64 research outputs found
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Energetic particle effects as an explanation for the low frequencies of Alfvén modes in the DIII-D tokamak
During beam injection in the DIII-D tokamak, modes with lower frequencies than expected for toroidicity induced Alfvén eigenmodes (TAEs) are often observed. The experimental 'TAE' frequency is often ≈0.8 of the nominal theoretical frequency of the TAE, f , while the typical frequency of beta induced Alfvén eigenmodes (BAEs) is (0.2-0.4) f . An analysis is presented of an unstable discharge with a high n stability code, HINST, that includes the effect of energetic ions on mode frequency. The analysis shows that the experimental 'TAE' and 'BAE' could be resonant branches of the TAE and the kinetic ballooning mode, respectively. TAE TA
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Beam-driven energetic particle modes in advanced tokamak plasmas
A major goal of the DIII-D program is to study 'advanced tokamak' plasmas with good confinement, large normalized β, and a large fraction of self-sustained current. Many of these plasmas have large beam pressures (≲1/3 of the total pressure) and weak magnetic shear; Alfvén instabilities with laboratory frequencies of 100-250 kHz are often observed. The instabilities correlate with reductions in the neutron rate below the classically expected value, complicating determination of the pressure and current profiles. Quantitative analysis of one case suggests that two types of energetic particle modes are destabilized: the resonant toroidicity-induced Alfvén eigenmode and the resonant kinetic ballooning mode. The strong dependence on neutral beam injection parameters and the variability in mode frequency are qualitatively consistent with this identification. Further analysis and measurements are planned
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Erratum: 1.5D quasilinear model and its application on beams interacting with Alfvén eigenmodes in DIII-D (Physics of Plasmas (2012) 19(092511))
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Observation of compressional Alfvén eigenmodes (CAE) in a conventional tokamak
Fast-ion instabilities with frequencies somewhat below the ion cyclotron frequency occur frequently in spherical tokamaks such as the National Spherical Torus Experiment (NSTX). NSTX and the DIII-D tokamak are nearly ideal for fast-ion similarity experiments, having similar neutral beams, fast-ion to Alfvén speed vf/vA, fast-ion pressure, and shape of the plasma but with a factor of two difference in major radius. When DIII-D is operated at low field (0.6 T), compressional Alfvén eigenmode (CAE) instabilities appear that closely resemble the NSTX instabilities. In particular, the mode frequencies, polarization and beam-energy threshold are nearly identical to NSTX. CAE in high-field discharges and emission at cyclotron harmonics are also observed. As on NSTX, the basic stability properties are consistent with the idea that the instability is driven by anisotropy in the fast-ion velocity distribution and is damped predominantly by Landau damping of electrons. The results suggest that these modes might be excited in ITER. © 2006 IAEA, Vienna
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1.5D quasilinear model and its application on beams interacting with Alfvén eigenmodes in DIII-D
We propose a model, denoted here by 1.5D, to study energetic particle (EP) interaction with toroidal Alfvenic eigenmodes (TAE) in the case where the local EP drive for TAE exceeds the stability limit. Based on quasilinear theory, the proposed 1.5D model assumes that the particles diffuse in phase space, flattening the pressure profile until its gradient reaches a critical value where the modes stabilize. Using local theories and NOVA-K simulations of TAE damping and growth rates, the 1.5D model calculates the critical gradient and reconstructs the relaxed EP pressure profile. Local theory is improved from previous study by including more sophisticated damping and drive mechanisms such as the numerical computation of the effect of the EP finite orbit width on the growth rate. The 1.5D model is applied on the well-diagnosed DIII-D discharges #142111 [M. A. Van Zeeland, Phys. Plasmas 18, 135001 (2011)] and #127112 [W. W. Heidbrink et al., Nucl. Fusion. 48, 084001 (2008)]. We achieved a very satisfactory agreement with the experimental results on the EP pressure profiles redistribution and measured losses. This agreement of the 1.5D model with experimental results allows the use of this code as a guide for ITER plasma operation where it is desired to have no more than 5% loss of fusion alpha particles as limited by the design. © 2012 American Institute of Physics
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Discrete compressional Alfvén eigenmode spectrum in tokamaks
The spectrum of compressional Alfvén eigenmodes (CAE) is analysed and shown to be discrete in tokamaks with low aspect ratio, such as the National Spherical Torus Experiment (NSTX), as well as in conventional tokamaks, such as DIII-D. The study is focused on recent similarity experiments on NSTX and DIII-D in which sub-cyclotron frequency instabilities of CAEs were observed at similar plasma conditions (W.W. Heidbrink et al 2006 Nucl. Fusion 46 324). The global ideal MHD code NOVA recovers the main properties of these modes predicted by theory and observed in both devices. The discrete spectrum of CAEs is characterized by three quantum mode numbers for each eigenmode, (M, S and n), where M, S and n are poloidal, radial and toroidal mode numbers, respectively. The expected mode frequency splitting corresponding to each of these mode numbers seems to be observed in experiments and is consistent with our numerical analysis. The polarization of the observed magnetic field oscillations in NSTX was measured and is also consistent with the numerical analysis, which helps to identify them as CAE activity. CAE mode structure was obtained and shown to be localized in both radial and poloidal directions with typical radial localization toward the plasma edge and poloidal localization at the low field side of the plasma cross section. © 2006 IAEA, Vienna
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An Alfvén eigenmode similarity experiment
The major radius dependence of Alfvén mode stability is studied by creating plasmas with similar minor radius, shape, magnetic field (0.5 T), density (n ≃ 3 × 10 m ), electron temperature (1.0 keV) and beam ion population (near-tangential 80 keV deuterium injection) on both NSTX and DIII-D. The major radius of NSTX is half the major radius of DIII-D. The super-Alfvénic beam ions that drive the modes have overlapping values of v /v in the two devices. Observed beam-driven instabilities include toroidicity-induced Alfvén eigenmodes (TAE). The stability threshold for the TAE is similar in the two devices. As expected theoretically, the most unstable toroidal mode number n is larger in DIII-D. e f A 19 -
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Validating predictive models for fast ion profile relaxation in burning plasmas
The redistribution and potential loss of energetic particles due to MHD modes can limit the performance of fusion plasmas by reducing the plasma heating rate. In this work, we present validation studies of the 1.5D critical gradient model (CGM) for Alfvén eigenmode (AE) induced EP transport in NSTX and DIII-D neutral beam heated plasmas. In previous comparisons with a single DIII-D L-mode case, the CGM model was found to be responsible for 75% of measured AE induced neutron deficit [1]. A fully kinetic HINST is used to compute mode stability for the non-perturbative version of CGM (or nCGM). We have found that AEs show strong local instability drive up to violating assumptions of perturbative approaches used in NOVA-K code. We demonstrate that both models agree with each other and both underestimate the neutron deficit measured in DIII-D shot by approximately a factor of 2. On the other hand in NSTX the application of CGM shows good agreement for the measured flux deficit predictions. We attempt to understand these results with the help of the so-called kick model which is based on the guiding center code ORBIT. The kick model comparison gives important insight into the underlying velocity space dependence of the AE induced EP transport as well as it allows the estimate of the neutron deficit in the presence of the low frequency Alfvénic modes. Within the limitations of used models we infer that there are missing modes in the analysis which could improve the agreement with the experiments
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