Cutoff Layer Research Articles

Electron density profile measurement using Ka-band microwave impulse radar reflectometer on LHD

We have installed a ten channel Ka-band ultrashort pulsed radar reflectometer system in the Large Helical Device. The main objective of this diagnostic is to provide reliable information about the edge electron density profile. The time-of-flight measurement technique is used for the delay time measurement of the reflected pulses from each cut-off layer in the plasma, in order to avoid a mixture of spurious radiation such as electron cyclotron emission and undesired reflection such as another polarization wave mode. As for the signal detection section, a filter bank system is applied; the reflected pulse shape is affected by the bandwidth of band pass filter; thus, the optimum bandwidth must be found experimentally. The reconstructed electron density profile is obtained using an Abel inversion method from the profile of the delay time as a function of the probing frequency. In density modulation experiments, the static island width can be estimated from the temporal change of the density profile.

Doppler reflectometry studies using a two-dimensional full-wave code

A two-dimensional full-wave numerical code in the extraordinary mode of propagation has been developed to simulate reflectometry. The code uses the finite-difference time-domain technique to solve wave propagation in magnetized and turbulent plasmas. The code has been used to study the viability of the Doppler reflectometry technique in determining the perpendicular rotation velocity of density fluctuations. The influence of parameters like plasma curvature, probing beam characteristics, turbulence wave-number spectral width and antenna tilt angle is studied. Accurate Doppler measurements can be obtained using Gaussian beams with low divergence and optimum beam waist in slab plasmas and also in plasmas with high curvature. Gaussian beam antennas with optimum spot size provide accurate Doppler frequency values for a wide range of wave-number spectral widths of the turbulence. Doppler measurements at large tilt angles—high probed wave-numbers—are able to give accurate results on the Doppler frequency keeping the enhancement factor of the probing electric field close to the cutoff layer and therefore its contribution to the spatial localization of the measurement; however, the efficiency of the Bragg backscattering process—defined as the ratio between the backscattered and incident power—is low at large tilt angles.

Microwave Reflectometry of Turbulence Spectrum in Tokamak Plasmas

The recently proposed method of plasma turbulence spectrum measurements in tokamaks by means of conventional microwave reflectometry is extended to the case of arbitrary density profiles. The localization property and sensitivity of the method to the microwave scattering by fluctuations far from the cut-off layer is studied, with a conclusion that, depending on the turbulence spectrum and amplitude, such a scattering can deteriorate the results of spectrum reconstruction. However, if turbulence spectrum parameters do not reveal drastic radial variations, the cumulative effect of fluctuations far from the reflection area is not significant. In such a case the reconstructed poloidal wavenumber spectrum of density fluctuations is well localized near the cut-off layer. The studied cases correspond to the geometry and plasma parameters on the JT-60U tokamak.

Fluctuation Measurement Using an Ultrashort-Pulse Reflectometry on the GAMMA 10

Fluctuation of electron density is observed by using a microwave reflectometry in the central cell of GAMMA 10 tandem mirror. An ultrashort-pulse train, whose pulse width is 65 ps, is transmitted into the plasma in the ordinary-wave mode and reflected at the cut-off layer. The reflected wave is detected by the receiving system, and its time-of-flight, i.e., round trip time, is measured. Fluctuation of the time-of-flight is fluctuation of the cutoff layer and it means density fluctuation. The pulse has a broad frequency spectrum, so each frequency component is reflected at different layer corresponding to its frequency. The frequency range of the receiving system is 7 to 11 GHz, and cut-off density ranges 0.61 to 1.5 × 1018 m-3. Density on the central axis of the plasma is about 2 × 1018 m-3. Radial intensity distribution of the fluctuation is observed without any perturbation. Frequency of the fluctuation is around several kHz.

Numerical Study of Microwave Imaging Reflectometer for a Tandem Mirror Device

Applicability of microwave imaging reflectometry (MIR) for the case of mirror device was numerically studied. The questions concerning radial localization of measurements, optimal position of optical system and imaging properties of MIR system were considered. The high curvature of the plasma cutoff layer and a relatively small distance from the plasma to the port are known to improve the performance of a conventional reflectometer. Nevertheless in the case of density fluctuations with wide wavenumber spectrum and large amplitude, the imaging reflectrometry shows better results than the conventional one.

Ion species mix measurements in DIII-D and International Thermonuclear Experimental Reactor using ion–ion hybrid layer reflectometry

A superheterodyne reflectometer can provide a direct and inexpensive measurement of the concentrations of ion species with different charge to mass ratios. The ion–ion hybrid cutoff frequency is uniquely determined by the cyclotron frequencies and concentrations of the different species. The phase of a ∼20 MHz wave that travels from a launching antenna on the low-field side of a tokamak, reflects off the cutoff layer, then travels to a receiving antenna provides a direct measure of the species mix. Hydrogen concentrations between 3% and 67% are measured in DIII-D using this technique. In theory, the technique can measure the spatial profile of the tritium concentration in the International Thermonuclear Experimental Reactor. Possible practical difficulties include attenuation of the wave in the evanescent layer near the antenna.

