Radial Electric Field Fluctuations Research Articles

I-mode investigation on the Experimental Advanced Superconducting Tokamak

By analyzing large quantities of discharges in the unfavorable ion drift direction, I-mode operation has been confirmed in the Experimental Advanced Superconducting Tokamak. During the L-mode to I-mode transition, the energy confinement is improvement substantially by the formation of a high-temperature edge pedestal, while the particle confinement remains almost identical to that in the L-mode. Similar with I-mode observations on other devices, the Er profiles obtained by the eight-channel Doppler backscattering system (DBS8) (Hu et al 2017 Rev. Sci. Instrum. 88 073504) show a deeper edge Er well in the I-mode than that in the L-mode. Additionally a weak coherent mode (WCM) with the frequency range of 40–150 kHz is observed at the edge plasma with the radial extent of about 2–3 cm. WCM could be observed in both density fluctuation and radial electric field fluctuation, and the bicoherence analyses showed significant couplings between WCM and high frequency turbulence, implying that the Er fluctuation and the caused flow shear from WCM should play an important role during I-mode operation. In addition, a low-frequency oscillation with a frequency range of 5–10 kHz is always accompanied with WCM, where the geodesic acoustic model intensity decreases or disappears. The evidence shows that the a low-frequency oscillation may be a novel kind of limited cycle oscillation but further investigation is needed to explain the new properties such as the harmonics and obvious magnetic perturbations.

Observation of geodesic acoustic mode in SINP-tokamak and its behaviour with varying edge safety factor

The spectral analysis of floating potential fluctuations measured in the edge plasma region (0.87 < r/a < 1.0) of Saha Institute for Nuclear Physics tokamak (SINP-tokamak) using Langmuir probes reveals the existence of a highly coherent mode with a frequency in the range of 15–21 kHz. Long range correlations in poloidal and toroidal directions are observed over a wide range of plasma discharges having different values of the edge safety factor, from very low qedge (<2) to high qedge (>3). These coherent modes are simultaneously observed in density and radial electric field fluctuation spectra as well. These coherent modes are identified as geodesic acoustic modes (GAMs) having different characteristics over the entire qedge range. In discharges with qedge greater than 3, the local wave number spectra of the mode exhibit the properties of continuum GAM with the observed poloidal and toroidal mode numbers of m ∼ 0 and n ∼ 0, and the mode is radially localized. The observed frequency and its variation with the safety factor for qedge > 3 closely agree with the theoretical predictions using the measured values of temperature. In contrast, for qedge < 3.0, the GAM nature changes from continuum to the Eigenmode as the associated GAM frequency remained uniform at ∼13–17 kHz over the q edge range of 1.5 to 3 and ceased to depend on local temperature. Furthermore, the poloidal wave number of the coherent mode no longer remains zero and is observed to increase when qedge falls below 2.5. Coherent modes in magnetic fluctuations having similar frequencies to those of electrostatic fluctuations are also observed in the discharges with the q edge below 2.5. The coupling of these electrostatic and magnetic modes may be responsible for triggering the Eigenmode GAM.

Spatiotemporal characterization of zonal flows with multi-channel correlation Doppler reflectometers in the HL-2A Tokamak

The oscillations of poloidal plasma flows induced by radially sheared zonal flows are investigated by newly developed correlation Doppler reflectometers in the HL-2A tokamak. The non-disturbing diagnostic allows one to routinely measure the rotation velocity of turbulence, and hence the radial electric field fluctuations. With correlation Doppler reflectometers, a three-dimensional spatial structure of geodesic acoustic mode (GAM) is surveyed, including the symmetric feature of poloidal and toroidal Er fluctuations, the dependence of GAM frequency on radial temperature and the radial propagation of GAMs. The co-existence of low-frequency zonal flow and GAM is presented. The temporal behaviors of GAM during ramp-up experiments of plasma current and electron density are studied, which reveal the underlying damping mechanisms for the GAM oscillation level.

Gyrokinetic simulations of interplay between geodesic acoustic modes and trapped electron mode turbulence

Radial electric field dynamics are studied in gyrokinetic full-f simulations of tokamak plasmas. Calculations use parameters similar to a TEXTOR L-mode discharge in the presence of a steep density gradient and dominating trapped electron mode turbulence in the edge pedestal of the plasma. Temperature, density gradient and ion species are varied to study the radial electric field fluctuations and their relationship with thermal transport. The dominant frequency of electric field oscillations is within the geodesic acoustic mode range, and their amplitude is found to be clearly correlated with transport across the parametric scan.

Damping of radial electric field fluctuations in the TJ-II stellarator

The drift-kinetic equation is solved for low density TJ-II plasmas employing slowly varying, time-dependent profiles. This allows to simulate density ramp-up experiments and to describe from first principles the formation and physics of the radial electric field shear, which is associated to the transition from electron to ion root. We show that the range of frequencies of plasma potential fluctuations in which zonal flows are experimentally observed is neoclassically undamped in a neighbourhood of the transition. This makes the electron root regime of stellarators, close to the transition to ion root, a propitious regime for the study of zonal-flow evolution. We present simulations of collisionless relaxation of zonal flows, in the sense of the Rosenbluth and Hinton test, that show an oscillatory behaviour in qualitative agreement with the experiment close to the transition.

