Electron Cyclotron Resonance Layer Research Articles

Experimental optimization in ion source configuration of a miniature electron cyclotron resonance ion thruster

A miniature ion thruster has been proposed in recent years for a small propulsion system applied in space missions such as deep space exploration, precise high-stability attitude and position control. An electron cyclotron resonance (ECR) ion thruster is free from contamination and degradation of electron emission capacity and will offer a potentially longer thruster lifetime than that in the electron bombardment type. The microwave ECR ion source with a 20-mm diameter designed here consists of two annular permanent magnets (SmCo), ring coupling antenna and a grid system including screen and acceleration. For the ion source performance optimization, with a fixed magnetic structure, the antenna position and cavity length in the discharge chamber can be adjusted to strengthen electron ECR heating and increase ion beam extraction. According to the distribution of static magnetic field and the ECR layer measured by Gauss meter, three possible sizes of antenna position (L1) are set; depending on the cut-off characteristics of the discharge chamber and the distribution of microwave electric field calculated by finite element method, six candidate sizes of cavity length (L2) are set. By comparing the difference in plasma discharge and ion beam extraction, the optimal structure of ion source can be obtained. Experimental results show that for a given antenna position, there is a cavity length not too long or too short to extract the maximum ion beam. And the launch of microwave from strong magnetic field near ECR layer is conductive to lossless wave propagation in plasma and highly efficient electron ECR heating. To maintain a plasma in very low power and flow, the size combination of 0.6-mm in L1 and 5-mm in L2 is selected as the preferred structure. The performances of miniature ECR ion source, that is, ion beam current, discharge loss, propellant utilization efficiency, thrust and specific impulse are 5.4 mA, 389 W/A, 15%, 163 μup N and 1051 s, respectively, at an incident power of 2.1 W and argon flow of 14.9 μg/s.

Non-linear effects in electron cyclotron current drive applied for the stabilization of neoclassical tearing modes

Due to the smallness of the volumes associated with the flux surfaces around the O-point of a magnetic island, the electron cyclotron power density applied inside the island for the stabilization of neoclassical tearing modes (NTMs) can exceed the threshold for non-linear effects as derived previously by Harvey et al (1989 Phys. Rev. Lett. 62 426). We study the non-linear electron cyclotron current drive (ECCD) efficiency through bounce-averaged, quasi-linear Fokker–Planck calculations in the magnetic geometry as created by the islands. The calculations are performed for the parameters of a typical NTM stabilization experiment on ASDEX Upgrade. A particular feature of these experiments is that the rays of the EC wave beam propagate tangential to the flux surfaces in the power deposition region. The calculations show significant non-linear effects on the ECCD efficiency, when the ECCD power is increased from its experimental value of 1 MW to a larger value of 4 MW. The nonlinear effects are largest in the case of locked islands or when the magnetic island rotation period is longer than the collisional time scale. The non-linear effects result in an overall reduction of the current drive efficiency for this case with absorption of the EC power on the low-field side of the electron cyclotron resonance layer. As a consequence of the non-linear effects, also the stabilizing effect of the ECCD on the island is reduced from linear expectations.

Electron cyclotron resonance near the axis of a quadrupole linear trap

The quasi-longitudinal propagation of an extraordinary electromagnetic wave in the vicinity of the electron cyclotron resonance layer in an open linear trap with a quadrupole magnetic field is studied analytically, taking into account the inhomogeneity of the magnetic field in a paraxial approximation. The ray trajectories are derived from a simplified dispersion equation, that is, nonetheless able to accurately describe the transition from finite to zero perpendicular refractive index. A criterion for an on-axis resonance point to be an attractor for the ray trajectories is formulated, which generalizes a similar criterion for axisymmetric linear traps derived in a recent paper [D. S. Bagulov and I. A. Kotelnikov, Phys. Plasmas 19, 082502 (2012)].

Measurement of the Electron Bernstein Wave Emission with One of the Power Transmission Lines for ECH in LHD

Possibility of the measurement of radiated waves derived from the thermally emitted electron Bernstein wave (EBW) is numerically investigated based on the assumption of the super dense core (SDC) plasma generated in LHD. EBW that is thermally emitted in the electron cyclotron resonance (ECR) layer may couple with the electromagnetic wave and be emitted to the vacuum via the EBW-extraordinary-ordinary (B-X-O) mode conversion process. We consider the use of one of the transmission lines for electron cyclotron heating (ECH) in LHD as a receiving system of the emission. It is derived that the waves in the fundamental cyclotron frequency range are emitted as the EBW near their upper hybrid resonance (UHR) layer outside the last close flux surface (LCFS). On the other hand, waves in the second harmonics cyclotron frequency range are emitted in the core region. It means that successful measurement of waves of the second harmonic frequency range emitted from extremely high dense core plasma with setting an aim angle for receiving indicates a possibility of the second harmonic ECH by EBW in the core region with setting the same aim angle and the same polarization for launching.

