Alfven Eigenmodes Research Articles

Smoothly Varying Injected Neutral Beam Voltage and Current Provides New Capability on the DIII-D Tokamak

A significant engineering upgrade of the DIII-D neutral beam system has recently completed, providing ability to smoothly and independently vary the beam voltage and current. This enables greater control of the injected beam power, torque, and plasma instability drive for fusion experiments. Modifications to the high-voltage equipment and the plasma control system were made and tested over the last year to allow beam energy to vary by as much as 20 kV over a 0.5-s period anywhere within the 45–85 kV operating range of the beams. The beam current can be made to track the voltage (keeping the perveance constant), or the current and voltage can be varied independently (scanning the perveance). The smooth variation of beam energy avoids the extremely perturbative effects of pulsewidth modulation, the only tool previously used for regulating the injected neutral beam power. With independent control of voltage and current, the beam ion velocity space can be tailored to facilitate new experiments that explore, for example, the detrimental effects of Alfven eigenmodes. These modes are driven by energetic particles and can possibly be avoided in steady-state scenarios by timely variation of the beam energy while maintaining constant input power. A description of the modifications made to the power supply and beam controls will be presented, as well as some initial physics results employing the new variable beam energy system.

Analysis of resonant fast ion distributions during combined ICRF and NBI heating with transients using neutron emission spectroscopy

ICRF heating at the fundamental cyclotron frequency of a hydrogen minority ion species also gives rise to a partial power absorption by deuterium ions at their second harmonic resonance. This paper studies the deuterium distributions resulting from such 2nd harmonic heating at JET using neutron emission spectroscopy data from the time of flight spectrometer TOFOR. The fast deuterium distributions are obtained over the energy range 100 keV to 2 MeV. Specifically, we study how the fast deuterium distributions vary as ICRF heating is used alone as well as in combination with NBI heating.When comparing the different heating scenarios, we observed both a difference in the shapes of the distributions as well as in their absolute level. The differences are most pronounced below 0.5 MeV. Comparisons are made with corresponding distributions calculated with the code PION. We find a good agreement between the measured distributions and those calculated with PION, both in terms of their shapes as well as their amplitudes.However, we also identified a period with signs of an inverted fast ion distribution, which showed large disagreements between the modeled and measured results. Resonant interactions with tornado modes, i.e. core localized toroidal alfven eigenmodes (TAEs), are put forward as a possible explanation for the inverted distribution.

Numerical simulation of the multiple core localized low shear toroidal Alfvenic eigenmodes

In modern tokamak experiments, scenarios with weak central magnetic shear has been proposed. It is necessary to study the Alfvenic mode activities in such scenarios. Theoretical researches have predicted the multiplicity of core-localized toroidally induced Alfvenic eigenmodes for ε/s > 1, where ε is the inverse aspect ratio and s is magnetic shear. We numerically investigate the existence of multiplicity of core-localized TAEs and mode characteristics using NOVA code in the present work. We firstly verify the existence of the multiplicity for zero beta plasma and the even mode at the forbidden zone. For finite beta plasma, the mode parities become more distinguishable, and the frequencies of odd modes are close to the upper tip of the continuum, while the frequencies of even modes are close to the lower tip of the continuum. Their frequencies are well separated by the forbidden zone. With the increasing value of ε/s, more modes with multiple radial nodes will appear, which is in agreement with theoretical prediction. The discrepancy between theoretical prediction and our numerical simulation is also discussed in the main text.

Zonal structure effect on the nonlinear saturation of reverse shear Alfven eigenmodes

A gyrokinetic ion/fluid electron hybrid model is used to study the nonlinear evolution of reverse shear Alfven eigenmodes (RSAE) driven by energetic particles. When only the energetic particle nonlinear effects are included, the saturation amplitude of a single-n RSAE follows the trapping scaling, δB/B∼(γ/ω)2. When the nonlinear effects of thermal ions and electrons are included, zonal structures are force generated but do not affect the saturation amplitude for γ/ω≤0.03. No spontaneous generation of zonal structures is observed, in contrast to ion-temperature-gradient-driven modes. At larger energetic particle drive, the effects of zonal structures cause a significant reduction in the RSAE saturation amplitude. The reduction is not caused by the zonal flow shearing of the RSAE, but by the force-generated n = 0 component in the thermal ion distribution function and the electron density. These n = 0 perturbations lead to nonlinear evolution of the RSAE mode structure and enhance damping.