Electron density profile measurement using an ultrashort-pulsed radar reflectometer on large helical device

We have installed a six channel ultrashort-pulsed radar reflectometer system on the large helical device and performed electron density profile measurements. The delay time of the reflected pulses from each cutoff layer in the plasma is measured by a time-of-flight measurement technique in order to avoid the mixture of radiation effects and spurious reflections. The electron density profile is reconstructed using an Abel inversion method from the profile of the delay time as a function of the probing frequency. The reconstructed density profile is compared with the profile measured with the far-infrared (FIR) interferometer. It is found that the arrival time of each reflected pulse differs from the estimated time measured with the FIR interferometer.

ICRF coupling and edge density profile on Tore Supra

In addition to the propagation and absorption of the waves into the plasma, a crucial point in ion cyclotron radiofrequency heating (ICRH) is to know how the energy is coupled to the plasma. The coupling efficiency is experimentally estimated through measurements of the antenna coupling resistance and it is of primary importance to precisely establish its dependence on the plasma parameters. The coupling resistance is highly connected to the density profile, which determines the cut-off positions of the magnetosonic waves for a given radiated wave number spectrum. For a large size tokamak like Tore Supra these cut-off densities range between 1018 and 1019 m−3, corresponding to the plasma edge. In this study, the RF coupling is investigated with respect to the plasma edge density profile measured by X-mode reflectometry and a characteristic cut-off position of the magnetosonic wave. We show that the edge density is not only a function of the average density but also depends in a complex way, on various edge plasma regimes, the power deposition, or the kind of limiter materials or divertor perturbation used to control the edge. Over a large variety of plasma edge regimes, it is shown how the distance between a characteristic ICRH cut-off density layer and the antenna is the relevant parameter to address the problem of the RF coupling efficiency. Simulation of the coupling process with the three-dimensional antenna code Ion Cyclotron ANTenna compares well with RF measurements, provided that the experimental density profiles are used in the computations.

Plasma rotation profile measurements using Doppler reflectometry

High spatial resolution radial profiles of the perpendicular plasma rotation velocity u⊥ using a dual channel 50–75 GHz Doppler reflectometer system on the ASDEX Upgrade tokamak are presented for a variety of discharge scenarios, including Ohmic, L-mode, H-mode, etc with forward and reversed magnetic field and co- and counter neutral beam injection. The reflectometers have steppable launch frequencies fo = c/λo, with selectable O- or X-mode polarization, giving tokamak edge to mid-radius coverage. Low-field-side antennae (hog-horn antenna pairs) with deliberate tilting (primarily poloidally) produce a Doppler shifted spectrum directly proportional to the perpendicular velocity fD = u⊥k⊥/2π = u⊥ 2sinθt/λo. The incident angle θt between the beam and cut-off layer normal varies with plasma shape, cut-off layer position and refraction. However, typical angles range from 5° to 27° giving a probed turbulence wavenumber, k⊥, range of 1.8–14.3 cm−1, with resulting Doppler shifts fD of up to 5 MHz. The measured perpendicular velocity is u⊥ = vE × B + vphase, which for a typical H-mode is slightly positive in the tokamak scrape-off-layer with a deep negative well across the H-mode steep pressure gradient pedestal region and then following the perpendicularly projected toroidal fluid velocity in the core, should be dominated by the E × B velocity, as the intrinsic phase velocity is predicted to be small, which may allow u⊥ to be interpreted directly as the radial electric field Er profile.

Fast dynamics of type I ELMs and transport of the ELM pulse in JT-60U

Simultaneous fast ELM measurements using a reflectometer, interferometers, Dα intensity and a magnetic probe reveal the detailed characteristics of type I ELMs. From the phase signal of the reflectometer indicating the radial movement of the cut-off layer, four different phases in the ELM event, i.e. a precursor phase, collapse phase, recovery phase and a relaxation phase, were observed. In the collapse phase, the radial extent of the collapse of the density pedestal reached up to twice the pedestal width. A fast drop of the horizontal interferometer signal was observed, while the vertical interferometer on the high-field side edge plasma only exhibited a small and delayed reduction. These features of the plasma response due to ELMs might indicate a non-uniform collapse of the density pedestal in the poloidal direction, localized near the low-field side (LFS) midplane. Expelled particles from the LFS midplane were measured by using scrape-off layer Mach probes, and the heat flux to the divertor target was measured using a fast infrared TV camera. The time constant of the peak heat load was comparable to that of the enhancement of the plasma flow towards the divertor target. The normalized ELM energy loss by pedestal stored energy in JT-60U was found to be less than 10% of the pedestal stored energy. It shows weak dependence on the pedestal parameters such as electron collisionality, parallel ion loss time and the ratio of the pedestal density to the Greenwald density limit.

