Electron Cyclotron Resonance Layer Research Articles

Heating characteristic of electron Bernstein wave using high-field side X-mode injection in QUEST

Abstract A comparative investigation was conducted to assess the impact of high-field side (HFS) extraordinary (X) mode (HFS X) injection and low-field side (LFS) ordinary (O) mode (LFS O) injection of 8.2 GHz radio frequency (RF) power in the Q-shu University experimental steady-state spherical tokamak. In the case of the HFS X injection, the ratio of electron plasma frequency and RF frequency f pe / f RF was 1.3, indicating an overdense plasma, whereas in the LFS O injection, this ratio was 0.9. Distinctive features emerged in the electron temperature and density profiles between the two injection scenarios. In the HFS X injection, the profiles peaked between the fundamental electron cyclotron resonance (ECR) layer and the upper hybrid resonance layer. Conversely, in the LFS O injection, the peaks were situated near the 1st ECR layer. This implies effective excitation of the electron Bernstein wave (EBW) in the case of the HFS X injection, with the wave power being absorbed through the doppler-shifted ECR of the EBW. Density and temperature oscillations were observed only in the start-up phase of the HFS X injection, which may correlate with the presence of the left-hand cutoff of the X-mode. These findings will be contributed to understand the distinctive characteristics associated with plasma heating through EBW excitation in the HFS X injection.

Numerical simulation of ultrashort-pulse reflectometry (USPR) on EAST

The microwave reflectometer is a popular non-intrusive plasma density diagnostic instrument on tokamaks that provides centimeter and millisecond level resolution. The ultrashort-pulse reflectometer (USPR) achieves plasma density measurement by emitting a chirped wave containing a broadband signal and measuring the time of flight from different frequency components. A USPR system is currently being built on EAST (Experimental Advanced Superconducting Tokamak) to meet the needs of diagnostic of the pedestal density evolution, such as high-frequency small edge-localized modes. In order to predict the density reconstruction of the EAST USPR system, this work presents a numerical simulation study of the beam propagation of the chirped wave of extraordinary waves (X-mode) in the plasma based on Python. The electron density profile has been successfully reconstructed by the reflection signal interpretation. The small gap between the right-hand cut-off layer and the electron cyclotron resonance layer, due to the low plasma density on the plasma edge, causes unexpected leakage from the transmitting microwave beam to the pedestal and the core region. This kind of ‘tunneling’ effect will cause the reflected signal to have energy loss in the low-frequency band. The study also discusses the influence of the poloidal magnetic field on the reflected signal. The spatial variation of the poloidal magnetic field will lead to the conversion between extraordinary (X) waves and ordinary (O) waves, which leads to energy loss in the reflected signals. The simulation results show that the ‘tunneling’ effect and the O-X mode conversion effect have little effect on the EAST USPR system. Therefore, the currently designed transmit power meets the working requirements.

Study on the effect of discharge chamber length on the performance of electron cyclotron resonance ion thruster

Discharge chamber length is one of the factors in optimizing the electron cyclotron resonance ion thruster performance. It adjusts the distance between bulk plasma and grid system to change the plasma density upstream of the screen grid, which will affect the ion beam current and focusing state to achieve optimization purpose. However, new evidence shows the discharge chamber length plays an important role in ionization during ion beam extraction, which means that the effect of discharge chamber length on the performance of electron cyclotron resonance ion thruster should be reexamined. After applying grid voltages, another high electron temperature region located upstream of the screen grid is observed in the integrated simulation using particle-in-cell with Monte Carlo collision method, but it is not observed in the traditional discharge chamber simulation. It is believed in the paper that the high electron temperature region exists objectively, because the Child-Langmuir sheath will repel electrons moving towards screen grid back to magnetic mirrors again. Those electrons will gain energy from microwave, and finally form a high electron temperature region along the Child-Langmuir sheath. This phenomenon implies that discharge chamber length can adjust the high electron temperature distribution upstream of screen grid to affect the plasma generation. Therefore, in this work, the effect of discharge chamber length on discharge and ion beam performance is systematically studied by adopting the integrated simulation. In this paper, three ion thrusters with different discharge chamber lengths are simulated. Under the conditions of same magnetic field and operation parameters, the comparisons of electron energy gain, plasma parameter distributions and ion beam current among the three ion thrusters are conducted. The results show that shorter discharge chamber length has higher electron energy gain, plasma density and voltage, but smaller ion beam current. This abnormal phenomenon can also be seen experimentally. By analyzing the ionization rate inside the chamber, it can be seen that high-temperature electrons upstream of the screen grid have a significant contribution to ionization. And thus, a little bit longer discharge chamber length with lower plasma density inside the chamber has bigger ion beam current for having higher plasma density upstream of the screen grid. According to this phenomenon, an electron heating mode is proposed: electrons gain energy by reciprocating through the electron cyclotron resonance layer between the Child-Langmuir sheath and magnetic mirrors. This heating mode can be used as a supplement to the electronic constraints outside the magnetic mirrors to improve the energy utilization efficiency of the thruster, which can provide a new insight into the electron cyclotron resonance ion thruster design in the future.