Optimization of Antenna Current Feeding for the Alfvén Eigenmodes Active Diagnostic System of JET

The possibility of exploring proper phasing of the feeding currents in the existing antenna of the Alfven Eigenmodes Active Diagnostic system of JET, to excite pure toroidal spectra of Toroidal Alfven Eigenmodes, is numerically investigated. Special attention is given to the actual perturbed fields excited in the plasma, which are calculated self-consistently using the antenna version of the CASTOR code. It is found that due to the close spacing of the JET antenna modules and quasi degeneracy of modes with medium to high values of the toroidal mode number n, although a proper choice of the phasing of the feeding currents of the antenna modules indeed leads to an increase of the perturbed fields of the selected mode, modes with nearby values of n are also excited with large amplitudes, so that a scheme to proper select the detected modes remains necessary. A scheme using different antenna position distribution is proposed to achieve successful optimization.

Toroidal Alfvén Eigenmodes in Globus-M Spherical Tokamak Plasma

Conditions of the excitation of toroidal Alfven eigenmodes (TAEs) and their inf luence on the confinement of fast particles into the Globus-M spherical tokamak have been studied by KINX code calculations of the magnetohydrodynamic spectra of reconstructed divertor equilibrium configurations with stability margin q0 > 1 on the magnetic axis. The sensitivity of the frequencies of TAE modes with toroidal wavenumber n = 1 to the type of boundary conditions and choice of boundary surface has been studied. It has been established that the frequencies of modes with dominating poloidal harmonics m = 1 and 2 in the gap of continuum are significantly higher than those observed in the spectra of signals measured in the Mirnov coil probes, especially under the assumption of free plasma boundary with allowance for its compressibility. The TAE modes with lower frequencies and higher poloidal wavenumbers localized near the plasma boundary may be responsible for the oscillations observed in the experiment. However, these modes are characterized by the interaction with continuum and, probably, exhibit related damping.

Destabilization of beta-induce Alfvén eigenmodes by energetic trapped ions in low-magnetic-shear plasma

The dispersion relation for high toroidal mode number n beta-induced Alfven eigenmodes (BAEs) excited by magnetically trapped energetic ions generated with ion-cyclotron resonance heating via precessional resonance in low magnetic shear (s) tokamak plasma is investigated analytically and numerically. The dynamics of the energetic particles (EPs) is treated non-perturbatively and finite drift orbit width (FOW) effects are taken into account. It is found that, depending on the plasma parameters, the most unstable mode can be either the BAE mode or the energetic particle mode (EPM). Both modes can resonate with the trapped-particles' magnetic precessional drifts and become unstable. The mode frequencies and growth rates depend strongly on the EP parameters. FOW can stabilize high-n BAE modes by reducing the wave-particle interaction. Magnetic shear has an important effect on the growth rate of the modes. For BAE, the growth rate presents the trend of first increase and following decrease with the increase of...

Analysis of Alfven eigenmodes destabilization by energetic particles in TJ-II using a Landau-closure modelaaThis 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

Alfvén Eigenmodes (AE) can be destabilized by energetic particles in neutral beam injection (NBI) heated plasmas through inverse Landau damping and couplings with gap modes in the shear Alfvén continua. We describe the linear evolution of the poloidal flux and the toroidal component of the vorticity in a full 3D system using the reduced MHD equations, density and parallel velocity moments for the energetic particles as well as the geodesic acoustic wave dynamics. A closure relation adds the Landau damping and resonant destabilization effects in the model. We apply the model to study the Alfvén modes stability in TJ-II, performing a parametric analysis in a range of realistic values of energetic particle β (), ratios of thermal/Alfvén velocities (), energetic particle density profiles and toroidal modes (n) including toroidal and helical couplings. The study predicts a large helical coupling between different toroidal modes and the destabilization of helical Alfvén eigenmodes (HAE) with frequencies similar to the AE activity measured in TJ-II, between 50–400 kHz. The analysis has also revealed the destabilization of GAE (global Alfvén eigenmodes), TAE (toroidal Alfvén eigenmodes) and EPM (energetic particle modes). For the modes considered here, optimized TJ-II operations require a profile in the range of to stabilize AEs in the inner and middle plasma. AEs in the plasma periphery cannot be fully stabilized, although for a configuration with , only AE are unstable with a growth rate 4 times smaller compared to the standard case and a frequency of 100 kHz. We reproduce the frequency sweeping evolution of the AE frequency observed in TJ-II as the profile is varied. The AE frequency sweeping is caused by consecutive changes of the instability dominant modes between different helical families.

