Comments on "Plasma current drive by injection of photons with helicity" [Comm. Plasma Phys. Controlled Fusion 12, 165 (1989)]

Ohkawa has proposed a tokamak current-drive scheme [Comm. Plasma Phys. Controlled Fusion 12, 165 (1989)], which relies on the injection of circularly polarized magnetohydrodynamic waves. It is shown here that the favorable current-drive efficiency predicted by Ohkawa is not attained because excessive power is dissipated by the fluctuating fields. The ratio of power absorbed by the plasma to the dc Ohmic power required to drive the same current is Pabsorbed/POhmic[approximately-equal-to]4(Ba/b)^2, where Ba is the static toroidal field strength and b is the strength of the fluctuating field

Can useful toroidal current be driven by classical viscoresistive Alfvén waves?

Simple, yet exact, analytic solutions for the shear and compressional Alfvén wave are obtained for helical magnetohydrodynamic (MHD) waves in cylindrical geometry with both resistivity and viscosity included. The current driven by all possible combinations of these waves is examined in the quasilinear regime (i.e., where the magnetic field produced by the driven current is not self-consistently included in the equilibrium where the wave equations are derived). It is found in all cases that it is not possible to drive significant bulk axial current with small amplitude wave fields. Thus, any useful low-frequency current drive scheme will have to be based on phenomena more complicated than those discussed here

A quick-retrieval high-speed digital framing camera

A new high-speed digital framing camera is described. The design is built around a rotating polygon mirror that provides a framing rate of 24 000 frames/s. The camera electronics digitizes an image into a 32×104 grid of pixels, where the second dimension of the grid can be varied and is determined by the 8 bit computer-aided measurement and control digitizer sampling rate. Available digitizer memory provides for 314 frames at this horizontal resolution. The advantages over other available high-speed framing cameras are (1) low cost of the system provided the digitizers are available, (2) rapid retrieval of a recorded event, and (3) the ease with which the system can be used. Sample results from an application in high-power arc photography are given to illustrate the system's spatial and temporal resolution

Poincaré maps define topography of Vlasov distribution functions consistent with stochastic dynamics

In a recent paper [A. D. Bailey et al., Phys. Rev. Lett. 34, 3124 (1993)], the authors presented direct planar laser induced fluorescence measurements of the oscillatory ion fluid velocity field in the presence of a large amplitude drift-Alfven wave. Surprisingly, the measured speeds were an order of magnitude lower than predicted by standard fluid theory, yet the flow pattern was consistent with the fluid theory. A new model, based on the connection between stochasticity and bulk behavior, is presented which gives insights into the cause of this behavior. It is shown that when particle motion is stochastic, invariant sets of a 'Poincaré map' define a flat-topped particle distribution function consistent with both the electromagnetic field driving the Vlasov equation and the fine-scale single particle dynamics. The approach is described for the general case and explored for a slab model of the observed drift wave

Observations of fast anisotropic ion heating, ion cooling, and ion recycling in large-amplitude drift waves

Large-amplitude drift wave fluctuations are observed to cause severe ion temperature oscillations in plasmas of the Caltech Encore tokamak [J. M. McChesney, P. M. Bellan, and R. A. Stern, Phys. Fluids B 3, 3370 (1991)]. Experimental investigations of the complete ion dynamical behavior in these waves are presented. The wave electric field excites stochastic ion orbits in the plane normal (perpendicular to) to B, resulting in rapid perpendicular to heating. Ion-ion collisions impart energy along (parallel to) B, relaxing the perpendicular to-parallel to temperature anisotropy. Hot ions with large orbit radii escape confinement, reaching the chamber wall and cooling the distribution. Cold ions from the plasma edge convect back into the plasma (i.e., recycle), causing further cooling and significantly replenishing the density depleted by orbit losses. The ion-ion collision period tau(ii)similar to Tau(3/2)/n fluctuates strongly with the drift wave phase, due to intense (approximate to 50%) fluctuations in n and Tau. Evidence for particle recycling is given by observations of bimodal ion velocity distributions near the plasma edge, indicating the presence of cold ions (0.4 eV) superposed atop the hot (4-8 eV) plasma background. These appear periodically, synchronous with the drift wave phase at which ion fluid flow from the wall toward the plasma center peaks. Evidence is presented that such a periodic heat/loss/recycle/cool process is expected in plasmas with strong stochastic heating

Real-time phase-selective data acquisition system for measurement of wave phenomena in pulsed plasma discharges

A novel data acquisition system and methodology have been developed for the study of wave phenomena in pulsed plasma discharges. The method effectively reduces experimental uncertainty due to shot-to-shot fluctuations in high repetition rate experiments. Real-time analysis of each wave form allows classification of discharges by wave amplitude, phase, or other features. Measurements can then be constructed from subsets of discharges having similar wave properties. The method clarifies the trade-offs between experimental uncertainty reduction and increased demand for data storage capacity and acquisition time. Finally, this data acquisition system is simple to implement and requires relatively little equipment: only a wave form digitizer and a moderately fast computer

Observation of fast stochastic ion heating by drift waves

Anomalously fast ion heating has been observed in the Caltech Encore tokamak [Phys. Rev. Lett. 59, 1436 (1987)], with the use of laser-induced fluorescence. This heating was found to be independent of electron temperature, but was well correlated with the presence of large-amplitude drift-Alfvén waves. Evidence is presented that suggests that the heating is stochastic and occurs when the ion displacement due to polarization drift becomes comparable to the perpendicular wavelength, i.e., when k[perpendicular] (mik[perpendicular] phi0/qB^2)~1. Stochastic heating may also be the cause of the anomalously high ion temperatures observed in reversed-field pinches

Heterodyne interferometer with unequal path lengths

Laser interferometry is an extensively used diagnostic for plasma experiments. Existing plasma interferometers are designed on the presumption that the scene and reference beam path lengths have to be equal, a requirement that is costly in both the number of optical components and the alignment complexity. It is shown here that having equal path lengths is not necessary - instead what is required is that the path length difference be an even multiple of the laser cavity length. This assertion has been verified in a heterodyne laser interferometer that measures typical line-average densities of $\sim 10^{21}/\textrm{m}^2$ with an error of $\sim 10^{19}/\textrm{m}^2$.Comment: 15 pages, 9 figures, to be published in Rev. Sci. Instrum. 77 (2006

Characterization of a spheromak plasma gun: The effect of refractory electrode coatings

In order to investigate the proposition that high-Z impurities are responsible for the anomalously short lifetime of the Caltech spheromak, the center electrode of the spheromak plasma gun has been coated with a variety of metals (bare steel, copper, nickel, chromium, rhodium, and tungsten). Visible light (230–890 nm) emitted directly from the plasma in the gun breech was monitored for each of the coated electrodes. Plasma density and temperature and spheromak lifetime were compared for each electrode. Results indicate little difference in gun performance or macroscopic plasma parameters. The chromium and tungsten electrodes performed marginally better in that a previously reported helicity injection effect [Phys. Rev. Lett. 64, 2144 (1990)] is only observed in discharges using these electrode coatings. There are subtle differences in the detailed line emission spectra from the different electrodes, but the spectra are remarkably similar. The fact that (1) contrary to expectations, attempts to reduce high-Z impurities had only marginal effect on the spheromak lifetime coupled with (2) an estimate of Zeff<2 based on a 0-D model suggests that it is not impurities but some other mechanism that limits the lifetime of small, cold spheromaks. We will discuss the general characteristics of the spheromak gun as well as effects due to the coatings