First results of fast visible imaging diagnostic in Aditya-U tokamak.

A Fast Visible Imaging Diagnostic (FVID) system, measuring the visible light emission spectrum (400-1000nm) from tokamak plasma, has been installed on the Aditya-U tokamak to monitor the two-dimensional dynamics of the poloidal cross section of the plasma. In this work, we present the design and installation of the FVID system on the Aditya-U tokamak. The evolution of plasma and plasma-wall interactions is described. The signature of the runaway electron beam in visible imaging and its correlation with other diagnostics is presented. The estimation of the electron cyclotron resonance layer position during pre-ionization is also discussed in this work.

Numerical simulation of electron extraction from micro electron cyclotron resonance neutralizer under different magnetic circuits

The electron cyclotron resonance (ECR) neutralizer is an important part of the micro ECR ion thruster. The electrons extracted from the neutralizer are used to neutralize the ions extracted from the ECR ion source, thereby avoiding the surface charges accumulating on the spacecraft, and the behaviour of electron extraction affects the overall performance of the thruster. In order to investigate the electron extraction through the orifices of the micro ECR neutralizer, a two-dimensional particle-in-cell with Monte Carlo collision (PIC/MCC) model is established in this work. The effects of different magnetic circuits on the electron extraction of the neutralizer and the influence of different cavity lengths on the wall current loss are studied through numerical simulation. The effects of different magnetic circuit structures on the electron extraction and wall current loss of the neutralizer are studied. The calculation results show that the position of the ECR layer and the magnetic flux lines near the extraction orifices are very important for the electron extraction performance of the neutralizer. When the ECR layer is located upstream of the antenna, electrons are easily lost in migration and diffusion motion, and the energy required for the electrons to cross the potential well before the extraction hole is higher. If more magnetic flux lines pass parallelly through the extraction orifices, the neutralizer requires a small voltage to extract the same electron current. When the ECR layer is cut by the antenna or is located downstream of antenna, more electrons may migrate along the magnetic flux lines to the vicinity of the extraction orifices, thereby reducing the voltage of collector plate. The effects of different cavity lengths on the extraction of electrons under the same magnetic circuit structure are studied. It is found that increasing the length of the cavity allows more parallel-axis magnetic flux lines to pass through the extraction holes to avoid electron loss on the surface of the extraction plate, and thus increasing the extraction electron current. The research results conduce to designing a reasonable neutralizer magnetic circuit and cavity size.

On abnormal behaviors of ion beam extracted from electron cyclotron resonance ion thruster driven by rod antenna in cross magnetic field

In a 2.45 GHz electron cyclotron resonance (ECR) ion thruster powered with rod antenna under a cross magnetic field, abnormal behaviours such as sudden drop of ion beam current (I b) and larger increasing-rate of I b in the high microwave power (P w) discharges at high gas flow rates were observed. A differential method was proposed to reveal the changes in the radial profiles of gray values extracted from the end-view discharge images. The increasing-rate of I b with respect to P w was used to evaluate efficiencies of ion production and transport. Analyses indicate that discharges are dominantly sustained by ordinary wave via electron heating in the electron plasma resonance layer that can shift along the rod-antenna, and extraordinary wave can only ignite a discharge in the ECR layer in the low gas flow rate regime. In terms of the confinement region defined by the magnetic field lines intercepting with the screen grid, the confinement region of the optimized 2.45 GHz cross magnetic field takes the shape of hourglass, enabling the high increasing-rate of I b with respect to P w in high power discharges at high gas flow rates. Correlated with the accompanied bright boundary layer appearing in the differentiated image, the sudden drop of I b in the low gas flow rate regime is attributed to the discharge ignited by the enhanced extraordinary wave in the ECR layer neighbouring the narrowest confinement region, where the produced ions can promptly enter the loss region.

