Toroidal Alfven Eigenmodes Research Articles

Discrete Alfvén eigenmodes under the condition <_ufe:lines-br xmlns:_ufe="http://www.arbortext.com/namespace/Styler/UserFormattingElements" xml:base="-7mm"> </_ufe:lines-br>of bootstrap current in EAST

Basing on experimental advanced superconducting Tokamak (EAST), we have researched relation between bootstrap current and α -induced toroidal Alfven eigenmode ( α TAE, α is a measure of plasma pressure gradient), which is investigated, in this paper, for the EAST tokamak operation condition with the bootstrap current. Our results show that the α TAE may exist in a relatively broad region of minor radius, where a large bootstrap current density is observed. In the presence of energetic particles, produced by the NBI experiments, our MHD-gyrokinetic hybrid simulations demonstrate that this quasi-marginally stable α TAE (within MHD description) can be readily destabilized by the kinetic compression upon the wave-particle resonances.

Alfvén eigenmode evolution computed with the VENUS and KINX codes for the ITER baseline scenario

A new application of the VENUS code is described, which computes alpha particle orbits in the perturbed electromagnetic fields and its resonant interaction with the toroidal Alfven eigenmodes (TAEs) for the ITER device. The ITER baseline scenario with Q = 10 and the plasma toroidal current of 15 МА is considered as the most important and relevant for the International Tokamak Physics Activity group on energetic particles (ITPA-EP). For this scenario, typical unstable ТАЕ-modes with the toroidal index n = 20 have been predicted that are localized in the plasma core near the surface with safety factor q = 1. The spatial structure of ballooning and antiballooning modes has been computed with the ideal MHD code KINX. The linear growth rates and the saturation levels taking into account the damping effects and the different mode frequencies have been calculated with the VENUS code for both ballooning and antiballooning TAE-modes.

Linear global gyrokinetic simulations of toroidal Alfven eigenmodes in KSTAR plasmas

Excitation of toroidal Alfven eigenmodes (TAEs) in KSTAR tokamak plasmas has been studied by using the GENE code. Verification and benchmark analysis are performed for Alfven eigenmodes (AEs) excited by the energetic particles (EPs) in comparison with the AEs from the GYGLES code, and excellent agreements are found. In addition, the threshold value of the EP density gradient to destabilize the TAE has been investigated. For the plasma equilibrium of KSTAR discharge (10574), TAEs of n = 2 are found to be excited by coupling of adjoining poloidal harmonics (5, 6), (6, 7), and (7, 8). The dependence of the growth rate and frequency of the TAE on the EP density gradient is examined. It is found that the threshold value of EP density gradient increases with the higher poloidal mode coupling, of which location moves outward in the radial direction. The growth rates of TAEs with higher poloidal mode numbers are smaller than those with lower poloidal mode numbers, indicating that perpendicular wavenumbers play an...

Multiple toroidal Alfven eigenmodes with a single toroidal mode number in KSTAR plasmas

Simultaneous excitation of multiple discrete toroidal Alfven eigenmodes (TAEs) for a single toroidal mode number have been observed in KSTAR plasmas. Excitation and characteristics of these modes are studied by using a global gyrokinetic particle-in-cell simulation code. It is shown that compared to a single core-localized mode, excitation of two modes is difficult. The frequency difference between the double TAEs studied from simulation seems to agree well with the experimental value. Details of studies on the frequency, growth rate, mode structures, etc, using the GTC simulation are presented.

The influence of toroidal Alfvén modes on the confinement of fast particles in the Globus-M spherical tokamak

Neutral beam injection into the Globus-M spherical tokamak at the early stage of discharge leads to the development of instabilities in a frequency range of 50–200 kHz, which have been identified as toroidal Alfven eigenmodes (TAEs) [1]. The influence of these modes on the confinement of fast particles has been studied with the aid of a neutral particle analyzer (NPA) and a neutron detector. The isotope effect was studied using hydrogen and deuterium both in the injected beam and in the target plasma. A correlation analysis of signals from magnetic probes showed that the observed modes in most cases contain a single harmonic with toroidal number n = 1. Upon the injection of deuterium into deuterium plasma, the development of TAEs led to a decrease in the neutron flux by 25%, whereas the fluxes of high-energy recharge atoms decreased by 75%. After the injection of hydrogen, a decrease in the flux measured by NPA did not exceed 25%.