Impact of energetic particle orbits on long range frequency chirping of BGK modes

Long range frequency chirping of Bernstein–Greene–Kruskal modes, whose existence is determined by the fast particles, is investigated in cases where these particles do not move freely and their motion is bounded to restricted orbits. A nonuniform equilibrium magnetic field is included into the bump-on-tail instability problem of a plasma wave. The parallel field gradients account for the existence of different orbit topologies of energetic particles. With respect to fast particles dynamics, the extended model captures the range of particles motion (trapped/passing) with energy and thus represents a more realistic 1D picture of the long range sweeping events observed for weakly damped modes, e.g. global Alfven eigenmodes, in tokamaks. The Poisson equation is solved numerically along with bounce averaging the Vlasov equation in the adiabatic regime. We demonstrate that the shape and the saturation amplitude of the nonlinear mode structure depends not only on the amount of deviation from the initial eigenfrequency but also on the initial energy of the resonant electrons in the equilibrium magnetic field. Similarly, the results reveal that the resonant electrons following different equilibrium orbits in the nonuniform field lead to different rates of frequency evolution. As compared to the previous model (Breizman 2010 Nucl. Fusion 50 084014), it is shown that the frequency sweeps with lower rates. The additional physics included in the model enables a more complete 1D description of the range of phenomena observed in experiments.

Excitation of low frequency Alfven eigenmodes in toroidal plasmas

Global gyrokinetic simulations find that realistic density gradients of energetic particles can simultaneously excite low frequency Alfven eigenmodes in toroidal geometry, beta-induced Alfven-acoustic eigenmode (BAAE) and beta-induced Alfven eigenmode (BAE), with similar radial mode widths and comparable linear growth rates even though damping rate of BAAE is much larger than BAE in the absence of energetic particles. This surprising result is attributed to non-perturbative effects of energetic particles that modify ideal BAAE mode polarizations and nonlocal geometry effects that invalidate radially local dispersion relation. Dominant mode changes from BAAE in a larger tokamak to BAE in a smaller tokamak due to the dependence of wave-particle resonance condition on the tokamak size.

Overview of NSTX Upgrade initial results and modelling highlights

The National Spherical Torus Experiment (NSTX) has undergone a major upgrade, and the NSTX Upgrade (NSTX-U) Project was completed in the summer of 2015. NSTX-U first plasma was subsequently achieved, diagnostic and control systems have been commissioned, the H-mode accessed, magnetic error fields identified and mitigated, and the first physics research campaign carried out. During ten run weeks of operation, NSTX-U surpassed NSTX record pulse-durations and toroidal fields (TF), and high-performance ~1 MA H-mode plasmas comparable to the best of NSTX have been sustained near and slightly above the n = 1 no-wall stability limit and with H-mode confinement multiplier H98y,2 above 1. Transport and turbulence studies in L-mode plasmas have identified the coexistence of at least two ion-gyro-scale turbulent micro-instabilities near the same radial location but propagating in opposite (i.e. ion and electron diamagnetic) directions. These modes have the characteristics of ion-temperature gradient and micro-tearing modes, respectively, and the role of these modes in contributing to thermal transport is under active investigation. The new second more tangential neutral beam injection was observed to significantly modify the stability of two types of Alfven eigenmodes. Improvements in offline disruption forecasting were made in the areas of identification of rotating MHD modes and other macroscopic instabilities using the disruption event characterization and forecasting code. Lastly, the materials analysis and particle probe was utilized on NSTX-U for the first time and enabled assessments of the correlation between boronized wall conditions and plasma performance. These and other highlights from the first run campaign of NSTX-U are described.

Numerical study of the existence criterion for the reversed shear Alfven eigenmode in the presence of a parallel equilibrium current

We have re-visited the existence criterion of the reverse shear Alfven eigenmodes (RSAEs) in the presence of the parallel equilibrium current by numerically solving the eigenvalue equation using a fast eigenvalue solver code KAES. The parallel equilibrium current can bring in the kink effect and is known to be strongly unfavorable for the RSAE. We have numerically estimated the critical value of the toroidicity factor Qtor in a circular tokamak plasma, above which RSAEs can exist, and compared it to the analytical one. The difference between the numerical and analytical critical values is small for low frequency RSAEs, but it increases as the frequency of the mode increases, becoming greater for higher poloidal harmonic modes.