Characteristics of microwave ECR ion thruster powered with plate antenna in cross-magnetic field: Standing wave, skin effect, and mode transition

In a 2.45 GHz electron cyclotron resonance xenon ion source powered with circular plate-antenna, a rapid evolution of radial plasma-profile with discharge power in a region below 35 W and the abrupt jump of ion beam current accompanied by sudden radial-expansion of discharge glow were observed. Based on analyses about quantified glow images captured from the end-view of the discharge chamber and the ion beam current against the discharge power, the fast evolution of the radial plasma-profile is attributed to the counteraction of standing wave and the skin effect, the coincidence of enhanced microwave absorption in the electron plasma resonance layer, and the transverse magnetic confinement of electrons. The jump of ion beam current and the sudden radial-expansion of discharge glow are confirmed to be originated from the extraordinary-wave discharge at the electron cyclotron resonance layer beyond the plate antenna when the skin effect is dominant.

Topology of the warm plasma dispersion relation at the second harmonic electron cyclotron resonance layer

The warm plasma dispersion relation, for waves in the electron cyclotron resonance range of frequencies, can be cast into the form of a bi-quadratic equation for N⊥, where the coefficients are a function of N⊥2 and an iterative procedure is required to obtain a solution. However, this iterative procedure is not well understood and fails to converge toward a solution at the second harmonic resonance layer, in particular at higher densities where the wave can couple to an electron Bernstein wave. This paper focuses on a solution to the poor convergence of the iterative method, enabling determination of the topology of the dispersion relation around the second harmonic using a fully relativistic code for oblique waves. A feed-forward controller is proposed with the ability to adjust the rotation of a step of N⊥2 within the complex plane, while also limiting the step-size. It is shown that implementation of the controller stabilizes unstable solutions, while improving overall robustness of the iteration. This allows the evaluation of the coupling between the fast extraordinary mode and electron Bernstein waves at the second harmonic electron cyclotron resonance layer, for non-perpendicularly propagating waves.

Experimental study on effect of the toroidal magnetic field ripple on the ECR layer in TABAN tokamak

Abstract The study of ECR assisted breakdown on TABAN tokamak (BT = 7000 G, R = 45 cm, a = 15 cm) has been directed to find the best conditions for ECR pre-ionization and plasma generation. The toroidal magnetic field ripple ( δ ) was investigated to obtain optimum pressure at pre-ionization phase. The internal chamber can be divided into two areas considering the δ value: underneath and between the coils.The toroidal magnetic field ripple was used to determine the moment of the Electron Cyclotron Resonance (ECR) layer entrance into the chamber, and consequently, calculate the accurate breakdown time and then optimum pressure range (3 × 10−2 Pa ∼ 5 × 10−2 Pa) was obtained at pre-ionization phase. In addition, the toroidal magnetic field and its ripple have been experimentally measured using magnetic probes and simulated by COMSOL software as well.

