Theory of free-electron-laser heating and current drive in magnetized plasmas

Abstract

The introduction of a powerful new microwave source, the free-electron laser, provides new opportunities for novel heating and current-drive schemes to be used in toroidal fusion devices. This high-power, pulsed source has a number of technical advantages for these applications, and its use is predicted to lead to improved current-drive efficiencies and opacities in reactor-grade fusion plasmas in specific cases. The Microwave Tokamak Experiment at the Lawrence Livermore National Laboratory will provide a test for some of these new heating and current-drive schemes. Although the motivation for much of this research has derived from the application of a free-electron laser to the heating of a tokamak plasma at a frequency near the electron cyclotron frequency, the underlying physics, i.e., the highly nonlinear interaction of an intense, pulsed, coherent electromagnetic wave with an electron in a magnetized plasma including relativistic effects, is of general interest. Other relevant applications include ionospheric modification by radiofrequency waves, high-energy electron accelerators, and the propagation of intense, pulsed electromagnetic waves in space and astrophysical plasmas. This review reports recent theoretical progress in the analysis and computer simulation of the absorption and current drive produced by intense pulses, and of the possible complications that may arise, e.g., parametric instabilities, nonlinear self-focusing, trappedparticle sideband instability, and instabilities of the heated plasma. CONTENTSI. Introduction 949 A. Motivation and scope 949 B. Free-electron laser 950 C. Basic physics 951 D. Outline of succeeding sections 954 II. Wave-Particle Interaction 954 A. Relativistic Hamiltonian and particle dynamics 954 B. Heating and current-drive mechanisms 958 III. Current-Drive Applications 965 A. Pulsed current drive 965 B. Trapped-particle effects 966 C. Current-drive efficiencies 967 IV. Stability of an Intense Electron Cyclotron Wave 971 A. Parametric instabilities 971 B. Trapped-particle sideband instability 974 C. Nonlinear self-focusing 975 D. Stability of the heated plasma 977 V. Simulations of FEL Heating and Current Drive 978 A. Monte Carlo and self-consistent particle simulations 978 B. Nonlinear electron cyclotron heating 978 C. Stochastic heating and current drive 980 D. Rising buckets 981 E. Beat-wave current drive 984 F. Parametric instabilities 985 VI. Experimental Tests 987 VII. Conclusions 987 Acknowledgments 988 References 988

Positive and negative effective mass of classical particles in oscillatory and static fields

“…IV C), all the three equilibria are stable here (Fig. 3), unlike for a one-dimensional (1D) nonlinear oscillator [60], as well as in contrast to the 3D cyclotron resonance in a quasi-static field [56] or any wave with a parallel refraction index n || other than unity [61,62,63,64]. The difference from Refs.…”

“…The difference from Refs. [56,61,62,63,64,65] is understood from the angle-action equations for the transverse oscillations, which now are governed by the Hamiltonian Here J = ρ 2 ⊥ /2σ is the action conjugate to the angle ψ, and λ = a 0 / √ σ; the effective time is ξ, dξ = (ωu/γ) dt. Since ρ z = (1 − u 2 + 2σJ)/2u [Eq.…”

On possibility of low-threshold two-plasmon decay instability in 2ndharmonic ECRH experiments at toroidal devices

“…As having been revealed by theoretical analysis a couple of decades ago [1] - [3] parametric decay instabilities (PDI) which may accompany ECR fundamental harmonic ordinary (O) mode and 2 nd harmonic extraordinary (X) mode heating experiments are believed to be deeply suppressed by convective losses of daughter waves both along the magnetic field and in radial direction. Thus, the EC wave propagation and absorption are thought to be well described by linear theory and predictable in detail.…”

Low Threshold Parametric Decay Backscattering Instability in Tokamak Electron Cyclotron Resonance Heating Experiments

“…The IB wave amplification coefficient and the PDI threshold are obtained. The later is sh standard theory [1][2][3].…”

“…According to theoretical analysis of PDI thresholds [1][2][3], the typical RF power at which these nonlinear effects can be excited at tokamak plasma parameters is around 1 GW, which is only possible with a free electron laser microwave source. The physical reason for such a deep suppression is provided by strong convective losses of daughter waves from the decay region either in the plasma inhomogeniety direction or along the magnetic field.…”