Electron Bernstein Wave Emission Research Articles

On correlation measurements of electron Bernstein wave emission

A systematic study of the electron Bernstein wave (EBW) emission from either overdense plasmas or plasmas with moderate electron temperatures is presented. Starting from description of the bare electrons microscopic thermal radiation we finally arrive at the macroscopic expression for the radiation temperature. The latter is shown to be a sum of the electron temperature and a fluctuating piece caused by the fluctuations of both the EBW emission source and the EBW damping rate within the ECR layer. It is shown that the correlation measurements of the EBW emission at two separate frequencies reveal information on the correlation length of the low frequency turbulence.

First results from EBW emission diagnostics on COMPASS

COMPASS tokamak shots at low magnetic field feature overdense plasmas during the extended current flat-top phase. The first harmonic of the electron cyclotron emission is completely cutoff for O and X modes and so the emission caused by electron Bernstein waves (EBWs) propagating obliquely with respect to the magnetic field and undergoing so called EBW-X-O conversion process can be observed. We perform an angular scan of the EBW emission during a set of comparable shots in order to determine the optimum antenna direction. A weak dependence of the radiative temperature on the antenna angles indicates an influence of multiple reflections from the vessel wall. The low temperature at the mode conversion region is responsible for the collisional damping of EBW, which can explain several times lower measured radiative temperature than the electron temperature measured by the Thomson scattering system.

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.

Two-Dimensional Studies of Electron Bernstein Wave Emission in MAST

Angular scanning of electron Bernstein wave emission (EBE) has been conducted in MAST. From EBE measurements over a range of viewing angles, the angular position and orientation of the B-X-O mode conversion (MC) window can be estimated, giving the pitch angle of the magnetic field in the MC layer. The radial position of the corresponding MC layer is found from Thomson scattering measurements. Measurements at several frequencies can provide a pitch angle profile. Results of pitch angle profile reconstruction from EBE measurements are presented in comparison with motional Stark effect measurements. Microwave imaging of the B-X-O MC window is proposed as an alternative to angular scanning. The proposed scheme is based on an imaging phased array of antennas allowing the required angular resolution. Image acquisition time is much shorter than magnetohydrodynamic (MHD) time scales so the EBE imaging can be used for pitch angle measurements even in the presence of MHD activity.

A spinning mirror for fast angular scans of EBW emission for magnetic pitch profile measurements

A tilted spinning mirror rapidly steers the line of sight of the electron Bernstein wave (EBW) emission radiometer at the Mega-Amp Spherical Tokamak (MAST). In order to resist high mechanical stresses at rotation speeds of up to 12,000 rpm and to avoid eddy current induced magnetic braking, the mirror consists of a glass-reinforced nylon substrate of a special self-balanced design, coated with a reflecting layer. By completing an angular scan every 2.5-10 ms, it allows one to characterize with good time resolution the Bernstein-extraordinary-ordinary mode-conversion efficiency as a function of the view angles. Angular maps of conversion efficiency are directly related to the magnetic pitch angle at the cutoff layer for the ordinary mode. Hence, measurements at various frequencies provide the safety factor profile at the plasma edge. Initial measurements and indications of the feasibility of the diagnostic are presented. Moreover, angular scans indicate the best launch conditions for EBW heating.

Electron cyclotron-electron Bernstein wave emission diagnostics for the COMPASS tokamak

The COMPASS tokamak recently started operation at the Institute of Plasma Physics AS CR, v.v.i., Prague. A new 16-channel radiometer, operating alternatively in three frequency bands, has been designed and constructed. The system is prepared for detection of normal electron cyclotron emission (O1 or X2) or oblique electron Bernstein wave emission. The end-to-end calibration method includes all components that influence the antenna radiation pattern. A steady recalibration is possible using a noise generator connected to the radiometer input through a fast waveguide PIN-switch. Measurements of the antenna radiation characteristics (2D electric field) were performed in free space as well as in the tokamak chamber, showing the degradation effect of structures on the Gaussian beam shape. First plasma radiation temperature measurements from low-field circular plasmas are available.

