenna ECR ion source. From the plasma simulation results, the transmission efficiency of microwaves increased from 32% to 89% (coupled to plasma), 15 mA H+ @ 150 W can be expected in the future.

The influence of deuterium ions on the deuterium permeation and retention behavior in V-4Cr-4Ti alloy under plasma loading

Deuterium (D) permeation and retention behavior in V-4Cr-4Ti alloy under ECR plasma or only gas loading has been studied. The gas pressure for plasma discharge was 0.6 Pa, which was the same as that for D2 gas loading. Different bias voltages of 0 V, -50 V and -100 V were applied to the samples to investigate the effect of ion incident energy on the D permeation behavior. D retention behavior was studied by thermal desorption spectroscopy (TDS). X-ray photoelectron spectroscopy (XPS) with depth profile was utilized to characterize the distribution of surface impurities and the chemical states of vanadium element. When the ion incident energy increases under plasma loading, D permeation flux decreases. D permeation fluxes under plasma loading are generally higher than those under only gas loading except those under D plasma loading with a bias of -100 V below 650 K. In addition, the total D retention in the plasma loaded sample is lower than that in the gas loaded one. XPS analyses show that the plasma loaded sample surface has a shallower V2O3 layer. Additionally, the percentage of VC in the plasma loaded sample is less than that in the gas loaded one. It suggests that D ions especially those with high ion incident energy could reduce the content of the oxides and carbides on the sample surface, which may finally result in the abnormal D permeation and retention behavior under or after plasma loading.

Design and development of a compact ion implanter and plasma diagnosis facility based on a 2.45GHz microwave ion source.

A project on developing a 2.45GHz microwave ion source based compact ion implanter and plasma diagnostic facility has been taken up by the Central University of Punjab, Bathinda. It consists of a double-wall ECR plasma cavity, a four-step ridge waveguide, an extraction system, and an experimental beam chamber. The mechanical design has been carried out in such a way that both types of experiments, plasma diagnosis and ion implantation, can be easily accommodated simultaneously and separately. To optimize microwave coupling to the ECR plasma cavity, a four-step ridge waveguide is designed. Microwave coupling simulation for the ECR plasma cavity has been performed at different power inputs using COMSOL Multiphysics. An enhanced electric field profile has been obtained at the center of the ECR plasma cavity with the help of a four-step ridge waveguide compared to the WR284 waveguide. The magnetic field distribution for two magnetic rings and the extraction system's focusing properties have been simulated using the computer simulation technique. A tunable axial magnetic field profile has been obtained with a two permanent magnetic ring arrangement. The dependency of the beam emittance and beam current on accelerating voltages up to 50 kV has been simulated with different ions. It shows that ion masses have a great impact on the beam emittance and output current. This facility has provision for in situ plasma diagnosis using a Langmuir probe and optical emission spectroscopy setups. This system will be used for ion implantation, surface patterning, and studies of basic plasma sciences.

Recovery of the Electron Energy Distribution Function under Conditions of High-Power ECR Plasma Heating at the L-2M Stellarator

Recovery of the Electron Energy Distribution Function under Conditions of High-Power ECR Plasma Heating at the L-2M Stellarator

The curling probe: A numerical and experimental study. Application to the electron density measurements in an ECR plasma thruster

The measurement of electron density is a key issue in understanding and controlling plasma applications. To date, plasma density in electric thrusters has been mainly evaluated with electrostatic techniques, such as the Langmuir probe, which could be quite invasive. In this paper, we propose the application of a microwave resonant probe, the curling probe, to the diagnostic of an electrodeless plasma thruster. The measurable electron density range and the probe accuracy are found to be limited by the probe natural frequency. We present the numerical study and the experimental characterization of three curling probes with different natural frequencies (700, 1400, and 3000 MHz, approximately). First, an analytical equation of the natural frequency as a function of geometrical parameters is drawn from 2D numerical simulations. Then, a procedure based on solid dielectric etalons is proposed for the absolute calibration of the probe. Finally, measurements are performed in the plume of an electron cyclotron resonance plasma thruster. Electron densities from 108 to 1011 cm-3 have been obtained in agreement with hairpin and Langmuir probes results. A wall-embedded probe has enabled measurements inside the thruster source with minimal plasma perturbation. A possible curling probe configuration, embedded in a reactor wall, is proposed as a fully non-invasive diagnostic for plasma sources.

Design and Analysis of Slotted Waveguide Antenna Radiating in a “Plasma-Shaped” Cavity of an ECR Ion Source

The design of a microwave antenna sustaining a high-energy-content plasma in Electron Cyclotron Resonance Ion Sources (ECRISs) is, under many aspects, similar to the design of a conventional antenna but presenting also peculiarities because of the antenna lying in a cavity filled by an anisotropic plasma. The plasma chamber and microwave injection system design plays a critical role in the development of future ECRISs. In this paper, we present the numerical study of an unconventionally shaped plasma cavity, in which its geometry is inspired by the typical star-shaped ECR plasma, determined by the electrons trajectories as they move under the influence of the plasma-confining magnetic field. The cavity has been designed by using CST Studio Suite with the aim to maximize the on-axis electric field, thus increasing the wave-to-plasma absorption. As a second step, an innovative microwave injection system based on side-coupled slotted waveguides is presented. This new launching scheme allows an uniform power distribution inside the plasma cavity which could lead to an increase of ion source performances in terms of charge states and extracted currents when compared to the conventional axial microwave launch scheme. Finally, the use of both the “plasma-shaped” cavity and the microwave side coupled scheme could make the overall setup more compact.