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...

Radially localized kinetic beta induced Alfven eigenmodes in tokamak plasmas

We have studied the kinetic beta induced Alfven eigenmodes (KBAEs) by developing a fast global eigenvalue solving code. Even and odd parity KBAEs radially localized at the rational surface are found with the presence of the kinetic effects, namely, the finite ion Larmor radius effects. The KBAE mode structures become broader in the presence of the parallel equilibrium current. The KBAE frequency increases with the increase in the plasma β and the ion Larmor radius (ρi). Damping of the KBAE increases with the finite Larmor radius and decreases with the plasma beta and mode with a higher radial mode number has a stronger damping. Numerical results reveal that frequency and kinetic damping of KBAEs are independent of the tokamak size, and that kinetic damping in the core region is significantly smaller by an order of magnitude compared to that in the radially outward region of tokamak.

Study of Reversed Shear Alfvén Eigenmodes with Down Sweeping Frequency in HL-2A Experiment atqmin∼ 1

The reserved shear Alfven eigenmodes (RSAEs) whose frequencies sweep down have been observed in HL-2A plasmas at qmin ∼ 1 during current ramp-up and NBI+ECRH recently. The kinetic Alfven eigenmode code (KAEC), which is a non-perturbative kinetic/magnetohydrodynamic eigenvalue code for calculating kinetic damping of shear Alfven eigenmodes, is used to investigate the modes according to the experimental data. It is found that the kinetic reversed shear Alfven eigenmodes (KRSAEs) with downsweeping frequency and localized radial structure would appear, which are consistent with the observations. The theoretical analysis for the existence of the mode and the scaling of the mode width are provided. It is shown that the mode width of KRSAEs is proportional to the square root of thermal ion Larmor radius.

The upgraded heavy ion beam probe diagnostics on the T-10 tokamak

The upgraded Heavy Ion Beam Probe (HIBP) diagnostics on the T-10 tokamak (National Research Center ‘Kurchatov Institute’) is presented. HIBP is a powerful tool to study electric potential in the core and edge plasmas along with broadband turbulence and quasicoherent modes such as Geodesic Acoustic Mode (GAM) and Alfven Eigenmode (AE). To study broadband turbulence and AEs, which can be driven by fast electrons in regimes with auxiliary Electron Cyclotron Resonance Heating the frequency range of about several hundred kHz is needed. The upgrade is focused on the extension of the frequency range of HIBP signals up to 500 kHz, and on increasing of density operating limit up to 5-1019 m-3. It becomes possible due to a newly designed emitter-extractor unit of HIBP accelerator aiming to provide the primary beam with the current of 300 pA at the energy of 300 keV and diameter of 7-10 mm. The new in-vessel elements of a primary beamline - wire sensor and Faraday cup - were upgraded accordingly to be able to deliver the probing beam with advanced parameters to the plasma.

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.

Development and validation of a critical gradient energetic particle driven Alfven eigenmode transport model for DIII-D tilted neutral beam experiments

Recent experiments with the DIII-D tilted neutral beam injection (NBI) varying the beam energetic particle (EP) source profiles have provided strong evidence that unstable Alfven eigenmodes (AE) drive stiff EP transport at a critical EP density gradient [Heidbrink et al 2013 Nucl. Fusion 53 093006]. Here the critical gradient is identified by the local AE growth rate being equal to the local ITG/TEM growth rate at the same low toroidal mode number. The growth rates are taken from the gyrokinetic code GYRO. Simulation show that the slowing down beam-like EP distribution has a slightly lower critical gradient than the Maxwellian. The ALPHA EP density transport code [Waltz and Bass 2014 Nucl. Fusion 54 104006], used to validate the model, combines the low-n stiff EP critical density gradient AE mid-core transport with the Angioni et al (2009 Nucl. Fusion 49 055013) energy independent high-n ITG/TEM density transport model controling the central core EP density profile. For the on-axis NBI heated DIII-D shot 146102, while the net loss to the edge is small, about half the birth fast ions are transported from the central core r/a < 0.5 and the central density is about half the slowing down density. These results are in good agreement with experimental fast ion pressure profiles inferred from MSE constrained EFIT equilibria.