Initial Electron Bernstein Wave Emission Measurements on the TJ-II Stellarator

Thermal electron emission at 28 GHz has been measured on the TJ-II stellarator. The emission from neutral beam-heated overdense plasmas, where the plasma density is greater than the ordinary-mode (O-mode) cutoff density, is consistent with electron thermal emission from mode-converted electron Bernstein waves (EBWs) via the Bernstein wave to extraordinary mode to ordinary mode scenario (B-X-O). Emission from underdense plasmas without neutral beam injection is consistent with the measurement of oblique electron cyclotron emission. Electron Bernstein wave emission measurements are being made to determine the optimum launch angle for planned EBW heating experiments and also to provide an indication of electron temperature evolution in overdense plasmas on TJ-II.

Overview of results from the National Spherical Torus Experiment (NSTX)

The mission of the National Spherical Torus Experiment (NSTX) is the demonstration of the physics basis required to extrapolate to the next steps for the spherical torus (ST), such as a plasma facing component test facility (NHTX) or an ST based component test facility (ST-CTF), and to support ITER. Key issues for the ST are transport, and steady state high β operation. To better understand electron transport, a new high-k scattering diagnostic was used extensively to investigate electron gyro-scale fluctuations with varying electron temperature gradient scale length. Results from n = 3 braking studies are consistent with the flow shear dependence of ion transport. New results from electron Bernstein wave emission measurements from plasmas with lithium wall coating applied indicate transmission efficiencies near 70% in H-mode as a result of reduced collisionality. Improved coupling of high harmonic fast-waves has been achieved by reducing the edge density relative to the critical density for surface wave coupling. In order to achieve high bootstrap current fraction, future ST designs envision running at very high elongation. Plasmas have been maintained on NSTX at very low internal inductance li ∼ 0.4 with strong shaping (κ ∼ 2.7, δ ∼ 0.8) with βN approaching the with-wall β-limit for several energy confinement times. By operating at lower collisionality in this regime, NSTX has achieved record non-inductive current drive fraction fNI ∼ 71%. Instabilities driven by super-Alfvénic ions will be an important issue for all burning plasmas, including ITER. Fast ions from NBI on NSTX are super-Alfvénic. Linear toroidal Alfvén eigenmode thresholds and appreciable fast ion loss during multi-mode bursts are measured and these results are compared with theory. The impact of n > 1 error fields on stability is an important result for ITER. Resistive wall mode/resonant field amplification feedback combined with n = 3 error field control was used on NSTX to maintain plasma rotation with β above the no-wall limit. Other highlights are results of lithium coating experiments, momentum confinement studies, scrape-off layer width scaling, demonstration of divertor heat load mitigation in strongly shaped plasmas and coupling of coaxial helicity injection plasmas to ohmic heating ramp-up. These results advance the ST towards next step fusion energy devices such as NHTX and ST-CTF.

Investigation of electron Bernstein wave (EBW) coupling and its critical dependence on EBW collisional loss in high-β, H-mode ST plasmas

High-β spherical tokamak (ST) plasma conditions cut off propagation of electron cyclotron (EC) waves used for heating and current drive in conventional aspect ratio tokamaks. The electron Bernstein wave (EBW) has no density cutoff and is strongly absorbed and emitted at the EC harmonics, allowing EBWs to be used for heating and current drive in STs. However, this application requires efficient EBW coupling in the high-β, H-mode ST plasma regime. EBW emission (EBE) diagnostics and modelling have been employed on the National Spherical Torus Experiment (NSTX) to study oblique EBW to O-mode (B–X–O) coupling and propagation in H-mode plasmas. Efficient EBW coupling was measured before the L–H transition, but rapidly decayed thereafter. EBE simulations show that EBW collisional damping prior to mode conversion (MC) in the plasma scrape off reduces the coupling efficiency during the H-mode phase when the electron temperature is less than 30 eV inside the MC layer. Lithium evaporation during H-mode plasmas was successfully used to reduce this EBW collisional damping by reducing the electron density and increase the electron temperature in the plasma scrape off. Lithium conditioning increased the measured B–X–O coupling efficiency from less than 10% to 60%, consistent with EBE simulations.

Collisional Damping of Electron Bernstein Waves and its Mitigation by Evaporated Lithium Conditioning in Spherical-Tokamak Plasmas

The first experimental verification of electron Bernstein wave (EBW) collisional damping, and its mitigation by evaporated Li conditioning, in an overdense spherical-tokamak plasma has been observed in the National Spherical Torus Experiment (NSTX). Initial measurements of EBW emission, coupled from NSTX plasmas via double-mode conversion to O-mode waves, exhibited <10% transmission efficiencies. Simulations show 80% of the EBW energy is dissipated by collisions in the edge plasma. Li conditioning reduced the edge collision frequency by a factor of 3 and increased the fundamental EBW transmission to 60%.