Plasma species mix diagnostic using ion–ion hybrid layer reflectometry

A superheterodyne reflectometer could provide a direct and inexpensive measurement of ion species mixes with different charge-to-mass ratios. Using the cold plasma dispersion relation, the ion–ion hybrid cutoff frequency is uniquely determined by the density ratio and cyclotron frequencies of the two different species. The phase of a 20 MHz wave that travels from the launching point to the cutoff layer to the receiving antenna provides a direct measure of the hydrogen : deuterium species mix. In the first experiment, a fast Alfvén wave is launched perpendicular to a hydrogen–deuterium plasma from the low field side of the DIII-D tokamak. Quantitative measurements observe a hydrogen concentration range of 3–67% and a maximum penetration depth of 0.60 m. Corroborative values are obtained from two independent diagnostics. In the second experiment, the fast Alfvén wave is launched from the high field side (HFS) during a hydrogen puffing experiment. The results suggest that a wave launched from the HFS is able to tunnel through the resonance layer and reflect back to the receiving antenna.

Full-Wave Maxwell Simulation on R-Mode Tunneling in Electron Cyclotron Resonance Heating Study

A new numerical scheme for electromagnetic wave tracing is presented in place of the standard ray-tracing method in studies of electron cyclotron resonance heating. The new method solves the full-wave Maxwell equation, and can take into account wave diffraction, mode conversion (or, cross-polarization scattering), and wave tunneling across an evanescent region between resonance and cutoff layers, in addition to estimating energy absorption due to wave-particle resonances. One and two-dimensional simulations of electromagnetic wave tunneling are also

Asymmetry of collapse of density pedestal by type I ELM in JT-60U

The edge plasma density at the high-field side (HFS) and low-field side (LFS) was measured using a FIR interferometer and an O-mode reflectometer simultaneously in order to study the asymmetry of collapse of the density pedestal by type I ELM. From fast time series analysis using the same digitizer system for the reflectometer, Dα intensity, interferometer and magnetic probe, experimental evidence for poloidal asymmetry of the collapse was clearly observed in JT-60U ELMy H-mode discharges for the first time. The collapse of the pedestal density is manifested as a fast inward displacement of the cutoff layer at the LFS, the increase of Dα intensity and large magnetic oscillations. No immediate response, however, was observed in the line-integrated density at the HFS of the plasma (FIR-U1). Instead, the signal showed a small reduction after a time delay of about L∥/Cs, which was attributed to the parallel flow from the inside to outside in order to fill the density gap as a result of localized particle expulsion from the LFS. Furthermore, no density increase in the HFS scrape-off layer (SOL) was observed during the horizontal plasma sweep experiments corresponding to particles expelled from the HFS midplane to the SOL directly, at the onset of density collapse and before the increase of Dα intensity. Together, these results indicate that the collapse of the density pedestal during type I ELMs is localized at the LFS midplane.

Reflectometry study of mode coupling in fusion plasma turbulence

The use of microwave reflectometry in the estimation of the bicoherency of the turbulent modes in plasmas is theoretically investigated. The full-wave two-dimensional model of microwave scattering in plasma is applied to simulate the reflectometer response to the turbulent fluctuations, consisting of random and coherent modes, and the simulated data ensemble is further processed to obtain the auto-bicoherence function. The sensitivity of the method was analysed for different ambient plasma profiles, geometry, fluctuation spectra and amplitudes.It was found that for plasma conditions similar to the edge JT-60U tokamak plasma, reflectometry-based estimations of the bicoherency can prove the existence of the phase-coupled modes only if the ratio of coherent to random amplitudes is larger than unity. Reflectometry in plasmas with stronger curvature of the cut-off layer has a better sensitivity to coherent fluctuations as compared to weak-curvature profiles.As an illustration, the results obtained were applied to the data ensemble acquired by the O-mode reflectometer on the JT-60U tokamak to demonstrate the existence of strong phase coupling in the H-mode plasma near the separatrix.

