Abraham Bers

Kinetic enhancement of Raman backscatter, and electron acoustic Thomson scatter

One-dimensional Eulerian Vlasov-Maxwell simulations are presented that show kinetic enhancement of stimulated Raman backscatter (SRBS) due to electron trapping in regimes of heavy linear Landau damping. The conventional Raman Langmuir wave is transformed into a set of beam acoustic modes [L. Yin et al., Phys. Rev. E 73, 025401 (2006)]. A low phase velocity electron acoustic wave (EAW) is seen developing from the self-consistent Raman physics. Backscatter of the pump laser off the EAW fluctuations is reported and referred to as electron acoustic Thomson scatter. This light is similar in wavelength to, although much lower in amplitude than, the reflected light between the pump and SRBS wavelengths observed in single-hot-spot experiments, and previously interpreted as stimulated electron acoustic scatter [D. S. Montgomery et al., Phys. Rev. Lett. 87, 155001 (2001)]. The EAW observed in our simulations is strongest well below the phase-matched frequency for electron acoustic scatter, and therefore the EAW is ...

Coherent acceleration of magnetized ions by electrostatic waves with arbitrary wavenumbers

This paper studies the coherent acceleration of ions interacting with two electrostatic waves in a uniform magnetic field B0. It generalizes an earlier analysis of waves propagating perpendicularly to B0 to include the effect of wavenumbers along B0. The Lie transformation technique is used to develop a perturbation theory describing the ion motion, and results are compared with numerical solutions of the complete equations of motion. Coherent energization occurs when the Doppler-shifted wave frequencies differ by nearly an integer multiple of the ion cyclotron frequency. When the difference in the parallel wavenumbers of the two waves is increased the coherent energization of ions is limited to a small part of the phase space. The energization of ions and its dependence on wave parameters is discussed.

Study of laser plasma interactions using an Eulerian Vlasov code

We present a one-dimensional Eulerian Vlasov code for performing kinetic simulations of laser-plasma interactions. We use the code to study parametric instabilities, in particular stimulated Raman scattering. For conditions similar to those of singlehot-spot experiments, we flnd that kinetic efiects are important in the saturation of this instability. Work is underway to make the code to 1 1 and 2D (resolving y and py) and parallelize it.

Vlasov simulations of trapping and inhomogeneity in Raman scattering

We study stimulated Raman scattering (SRS) in laser-fusion conditions with the Eulerian Vlasov code ELVIS. Back SRS from homogeneous plasmas occurs in subpicosecond bursts and far exceeds linear theory. Forward SRS and re-scatter of back SRS are also observed. The plasma wave frequency downshifts from the linear dispersion curve, and the electron distribution shows flattening. This is consistent with trapping and reduces the Landau damping. There is some acoustic (! / k) activity and possibly electron acoustic scatter. Kinetic ions do not affect SRS for early times but suppress it later on. SRS from inhomogeneous plasmas exhibits a kinetic enhancement for long density scale lengths. More scattering results when the pump propagates to higher as opposed to lower density.

Symbolic computation of nonlinear wave interactions on MACSYMA

Abstract In this paper we describe the use of a large symbolic computation system - MACSYMA - in determining approximate analytic expressions for the nonlinear coupling of waves in an anisotropic plasma. MACSYMA was used to implement the solutions of a fluid plasma model nonlinear partial differential equations by perturbation expansions and subsequent iterative analytic computations. By interacting with the details of the symbolic computation, the physical processes responsible for particular nonlinear wave interactions could be uncovered and appropriate approximations introduced so as to simplify the final analytic result. Details of the MACSYMA system and its use are discussed and illustrated.

Electron Cyclotron Resonance Heating of Plasmas in Spherical Tori

The conventional ordinary and extraordinary modes in the electron cyclotron range of frequencies are not suitable for heating of and/or driving currents in spherical tori (ST) plasmas. However, electron Bernstein waves offer an attractive possibility for heating and current drive in this range of frequencies. In this paper, we summarize our theoretical and numerical results which describe the excitation and propagation of electron Bernstein waves in ST plasmas. INTRODUCTION Plasmas in spherical tori, e.g. in MAST and NSTX, present a special experimental challenge when considering heating and current drive by waves in the electron cyclotron range of frequencies. This is primarily due to such plasmas being overdense, i.e., ωpe/ωce 1 where ωpe and ωce are the electron plasma and electron cyclotron frequencies, respectively. For fundamental or second harmonic heating in such plasmas, the conventional X mode and O mode are cutoff near the edge of the plasma and cannot access the core. For higher harmonics the plasma is essentially transparent to the X and O modes. Mode conversion near the plasma edge allows X or O mode polarized power, incident from free space, to couple to electron Bernstein waves (EBW). The X mode couples to EBWs in the vicinity of the upper hybrid resonance (UHR). The O mode coupling to EBWs is via the slow X mode whereby power from the externally excited O mode is first mode converted to the slow X mode which subsequently mode converts to EBWs near the UHR. The propagation of EBWs is not density limited and the waves damp effectively on electrons in the vicinity of the Doppler shifted electron cyclotron resonance (or its harmonics) [1]. In this paper we discuss theoretical details of the constraints imposed on the mode conversion of X and O modes to the EBWs in spherical tori. We also demonstrate interesting features of EBWs as they

Electron Bernstein Wave Research on the National Spherical Torus Experiment

Off‐axis electron Bernstein wave current drive (EBWCD) may be critical for sustaining non‐inductive high β NSTX plasmas. Modeling results predict that the ∼ 100 kA of off‐axis current needed to stabilize a solenoid‐free high β NSTX plasma could be generated by by 3 MW of 28 GHz EBW power. Synergy with the bootstrap current may enhance CD efficiency by ∼ 10%. EBW radiometry measurements on NSTX support coupling to EBWs by launching elliptically polarized electromagnetic waves oblique to the confining magnetic field. Plans are being developed to implement a 1 MW, 28 GHz proof‐of‐principle EBWCD system to test the EBW coupling, heating and CD physics at high rf power densities on NSTX.

Electron Bernstein Wave Research on NSTX and CDX-U

Studies of thermally emitted electron Bernstein waves (EBWs) on CDX‐U and NSTX, via mode conversion (MC) to electromagnetic radiation, support the use of EBWs to measure the Te profile and provide local electron heating and current drive (CD) in overdense spherical torus plasmas. An X‐mode antenna with radially adjustable limiters successfully controlled EBW MC on CDX‐U and enhanced MC efficiency to ∼ 100%. So far the X‐mode MC efficiency on NSTX has been increased by a similar technique to 40–50% and future experiments are focused on achieving ⩾ 80% MC. MC efficiencies on both machines agree well with theoretical predictions. Ray tracing and Fokker‐Planck modeling for NSTX equilibria are being conducted to support the design of a 3 MW, 15 GHz EBW heating and CD system for NSTX to assist non‐inductive plasma startup, current ramp up, and to provide local electron heating and CD in high β NSTX plasmas.