JT-60U Tokamak Plasmas Research Articles

On the Effect of Beating during Nonlinear Frequency Chirping

Spectral analyses of energetic particle (EP) driven bursts of MHD fluctuations in magnetically confined plasmas often exhibit multiple simultaneous chirps. While the superposition of oscillations at multiple frequencies necessarily causes beating in the signal acquired by a localized external probe, self-consistent hybrid simulations of chirping EP modes in a JT-60U tokamak plasma have demonstrated the possibility of global beating, where the electromagnetic field vanishes globally between beats and reappears with opposite phase. This implies that there can be a single field mode that oscillates at multiple frequencies simultaneously when resonantly driven by multiple density waves in EP phase space. Conversely, this means that the EP density waves are mutually coupled and interfere with each other via the jointly driven field, a mechanism ignored in some theories. In this treatise, we study the role of field pulsations in general and beating in particular using the Hamiltonian guiding center orbit-following code ORBIT with a reduced wave-particle interaction model in realistic geometry. Through amplitude pulsations and phase jumps, beating is found to drive the evolution of EP phase space structures. Observations: (1) Beating causes density wave fronts to advance radially in pulses. The resulting chirps become staircase-like. (2) The beats facilitate convective transfer of material between neighboring layers of phase space density waves. On the one hand, this may delay detachment of solitary vortices. On the other hand, it facilitates the accumulation of hole and clump fragments into larger structures. (3) Long-range chirping occurs when massive holes or clumps detach and drift away from the turbulent belt around the seed resonance. The detached vortices can remain robust and, on average, maintain their concentric nested layers while being perturbed by the field's continued beating.

Gyrokinetic analysis of low-n shear Alfvén and ion sound wave spectra in a high-beta tokamak plasma

Using the linear gyrokinetic code LIGKA, we study the structure of the continuous spectra of shear Alfvén waves (SAW) and ion sound waves (ISW) in a high-beta JT-60U tokamak plasma and look for evidence of Alfvén acoustic couplings or mode conversion. Here, is the complex local eigenfrequency, ρ is a radial coordinate, and we consider waves with low toroidal mode number . We focus on the frequency range between the beta-induced and toroidicity-induced Alfvén frequency gaps. The real frequencies of the gyrokinetic ISW continua are remarkably similar to MHD results. The kinetic damping rates are of order for , and reduce to when the temperature ratio is raised to . It is shown that SAW and ISW continua can be simultaneously excited with an antenna and that the global response of the ISWs is significantly enhanced when the on-axis beta value is raised from to 3.6% while keeping . In contrast, when the ion temperature is increased such that , ISW branches become undetectable in spite of higher . At the same time, a large part of the SAW continuum is locally destabilized by ion temperature gradients and a set of discrete global modes was found, some of which are weakly damped or unstable and interpreted as kinetic beta-induced Alfvén eigenmodes. It is estimated that the kinetic damping of such low-n Alfvénic modes contributes much more to the anomalous bulk ion heating than the excitation of nearby ISW continua, so that Alfvén acoustic couplings in the frequency band are irrelevant within the scope of the model used.

Benchmark of multi-phase method for the computation of fast ion distributions in a tokamak plasma in the presence of low-amplitude resonant MHD activity

The transport of fast ions in a beam-driven JT-60U tokamak plasma subject to resonant magnetohydrodynamic (MHD) mode activity is simulated using the so-called multi-phase method, where 4 ms intervals of classical Monte-Carlo simulations (without MHD) are interlaced with 1 ms intervals of hybrid simulations (with MHD). The multi-phase simulation results are compared to results obtained with continuous hybrid simulations, which were recently validated against experimental data (Bierwage et al., 2017). It is shown that the multi-phase method, in spite of causing significant overshoots in the MHD fluctuation amplitudes, accurately reproduces the frequencies and positions of the dominant resonant modes, as well as the spatial profile and velocity distribution of the fast ions, while consuming only a fraction of the computation time required by the continuous hybrid simulation. The present paper is limited to low-amplitude fluctuations consisting of a few long-wavelength modes that interact only weakly with each other. The success of this benchmark study paves the way for applying the multi-phase method to the simulation of Abrupt Large-amplitude Events (ALE), which were seen in the same JT-60U experiments but at larger time intervals. Possible implications for the construction of reduced models for fast ion transport are discussed.

