Effect Of Electron Cyclotron Current Research Articles

Effects of the resonance modification by electron cyclotron current drive on the linear and nonlinear dynamics of energetic particle driven magnetohydrodynamics modes in Heliotron J

This study investigates the effects of electron cyclotron current drive (ECCD) on the linear and nonlinear dynamics of energetic-particle (EP)-driven magnetohydrodynamics (MHD) instability (mode) in Heliotron J (HJ), using MEGA, a nonlinear hybrid EP-MHD simulation code. The effects of ECCD are included through the modification of the equilibrium magnetic field. Five MHD equilibria with plasma currents ( Ip ) of −2.00, −1.00, 0.00, 0.50, and 1.00 kA are considered, where negative and positive Ip represent counter and co-ECCD, respectively. To account for the EP finite charge-exchange loss observed in HJ, a bump-on-tail velocity distribution is assumed for EP. The study finds that the stabilization trends in terms of the linear growth rate ( γωA−1 ) are qualitatively similar to the FAR3D linear simulation and experimental results. In addition to the enhancements of the local magnetic shear and continuum damping by ECCD as proposed in previous studies, the study suggests that changes in the linear EP and shear Alfvén wave resonance are another important factor, especially for the n/m=1/2 mode. The study shows that in the currentless equilibrium, the high-velocity toroidicity-induced resonance between the n/m=1/2 mode and the co-passing EP ( L=−1 ) has the highest EP energy transfer rate, but is located slightly below the NBI injection velocity. The co-ECCD and cntr-ECCD can shift this resonance into higher and lower velocity regions, respectively. This can lead to resonance removal for co-ECCD, while cntr-ECCD can initially increase the number of resonant EPs for a small value of shift. If the shift of the resonance by cntr-ECCD is sufficiently large, the study observes stabilization due to the reduction in the EP population in the low-velocity region. The study also finds that changes in resonance affect nonlinear dynamics such as EP transport and frequency chirping. The convective transport of the co- and counter-passing EPs can be controlled, which can only be achieved for energetic-particle-mode due to the asymmetry in the resonance velocity between co- and cntr-passing EPs. The changes in the frequency chirping from the downward chirping branch to the upward chirping branch can also be explained by the changes in the resonance velocity caused by ECCD.

Control of multi-helicity neo-classical tearing modes by electron cyclotron current drive in tokamak plasmas

The effect of electron cyclotron current drive (ECCD) on the multi-helicity neo-classical tearing modes (2/1 and 3/2 NTMs) in tokamak plasmas is numerically studied by adopting a reduced magnetohydrodynamic (MHD) model. It is found that the control process of certain single helicity NTM by ECCD can be influenced by nonlinear multi-helicity interaction. In the case without plasma rotation, the ‘phase flip’ of 2/1 NTM, which happens in the control of single helicity situation, has been modified by multi-helicity interaction and consequently behaves periodicity. And the ‘phase flip’ of 3/2 NTM disappears. When plasma rotation is considered, the direct control of 3/2 NTM becomes difficult. Moreover, a dual role of 5/3 mode is found in the multi-helicity interaction between 2/1 and 3/2 modes. The nonlinear mode coupling among 2/1 and 3/2 modes through the 5/3 mode will increase the difficulty of the suppression of 3/2 mode by ECCD, while the competition effect between multi-helicity NTMs leads to the relatively weak amplitude of 2/1 mode. In addition, investigations of multiple ECCDs are also carried out. The numerical results indicate that the control strategy for controlling multi-helicity NTMs by ECCD in experiments may require well-designed aiming to gain high effectiveness and efficiency.

Synergistic effect of electron cyclotron current drive and poloidal shear flow on the tearing mode

The separate and synergistic effects of both the electron cyclotron current drive (ECCD) and poloidal shear flow on the tearing mode are investigated numerically by using two-dimensional compressible magnetohydrodynamics equations in slab geometry. For the misaligned ECCD, effects of radial and poloidal misalignments have been compared emphatically. It is found that the suppression effect of ECCD is weakened with the increase of malposed ratio and it is more sensitive to the radial misalignment. The stability effect of shear flow is not positively related to the flow shear; the effects of starting moment of ECCD and shear flow are similar but not identical. The synergistic stability effect of ECCD and shear flow is more effective than ECCD or shear flow acts alone on the tearing mode without considering the “flip” instability. Furthermore, the combinatorial stability effect is more obvious when ECCD has a radial misalignment as a result of the continuous poloidal shift of magnetic island.

Effects of electron cyclotron current drive on magnetic islands in tokamak plasmas

The effects of the electron cyclotron current drive on magnetic islands in tokamak plasmas are studied using gyrokinetic simulations. By investigating the effects of different characteristics of the driven current, such as current density distribution and deposition location, the factors which can determine the suppression effect on the resistive tearing modes have been explored. It is found that an electron cyclotron wave (ECW) driven current with a larger peak value and more focused deposition region has a better stabilization effect. When the ECW-driven current is closer to the rational surface, it has a better stabilizing effect. These gyrokinetic toroidal code (GTC) linear simulations in the electron fluid limit of the tearing modes in the cylindrical geometry agree well with the magnetohydrodynamic codes. Furthermore, the optimal timing control of the current deposition on resistive tearing modes is demonstrated.

