Perturbation of a Plasma by a Probe

Abstract

Indications from voltage-current curves of thermionic power converters suggest increased ionization at, or very near, cesium-coated surfaces by low-energy electrons. Experiments have been conducted to study this phenomenon quantitatively for a low cesium concentration. The tube contained three indirectly heated tantalum surfaces surrounded by a shield. These heated surfaces, El, E2, and E3, were tantalum tubes located at the three corners of an equilateral triangle. The extension of a line from the corner EL perpendicular to the base of the triangle formed by the line E2 and E3 served to locate the position of a fine-wire ion collector. Electrons were accelerated from surfaces E2 and E3 to bombard El. Ions produced at or near El were then accelerated toward the ion collector to serve as a measure of the ion production. Results of this experiment indicate that no appreciable ion current was produced in cesium vapor controlled by a TCs = 350°K, by electrons having an energy less than 3.89 volts. The square root of the ion-to-electron current ratio was a linear function of the surface electron energy in excess of the ionization potential. Although not accurately determined, the ionization coefficient is given by Pi = P (V-Vl) with P1 = 20 cm v for the standard condition of 3.54 X 1016 atoms/cm 3 II. The Energy Distribution for Electrons in a Thermionic Diode Plasma Cannot Be Truly Maxwellian AbstractFor the refractory emitters, such as tungsten, tantalum, and molybdenum, operated in an energy-conversion diode, the ratio of the emitter temperature to the cesium temperature strongly influences the operating properties. If this ratio exceeds 3. 2, an ionrich sheath usually forms at the emitter surface. Electrons are accelerated in the plasma space by this injection potential. An analysis made here shows that at the opencircuit condition, the energy distribution of the electrons at the plasma edge of the emitter sheath cannot be a true Maxwellian over the entire range in electron energy. Instead, it is thought to be made up of two quasi-Maxwellians. The low-energy electrons are trapped and may have a high average energy close to that of the injection potential. The untrapped electrons will have an electron temperature equal to that of the emitter and their density will be that associated with an apparent or fictitious density many times that of the actual density.A theory has been developed to describe quantitatively the relations that must be satisfied for the description above to apply. From typical numerical results that are applicable to specific diode configurations it is concluded that if the emitter-to-cesium temperature ratio exceeds 3.6, volume ionization is not required for a stable sheath. In the range 3.2-3. 6 surface ionization is usually insufficient, and it is necessary to have volume ionization to sustain the sheath. Further studies will be required to establish the mechanism. It may be direct ionization by high-energy electrons, or ionization from excited states of the cesium atoms.

The effect of ionization on heat transfer to wires immersed in a highly thermallyionized plasma

“…A more direct treatment of the interaction between a plasma and a solid is available in the literature in conjunction with electrostatic probes [7][8][9][10][11]. The purpose of these probes is to infer, from the probe response, some properties of the plasma surrounding the probe.…”

“…The purpose * Numbers in brackets refer to entries in References. of the recent literature on electrostatic probes [8][9][10][11] is to theoretically account for the disturbance of the plasma by the probe and to predict the currentvoltage characteristic as a function of conditions in the undisturbed plasma. Interpretation of the currentvoltage characteristic in terms of plasma properties is possible only under very simplified theoretical assumptions for the charge carrier flow to the probe.…”

“…This potential, aptly termed the floating potential in electrostatic probe theory, accomplishes the required restriction of no net current flow to the entire probe immersed in the plasma. For conditions of this study, the characteristic time for the wire to respond to immersion in the plasma through a chemically frozen boundary layer is estimated [ 11 ] by 100 r~…”

“…The flow of charged particles, which determines the value of this energy transfer, is governed by the density and temperature gradients which, in turn; establish a certain potential difference between the plasma and the wire. The values of ion and electron currents are calculated from a model due to Waymouth [ 11 ] which is valid for the range of plasma and probe conditions encountered in this study. Values of the parameters describing these conditions are listed in Table 2.…”

Langmuir Probe Diagnostics for Medium Pressure and Magnetised Low-Temperature Plasma

“…Two collisionless models are used: Laframboise's [22] model that includes the classical Langmuir's orbital motion limited model (OML) [3] and the radial motion cold ion approximation model, so-called ABR-Chen [19,20]. We shall not present the thin sheath collision models such as "continuum plus free fall theory" [41,42] since they require higher plasma density than what is usually used in plasma-aided technologies.…”