Electron Bernstein wave conversion of high-field side injected X-modes in QUEST

This paper presents a detailed design of the Q-shu University experimental steady state spherical tokamak’s (QUEST’s) high-field side (HFS) injection system for electron Bernstein wave (EBW) excitation and the results of an experimental comparison of the HFS eXtraordinary X-mode and low-field side (LFS) ordinary O-mode injection of 8.2 GHz radio frequency (RF) power. Waveguides, as an alternative to mirror polarizers for transmitting RF X-mode power from LFS to HFS for EBW conversion, were used instead of the installation of an RF mirror. Testing of LFS-to-HFS RF power transmission at 8.2 GHz, using an RG-50-type vacuum waveguide in a bench-scale device filled with SF6 gas at 0.03 Mpa, revealed that an RF power of 10.8 kW could traverse the fundamental electron cyclotron resonance layer for 60 s without breakdown. The short-length, open-ended waveguide antenna used in the HFS injection-induced wave diffraction reduced the efficiency of power delivery to the upper hybrid resonance (UHR) by approximately 7% at an electron temperature of 50 eV. The HFS injection was able to produce brighter camera images than the standard LFS injection. The location of the UHR, as estimated by measuring the density with an interferometer, agreed with its location as measured by plasma radiation low-field, side-edge positions shown by fast camera imaging. This indicates that the plasma was produced by mode-converted EBW. The HFS injection had an absorption efficiency of 96%, compared to 40% for LFS. A greater fluctuation of floating potential adjustable to the lower hybrid wave (LHW) was observed in the HFS case by installing a Langmuir probe, confirming that EBW conversion efficiency was higher in the HFS case. Moreover, after setting the poloidal field to BPF = 7.6 mT, plasma current (IP) in the HFS peaked at 1.3 kA, as opposed to 0.3 kA for LFS, despite LFS injection having a total power of 55 kW, compared to 40 kW for HFS. However, as the impurity level was comparatively high, it is believed that this IP is dominated by pressure-drive, which makes it difficult to analyze EBWCD. Finally, the line-integrated density in the HFS injection peaked at 1.6 × 1018 m−2, compared to 8 × 1017 m−2 in the LFS one.

Mapping of power deposition zone of electron Bernstein waves externally excited in tokamak plasmas

In toroidal plasmas, electron Bernstein (EB) waves mode-converted from X waves at the upper hybrid resonance (UHR) layer propagate toward the higher field side and are cyclotron-damped away at Doppler shifted frequency before arriving at the electron cyclotron resonance (ECR) layer. Resulting power deposition profile depends on local density, temperature and field profiles and is usually analyzed using the ray tracing technique for EB wave propagation and absorption in inhomogeneous magnetized plasmas. In axisymmetric tokamak plasmas, the ray tracing equation of EB waves can be cast into the form of equation of particle motion in a potential field. The particle trajectory and its momentum correspond to the wave trajectory and the refractive index, respectively. The potential field reflects the dispersion characteristics of EB waves, that is, the perpendicular refractive index strongly increases as the waves approach to the ECR layer from the lower field side, making a deep trench of potential along the ECR layer. Therefore the EB waves perpendicularly approach the ECR layer, which enable us to estimate the parallel refractive index and the parallel velocity of resonance electrons near the ECR layer. Thus, possible power deposition zone can be mapped and the associated current drive efficiency is roughly estimated directly from the information on wave frequency and plasma profiles without ray tracing calculations (direct mapping). The mapped zone, in the case of a moderately over dense plasma, is shown to be coincident with the ray tracing results for both XB and OXB schemes of mode conversion. In the cases of highly over dense, low safety factor and low aspect ratio plasmas, ω/Ωce is rather close to two at the UHR layer and the second harmonic cyclotron damping can take place along the ray trajectory just after OXB mode conversion, depending on the parameters. Here, ω is the wave frequency and Ωce is the local cyclotron frequency. It is shown that direct mapping also works in this case as well. The direct mapping technique is based on above picture of EB wave propagation and absorption, and is useful to guide the ray tracing analyses and cross-check the ray tracing results.

28-GHz ECHCD system with beam focusing launcher on the QUEST spherical tokamak

New polarizer and launcher systems on a 28-GHz electron-cyclotron (EC) heating and current drive (HCD) system have been developed for non-inductive second-harmonic EC-plasma-current ramp-up in the QUEST spherical tokamak. A launcher system with two quasi-optical mirrors providing beam steering capability was designed to focus the incident beam to a small-sized waist of ∼0.05 m at the second-harmonic EC resonance layer. A relatively large focusing mirror was designed based on a Kirchhoff integral code developed to derive wave solutions. The focusing property of the launched beam was first confirmed with a 3-dimensional (3D) electromagnetic-wave simulator. Focusing characteristics were also checked at low-power test facilities, together with the steering capability. The performance of this launcher system was demonstrated to work as designed, and assembled in the QUEST device. The system was applied to the non-inductive second-harmonic EC plasma ramp-up experiments with no optimization required regarding the incident polarization. The results obtained for the non-inductive plasma ramp-up are also presented.

