Search

Enhancements of edge zonal flows, radial electric fields, and turbulence are observed in electron cyclotron resonance heating-heated plasmas (Zhao et al 2013 Nucl. Fusion 53 083011). In this paper, the effects of sawtooth heat pulses on flows and turbulence are presented. These experiments are performed using multiple Langmuir probe arrays in the edge plasmas of the HL-2A tokamak. The edge zonal flows, radial electric fields, and turbulence are all enhanced by sawteeth. Propagation of the zonal flow and turbulence intensities is also observed. The delay time of the maximal intensity of the electric fields, zonal flows, and turbulence with respect to the sawtooth crashes is estimated as ∼1 ms and comparable to that of the sawtooth-triggered intermediate phases. Not only the zonal flows but also the radial electric fields lag behind the turbulence. Furthermore, the intensities of both the zonal flows and electric fields nearly linearly increase/decrease with the increase/decrease of the turbulence intensity. A double-source predator–prey model analysis suggests that a relatively strong turbulence source may contribute to the dominant zonal flow formation during sawtooth cycles.

The abrupt and strong excitation of the geodesic acoustic mode (GAM) has been found in the large helical device (LHD), when the frequency of a chirping energetic particle-driven GAM (EGAM) approaches twice that of the GAM frequency. The temporal evolution of the phase relation between the abrupt GAM and the chirping EGAM is common in all events. The result indicates a coupling between the GAM and the EGAM. In addition, the nonlinear evolution of the growth rate of the GAM is observed, and there is a threshold in the amplitude of the GAM for the appearance of nonlinear behavior. A threshold in the amplitude of the EGAM for the abrupt excitation of the GAM is also observed. According to one theory (Lesur et al 2016 Phys. Rev. Lett. 116 015003, Itoh et al 2016 Plasma Phys. Rep. 42 418) the observed abrupt phenomenon can be interpreted as the excitation of the subcritical instability of the GAM. The excitation of a subcritical instability requires a trigger and a seed with sufficient amplitude. The observed threshold in the amplitude of the GAM seems to correspond with the threshold in the seed, and the threshold in the amplitude of the EGAM seems to correspond with the threshold in the magnitude of the trigger. Thus, the observed threshold supports the interpretation that the abrupt phenomenon is the excitation of a subcritical instability of the GAM., source:T. Ido et al 2017 Nucl. Fusion 57 072009, source:https://doi.org/10.1088/1741-4326/aa665a

In a collisionless plasma, it is known that linearly stable modes can be destabilized (subcritically) by the presence of structures in phase space. However, nonlinear growth requires the presence of a seed structure with a relatively large threshold in amplitude. We demonstrate that, in the presence of another, linearly unstable (supercritical) mode, wave-wave coupling can provide a seed, which is significantly below the threshold, but can still grow by (and only by) the collaboration of fluid and kinetic nonlinearities. By modeling the subcritical mode kinetically, and the impact of the supercritical mode by simple wave-wave coupling equations, it is shown that this new kind of subcritical instability can be triggered, even when the frequency of the supercritical mode is rapidly sweeping. The model is applied to the bursty onset of geodesic acoustic modes in a LHD experiment. The model recovers several key features such as relative amplitude, time scales, and phase relations. It suggests that the strongest bursts are subcritical instabilities, driven by this mechanism of combined fluid and kinetic nonlinearities., source:Nonlinear Excitation of Subcritical Instabilities in a Toroidal Plasma M. Lesur, K. Itoh, T. Ido, M. Osakabe, K. Ogawa, A. Shimizu, M. Sasaki, K. Ida, S. Inagaki, S.-I. Itoh, and the LHD Experiment Group Phys. Rev. Lett. 116, 015003 – Published 8 January 2016, source:https://doi.org/10.1103/PhysRevLett.116.015003

