Electron Cyclotron Resonance Surface Research Articles

Controlled turbulence regime of electron cyclotron resonance ion source for improved multicharged ion performance

Fundamental studies of excitation and non-linear evolution of kinetic instabilities of strongly nonequlibrium plasmas confined in open magnetic traps suggest new opportunities for development of electron cyclotron resonance (ECR) ion sources. These devices are widely used for the production of particle beams of high charge state ions. Operating the ion source in controlled turbulence regime allows increasing the absorbed power density and therefore the volumetric plasma energy content in the dense part of the discharge surrounded by the ECR surface, which leads to enhanced beam currents of high charge state ions. We report experiments at the ECR ion source at the JYFL accelerator laboratory, in which adopting of a new approach allows to increase the multicharged ion beam current up to two times, e.g. to 95 µA of achieved with mere 280 W power at 11.56 GHz. A theoretical model supporting and explaining the experimental findings is presented. The study suggests that the controlled turbulence regime has the potential to enhance the beam currents of modern high-performance ion sources, including state-of-the-art superconducting devices.

Modeling of plasma density, argon ion energy and ion velocity functions in a dipolar electron cyclotron resonance plasma source

Improving the surface feature precision of semiconductor devices largely depends on the ion distribution functions, which are non-Maxwellian and varying in processing plasmas spatially. In this work, a hybrid plasma model (HPM) is developed for an Ar discharge of dipolar electron cyclotron resonance (ECR) to explore the characteristics of ion energy distributions (IEDs), ion angular distributions (IADs) and ion velocity distributions (IVDs). The hybrid model contains a fluid model that determines the electric space characteristics of plasmas, as well as a Particle in Cell/Monte Carlo (PIC/MC) model that simulates ion distributions in consideration of the collisions between ions and neutrals. The influences of the pressure, magnet current, and position on the ion distributions are also discussed. The simulation results reveal the features of IEDs which transforms from a single energy peak into a bimodal distribution with the increasing pressure. IADs have a significant peak at a small angle, and most ions strike to the substrate surface with an angle less than 10°. Increasing the magnet current contributes to broader IEDs yet does not significantly alter neither IADs nor IVDs. The sensitive position dependence of ion distributions reveals considerable anisotropy of ions on the ECR surface.

On the Spatial Uniformity of the Degree of Ionization in a Helium ECR Plasma Produced under a Simple Cusp Field

Production of a plasma that has a large degree of ionization (DOI), volume, and spatial and temporal uniformities is a challenge for the improvement of the performance of plasma-based vapor deposition processes. As a potential candidate for the discharge, we investigate plasma parameters arising in helium electron cyclotron resonance (ECR) discharges due to a simple cusp field. Two-dimensional distributions of helium atom emission-line intensities were measured using spectroscopy with multiple viewing chords and then de-convoluted by Abel inversion. The local plasma parameters, including the atomic density, were evaluated using collisional-radiative model analysis. The DOI calculated from the electron and atomic densities reached up to 35% and, in most of the region inside the ECR surface, it was more than 10%.

Spectroscopic measurement of the degree of ionization in a helium electron cyclotron resonance discharge in a simple cusp field

For an electron cyclotron resonance (ECR) discharge, a simple cusp field can improve electron confinement and enhance the degree of ionization (DOI) without sacrificing accessibility to the plasma. In this study, the spatial distribution of the DOI is experimentally revealed in a helium plasma produced with widely used 2.45 GHz and 800 W microwaves. The DOI is evaluated from the electron density and ground state atom density measured using HeI emission line intensities and by collisional-radiative model analysis. It is found that the DOI increases to more than 15% within a reasonably large volume surrounded by the ECR surface and locally reaches as high as 25%.

Measurement of linear polarization in the HeI 21P–31D emission line in an electron cyclotron resonance discharge plasma

The magnitude and direction of linear polarization in the HeI 21P–31D emission line are simultaneously measured using a beam-splitting-type polarization spectroscopy system. The polarization measured in the steady-state phase of an electron cyclotron resonance (ECR) discharge produced in a simple cusp field is directed closely perpendicular to the magnetic field at the ECR surface, and the degree of polarization increases up to 8% when the neutral pressure is reduced to less than 23 mPa. These results suggest that the cause of the polarization is attributable to impact excitation by high-energy electrons produced at the ECR surface.

