F. Skiff

Measurements of electron parallel‐momentum distributions using cyclotron wave transmission*

Practical aspects of the implementation of cyclotron wave transmission diagnostics are considered in three specific areas. Questions raised by the calculation of the diagnostic resolution and sensitivity, elimination of reflections through swept frequency and phase‐coherent detection, and correcting for the effects of wave refraction through the use of equivalent rays are addressed. Results of preliminary tests and experiments as well as comments on future implementations are used to illustrate the discussion.

Plasma Diagnostics with Spin-Polarized Ions

Abstract Ionic spin polarization by optical pumping is demonstrated as a technique for plasma test-particle diagnostics. “Spin-tagging” is used to observe the magnetized plasma tag-particle transfer function.

A nonlocal analysis of electrostatic waves in hot inhomogeneous bounded plasmas

A numerical code, seal, solving the full form of a second‐order integrodifferential equation that describes electrostatic waves in a slab plasma is presented. No expansion in the smallness of the ion Larmor radius is made. The plasma may have arbitrary density and temperature profiles and is immersed in a nonuniform magnetic field. Only small magnetic field gradients, Maxwellian equilibrium distribution functions, and ky =0 are assumed. First the integral equation is derived in Fourier space using the linearized Vlasov and Poisson equations, and then it is transformed back into real space, which enables us to treat the case of bounded plasmas. The two boundary conditions specified simulate an antenna at one end of the plasma and wave‐reflecting walls. Solutions having wavelengths smaller than the ion Larmor radius have been found. Comparison with experiments where ion Bernstein waves are launched in argon and barium plasmas shows very good agreement with the solution of the code seal. A positive‐definite ...

Measurements of electron dynamics during lower hybrid current drive

The method of oblique-incidence electron cyclotron wave transmission is studied in a lower hybrid driven tokamak plasma. Cyclotron wave transmission measures the asymmetry in the suprathermal electron parallel-momentum distribution which may be integrated to obtain a measure of the suprathermal electron current. Questions of resolution, sensitivity, and technique are addressed experimentally. The results indicate that cyclotron wave transmission is well suited for studies of electron velocity-space phenomena and may be used to detect small perturbations in the suprathermal current.

Observation and analysis of a two‐parallel‐temperature electron tail during lower‐hybrid current‐drive on a tokamak

A ‘‘two‐parallel‐temperature’’ feature of the fast‐electron distribution function has been observed during lower‐hybrid current‐drive experiments on the Versator‐II tokamak (B. Richards, Ph.D. thesis, Massachusetts Institute of Technology, 1981). Computational modeling predicts this feature and indicates that the colder tail is initiated by low‐power, high‐N∥ waves from the sidelobes of the antenna N∥ spectrum. The modeling indicates these waves bridge the ‘‘spectral gap,’’ enabling current drive even though the wave parallel phase velocity greatly exceeds the electron thermal velocity for the majority of the wave power.

Test-ion diffusion in a magnetized plasma

Cross-field transport of barium test ions is studied in a Q-machine plasma. Test particles are produced and tracked in their motion by an optical tagging method. The comparison between the measured tag signal and a simple theoretical simulation yields an evaluation of the ion diffusivity. Diffusion is supported by classical mechanisms. The injection of different noble buffer gases modifies the plasma parameters in such a way that the diffusion remains classical but its magnitude is reduced. >

Optical Carriage for Laser-Induced Fluorescence in a Magnetized Plasma

An ‘‘optical carriage’’ has been developed to improve plasma access for LIF diagnostics. Laser light inducing the fluorescence is transported through an optical fiber to the carriage. A telescope fixed on the carriage collects the plasma fluorescence light and sends it through a fiber bundle to an external PMT. The whole carriage is mounted on rails and can be scanned along and across the magnetic field.

Rapid cross-field ion transport in a plasma with sheared parallel flow

Note: 34th Annual Meeting APS, Division of Plasma Physics, Seattle, WA, USA Reference CRPP-CONF-1992-026 Record created on 2008-05-13, modified on 2016-08-08

Stochastic Plasma-Heating by Electrostatic-Waves - a Comparison between a Particle-in-Cell Simulation and a Laboratory Experiment

Abstract Dynamical chaos is produced by the interaction between plasma particles and two electrostatic waves. Experiments performed in a linear magnetized plasma and a 1D particle-in-cell simulation agree qualitatively: above a threshold wave amplitude, ion stochastic diffusion and heating occur on a fast time scale. Self-consistency appears to limit the extent of the heating process.

Ion-Cyclotron Wave Excitation by Double-Resonance Coupling

A modulated high frequency wave is used to remotely excite low frequency oscillations in a linear, strongly magnetized plasma column. An electromagnetic wave is launched as an extraordinary mode across the plasma by an external waveguide in the upper‐hybrid frequency regime f≊fUH≊fce≊8 GHz, with P≤2 W. By frequency modulating (at fFM≊1–60 kHz, with fci≂30 kHz) the pump wave, the resonant layer is swept radially across the profile and perpendicularly to the field lines at f=fFM. The resulting radial oscillation of the electron linear and nonlinear pressure can be considered to act as a source term for the ion wave. A localized virtual antenna is thereby created inside the plasma. Measurements of the ion dielectric response (interferograms and perturbed distribution functions) via laser‐induced fluorescence identify the two branches (forward, or ion‐acoustic‐like, and backward, or Bernstein, modes) of the electrostatic dispersion relation in the ion cyclotron frequency range. By changing the modulation band...