Self-consistent long-time simulation of chirping and beating energetic particle modes in JT-60U plasmas

Recurring bursts of chirping Alfvén modes that were observed in JT-60U tokamak plasmas driven by negative-ion-based neutral beams (N-NB) are reproduced in first-principle simulations performed with an extended version of the hybrid code MEGA. This code simulates the interactions between gyrokinetic fast ions and magnetohydrodynamic (MHD) modes in the presence of a realistic fast ion source and collisions, so that it self-consistently captures dynamics across a wide range of time scales (0.01–100 ms). The simulation confirms that the experimentally observed phenomena known as ‘fast frequency sweeping (fast FS) modes’ are caused by bursts of energetic particle modes (EPM) with dominant toroidal mode number n = 1. On the long time scale (1–10 ms), the simulation reproduces the chirping range (40–60 kHz), the burst duration (few ms) and intervals (5–10 ms). On the short time scale (0.01–0.1 ms), it reproduces pulsations and phase jumps, which we interpret as the result of beating between multiple resonant wave packets. Having reproduced at multiple levels of detail the dynamics of low-amplitude long-wavelength Alfvén modes driven by N-NB ions, the next goal is to reproduce and explain abrupt large-amplitude events (ALE) that were seen in the same experiments at longer time intervals (10–100 ms).

Scrape-off Layer Plasma Flow in L- and H-Mode Plasmas on JT-60U

Significant progress has been made in understanding the scrape-off layer (SOL) mass transport along magnetic field lines — the SOL flow. Understanding the driving mechanisms of the SOL flow was summarized based on experiments in the JT-60U tokamak plasmas. Fast SOL flow with parallel Mach numbers of 0.2-1 was generated from the low magnetic field side (LFS) SOL to the high magnetic field side (HFS) divertor for the ion ∇B drift direction toward the divertor. The SOL flow pattern was formed mainly by the LFS enhanced in-out asymmetry in diffusion and by classical drifts in the torus. Detachment of the divertor plasma affected enhancement of the SOL flow at the HFS SOL. Dynamics of the SOL flow were measured during the transient event of edge localized modes (ELM), and the flow pattern of the plasma filaments was clarified at both SOLs. The radial movement of the ELM filaments at the LFS SOL was sometimes faster than the parallel convective transport to the divertor target, which caused the heat loading to the first wall. Filament structures with temporal peaks and flow velocities comparable to the ion sonic level were also determined in the HFS SOL, but they appeared only near the separatrix. The delay after start of the ELM was shorter than the parallel convection time from the LFS midplane, suggesting that part of the ELM filaments was ejected into the HFS SOL.

Transition between Internal Transport Barriers with Different Temperature-Profile Curvatures in JT-60U Tokamak Plasmas

A spontaneous transition phenomena between two states of a plasma with an internal transport barrier (ITB) is observed in the steady-state phase of the magnetic shear in the negative magnetic shear plasma in the JT-60U tokamak. These two ITB states are characterized by different profiles of the second radial derivative of the ion temperature inside the ITB region (one has a weak concave shape and the other has a strong convex shape) and by different degrees of sharpness of the interfaces between the L mode and the ITB region, which is determined by the turbulence penetration into the ITB region.

Fast data acquisition system based on digital oscilloscopes for fluctuation measurements in a long pulse JT-60U tokamak plasma

We have developed a new data acquisition (DAQ) system with fast sampling rate for fluctuation measurements in a long pulse JT-60U tokamak plasma. This system is based on a powerful digital oscilloscope, which has a large acquisition memory up to 50 Mwords/ch, 1 MHz sampling rate and 16 bits high resolution AD convertors. The system is composed of plural digital oscilloscopes and mass storages. On this system, most of data acquisition processes are executed at each digital oscilloscope. This feature of the system leads to an advantage that the processing loads are distributed among the digital oscilloscopes. This system has been successfully employed for measurements of various fluctuations obtained through magnetic probes, beam emission spectroscopy and so on. The size of the acquired data using this system has reached up to 10 GB/shot so far. It has demonstrated that this system is very powerful for data acquisition of multi-channeled signals with high time resolution in a long pulse plasma.

Reflectometry study of mode coupling in fusion plasma turbulence

The use of microwave reflectometry in the estimation of the bicoherency of the turbulent modes in plasmas is theoretically investigated. The full-wave two-dimensional model of microwave scattering in plasma is applied to simulate the reflectometer response to the turbulent fluctuations, consisting of random and coherent modes, and the simulated data ensemble is further processed to obtain the auto-bicoherence function. The sensitivity of the method was analysed for different ambient plasma profiles, geometry, fluctuation spectra and amplitudes.It was found that for plasma conditions similar to the edge JT-60U tokamak plasma, reflectometry-based estimations of the bicoherency can prove the existence of the phase-coupled modes only if the ratio of coherent to random amplitudes is larger than unity. Reflectometry in plasmas with stronger curvature of the cut-off layer has a better sensitivity to coherent fluctuations as compared to weak-curvature profiles.As an illustration, the results obtained were applied to the data ensemble acquired by the O-mode reflectometer on the JT-60U tokamak to demonstrate the existence of strong phase coupling in the H-mode plasma near the separatrix.