The Features of Refraction of Electromagnetic Waves Near the Electron Cyclotron Resonance Surface in an Open Magnetic Trap

Some features of the electromagnetic wave propagation near the electron cyclotron resonance region in axisymmetric magnetic traps are studied both analytically and numerically. The ray paths are obtained with allowance for the plasma density inhomogeneities, magnetic field direction, and magnetic field strength. Criteria for the limiting cases of constructive refraction (the ray paths are attracted to the trap axis, where effective absorption takes place) and destructive refraction (the ray paths are expelled from the trap axis, and efficient heating of the central part of the plasma column is not possible) are found. It is shown that the inhomogeneities of the magnetic field direction and plasma density, which are typical of axisymmetric magnetic traps, as well as the spatial dispersion effects, can strongly affect the electromagnetic wave propagation near the electron cyclotron resonance surface.

Quasi-steady carbon plasma source for neutral beam injector

Carbon plasma is successfully sustained during 1000 s without any carrier gas in the bucket type ionization chamber with cusp magnetic field. Every several seconds, seed plasmas having ∼3 ms duration time are injected into the ionization chamber by a shunting arch plasma gun. The weakly ionized carbon plasma ejected from the shunting arch is also ionized by 2.45 GHz microwave at the electron cyclotron resonance surface and the plasma can be sustained even in the interval of gun discharges. Control of the gun discharge interval allows to keep high pressure and to sustain the plasma for long duration.

Electron Cyclotron Resonance Near the Axis of a Mirror Trap

Propagation of an extraordinary electromagnetic wave in the vicinity of electron cyclotron resonance surface in an open linear trap is studied analytically, taking into account inhomogeneity of the magnetic field in paraxial approximation. Ray trajectories are derived from a reduced dispersion equation that makes it possible to avoid the difficulty associated with a transition from large propagation angles to the case of strictly longitudinal propagation. As an example, results of general theory are applied to the electron cyclotron resonance heating experiment which is under preparation on the Gas Dynamic Trap in the Budker Institute of Nuclear Physics.

Electron cyclotron resonance near the axis of the gas-dynamic trap

Propagation of an extraordinary electromagnetic wave in the vicinity of electron cyclotron resonance surface in an open linear trap is studied analytically, taking into account inhomogeneity of the magnetic field in paraxial approximation. Ray trajectories are derived from a reduced dispersion equation that makes it possible to avoid the difficulty associated with a transition from large propagation angles to the case of strictly longitudinal propagation. Our approach is based on the theory, originally developed by Zvonkov and Timofeev [Sov. J. Plasma Phys. 14, 743 (1988)], who used the paraxial approximation for the magnetic field strength, but did not consider the slope of the magnetic field lines, which led to considerable error, as has been recently noted by Gospodchikov and Smolyakova [Plasma Phys. Rep. 37, 768–774 (2011)]. We have found ray trajectories in analytic form and demonstrated that the inhomogeneity of both the magnetic field strength and the field direction can qualitatively change the picture of wave propagation and significantly affect the efficiency of electron cyclotron heating of a plasma in a linear magnetic trap. Analysis of the ray trajectories has revealed a criterion for the resonance point on the axis of the trap to be an attractor for the ray trajectories. It is also shown that a family of ray trajectories can still reach the resonance point on the axis if the latter generally repels the ray trajectories. As an example, results of general theory are applied to the electron cyclotron resonance heating experiment which is under preparation on the gas dynamic trap in the Budker Institute of Nuclear Physics [Shalashov et al., Phys. Plasmas 19, 052503 (2012)].

Ray trajectories near the electron cyclotron resonance surface in an axisymmetric magnetic trap

Characteristic features of the propagation of electromagnetic electron cyclotron waves in the vicinity of the electron cyclotron resonance surface are investigated both analytically and numerically with allowance for variation in the magnetic field strength and a corresponding variation in the magnetic field direction. It is demonstrated that variation in the magnetic field direction can qualitatively change the wave propagation pattern and can markedly affect the efficiency of electron cyclotron resonance plasma heating in an axisymmetric magnetic trap.