Bended probe diagnostics of the plasma characteristics within an ECR ion source with a rectangular waveguide

To reveal the argon plasma characteristics within the entire region of an electron cyclotron resonance (ECR) ion source, the plasma parameters were diagnosed using a bended Langmuir probe with the filament axis perpendicular to the diagnosing plane. Experiments indicate that, with a gas volume flow rate and incident microwave power of 4 sccm and 8.8 W, respectively, the gas was ionized to form plasma with a luminous ring. When the incident microwave power was above 27 W, the luminous ring was converted to a bright column, the dark area near its axis was narrowed, and the microwave power absorbing efficiency was increased. This indicates that there was a mode transition phenomenon in this ECR ion source when the microwave power increased. The diagnosis shows that, at an incident microwave power of 17.4 W, the diagnosed electron temperature and ion density were below 8 eV and 3 × 1017 m−3, respectively, while at incident microwave power levels of 30 W and 40 W, the maximum electron temperature and ion density were above 11 eV and 6.8 × 1017 m−3, respectively. Confined by magnetic mirrors, the higher density plasma region had a bow shape, which coincided with the magnetic field lines but deviated from the ECR layer.

Detection of anisotropy in the electron velocity distribution produced by electron cyclotron resonance heating using the polarization of helium atom emission lines

The anisotropy in the electron velocity distribution (EVD) was measured using the polarization of two helium atom emission lines, 21P–31D (668 nm) and 23P–33D (588 nm), in a helium electron cyclotron resonance (ECR) discharge plasma. A small polarization degree of less than 4% was measured by adopting a temporal modulation technique. It was found that the polarization originated locally from around the ECR layer and that the anisotropic component of the EVD produced by ECR heating had an average kinetic energy of approximately 40 eV.

Real-time control of electron cyclotron wave polarization in the LHD

Peripheral plasma with finite electron density gradients and finite magnetic shear is known to affect polarization of electron cyclotron (EC) waves. Calculation of the ratio between the ordinary (O) mode and the extraordinary (X) mode, integrated in the ray-tracing code developed for EC heated plasmas in the Large Helical Device (LHD), enables the search for the optimum EC wave polarization in order to excite the pure O/X mode at the EC resonance layer. The real-time control system of the incident polarization was developed for maximum single-pass absorption of EC waves, based on the dependence of optimum EC wave polarization on peripheral density profiles. The polarization control system is equipped with a fast field programmable gate array, which processes in real time the calculation of the peripheral electron density profile and the optimum EC wave polarization for motion control of the polarization rotator and the elliptical polarizer on the transmission line. The real-time control in LHD experiments functioned properly in maintaining the absorbed power of the EC wave higher than that without the control, demonstrating that the purer heating mode was successfully excited.

Analysis of the primary experimental results on a 5 cm diameter ECR ion thruster

An ECR ion thruster with a diameter of 5 cm has been developed and tested. Four different antenna positions were experimentally and numerically investigated, and the results suggest that the optimal location for the antenna is where it is perfectly surrounded by the electron cyclotron resonance layer. We also evaluated two different antenna configurations, and found that the star configuration is preferable to the circular configuration, and also that the circular antenna is only 40% as efficient as the star antenna. The experimental curve of the ion beam current and voltage agrees with the fitting results from the analytic solution. The simulation of the magnetic topology in the discharging chamber with different back yoke heights indicates that it needs to be further verified.

Origin and Evolution of Spontaneous Rotation in Plasma Under Different Magnetic Field Geometries in Tokamak QUEST

Spontaneous toroidal rotation of the plasma is observed in the spherical tokamak QUEST with the help of electron cyclotron resonance (ECR) heating and without the use of any externally injected momentum. Several vertical magnetic field ( $B_{z}$ ) configurations with varying mirror ratio ( $M$ ) (a measure of field curvature) are applied and evolution of rotation is studied with the help of Doppler spectroscopy of bulk and impurity ions. Significant toroidal rotation ( $V_{\phi } \sim 6$ km/s) is initiated in the open magnetic field configuration during the initial plasma breakdown phase, which is later sustained ( $V_{\phi } \sim 20$ km/s) in a closed magnetic field configuration in steady state. Rotational velocity is primarily along the cocurrent direction and is found to be proportional to the $B_{z}$ strength and the resulting plasma current. High $M$ and $B_{z}$ are demonstrated to be the two specific external controls by which rotation can be initiated in the plasma. The rotation in open field lines is found to be initiated at the ECR layer in the slablike plasma, which is evolved to produce a sustained rotation in the natural divertor inboard poloidal field null equilibrium in QUEST.

