Chord-averaged Density Research Articles

Characterization of pedestal burst instabilities during I-mode to H-mode transition in the EAST tokamak

Quasi-periodic pedestal burst instabilities (PBIs), featuring alternative turbulence suppression and bursts, have been clearly identified by various edge diagnostics during I-mode to H-mode transition in the EAST Tokamak. The radial distribution of the phase perturbation caused by PBI shows that PBI is localized in the pedestal. Prior to each PBI, a significant increase of density gradient close to the pedestal top can be clearly distinguished, then the turbulence burst is generated, accompanied by the relaxation of the density profile, and then induces an outward particle flux. The relative density perturbation caused by PBIs is about 6%–8%. Statistic analyses show that the pedestal normalized density gradient R/L n triggering the first PBI has a threshold value, mostly in the range of 22–24, suggesting that a PBI triggering instability could be driven by the density gradient. And R/L n triggering the last PBI is about 30–40 and seems to increase with the loss power and the chord-averaged density. In addition, the frequency of PBI is likely to be inversely proportional to the chord-averaged density and the loss power. These results suggest that PBIs and the density gradient prompt increase prior to PBIs can be considered as the precursor for controlling I–H transition.

Gas-puff induced cold pulse propagation in ADITYA-U tokamak

Short bursts (∼1 ms) of gas, injecting ∼1017–1018 molecules of hydrogen and/or deuterium, lead to the observation of cold pulse propagation phenomenon in hydrogen plasmas of the ADITYA-U tokamak. After every injection, a sharp increase in the chord-averaged density is observed followed by an increase in the core electron temperature. Simultaneously, the electron density and temperature decrease at the edge. All these observations are characteristics of cold pulse propagation due to the pulsed gas application. The increase in the core temperature is observed to depend on the values of both the chord-averaged plasma density at the instant of gas-injection and the amount of gas injected below a threshold value. Increasing the amount of gas-puff leads to higher increments in the core-density and the core-temperature. Interestingly, the rates of rise of density and temperature remain the same. The gas-puff also leads to a fast decrease in the radially outward electric field together with a rapid increase in the loop-voltage suggesting a reduction in the ion-orbit loss and an increase in Ware-pinch. This may explain the sharp density rise, which remains mostly independent of the toroidal magnetic field and plasma current in the experiment. Application of a subsequent gas-puff before the effect of the previous gas-pulse dies down, leads to an increase in the overall electron density and consequently the energy confinement time.

Two-temperature effects in Hall-MHD simulations of the HIT-SI experiment

A two-temperature Hall-magnetohydrodynamic (Hall-MHD) model, which evolves the electron and ion temperatures separately, is implemented in the PSI-Tet 3D MHD code and used to model plasma dynamics in the Helicity Injected Torus–Steady Inductive (HIT-SI) experiment. The two-temperature model is utilized for HIT-SI simulations in both the PSI-Tet and NIMROD codes at a number of different injector frequencies in the 14.5–68.5 kHz range. At all frequencies, the NIMROD two-temperature model results in increased toroidal current, lower chord-averaged density, higher average temperatures, outward radial shift of the current centroid, and axial symmetrization of the current centroid, relative to the single-temperature NIMROD simulations. The two-temperature PSI-Tet model illustrates similar trends, but at high frequency operation, it exhibits lower electron temperature, smaller toroidal current, and decreased axial symmetrization with respect to the single-temperature PSI-Tet model. With all models, average temperatures and toroidal currents increase with the injector frequency. Power balance and heat fluxes to the wall are calculated for the two-temperature PSI-Tet model and illustrate considerable viscous and compressive heating, particularly at high injector frequency. Parameter scans are also presented for artificial diffusivity, wall temperature, and density. Both artificial diffusivity and the density boundary condition significantly modify the plasma density profiles, leading to larger average temperatures, toroidal current, and relative density fluctuations at low densities. A low density simulation achieves sufficiently high current gain (G > 5) to generate significant volumes of closed flux lasting 1–2 injector periods.

