Reversed Shear Discharges Research Articles

Experimental study of the core instability before and after internal transport barrier formation in EAST

In a reversed shear discharge in the Experimental Advanced Superconducting Tokamak (EAST), an internal transport barrier (ITB) was formed. For the first time, the turbulence in the plasma core before and after the formation of the ITB in EAST was investigated by microwave reflectometry. It was found that during the formation of the ITB, the turbulence inside the barrier was not completely suppressed. The density fluctuation level decreased in the early phase of ITB and significantly increased later. It was found that the density fluctuation level increased with the density gradient after the appearance of reversed shear Alfvén eigenmodes (RSAEs). The change in turbulence, measurable by the reflectometer, did not affect the rate of increase in core density. Through the measurement of RSAEs, it was found that the formation of this ITB may be related to the minimum q (q min). q min was close to 2.

Hybrid simulations of reversed shear Alfvén eigenmodes and related nonlinear resonance with fast ions in a tokamak plasma

Reversed shear Alfvén eigenmodes (RSAEs) are often observed in HL-2A reversed shear discharges with . The modes are highly localized near , and their frequencies sweep downward or upward as decreases. In this work, kinetic-magnetohydrodynamics (MHD) hybrid simulations based on experimental parameters in the HL-2A tokamak are carried out to investigate the linear stability and nonlinear dynamics of RSAEs. Linear simulations show that the frequencies of RSAEs sweep downward () first and then upward () as decreases, which is rarely seen in previous simulation work. The frequency and mode structure of the instabilities are consistent with experimental observations. For the same , the critical value of the fast ion beta required to destabilize down-sweeping RSAEs is higher than that of up-sweeping RSAEs. Both the up-sweeping and down-sweeping RSAEs are mainly driven by co-passing fast ions. The nonlinear simulation results of n = 3 and show that the frequency of the mode chirps up and down with corresponding hole-clump structures of energy perturbation appearing in the phase space. In the nonlinear phase, the dominant resonance condition becomes instead of the linear resonance condition , which can be the result of a nonlinear interaction between wave and particles. This work provides a new proof for the theory of nonlinear wave-particle resonance proposed recently (Chen and Zonca 2019 Plasma Sci. Technol. 21 125101).

Subdominant modes and optimization trends of DIII-D reverse magnetic shear configurations**This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05- 00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce

Alfvén Eigenmodes (AE) and magneto-hydrodynamic (MHD) modes are destabilized in DIII-D reverse magnetic shear configurations and may limit the performance of the device. We use the reduced MHD equations in a full 3D system, coupled with equations of density and parallel velocity moments for the energetic particles (with gyro-fluid closures) as well as the geodesic acoustic wave dynamics, to study the properties of instabilities observed in DIII-D reverse magnetic shear discharges. The aim of the study consists in finding ways to avoid or minimize MHD and AE activity for different magnetic field configurations and neutral beam injection (NBI) operational regimes. The simulations show at the beginning of the discharge, before the reverse shear region is formed, a plasma that is AE unstable and marginally MHD stable. As soon as the reverse shear region appears, ideal MHD modes are destabilized with a larger growth rate than the AEs. Both MHD modes and AEs coexist during the discharge, although the MHD modes are more unstable as the reverse shear region deepens. The simulations indicate the destabilization of Beta induced AE (BAE), Toroidal AE (TAE), elliptical AE (EAE) and reverse shear AE (RSAE) at different phases of the discharges, showing a reasonable agreement between the frequency range of the dominant modes in the simulations and the diagnostic measurements. A further analysis of the NBI operational regime indicates that the AE stability can be improved if the NBI injection is off axis, because on-axis injection leads to AEs with larger growth rate and frequency. In addition, decreasing the beam energy or increasing the NBI relative density (the ratio between the energetic particle and thermal plasma density) leads to AEs with larger growth rate and frequency, so an NBI operation in the weakly resonant regime requires higher beam energies than in the experiment (). The MHD linear stability can be also improved if the reverse shear region and the q profile near the magnetic axis are in between the rational surfaces q = 2 and q = 1, particularly if there is a region in the core with negative shear, avoiding a flat q profile near the magnetic axis. The simulations also show a smooth transition between MHD modes and low frequency AE, no critical , pointing out an overlap between MHD and AE activity for modes with frequencies lower than 30 kHz. This is in the range of beta acoustic Alfven eigenmodes (BAAE) and BAE.