Non-linear modulation of short wavelength compressional Alfvén eigenmodes

Most Alfvénic activity in the frequency range between toroidal Alfvén eigenmodes and roughly one half of the ion cyclotron frequency on National Spherical Torus eXperiment [Ono et al., Nucl. Fusion 40, 557 (2000)], that is, approximately 0.3 MHz up to ≈1.2 MHz, are modes propagating counter to the neutral beam ions. These have been modeled as Compressional and Global Alfvén Eigenmodes (CAE and GAE) and are excited through a Doppler-shifted cyclotron resonance with the beam ions. There is also a class of co-propagating modes at higher frequency than the counter-propagating CAE and GAE. These modes have been identified as CAE, and are seen mostly in the company of a low frequency, n = 1 kink-like mode. In this paper, we present measurements of the spectrum of these high frequency CAE (hfCAE) and their mode structure. We compare those measurements to a simple model of CAE and present a predator-prey type model of the curious non-linear coupling of the hfCAE and the low frequency kink-like mode.

Simulation Study of Alfvén-Eigenmode-Induced Energetic Ion Transport in LHD

The interaction between toroidal Alfven eigenmode (TAE) and energetic ions in the Large Helical Device (LHD) is investigated using a reduced version of theMEGA code that implements a realistic equilibriummagnetic field with the HINT code and corresponding TAE profile with the AE3D code. In simulations, the linear growth rate of TAE amplitude is proportional to energetic ion density; consequently, the nonlinear saturation level of the TAE amplitude is enhanced by the increase in the energetic ion density. Energy transfer analysis is performed to clarify destabilization and saturation mechanisms of the TAE and to identify resonant energetic ions. An analysis of test particles in the electromagnetic field perturbed by the TAE shows that the magnitude of fluctuations in the energetic ion orbits is proportional to the square root of the TAE amplitude. Our results qualitatively reproduce the radial transport of energetic ions by the TAE in the LHD.

Comparison of Measurement and Modeling of Current Profile Changes due to Neutral Beam Ion Redistribution during TAE Avalanches in NSTX

Brief avalanches of toroidal Alfven eigenmodes (TAEs) are observed in NSTX plasmas with several different n numbers simultaneously present. These affect the neutral beam ion distribution as evidenced by a concurrent drop in the neutron rate and, sometimes, beam ion loss. Guiding center orbit modeling has shown that the modes can transiently render portions of the beam ion phase space stochastic. The resulting redistribution of beam ions can also create a broader beam-driven current profile and produce other changes in the beam ion distribution function

Observation of Low Frequency MHD Mode Driven by Energetic Particles in Large Helical Device Plasmas with Strong Interchange Mode Activities

The beta-induced Alfven eigenmode (BAE) like modes during strong interchange mode, whose mode- numbers are m/n = 2/1, have been recently observed for the first time in Large Helical Device (LHD). The first harmonic frequencies of these oscillations range from 30 to 70 kHz, much lower than the toroidal-Alfven- eigenmode (TAE) frequency, and are provided with the same order of the low-frequency gap induced by finite beta effects. The magnetic fluctuation spectrogram indicates that the BAEs often occur in pairs, and their mode- numbers are m/n = 2/ 1a nd−2/−1. The analysis reveals that the modes propagate poloidally and toroidally in opposite directions, and form standing-wave structures in interchange-mode rest frame. The frequencies of the pair mode are associated with the Te/Ti ratio, and the frequency difference of the pair modes is determined by the frequency of interchange mode. The new finding shed light on the underlying physics mechanism for the excitation of the low frequency Alfvenic fluctuation.