Abrupt and strong excitation of a mode has been observed when the frequency of a chirping energetic-particle driven geodesic acoustic mode (EGAM) reaches twice the geodesic acoustic mode (GAM) frequency. The frequency of the secondary mode is the GAM frequency, which is a half-frequency of the primary EGAM. Based on the analysis of spatial structures, the secondary mode is identified as a GAM. The phase relation between the secondary mode and the primary EGAM is locked, and the evolution of the growth rate of the secondary mode indicates nonlinear excitation. The results suggest that the primary mode (EGAM) contributes to nonlinear destabilization of a subcritical mode., source:Strong Destabilization of Stable Modes with a Half-Frequency Associated with Chirping Geodesic Acoustic Modes in the Large Helical Device T. Ido, K. Itoh, M. Osakabe, M. Lesur, A. Shimizu, K. Ogawa, K. Toi, M. Nishiura, S. Kato, M. Sasaki, K. Ida, S. Inagaki, S.-I. Itoh, and the LHD Experiment Group Phys. Rev. Lett. 116, 015002 – Published 8 January 2016, source:https://doi.org/10.1103/PhysRevLett.116.015002

In recent deuterium experiments on the large helical device (LHD), we succeeded in expanding the temperature domain to higher regions for both electron and ion temperatures. Suppression of the energetic particle driven resistive interchange mode (EIC) by a moderate electron temperature increase is a key technique to extend the high temperature domain of LHD plasmas. We found a clear isotope effect in the formation of the internal transport barrier in high temperature plasmas. A new technique to measure the hydrogen isotope fraction was developed in the LHD in order to investigate the behavior of the isotope mixing state. The technique revealed that the non-mixing and the mixing states of hydrogen isotopes can be realized in plasmas. In deuterium plasmas, we also succeeded in simultaneously realizing the formation of the edge transport barrier (ETB) and the divertor detachment. It is found that resonant magnetic perturbation plays an important role in the simultaneous formation of the ETB and the detachment. Contributions to fusion reactor development from the engineering point of view, i.e. negative-ion based neutral beam injector research and the mass balance study of tritium, are also discussed., source:https://doi.org/10.1088/1741-4326/ac3cda, identifier:0000-0001-5220-947X

A new Doppler radar using millimeter-waves in the Ka-band, named the “dual-comb Doppler reflectometer”, has been developed to measure the turbulence intensity and its velocity in high-temperature plasmas. For the realization of a fusion power generation, it is very important to know the spatial structure of turbulence, which is the cause of plasma confinement degradation. As a non-invasive and high spatial resolution measurement method for this purpose, we apply a multi-frequency Doppler radar especially with simultaneous multi-point measurement using a frequency comb. The newly developed method of synchronizing two frequency combs allows a lower intermediate frequency (IF) than the previously developed frequency comb radar, lowering the bandwidth of the data acquisition system and enabling low-cost, long-duration plasma measurements. In the current dual-comb radar system, IF bandwidth is less than 0.5 GHz; it used to be 8 GHz for the entire Ka-band probing. We applied this system to the high-temperature plasma experimental device, the Large Helical Device (LHD), and, to demonstrate this system, verified that it shows time variation similar to that of the existing Doppler radar measurements., source:Appl. Sci. 2022, 12(9), 4744; https://doi.org/10.3390/app12094744, source:https://doi.org/10.3390/app12094744, identifier:0000-0001-5473-2109

It is demonstrated that the zonal flow is controlled by the parallel flow in the cylindrical plasmas. A three-dimensional turbulence simulation is performed, based on the reduced fluid model. The background parallel flow is applied by introducing the time independent parallel momentum source. Changing the magnitude of the momentum source, the behavior of the zonal flow is investigated. The drift wave turbulence is affected by the parallel flow, and it shows the spatial competition between turbulence modes. The spatial competition appears/disappears abruptly, depending on the intensity of the parallel momentum source. As a consequence of the transition between the turbulence state, the radial profile of the turbulence drastically changes, which leads to the change of the turbulence force to drive the zonal flow. In this way, the parallel flow indirectly affects the zonal flow through the deformation of the turbulence., source:https://doi.org/10.1585/pfr.14.1401161