Dynamics of an n = 1 explosive instability and its role in high-β disruptions

Some low-n kink-ballooning modes not far from marginal stability are shown to exhibit a bifurcation between two very distinct nonlinear paths that depends sensitively on the background transport levels and linear perturbation amplitudes. The particular instability studied in this work is an mode dominated by an component. It is driven by a large pressure gradient in weak magnetic shear and can appear in various high- hybrid/advanced scenarios. Here it is investigated in reversed shear equilibria where the region around the safety-factor minimum provides the low-shear conditions necessary for instability. For a certain range of parameters, a relatively benign path results in a saturated ‘long-lived mode’ (LLM) that causes little confinement degradation. At the other extreme, the quadrupole geometry of the perturbed pressure field evolves into a ballooning finger that subsequently transitions from exponential to explosive growth. The finger eventually leads to a fast disruption with precursors too short for any mitigation effort. Interestingly, the saturated LLM state is found to be metastable; it also can be driven explosively unstable by finite-amplitude perturbations. Similarities to some high-β disruptions in reversed-shear discharges are discussed.

Excitation of Alfvén eigenmodes by low energy beam ions in the DIII-D and JET tokamaks

Core localized Alfvén eigenmodes in DIII-D [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] and Joint European Torus (JET) [P. H. Rebut and B. E. Keen, Fusion Technol. 11, 13 (1987)] plasmas are driven by deuterium neutral beam ions traveling well below the Alfvén speed. Modes are observed in reverse magnetic shear discharges with deuterium ion velocities as low as 0.23 and 0.16 of the Alfvén speed parallel to the magnetic field in DIII-D and JET plasmas, respectively. Ellipticity-induced Alfvén eigenmodes in DIII-D and toroidicity-induced Alfvén eigenmodes in JET are excited by deuterium ions traveling well below the fundamental passing ion resonance condition, indicating the role of high-order resonances in driving these modes. NOVA-K analysis reveals many high-order resonances as contributing to the mode drive at high central safety factor due to the correspondingly large poloidal orbit width and the decrease in the perpendicular scale length of the modes.

Intermittent β collapse after NBCD turn-off in JT-60U fully non-inductive reversed shear discharges

Non-disruptive β collapses with a regular intermittency have been observed after a forced turn-off of neutral beam current drive (NBCD) in JT-60U fully non-inductive, reversed shear (RS) discharges. Self-consistent transport simulations with improved core confinement have first clarified that the redistribution of ‘return current’ induced after the NBCD turn-off lowers the safety factor of magnetic shear reversal, qmin. Linear MHD stability analysis has also revealed that this lowering of safety factor leads to n = 1 kink-ballooning instability with the modes localized around the internal transport barrier (ITB) of higher qmin (∼6) discharges. It was also pointed out that an increase in the bootstrap current under continuous NB heating can lead to ITB reconstruction and thus causes subsequent β collapses.

Alfvén cascades in JET discharges with NBI-heating

Alfvén cascade (AC) eigenmodes excited by energetic ions accelerated with ion-cyclotron resonance heating in JET reversed-shear discharges are studied experimentally in high-density plasmas fuelled by neutral beam injection (NBI) and by deuterium pellets. The recently developed O-mode interferometry technique and Mirnov coils are employed for detecting ACs. The spontaneous improvements in plasma confinement (internal transport barrier (ITB) triggering events) and grand ACs are found to correlate within 0.2 s in JET plasmas with densities up to ∼5 × 1019 m−3. Measurements with high time resolution show that ITB triggering events happen before ‘grand’ ACs in the majority of JET discharges, indicating that this improvement in confinement is likely to be associated with the decrease in the density of rational magnetic surfaces just before qmin(t) passes an integer value. Experimentally observed ACs excited by sub-Alfvénic NBI-produced ions with parallel velocities as low as V‖NBI ≈ 0.2 · VA are found to be most likely associated with the geodesic acoustic effect that significantly modifies the shear-Alfvén dispersion relation at low frequency. Experiments were performed with a tritium NBI-blip (short time pulse) into JET plasmas with NBI-driven ACs. Although considerable NBI-driven AC activity was present, good agreement was found both in the radial profile and in the time evolution of DT neutrons between the neutron measurements and the TRANSP code modelling based on the Coulomb collision model, indicating the ACs have at most a small effect on fast particle confinement in this case.