Experimental study of toroidal Alfvén modes in the Globus-M spherical tokamak

In the experiments carried out on the Globus-M tokamak in regimes with injection of 26-keV neutral beams with a power of 0.75–0.85 MW, two branches of instabilities excited by fast ions were observed in the early stage of a discharge: a low-frequency energetic particle mode (EPM) in the frequency range of 5–30 kHz and a high-frequency mode in the range of 50–200 kHz, identified as a toroidal Alfven eigenmode (TAE). The TAE developed in the initial phase of the discharge at q(0) > 1 and terminated when sawtooth oscillations were excited at q(0) < 1. The spectrum and spatial localization of the mode agree with predictions of the linear theory. The modes observed in the Globus-M tokamak possess both properties common to other tokamaks and their own specific features.

New developments in the diagnostics for the fusion products on JET in preparation for ITER (invited)

Notwithstanding the advances of the past decades, significant developments are still needed to satisfactorily diagnose “burning plasmas.” D–T plasmas indeed require a series of additional measurements for the optimization and control of the configuration: the 14 MeV neutrons, the isotopic composition of the main plasma, the helium ash, and the redistribution and losses of the alpha particles. Moreover a burning plasma environment is in general much more hostile for diagnostics than purely deuterium plasmas. Therefore, in addition to the development and refinement of new measuring techniques, technological advances are also indispensable for the proper characterization of the next generation of devices. On JET an integrated program of diagnostic developments, for JET future and in preparation for ITER, has been pursued and many new results are now available. In the field of neutron detection, the neutron spectra are now routinely measured in the energy range of 1–18 MeV by a time of flight spectrometer and they have allowed studying the effects of rf heating on the fast ions. A new analysis method for the interpretation of the neutron cameras measurements has been refined and applied to the data of the last trace tritium campaign (TTE). With regard to technological upgrades, chemical vapor deposition diamond detectors have been qualified both as neutron counters and as neutron spectrometers, with a potential energy resolution of about one percent. The in situ calibration of the neutron diagnostics, in preparation for the operation with the ITER-like wall, is also promoting important technological developments. With regard to the fast particles, for the first time the temperature of the fast particle tails has been obtained with a new high purity Germanium detector measuring the gamma emission spectrum from the plasma. The effects of toroidal Alfven eigenmodes modes and various MHD instabilities on the confinement of the fast particles have been determined with a combination of gamma ray cameras, neutral particle analyzers, scintillator probe, and Faraday cups. From a more technological perspective, various neutron filters have been tested to allow measurement of the gamma ray emission also at high level of neutron yield.

Measurements of fast ions and their interactions with MHD activity using neutron emission spectroscopy

Ion cyclotron radio frequency (ICRF) heating can produce fast ion populations with energies reaching up to several megaelectronvolts. Here, we present unique measurements of fast ion distributions from an experiment with 3rd harmonic ICRF heating on deuterium beams using neutron emission spectroscopy (NES). From the experiment, very high DD neutron rates were observed, using only modest external heating powers. This was attributed to acceleration of deuterium beam ions to energies up to about 2–3 MeV, where the DD reactivity is on a par with that of the DT reaction. The high neutron rates allowed for observations of changes in the fast deuterium energy distribution on a time scale of 50 ms. Clear correlations were seen between fast deuterium ions in different energy ranges and magnetohydrodynamic activities, such as monster sawteeth and toroidal Alfven eigen modes (TAE). Specifically, NES data showed that the number of deuterons in the region between 1 and 1.5 MeV were decaying significantly during strong TAE activity, while ions with lower energies around 500 keV were not affected. This was attributed to resonances with the TAE modes.