Changing the experimental conditions such as the neutral pressure and the magnetic field strength provides a drastic change in the turbulence states in linear magnetized helicon plasmas. In order to define the turbulence states and their occurrence region a throughout parameter scan in the two-dimensional parameter space was performed. The classification of the turbulence states is carried out phenomenologically based on the turbulence spectrum and the waveform., source:https://doi.org/10.1585/pfr.12.1401019, identifier:0000-0001-5669-1937

In this overview, we explain methods to identify hysteresis in the gradient-flux relation for energy transport in a core plasma, which is revealed by a power modulation experiment. Two approaches to experimentally observe the hysteresis relation are discussed in detail. Advanced data analysis methods are also illustrated. A theoretical analysis of the process that couples heat to turbulence is also addressed., source:https://doi.org/10.1585/pfr.11.2503086, identifier:0000-0001-5093-6331

Multiple free energy sources for instabilities coexist in magnetized plasmas with density gradient and velocity shear. Linear stabilities are investigated, and the mutual relation between resistive drift wave, D'Angelo mode and flute mode is systematically clarified. By evaluating the linear growth rates, dominant instability is categorized in a parameter space. The parallel wavenumber spectrum could be used as a guideline for the identification of the instabilities., source:https://doi.org/10.1585/pfr.12.1401042

We have proposed and developed a digital correlation ECE (DCECE) measurement method using a high-speed sampling digitizer. This is the first report of the validation of the DCECE using actual data of plasma. The experiment was conducted in the 18th cycle experiments in the large helical device (2014 fiscal year) and a several kHz electron temperature fluctuation was detected in the high ion temperature plasma. The results were compared to the conventional radiometer type ECE measurement. Moreover, it was found that the correlation profile depends on the analytical parameters of the IF conversion., source:https://doi.org/10.1585/pfr.11.2402072, identifier:0000-0002-8626-129X

A ka-band multi-channel Doppler reflectometer system was constructed for the Large Helical Device (LHD) using a comb frequency generator as a source. A filter bank system is utilized for precise quadrature phase detection, and preliminary back-scattered waves were obtained in LHD plasma experiments. In addition, a direct digital signal acquisition system was successfully demonstrated for providing a greater number of multi-channel measurements., source:https://doi.org/10.1585/pfr.9.1402149, identifier:0000-0001-5473-2109

The diagnostics method of a tracer-encapsulated solid pellet (TESPEL) has been developed. TESPEL consists of polystyrene as an outer shell and of specific material as a tracer in the core. Owing to the advantages of the TESPEL, the following results have been successfully obtained: (1) distinctive different feature of impurity transport between the plateau and Pfirsch-Schlüter regimes depending on the impurity source location which is analyzed with the STRAHL code, (2) specific feature of a non-local thermal transport such as abrupt increase of electron temperature in the plasma core in case of plasma cooling in the plasma periphery due to a small TESPEL injection, (3) spatially resolved energy distribution of the high energy particles obtained by a pellet charge exchange method, (4) longer impurity containment inside the magnetic island which is observed by depositing the tracers in the magnetic island by means of the TESPEL, and (5) identification of new spectral lines using interested atoms contained in the core of the TESPEL., source:https://doi.org/10.1585/pfr.9.1402039

By employing a new method for studying dynamical transport, hysteresis in the flux-gradient relation was recently discovered for modulated heating. In this paper, the new method is compared to the conventional heat-pulse propagation method. We also demonstrate integration of results from the new method with the power balance thermal conductivity., source:https://doi.org/10.1585/pfr.8.1202172

A new method is presented for identifying waveforms of fluctuations in turbulent plasmas. The method is based on heartbeat analysis in which the convolution of a waveform is obtained by employing the phase tracking method. Phase tracking is performed by correlating raw time-series data with a template waveform; the template is evaluated through iteration procedure. The method is applied to fluctuations in a PANTA plasma, and the nonlinear waveform and its distribution of periods are obtained., source:https://doi.org/10.1585/pfr.9.1201016