Reversed shear Alfvén eigenmodes associated with the ellipticity and triangularity Alfvén gaps

Based on numerical simulations with the magnetohydrodynamic code NOVA, reversed shear Alfvén eigenmodes (RSAE) associated with the ellipticity-induced and non-circular triangularity-induced Alfvén eigenmode gaps were found. An analytical model for large aspect ratio plasmas confirms the existence of those modes with a pressure gradient threshold that agrees very well with the NOVA calculations. It is predicted that these higher gap RSAEs might be observed in existing large tokamaks in the early phase of reversed shear discharges when the minimum value of the magnetic safety factor is high (larger than two) and in ITER advanced scenario discharges.

A Shear-Flow Instability Relevant to Advanced Tokamak Operation

An ideal magnetohydrodynamic instability which is driven by sheared flow and which may be relevant to reversed-shear advanced tokamak operation is described. If there is flow shear at the qmin surface, relatively weak velocity shear can drive this instability, with a time scale of the flow. The flows may be significantly slower than the ambient poloidal Alfven velocity, and no inflection point is needed. Thus the time scale of the instability may be significantly longer than that of the poloidal Alfven transit time, and it might account for disruption of reversed-shear discharges recently observed in JT-60U.

Advanced tokamak research on JT-60

The Japanese large tokamak JT-60 has been focusing its research emphasis on the development of high performance plasmas, with high confinement, high temperature and high density and non-inductive sustainment for a long time with possible minimization of external power input. The first demonstration of high bootstrap current discharges up to 80% of the total plasma current in a high-poloidal-beta (high-βp) mode and the concept development of a steady-state tokamak reactor (SSTR) based on this experimental achievement initiated the so-called ‘advanced tokamak research’. The internal transport barriers (ITBs), discovered in the JT-60 high-βp mode, have been followed by worldwide explorations of reversed shear discharges associated with ITBs. The highest DT equivalent energy gain of was achieved in the JT-60 reversed shear H-mode discharges. The highest ion temperature of Ti = 45 keV and the highest fusion triple product of ni(0)τETi(0) = 1.5 × 1021 m−3 s keV were obtained in high-βp discharges. Advanced tokamak research is now the major trend of the current tokamak development. A new concept of compact ITER has been developed and proposed in the context of this advanced tokamak approach pursued on JT-60. Prospects for burning plasma physics have been investigated along the progress made in these modern tokamak experiments on JT-60 and related computer simulation analyses.

Effect of hysteresis in JET ITB plasma with LHCD

A strong ITB was sustained in a reversed shear discharge in JET during a long time interval after a significant reduction in plasma heating power. The observed ITB evolution is reminiscent of the effect of hysteresis, associated in theory with turbulence suppression due to shear of the plasma rotation. The mechanism of ITB sustainment was analysed. Modelling of the plasma heating and current profile was done using the TRANSP and JETTO transport codes. The resulting q-profile evolution was verified by comparison with the pitch angle δ = Bp/Bt deduced from motional Stark effect measurements (where Bp and Bt are the poloidal and toroidal magnetic field, respectively). Turbulence stability was analysed using the gyro-kinetic code Kinezero. It was shown that the strong negative magnetic shear produced by LHCD and sustained mainly by the bootstrap current in the plasma core is responsible for the turbulence stabilization. Stabilization due to shear of the plasma rotation and finite β -stabilization play a complimentary role. The effect of bootstrap and LH driven currents on magnetic shear in JET discharges was analysed.