Influence of Magnetic Field Directions Inhomogeneity on Longitudinal Propagation of Wave Beams

In present communication analytical and numerical investigations of electromagnetic waves propagation near electron cyclotron resonance surface are presented taking into account variation of both magnetic field and magnetic field direction. Differences from analytical solution for constant magnetic field direction are revealed.

Electron cyclotron resonance breakdown studies in a linear plasma system

Electron cyclotron resonance (ECR) plasma breakdown is studied in a small linear cylindrical system with four different gases — hydrogen, helium, argon and nitrogen. Microwave power in the experimental system is delivered by a magnetron at 2.45 ± 0.02 GHz in TE10 mode and launched radially to have extra-ordinary (X) wave in plasma. The axial magnetic field required for ECR in the system is such that the fundamental ECR surface (B = 875.0 G) resides at the geometrical centre of the plasma system. ECR breakdown parameters such as plasma delay time and plasma decay time from plasma density measurements are carried out at the centre using a Langmuir probe. The operating parameters such as working gas pressure (1 × 10−5−1× 10−2 mbar) and input microwave power (160–800 W) are varied and the corresponding effect on the breakdown parameters is studied. The experimental results obtained are presented in this paper.

Electron Bernstein waves in the mid-plane region of a spherical tokamak

The propagation of electron Bernstein waves (EBWs) close to the tokamak mid-plane is considered. It has been shown that in the case of a concave electron cyclotron resonance (ECR) surface, typical for an ECR located between the upper hybrid resonance layer and the discharge axis, there is a plasma waveguide for EBWs, inhomogeneous in the poloidal direction and slowly varying in the radial direction, where the waves propagate in the form of discrete eigenmodes. Since the WKB theory is inapplicable to small mode number eigenfunctions, this means that the ray method is inapplicable to a part of the waves. To investigate this effect, a solution to the wave equation is found using a generalization of the parabolic equation method. For the Gaussian initial condition used, the solution is often written explicitly. Close to the ECR it has the form of an adiabatic eigenmode superposition. For illustration, the eigenmode spectrum for the case of the Gaussian beam strongly elongated in toroidal direction is investigated as a function of the initial poloidal beam width. For the usual experimental poloidal beam width of several centimetres, the eigenmode spectrum is rather wide; nonetheless, about half of the launched RF power is deposited into non-WKB modes. With the initial beam width diminishing, this share increases rapidly. The damping of the non-WKB modes is analysed in the relativistic approximation. In contrast with ‘relativistic’ rays, these modes lose their energy before reaching the relativistic ECR layer, in the region where damping is weak, so that the power deposition profile is rather smooth. In the case of a convex ECR surface there is a ‘potential hump’ in the mid-plane region and the EBW behaviour is adequately described by the ray method.

Electron Bernstein wave generation in a linear plasma system

Electron Bernstein wave (EBW) generation via mode conversion is observed in a short cylindrical electron cyclotron resonance (ECR) plasma system. ECR plasma with input microwave power varying from 160 to 800 W is produced at an operating pressure of 10−3 mbar with hydrogen gas with a maximum plasma density of 3.1×1010 cm−3 at the center and plasma temperature of 10 eV. The axial magnetic field required for ECR in the system is produced by two identical magnetic coils arranged in the Helmholtz configuration. The fundamental ECR surface (B=875 G) is located at the geometrical center of the plasma system. Microwaves of 2.45±0.02 GHz from a magnetron in TE10 mode are launched radially to have extraordinary (X) wave in plasma. The EBW is observed to be generated at the upper hybrid resonance layer at a distance of 2 cm from the center towards the microwave source. The EBW is assumed to be generated through X-B mode conversion which is inferred by the observation of the parametric decay of the incident wave. The experimental results pertaining to the EBW detection and characterization are discussed in this paper.