Current circulation and equilibrium in toroidal electron cyclotron resonance (ECR) plasmas in the LATE device

In toroidal plasmas immersed in a toroidal field of Bϕ, the electrons drift downward, while the ions drift upward due to the field gradient and curvature. The electrons that have reached the bottom wall return through the conducting vessel to the top, on which they recombine with the ions, completing the current circulation. Helical field lines by the superposition of a vertical field BZ provide another return path of internal current circulation, along which the electrons flow toroidally, generating a toroidal current. These conjectures have been examined on electron cyclotron resonance (ECR) plasmas in the Low Aspect ratio Torus Experiment (LATE) device with current-collecting electrodes at the top and bottom of the vacuum chamber. Upon the blocking of the external return path, the discharges terminate when BZ = 0, while they survive with a toroidal current when BZ is applied, showing that current circulation is vital to maintain the discharge. Detailed studies reveal the characteristics of current circulation and equilibrium. When BZ = 0, the electron pressure profile is a uniform vertical ridge along the ECR layer in accordance with the electron vertical drift current circulating via the external circuit, while an upwardly shifted potential hill arises, providing the ions E × B drift paths around the potential peak to the vicinity of the top electrode. When BZ is applied, the central electron pressure rises with an upwardly shifted peak, being relaxed from the vertical uniformity by addition of the internal return circuit. The space potential VS decreases and the electron density obeys the Boltzmann law under VS. The vertical and toroidal currents are found to be equilibrium currents driven by the base and excess portions of electron pressure, respectively, to ensure the radial force balance of the plasma torus.

ECCD calculations in ITER by means of the quasi-optical code

Electron cyclotron current drive (ECCD) calculations for the case of the ITER electron cyclotron resonant heating upper port launcher are presented making use of a quasi-optical (QO) code (Balakin et al 2008 Nucl. Fusion 48 065003). The QO code describes accurately the behaviour of the wave beam in the electron cyclotron resonance layer, taking into account spatial inhomogeneity and dispersion. The ECCD efficiency is obtained using the adjoint calculation as presented by (Lin-Liu et al 2003 Phys. Plasmas 10 4064). The results show a broadening of the EC driven current density profiles in the range 15–30% as compared with beam-tracing calculations using TORBEAM. Consistently, peak driven current density values are found to be decreased by 10–20%. These results have significant consequences for the determination of the requirements on ECCD power to control magnetohydrodynamic instabilities such as neoclassical tearing modes and sawteeth.

Measurements of electron density and temperature in a miniature microwave discharge ion thruster using laser Thomson scattering technique

In order to improve the thrust performance of a miniature microwave discharge ion thruster, the relationship between electron number density/temperature and operational conditions, mass flow rate, incident microwave power and magnetic field strength were measured by means of laser Thomson scattering. A photon counting method and a triple grating spectrometer were used against a small Thomson scattering signal and a strong stray laser light. Electron number density increased with incident microwave power and was saturated at critical incident microwave power; it was about 1.2 × 1018 m−3 at incident microwave power >8 W. In addition, electron number density increased with mass flow rate and became saturated; it was about 1.7 × 1018 m−3 at mass flow rate > 0.04 mg s−1. The electron number density gradually increased with an increase in the number of magnets, i.e. magnetic field strength. There was a sudden jump at thirteen magnets, although the thruster failed to ignite at fourteen magnets. This is because there is an optimum distance between the antenna and the electron cyclotron resonance layer. These results suggest that future improvement in thrust efficiency in miniature microwave discharge ion thrusters may come from the fine adjustment of the magnetic field configuration inside the discharge chamber.

