R. A. Stern

Temporal evolution of ion temperatures in the presence of ion cyclotron instabilities

Spectroscopic techniques to determine the temporal evolution of the ion temperature in a collisionless barium plasma are presented. The theory of temperature relaxation in plasmas with external magnetic field is modified to include wave‐induced collisions. Experiment and theory are compared. The current‐driven electrostatic ion cyclotron instability is shown to be an effective method for wave heating of ions.

Direct ion-transport measurement by optical tagging

Abstract Optical pumping of long-lived quantum states is used to identify groups of ions in phase space and follow their transport. The principles and variants of a novel diagnostic method, as well as new physical aspects and applications, including space-dependent line narrowing, are demonstrated.

Photodetachment technique for measuring H− velocities in a hydrogen plasma

This article reports work in progress on laser diagnostics of negative‐ion transport velocity in H−‐ion volume sources. The plasma dynamics after the laser shot is discussed in detail, and the effect of the potential perturbation on the H− velocities is evaluated. A method of evaluation of the H− transport velocity from single‐laser‐beam photodetachment experiments is proposed. To substantiate this method, two‐laser‐beam photodetachment experiments have been effected. The velocities thus determined are pressure dependent; they correspond to H− energies in the range 0.23–0.08 eV.

Beam-plasma interactions in a positive ion-negative ion plasma

An electron‐free plasma consisting of negative ions (SF6 −) and positive ions (Ar+), and negligible neutral‐ion collision frequencies has been created in the laboratory. This plasma has a mass ratio of approximately 3.5‐similar to many computer particle‐in‐cell simulated systems. A fluid description of this positive and negative ion confinement (PANIC) plasma is given and compared to experimental measurements of a beam–plasma instability for both beam species and a wide range of beam energies. The fluid dispersion relation and most growing modes are predicted to be insensitive to many parameters of the PANIC beam–plasma system, and found to the consistent with the data.

Measurement of the H− thermal energy in a volume ion source plasma

The H− negative ion thermal energy measured using the two‐laser‐pulse photodetachment technique is reported to be in the range from 0.1 to 0.7 eV for various conditions of volume ion source operation (pressure−from 2 to 7 mTorr, discharge current−from 1.5 to 20 A). The hydrogen pressure has a significant effect in lowering the negative ion temperature, while the increase of the discharge current leads to a rise in T−. It is found that T− is a fraction of the electron temperature, Te. This fraction is strongly dependent on the gas pressure. T− scales linearly with the electron temperature and exceeds the highest values predicted by the theory of dissociative attachment. The possible mechanisms for H− ion heating are discussed.

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-Alfven 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.

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.

H- and D- temperature in volume sources

A new technique using laser induced photodetachment has been developed for measuring the negative ion temperature in H− and D− sources. Using this technique, we have investigated the dependence of the negative ion temperature on source parameters such as pressure, discharge current, and electron temperature. Simultaneous measurements of negative ion density, temperature, and extracted current lead to the conclusion that the extracted negative ion current is, at most, equal to the thermal flux.

Experimental studies of the propagation of electrostatic ion perturbations by time-resolved laser-induced fluorescence

Effects induced by the propagation of several kinds of electrostatic perturbation in a low-density collisionless argon plasma are observed with space, time, and velocity-resolved laser-induced fluorescence (LIF). The propagation of strong self-organized ion structures is observed and the associated electric field is determined. Snap shots of the ion phase space with a time resolution of 2 μs can be reconstructed from the experimental data. All the terms of the kinetic equation can also be determined from the data. A one-dimensional (1D) numerical simulation reproduces qualitatively the experimentally observed ion phase space behavior.

A fundamental limit to the polarized current density produced by the lamb-shift polarized ion source

Abstract The Lamb-shift polarized ion source has been shown to be able to produce only 2–3 μA of polarized beam current. The mechanism that is responsible for this limitation has proved elusive over the years. In this paper, we present an explanation for this limitation. We have conducted a proof-of-principle-type of experiment that verifies our hypothesis. The results from the experiment are then used along with well-known properties of the source to predict the limiting current within a factor of two of the observed value.