Electron Temperature Profiles Formed under Conditions of the Axial ECR Plasma Heating at the L-2M Stellarator

The shape is analyzed of the electron temperature profiles measured in experiments on the axial ECR heating at the L-2M stellarator at different densities and heating powers. It is ascertained that when the reduced ECR heating power exceeds some threshold value, the electron temperature profiles become broadened and flat in the core region of the plasma column. The correlation is revealed between the appearances of the flat temperature profiles and the density profiles with a dip in the axial region (the dip-shaped profiles) occurring as a result of the density pump-out effect. It is concluded that the appearance of the dip-shaped density profiles in the plasma results in a change in the mechanism of microwave radiation absorption. Hypothetically, the cyclotron absorption of the electron Bernstein waves becomes the dominant mechanism for the microwave radiation absorption that, in turn, results in the formation of the broad and flat (in the core region) electron temperature profiles.

Efficiency investigation of a negative hydrogen ion beam production with the use of the gasdynamic ECR plasma source

Negative hydrogen ion sources are of great demand in modern physics as injectors into accelerators and drivers for neutral beam injectors for fusion devices. It has been shown earlier that the use of the gasdynamic ECR discharge provides the opportunity to extract up to 80 mA/cm2 of negative ion current density. We studied experimentally the volumetric negative hydrogen ion production and vacuum ultraviolet emission in a gasdynamic ECR discharge. The high-density plasma was sustained by the pulsed 37 GHz / 100 kW gyrotron radiation in a magnetic configuration consisting of two consecutive simple mirror traps. The future prospects of the volumetric H− source based on the gasdynamic ECR discharge related to the transition from pulsed to continuous operating mode with the use of an improved magnetic confinement system are discussed. Numerical simulation of the negative hydrogen ion beam extraction at the continuous operating mode facility was performed. Optimal configuration of the extraction electrodes and the electron damping magnets was found.

Microwave Setup of a Megawatt Power Level for the ECR Plasma Heating and Current Drive System of the T-15MD Tokamak

Microwave Setup of a Megawatt Power Level for the ECR Plasma Heating and Current Drive System of the T-15MD Tokamak

Formation of flat electron density profiles at the L-2M stellarator at high ECRH powers

The shapes of the electron temperature profiles were analyzed at the L-2M stellarator based on the data obtained in experiments on the axial ECR plasma heating performed at different plasma densities and heating powers. It was ascertained that, at the reduced ECRH power exceeding some threshold value, the electron temperature profiles become broadened and flat in the axial region of the plasma column. The appearance of the flat temperature profiles correlates with the occurrence of the density profiles with a dip in the axial region forming at high reduced heating powers as a result of the density pump-out effect. It was concluded that, as a result of occurrence of the density profiles with a dip in the axial region, the mechanism for the microwave radiation absorption changes. Apparently, the cyclotron absorption of the electron Bernstein waves becomes the main mechanism for the microwave radiation absorption, which, in turn, results in the occurrence of the broad and flat (in the axial region) electron temperature profiles.

Thrust evaluation of compact ECR plasma source using 2-zone global model and plasma measurements

Plasma thrusters are required for deep space applications for reducing the fuel payload. This paper evaluates the capability of a compact ECR plasma source (CEPS) as a standalone plasma thruster in space. The unique magnetic field profile of CEPS’ ring magnets enables very efficient plasma production and electron heating within the CEPS plasma source section (ID ≈ 8.5 cm, length ≈ 11.6 cm) resulting in high plasma potentials. Experiments were undertaken with the CEPS attached coaxially to a larger expansion chamber (ID ≈ 48.2 cm, length ≈ 75 cm). Diagnostics included specially designed Langmuir probes (LP) and ion energy analyzers (IEA) that are capable of withstanding the harsh plasma conditions in front of and within the CEPS. LP measurements reveal high-density argon plasma (≈1012 cm−3), with high bulk electron temperatures (≈20 eV) and plasma potentials (≈100 V) at very modest microwave power (≈600 W) over a wide range of pressures (0.3–1 mTorr). An important observation is the fall of almost the full plasma potential inside the CEPS within a short distance close to its exit, giving rise to strong ambipolar electric fields suitable for accelerating ions. IEA measurements confirmed the presence of streaming ions with energies ≈87 eV at ≈2 cm in front of the CEPS. Estimates based on the IEA results yield thrust values ≈50 mN at ≈0.5 mTorr. A 2-zone, global model was developed to determine the steady state plasma parameters (including plasma flow velocity) in the geometry of the experimental system (without magnetic field). The model reproduced the key experimental plasma parameters and was extended to compute plasma thrust under actual space-like conditions. The computed thrust values for xenon are higher (≈80 mN) than that for argon (≈40–45 mN) at ≈600 W of microwave power.

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.