Progress of microwave collective Thomson scattering in LHD

Microwave collective Thomson scattering (CTS) by using a 77 GHz gyrotron is routinely working in LHD and the improvements of the system is now underway. The targets of this diagnostic are measurements of energetic fast ion distribution and ion ratio. In the present system, 800kW 77 GHz gyrotron is injected horizontally and scattered radiation is received changing scattering angle. The system works with existence of electron cyclotron resonance layer. Thus, most of the power is absorbed at the layer like beam damping and stray radiation dramatically decreases. Gyrotron is modulated at 40 Hz, then, background ECE, which is signal in gyrotron off phase, is subtracted from scattered signal in gyrotron on phase. The perturbation of electron temperature due to the gyrotron injection is almost negligible. Temporal evolution of CTS spectrum is obtained by 32ch filter bank receiver through discharge and fine spectrum is obtained by 10 GHz sampling fast digitizer for 80 ms. Change of the width and asymmetry of CTS spectrum is observed after turning off of tangentially injected neutral beam (NB). This is qualitatively consistent with reduction of fast ion density. Preliminary data of ion ratio between hydrogen and helium are also obtained.

High power 303 GHz gyrotron for CTS in LHD

A high-power pulsed gyrotron is under development for 300 GHz-band collective Thomson scattering (CTS) diagnostics in the Large Helical Device (LHD). High-density plasmas in the LHD require a probe wave with power exceeding 100 kW in the sub-terahertz region to obtain sufficient signal intensity and large scattering angles. At the same time, the frequency bandwidth should be less than several tens of megahertz to protect the CTS receiver using a notch filter against stray radiations. Moreover, duty cycles of ∼ 10% are desired for the time domain analysis of the CTS spectrum. At present, a 77 GHz gyrotron for electron cyclotron heating is used as a CTS wave source in the LHD. However, the use of such a low-frequency wave suffers from refraction, cutoff and absorption at the electron cyclotron resonance layer. Additionally, the signal detection is severely affected by background noise from electron cyclotron emission. To resolve those problems, high-power gyrotrons in the 300 GHz range have been developed. In this frequency range, avoiding mode competition is critical to realizing high-power and stable oscillation. A moderately over-moded cavity was investigated to isolate a desired mode from neighbouring modes. After successful tests with a prototype tube, the practical one was constructed with a cavity for TE22,2 operation mode, a triode electron gun forming intense laminar electron beams, and an internal mode convertor. We have experimentally confirmed single mode oscillation of the TE22,2 mode at the frequency of 303.3 GHz. The spectrum peak is sufficiently narrow. The output power of 290 kW has been obtained at the moment.

Cross-field potential hill arisen eccentrically in toroidal electron cyclotron resonance plasmas in the Low Aspect ratio Torus Experiment device to regulate electron and ion flows from source to boundary

We have investigated the electron and ion flows in toroidal electron cyclotron resonance (ECR) plasmas maintained by a 2.45 GHz microwave power around 1 kW under a simple toroidal field in the low aspect ratio torus experiment (LATE) device. We have found that a vertically uniform ridge of electron pressure that also constitutes the source belt of electron impact ionization is formed along just lower field side of the ECR layer and a cross-field potential hill (30 V while 10 eV), eccentrically shifted toward the corner formed by the top panel and the ECR layer, arises. Combination of the hill-driven drift and the vertical drift due to the field gradient and curvature, being referred to as vacuum toroidal field (VTF) drift, realizes steady flows of electrons and ions from the source to the boundary. In particular, the ions, of which VTF drift velocity is much slower than the electron VTF drift velocity near the source belt, are carried by the drift around the hill to the vicinity of the top panel, where the ion VTF drift is enhanced on the steep down slope of potential toward the top panel. On the other hand the electron temperature strongly decreases in this area. Thus the carrier of VTF drift current is replaced from the electrons to the ions before the top panel, enabling the current circulation through the top and bottom panels and the vessel (electrons mainly to the bottom and ions mainly to the top) that keeps the charge neutrality very high. A few percent of electrons from the source turn around the hill by 360 degree and reentry the source belt from the high field side as seed electrons for the impact ionization, keeping the discharge stable.