We have used the convolution method to obtain higher harmonics of the temperature perturbation in a heating modulation experiment on LHD plasma. In comparison with predictions based on the diffusive model, amplitudes of higher harmonics (such as the seventh) decayed much more slowly as they propagated radially. Changes in the time derivative of temperature at the onset and turning-off of heating power remained much sharper after radial propagation than those predicted by the diffusion model., source:https://doi.org/10.1585/pfr.8.1202173

We propose the use of a microwave comb for measuring the density profile as a continuous function of radius with high temporal resolution. The analysis focuses on phase-sensitive convolution of the reflected waves: convolution of pulses can eliminate statistical noise. The time resolution for constructing the density profile is also discussed., source:https://doi.org/10.1585/pfr.8.1201171

To study the nonlinear saturation mechanism in turbulent plasmas, we plan to obseve ion temperature gradient (ITG) modes in the Plasma Assembly for Nonlinear Turbulence Analysis (PANTA). Linear growth rates of ITG modes in the linear device are analyzed to determine their excitation conditions using a three-field fluid model to describe the ion motion. Parameter scans of the linear growth rate show the threshold for ITG mode excitation, and the typical plasma parameters of the PANTA are found in the unstable domain., source:https://doi.org/10.1585/pfr.8.2403133

Turbulence Diagnostic Simulator is an assembly of simulation codes to clarify the formation mechanism of turbulent structures by numerical diagnostics in magnetically confined plasmas. Global simulations are carried out using a reduced MHD model of drift-interchange mode in helical plasmas, and time series data of 3-D fluctuation fields are produced. It includes localized modes in their rational surfaces, and broad modes spread in the radial direction. Magnitudes of nonlinear couplings from the convective derivative are evaluated in the nonlinear saturated states. The radial profile shows that there exist strong mode excitation near the center, various modes and nonlinear couplings with higher m modes in the middle radius, small number of propagating modes near the edge, which contribute to the pressure profile modification. For the detection of the different features, combination of several diagnostics is necessary., source:https://doi.org/10.1585/pfr.8.2403070, identifier:0000-0001-8210-5700

A nonlinear simulation of resistive drift wave turbulence in a cylindrical plasma is carried out. Long time evolution of turbulence with formation of a zonal flow is obtained for more than 1000 times of the typical drift wave period, which is sufficient for statistical analyses. Dynamical particle and momentum balance for the formation of the mean reveals that the radial turbulent fluxes are dominant contributors for the evolution of fluctuations. The particle flux is found to precede the momentum flux for 0.4 times of the typical drift wave period with large temporal variance. The analyses of the time series data of 3-D fields give the understanding of the dynamical structural formation mechanism., source:https://doi.org/10.1585/pfr.8.2401113, identifier:0000-0001-6835-1569

Measuring the time variation of the wavenumber spectrum of turbulence is important for understanding the characteristics of high-temperature plasmas, and the application of a Doppler reflectometer with simultaneous multi-frequency sources is expected. To implement this diagnostic in future fusion devices, the use of a phased array antenna (PAA) that can scan microwave beams without moving antennas is recommended. Since the frequency-scanning waveguide leaky-wave antenna-type PAA has a complex structure, we have investigated its characteristics by modeling it with 3D metal powder additive manufacturing (AM). First, a single waveguide is fabricated to understand the characteristics of 3D AM techniques, and it is clear that there are differences in performance depending on the direction of manufacture and surface treatment. Then, a PAA is made, and it is confirmed that the beam can be emitted in any direction by frequency scanning. The plasma flow velocity can be measured by applying the 3D manufacturing PAA to plasma measurement., source:Review of Scientific Instruments 93, 113535 (2022); https://doi.org/10.1063/5.0101723, source:https://doi.org/10.1063/5.0101723, identifier:0000-0001-5473-2109