Measurements of density fluctuations in reflectometry of cylindrical plasmas

An analytical model of fluctuation reflectometry is developed, which is capable of handling the plasma profiles of arbitrary shape and curvature. The experimental profiles are piece-wise approximated by the functions, which allow for the integration of the full-wave O-mode equation. The model is applied to the reflectometry of the JT-60U tokamak plasma to provide a preliminary estimation of the fluctuation amplitude and spectral width.

Instantaneous Correlation between ELM-like MHD Activity and Abrupt Non-local Reduction of Transport at Internal Transport Barrier Formation in JT-60U High-.BETA.p Plasmas

Transport evolution in normal and reversed shear JT-60U tokamak plasmas with internal transport barrier (ITB) often occurs via non-local transport bifurcations. In the present paper, we highlight new type of edge-core instantaneous interplay in JT-60U high-βp plasmas. Abrupt in time (ms timescale) and wide in space (˜0.4 of minor radius) reduction of electron heat flux in the core (ITB-event) is correlated in ms timescale with the start of an ELM that creates enhanced level of Hα. This result gives a hint of the control to form the ITB immediately and non-locally by inducing the ELM-like MHD activity.

Dynamics and interplay of L-H-L transitions and ITB events in reversed shear plasmas withinternal barriers in JT-60U

Abrupt in time (correlated within a few ms with L-H and H-L transitions) and wide in space (from the edge up to ~0.3 of minor radius in the extreme case) variations of electron heat diffusivity δχe were found in reversed shear (RS) JT-60U tokamak plasmas with internal transport barrier (ITB) at various profiles of safety factor q. The δχe profile follows the position of q minimum (qmin ) and penetrates into the RS region deeper for the weak ITB than for the strong one, suggesting the presence of `global' edge-core connection throughout qmin in a ms timescale at L-H and H-L confinement bifurcations. The δχe value at the position of qmin is small for the strong ITB (~0.05 m2 s-1) and increases by an order of magnitude for the weak ITB. The ITB event is another non-local transport bifurcation (improvement or degradation) inside and around ITB. The case of strong ITB formation in H-mode via a series of ITB events in ELM-induced L-mode is described. ITB event degradation in the positive shear zone causes L-H transition.

Generation and termination of runaway electrons at major disruptions in JT-60U

The operation conditions to avoid runaway electron generation at the major disruption have been investigated in JT-60U tokamak plasmas. It has been found that runaway electrons are not observed for low Bt of ⩽ 2.2 T or low plasma current quench rates (Iγ ≡ -(dIp/dt)/Ip) of <50 s-1. Furthermore, they are not observed for low effective safety factors defined at the plasma edge (qeff) of ⩽ 2.5 even for high Iγ of 300-400 s-1, which is the case for uncontrolled disruptions accompanied by large plasma displacements (e.g., vertical displacement events (VDEs)). On the other hand, in controlled disruptions with small plasma shifts, qeff easily increases above 8, and runaway electrons are observed even for low current quench rates of 50-100 s-1. Furthermore, it has been found that in these position controlled disruptions the runaway current tail can rapidly decay even for zero or weakly positive plasma surface voltages. These observations of the avoidance and termination of runaway electrons suggest an anomalous loss mechanism for runaway electrons.

High time resolution ion temperature and rotation velocity measurements by charge exchange spectroscopy using three interference filters

The ion temperature and toroidal rotation velocity with high time resolution (τ∼1 ms) using charge exchange spectroscopy in JT-60U tokamak plasma have been measured. The method uses three interference filters that have slightly different peak wavelengths. These filters are selected according to the ion temperature and rotation velocity expected at the measured position. The system with a time resolution of 1 ms was applied to the analysis of fast plasma phenomena. As an example, the time evolution of the C VI ion temperature and toroidal rotation velocity across the L–H transition has been measured. The toroidal rotation at a normalized minor radius (r/a=0.3) increases gradually after the L–H transition, while that at the edge changes from a speed of 5 km/s before the transition to −40 km/s after the transition within a time of 5–10 ms.