X-mode Radiation Research Articles

Role of ion-acoustic wave energy in enhanced X-mode radiation phenomena in magnetospheric plasma

Role of ion-acoustic wave energy in enhanced X-mode radiation phenomena in magnetospheric plasma

Kinetic theory of parametric decay instabilities near the upper hybrid resonance in plasmas

Parametric decay instabilities (PDIs) near the upper hybrid resonance layer are studied with a 1D framework. In a uniform plasma, the kinetic nonlinear dispersion relation of PDI is numerically calculated for parameters corresponding to electron cyclotron heating experiments at the ASDEX-U tokamak, in which O-mode radiation was converted to X-mode radiation by reflection from the high-field sidewall. The forward scattering processes driven by X-mode and linearly converted electron Bernstein waves (EBWs) are investigated and found to lead to a primary PDI where the pump waves decay into lower hybrid waves and sideband EBWs. A frequency shift of 930 MHz is obtained for the sideband EBWs in the primary PDIs. Subsequently, the sideband EBWs can decay into a low-frequency ion Bernstein quasi-mode (IBQM) and a secondary EBW, where the dominant forward scattering channel is the first-order IBQM with a frequency close to twice the ion cyclotron frequency. The decay channels obtained by numerical calculation can explain the characteristics of the signal observed in ASDEX-U experiments. The threshold of the pump electric field strength required to excite the primary PDI in the presence of plasma inhomogeneity is also estimated.

Parametric Decay Instabilities during Electron Cyclotron Resonance Heating of Fusion Plasmas, Problems and Possibilities

We review parametric decay instabilities (PDIs) expected in connection with electron cyclotron resonance heating (ECRH) of magnetically confined fusion plasmas, with a specific focus on conditions relevant for the ITER tokamak. PDIs involving upper hybrid (UH) waves are likely to occur in O-mode ECRH scenarios at ITER if electron density profiles allowing trapping of UH waves near the ECRH frequency are present. Such PDIs may occur near the plasma center in ITER full-field scenarios heated by 170 GHz O-mode ECRH and on the high-field side of half-field ITER plasmas heated by 110 GHz or 104 GHz O-mode ECRH. Additionally, 110 GHz O-mode ECRH of half-field ITER scenarios may have low ECRH absorption, due to the electron cyclotron resonance being located on the high-field side of the main plasma. This potentially allows PDIs driven by a significant amount of ECRH radiation reaching the UH resonance in X-mode to occur, as X-mode radiation can be generated by reflection of unabsorbed O-mode radiation from the high-field side wall. The occurrence of PDIs during ECRH may damage microwave diagnostics, such as the electron cyclotron emission and low-field side reflectometer systems at ITER, as well as complicate the calculation of heating and current drive characteristics. However, if PDIs are induced in a controlled manner, they may provide novel diagnostic tools and allow the generation of a moderate fast ion population in plasmas heated only by ECRH.

Radio Emission from UV Cet: Auroral Emission from a Stellar Magnetosphere

The archetypical flare star UV Cet was observed by MeerKAT on 2021 October 5–6. A large radio outburst with a duration of ∼2 hr was observed between 886 and 1682 MHz, with a time resolution of 8 s and a frequency resolution of 0.84 MHz, enabling sensitive dynamic spectra to be formed. The emission is characterized by three peaks containing a multitude of broadband arcs or partial arcs in the time-frequency domain. In general, the arcs are highly right-hand circularly polarized. At the end of the third peak, brief bursts occur that are significantly elliptically polarized. We present a simple model that appears to be broadly consistent with the characteristics of the radio emission from UV Cet. Briefly, the stellar magnetic field is modeled as a dipole aligned with the rotational axis of the star. The radio emission mechanism is assumed to be due to the cyclotron maser instability, where x-mode radiation near the electron gyrofrequency is amplified. While the elliptically polarized bursts may be intrinsic to the source, rather stringent limits are imposed on the plasma density in the source and along the propagation path. We suggest that the elliptically polarized radiation may instead be the result of reflection on an overdense plasma structure at some distance from the source. The radio emission from UV Cet shares both stellar and planetary attributes.