Observations of toroidal plasma rotation reversal in the Aditya-U tokamak

An impurity ion toroidal rotation profile has been observed in the ohmic discharges of the Aditya-U tokamak from the Doppler-shifted passive charge exchange line emission of C5+ at 529 nm. The line has been monitored using a space-resolved visible spectroscopy diagnostic consisting of a 1 m multi-track spectrometer coupled with a charge coupled device (CCD) detector. In typical discharges with central chord-averaged electron density ≲2.5 × 1019 m−3, the maximum toroidal rotation velocity at the plasma core is found to be −20 km s−1 in the counter-current direction. Reversal of plasma toroidal rotation is observed for high electron density discharges with a central chord-averaged density of ~5 × 1019 m−3. During the rotation reversal, the direction of toroidal rotation changes to the co-current direction in the core. Interestingly, in the discharges with a central chord-averaged electron density of ≲2.5 × 1019 m−3, where neon gas is injected at the plasma current flat-top, the toroidal rotation reversal has been observed after the gas puff.

Density modulation experiments within the full metallic wall and lithium limiter on the HT-7 tokamak

The methods to study the particle diffusion coefficient and the convection velocity were primarily based on the density modulation and the use of D2 gas puffing in the HT-7 tokamak. The density was measured by a five-channel far-infrared interferometer. The density modulation amplitude was 10% of the central chord average background density, and the modulation frequency was 7 Hz in experiments. For the reason that the wall condition and the types of the limiter will influence the transport coefficients, during this new experimental cycle we have improved the device on HT-7 such as updating the full metallic wall and making use of the lithium limiter, forming the new experimental condition we take advantage by determining the particle diffusion coefficient (D) and the convection velocity (V) and testing the coefficient changes under different wall conditions and the limiter. Apparently, D and V were obtained at different background plasmas (the central chord average density ⟨ne⟩=1.5×1019 and 2.2×1019 m−3). It is observed that the influence of density modulation on the main plasma parameters is very weak, and a comparison will be discussed between the experimental data D, V under the lithium limiter and the ones under the carbon limiter.

Abel inversion of asymmetric plasma density profile at Aditya tokamak

In Aditya tokamak, at Institute for Plasma Research, till now, multi-channel microwave interferometer system is used to measure the cord averaged plasma density at predefined radial position. An inversion code is developed to determine the local density profile from the chord average density measurement of radially asymmetric plasma. The radial density profile is interpolated using Spline interpolation analytical technique for symmetric plasma density profile. Code implements the Slice and Stack method to determine localized density from asymmetric averaged plasma density measurement from interferometer. Inverted results are tested with various monotonically varying asymmetric radial density profiles of the plasma shots. It also provides the poloidal picture of plasma density distribution with circular constant density surfaces. Localized density measurements, which is very important for successful operation of tokamak, is in agreement with observation of other diagnostics.

Density Modulation Experiments to Determine Particle Transport Coefficients on HT-7 Tokamak

The particle diffusion coefficient and the convection velocity were studied based on the density modulation using D2 gas puffing on the HT-7 tokamak. The density was measured by a five-channel FIR interferometer. The density modulation amplitude was 10% of the central chord averaged background density and the modulation frequency was 10 Hz in the experiments. The particle diffusion coefficient (D) and the convection velocity (V) were obtained for different background plasmas with the central chord averaged density <ne> = 1.5×1019 m−3 and 3.0×1019 m−3 respectively. It was observed that the influence of density modulation on the main plasma parameters was very weak. This technology is expected to be useful for the analysis of LHW and IBW heated plasmas on HT-7 tokamak in the near future.

A monitor of chord-averaged plasma density

A single-channel two-frequency chord-averaged density monitor for measuring the plasma density in tokamaks is described. The operating principle of the monitor is based on measuring the phase shift between two microwave signals at close frequencies transmitted through plasma along one and the same chord. At an appropriately chosen difference between these frequencies, the measured phase difference is <2π in the entire range of measured densities, thereby ensuring measurements without fringe jumps. The advantage of this method for measuring the delay time compared to the pulsed method consists in an increased sensitivity of measurements owing to the possibility of filtering signals at fairly high intermediate frequency (∼0.5–1 GHz). It is additionally possible to adjust the dynamic range of density measurements by selecting (optimizing) the frequency difference. The schematic diagram and the main performance characteristics of the monitor are presented. The results from laboratory tests of the monitor and those obtained with it on the T-11M facility are presented. The range of density measurements is 0.7 × 1012–0.7 × 1014 cm-3, the measurement accuracy is no worse than 1%, and the temporal resolution reaches 30 ns.