Anomalous transport and multi-scale drift turbulence dynamics in tokamak ohmic discharge as measured by high resolution diagnostics and modeled by full-f gyrokinetic code

Quantitative reproduction of selected micro-, meso- and macro-scale transport phenomena as measured in the FT-2 tokamak is reached by Elmfire global full-f nonlinear gyrokinetic particle-in-cell simulation predictions. A detailed agreement with mean equilibrium flows, oscillating fine-scale zonal flows and turbulence radial correlation length observed by a set of sophisticated microwave backscattering techniques, as well as a good fit of the thermal diffusivity data in the central and gradient region of discharge are demonstrated. Both the shift and the broadening of the power spectrum of synthetic and experimental Doppler reflectometry diagnostics have been found to overlap perfectly at various radial positions, indicating similar rotation and spreading of the selected density fluctuations. At the same time similar radial electric field dynamics, spatial structure and outward geodesic acoustic mode (GAM) propagation have been observed by comparisons of the probability distribution function, the dominant frequency, the coherence and the cross-phase of the simulated and experimentally measured radial electric field fluctuations, identifying the turbulent driven GAM as a key contributor to the observed strong temporal variation of the radial electric field affected by impurity ions.

Potential fluctuation associated with the energetic-particle-induced geodesic acoustic mode in the Large Helical Device

Geodesic acoustic modes (GAM) driven by energetic particles are observed in the Large Helical Device (LHD) by a heavy ion beam probe. The GAM localizes near the magnetic axis. It is confirmed that the energetic-particle-induced GAM is accompanied by an electrostatic potential fluctuation and radial electric field fluctuation. The amplitude of the potential fluctuation is several hundred volts, and it is much larger than the potential fluctuation associated with turbulence-induced GAMs observed in the edge region in tokamak plasmas. The energetic-particle-induced GAM modulates the amplitude of the density fluctuation in a high-frequency range. The observed GAM frequency is constant at the predicted GAM frequency in plasmas with reversed magnetic shear. On the other hand, it shifts upwards from the predicted GAM frequency in plasmas with monotonic magnetic shear.

A nine-electrode probe for simultaneous measurement of all terms in the ideal radial Ohm’s law

A Nine-Electrode Probe (NEP) has been developed for simultaneous measurement of all terms in the ideal Ohm’s law E+U×B=0 in the radial (r̂) direction in cylindrical geometry, where E is the electric field, U is the plasma flow velocity, and B is the magnetic field. The probe consists of two pairs of directional Langmuir probes (“Mach” probes) to measure the axial (ẑ) and azimuthal (θ̂) plasma flows, two pairs of floating Langmuir probes at different radial positions to measure the radial electric field, and two B-dot coils to measure the axial and azimuthal magnetic field. The measurement is performed in the Flowing Magnetized Plasma (FMP) experiment. Two flow patterns are identified in the FMP experiment by the NEP. The peak-to-peak values of radial electric field fluctuation is 1.5–4 times of the mean values. Comparisons of ∣U×B∣r and Er show that Er+ ∣U×B∣r is not zero within some periods of discharge. This deviation suggests non-ideal effects in Ohm’s law can not be neglected.

Mechanism of Annular Two-Phase Flow Heat Transfer Enhancement and Pressure Drop Penalty in the Presence of a Radial Electric Field—Turbulence Analysis

The mechanism of heat transfer enhancement and pressure drop penalty in the presence of a radial electric field for the two-phase (liquid/vapor) annular flow is presented. The turbulence spectral theory shows that the radial electric field fluctuation changes the turbulent energy distribution, especially in the radial direction. Consequently, the Reynolds stresses are directly affected by the applied electric field. The analysis reveals that the influence of the applied electric field on the turbulence distribution in an annular two-phase flow leads to the changes in the heat transfer and the pressure drop. The magnitudes of the heat transfer enhancement and the pressure drop penalty are strongly related to the ratio of the radial pressure difference generated by the EHD force to the axial frictional pressure drop. The existing experimental data agree with the predictions of the analysis presented in this paper. The analysis developed here can be a valuable tool in properly predicting the two-phase annular flow heat transfer enhancement and pressure drop penalty in the presence of a radial electric field for both convective boiling and condensation processes.

Electron Kinetics of Ionization Waves in Helium Positive Columns

Electron kinetics related to ionization waves is discussed using the results of temporally and radial-spatially resolved Langmuir probe measurements during propagation of an artificially excited ionization wave packet along a helium positive column at 0.55 Torr pressure and 200 mA discharge current. The results show a strong interdependence between the axial and radial electric field fluctuations and the electron energy distribution fluctuations. Fluctuations in the electron energy distribution function related to the ionization waves revealed a complex energy spectrum whose low (0–10 eV) and high (10–25 eV) energy regions oscillate with opposite phases.