Design and characterization of a 32-channel heterodyne radiometer for electron cyclotron emission measurements on experimental advanced superconducting tokamak.

A 32-channel heterodyne radiometer has been developed for the measurement of electron cyclotron emission (ECE) on the experimental advanced superconducting tokamak (EAST). This system collects X-mode ECE radiation spanning a frequency range of 104-168 GHz, where the frequency coverage corresponds to a full radial coverage for the case with a toroidal magnetic field of 2.3 T. The frequency range is equally spaced every 2 GHz from 105.1 to 167.1 GHz with an RF bandwidth of ~500 MHz and the video bandwidth can be switched among 50, 100, 200, and 400 kHz. Design objectives and characterization of the system are presented in this paper. Preliminary results for plasma operation are also presented.

Conceptual design of ITER ECE receiver systems and their performance parameters

The Electron Cyclotron Emission diagnostic on ITER requires electron temperature profile with 10 μsec time and 6 - 7 cm spatial resolution. The diagnostic is also useful to study many plasma physics phenomenon like temperature fluctuation, non-thermal electrons population and the power loss due to ECE. The conceptual design being considered for ITER ECE consist of a radiometer of frequency range 122 to 220 GHz for O-mode radiation measurement and two Fourier transform spectrometers of frequency range 70 to 1000 GHz as measuring instruments for O-mode and X-mode radiation measurement. There are two lines of sight for the collection of cyclotron radiation. One is radial view and other is oblique view at 10 degree (1). The radiation collected by both view will be transmitted through the polarization splitter boxes and four transmission lines to the diagnostic area. In this paper we will summarize the conceptual design details and changes proposed at the Conceptual Design Review. The required calibration time for the ECE measuring instrument will be discussed in the paper.

Characterization of a Penning discharge for investigation of auroral radio wave generation mechanisms

Auroral Kilometric Radiation (AKR), observed by satellites in the Earth's magnetosphere, is naturally generated in regions of partial plasma depletion (auroral density cavity) in the polar magnetosphere at approximately 3200 km altitude. As an electron descends through these regions of partial plasma depletion along magnetic field lines towards the Earth's ionosphere, the field lines increases and, through conservation of the magnetic moment, the electron gives up axial velocity in favour of perpendicular velocity. This results in a horseshoe-shaped distribution function in parallel/perpendicular-velocity space which is unstable to X-mode radiation, near the cyclotron frequency. Power levels as high as GW levels have been recorded with frequencies around 300 kHz. The background plasma frequency within the auroral density cavity is approximately 9 kHz corresponding to a plasma density 1 cm−3. A laboratory experiment scaled from auroral frequency to microwave frequency has previously been reported. Here, the addition of a Penning trap to simulate the background plasma of the density cavity is reported, with measurements ne ∼ 2 × 1014–2.17 × 1015 m−3, fpe ∼ 128–418 MHz and fce ∼ 5.21 GHz giving a ratio of ωce/ωpe comparable to the magnetospheric AKR source region.

Parametric decay of X-mode radiation into electron Bernstein and lower hybrid waves in a plasma

A large-amplitude electromagnetic wave, propagating perpendicular to ambient magnetic field in the X-mode, is shown to be susceptible to parametric decay into a short-wavelength electron Bernstein wave and a lower-hybrid wave. The density perturbation associated with the lower-hybrid wave couples to the oscillatory velocity due to the pump to produce a density perturbation driving the Bernstein wave. The Bernstein wave and the pump exert a low-frequency ponderomotive force on electrons, driving the sideband.

