T. Intrator

Techniques for using emitting probes for potential measurement in rf plasmas

Investigations of the effects of rf on plasma potential measurements with electron emitting probes and methods for interpreting data are presented. Techniques correspond to the floating and inflection point methods of single‐emitting and differential emitting probes, respectively. A simple method of measurement of plasma potential fluctuations is given which makes use of time‐averaged emitting probe I–V characteristics.

The virtual cathode as a transient double sheath

The two‐dimensional plasma potential measurements are given of a space‐charge dominated double sheath near a hot cathode. Laboratory data show that a virtual cathode is a self‐consistent solution only for a transient cathode‐plasma system. Slow charge exchange ions get trapped in the potential dip that forms the virtual cathode and eventually destroy it.

Emissive probe current bias method of measuring dc vacuum potential

It is experimentally demonstrated that, with proper current bias, emissive probes can accurately measure dc electric potential in a vacuum. A comparison is made of the accuracy and time response of this ‘‘vacuum current bias’’ method with two other emissive probe techniques, the inflection point in the limit of zero emission, and the floating potential of a strongly heated probe.

Experimental observations of self-similar plasma expansion

The potential profiles of an expanding plasma have been measured experimentally. The profiles are shown to be self‐similar.

Temporal evolution of collisionless sheaths

We have performed experimental measurements to determine the temporal evolution of Langmuir sheaths near an electrode to which a negative step bias is applied in a collisionless argon plasma. The plasma was produced by a hot‐filament discharge in a multidipole device. Plasma potential data were obtained using emissive probes with two different techniques: time resolved sampling and time averaged techniques. The sheath is found to initially form close to the electrode, to extend to a maximum separation, and to contract to a steady‐state value. The time scale required to reach a steady state is close to the time scale of the presheath relaxation. Characteristics of sheaths in rf plasmas are also measured using a parallel‐plate plasma capacitor. It is observed that the plasma potential profile has significant variation with frequency, even for frequencies as low as 1 kHz which are far below the ion plasma frequency (∼1 MHz).

Direct indication technique of plasma potential with differential emissive probe

We present a new method of plasma potential measurement with a differential emissive probe. The dc heated probes are combined with a feedback loop control circuit to measure plasma potential automatically. In addition, it is shown that connecting to the central point of the filaments reduces the effect on the emissive currents of voltage drops across the filaments. The decay time constant of half‐cycle heated emissive probe current during the off heating cycle is shown to depend on the initial emission current and on the bias voltage.

Langmuir probe characteristics in RF glow discharges

The current-voltage characteristics of Langmuir probes in rf glow discharges can be misinterpreted by the effects of rf time averaging, ionization near a probe, and expansion of the probe sheath. This paper presents a new Langmuir probe technique which can be used to determine the plasma parameters in rf glow discharges. Simple expressions for the time averaged I-V characteristics are derived for the cases of fully sinusoidal and partially rectified plasma potential waveforms. Examples of corresponding I-V characteristics obtained from argon rf glow discharges are also illustrated.

Measurement of vacuum space potential by an emissive probe

Using the inflection point method of interpreting emissive probes, it is shown that it is possible to measure the electric space potential established between two parallel metal plates in a vacuum.

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.

Multiple valued floating potentials of Langmuir probes

It is shown that Langmuir probes can have three different floating potentials in plasmas produced by a hot filament discharge in a multi‐dipole device when the primary and secondary electron currents are comparable. The measured floating potential depends on the probe’s initial condition—the most negative and the least negative potentials are found to be stable and the in‐between value is found to be unstable. Results are compared to a simple theoretical model.