Steven L. Cartier

Anode-type double layers in a nonuniform magnetic field

The properties of strong, magnetized, three‐dimensional double layers are studied. The double layers are produced by drawing a discharge to a large anode plate located in the diverging magnetic field region of a cylindrical argon discharge. If the anode voltage is sufficiently high, the electrons that are accelerated through the anode sheath may become sufficiently energetic to ionize the background neutral gas and transform the anode sheath into a strong double layer. The resulting conical‐shaped structures, which extend outward from the plate, have parallel, oblique, and perpendicular electric field components with respect to the magnetic field. The axial extent of these structures depends on the plate bias voltage, neutral gas pressure, and the magnetic field. At neutral gas pressures of a few millitorr, the double‐layer structures are visually apparent because of the enhanced light emission from neutrals excited by the energetic electrons. Color photographs of some of these structures are shown. The s...

Surface trapping of primary electrons by multidipole magnetic fields

Multidipole plasma devices are operated in an unconventional manner with primary electrons prevented by surface magnetic fields from entering the main plasma volume. Applications of this technique, to provide primary electron‐free unmagnetized plasmas and to furnish a source for a large diameter magnetized plasma column, are given.

Electrostatic ion-cyclotron waves in a nonuniform magnetic field

The properties of electrostatic ion‐cyclotron waves excited in a single‐ended cesium Q machine with a nonuniform magnetic field are described. The electrostatic ion‐cyclotron waves are generated in the usual manner by drawing an electron current to a small exciter disk immersed in the plasma column. The parallel and perpendicular (to B) wavelengths and phase velocities are determined by mapping out two‐dimensional wave phase contours. The wave frequency  f  depends on the location of the exciter disk in the nonuniform magnetic field, and propagating waves are only observed in the region where  f≳fci, where  fci is the local ion‐cyclotron frequency. The parallel phase velocity is in the direction of the electron drift. From measurements of the plasma properties along the axis, it is inferred that the electron drift velocity is not uniform along the entire current channel. The evidence suggests that the waves begin being excited at that axial position where the critical drift velocity is first exceeded, con...

Hysteresis in a low‐pressure argon discharge

Hysteresis in the discharge characteristics of a low pressure (p≂10−4 Torr), magnetized argon plasma column is reported. The hysteresis is associated with sudden jumps in plasma density and discharge current as either the discharge voltage or magnetic field strength is varied. A substantial change in the plasma density profile and the appearance of coherent plasma oscillations are also observed.

Filamental quenching of the current-driven ion-cyclotron instability

Experimental evidence is presented on the effect of the finite width of the current channel for the excitation of the current-driven ion-cyclotron instability. The results are in agreement with the non-local theory of Bakshi, Ganguli, and Palmadesso.

Observations of Nonlinear Behavior in a Low-Pressure Discharge Column

Sudden and abrupt jumps in the plasma density and discharge current of low-pressure magnetized argon and helium plasmas are observed. These jumps are found to depend on the discharge bias voltage, the neutral gas pressure, and the magnetic field strength and occur with a substantial hysteresis in those parameters. These jumps are accompanied by the onset of intense and coherent low-frequency plasma oscillations. In addition, under certain conditions, the radial density profile of the plasma is found to be significantly different following a jump. Some possibly related plasma instabilities are discussed.

Influence of the ion/neutral atom mass ratio on the damping of electrostatic ion-cyclotron waves

The damping of electrostatic ion–cyclotron waves by ion–neutral collisions was studied in a single‐ended Q machine. The amplitudes of K+ and Cs+ electrostatic ion‐cyclotron waves were measured as a function of neutral pressure in helium, neon, argon, krypton, and xenon. For each ion/neutral atom combination, the electrostatic ion‐cyclotron wave amplitude maximizes at a neutral pressure that scales monotonically with the m+/mn mass ratio. This result is interpreted by considering the dynamics of elastic collisions between the ions and the neutral atoms.

Wide‐range Langmuir probe sweep circuit

A circuit is described which facilitates measurements of Langmuir probe currents from 1 μA to 400 mA with sweep voltages as high as 70 V.

Comments on the ‘‘New method to measure plasma potential with emissive probes’’ [M. A. Makowski and G. A. Emmert, Rev. Sci. Instrum. 54, 830 (1983)]

A caution is raised concerning the use of the ‘‘droop method’’ [M. A. Makowski and G. A. Emmert, Rev. Sci. Instrum. 54, 830 (1983)] to measure plasma potentials with emissive probes. We show that other effects may be responsible for the observed droop upon which the method is based.