Trapped electron stabilization of ballooning modes in low aspect ratio toroidal plasmas

The kinetic effects of trapped electron dynamics and finite gyroradii and magnetic drift motion of ions are shown to give rise to a large parallel electric field and hence a parallel current that greatly enhances the stabilizing effect of field line tension for ballooning modes in low aspect ratio toroidal plasmas. For large aspect ratio the stabilizing effect increases (reduces) the β(=2P/B2) threshold for the first (second) stability of the kinetic ballooning mode (KBM) from the magnetohydrodynamics (MHD) β threshold value by a factor proportional to the trapped electron density fraction. For small aspect ratio the stabilizing effect can greatly increase the β threshold of the first stability of KBMs from the MHD β threshold by Sc≃1+(ne/neu)δ, where ne/neu is the ratio of the total electron density to the untrapped electron density, and δ depends on the trapped electron dynamics and finite gyroradii and magnetic drift motion of ions. If ne/neu≫1 as in the National Spherical Torus Experiment (NSTX) [M. Ono, Nucl. Fusion 40, 557 (2000)] with an aspect ratio of ≃1.4, the KBM should be stable for β⩽1 for finite magnetic shear. Therefore, unstable KBMs are expected only in the weak shear region near the radial location of the minimum of the safety factor in NSTX reverse shear discharges.

Kinetic reversed-shear Alfvén eigenmodes

Alfvén eigenmodes in tokamak discharges with reversed shear are theoretically studied taking into account the effects of finite Larmor radius. It is predicted that, similarly to the kinetic toroidal Alfvén eigenmodes in positive-shear discharges, a new branch of eigenmodes in reversed-shear discharges exists, called here the kinetic reversed-shear Alfvén eigenmodes.

Chapter 9: Performance-Limiting MHD Activity and Possibilities for Its Stabilization in ASDEX Upgrade

Performance-limiting magnetohydrodynamic (MHD) instabilities on ASDEX Upgrade are discussed. In the conventional H-mode scenario, the main MHD performance limitation is found to be the neoclassical tearing mode (NTM). The onset β of NTMs in ASDEX Upgrade scales with the poloidal ion gyroradius, in agreement with theoretical expectations. At higher β values, NTMs occur in a more benign form, the frequently-interrupted-regime NTMs, which lead to a smaller confinement degradation than normal NTMs. Active control of NTMs by electron cyclotron current drive in the island has been demonstrated on ASDEX Upgrade. In advanced tokamak regimes with reversed shear, a variety of performance-limiting instabilities has been observed. The shear reversal zone can be unstable to double tearing modes or to infernal modes; both have been identified in ASDEX Upgrade. Due to the broad current profile in advanced tokamak discharges, the ideal external kink mode can be unstable at relatively low βN ≤ 2; this is a main limitation to strongly reversed shear discharges with peaked pressure profiles. Finally, it is shown that fast-particle-driven modes such as fishbones can also have beneficial effects, such as providing stationary current profiles or triggering internal transport barriers.

Accumulation of impurities in advanced scenarios

Impurity behaviour in advanced scenarios at ASDEX Upgrade and JET has been investigated. For ASDEX Upgrade improved H-mode plasmas at δ=0.35, accumulation of tungsten and metallic impurities was observed inside r/ a=0.3 in cases with weak central heating. Central ECRH suppressed the accumulation by increasing the anomalous diffusion and by flattening the profile of the main plasma density which reduces neoclassical inward convection for the impurities. These effects are well described by impurity transport simulation and calculated v/ D values agree with the measurements within a factor of 2. In JET reversed shear discharges with internal transport barriers, metallic impurities accumulate in cases with too strong peaking of the density profile. The peaking increases with the impurity charge and is low for the low- Z elements C and Ne as expected from neoclassical convection.

Reduction of energetic particle loss by ferritic steel inserts in ITER

Alpha particle loss due to the toroidal field ripple is significantly enhanced in reversed shear plasmas, especially in a high-q regime. To ensure a tolerable heat load on the wall due to the loss of alpha particle in reversed shear discharges with various q profiles, ferritic steel (FS) inserts are considered as a countermeasure for reducing the ripple amplitude at a low cost. With this being the situation, an assessment of ripple loss for various conditions in ITER is an important study that should to be conducted in the International Tokamak Physics Activities. This paper describes an assessment result, indicating that FS inserts lead to a dramatic reduction of ripple loss. When the arrangement of the FS inserts is optimized, the resulting alpha particle loss is anticipated to be reduced by an order of magnitude or more, and the power loss fraction is as low as 1% even for a reversed shear plasma with a relatively high qmin of 3.