Boundary plasma behavior under different wall conditions on HT-7 tokamak

The boundary plasma behaviors under different wall conditions on the HT-7 tokamak were investigated using a reciprocating Langmuir probe system. The ion cyclotron radio frequency (ICRF) boronization has become a routine wall conditioning technique on HT-7. After ICRF boronization, the turbulence de-correlation and radial electric field shear in the plasma edge region accompanied the strong reduction of impurity radiation, which resulted in the enhancement of edge transport barrier. The central chord average density scanning experiment had been carried out on HT-7. Accompanying the increase of density an evident increase of radial electric field shear could be found in the plasma edge region, which could be responsible for the turbulence de-correlation and improved particle confinement. The results presented here suggest a link between wall conditions and boundary plasma physics; especially interplay between atomic processes and turbulence through the formation of radial electric field shear in the plasma edge region.

An investigation of density fluctuations and electron transport in the Madison Symmetric Torus reversed-field pinch

The origin of large scale density fluctuations and their role in electron transport has been investigated in the Madison Symmetric Torus reversed-field pinch [R. N. Dexter et al., Fusion Tech. 19, 131 (1991)]. At medium frequencies (10–30 kHz), ñ is associated with core-resonant magnetic fluctuations having poloidal mode number m=1 and toroidal mode numbers n=6–10. These chord-averaged density fluctuations are shown to be coherent with large scale radial flow fluctuations, permitting fluctuation-induced particle flux of the form 〈ñṽr〉 to be studied. In the edge, although amplitudes are large, ñ and ṽr are out of phase, consistent with advection of the mean density gradient by perturbed magnetic field lines and no electron transport. During improved confinement discharges, produced by inductive current drive, both fluctuation amplitudes and the total radial electron flux are greatly reduced over the entire plasma cross section.

Calculation of Neutral Particle Energy Spectra in Tokamak by Using the Monte Carlo Method

Neutral particle energy spectra in the HT-7 tokamak are calculated by using the Monte Carlo method. It can reproduce the spectra measured in experiment. Differences of neutral particle energy spectra in higher and lower electron density plasma are discussed. Results show that the ion temperature given by neutral particle energy spectra is lower than the real ion temperature, but the deviation is within 10% if the ion temperature is less than 800 eV and the central chord-averaged electron density does not exceed 3 × 1013 cm-3. But for ion temperature higher than 1000 eV at the central chord-averaged density limit up to 5 × 1013 cm-3, the neutral particle energy spectra can still give the ion temperature within 10% deviation.

Observation of Low Frequency Ion Mode Turbulence in Tokamak Plasma

Features of a low frequency ion mode were observed together with the electron drift wave type fluctuations by a Langmuir probe array in the bulk plasma inside the velocity shear layer in the KT-5C tokamak. The measured ion mode wavenumber, scaling of the ion mode with chord-averaged density, and the estimated ηi value suggest that the observed ion mode may be the theoretically predicted ηi mode driven by ion temperature gradient.

Microturbulence in HT-6M Tokamak

The small scale density fluctuations in the interior of HT-6M Ohmic plasma have been studied by CO2 laser collective scattering system in deuterium discharges covering a range of n̄eqa (chord-average density times safety factor at the limiter) and energy confinement time. The relative density fluctuation level in the interior is inversely proportional to the toroidal magnetic field and average density, and the energy confinement time τE decreases with the fluctuation level increasing in the region where τE linearly increases with n̄eqa and statisfies the Goldston scaling law. It is suggested that the microturbulence in the interior zone is responsible for anomalous transport in tokamaks.

Effects of NBI power modulation on pellet injection into a current free plasma in Heliotron E

Hydrogen pellets are injected into the current free neutral beam injection (NBI) heated plasma in Heliotron E to effectively increase the plasma density and the internal energy. The chord averaged density of the target plasma is about 2 × 1019 m−3, and the density increase by pellet injection is (4 – 6) × 10l9 m−3. During this process the plasma remains stable. Under the present operational conditions, the pellet reaches the central axis: the penetration depth ranges from 27 cm to 35 cm (the magnetic axis corresponds to 30 cm). To optimize the plasma parameters obtainable by pellet injection, the NBI heating power is modulated. It is found that the mode of operation in which pellets are injected at relatively low NBI power followed by a higher-NBI-power phase is preferable to that in which pellets are injected during a constant high power phase. The main reason for this difference is the penetration depth of the pellet in the plasma which determines the deposition profile of particles in the plasma.