EBW power deposition and current drive in WEGA—comparison of simulation with experiment

Detailed computational studies of electrostatic electron Bernstein waves (EBWs) propagation in the WEGA stellarator are performed and compared with experimental results. Using the WEGA antenna, the two O-/X-mode radiation lobes are modelled by sets of rays whose intensities are proportional to the measured radiation pattern. After projecting these rays onto the plasma periphery, the O–X-EBW mode conversion efficiency around the upper hybrid resonance is determined from a full wave adaptive mesh solver of the cold plasma equations. From the roots of the electrostatic EBW dispersion relation, ray tracing is performed to determine the power absorption on the first or second cyclotron harmonic as well as current drive assuming the Fisch–Boozer mechanism. Good agreement is achieved between our EBW simulations on specific WEGA equilibria and the experimental results from the antenna launch of 2.45 GHz waves. The experimentally observed off-axis power deposition and the outward shift dependence of the absorption maxima on increasing magnetic field can only be explained by the existence of a hot electron component in the WEGA plasma. It is this hot electron component that permits wave absorption at the second harmonic near the plasma boundary. Moreover, the simulations not only reproduce the current density reversal at the plasma centre for low magnetic fields but also the destruction of this current density reversal for larger magnetic fields.

Mode conversion of Langmuir to electromagnetic waves at magnetic field-aligned density inhomogeneities: Simulations, theory, and applications to the solar wind and the corona

Linear mode conversion of Langmuir waves to radiation near the plasma frequency at density gradients is potentially relevant to multiple solar radio emissions, ionospheric radar experiments, laboratory plasma devices, and pulsars. Here we study mode conversion in warm magnetized plasmas using a numerical electron fluid simulation code with the density gradient parallel to the ambient magnetic field B0 for a range of incident Langmuir wavevectors. Our results include: (1) both o- and x-mode waves are produced for Ω=(ωL∕c)1∕3(ωc∕ω)≲1, contrary to previous ideas. Only the o mode is produced for Ω≳1.5. Here ωc is the (angular) electron cyclotron frequency, ω is the angular wave frequency, L is the length scale of the (linear) density gradient, and c is the speed of light. A WKB-style analysis accounts semiquantitatively for the production and relative conversion efficiencies of the o and x modes in the simulations. (2) In the unmagnetized limit, equal amounts of o- and x-mode radiation are produced. (3) The mode conversion window narrows as Ω increases. (4) As Ω increases the total electromagnetic field changes from linear to circular polarization, with the o- and x-mode signals remaining circularly polarized. (5) The conversion efficiency to the x mode decreases monotonically as Ω increases while the o-mode conversion efficiency oscillates due to an interference phenomenon between incoming and reflected Langmuir/z modes. (6) The maximum total conversion efficiencies for wave power from the Langmuir/z mode to radiation are of order 50%–70%. They depend strongly on the wave frequency when close to the background plasma frequency but weakly on the electron temperature T0 and β=T0∕mc2. The corresponding energy conversion efficiencies are favored since they allow separation into o and x modes, use directly measured experimental quantities, and generalize easily for wave packets. The total energy conversion efficiency differs from the power conversion efficiency by the ratio of the group speeds for each mode, is less than 10% for the value of β=0.01 simulated, and decreases linearly with β. Since β≈10−5–10−4 in the solar wind and corona, this β dependence is important in applications. (7) The interference effect and the disappearance of the x mode at Ω≳1 can be accounted for semiquantitatively using a WKB-type analysis. (8) Constraints on density turbulence are developed for the x mode to be generated and be able to propagate from the source. (9) Standard parameters for the corona and the solar wind near 1 AU suggest that linear mode conversion should produce both o- and x-mode radiation for solar and interplanetary radio bursts. It is therefore possible that linear mode conversion under these conditions might explain the weak total circular polarizations of type II and III solar radio bursts.