Electron Bernstein wave heating via the slow X–B mode conversion process with direct launching from the high field side in LHD

In the Large Helical Device (LHD), direct oblique launching of the fundamental extraordinary (X-) mode from the high magnetic field side (HFS) is available without installation of any additional launching equipment on the inner side wall of the torus. In the experiment, power absorption was observed in two separated regions by the X-mode launching. The central electron density was about 8% of the cutoff density. The result of numerical analysis with the ray-tracing calculation suggests that most power of the launched X-mode is damped out as the X-mode in the fundamental electron cyclotron resonance (ECR) layer before it reaches the upper hybrid resonance (UHR) layer where the electron Bernstein wave (EBW) occurs which is excited via the slow X (SX-) B mode conversion process. Only about 0.2% of the launched power is mode converted to the EBW and is then absorbed at a maximum. One of the two separated power absorption regions observed in the experiment agrees well with the power absorption region of the X-mode suggested by the ray tracing. The other one agrees well with that of the O-mode despite the setting of the X-mode launching. Supposedly mixed waves of the X- and the O-mode might be launched in the experiment. We assumed that the incident transverse electromagnetic waves in vacuum couple with the electromagnetic modes in the plasma at the last closed flux surface (LCFS). However, the coupling point was supposedly located outside the LCFS.For the few rays that can reach the UHR layer we have recognized that the parallel component of the refractive index N∥ becomes close to zero and power absorption as the X-mode weakens when the rays pass through the fundamental ECR layer. A numerical investigation assuming a higher central electron density, that is 23% of the cutoff density, suggests a scenario of effective EBW excitation. The ray that passes through the centre of the focused Gaussian beam launched from the HFS so that N∥ becomes close to zero near the ECR layer, Z can reach the UHR layer without being damped out and excite EBW. About 71% of power of the launched X-mode is mode converted to the EBW and absorbed in the Doppler-shifted ECR layer.Observation of the parametric decay waves suggests that the decay wave was excited in the ‘exterior’ UHR layer which is located outside the LCFS before the launched X-mode reaches the HFS. The combination of tunnelling, reflection and mode-coupling processes, the so-called ‘Budden's problem’, is suggested to occur in the evanescent region between the ‘exterior’ right handed cyclotron cutoff and the UHR layer outside the LCFS. The tunnelling rate should be considered for estimating the power that can penetrate inside the LCFS from the HFS, in particular in the higher density regime where the excitation of the EBW is expected.

Investigation of Experimental Configuration for Electron Bernstein Wave Heating on LHD

Investigation of experimental configuration for the electron Bernstein wave (EBW) heating by using the existing electron cyclotron heating (ECH) antennas on LHD was performed. By using an antenna installed in the lower port, direct oblique launching of the extraordinary (X-) mode from the high magnetic field side (HFS) is available. Since the parallel component of the refractive index (N||) varies during propagation because of the inhomogeneity of the magnetic field, N|| can be zero when the launched X-mode crosses the fundamental electron cyclotron resonance (ECR) layer even N|| is noonzero initially. In such a condition, if the electron density is above a certain level the obliquely launched X-mode can pass the fundamental ECR layer without being damped out and can be mode-converted to EBW that is absorbed at the Doppler shifted ECR layer. By using an antenna installed in the horizontal port, oblique launching from the lower magnetic field side (LFS) toward the over-dense plasma is available. Excitation of EBW via the mode conversion process of ordinary mode(O)-extraordinary mode(X)-electron Bernstein wave (B) is expected with the O-mode launching toward an appropriate direction. The O-X-B mode conversion rate and the region of power deposition were surveyed by varying the magnetic field strength and the launching direction. The results of the survey suggest that efficient heating in the core region is difficult by using the existing antenna. Rearrangement of the final mirror of the launching antenna may be needed.

Two Dimensional Li Beam Imaging to Study the Magnetic Field Configuration Effects on Plasma Confinement in Spherical Tokamak CPD

Two dimensional lithium beam imaging technique has been applied in the spherical tokamak, CPD (Compact Plasma wall interaction experimental Device), to study the effects of various magnetic field configurations on RF plasma confinement topology. The performance of the lithium sheet beam is absolutely calibrated by a quartz crystal monitor. Experimental results show that plasma initiation takes place at the electron cyclotron resonance layer in a simple torus configuration and then it expands quickly to the low magnetic field side. Different magnetic field configurations critically affect the RF plasma confinement topology. A sharp lower boundary exists for the RF plasma in magnetic null configuration. Magnetic connection length plays the key role in defining plasma boundary and the critical value of connection length for plasma to exist in CPD is found to be ∼ 5-6 meter for a given pressure condition.