Observation of parametric decay spectrum in a cylindrical electron cyclotron resonance plasma system

Nonlinear wave coupling phenomena in an electron cyclotron resonance (ECR) produced plasma are observed. Experiments are carried out in a small linear cylindrical system with hydrogen plasma. Microwave power in the system is delivered by a magnetron at 2.45 GHz in TE10 mode and is launched radially to have extraordinary (X) wave in plasma. The axial magnetic field configuration, required for ECR to occur in the system, is such that the first ECR surface (B=875 G) resides at the center of the system. Parametric decay spectrum of the launched electromagnetic wave is observed at the upper hybrid resonance (UHR) layer with the appearance of sidebands on the higher and lower side of the launched frequency. Experiments show that the appearance of parametric decay spectrum has a threshold on input microwave power. The decay spectrum is observed only at the UHR surface indicating strong spatial dependence. The parametric decay is also verified in argon and helium plasmas.

Confinement of Ions in Sources Based on Electron Cyclotron Resonance Phenomenon

The problem of the multicharged ion generation in the sources based on the electron cyclotron resonance phenomenon is discussed. Such important peculiarities as the existence of three electron groups and the closeness of the electron cyclotron resonance surface in magnetic fields of “minimum B” type are taken into consideration. The degree of electrical neutrality of plasma and the depth of a potential well for ions of different charge states are estimated. Consideration is given to conditions necessary for a negative potential well formation in the core plasma. It is shown that the effective temperatures as well as the life times of ions with different charges are distinct and moreover, they are functions of the ion charge state distribution.

Electron cyclotron resonance plasma ion source for material depositions

An electron cyclotron resonance (ECR) plasma has been used in conjunction with solid metal targets for deposition of various metal and oxide compounds. A large microwave system provides dense and high-quality films at low gas pressures with a plasma generation independent on the sputtering process. The 2.45 GHz microwave power is introduced in the reactor via two separate rectangular waveguides and the plasma cross section is 20×5 cm2 in area. High plasma densities over 4×1011 cm−3 were measured by interferometry. An iron deposition rate of 2000 Å/mn has been achieved. Transparent aluminum oxide Al2O3 films with a refractive index of 1.57 were obtained under a deposition rate of 150 Å/mn. The association of several rectangular plasma chambers similar to the elementary plasma source used in this work is discussed. Direct metal ion production was also investigated in such a waveguide plasma source. Metal vaporization and ionization have been provided by energetic electrons from the ECR surface. Thus, a Zn+ ion current density of 30 mA/cm2 has been reached without support gas.

Development of a 2.45 GHz electron cyclotron resonance ion source for ion-implanter application

A compact electron cyclotron resonance (ECR) ion source has been designed at 2.45 GHz microwave frequency for ion implanter and is being tested. The design is based on a straightforward application of the method adopted for conventional ECR ion sources for multiply charged ions: a closed ECR surface completely contained in the plasma chamber. Initial tests have shown a promising indication: a total current of about 7 mA of Ar ions has been extracted at 25 kV extraction voltage. Design features and preliminary performance are reported.

Generation of large-area disk-shaped ECR plasma for diamond deposition

An electron cyclotron resonance (ECR) plasma source 16 cm in diameter using a planar ring-cusp magnetic field and a reentrant coaxial cavity was developed. The planar ring-cusp magnetic field produces a large-area ECR surface, and the reentrant coaxial cavity efficiently introduces microwaves into the ECR surface. Large-area, uniform and stable plasmas were generated with a mixture of H 2+CH 4+O 2 for diamond synthesis. The film deposited by means of the plasma CVD method was made of particles with high-quality crystal habit and with the typical diamond Raman spectrum.

First operation of HiECR-3 ion source for 14–18 GHz

The new HiECR-3 ion source has been designed to have a mirror field and a hexapole field strong enough for both 14 and 18 GHz operation. We designed, manufactured, and operated the HiECR-3, for studying characteristics of volume and surface dependence on the electron cyclotron resonance (ECR) plasma zone. From these measurements of the ECR plasma, we examined many problems. They are the ECR plasma zone, the extraction efficiency from the plasma zone, electron injection from another ECR plasma zone, and volume and surface of the ECR plasma zone.