Extraordinary-Mode Radiation Produced by Linear-Mode Conversion of Langmuir Waves

Linear-mode conversion (LMC) of Langmuir waves to radiation near the plasma frequency at density gradients is important for space and astrophysical phenomena. We study LMC in warm magnetized plasmas using numerical electron fluid simulations when the density gradient is parallel to the ambient magnetic field (B0). We demonstrate that LMC can produce extraordinary- (x-) as well as ordinary- (o-) mode radiation from Langmuir waves, contrary to earlier expectations of o mode only. Equal amounts of o- and x-mode radiation are produced in the unmagnetized limit. The x-mode efficiency decreases as B0 increases, while the o-mode efficiency oscillates due to interference between incoming and reflected Langmuir waves. Both x and o modes should be produced for typical coronal and interplanetary parameters, alleviating the depolarization problem for type III solar radio bursts.

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.

Second‐Harmonic Plasma Radiation of Magnetically Trapped Electrons in Stellar Coronae

Plasma radiation near the second harmonic of the plasma frequency driven by the loss-cone instability of magnetically trapped energetic electrons in stellar coronae is considered. Growth rates of longitudinal waves near the upper hybrid frequency are determined for warm background plasma and sufficiently high plasma densities, ωp > ωc, where the electrostatic instability prevails over the electromagnetic cyclotron maser instability, with particular attention given to the intermediate magnetic field condition, 1 < ω/ω 5. The plasma turbulence level and the brightness temperature of the second-harmonic plasma radiation arising from the coalescence of upper hybrid waves are estimated. The brightness temperature can reach ~1014 K for spontaneous conversion of the waves and ~1016 K for induced conversion. The radiation pattern of the second-harmonic plasma emission is also calculated; it shows a prevalence of the extraordinary mode. Analyzing the problem of the escape of radiation from stellar coronae, it is found that the escape window is wider for the o-mode because the x-mode radiation is strongly absorbed by the warm background plasma at the low harmonic gyrolevels, and thus the observed radiation can be polarized in the ordinary sense in the intermediate magnetic field case. Because of the high temperature of the plasma in the coronae of X-ray-emitting stars, the characteristic length scale of the wave conversion and the efficiency of the plasma radiation mechanism can be much higher than on the Sun. The results are discussed in the context of nonthermal quiescent and flare radio emission from active stars.

Electron cyclotron emission imaging diagnostic for the helically symmetric experiment stellarator (abstract)

An eight channel electron cyclotron emission imaging (ECEI) diagnostic system is being developed for use on the helically symmetric experiment (HSX) stellarator. The system utilizes a linear Schottky diode mixer/receiver array, coupled with low cost conventional ECE radiometer IF boards under development at U.C. Davis, to measure the second harmonic x-mode radiation from the plasma. The array is fed with a fixed local oscillator, with single sideband operation enabled via a dichroic plate high pass filter whose cutoff frequency equals that of the local oscillator. A multithrow, high speed (&amp;lt;100 ns switching times) microwave switch is utilized to sequentially connect the output from each mixer channel to the broadband IF receiver system, thus making the array capable of generating a high resolution two-dimensional image of the HSX electron temperature profile at rates exceeding 50 kHz. Instrument details of the system will be described.

Effects of turbulence on the electron cyclotron-maser mechanism for solar microwave spike bursts

It is shown that small magnetic perturbations can significantly alter the rates of cyclotron growth, absorption, mode conversion, and refraction because of the sensitive dependence of these processes on the field strength in narrow layers. In particular, growth lengths are increased, absorption depths decreased, mode conversion becomes more effective, and turbulent refraction leads to isotropization of the emission. The criteria for significant effects to occur are derived and it is shown that they can be met by the few-percent field perturbations observed in coronal loops. Relative to the theory of cyclotron-maser emission in smoothly varying plasmas, perturbations enable fundamental o-mode (o1) and second-harmonic x-mode (x2) radiation to saturate more effectively, increase the chance of x1, o1, and x2 radiation escaping to infinity through absorption and mode-coupling windows, and partially isotropize radiation emitted near the x-mode cutoff. It is concluded that o1 and x1 emission are both likely to be present, and that x2 emission is possible under some circumstances. However, x1 radiation can escape only at near-parallel propagation (θ ≈ 0) or via mode conversion to the o-mode at θ ≈ 90°, whereas o1 and x2 emission can escape for a wide range of angles around θ = 0 and, under many circumstances, near θ = 90°.