Electron Bernstein wave emission based diagnostic on National Spherical Torus Experiment (invited)

National Spherical Torus Experiment (NSTX) is a spherical tokamak (ST) that operates with n(e) up to 10(20) m(-3) and B(T) less than 0.6 T, cutting off low harmonic electron cyclotron (EC) emission widely used for T(e) measurements on conventional aspect ratio tokamaks. The electron Bernstein wave (EBW) can propagate in ST plasmas and is emitted at EC harmonics. These properties suggest thermal EBW emission (EBE) may be used for local T(e) measurements in the ST. Practically, a robust T(e)(R,t) EBE diagnostic requires EBW transmission efficiencies of >90% for a wide range of plasma conditions. EBW emission and coupling physics were studied on NSTX with an obliquely viewing EBW to O-mode (B-X-O) diagnostic with two remotely steered antennas, coupled to absolutely calibrated radiometers. While T(e)(R,t) measurements with EBW emission on NSTX were possible, they were challenged by several issues. Rapid fluctuations in edge n(e) scale length resulted in >20% changes in the low harmonic B-X-O transmission efficiency. Also, B-X-O transmission efficiency during H modes was observed to decay by a factor of 5-10 to less than a few percent. The B-X-O transmission behavior during H modes was reproduced by EBE simulations that predict that EBW collisional damping can significantly reduce emission when T(e)<30 eV inside the B-X-O mode conversion (MC) layer. Initial edge lithium conditioning experiments during H modes have shown that evaporated lithium can increase T(e) inside the B-X-O MC layer, significantly increasing B-X-O transmission.

Chapter 3: Microwave Diagnostics

Microwave-based diagnostics have found broad application in magnetic fusion plasma diagnostics and are expected to be widely employed in future burning plasma experiments (BPXs). Most of these techniques are based directly on the dispersive properties of the plasma medium that, as shown in the body of the paper, results in the microwave/millimeter wave portion of the electromagnetic spectrum being particularly well suited for a variety of measurements of both magnetic fusion plasma equilibrium parameters and their fluctuations. Electron cyclotron emission provides a measurement of electron temperature and its fluctuations while electron cyclotron absorption potentially can provide a measurement of electron pressure (the product of electron density and temperature) as well as information on the suprathermal electron distribution. Electron Bernstein wave emission is also employed for electron temperature radiometric measurements in devices including reversed field pinches, spherical tori, and higher-aspect-ratio tokamaks and stellarators that operate at high β. The radar-based microwave reflectometry technique measures the electron density profile and its fluctuations by means of the reflection of electromagnetic waves at the plasma cutoff layer. Coherent Thomson scattering in the microwave region yields information on the fast ion population. Wave number resolved microwave collective scattering is also widely employed for measuring nonthermal (turbulent) density fluctuations or coherent electrostatic waves. The approach taken in this review is to address each technique separately beginning with the physical principles followed by representative implementations on magnetic fusion devices. In each case, the applicability to future BPXs is discussed. It is impossible in a short review to capture fully the numerous significant accomplishments of the many clever scientists and engineers who have advanced microwave plasma diagnostics technology over many decades. Therefore, in this paper, we can reveal only the basic principles together with some of the most exciting highlights while outlining the major trends, and we hope it will serve as an exciting introduction to this rich field of plasma diagnostics.