Magnetohydrodynamic Stability of Improved Confinement Plasmas in JT-60U

Progress in the understanding of magnetohydrodynamic (MHD) stability is summarized on JT-60U tokamak discharges with improved confinement such as the (hot-ion) H-mode, high-βp mode, high-βp H-mode, and reversed shear discharges. Transport barriers, which are essential for the improved confinement, play key roles in the local and global MHD stability owing to the local large pressure gradient and the related bootstrap current. Disruptive β limits of these discharges are consistent with theoretical ideal kink-ballooning stability limits with low toroidal mode numbers n. Achievable β limit is improved by broadening of the pressure profile with high plasma internal inductance, plasma shaping, and wall stabilization. Edge localized modes (ELMs) and barrier localized modes (BLMs), which are associated with edge and internal transport barriers, respectively, are analyzed carefully. Resistive interchange modes with n ≤ 3 are excited in the negative shear region in reversed shear discharges with the internal transport barrier and lead to major collapse occasionally through nonlinear coupling with a tearing mode in the positive shear region. MHD characteristics of low m/n (m: poloidal mode number) tearing modes, which are attributed to the neoclassical tearing mode, are investigated. Stabilization of tearing modes and control of sawtooth activity are demonstrated using the fundamental O-mode electron cyclotron wave injection. Resistive wall modes associated with current-driven and pressure-driven low n external kink modes are identified.

Turbulence behaviour during electron heated reversed shear discharges in JET

Low-frequency/long-wavelength density turbulence is found to be reduced withinthe volume enclosed by an electron thermal internal transport barrier generated with lower hybrid microwave heating in the JET tokamak. Theturbulence reduction coincides with a region (typically r/a<0.4) ofnegative, or reversed magnetic shear s and reduced electron thermal diffusionχe. In the tokamak edge region (r/a>0.8), the turbulenceamplitude is also reduced coinciding with large positive magnetic shears>1. Estimates of the core turbulence wavelengths (λ⊥>0.1 m) are not in agreement with expectations from electron temperaturegradient instabilities.

A 20-channel electron cyclotron emission detection system for a grating polychromator in JT-60U

A 20-channel electron cyclotron emission (ECE) detection system for the grating polychromator in JT-60U has been developed to measure electron temperature and its perturbations. Indium antimonide (InSb) detectors aligned in a row are cooled by liquid helium held by a 50 cm-diam cryostat, in which a hold time of the liquid helium reaches 7 weeks. Twenty-channel preamplifiers are dc-coupled ones, which enable to measure low-frequency ECE. From the comparison between the new system and the existing 20-channel detector system, where a refrigerator for cooling InSb detectors and ac-coupled preamplifiers are used, it is found that the signal-to-noise ratio is improved, and that small perturbations can be detected. By using this system, the electron temperature and its perturbations near the internal transport barrier in a reversed shear discharge are measured in detail.

Resistive instabilities in reversed shear discharges and wall stabilization on JT-60U

Resistive instabilities and wall stabilization of ideal kink modes with low toroidal mode number n are investigated in JT-60U reversed shear discharges. Localized burst-like MHD activity with n⩽3 is observed in the negative shear region with large pressure gradient near the internal transport barrier; this activity is identified as the resistive interchange mode.The burst-like MHD activity is benign with respect to the internal transport barrier and no clear degradation of plasma stored energy by the burst-like MHD activity is observed. The resistive interchange mode, however, results in a major collapse through non-linear coupling with a tearing mode in the positive shear region. A resistive wall mode (RWM) followed by a major collapse is observed in wall stabilized high β discharges exceeding the ideal stability limit with the wall at infinity (βN>βNno-wall).No clear continuous slowing down of the plasma toroidal rotation is observed before the growth of an RWM.