Innovative signal processing and data analysis methods on JET for control in the perspective of next-step devices

In the last few years, it has been realized that more sophisticated control schemes are necessary to push the boundaries of tokamak operation and the performance of reactor-like machines. In addition, JET needs to operate safely with the new metallic wall and such protection will be needed for ITER. These objectives have motivated the development, benchmark and validation of new signal processing and data analysis methods. Two new approaches for the determination of the magnetic topology in real time have been validated on an extensive database of JET discharges, including advanced tokamak scenarios. Robust methods of confinement regime identification and disruption prediction are a prerequisite for safe, general control schemes. New identifiers have been developed and their success rates exceed 99% in determining whether plasmas are in the L or H mode. A new disruption predictor is being developed and has already provided success rates higher than 90% in realistic real-time conditions. Moreover, the generalization capability of this new predictor has been confirmed by applying it to new experimental campaigns not used for the training. The success rate remains high even more than ten campaigns, or about four years, after the last one used for the training. The deployment of video cameras in real time requires the development of new image processing algorithms, which have already been implemented and validated successfully on JET for the real-time identification of hot spots with a time resolution of tens of milliseconds. A series of new feedback schemes has also been explicitly developed not much to control the plasma but to really improve the physics understanding of some phenomena. Particularly interesting are the simultaneous control of the safety factor and pressure profiles and the real-time tracking of toroidal Alfven eigenmode instabilities. These advanced feedback schemes for physics understanding often require more advanced signal processing techniques like adaptive filtering, which have already been implemented. The paper concludes by discussing the use of these real-time analysis and control developments in next-step machines such as ITER.

A Real-Time Synchronous Detector for the TAE Antenna Diagnostic at JET

Routine studies are performed on JET using a new set of antennas to excite Toroidal Alfve?n Eigenmodes (TAE). A TAE resonance footprint is observed in the plasma response measurement when there is a noticeable variation in both the amplitude and the phase of the response with respect to the excitation. An algorithm for real-time identification of TAE resonances, based on a hardware lock-in amplifier, is presently used at the Joint European Torus (JET) tokamak for detecting such variations. In this paper, we revisit the problem of estimating the I-Q characteristics from a known non-stationary frequency mode, with a resonant-like phase response, embedded in a digital signal. A non-stationary linear model is used in a recursive filter implementation of a lock-in amplifier. We propose it as a viable alternative to hardware synchronous detectors such as the one in use at the JET and compare its' performance with standard digital lock-in techniques in terms of bandwidth and phase response under high throughput rates requirements.

Observation of Fast Ion Losses Induced by Various MHD Modes Driven by Fast Ions and Bulk Plasma Pressure in the Large Helical Device

Results from a scintillator-based lost-fast ion probe newly installed at the horizontally elongated outboard side of the Large Helical Device are presented. Correlating with bursts of toroidal Alfven eigenmodes (TAEs) and energetic-particle continuum modes (EPMs), recurrent increases in beam ion losses were observed during co-neutral beam injections. Beam ion loss rate was also enhanced, correlating with low-frequency interchange modes driven by the bulk plasma pressure gradient near the plasma edge.

Observation of the Internal Structure of Energetic Particle-Driven MHD Modes in a Large Helical Device

For the preparation study of a future burning plasma, experimental information about the internal structure of energetic particle-driven MHD instabilities, such as toroidal Alfven eigenmodes, is important. In the Large Helical Device, three microwave reflectometer systems have been recently installed for measuring the density fluctuation in the wide radial region with a high spatial resolution. One of the three systems has three channels with fixed frequencies. The others are operated in a frequency-hopping mode. These systems are used for measuring the internal structures of an MHD mode and its temporal behaviors. Using these systems, strongly localized MHD instabilities driven by the energetic particles are measured in the plasma core region, and the measured values are consistent with the theoretical expectation.

Excitation of Stable Alfvén Eigenmodes by Application of Alternating Magnetic Field Perturbations in the Compact Helical System

Alternating magnetic perturbations, generated by external electrodes, are applied to a neutral-beam-heated plasma in the Compact Helical System, to excite stable Alfven eigenmodes (AEs). Resonant peaks in the frequency range of the toroidal Alfven eigenmode (TAE) have been observed in a transfer function defined by the ratio between the magnetic probe signal and the alternating electrode current. The resonant frequency fo agrees well with that of the TAE gap located near the plasma edge, where the strength of energetic ion drive of AEs will be sufficiently low. The damping rates γ derived from the width of the resonant peak are fairly large, i.e., γ/(2π fo) ∼ 20%.