Nonthermal Electron Bernstein Emission in NSTX-Like Discharges

In overdense plasma for which the plasma frequency exceeds the cyclotron frequency, X-mode, near-perpendicular cyclotron emission does not propagate to the outboard plasma edge. However, under these conditions it remains possible for electron Bernstein waves (EBWs) to transmit emitted radiation from central plasma to the plasma exterior via a mode conversion to electromagnetic waves near the plasma edge. GENRAY is an all-frequencies, three-dimensional ray-tracing code and also calculates EBW emission (EBWE) from thermal or nonthermal relativistic distributions. The numerical methods are based on the earlier HORACE circular plasma code (R.W. Harvey et al., Proc. 7th Joint Workshop and International Atomic Energy Agency Technical Committee Meeting on Electron Cyclotron Emission and Electron Cyclotron Resonance Heating, Hefei, China, 1989), generalized to noncircular plasmas and to electromagnetic EBWs, including a parallel refractive index greater than 1. Emission and absorption are calculated on an array of points along EBW rays emanating from the antenna, and the radiation transport equation is backsolved along the EBW rays to the antenna. Hot plasma dispersion is used along with a relativistic calculation of the thermal or nonthermal emission and absorption. This paper describes the calculation and reports new results for nonthermal EBWE. Along with detailed numerical analysis, EBWE can be used to measure both thermal and nonthermal properties of the electron distribution function.

Recent Developments in Electron Cyclotron Emission Research on Magnetically Confined Plasmas

Electron cyclotron emission (ECE) has been an important diagnostic for measuring the temporal evolution of the electron temperature profile in magnetically confined plasma devices for more than 25 years. Recent advances in ECE measurements, such as two-dimensional ECE imaging and ECE intensity correlation techniques, have provided detailed information on sawtooth reconnection, neoclassical tearing mode behavior, electron heat transport, fast electron dynamics, and fast particle-driven Alfvén eigenmodes. ECE spectral analysis is benefiting from improved ECE modeling and significant increases in computational power that allow fast, real-time, temperature measurements. Mode-converted electron Bernstein wave emission (EBE) diagnostics are being developed to study overdense (ɷpe ≫ ɷce) plasmas, a regime where conventional ECE diagnostics cannot be applied and one commonly encountered in high-β devices, such as the spherical torus and reversed-field pinch. While ECE diagnostic techniques are now well established on many existing magnetically confined plasmas, significant challenges lie ahead for applying ECE techniques to reactor-grade plasmas such as ITER, where Te(0) is expected to reach 20 to 40 keV. This paper reviews the recent advances in ECE, electron cyclotron absorption, and EBE diagnostics and discusses the challenges for ECE measurements on ITER.

Electron Bernstein Wave Heating and Emission in the TCV Tokamak

Electron cyclotron resonance heating (ECRH) of high-density tokamak plasmas is limited because of reflections of the waves at so-called wave cutoffs. Electron Bernstein wave (EBW) heating (EBWH) via a double mode conversion process from ordinary (O)-mode, launched from the low field side, to extraordinary (X)-mode and finally to Bernstein (B)-mode offers the possibility of overcoming these density limits.In this paper, the O-X mode conversion dependence on the microwave injection angle is demonstrated experimentally. The dependence on the injection angle is studied in high-density plasmas in H-mode, in the presence of magnetohydrodynamic activity, edge-localized modes, and sawteeth. The results of localized heat deposition at an overdense location are presented, demonstrating EBWH for the first time via the O-X-B mode conversion process in a standard aspect-ratio tokamak. The results of global and local power deposition are compared with ray-tracing calculations. Moreover, a temperature increase due to EBWH is observed.Initial EBW emission measurements with a newly installed ECRH reception launcher are presented. The inverse double mode conversion process B-X-O is observed by measuring the emission for several frequencies at an optimum angle.

Electron Bernstein wave simulations and comparison to preliminary NSTX emission data

Simulations indicate that during flattop current discharges the optimal angles for the aiming of the National Spherical Torus Experiment (NSTX) antennae are quite rugged and basically independent of time. The time development of electron Bernstein wave emission (EBWE) at particular frequencies as well as the frequency spectrum of EBWE as would be seen by the recently installed NSTX antennae are computed. The simulation of EBWE at low frequencies (e.g., 16GHz) agrees well with the recent preliminary EBWE measurements on NSTX. At high frequencies, the sensitivity of EBWE to magnetic field variations is understood by considering the Doppler broadened electron cyclotron harmonics and the cutoffs and resonances in the plasma. Significant EBWE variations are seen if the magnetic field is increased by as little as 2% at the plasma edge. The simulations for the low frequency antenna are compared to preliminary experimental data published separately by Diem et al. [Rev. Sci. Instrum.77 (2006)]