均一・高密度プラズマ生成アンテナを用いたマイクロ波放電型イオンスラスタの高性能化

The dependency of thrust performance on thruster configurations such as antenna length, antenna height, number of antenna, magnetic field configuration, and microwave frequency, was investigated with the objective of improving the thrust performance of microwave discharge ion thruster using antennas for uniform and dense plasma production. The experimental results showed that there was an optimum length of the antennas, and it was 3/4 times the wave length of incident microwaves. The ion beam current reaches its maximum value when the antenna was set at 2mm downstream of an electron cyclotron resonance layer. There was an optimum number of the antennas. This is due to the tradeoff between the coupling of plasma with microwave and the surface recombination on the antenna. The expansion of ionization zone was made successfully by changing magnetic field configuration. In addition, the thrust performance was slightly improved with increase in incident microwave frequency from 2.45GHz to 4.2GHz. A value for the ion beam current with 2.45GHz is compensated by high electron number density and less magnetized ions for the disadvantage of small plasma number density. Overall, the propellant utilization efficiency, ion production cost, and estimated thrust were found to be 0.62, 300W/A and 6.2mN, respectively at mass flow rate of 0.22mg/s for xenon, ion beam voltage of 1,500V and 2.45GHz microwave incident power at 32W.

First Observation of ECH by Electron Bernstein Waves Excited via X-B Mode Conversion Process in LHD

In a magnetic field configuration of the Large Helical Device (LHD), when the extraordinary mode (X-mode) waves are obliquely injected from a bottom antenna, it can directly access the upper hybrid resonance (UHR) layer from the high field side and excite electron Bernstein waves (EBWs) without the need for any additional reflecting mirror antenna. A localized power absorption is observed in the low field side of the electron cyclotron resonance (ECR) layer. This result suggests electron cyclotron heating (ECH) by mode converted EBWs excited via X-B mode conversion process.

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.

内部導体装置Ｍｉｎｉ−ＲＴにおけるプラズマ生成と高温超伝導コイルの磁気浮上実験

Plasma is produced using a 2.45 GHz microwave in a Mini-RT internal coil device, where a high-temperature superconductor is employed. The radius of the internal coil is 0.15 m and its weight is 16.8 kg. The maximum coil current is 50 kA, and the typical magnetic field strength at the internal coil is 0.1 T. First plasma experiments were carried out using a mechanically supported coil, and a typical plasma density in the range of 1016 m-3 was produced. When the internal coil current is decreased to less than 10 kA, plasma cannot be produced because of the disappearance of the electron cyclotron resonance layer of the 2.45 GHz microwave. Concerning to the levitation experiments of the internal coil, the coil position is monitored with laser sensors, and the levitation coil current is feedback-controlled. The HTS coil has been levitated for a period of one hour with an accuracy of 10 micrometers. Plasma production with a floating coil has been successfully initiated.

Doppler-shifted cyclotron absorption of electron Bernstein waves via N( parallel)-upshift in a Tokamak plasma.

Extraordinary (X) waves are perpendicularly injected for electron Bernstein (B) wave heating into an Ohmically heated plasma from the inboard side in the WT-3 tokamak. Measurements show that absorption does not take place at the electron cyclotron resonance layer nor the upper hybrid resonance layer, but does happen midway between them. This is consistent with the ray tracing prediction, i.e., the poloidal field and poloidal inhomogeneity of toroidal field lead the B waves to have a large parallel refractive index N( parallel) (>1), and the B waves are damped away via the Doppler-shifted cyclotron resonance.

Electron Bernstein wave electron temperature profile diagnostic (invited)

Electron cyclotron emission (ECE) has been employed as a standard electron temperature profile diagnostic on many tokamaks and stellarators, but most magnetically confined plasma devices cannot take advantage of standard ECE diagnostics to measure temperature. They are either “overdense,” operating at high density relative to the magnetic field (e.g., ωpe≫Ωce in a spherical torus) or they have insufficient density and temperature to reach the blackbody condition (τ>2). Electron Bernstein waves (EBWs) are electrostatic waves that can propagate in overdense plasmas and have a high optical thickness at the electron cyclotron resonance layers as a result of their large kperp. In this article we report on measurements of EBW emission on the CDX-U spherical torus, where B0∼2 kG, 〈ne〉∼1013 cm−3 and Te≈10–200 eV. Results are presented for electromagnetic measurements of EBW emission, mode converted near the plasma edge. The EBW emission was absolutely calibrated and compared to the electron temperature profile measured by a multipoint Thomson scattering diagnostic. Depending on the plasma conditions, the mode-converted EBW radiation temperature was found to be ⩽Te and the emission source was determined to be radially localized at the electron cyclotron resonance layer. A Langmuir triple probe and a 140 GHz interferometer were employed to measure changes in the edge density profile in the vicinity of the upper hybrid resonance where the mode conversion of the EBWs is expected to occur. Initial results suggest EBW emission and EBW heating are viable concepts for plasmas where ωpe≫Ωce.