Measurement of Radial Structure of Energetic-Ion-Driven MHD Modes with a Fast Response Hα-Detector Array in the Large Helical Device

In the Large Helical Device, coherent fluctuations related to energetic-ion-driven magneto-hydrodynamic (MHD) instabilities such as Alfven eigenmodes (AEs) were detected for the first time using an Hα detector array having high-frequency response up to 200 kHz. Correlation analysis between detector and magnetic probe signals has enabled us to successfully extract coherent fluctuations in the AE frequency range. The radial profile of the excess of the coherence from the background value (Δγ) has a clear peak around the predicted toroidal Alfven eigenmode (TAE) gap.

Overview of recent physics results from the National Spherical Torus Experiment (NSTX)

The National Spherical Torus Experiment (NSTX) has made considerable progress in advancing the scientific understanding of high performance long-pulse plasmas needed for future spherical torus (ST) devices and ITER. Plasma durations up to 1.6 s (five current redistribution times) have been achieved at plasma currents of 0.7 MA with non-inductive current fractions above 65% while simultaneously achieving βT and βN values of 17% and 5.7 (%m T MA−1), respectively. A newly available motional Stark effect diagnostic has enabled validation of current-drive sources and improved the understanding of NSTX ‘hybrid’-like scenarios. In MHD research, ex-vessel radial field coils have been utilized to infer and correct intrinsic EFs, provide rotation control and actively stabilize the n = 1 resistive wall mode at ITER-relevant low plasma rotation values. In transport and turbulence research, the low aspect ratio and a wide range of achievable β in the NSTX provide unique data for confinement scaling studies, and a new microwave scattering diagnostic is being used to investigate turbulent density fluctuations with wavenumbers extending from ion to electron gyro-scales. In energetic particle research, cyclic neutron rate drops have been associated with the destabilization of multiple large toroidal Alfven eigenmodes (TAEs) analogous to the ‘sea-of-TAE’ modes predicted for ITER, and three-wave coupling processes have been observed for the first time. In boundary physics research, advanced shape control has enabled studies of the role of magnetic balance in H-mode access and edge localized mode stability. Peak divertor heat flux has been reduced by a factor of 5 using an H-mode-compatible radiative divertor, and lithium conditioning has demonstrated particle pumping and results in improved thermal confinement. Finally, non-solenoidal plasma start-up experiments have achieved plasma currents of 160 kA on closed magnetic flux surfaces utilizing coaxial helicity injection.

Structural Analysis of the JET TAE Antenna

In this paper the mechanical design of the new active MHD antennas for JET is described and the structural/mechanical analysis for the antennas is presented. These new antennas replace the existing n = 1 or 2 saddle coils with a set of eight smaller antennas designed to excite Toroidal Alfven Eigenmodes (TAE’s) with high toroidal mode number (n ~ 10) in the frequency range of 30 kHz-500 kHz. TAE’s with these higher mode numbers are expected in ITER and could enhance the loss of fast alpha particles in a burning plasma regime. By studying the properties of stable TAE’s excited actively by these antennas, high performance regimes of operation avoiding unstable fast particle driven modes can be found. A more complete overview of the experiment may be found in Reference[1]. Two antenna assemblies will be installed at toroidally opposite positions. Antenna wires are protected from the plasma heat flux by CFC tiles mounted on mini-limiters, located between the individual windings. The main structural element is a box section. The support scheme utilizes cantilevered brackets that connect to the saddle coils, and “wing” brackets which add support to the top of the frame. Conservative estimates of the disruption currents in the MHD antennas and frame were used to calculate loading and resulting stress in the antenna structure. Fields, field transients, and halo current specifications were provided by JET. The frame originally was designed as a continuous loop, and was converted to an open structure to break eddy current loops. Antenna eddy currents were computed assuming the antenna is shorted. In the final design, frame forces primarily result from halo currents entering around the mini limiters that now protect the antenna windings. Accelerations due to the vessel disruption dynamic response were included in the loading. The antenna mechanical design has been shown to perform adequately for all identified disruption loading.