Measurement of the relaxation of electron parallel momentum in a tokamak

Measurements of the momentum relaxation rate of suprathermal electrons in the VERSATOR II tokamak have been made using a novel cyclotron resonance technique. Attenuation of probe beams of X-mode radiation is interpreted as cyclotron absorption, yielding a measure of the distribution of current carriers. Differential measurements of two counter-propagating beams eliminate nonresonant effects. A distribution of electrons is produced by lower hybrid current drive and is observed to relax in the absence of wave power. The rate of momentum relaxation is anomalous but consistent with the observed current drive efficiency.

Solar and stellar radio spikes - Limits on the saturation of the electron-cyclotron maser

The solar millisecond radio 'spikes' have been explained in terms of X-mode radiation generated by a maser near the electron gyrofrequency, acting on fast coronal electrons with a loss cone. This maser is a phenomenon described by quasi-linear theory. It is sensitive to the small first-relativistic correction to the gyrofrequency. Thus, it might be disrupted rather easily by nonlinear effects. The maximum radiation density that can be reached before the radiation entrains (phase-locks) the electrons and saturates the maser is discussed. If the observed durations of solar radio spikes are a measure of the rate of scattering into the loss-cone, then the inferred energy density is at least two orders of magnitude less than the energy density at which entrainment sets in. Also, maser emission from auroral kilometric radiation does not reach wave energies critical for electron entrainment. Maser emissions from flare stars, however, show 3-4 orders of magnitude higher radio fluxes and brightness temperatures than for the solar case and are likely to be saturated by entrainment.

A mechanism for bursty radio emission in planetary magnetospheres

Bursty radio emissions are often observed from the polar magnetospheres of the Earth, Jupiter, Saturn, and Uranus in addition to the smooth radio emissions commonly detected. We show that in plasma regimes in which the electron plasma frequency is less than the electron cyclotron frequency, anisotropic electron beams or gyrating electron beams can excite directly broadband electromagnetic radiation. The largest growth is for right‐hand X‐mode radiation with frequencies above the electron cyclotron frequency. This instability can produce bursty, broadband emission, consistent with some of the properties of the radiation observed from the magnetized planets.

Escape of fundamental electron-cyclotron maser emission from the sun and stars

view Abstract Citations (37) References (18) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Escape of Fundamental Electron-Cyclotron Maser Emission from the Sun and Stars Robinson, P. A. Abstract It is shown that fundamental x-mode emission from flaring regions can undergo partial mode conversion at the second-harmonic absorption layer, with a fraction emerging in the o mode through a window near theta = 90 deg; fundamental o-mode radiation can emerge through this window directly. The optical depth for mode-converted x-mode radiation is up to 200 times less than the depth tau(x) for unconverted radiation; the o-mode depth is up to c-squared/V-squared (roughly 1000) times smaller than tau(x), where V is the thermal velocity of the plasma. This mechanism is linear and threshold-free, requires little scattering or refraction of the emitted radiation, and permits the strongest instability (fundamental x-mode) to dominate in producing the observed emission. Publication: The Astrophysical Journal Pub Date: June 1989 DOI: 10.1086/185467 Bibcode: 1989ApJ...341L..99R Keywords: Masers; Radio Emission; Solar Radiation; Stellar Radiation; Absorption Spectra; Coupled Modes; Electron Cyclotron Heating; Escape (Abandonment); Radiative Transfer; Astrophysics; MASERS; STARS: RADIO RADIATION; SUN: RADIO RADIATION full text sources ADS |

Recent trend of plasma diagnostics using scattering/interferometer technique.

Recent research and development on plasma diagnostics related with the scattering/ interferometer technique are surveyed. In particular, the alpha particle dianostics using the collective scattering with the X-mode radiation and the polarimeter for measuring the current density profile in high density and field tokamak plasmas are described.