Strong narrow-band electron cyclotron emission from a mirror plasma heated by electron cyclotron waves

Strong electromagnetic radiation is observed in a narrow band slightly above the frequency of a heating wave that is absorbed by electrons near the electron cyclotron resonance layer in a magnetic beach. The frequency spectrum consists of a sharp component and a broad background, which is enhanced by more than 30 dB and 10 dB, respectively, above the radiation of thermal electrons. This observation is explained in terms of cyclotron radiation emitted by electrons, localized in a magnetic mirror, that are resonantly heated and bunched by a strong monochromatic wave.

Suppression of a pressure driven m=1 mode in a lower hybrid current drive plasma by electron cyclotron heating in the WT-3 tokamak

A pressure driven m=1/n=1 mode is excited by lower hybrid current drive in the WT-3 tokamak [T. Maehara et al., Nucl. Fusion 38, 39 (1998)]. The excitation of the mode is accompanied with the decrease of the magnetic shear and with the peaking of the soft x-ray emissivity profile inside the q=1 surface. The crescent-shaped mode structure appeared on the contour map of the soft x-ray emissivity is consistent with that of the quasi-interchange mode. The m=1 mode can be suppressed by electron cyclotron heating near the q=1 surface. The range of the location of the electron cyclotron resonance layer effective for the complete suppression is much wider and the time scale for the suppression is much faster than those in the case of the suppression of the tearing mode in the ohmic heating plasma.

Wave propagation through an electron cyclotron resonance layer

The propagation of a wave beam through an electron cyclotron resonance layer is analysed in two-dimensional slab geometry in order to assess the deviation from cold plasma propagation due to resonant, warm plasma changes in wave dispersion. For quasi-perpendicular propagation, , an O-mode beam is shown to exhibit a strong wiggle in the trajectory of the centre of the beam when passing through the fundamental electron cyclotron resonance. The effects are largest for low temperatures and close to perpendicular propagation. Predictions from standard dielectric wave energy fluxes are inconsistent with the trajectory of the beam. Qualitatively identical results are obtained for the X-mode second harmonic. In contrast, the X-mode at the fundamental resonance shows significant deviations from cold plasma propagation only for strongly oblique propagation and/or high temperatures. On the basis of the obtained results a practical suggestion is made for ray tracing near electron cyclotron resonance.

Mechanism of selective SiO2/Si etching with fluorocarbon gases (CF4, C4F8) and hydrogen mixture in electron cyclotron resonance plasma etching system

A mixture of fluorocarbon gases having high C/F ratio is commonly used to obtain the high selective etching of SiO2 to Si in high-density plasmas. We investigated the etch rate of SiO2 and Si in an electron cyclotron resonance (ECR) plasma etching system as functions of gas pressure, gap distance between the ECR layer and the substrate, and hydrogen percentage in fluorocarbon gases such as CF4 and C4F8. The selectivity increases considerably in the case of the C4F8 plasma as the gas pressure, hydrogen percentage, and the gap distance increase. Actinometric optical emission spectroscopy (OES) and appearance mass spectrometry (AMS) were employed to study the mechanism of selective SiO2 etching. Ar was used as the actinometer, and the behavior of the F atom and various fluorocarbon radicals was investigated for CF4 and C4F8 ECR plasmas. The following results were obtained. First, the CF2 radical behavior investigated by OES coincides well with that by AMS. Second, the behavior of CF2 radical density as a function of hydrogen percentage in the CF4 ECR plasma is very similar to that in the rf capacitively coupled plasma (RFCCP), but its increasing rate with H2 addition is smaller by an order of magnitude compared to using the CF4 in RFCCP system. Third, the dominant polymer precursor which is important for the selective SiO2 to Si etching is CF in the C4F8+H2 ECR plasma, and as the hydrogen percentage increases, the decreasing rate of the relative concentration of atomic F is higher in the CF4 ECR plasma than in the C4F8 ECR plasma. Therefore, it is suggested that the F abstraction reaction in gas phase process such as CF2+H→CF+HF is more dominant than the F scavenging reaction such as CF2+F+H→CF2+HF in the C4F8 ECR plasma in the case with hydrogen addition. Fourth, the densities of all the fluorocarbon radicals increase contrary to the decrease of atomic F when the pressure increases from 2.5 to 10 mTorr when the H2 addition is used. Last, the relative concentration of the F atom decreases as the distance from the ECR layer increases axially, but the fluorocarbon radicals do not show considerable changes.