Effects of electron cyclotron current drive on the evolution of double tearing mode

The effects of electron cyclotron current drive (ECCD) on the double tearing mode (DTM) in slab geometry are investigated by using two-dimensional compressible magnetohydrodynamics equations. It is found that, mainly, the double tearing mode is suppressed by the emergence of the secondary island, due to the deposition of driven current on the X-point of magnetic island at one rational surface, which forms a new non-complete symmetric magnetic topology structure (defined as a non-complete symmetric structure, NSS). The effects of driven current with different parameters (magnitude, initial time of deposition, duration time, and location of deposition) on the evolution of DTM are analyzed elaborately. The optimal magnitude or optimal deposition duration of driven current is the one which makes the duration of NSS the longest, which depends on the mutual effect between ECCD and the background plasma. Moreover, driven current introduced at the early Sweet-Parker phase has the best suppression effect; and the optimal moment also exists, depending on the duration of the NSS. Finally, the effects varied by the driven current disposition location are studied. It is verified that the favorable location of driven current is the X-point which is completely different from the result of single tearing mode.

Consequences of plasma rotation for neoclassical tearing mode suppression by electron cyclotron current drive

In the generalized Rutherford equation describing the nonlinear evolution of the width of the magnetic island associated with a neoclassical tearing mode, the effect of localized current drive is represented by a term ΔCD′. We investigate oscillations in ΔCD′ originating from the rotation of the island through the electron cyclotron power deposition region and their dependence on the collisional time scale on which the driven current is generated, the rotation period, the island size, and the power deposition width. Furthermore, their consequences for the island growth or the stabilization are analyzed. This work shows that the net result of the oscillations in ΔCD′ is a slight increase in the stabilizing effect of electron cyclotron current drive and consequently, a reduction in the minimum power requirement to fully suppress an island.

Calculating electron cyclotron current drive stabilization of resistive tearing modes in a nonlinear magnetohydrodynamic model

A model which incorporates the effects of electron cyclotron current drive (ECCD) into the magnetohydrodynamic equations is implemented in the NIMROD code [C. R. Sovinec et al., J. Comput. Phys. 195, 355 (2004)] and used to investigate the effect of ECCD injection on the stability, growth, and dynamical behavior of magnetic islands associated with resistive tearing modes. In addition to qualitatively and quantitatively agreeing with numerical results obtained from the inclusion of localized ECCD deposition in static equilibrium solvers [A. Pletzer and F. W. Perkins, Phys. Plasmas 6, 1589 (1999)], predictions from the model further elaborate the role which rational surface motion plays in these results. The complete suppression of the (2,1) resistive tearing mode by ECCD is demonstrated and the relevant stabilization mechanism is determined. Consequences of the shifting of the mode rational surface in response to the injected current are explored, and the characteristic short-time responses of resistive tearing modes to spatial ECCD alignments which are stabilizing are also noted. We discuss the relevance of this work to the development of more comprehensive predictive models for ECCD-based mitigation and control of neoclassical tearing modes.

Simulation of Hybrid Operation Modes in ITER

As one of the international thermonuclear experimental reactor (ITER) primary operation modes, the hybrid mode aims at establishing plasmas with significant fusion power and low loop voltage to drive an inductive current to test reactor-relevant components in extended pulse lengths at high neutron fluence. In this paper, predictive modeling of the hybrid mode is presented. The potential of hybrid modes is investigated with respect to fusion performance and the non-inductive current drive fraction in ITER. Simulations are performed with the ASTRA transport code by employing a physics-based heat transport model. Here, the particle transport is prescribed. The effect of electron cyclotron current drive (ECCD) to establish a low magnetic shear in the center of the plasma is also discussed. The simulations show that fusion gains and the non-inductive current drive fractions of up to 8.4 and 49 %, respectively, can be achieved in hybrid modes at ITER.

The m=2, n=1 mode suppression by ECRH on the T-10 tokamak

Experiments on m=2, n=1 tearing mode suppression and on avoidance of density limit disruptions by electron cyclotron resonance heating (ECRH) were performed on the T-10 tokamak. Partial suppression of the m=2, n=1 mode by the high frequency (HF) power deposition in the vicinity of the q=2 surface was observed. Development of external kink modes with HF power injection can result in m=2, n=1 mode destabilization under specific operating conditions. ECRH suppresses m=2, n=1 mode activity at extremely high values of electron densities and prevents the density limit disruptions practically independently of EC resonance position. Complete compensation of the additional peripheral heat losses near the density limit by ECRH should be responsible for this result. No effect of electron cyclotron current drive (ECCD) on m=2, n=1 mode stability has been observed because of insufficient values of HF driven current in the vicinity of the q=2 surface under the operating conditions of the experiment

Theory and calculation of current profile control near rational surfaces in a tokamak by electron cyclotron waves

The effect of electron cyclotron current drive (ECCD) on nonlinear resistive magnetohydrodynamic (MHD) perturbations in a tokamak is analyzed. Along the ray trajectories of electron cyclotron wave (ECW) beams injected from the low-field side in the ordinary mode (O-mode) polarization, the power deposition, and current drive efficiencies are calculated using fully relativistic analytical expressions. The driven current is inserted in a consistent theory of nonlinear magnetic island evolution, which provides the scaling of the radio-frequency (RF) driven current required to control magnetic reconnection in terms of the plasma equilibrium parameters and optical properties. It is shown that the threshold for error field driven islands can be increased by ECCD and that the current profile around the q=2 rational surface can be shaped to control the evolution of rotating magnetic islands by modulation of the gyrotron power source.