Progress towards steady state on NSTX

In order to reduce recirculating power fraction to acceptable levels, the spherical torus concept relies on the simultaneous achievement of high toroidal β and high bootstrap fraction in steady state. In the last year, as a result of plasma control system improvements, the achievable plasma elongation on NSTX has been raised from κ ∼ 2.1 to κ ∼ 2.6—approximately a 25% increase. This increase in elongation has led to a substantial increase in the toroidal β for long pulse discharges. The increase in β is associated with an increase in plasma current at nearly fixed poloidal β, which enables higher βt with nearly constant bootstrap fraction. As a result, for the first time in a spherical torus, a discharge with a plasma current of 1 MA has been sustained for 1 s (0.8 s current flat-top). Data are presented from NSTX correlating the increase in performance with increased plasma shaping capability. In addition to improved shaping, H-modes induced during the current ramp phase of the plasma discharge have been used to reduce flux consumption and to delay the onset of MHD instabilities. Based on these results, a modelled integrated scenario, which has 100% non-inductive current drive with very high toroidal β, will also be discussed. The NSTX poloidal field coils are currently being modified to produce the plasma shape which is required for this scenario, which requires high triangularity (δ ∼ 0.8) at elevated elongation (κ ∼ 2.5). The other main requirement of steady state on NSTX is the ability to drive a fraction of the total plasma current with RF waves. The results of high harmonic fast wave heating and current drive studies as well as electron Bernstein wave emission studies will be presented.

Measurement of the magnetic field in a spherical torus plasma via electron Bernstein wave emission harmonic overlap

Measurement of the magnetic field in a spherical torus by observation of harmonic overlap frequencies in the electron Bernstein wave (EBW) spectrum has been previously suggested [V. F. Shevchenko, Plasma Phys. Rep. 26, 1000 (2000)]. EBW mode conversion to X-mode radiation has been studied in the Current Drive Experiment-Upgrade spherical torus (T. Jones, Ph.D. thesis, Princeton University, 1995) with emission measured at blackbody levels [B. Jones et al., Phys. Rev. Lett. 90, 165001 (2003)]. Sharp transitions in the thermally emitted EBW spectrum have been observed for the first two harmonic overlaps. These transition frequencies are determined by the magnetic field and electron density at the mode conversion layer in accordance with hot-plasma wave theory. Prospects of extending this measurement to higher harmonics, necessary in order to determine the magnetic field profile, and high-β equilibria are discussed for this proposed magnetic field diagnostic.

Electron Bernstein wave emission diagnostic assisted by reflectometry on TST-2 spherical tokamak

A new electron Bernstein wave emission diagnostic consisting of a heterodyne radiometer and a reflectometer was installed on TST-2. The instrument has a frequency range from 5 to 12 GHz which covers partially the fundamental and fully covers the second and third harmonic emissions. X-mode emission perpendicular to the flux surface, generated by B–X–FX mode-conversion scenario, is measured. The mode-conversion efficiency is calculated using a one-dimensional full-wave code with the density profile obtained simultaneously from the reflectometer, and the electron temperature (Te) is estimated. The levels corresponding to the 100–150 eV range are measured from the plasma core region, and the mode-conversion efficiency is calculated to be 0.5–0.8, leading to Te of approximately 200 eV. In addition, it is found that the reflectivity measured by the reflectometer is consistent with the calculated mode-conversion efficiency. This property can be used for a direct measurement of the mode-conversion efficiency.

BXO mode-converted electron Bernstein emission diagnostic (invited)

Electron temperature profiles at densities above the electron cyclotron emission (ECE) cutoff are measured at the W7-AS stellarator by a novel diagnostic based on black body emission and Bernstein-extraordinary-ordinary mode conversion of electron Bernstein waves (EBWs). The radiation is collected along a special oblique line of sight by an antenna with gaussian optics. This was optimized for maximal conversion efficiency and minimal Doppler broadening by means of EBW ray tracing calculations in full stellarator geometry. The elliptical O-mode polarization detected along the oblique line of sight is changed into a linear polarization by a broadband quarter wave shifter, namely an elliptical waveguide. The signal is spectrum analyzed by an heterodyne radiometer and temperature profiles are derived from spectra by means of ray tracing. The diagnostic was applied to measurements of edge-localized modes to illustrate its advantages in terms of spatial and temporal resolution. Moreover, for the first time, the heat wave propagation method for the determination of local heat transport coefficients was extended beyond the ECE cutoff density by combining EBW emission measurements at the first harmonic (f=66–78 GHz) with modulated EBW heating at the second harmonic (140 GHz).