Design of an X-mode fast-scanning reflectometry for edge density profile measurement on HT-7 tokamak

An amplitude modulation reflectometry system with fast-scanning microwave source is designed for edge density profile measurement on the HT-7 superconducting tokamak (R0=1.22 m, a=0.28 m, Bt: 1.8–2.3 T, plasma pulses of 1–20 s). The reflectometer launches microwave power from a very fast-scanning backward wave oscillator in 53–78 GHz to probe the plasma and receive the reflection by the density cutoff layer. It is an extraordinary mode reflectometer, which can measure the edge density profiles in normalized radius r/a from scrape-off layer up to 0.5 when Bt=2.2 T, Ne0=3×1019 m−3 in typical HT-7 operation parameters. The microwave amplitude modulation frequency is 200 MHz. A 2 MHz reference signal and the probing signal are obtained by mixing a 198 MHz local oscillator power with the demodulated microwave signals. Phase comparison is then accomplished at 2 MHz by I/Q phase detector. The scanning time over the full band of 53–78 GHz is about 0.5 ms. The reflectometry will fill in the space left by the five-chord interferometer, which measures in the core plasma region and the Langmuir probes, which work in the scrape-off layer; especially the reflectometry is used to study the lower hybrid wave coupling into plasmas for the current drive and the radio frequency wave coupling problems for heating on HT-7. The reflectometry is also considered to be useful on the future HT-7U superconducting tokamak.

Radial wave number spectrum of density fluctuations deduced from reflectometry phase signals

A method to extract the radial wave-number spectrum of the density fluctuations from the reflectometry phase signal is proposed and the main points to interpret it are described. The method buildup is based on reflectometry studies devoted to phase or amplitude variations induced by coherent density perturbations. These previous analyses show that all the wave numbers up to the Bragg detection limit kf (fluctuation radial wave number) <2ko (incident wave number in vacuum) contribute to the phase fluctuations. Although the main part comes from the vicinity of the cutoff layer the entire probing zone contributes to the phase fluctuations. The phase fluctuations dependencies in kf and L (length of the density gradient) found in the literature for the O mode have been estimated for the X mode in the Bragg backscattering zone in the case of extensive density perturbations. The assumptions used to establish this diagnostic and its limitations are discussed. The numerical study with a FDTD code in two-dimensional configuration including the reflectometer antenna shows that the contribution of the poloidal wave number can be neglected in the studied cases. The method used to recover the kf spectrum from the Tore Supra swept reflectometer data and the results are presented.

How to Make a Microwave Reflectometer

Microwave reflectometry is a plasma diagnostic that launches microwaves to a plasma and measures the wave reflected at a cutoff layer in the plasma. It is used mainly for density profile and density fluctuation measurements. This lecture note is written for those who want to make a reflectometer and apply it to a target plasma. Various types of microwave reflectometers and a simple numerical tool useful for designing a system and for analyzing data are described. Details of a virtual system are also explained.

Microwave Reflectometry for Plasma Diagnostics

Microwave reflectometry is a plasma diagnostic that launches microwaves to a plasma and measures the wave reflected at a cutoff layer in the plasma. It is used mainly for density profile and density fluctuation measurements. Because of its high radial resolution, high sensitivity, and relatively easy implementation, microwave reflectometry has been used in many devices. In this report, highlights of the measurements, as well as the features of reflectometry, are described.

Behavior of reflected extraordinary mode in the fundamental electron cyclotron heating and current drive in JT-60U

Unwanted X-mode behavior in the fundamental harmonic electron cyclotron (EC) wave launching from the low field side with an oblique toroidal injection angle is experimentally studied on JT-60U by changing the poloidal injection angle. It is found that there is a critical region in the poloidal injection angle to cause the rise in vacuum pressure at the NBI port adjacent to the ECRF antenna port. The typical EC beam power is about 1 MW and the pulse length is 3 s in this experiment. The ray-trace calculation indicates that the X-mode launched from the low field side is guided into the NBI port after reflection at the cut-off layer for the critical poloidal injection angle. The results suggest that the O-mode injection with quite high mode purity, that is, with a very low fraction of X-mode is required for the avoidance of the overheat of the in-vessel components due to the reflected X-mode in the case of the EC beam launching from the low field side, especially for long pulse operation.

Fluctuation reflectometry of azimuthally symmetric plasmas

The results of analytical solution of time-dependent two-dimensional full-wave equation are presented here for the case of fluctuation reflectometry. We consider a family of plasma density profiles with cylindrical symmetry, which allow for an integration of the microwave scattering equation.Based on the results of analytical modelling, Doppler reflectometry as a method of plasma velocity measurements is investigated. It is shown that the accuracy of Doppler shift measurements is sensitive to the spectral content of the broadband turbulence, as well as to the curvature of the plasma cut-off layer. Addressing the problem of fluctuation amplitude measurements, it is suggested that a power-related parameter of the microwave signal, called fluctuation index, is suitable to quantify the plasma turbulence level provided that the fluctuation spectrum is a priori estimated. As an illustration, we apply our model to estimate the density fluctuation level in the edge plasma of the JT-60U tokamak.